Photochemical Purification of Water and Air: Advanced Oxidation Processes (AOPs): Principles, Reaction Mechanisms, Reactor Concepts

Thomas Oppenländer Photochemical Purification of Water and Air Thomas Oppenländer Photochemical Purification of Wate...

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Thomas Oppenländer

Photochemical Purification of Water and Air

Thomas Oppenländer Photochemical Purification of Water and Air

Thomas Oppenländer

Photochemical Purification of Water and Air

Author Thomas Oppenländer University of Applied Sciences (Fachhochschule) Furtwangen Department of Mechanical and Chemical Engineering Jakob-Kienzle-Str. 17 78054 Villingen-Schwenningen Germany Email: [email protected] José Franco-Pereira, who received the degree of a Magister Artium (M.A.) in film and media sciences at the University of Frankfurt (Germany), kindly designed the cover picture.

n This book was carefully produced. Nevertheless, author and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: Applied for. British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at . 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany. Printed on acid-free paper. Composition K + V Fotosatz GmbH, Beerfelden Printing betzdruck GmbH, Darmstadt Bookbinding Litges & Dopf Buchbinderei GmbH, Heppenheim ISBN 3-527-30563-7

V

Foreword The field of Advanced Oxidation Processes (AOPs) or Advanced Oxidation Technologies (AOTs) is of relatively recent vintage. However, AOPs/AOTs are proving to be important contributions to the treatment of water and air to remove harmful and toxic contaminants. Thus, it is gratifying to see this very comprehensive and authoritative treatise appear covering all aspects of AOPs/AOTs. These range from the treatment of industrial wastewaters, drinking water (both contaminant removal and disinfection), ultrapure water to the purification and deodorizing of air. Light sources include low pressure and medium pressure UV lamps, flash lamps, incoherent excimer lamps and sunlight itself. This treatise includes 146 figures, 17 worked out examples and 37 tables (many of which contain valuable collections of data and references). Each of the nine chapters cites extensive references, including very recent papers where the reader can follow up information in more detail. Professor Oppenländer is well qualified to write about AOPs/AOTs, since he has contributed to this literature in a very significant manner. It is thus particularly valuable to see the application of incoherent excimer lamps covered so well, a subject that has been a central part of his research. Finally, it is good to see careful attention being made to consistent terms, definitions and units in accord with IUPAC recommendations. This book will be of considerable value to graduate students, science and engineering faculty, scientists, process engineers and sales engineers in industry, government regulators and health professionals and all others with an interest in purifying our water and air. James R. Bolton President, Bolton Photosciences Inc. 628 Cheriton Cres., NW, Edmonton AB, Canada T6R 2M5

VI

Curriculum vitae Thomas Oppenländer studied chemistry at the University of Würzburg (Germany). He received his doctoral degree (Dr. rer. nat.) in 1984 working in the research group of Waldemar Adam on 185-nm-photochemistry of azoalkanes in solution. The following year was spent as a postdoctoral fellow at Colorado State University (Fort Collins, USA) in the group of Albert I. Meyers being involved in the asymmetric synthesis of dihydropyridines and NADH mimics. From 1985 to 1991 he was affiliated with F. Hoffmann-La Roche in Basel (Switzerland). Here he was engaged in research on drug phototoxicity and on the photochemical production of fine chemicals. Since 1991 he has been a full professor at the University of Applied Sciences (Fachhochschule) of Furtwangen (Germany) in the Department of Mechanical and Chemical Engineering, where he has been involved in photoinitiated advanced oxidation technologies. His main interests concentrate on the potentials of novel incoherent excimer lamps for photochemical treatment of water and air.

VII

Preface This book is aimed at researchers, engineers, and students who deal with problems in the expanding field of photo-initiated process technologies of water or air treatment. It lays claim to be a mine of information related to these interdisciplinary topics and presents a selection of the most important primary and secondary literature in this area of research and development. Clearly not all references of the vast research literature could be incorporated. The detailed discussion of many examples taken from the current scientific literature facilitates access to research topics of particular interest from different but interrelated point of views. This is supported by tables that collect together interesting studies, which deal with new developments of photochemical advanced oxidation processes and technologies allowing easy access to the primary scientific literature. A most important feature of this book is that it presents many detailed figures for the purpose of visualization of photochemical and photophysical phenomena, for the presentation of the underlying basic principles and for the interpretation of important research results. Additionally, several helpful example calculations are presented to demonstrate the use of quoted equations dealing, for example, with photons and reaction kinetics. This idea is thought to be very helpful in the understanding of the mathematical relationships presented Thus, the reader will become acquainted with the basic knowledge that is essential for an understanding of photochemical processes. This will enable him/her to carry out laboratory and large-scale experiments with the aim of developing and improving AOPs. There are many researchers and engineers in water and air industries who require more extensive and novel information than that which is obtainable from textbooks that have their emphasis placed on tradition. Therefore, this book intends to supply such missing information to as great an extent as is possible. To avoid any misinterpretations concerned with the digit style of numbers, the decimal point is used throughout the book instead of a comma (i.e. computer notation: 1.03 instead of 1,03, except for some graphical representations). In representative molecular structures, spin-paired non-bonding electrons around an atom of a molecule are represented (if necessary) by a bold line “7”, in accord with commonly used Lewis-structures. Single electrons are represented by a dot “•”. A full arrow (?) indicates shifts of electron pairs, whereas single electron shifts are

VIII

Preface

shown by using a half arrow (*). In photoscience an asterisk (*) usually denotes an electronically exited state of a molecule or atom, whereas in molecular orbital (MO) theory it marks an anti-bonding molecular orbital. The occasional use of commercial names to identify materials and instruments in no way implies endorsement or recommendation by the author. Thomas Oppenländer, November 2002

IX

Contents Introduction

1

1

AOPs and AOTs 5

2

Why UV and Oxidation/Disinfection?

19

2.1 Global Water Resources and Resulting Water Market 19 2.2 Present and Historical Dimensions of the Radiation Concept 24 2.3 Some Historical Landmarks of the Development of AOTs 29 References 32 3

Interaction of UV/VIS Radiation with Matter 37

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Photoscience in Research and Development 37 Physical Constants and Standard Values Used in Photochemistry 40 The Electromagnetic Wave 40 The Photon Stream and Planck’s Equation 41 Electromagnetic Spectral Ranges of Interest in Photochemistry 44 Conversion of Energy Units and Other Useful Conversions 46 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset of Molecules 51 Absorption of UV Radiation by Molecules 55 The Beer–Lambert Relationship 56 The Nature of Electronically Excited States 60 The Jablonski Diagram 64 Absorption of UV/VIS Radiation by Solids 66 UV/VIS Radiation as a Specific Reagent: Quantum Yield, Quantum Efficiency, Actinometry and Photoreaction Kinetics 68 Terms Associated with the Emission and Receipt of Electromagnetic Radiation 73 Safety Precautions for Radiant Sources 75

3.8 3.9 3.10 3.11 3.12 3.13 3.14

3.15 References 76 4

VUV and UV Radiant Sources and their Characteristics 79

4.1 4.2

Types of Lamps used in AOP Research and Development 79 Specific Properties of Mercury Arc Lamps 83

X

Contents

4.3 Development of Incoherent Excimer Lamps 87 4.4 Typical Photon Flow of VUV or UV Lamps 94 4.5 The Sun as Radiation Source 94 References 97 5

5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.4

Photochemical Processes of Water Treatment 101 Description of Aqueous Systems 101 Classification of Water Constituents 101 Analytical Parameters of Water Quality 105

Synopsis: Water and Wastewater Treatment Technologies 112 Synopsis: Oxidative Water Treatment Technologies and Methods of Hydroxyl Radical Production 114 Nomenclature of Photochemical AOPs 118 Photooxidation Reactions 119 Photocatalytic Reactions 120 General Reaction Schemes 122 Photo-Initiated Oxidations 122 Heterogeneous Photocatalysis 123 Homogeneous Photocatalysis 124 Photolysis of Water 126 Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies 127 Photodegradation of Water Pollutants 130

5.5 References 136 6

6.1 6.2 6.3

Properties, Reactivity and Photochemistry of Auxiliary Chemicals 145 Electronic Structures of Oxygen Species Involved 145 Reduction Potentials 149

UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates 149 Physical-chemical Properties of Ozone and Hydrogen Peroxide 155 Ozone 155 Hydrogen Peroxide 155 Photochemistry of Hydrogen Peroxide 157 Photocatalysts 159 Quantum Yields 162 Primary and Secondary Reactive Species 166 Hydrated Electrons 167 Hydrogen Atoms 168 Hydroxyl Radicals 168 Oxidative Degradation of Organic Matter by Hydroxyl Radicals 172 Aliphatic and Aromatic Hydrocarbons 172 Chlorinated Hydrocarbons 177 Rate Law and Rate Constants of Hydroxyl Radical Reactions 178

6.4 6.4.1 6.4.2 6.4.3 6.5 6.6 6.7 6.7.1 6.7.2 6.7.3 6.8 6.8.1 6.8.2 6.9 References 183

Contents

7

Photooxidation and Photomineralization of Organic Matter in Water and Air 189 Aqueous Systems 189 Substrate Oxidation and Substrate Mineralization 189 Reaction and Kinetic Modeling of H2O2-UV AOPs 191 Vacuum-UV Oxidation: The H2O-VUV AOP 200 Comparative Studies of Photo-Initiated AOPs 213 Biodegradability and Toxicological Studies 215 Gaseous Systems 219 Applicability 219

7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.2 7.2.1 7.2.2 Chemical and Physical-Chemical Aspects of Air Pollution 221 7.2.3 Process Technologies of Air Treatment 225 References 231 8

8.1 8.2 8.3 8.4 8.5

Process Engineering and Reactor Concepts 239 Chemical Engineering 239 Photoreactor Concepts 243 Reactor Materials 250

Influences on the Process Performance and Design Criteria 254 Economic Factors and Design Parameters (Figures-of-Merit) of PhotoInitiated AOPs 257 Selected Industrial Applications 268 Aqueous Systems 268 Gaseous Systems 269

8.6 8.6.1 8.6.2 References 273 9

UV Disinfection 279

References 290 10

Alternative Glossary 295

References 297 Glossary of Terms Used in Photochemistry 299 Index 353

XI

XIII

Abbreviations and Symbols

Abbreviations/Symbols a)

Units

Definition

[C], c 2,4-DCP A a A10, e abs (superscript) ACM ACO ads (subscript) AO AOP AOT a-Ox aq (subscript) c0 cb CFBR CFC CFU CHC CSTR DBE DBP DDT DO DOM E e (subscript) Ebg EEA EEM EEO el (subscript) EOP Ep

mol L–1 or g L–1 – m2 m–1 (SI) or cm–1 – – m2 kg–1 m2 m–3 order–1 – – – – – – m s–1 – – – m–3 (SI), L–1, cm–3 – – – – – – – V – eV – kW h kg–1 kW h m–3 order–1 – – m–2 s–1 or mol m–2 s–1

Concentration b) 2,4-Dichlorophenol Area (Decadic) absorption coefficient Absorbance (base 10 or base e) Absorbed Collector area per mass Collector area per order Adsorbed Atomic orbital Advanced oxidation process Advanced oxidation technology Auxiliary oxidant Aqueous, hydrated Speed of light in vacuum Conduction band Contact-free batch reactor Chlorinated fluorocarbon Colony forming units Chlorinated hydrocarbon Continuously stirred tank reactor Dielectric-barrier discharge Disinfection by-product Dichlorodiphenyltrichloroethane Dissolved molecular oxygen Dissolved organic matter Reduction potential Energetic Band gap energy European Environment Agency Electric energy per mass Electric energy per order Electric Enhanced oxidation processes Photon irradiance (cf. Tab. 3-9)

XIV

Abbreviations and Symbols Abbreviations/Symbols a)

Units

Definition

Ep eV FWHM g (subscript) h H0 HC Hp,0 IC IC ISC IUPAC

J eV – – Js J m–2 – mol m–2 ppm, mg L–1 – – –

kM kN,M

s–1 L mol–1 s–1

l

m

LP Hg lamp m M (MM)

– kg kg mol–1 (SI) or g mol–1

MO MP Hg lamp NA NOM Np; Nk (k specified) OM ou P P (subscript) PAHs PCBs PCDDs PCDFs PCO PCP in Pel PIEP

– – mol–1 – mol – – W – – – – – – – W –

POP ppb (10–9 parts/unit) ppm (10–6 parts/unit) ppmv (obsolete) Qk R r Rf

– ppb ppm ppmv J – e) –

Energy of a photon Electron volt Full width at half maximum (half-width) Gaseous Planck constant Fluence (cf. Tab. 3-9) Hydrocarbon Photon fluence (cf. Tab. 3-9) Inorganic carbon Internal conversion (Jablonski diagram) Inter system crossing (Jablonski diagram) International Union of Pure and Applied Chemistry Rate constant (first order) Rate constant (second order) of reaction between molecules N and M Lengthc), penetration depth, optical pathlength, photoreaction zone Low-pressure mercury lamp Mass Molar mass (the subscript M specifies a substrate molecule M)d) Molecular orbital Medium-pressure mercury lamp Avogadro constant Natural organic matter Amount of photons Organic matter Odor unit Radiant power Photon Polycyclic aromatic hydrocarbons Polychlorinated biphenyls Polychlorinated dibenzo-p-dioxines Polychlorinated dibenzo-p-furanes Photocatalytic oxidation Pentachlorophenol (C6Cl5HO) Electric input power Production-integrated environmental protection Persistent organic pollutant Parts per billion Parts per million Parts per million by volume Radiant energy Alkyl-skeleton of an organic molecule Reaction rate Reduction factor

Abbreviations and Symbols Abbreviations/Symbols a)

Units

Definition

ROS S SC Si SOC SUWOX t T T TC TOC trans (superscript) UNEP UV V v (subscript) vb vib VIS VOC VUV X Up U V˙ g) ek

– – – – – – h, min, s K (SI) or 8C – ppm, mg L–1 ppm, mg L–1 – – – m3 (SI) or L – – – – – – – mol s–1 (Einstein s–1) – m3 s–1 or L h–1 m2 mol–1 (SI) or L mol–1 cm–1, or M–1 cm–1 – – nm Hz, s–1 cm–1

Refractory organic substances Singlet state (Jablonski diagram) Semiconductor Scavenger molecule Semivolatile organic compounds Super critical water oxidation Time Temperature Triplet state (Jablonski diagram) Total carbon Total organic carbon Transmitted United Nations Environment Program Ultraviolet (200 nm ≤ k ≤ 400 nm) Volume Visible Valence band Vibrational Visible electromagnetic radiation = light Volatile organic compound Vacuum-UV (100 nm ≤ k ≤ 200 nm) Halogen Photon flow Quantum yield f) Volume flow rate Molar (decadic) absorption coefficient b)

g g k m m

Efficiency Quantum efficiency Wavelength Frequency Wavenumber

a) Additional terms used for the description and measurement of electromagnetic radiation are collected in Tab. 3-9, abbreviations of water analytical parameters are summarized in Tab. 5-1, and abbreviations and definitions of design parameters (“figures-of-merit”) are given in Tab. 8-4. b) Many authors prefer the symbol “M” for molar concentration, rather than mol L–1. c) To avoid any misinterpretations, the physical quantity length is symbolized by l in contrast to the numerical value 1. d) M is also used in chemical equations to denote a molecule or substrate. e) The unit depends on the reaction order. f) To distinguish the symbol for quantum yield U from photon flow Up, it is printed italic (if possible). g) Many authors use other symbols e. g., F for flow rate.

XV

1

Introduction Browsing the vast primary and secondary literature that deals with water or air treatment processes and technologies very quickly leads to a point of dissatisfaction and confusion concerning vague interpretations of experimental results, reaction mechanisms, or descriptions of reactor specifications. Because water and air are essential to our life, they are of special interest not only to researchers and engineers but also to economists and many others. Thus, extra-scientific considerations such as media attention are enormous. Many of these activities generate considerable confusion related to water and its treatment technologies, especially in the case of photochemical methods. From the chemist’s point of view, however, water in its purest form does not exist in the environment. It is very difficult and costly to produce and has anomalous solvent properties, so that it is contaminated immediately by dissolving trace elements or compounds from its environment. On the other hand, “pure” air is simply a mixture of several gaseous compounds of well-known composition. By thinking of nature as a huge photochemical reactor with the sun as an intensive source of electromagnetic radiation, the air as the gaseous component and water as the liquid medium, the bridge to photochemical reactions in water and air is built. This led to the investigation of photo-induced and photo-initiated processes within these media. As a result of these understandings, photochemical technologies for water and air treatment were developed over recent decades. Because of the interdisciplinary nature of this research area, which connects aqueous and gaseous chemistry and photochemistry with engineering concepts, an almost Babylonian expansion of terms, definitions, designations and abbreviations has been observed, such as AOPs, AOTs, EOTs, EEO, EEM, ACM, ACO, H2O2-UV, H2O-VUV, photocatalysis, photooxidation, photo-initiated oxidation, fluence, “UV light”, “UV dose”, etc. Consequently, it is one objective of this book to outline, strictly define, and unify the corresponding nomenclature used in the literature to describe the various processes and techniques (in accord with recommendations of the International Union of Pure and Applied Chemistry, IUPAC), and to address severe misinterpretations that abound throughout the general literature, such as for example the misleading, often used and fundamentally incorrect term “UV catalyzed” oxidation. Another important goal is the elaboration, clarification and discussion of the underlying chemical, photophysical and photochemical processes and mecha-

2

Introduction

nisms, with distinct emphasis on the last, based on the historical development of the photon concept. Hence, the complexity arising from the diversity of reactions in aqueous or in gaseous media with or without catalysts or auxiliary oxidants, in the presence or absence of microorganisms, will be structured and classified on an accurate basis. In addition, this task requires knowledge of the language of a chemist who is usually wild about chemical formulas and structures that are composed of line drawings. Hoffmann and Laszlo (1991) enthusiastically outlined this concept. These authors stressed that the language of chemistry parallels the grammar of our natural language (with several restrictions of course). This is also true if we introduce the specialized language of photochemistry, so that the scheme of Hoffmann and Laszlo may be extended to a “photochemical sentence” (excluding photophysical deactivation processes): Language: Chemistry: Photochemistry:

subject substrate substrate

–– transitive verb –– object –– reagent –– product –– absorbed photon –– photoproduct

The interpretation of an absorbed photon as a specific reagent that transforms a substrate under special conditions to a distinct photoproduct, i.e. which causes a photochemical reaction, has an enormous impact on the understanding and design of photochemical processes and their scaling-up. In addition, it clearly demonstrates that light or UV radiation can never act as a catalyst, because the energy of the absorbed photon is used to produce an electronically excited state of a molecule, and therefore its energy is dissipated and the photon is lost. Furthermore, one should notice that the product of a chemical (thermal) reaction is not usually identical with the photoproduct that is produced in a photoreaction. This is a direct consequence of the higher energy introduced by photons into a molecular entity compared with thermal chemistry, i.e. excited state versus ground state reactivity. This again has an important influence on the understanding of photochemical processes used for the purification and detoxification of water and air. After having discussed the fundamentals of the interaction of UV/VIS radiation with matter, the photochemical processes of water treatment will be analyzed in relation to the photochemistry of the oxidative auxiliaries. The behavior of the primary and secondary reactive species (mainly oxygen containing radicals and carbon radicals, respectively) is outlined in detail. After a systematic classification of photo-initiated oxidations, reactions related to the used oxidants, catalysts and applied excitation wavelengths, a detailed description of photooxidation and photomineralization of organic matter from a chemistry point of view follows. It includes the analysis of the level of reactive intermediates, the level of oxidation products and a discussion of toxicological studies. Additionally, several recent research activities and potential applications of novel incoherent excimer lamps including the expanding field of vacuum-UV (VUV) photochemistry of water are reported in detail. Another section of the book covers the engineering concepts based on the underlying photochemical and chemical processes. It assesses the different types of

Acknowledgements

photoreactors and the various influences on the performance of the process. Most importantly, the economic factors of photo-initiated oxidations are worked out according to the Bolton concept of design parameters (Bolton’s “figures-of-merit”) that are directly related to the observed degradation kinetics, and thus to the cost of the advanced oxidation process (AOP). Short reviews of the recent research activities into UV disinfection of water and air and of the UV treatment techniques of polluted air are enriched by the description and discussion of several selected examples of published industrial applications. Many questions related to the selection of UV processes will be answered and common pitfalls referring to UV applications addressed. Until now, many technical and research papers concerning photo-initiated treatment processes of water or air are almost useless for synoptic discussions due to the lack of precise data describing the photoreactor, its components and a comprehensive set of the analytical and engineering parameters of the system. Thus, a detailed analysis of the experimental information necessary for the correct description of photoprocesses (for example in technical publications) will help to facilitate future work and communication in this interdisciplinary field of research and development. Thus, the motto of this book is, “to purify, irradiate!”

Reference Hoffmann R, Laszlo P (1991) Darstellungen in der Chemie – die Sprache der Chemiker, Angew. Chem. 103: 1–16; similar article in French: Hoffmann R, Laszlo P (1989) La Représentation en Chimie, Diogène, No. 147: 24–54.

Acknowledgments

I wish to express my sincere gratitude to several companies and institutions that provided information concerning their products and research efforts. The following companies, institutions, and persons provided specific information for me: • a.c.k. aqua concept GmbH (Karlsruhe, Germany), M. Sörensen: case studies and a photograph of a photoreactor. • ABB (Baden, Switzerland), U. Kogelschatz: incoherent excimer lamps and corresponding emission spectra. Many stimulating discussions with Dr. Kogelschatz initiated research concerning water and air treatment with VUV/UV excimer lamps. • C & C Consult (Schleiden/Eifel, Germany): R. S. Chatty: technical information, investigations, and a photograph of a mobile air treatment system. • Calgon Carbon Inc. (Pittsburgh, PA, USA), W. Lem: photograph and description of a photoreactor and treatment examples. • Dr. Hönle AG UV-Technologie (Planegg/München, Germany), P. Wind: information and a photograph concerning the surface irradiation system used for bottle screw cap disinfection.

3

4

Introduction

• GMBU e.V. Gesellschaft zur Förderung von Medizin-, Bio- und Umwelt-Technologien, Fachsektion Sensorik (Jena, Germany), D. Faßler: electronic release of the Conference Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, May 28–31 (2000), Clausthal-Zellerfeld (Germany). • HDN-Technik GmbH (Rednitzhembach, Germany), K. W. Hofmann: information concerning odor abatement (gaseous systems) and waste air treatment and a photograph of a photoreactor. • Heraeus Noblelight AG (Kleinostheim, Germany), J. P. Schulz: product information and spectra of the low-pressure and medium-pressure mercury vapor lamp types. • Institute for Sanitary Engineering, Water Quality and Waste Management, University of Stuttgart (Germany), E. Thomanetz: descriptions and photographs of photoreactors, and treatment examples. • UMEX GmbH (Dresden, Germany), S. Johne: electronic release of the Conference Proceedings of the First International Congress on Ultraviolet Technologies, IUVA, June 14–16 (2001), Washington, DC; photoreactor specifications and photographs of UMEX reactors. • VitaTec UV-Systeme GmbH (Freigericht, Germany), M. Trageser: photographs of photoreactors and treatment examples. • WaCo Wassertechnik Consult GmbH (Hamburg, Germany), O. Debus: supplementary information regarding the AOX elimination with a contact-free batch photoreactor. The editorial assistance of Wiley-VCH is also acknowledged. In addition, I wish to thank the Willy-Hager-Stiftung (Stuttgart, Germany) for their financial support. This foundation arranged contacts with E. Thomanetz at the Institute for Sanitary Engineering, Water Quality and Waste Management, University of Stuttgart. We had close discussions on the internal project results concerning the development of AOTs. The exchange of stimulating ideas and the numerous scientific discussions with Prof. Thomanetz during the past few years were extremely fruitful and encouraging to the planning and realization of the current book project. He made many practical suggestions, and fascinating insights into the technical applications and problems of advanced oxidation technologies were opened up. Finally, yet importantly, I wish to acknowledge the direct support of James R. Bolton. He read the entire manuscript and offered critical and constructive comments, and made detailed suggestions concerning mechanistic interpretations and technical applications. James Bolton, Professor Emeritus of Chemistry, University of Western Ontario, Department of Chemistry (London ON, Canada), is currently the Executive Director of the International Ultraviolet Association (IUVA), Adjunct Professor at the Department of Civil & Environmental Engineering, University of Alberta (Edmonton AB, Canada), and he is the President of Bolton Photosciences Inc. (Ayr ON, Canada).

5

1

AOPs and AOTs Students and experts who concentrate seriously on research and development strategies that are related to the interdisciplinary topics of UV radiation/light, the environment and technology are increasingly confronted with an enormous flood of information that is spread throughout the scientific and technical literature. This is especially the case since the introduction of the Internet, which enables easy access to journalistic, and primary and secondary scientific literature. Hence, Tab. 1-1 summarizes several introductory and sophisticated links to Internet pages of special interest to these research areas. The link collection of Tab. 1-1 should facilitate the entry of newcomers into the complex field of ultraviolet (UV) radiation or light-induced environmental technologies and photochemical or photophysical research concepts. In addition, it should help the researcher and developer to specify their interests. In this connection, it is worth mentioning that often scientists and engineers do not differentiate correctly between the terms light and ultraviolet radiation in everyday usage. The term “light” strictly stands for the visible part (VIS) of the electromagnetic spectrum and it covers only a short wavelength segment between 380 nm ≤ k ≤ 780 nm. Nonetheless, the term “UV-light” is used in the common vocabulary of (photo)scientists and engineers, but for the sake of clarity, it should be avoided. In this book, the abbreviation UV/VIS is used to describe UV radiation and light. However, the term “radiation” has a negative public reaction (it means “high-energy radiation”, something strange and dangerous) and it should be used carefully in the public domain. The cross sections of the subject areas UV radiation/light and technology, UV radiation/light and environment, and environment and technology (Fig. 1-1) are interrelated and extraordinarily versatile with respect to their future development potentials. Therefore, the use of light and UV radiation in diverse applications represents one of the advanced technologies of the 21st century. This is particularly the case for the utilization of solar energy and any research activities that are related to climate protection technologies. This book concentrates on the basic principles, reaction mechanisms and engineering concepts of technologies that use UV radiation (or light) as a selective reagent for the cleavage of chemical bonds and hence for the destruction of unwanted chemicals or microorganisms. Accordingly, these methods include UV

6

1 AOPs and AOTs Tab. 1.1 Link collection of Internet pages comprising research, development, and general infor-

mation in the field of UV radiation/light, environment and technology (all links have been tested regularly for their actual status) Index

Source/description

Link

Air quality guidelines, drinking water quality, water resources

World Health Organization (WHO)

http://www.who.int/home-page

Bibliography on heterogeneous photocatalysisa)

National Renewable Energy Laboratory

http://www.ott.doe.gov/coolcar/ pdfs/water_ air2.pdf

Chemical nomenclature

International Union of Pure and Applied Chemistry

http://www.iupac.org

Chemical structure database, structure viewing support by Chemscape Chime

U.S. National Library of Medi- http://sis.nlm.nih.gov/Chem/ cine, National Institute of ChemMain.html Health

Chemical-physical data, litera- Database catalog for chemists http://www.chemieture datenbanken.de Data collections of organic and small inorganic compounds

National Institute of Standards and Technology (NIST)

http://webbook.nist.gov

Drinking water standards of the U.S.

EPA Office of Water

http://www.epa.gov/safewater/

Economy, technology, profes- German Machinery and Plant http://www.vdma.de sion and other online services Manufacturers Association (VDMA) b) Environmental policy and reports

Federal Environment Ministry http://www.bmu.de/ (BMU, Germany)

Environmental products, tech- ECO Services International nology and services, global di- (Switzerland) rectory

http://eco-web.com

European Union Law and other activities

European Union On-line

http://europa.eu.int

Federal Legislation of Germany

Federal Law Gazette (Germany)

http://www.bundesgesetzblatt. de

http://www.rbv-koeln.de/ Gas and water relevant topics, Federal Association of Gas technical press releases of and Water Companies/Asso- figawa/index.php4 ciation of Pipework Construc- http://www.rbv-koeln.de brbv c) tors (Germany) German Standards

German Institute of Standardization (DIN)

http://www.din.de

Glossary of terms used in photochemistry, full text in pdf format

Report of the IUPAC d) Photo- http://www.unibas.ch/epa/ chemistry Commission (Ver- texts.html hoeven, 1996)

1 AOPs and AOTs Tab. 1.1 (continued)

Index

Source/description

Hydrogen peroxide: applicaUS Peroxide tions to water, wastewater and hazardous waste treatment

Link http://www.h2o2.com

International chemical safety cards

National Institute for Occupa- http://www.cdc.gov/niosh/ tional Safety and Health ipcs/icstart.html

International conference

Advanced Oxidation Technolo- http://www.aotsconference.com gies for Water and Air Remediation (AOTs)

International conference

Solar energy research and ap- http://photoenergy.org plied photochemistrye)

International conference, research

Clausthaler UmwelttechnikInstitut GmbH

International standards and guidelines

International Organization for http://www.iso.ch/iso/en/ Standardization (ISO) ISOOnline.frontpage

Journal finder and journal search, journal service

Wiley Interscience

http://www3.interscience. wiley.com

Journal tables of contents: regular email service

Elsevier

http://www.elsevier.nl

Journals and books, subject search and alerting service (email)

Wiley-VCH

http://www.wiley-vch.de

Journals and publishers

ChemPort

http://www.chemport.org

Laser development and applications, optics, photonics and imaging

The International Society for Optical Engineering (SPIE)

http://www.spie.org

Laws, regulations, information

Environmental Protection Agency (EPA)

http://www.epa.gov

Light and lighting, radiation

International Commission on http://www.cie.co.at/ciecb Illumination (CIE)f)

Light measurement handbook g)

International Light, Inc.

Literature search, general

KVK-virtual catalog: books and http://www.ubka.uni-karlsjournals ruhe.de/kvk.html

http://www.cutec.de

http://www.intl-light.com/ handbook

Literature search: photochem- Photochemistry/photobiology istry and related fields database, access to photochemistry literature (Grabner and Kuti, 1997)

http://www.chemres.hu/ pchem

Odor assessment, annoyance and research

http://odournet.com

OdourNet

7

8

1 AOPs and AOTs Tab. 1.1 (continued)

Index

Source/description

Link

Organic photochemistry

Home page of Turro’s h) research group

http://turmac13.chem. columbia.edu

Original publications of photo- International Journal of Photoe- http://www.photoenergy.org/ ijp science, full text of papers in nergyi) pdf format, online journal Ozone and its applications

International Ozone Association (IOA)

http://www.int-ozoneassoc.org

Persistent organic pollutants (POPs)

POPs related United Nations Environment Program

http://irptc.unep.ch/pops/

Photochemistry activities

European Photochemistry As- http://www.unibas.ch/epa sociation (EPA)

Photochemistry activities

Inter-American Photochemical http://www.chemistry. Society (I-APS) mcmaster.ca/~iaps

The Radiation Chemistry Data http://www.rcdc.nd.edu/icabr/ Radiation chemistry databases: radical and excited state Center at the Notre Dame Ra- RCDC.html processes, extensive data com- diation Laboratory Database pilations Radical reactions and rate con- The Radiation Chemistry Data http://www.rcdc.nd.edu/ stants (Buxton et al., 1988): Center at the Notre Dame Ra- Solnkin2 e–aq, H•, •OH, O•–, 1O2 diation Laboratory Database Regulations and standards re- Environmental legislation lated to environment and en- (Germany) vironmental technology Research projects, environment

http://www.umwelt-online.de/ regelwerk/index.htm

Federal Ministry of Education http://www.bmbf.de and Research (BMBF, Germany)

Search engine: only web sites Elsevier Science containing scientific information

http://www.scirus.com

http://www.bgvv.de Security data of chemicals etc. Federal Institute for HealthOriented Consumer Protection and Vetenary Medicine in Berlin (Germany) Solar radiation, relevant glossaries

Renewable Resource Data Center (RReDC)

http://rredc.nrel.gov

Technical standards for indus- American Society for Testing tries and Materials (ASTM)

http://www.astm.org

Technical standards, gas and water treatment

German Association of Gas and Water Experts (DVGW)

http://www.dvgw.de

Toxicological profile information sheet

Agency for Toxic Substances and Disease Registry

http://www.atsdr.cdc.gov/ toxpro2.html

1 AOPs and AOTs Tab. 1.1 (continued)

Index

Source/description

Link

Toxicology, environmental health, chemical information

Specialized Information Ser- http://sis.nlm.nih.gov/ vices, National Library of Med- index.html icine

Ultraviolet technologies, appli- International Ultraviolet Asso- http://www.iuva.org cations, research ciation (IUVA) Ultraviolet technologies: exper- Bolton Photosciences Inc. tise

http://www.boltonuv.com

UNEP chemicals

United Nations Environment Program (UNEP)

http://irptc.unep.ch

Water activities in Europe

European Water Association (EWA)

http://www.ewaonline.de

Water activities in USA

American Water Works Association AWWA Research Foundation Water Environment Federation

http://www.awwa.org http://www.awwarf.com http://www.wef.org

Water activities, international, International Water Associapublications tion

http://www.iawq.org.uk

Water and wastewater industry news

U.S. Water News, Inc.

http://www.uswaternews.com

Water management industries, search engine

Dutch journal H2O

http://www.h2o-scanner.com

Water programs

EPA’s Office of Water

http://www.epa.gov/ow

Water technology online

National Trade Publications Inc.

http://www.watertechonline. com

Water, marketplace for the water industry

VerticalNet web site

http://www.wateronline.com

Water, photochemistry, and chemistry in general

German Chemical Society and http://www.gdch.de divisions (GdCh)

Water, versatile information j)

Water quality

http://www.waterquality.de

Water, wastewater and waste treatment

German Association for Water, Wastewater and Waste

http://www.atv.de

a) M Blake (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, January 1999: 154 pp, also available from the National Technical Information Service, Springfield, VA 22161. b) VDMA: Verband Deutscher Maschinen- und Anlagenbau e. V. c) brbv: Berufsförderungswerk des Rohrleitungsbauverbandes. d) IUPAC: International Union of Pure and Applied Chemistry. e) Photoenergy Center, Ain Shams University, Cairo. f) CIE (French title): Commission Internationale de l’Eclairage. g) by A Ryer (1997), International Light, Inc. h) NJ Turro, Columbia University, New York. i) edited by MSA Abdel-Mottaleb (since 1999). j) collected by H Willmitzer.

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1 AOPs and AOTs

Some interdisciplinary research and development areas related to the terms UV radiation/light, environment and technology.

Fig. 1.1

Fig. 1.2

Main objectives of photochemical technologies for the UV treatment of water and air.

techniques for the treatment of contaminated media water and air, mostly in the presence of oxidative auxiliaries or photocatalysts, leading to their efficient detoxification, purification or remediation, and disinfection, including odor abatement strategies and surface treatment (Fig. 1-2). However, the most obvious goal of these photochemical technologies is to attain water and air quality standards that fulfill government regulations according to emission ordinances. Ultimately, if necessary the complete mineralization of the organic carbon content of water or air can be achieved according to the general Eq. 1-1. In general, the process of photomineralization of organic contaminants describes the oxidation of organic carbon atoms to carbon dioxide or carbonate species (CO2, H2CO3, HCO–3, CO2– 3 ) initiated by electromagnetic radiation. Simultaneously, hydrogen atoms are converted into water and heteroatoms X of organic molecules are trans-

1 AOPs and AOTs

formed into the corresponding mineral acids HX = HCl, HBr, HNO3, HNO2, H2SO4, etc. Cn Hm Xz

hm;O2

! nCO2 m

z=2H2 O zHX

(1-1)

For example, 2,4-dichlorophenol (C6H4Cl2O) mineralizes stoichiometrically according to Eq. 1-2 (Bolton and Cater, 1994). Even pollutants containing inorganic carbon such as cyanide (CN–) can be oxidized to less toxic compounds by adequate irradiation methods.

(1-2)

All of these photochemical processes and related technologies are usually classified as Advanced Oxidation Processes (AOPs) (Glaze et al., 1987) and Advanced Oxidation Technologies (AOTs) or sometimes as Enhanced Oxidation Processes (EOPs). The underlying oxidative reaction mechanisms are basically imitations of natural photo-initiated processes that take place in sunlit surface waters or in the Earth’s atmosphere. They rely mainly on the formation of reactive and short-lived oxygen containing intermediates such as hydroxyl radicals (•OH) and they exploit the high reactivity of these species. The hydroxyl radical is a powerful oxidant and a short lived, highly reactive, and non-selective reagent that is easy to produce (Fig. 1-3). It has electrophilic properties and its reactions with appropriate substrate molecules are kinetically controlled usually exhibiting very high second order rate constants, which are often close to (or even above) the diffusion-con-

Some characteristic features of the hydroxyl radical.

Fig. 1.3

11

12

1 AOPs and AOTs

Fig. 1.4

The family of advanced oxidation technologies for water and air remediation.

trolled limit (von Sonntag, 1996). Kinetic reaction control refers to competing irreversible reactions in which the product composition is determined by the relative rates of product formation. Furthermore, the •OH radical is a ubiquitous transient species in nature and it is a very important agent in numerous human diseases or disorders and in the aging process (Knight, 1998). The transfer of such natural processes into technical dimensions requires methods for the efficient production of the so-called “free” hydroxyl radicals at sufficient concentrations from appropriate precursors. Besides photochemical and solar photocatalytic processes many other innovative technologies have been proposed for treating liquid or gaseous wastes around abandoned sites (Freeman and Harris, 1995) and are currently under intensive investigation (Fig. 1-4), for example catalytic processes (e.g. Hofmann et al., 2000), electrochemical oxidation of organic water contaminants (Galla et al., 1999) and non-thermal plasma treatment methods for exhaust air processing. Further AOPs of special interest include sonolysis (Ince et al., 2001, Hua and Hoffmann, 1997), X-ray irradiation and -radiolysis (Ferradini and Jay-Gerin, 1999), electron beam irradiation (Pikaev, 2000, Cooper et al., 1998, Getoff, 1993) of contaminated water, radiation chemistry in general (Jonah and Rao, 2001) and the SUWOX process (Schmieder and Abeln, 1999) that uses super critical water conditions (Kritzer and Dinjus, 2001) for the oxidation of organic water contaminants, and many other remediation technologies (Freeman and Harris, 1995). Over several years of extensive research and development efforts in these fields a wide-ranging knowledge of environmental applications has accumulated, including analytical methodologies and process engineering of photochemical reactions, e.g. for the generation of •OH radicals. This fact manifests itself in the countless scientific and technical publications in various journals that deal with environmental technologies and sustainable development.

1 AOPs and AOTs

At this point it is advisable to note that all the AOTs mentioned above are “endof-the-pipe” technologies that are usually applied to the subsequent elimination of undesirable production side or waste products. However, in the sense of production-integrated environmental protection (PIEP) the process dependent pollution of media should be avoided by ecologically friendly process optimizations at an early stage of chemical production (Christ, 1999 a, b). In this connection, photochemical AOPs are perfectly placed for the decontamination of tributary wastewater streams to avoid and to reduce residues that accumulate directly during the production process. This is also true for the UV disinfection of process water, demonstrating great relevance to water recycling strategies and for the prevention of point source pollution of water. Because even PIEP cannot be realized without the formation of wastewater and waste gases (cf. Christ, 2000) and because biotechnological processes depend extensively on biochemical reactions in the water phase there seems to be a positive future for photochemical (and other) AOTs for industrial water purification and/or disinfection, for example in synergistic operation with biological treatment installations. The search for comprehensive book publications concerning photochemical water and air treatment technologies turns out, surprisingly, to be without very much success, especially in the case of the German technical literature. Usually, textbooks on environmental chemistry (Bliefert, 1997, vanLoon and Duffy, 2000) cover the basics of UV-induced processes in water or in the Earth’s atmosphere only briefly. The modern technologies of photochemical water and air treatment are not covered. Even in books on environmental technology the techniques of UV oxidation or UV disinfection are only treated superficially (e.g. Bank, 2000, Hartinger, 1995, Kunz, 1995). This lack of comprehensive information is extremely disappointing in textbooks concerning wastewater treatment technologies (Neitzel and Iske, 1998, Pöppinghaus et al., 1994, Hancke, 1991) or in books on waste gas or air purification (Ebeling, 1999, Fritz and Kern, 1992). On the other hand, the physico-chemical principles and basic mechanistic considerations of the ozone UV and of the hydrogen peroxide UV process have been reported recently by Gottschalk et al. (2000) within the context of ozonation of water and wastewater. The disinfection of drinking water by UV treatment has a long history and it was covered by Bartz and Wippler several years ago (1989). In Europe today, over 3000 drinking water facilities use UV disinfection technology (Rice, 2001). Most encouraging in this context is the fact that the world’s largest UV disinfection plant was recently built in Germany to disinfect the water of the Wahnbachtalsperre located southeast of Cologne (König, 2000) supplying about 720 000 residents with 110 to 120 million m3 of drinking water per year. This plant is designed for volume flow rates V˙ of water of between 13 000 and 14 000 m3 h–1. It consists of seven independent photoreactor modules with a total of 462 UV lamps, i.e. special high-intensity low-pressure mercury lamps (König, 2001). In the United States and Canada even biologically pretreated municipal waste waters (secondary sewage effluents) are usually UV disinfected by passing through huge irradiation channels before they are drained into the surface water and released into natural cycles. It has been shown that UV irradiation is a power-

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1 AOPs and AOTs Tab. 1.2 Selected candidates, applications and products of UV treatment technology

UV disinfection

UV oxidation

Air purifiers (apartment, vehicle, hospitals etc.)

Air stripper ventilation treatment

Air-conditioning installations

Chemical and biochemical laboratories

Aquaculture for hatcheries

Degradation of toxic explosives

Clean room air supply, air disinfection

De-odorization of air, odor abatement

Community water systems

Galvanic wastewater: cyanide oxidation

Drinking water supply

Ground water remediation

Food and beverage industry

High purity water production

Hospital water supply

Landfill leachate water treatment

Indoor or closed system air treatment

Oxidation of non-biodegradable compounds

Legionella control in warm water systems

Pharmaceutical wastewater treatment

Municipal water systems

Reuse of wastewater

Pharmaceutical and cosmetic industry

Semi-conductor manufacturing industry

Process water treatment

Textile industry wastewater treatment

Rain water utilization

Treatment of recalcitrant organics a)

Sewage water treatment

Ultra pure water production

Sterile boxes

Process water treatment

Surface disinfection units

Drinking water treatment

Under sink UV systems (households etc.)

Mineralization of toxic contaminants

a) PAHs, PCP, PCBs, Pesticides, POPs etc.

ful alternative to chemical disinfection by conventional chlorination avoiding hazardous disinfection side products. Ho et al. reported in 1998 on the existence of over 500 UV facilities for wastewater disinfection operated in the United States with water flow rates V˙ between 158 m3 h–1 and 3200 m3 h–1. Since 1986, UV technology has been used in Québec (Canada) for the disinfection of municipal wastewater treatment effluents in irradiation plants with up to 4640 (!) low-presin of 290 kW (Babineau et sure mercury lamps with a total electric input power Pel al., 1999). Large-scale UV systems with over 5000 low-pressure mercury lamps are commercially available (Schneider et al., 2001). At the same time, even in Europe some installations for the UV disinfection of secondary sewage effluents by the German North Sea, by the Baltic Sea and in Austria have been in operation for several years (Popp et al., 1996). At present, a huge wastewater disinfection plant has been installed in Manukau, Auckland in New Zealand. This UV installation consists of 12 open channels each of 17 m length containing almost 8000 high-efficiency low-pressure mercury lamps (type: Spectrotherm). A maximum water flow of 57 600 m3 h–1 can be effectively disinfected (WEDECO, 2001).

References

Furthermore, pretreated ground or process waters can be decontaminated efficiently and purified in the presence of hydrogen peroxide by powerful irradiation units that consist of 12 or more modules each containing a medium-pressure in of 30 kW (Bircher et al., 1996). mercury lamp with an electric input power Pel Hence, that is why the tiny and inconspicuous •OH radical has emerged as an economically and ecologically important “industrial chemical” (cf. vanLoon and Duffy, 2000) over the past two decades. It still attracts a growing interest of water consuming or wastewater producing industries around the world. Finally, an Internet search for term combinations such as UV/oxidation, UV/disinfection or UV/technology leads to a multitude of products and companies that offer a variety of interdisciplinary applications to water and air treatment in industry or in private households (see Tab. 1-2). All of the facts mentioned above and the observation that several photochemical AOTs for water and air remediation have been successfully commercialized during recent years, justify a comprehensive description of photochemically driven advanced oxidation processes for water and air treatment in this book, which includes a brief description of UV disinfection techniques.

References Babineau D, Payment P, Chartray D, Laporte A, Fournier-Martin A (1999) Ultraviolet Disinfection of Municipal Wastewater Treatment Effluents: Pilot Studies with a Low-Pressure System Using Thermotolerant Coliforms and Clostridium perfringens as Indicators, Vecteur Environ. 32, No. 6: 34–43. Bank M (2000) Basiswissen Umwelttechnik: Wasser, Luft, Abfall, Lärm und Umweltrecht, 4th ed., Vogel Buchverlag, Würzburg. Bartz WJ, Wippler E (eds.) (1989) Desinfektion von Trinkwasser durch UV-Bestrahlung, 2nd ed., Expert-Verlag, Ehningen. Bircher KG, Simms K, Lem W (1996) Rayox UV/Oxidation-An Integrated Approach, in: Proceedings of the Symposiums on Chemical Oxidation: Technology for the Nineties, Eckenfelder WW, Bowers AR, Roth JA (eds.), Vol. 6: 288–297. Bliefert C (1997) Umweltchemie, 2nd ed., Wiley-VCH, Weinheim. Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Aquatic and Surface Photochemistry, Helz G R (ed.), Lewis Publishers, Boca Raton, pp 467–490.

Buxton GV, Greenstock CL, Helman WP, Ross AB (1988) Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (•OH/•O–) in Aqueous Solution, J. Phys. Chem. Ref. Data 17: 513–886. Christ C (1999 a) Integrated Environmental Protection Reduces Water Pollution, Chem. Eng. Technol. 22, No. 8: 642–651. Christ C (2000) Umweltschonende Technologien aus industrieller Sicht – Verfahrensverbesserungen und Stoffkreisläufe, Chem.-Ing. Tech. 72, No. 1+2: 42–57. Christ C (ed.) (1999 b) Production-Integrated Environmental Protection and Waste Management in Chemical Industry, WileyVCH, Weinheim. Cooper WJ, Curry RD, O’Shea K (eds.) (1998) Environmental Applications of Ionizing Radiation, Wiley, New York. Ebeling N (1999) Abluft und Abgas, WileyVCH, Weinheim. Ferradini C, Jay-Gerin J-P (1999) La Radiolyse de l’Eau et des Solutions Aqueuses: Historique et Actualité, Can. J. Chem. 77: 1542–1575. Freeman HM, Harris EF (eds.) (1995) Hazardous Waste Remediation, Innovative

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1 AOPs and AOTs Treatment Technologies, Technomic Publishing, Lancaster/Basel. Fritz W, Kern H (1992) Reinigung von Abgasen, 3rd ed., Vogel Buchverlag, Würzburg. Galla U, Kritzer P, Bringmann J, Schmieder H (1999) Verfahren zum Abbau von organischen Schadstoffen durch indirekte Elektrooxidation, Chem.-Ing. Tech. 71, No. 6: 628–633. Getoff N (1993) Purification of Drinking Water by Irradiation. A Review, Proc. Indian Acad. Sci. (Chem. Sci.) 105, No. 6: 373–391. Glaze WH, Kang J-W, Chapin DH (1987) The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation, Ozone Sci. Eng. 9: 335–352. Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Grabner G, Kuti Z (1997) Literature Access and Search with the Photochemistry Database, Europ. Photochem. Assoc. Newslett. No. 61: 104–107. Hancke K (1991) Wasseraufbereitung, Chemie und chemische Verfahrenstechnik, 2nd ed., VDI-Verlag, Düsseldorf. Hartinger L (1995) Handbuch der Abwasser- und Recyclingtechnik, 2nd ed., Hanser, München. Ho C-F H, Pitt P, Mamais D, Chiu C, Jolis D (1998) Evaluation of UV Disinfection Systems for Large-scale Secondary Effluent, Water Environ. Res. 70, No. 6: 1142– 1150. Hofmann J, Tauchnitz H, Vanselow H (2000) Catalytic Oxidation – A New Method for the Degradation of Pollutants in Wastewater, Chem. Eng. Technol. 23, No. 2: 125–127. Hua I, Hoffmann MR (1997) Optimization of Ultrasonic Irradiation as an Advanced Oxidation Technology, Environ. Sci. Technol. 31, No. 8: 2237–2243. Ince NH, Tezcanli G, Belen RK, Apikyan IG (2001) Review: Ultrasound as a Catalyzer of Aqueous Reaction Systems: the State of the Art and Environmental Applications, Appl. Catal. B: Environ. 29: 167– 176.

Jonah CD, Rao BSM (eds.) (2001) Radiation Chemistry, Present Status and Future Trends, Elsevier, Amsterdam. Knight JA (1998) Free Radicals: Their History and Current Status in Aging and Disease, Ann. Clin. Lab. Sci. 28, No. 6: 331– 346. König R (2000) Trinkwasserentkeimung mit ultraviolettem Licht, Talsperrenverband beschreitet neue Wege, Umwelt 30, No. 10/ 11: 42–44. König R (2001) WEDECO AG, personal communication. Kritzer P, Dinjus E (2001) An Assessment of Supercritical Water Oxidation (SCWO) – Existing Problems, Possible Solutions and New Reactor Concepts, Chem. Eng. J. 83, No. 3: 207–214. Kunz PM (1995) Behandlung flüssiger Abfälle, Vogel Buchverlag, Würzburg. Neitzel V, Iske U (1998) Abwasser, WileyVCH, Weinheim. Pikaev AK (2000) Mechanism of Radiation Purification of Polluted Water and Wastewater, Conference Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, May 28–31 (2000), ClausthalZellerfeld (Germany); electronic release. Popp W, Fenn B, Roth D, Schade M (1996) Mikrobiologische Untersuchungen zur Abwasserdesinfektion mit UV-Strahlung, Forum Städte-Hygiene 47: 250–257. Pöppinghaus K, Fresenius W, Schneider W (eds.) (1994) Abwassertechnologie, 2nd ed., Springer-Verlag, Berlin. Rice RG (2001) Practical Applications of UV in Combination with Ozone, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC; electronic release. Schmieder H, Abeln J (1999) Supercritical Water Oxidation: State of the Art, Chem. Eng. Technol. 22, No. 11: 903–908. Schneider OD, Malanchuk D, Nickols D, Ashe CR (2001) Comparison of LargeScale UV Systems for Disinfection of an Unfiltered Drinking Water Supply, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC; electronic release.

References Sonntag C von (1996) Degradation of Aromatics by Advanced Oxidation Processes in Water Remediation: Some Basic Considerations, J. Water SRT-Aqua, 45, No. 2: 84– 91. vanLoon GW, Duffy SJ (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford.

Verhoeven JW (1996) Glossary of Terms Used in Photochemistry, 2nd ed., Pure Appl. Chem. 68, No. 12: 2223–2286. WEDECO (2001) WEDECO’s weltgrößte UVAbwasserdesinfektionsanlage geht in Betrieb, October: Internet, http:// www.wedeco.de.

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2

Why UV and Oxidation/Disinfection? 2.1

Global Water Resources and Resulting Water Market

Water is one of the essential chemicals for all forms of life on Earth and its quality is crucial for the future of mankind. However, resulting from the high population density and the level of industrialization the hydrosphere is increasingly polluted with inorganic and organic matter. The sensitive hydrosphere is a central part of the global ecosystem and it covers about 73% of the Earth’s surface. Its total water content is distributed between the main compartments of the atmosphere, the biosphere, the oceans and the continents. The mass of water in oceans has been estimated to be around 1.37 × 1021 kg (vanLoon and Duffy, 2000). This value corresponds to 97% of the Earth’s water (Fig. 2-1). The mass of freshwater resulting from 3% of the total global water resource is mostly bound as ice in polar caps and glaciers (79%). About 20% of the freshwater resources are found as groundwater and only 1% is thought to be easily accessible surface water located in biomass, rivers, lakes, soil moisture and distributed in the atmosphere as water vapor.

Distribution of global water resources: composition of the hydrosphere (data from vanLoon and Duffy, 2000: p. 187).

Fig. 2.1

20

2 Why UV and Oxidation/Disinfection?

However, this immense mass of freshwater is unevenly distributed throughout the world and its quality is highly variable with respect to chemical and microbiological aspects. Furthermore, ground- and surface-water resources are strongly influenced by anthropogenic processes. Hence, enormous efforts are necessary to preserve the quality of surface waters by development and improvement of municipal and industrial wastewater treatment technologies and by strengthening water quality monitoring and environmental legislation policy. Accordingly, the European Environment Agency (EEA) stresses approaches to adequate water management and sustainable water use in Europe that are unequivocally of great importance to the future development of European countries (see Smith and Scott, 2002, van de Kraats, 2001). A worldwide ban of persistent organic pollutants (POPs) will be implemented by the POP convention. It has been negotiated since 1998 by mandate of the Governing Council of the United Nations Environment Program (UNEP) (Schlottmann and Kreibich, 2001). At the international convention on POPs in Stockholm in May 2001 more than 90 countries signed a global treaty that controls the production, import, export, disposal and use of these chemicals (van de Kraats, 2001). POPs are compounds that are already prohibited or strictly regulated in industrialized countries. Their dangerous environmental potential is established, for example, by their bioaccumulation in the arctic food chain. They are distributed all over the world by seasonally dependent deposition and evaporation processes. The “dirty dozen” (Fig. 2-2) covers the chloro-organic compounds aldrin, dieldrin, endr-

The dirty dozen of persistent organic pollutants (POPs) and effects of photo-initiated processes (and of AOPs/AOTs in general) generating a convenient sink.

Fig. 2.2

2.1 Global Water Resources and Resulting Water Market

in, chlordane, mirex, toxaphene, heptachlor, hexachlorobenzene, DDT, polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (Schlottmann and Kreibich, 2001, see also Tab. 1-1, entry: persistent organic pollutants). These ubiquitous compounds are characterized by their persistence, bioaccumulation, toxicity and long-range transport potential (Koch and Wagner, 1989). They mostly belong to the class of “semivolatile organic compounds” (SOC) and are used as insecticides, pesticides or for their formulation, respectively (Kempter and Jumar, 1983). They pose a severe risk of causing adverse effects to the environment and to human health. However, photochemical degradation reactions are known to be sinks of pesticides in environmental compartments (Klöpffer, 1992). Many polychlorinated aliphatic and aromatic hydrocarbons are photolabile (Legrini et al., 1993) and halogenated dioxins are easily dehalogenated and hence detoxified by UV irradiation (Tschirley, 1986, Crosby, 1983). Furthermore, many POPs and similar compounds may be effectively degraded by photochemical AOPs applying auxiliary oxidants or photocatalysts (Zaleska et al., 2000, 1999, Wenzel et al., 1999, Silva et al., 1992, Slein and Sansone, 1980). Therefore, photo-initiated AOPs seem to be techniques that can annihilate POPs and many other pesticides (Chiron et al., 2000) in micropolluted aqueous solutions or as trace contaminants in waste gases by an effective POPs sink at their point of origin, for example in rinse waters resulting from washing pesticide containers (cf. Fallmann et al., 1999). There is still a tremendous need for research related to photo-initiated AOPs concerning the formation of bio-available by-products, and investigations at industrial scales (Chiron et al., 2000). According to the origin of the water from natural cycles and depending on the water quality requirements, the market for water processing mainly covers three important sectors (WEDECO, 2000), namely the treatment technologies of drinking water, wastewater and of industrial process water (Fig. 2-3). In addition, the desalination and purification of seawater and the use of rainwater are of increasing interest for future water management and the development of adequate treat-

Main segments of the water processing market with potentials for photochemical AOTs (modified according to WEDECO, 2000).

Fig. 2.3

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2 Why UV and Oxidation/Disinfection?

ment technologies. Owen (2002) has presented a comprehensive overview of the European water and wastewater services market including a description of regulatory and legal structures and management strategies. Accordingly, there exists a wide variety of process routes by which water can be treated in order to satisfy different specifications. Apart from biological treatment techniques these systems may incorporate desorption processes (air/steam stripping), adsorption processes (activated carbon, polymeric resins), membrane separation processes (e.g. Rosenwinkel et al., 2000) and alternative technologies such as evaporation, distillation, flocculation, precipitation, and others, and last but not least photochemical AOPs (Mersmann et al., 1995). The detailed design of the appropriate treatment system depends mainly on the raw water composition and of course on economic considerations. For example, swimming pool water, house and operational water require mainly UV disinfection treatment. On the other hand, drinking water is being increasingly micropolluted by fertilizers, pesticides and other biologically resistant pollutants, thus producing the demand for new and efficient treatment technologies. It has been shown, however, that many micropollutants can be photochemically degraded (Getoff, 1993). Furthermore, increasing drinking and wastewater charges defined by fiscal policy will favor concepts for the use of greywater in households in future. Greywater is defined as washwater excluding toilet wastes, i.e. bath, laundry and dishwater. These household waters may be collected and can be used after appropriate treatment as a valuable resource for toilet flushing systems or for operating washing machines, so reducing the consumption of valuable drinking water considerably. However, greywater in storage tanks soon becomes malodorous and anaerobic microbiological processes will prevail. For this purpose UV disinfection represents a rational treatment option. The selection of a treatment process for industrial and municipal wastewater and for industrial cooling, purging or product waters depends mainly on the specific use and the desired water quality. If necessary, it should include UV disinfection, UV detoxification, or UV purification. On the other hand, the production of pure and ultra pure water for pharmaceutical purposes or of cleaning water for computer chip manufacturing industries requires disinfection and total mineralization of the organic matter content (Bendlin, 1995). In addition, photochemical technologies are well suited for the disinfection and purification of contaminated air streams (Sebold, 1994, Ollis and Al-Ekabi, 1993). They are being increasingly applied to air pollution prevention caused by relatively small point sources, e.g. malodor treatment by UV processing (e.g. Schneider, 1997/98). Competing air purification technologies include thermal waste gas treatment, adsorption, membrane and catalytic processes, biological treatment and others (Börger et al., 2000, Ebling, 1999). The special features of UV treatment technologies of water and air that are favored by manufacturers, researchers and developers are collected in Tab. 2-1 (cf. FIGAWA, 1997, Prengle Jr et al., 1996, Freeman and Harris, 1995, von Sonntag, 1995, Meyer and Fischer, 1994). From this table it is evident that a remarkable difference in the numerical handling of UV disinfection and photo-initiated oxidation techniques ex-

2.1 Global Water Resources and Resulting Water Market Tab. 2.1 Several special features of photochemical water and air treatment technologies

UV disinfection

Photo-initiated oxidation techniques

• Inactivation of a broad range of microorganisms by adequate fluence H0/J m–2

• Cleavage of chemical bonds by adequate photon fluence Hp,0/mol m–2

• Simple and reliable disinfection procedure with high performance, safe and easy handling, storage and shipping

• Oxidative degradation of bactericidal or non-biodegradable compounds and of a broad range of toxic contaminants accompanied by diminution of water toxicity

• No concentration and accumulation of viruses, bacteria or other microorganisms

• Partial oxidation usually leads to the formation of innocuous and biodegradable low molecular weight products, such as carboxylic acids and aldehydes

• Minimum formation of chemical side prod- • Detoxification of persistent organic pollutants (POPs) possible ucts and absence of mutagenic or carcinogenic compounds • No hyper-salination effects • No influence on odor and taste of water or air

• No sludge formation, minimal treatment residues, no subsequent disposal

• No threat to maintenance staff due to use of aggressive chemicals • Point-of-use systems on household scale and for camping sites • Applications in health risk zones such as hospitals etc. • Use in food processing industries

• Photomineralization generates a real pollutant sink • Implementation of synergistic treatment concepts with conventional technologies (hybrid systems), e. g. carbon adsorption, biological treatment • Several alternatives in the selection of the auxiliary oxidant (H2O2, O3, or their mixtures) • High efficiency in removing micro-pollutants by oxidation or mineralization

Modular engineering facilitates scaling-up and systems are easily automated. Batch or continuous flow modes and treatment of large volume flows. Low preventative maintenance and operating requirements together with reliable safety during operation. Design of photocatalytic solar systems based on titanium dioxide. Decentralized and remote installations possible.

ists, i.e. the former is analyzed in terms of the fluence H0/J m–2 of germicidal UV radiation (Sommer et al., 1999), whereas the latter processes depend unequivocally on the absorbed photon fluence Hp,0/mol m–2. In this case, an absorbed photon is treated as a selective reagent used for the cleavage of chemical bonds enabling stoichiometric calculations. To avoid any confusion it must be stressed that several authors use the term “UV dose” in UV disinfection literature (Leuker, 1999, Chiu et al., 1999) instead of

23

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2 Why UV and Oxidation/Disinfection?

UV fluence. However, this is incorrect and misleading because the SI unit of the absorbed dose of radiation is gray/J kg–1 (Gy) (Mills et al., 1993). The unit Gy is related to a specific mass and it was defined for reasons of safeguarding human health in radiology. It is used in radiation protection. However, the overall positive statements of Tab. 2-1 succeed only by adapting optimal design and use to photochemical reactors and by taking into account the photochemical reaction mechanisms (cf. Braun et al., 1993). Hence, several disadvantages, problems and limitations related to the application of photochemical AOPs must be addressed at this point: the initial capital costs are usually higher than with competing technologies and often a pretreatment of the aqueous stream is necessary to enhance the optical properties of the water (Freeman and Harris, 1995). Furthermore, the lamps (or quartz sleeves containing the lamps) can foul substantially by formation of UV absorbing films. This fouling results in a severe decrease in the UV intensity that is received by the medium. The finite lifetime of UV lamps requires their periodic replacement and hence produces additional costs (Prengle Jr et al., 1996). Further implementation considerations, evaluation and design criteria will be discussed in later sections (cf. Chapters 8.4 and 8.5). The question in the title of the present chapter deals not only with mechanistic and technical aspects of photochemical AOPs and AOTs but also with the regulation of environmental policy. Federal authorities are responsible for concurrent legislation concerning waste management and air quality control and outline water resources management in Germany (Bosenius and Rechenberg, 1998). The Environment Ministry of the European Union (EU) prepares draft regulations and directives for the council of Environment Ministers of the member states of the EU. This complex topic, however, goes beyond the scope of this book. The amount of German literature that concerns environmental legislation is enormous: for example, over 2700 entries were collected by Lohse et al. (1996) during the period of July 1995 to June 1996! Stober (1998) collected important environmental laws. Updated versions can be found in the federal law gazette of Germany (for the Internet reference refer to Tab. 1-1).

2.2

Present and Historical Dimensions of the Radiation Concept

“In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. And God said, let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness.” (Genesis, verses 3–4). The historiographers of the Old Testament knew about the predominant role of light. They categorized light as a vital factor for the development of life on our planet after the creation of the Earth, water and the atmosphere. Even today, re-

2.2 Present and Historical Dimensions of the Radiation Concept

Fig. 2.4

Processes in everyday life associated with UV/VIS radiation.

search argues that UV radiation had an essential role during the prebiotic development phase on Earth, concerned with the formation of “life molecules”, i.e. during chemical evolution (Mason, 1991). Thus, the Bible writers pronounced the positive effects of light very early (Fig. 2-4). They demonstrated with their statements that UV radiation, light and life are interrelated and that positive effects should dominate. Hence, life on Earth is only possible due the ozone shield that is formed in the stratosphere, i.e. the region of the atmosphere between 12 and 50 km above the surface. Here the intensity of the high-energy solar radiation is strong enough to produce a photostationary equilibrium between molecular oxygen and ozone (vanLoon and Duffy, 2000). This steady-state situation is very sensitive to catalytic processes leading to ozone decomposition, e.g. by intervention of chlorinated fluorocarbons (CFCs). Owing to its broad and intense absorption band between k of 200 nm and 300 nm, ozone absorbs the harmful solar radiation below k of 300 nm. Therefore, the natural and toxic pollutant ozone protects life from dangerous solar radiation and acts as the “sun glasses” of the Earth (Feister, 1990). The decrease in the ozone concentration in the stratosphere, the so called “ozone hole” (Solomon, 1999), may lead to an increase in biologically active UV radiation on the Earth’s surface (Hester and Harrison, 2000, Madronich et al., 1998). On the other hand, ozone is one of the active constituents of photochemical smog observed on hot sunny days in cities with heavy traffic (vanLoon and Duffy, 2000, Heintz and Reinhardt, 1990). The photochemical production of ozone in a polluted troposphere (the region of the atmosphere between the surface and the stratosphere) is accomplished through a complex series of reactions involving nitrogen oxides (NOx) and many other volatile organic compounds (VOCs) (Kondratyev and Varotsos, 2001).

25

26

2 Why UV and Oxidation/Disinfection?

Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical (•OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature’s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). Several photobiological processes are fairly well understood and they are essential for life on Earth. For example, the processes of photosynthesis, phototropism (orientation of organisms to light) and photomorphogenesis (growth of plants as a function of light intensity) (Singhal, 1999) are important positive light-induced effects, which are even of industrial relevance. The process of vision, sun tanning of the human skin as an immanent sun protection system and photochemical vitamin D production in the skin of humans are further examples of the positive effects of UV/VIS radiation on life. One of the most fascinating photobiological processes is bioluminescence, the biological or biochemical production of light (Adam and Cilento, 1982). Typical negative effects of UV radiation on humans are the development of sunburn, the formation of skin cancer or eye damage. Additionally, phototoxicity of naturally occurring compounds and UV/VIS radiation-induced adverse effects of drugs (cf. Oppenländer, 1996, 1988) represent severe problems to human health. However, the controlled use of phototoxic mechanisms plays an important role in the photomedicinal treatment of several cancers (photodynamic therapy, PDT) or of skin diseases such as psoriasis (PUVA therapy). Knowledge of the healing capacity of plant extracts in combination with sunshine was known even in ancient times, as is exemplified by Fig. 2-5. Finally, human health may be influenced negatively, if the light intensity is too low. This phenomenon usually occurs during the winter season. The resulting psychological or physical status of individuals is designated as seasonal depression. At the end of this discussion, it has to be stressed that the beneficial and detrimental categorization of UV radiation- and light-induced effects is of a subjective nature and is only related to the needs and wishes of humans (Suppan, 1994). This assessment would read completely differently from the point of view of a microorganism that is to be inactivated by irradiation with germicidal UV radiation in a UV disinfection treatment plant. The historical development of the term light demonstrates its versatility and its significance for current developments related to AOPs (Tab. 2-2). It has been characterized by controversial discussions about the contradictory wave and particle nature of light and UV radiation from its very early beginnings. At the beginning of the 20th century, it culminated in the theory of wave/particle duality. The light or UV radiation particles are called photons (symbol: c; photochemists usually use hm). Simple physical experiments verify the dual nature of UV radiation and light (Fig. 2-6): interference and diffraction are typical attributes of electromagnetic

2.2 Present and Historical Dimensions of the Radiation Concept

Fig. 2.5 Ancient Egyptian papyrus demonstrating the combination of solar radiation and skin ointment for the treatment of skin disease (reproduced from the front page of the invitation to the 4th International Conference and Workshop on Solar Energy Storage and Applied Photochemistry, January 1997 by permission of M. S. A. Abdel-Mottaleb, Ain Shams University, Cairo, Egypt).

waves, whereas the Compton Effect and the photoelectric effect are experimental evidence for the corpuscular nature of electromagnetic radiation. In Compton scattering a photon of energy Ep = hkc collides with an electron and transfers some of its energy to the latter. Thus, the scattered photon has a lower frequency m (i.e. a greater wavelength k) than the incoming photon. This phenomenon is comparable to the balancing of energy during the collision of two billiard balls. The collision of a photon with an electron (i.e. of two bodies) can be described by the fundamental laws of physics, namely the conservation of energy and the conservation of momentum. The photoelectric effect is observed during irradiation of metals or metal alloys by electromagnetic radiation of suitable wavelength. It shows that the number of released electrons is proportional to the number of incident photons and that the kinetic energy of the ejected electrons is only dependent on the energy of the photons. The number of incident photons (i.e. the intensity of the ra-

27

28

2 Why UV and Oxidation/Disinfection? Tab. 2.2 Some outstanding historical developments related to the terms of light and UV radia-

tion Scientist

Period

Theme

Reference

Heisenberg, W. 1901–1976

Uncertainty principle and quantum me- Mason (1961) chanics

Jablonski, A.

1898–1980

Mechanisms of fluorescence and phosphorescence, Jablonski diagram (1933)

Nickel (1998)

Einstein, A.

1879–1955

The photon as quantum of the electromagnetic field

Mason (1961)

Planck, M.

1858–1947

Quantum theory of light and radiation: wave/particle duality

Mason (1961)

Hertz, H.

1857–1922

Experimental verification of the electro- Mason (1961) magnetic theory of light

Maxwell, J. C.

1831–1879

Electromagnetic theory of light

Faraday, M.

1791–1867

Search for a connection between electri- Bugge (1929) city, magnetism and light

Mason (1961)

Berzelius, J. J.

1779–1848

Electrochemical theory of light and heat Bugge (1929)

Ritter, J. W.

1778–1810

Discovery of chemically active radiation Schwedt (1999), beyond violet light, UV radiation Mascia and Tausch (2000)

Goethe von, J. W.

1749–1832

Experiments related to the chemical effects of colored illumination

Scheele, C. W.

1742–1786

Influence of parts of the solar spectrum Bugge (1929) on silver chloride

Pristley, J.

1733–1804

Paper on the history and the discoveries Bugge (1929) in the field of light and color research

Römer, O.

1644–1710

Determination of the speed of light from astronomical observations

Newton, I.

1642–1727

Particle theory mixed with elements of Mason (1961) wave theory for the explanation of opti- Wickert (1985) cal phenomena

Huygens, C.

1629–1695

Wave properties and speed of light

Mason (1961)

Grimaldi, F.

1618–1663

Wave theory of light

Mason (1961)

Descartes, R.

1596–1650

Particle character of light

Mason (1961)

Snellius, W.

1591–1626

Law of light refraction

Mason (1961)

Hero of Alexandria

125 BC

Linearity of a light beam and the princi- Von Laue (1958) ple of fastest optical pathway

Platon

427–348/347 BC

Theory of colors by interaction of a vi- Grassi (1959) sion beam with particles emitted by objects (dialog Timaios)

Schwedt (1999)

Von Laue (1958)

2.3 Some Historical Landmarks of the Development of AOTs Experiments verifying the wave/particle duality of electromagnetic radiation, and the bridge between corpuscular and wave physics: relation of the momentum p to the wavelength k.

Fig. 2.6

diation or the radiant power P) has no influence on the kinetic energy of the ejected electrons. Thus, the connecting bridge between corpuscular and wave physics is represented by the fundamental equation of De Broglie (Eq. 2-1). p h=k mc

(2-1)

Eq. 2-1 is generated by combination of Planck’s relationship Ep = hm = hc/k with Einstein’s equation Ep = m c2. This leads to the momentum p of a photon that is now related to the wavelength k (Suppan, 1994). Eq. 2-1 associates the wavelength k with the mass m of a photon and with its velocity c, the speed of light. Hence, the photon has no rest mass. Although it appears puzzling to our minds each photon is characterized by a wavelength k and a frequency m.

2.3

Some Historical Landmarks of the Development of AOTs

Only during the past three decades, have UV-based technologies emerged as an important class of treatment processes for water and air purification. They include advanced oxidation in the homogeneous and heterogeneous phase [UV oxidation, vacuum-UV (VUV) oxidation, photocatalysis] and disinfection. Their progress is definitely coupled to the breakthroughs in exploratory photochemistry, which had barely developed as a distinct science at the beginning of the 20th century. In addition, the development status of efficient UV lamps was in a rudimentary state (refer to Skaupy, 1924 and Vogl, 1937). Although the “first law of photochemistry” had already been recognized by Grotthus in 1817 and Draper in 1843, the quantum nature of UV/VIS radiation and their consequences for photochemistry were established only decades later by Planck and Einstein (see Tab. 2-2). Nowadays, the observation by Grotthus and Draper that only electromagnetic radiation that is absorbed by a molecule leads to a

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30

2 Why UV and Oxidation/Disinfection? Tab. 2.3 Selected historical landmarks related to the development of AOPs and AOTs and their

mechanisms Year

Research group

Specification of AOP or AOT

Reference

1874

Downes and Blunt

Bactericidal effect of the radiant energy Rice (2001) a), Ullmann’s (1954) a) of sunlight

1879

Downes and Blunt

H2O2 decomposition by sunlight

1894

Fenton

Fe2+ catalyzed oxidation of tartaric acid Fenton (1894) (Fenton reaction)

1900

Kistiakowsky

Effect of radiation on H2O2 decomposi- Lunak and Sedlak tion (1992)a)

1907

Thiele

H2O2 decomposition by UV light

Gmelin (1966)a)

1910

Henri et al.

Large scale UV disinfection of water and pilot plant installation

von Sonntag (1987) a)

1917

Smith

First full-scale UV disinfection of water Rice (2001) a) in the USA

1922

Kornfeld

H2O2 photolysis and reaction products Gmelin (1966) a)

1931

Herzberg

Water photolysis

1932

Haber and Weiss

Radical chain mechanism of the photo- Gmelin (1966)a) lysis of H2O2

1935

Weiss

HO•2 radical in solution

Glaze et al. (1987) a)

1936

Fricke and Hart

Photomineralization by VUV irradiation

Fricke and Hart (1936)

1949

Lea

Quantum yield of H2O2 photolysis as a Gmelin (1966) a) function of UV radiant power

1953

Seitz

Surface and air disinfection by UV radiation

Ullmanns (1954) a)

1956

Taube

Photoreactions of O3 in solution, formation of H2O2 and O2

Taube (1956)

1957

Baxendale and Wilson Photolysis of H2O2

Glaze et al. (1987) a)

1957

Chen and Taylor

Photolysis of water vapor

Chen and Taylor (1957)

1964

Eisenhauer

Fenton’s reagent and waste water treat- Eisenhauer (1964) ment

1968

Getoff and Schenck

H2O-VUV, vacuum-UV photolysis of water

1971

Formenti et al.

Photocatalytic oxidation of single com- Sauer and Ollis pounds in the gas phase (1996) a)

1975

Garrison et al.

O3-UV, pilot scale applications

Glaze et al. (1987) a)

1975

Hoigné and Bader

O3 in H2O: identification of •OH radicals as intermediates by competition experiments

Hoigné and Bader (1975)

Gmelin (1966) a)

Herzberg (1931)

Getoff and Schenck (1968)

2.3 Some Historical Landmarks of the Development of AOTs Tab. 2.3 (continued)

Year

Research group

Specification of AOP or AOT

Reference

1976

Oliver and Carey

UV disinfection as an alternative to chlorination

Oliver and Carey (1976)

1976

Carey et al.

TiO2 Photocatalytic oxidation of biphe- Jakob et al. (1993) a) nyl and chlorobiphenyls

1977

Farrell et al.

O3-UV, pilot scale applications

1977

Koubek

H2O2-UV, oxidation of refractory organ- Glaze et al. (1987) a) ics in aqueous waste streams

1977

Frank and Bard

TiO2 Photocatalytic oxidation of CN–

Fujishima et al. (2000) a)

1979

Leitis et al.

O3-UV, purification process

Glaze et al. (1987) a)

1979

Nakayama et al.

O3-H2O2-UV, waste water treatment

Glaze et al. (1987) a)

1979

Berglind et al.

H2O2-UV, oxidation of contaminants in Glaze et al. (1987) a) water

1980

Glaze et al.

O3-UV, refractory species

Glaze et al. (1987) a)

1980

Malaiyandi et al.

H2O2-UV, TOC diminution in tap water

Malaiyandi et al. (1980)

1981

Hango et al.

O3-H2O2-UV, waste water treatment

Glaze et al. (1987) a)

1982

Staehelin and Hoigné Mechanism of O3 decomposition in water

Glaze et al. (1987) a)

1982

Forni et al.

Mechanism of O3 decomposition in aqueous solution

Glaze et al. (1987) a)

1983

Bothe et al.

•

OH and secondary carbon radical chemistry

Peyton (1996) a)

1983

Hustert et al.

TiO2 photocatalytic oxidation

Matthews (1991) a)

1983

Pruden and Ollis

TiO2 photocatalytic oxidation

Matthews (1991) a)

1984

Glaze et al.

O3-UV, pilot scale application

Glaze et al. (1987) a)

1984

Brunet et al.

O3-H2O2-UV, drinking water treatment Glaze et al. (1987) a)

1985

Duguet et al.

O3-H2O2-UV, drinking water treatment Glaze et al. (1987) a)

1985

Matsunaga et al.

TiO2-UV, bactericidal activity

Rajeshwar (1996) a)

1986

Peyton and Glaze

O3-UV, mechanistic investigations

Glaze et al. (1987) a)

1987

Bollyky

O3-H2O2-UV, drinking water treatment Glaze et al. (1987) a)

1990

Braun

H2O-VUV, vacuum-UV photolysis of water with xenon excimer lamps

Braun et al. (1993) a)

1992

Glaze

H2O-VUV, vacuum-UV photolysis of water (xenon excimer lamp)

Legrini et al. (1993) a)

a) Cited within this reference.

Glaze et al. (1987) a)

31

32

2 Why UV and Oxidation/Disinfection?

distinct photoproduct (see Pfoertner, 1986) seems trivial. However, in combination with the Stark–Einstein “quantum equivalence law” (the “second law of photochemistry”) that “each absorbed quantum of radiation causes one equivalent of a chemical reaction” (Roth, 2001) the highly sophisticated photosciences of today have emerged. Two major exceptions to the Grotthus–Draper and the Stark–Einstein laws are the chemi-excitation process and the multiphoton excitations with laser radiation (VUV, UV, VIS and IR), respectively (Kopecky, 1992). The actual beginning of photochemistry as an independent scientific discipline is related undoubtedly to the work of Giacomo Ciamician and Paul Silber in Bologna who, between 1900 and 1915, initiated systematic investigations related to the interaction of light with matter. In a remarkable lecture at a conference in New York in 1912, Ciamician had already postulated the extraordinary concept of solar energy usage (cf. Roth, 1989), which, hopefully in the 21st century, will experience its broad realization. A historical review of photochemical research activities in the 20th century was presented by Heimgartner and Hansen (1993). Here it was demonstrated that synthetic organic photochemistry has enormous potentials for the design of novel molecular structures. However, a priori this review does not mention any developments related to AOPs or AOTs, which in contrast to photochemical synthesis are of a destructive nature with respect to organic contaminants. Unfortunately, this lack of information is characteristic for the photochemical and technical literature in this field. It is symptomatic for organic photochemistry that degradation pathways of organic chemicals in water have only been systematically investigated over the past few years. However, this field of photochemical research is at least as exciting as synthetic photochemistry with respect to the understanding and illustration of complex degradation pathways! Several years ago, Glaze et al. (1987) surveyed the research concepts and the historical backgrounds related to the ozone UV and hydrogen peroxide UV processes in water. Halmann (1995) reviewed and discussed the photodegradation of water pollutants. Some selected landmarks of the development of AOPs and AOTs are collected in Tab. 2-3. It is astonishing to recognize that several basic concepts and methods of modern AOTs are more than 100 years old!

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Management in Germany, Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Bonn. Braun A M, Jakob L, Oliveros E (1993) Advanced Oxidation Processes – Concepts of Reactor Design, J Water SRT-Aqua 42, No. 3: 166–173. Bugge G (1929) Das Buch der großen Chemiker, Vol. 1 and 2, 1st ed., unveränderter Nachdruck, Verlag Chemie, Weinheim 1974.

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Fricke H, Hart EJ (1936) Studies of Reactions Induced by the Photoactivation of the Water Molecule. I, J. Chem. Phys. 4: 418– 422. Fujishima A, Rao TN, Tryk DA (2000) Titanium Dioxide Photocatalysis, J. Photochem. Photobiol. C: Photochem. Rev. 1: 1–21. Getoff N (1993) Purification of Drinking Water by Irradiation. A Review, Proc. Indian Acad. Sci. (Chem. Sci.), 105, No. 6: 373–391. Getoff N, Schenck GO (1968) Primary Products of Liquid Water Photolysis at 1236, 1470 and 1849 Angstrom, Photochem. Photobiol. 8: 167–178. Glaze WH, Kang J-W, Chapin DH (1987) The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation, Ozone Sci. Eng. 9: 335–352. Gmelin (1966) Gmelins Handbuch der Anorganischen Chemie, Sauerstoff, 8th ed., Verlag Chemie, Weinheim, Part 3: 2275–2285. Grassi E (1959) Platon, Sämtliche Werke 5, Politikos, Philebos, Timaios, Kritias. Griechische Philosphie, Bd. 6, Rohwolt, Hamburg. Halmann MM (1995) Photodegradation of Water Pollutants, CRC Press, Boca Raton. Heimgartner H, Hansen H-J (1993) Organische Photochemie im Spiegel von Helvetica Chimica Acta, Helv. Chim. Acta 76: 1027–1114 Heintz A, Reinhardt G (1990) Chemie und Umwelt, Friedr. Vieweg & Sohn, Braunschweig. Herzberg G (1931) General Discussion, On Part I, Paper 3, Trans. Faraday Soc. 27:402– 403. Hester RE, Harrison RM (2000) Causes and Environmental Implications of Increased UV-B Radiation, Royal Society of Chemistry, Cambridge. Hoigné J, Bader H (1975) Ozonation of Water: Role of Hydroxyl Radicals as Oxidizing Intermediates, Science 190: 782–784. Jakob L, Oliversos E, Legrini O, Braun AM (1993) TiO2 Photocatalytic Treatment of Water. Reactor Design and Optimization Experiments, in: Ollis DF, Al-Ekabi H (eds.) Photocatalytic Purification of Water and Air, Elsevier, Amsterdam, pp 511–532.

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2 Why UV and Oxidation/Disinfection? Kempter G, Jumar A (1983) Chemie organischer Pflanzenschutz und Schädlingsbekämpfungsmittel, 2nd ed., VEB Deutscher Verlag der Wissenschaften, Berlin. Klöppfer W (1992) Photochemical Degradation of Pesticides and other Chemicals in the Environment: a Critical Assessment of the State of the Art, Sci. Total Environ., 123/124: 145–159. Koch R, Wagner BO (1989) Umweltchemikalien, VCH, Weinheim. Kondratyev KY, Varotsos CA (2001) Global Tropospheric Ozone Dynamics, Environ. Sci. Poll. Res. 8, No.1: 57–62. Kopecky J (1992) Organic Photochemistry: A Visual Approach, VCH, Weinheim. Laue von M (1958) Geschichte der Physik, Ullstein, Frankfurt, No. 222. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93, No. 2: 671–698. Leuker G (1999) Description and Application of Biodosimetry – a Testing Procedure for UV Systems, J Water SRT – Aqua 48, No. 4: 154–160. Lohse S, Dörner E, Hesterberg B (eds.) (1996) Neue Literatur zum Umweltrecht, No. 69, Umweltbundesamt, Berlin. Lunak S, Sedlak P (1992) Photoinitiated Reactions of Hydrogen Peroxide in the Liquid Phase, J. Photochem. Photobiol. A: Chem. 68: 1–33. Madronich S, McKenzie RL, Björn LO, Caldwell MM (1998) Changes in Biologically Active Ultraviolet Radiation Reaching the Earth’s Surface, J. Photochem. Photobiol. B: Biol. 46: 5–19. Malaiyandi M, Sadar MH, Lee P, O’Grady R (1980) Removal of Organics in Water Using Hydrogen Peroxide in the Presence of Ultraviolet Light, Wat. Res. 14: 1131– 1135. Mascia M, Tausch MW (2000) Ein historisches Experiment 200 Jahre danach. Die Entdeckung der UV-Strahlen durch J. W. Ritter, Naturwissenschaften im Unterricht 11, No. 57: 17–21. Mason SF (1961) Geschichte der Naturwissenschaft in der Entwicklung ihrer Denkweisen, Alfred Kröner Verlag, Stuttgart. Mason SF (1991) Chemical Evolution, Origin of the Elements, Molecules and Living Systems, Clarendon Press, Oxford.

Matthews RW (1991) Environment: Photochemical and Photocatalytic Processes. Degradation of Organic Compounds, in: Pelizzetti E, Schiavello M (eds.), Photochemical Conversion and Storage of Solar Energy, Kluwer Academic Press, pp 427– 449. Mersmann A, Kutzer S, Kajszika H, Wintrich H (1995) Nichtbiologische Reinigung organisch verschmutzten Wassers, Chem.-Ing.-Tech. 67, No.1: 41–49. Meyer A, Fischer R (1994) Moderne apparative Entwicklungen zur UV-Oxidationsmittelbehandlung, Gewässerschutz-Wasser-Abwasser 143: 261–274. Mills I, Cvitas T, Homann K, Kallay N, Kuchitsu K (1993) Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Scientific Publications, Oxford. Nickel B (1998) Pioneers in Photochemistry, From Wiedemann’s Discovery to the Jablonski Diagram, Europ. Photochem. Assoc. Newslett. No. 64: 19–72. Oliver BG, Carey JH (1976) Ultraviolet Disinfection: an Alternative to Chlorination, J Water Pollut. Contr. Fed. 48, No. 11: 2619– 2624. Ollis DF, Al-Ekabi H (eds.) (1993) Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam. Oppenländer T (1988) A Comprehensive Photochemical and Photophysical Assay Exploring the Photoreactivity of Drugs, Chimia 42, No. 10: 331–342. Oppenländer T (1996) The Contribution of Organic Photochemistry to Investigations of Phototoxicity, in: Tonnesen H (ed.) The Photostability of Drugs and Drug Formulations, Taylor & Francis, London, Chap. 11: 217–265. Owen DL (2002) The European Water Industry – Market Drivers and Responses, CWC Publishing, London, UK. Peyton GR (1996) Kinetic Modeling of FreeRadical Water Treatment Processes: Pitfalls, Practicality, and the Extension of the Hoigné/Bader/Staehelin Model, J. Adv. Oxid. Technol. 1, No. 2: 115–125. Pfoertner KH (1986) Practical Photochemistry: Scale-up, in: Photochemistry in Organic Synthesis, Coyle JD (ed.), The Royal Society of Chemistry, Burlington House, London, Chap. 13.

References Prengle Jr HW, Symons JM, Belhateche D (1996) H2O2/VisUV Process for Photo-Oxidation of Waterborne Hazardous Substances- C1–C6 Chlorinated Hydrocarbons, Waste Management 16, No. 4: 327–333. Rajeshwar K (1996) Photochemical Strategies for Abating Environmental Pollution, Chem. Ind. June: 454–458. Ravishankara AR, Hancock G, Kawasaki M, Matsumi Y (1998) Photochemistry of Ozone: Surprises and Recent Lessons, Science 280: 60–61. Rice RG (2001) Practical Applications of UV in Combination with Ozone, Conference Proceedings of the First International Congress on Ultraviolet Technologies, June 1416, Washington, DC, electronic release. Rosenwinkel K-H, Wagner J, Nagy J (2000) Membranverfahren in der industriellen Abwasserbehandlung, Chem.-Ing.-Tech. 72, No. 5: 433–440. Roth HD (1989) Die Anfänge der Organischen Photochemie, Angew. Chem. 101: 1220–1234; Angew. Chem. Int. Ed. Engl. 28: 1193–1207. Roth HD (2001) Selected Developments in the 20th Century Photochemistry: Historical Sketches, Europ. Photochem. Assoc. Newslett. No. 71: 37–57. Sauer ML, Ollis DF (1996) Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air, J. Catal. 158: 570–582. Schenck GO (1985) Is the New Forest Decline a Photodynamic Disease Caused by Light and Peroxyacetylnitrate, Ozone, Halocarbons and Others?, Europ. Photochem. Assoc. Newslett., No. 23/24: 15–41. Schlottmann U, Kreibich M (2001) Aus für das dreckige Dutzend, Nachr. Chem. 49, No. 5: 608–614. Schneider H (1997/98) Licht frißt Gestank, Gerüche kostengünstig ohne Rückstände beseitigen, Chemie Umwelt Technik, Special Issue: 16–18. Schwedt G (1999) Goethes Experimente mit ultraviolettem Licht, Phys. Unserer Zeit 30, No. 5: 207–210. Sebold M (1994) UV-Oxidation in der Gasphase – Verfahrensprinzip und Einsatzbereiche der UV-Oxidation zur Abreinigung kontaminierter Luft, Report No. 13/94, Landesanstalt für Umweltschutz, BadenWürttemberg.

Silva de M, Dullin J, Thiemann W (1992) Versuche zur Elimination von Lindan und Fluoranthen in Sickerwässern durch kombinierten Einsatz von UV-Licht und Wasserstoffperoxid, Vom Wasser 79: 333–353. Singhal GS (1999) Concepts in Photobiology: Photosynthesis and Photomorphogenesis, Kluwer, Boston. Skaupy F (1924) Die Entwicklung der elektrischen Lichtquellen seit der Zeit der Nernstlampe, Z. Angew. Chem. 37: 445– 447. Slein MW, Sansone EB (1980) Degradation of Chemical Carcinogens, An Annotated Bibliography, Van Nostrand Reinhold, New York. Smith PG, Scott JS (2002) Dictionary of Water and Waste Management, IWA Publishing, London, UK. Solomon S (1999) Stratospheric Ozone Depletion: A Review of Concepts and History, Rev. Geophys. 37, No. 3: 275–316. Sommer R, Cabaj A, Sandu T, Lhotsky M (1999) Measurement of UV Radiation Using Suspensions of Microorganisms, J. Photochem. Photobiol. B: Biol. 53: 1–6. Sonntag C von (1987) Disinfection with UVRadiation, in: Stucki S (ed.) Process Technologies for Water Treatment, Plenum Press, New York: 159–177. Sonntag C von (1995) Chemische Grundlagen der Oxidationsverfahren und neuere Erkenntnisse bei der Bildung von Zwischenprodukten, CUTEC-Schriftenreihe, Vogelpohl A (ed.) No. 20: 1–11. Stober R (ed.) (1998) Wichtige Umweltgesetze für die Wirtschaft, Verlag Neue Wirtschafts-Briefe, Herne/Berlin. Suppan P (1994) Chemistry and Light, Royal Society of Chemistry, Cambridge. Taube H (1956) Photochemical Reactions of Ozone in Solution, Trans. Faraday Soc. 53: 656–665. Tschirley FH (1986) Dioxin, Spektrum der Wissenschaft, April: 38–45. Ullmanns (1954) Encyklopädie der technischen Chemie, Urban & Schwarzenberg, München, 3rd ed., 5: 737. van de Kraats JA (ed.) (2001) Eur. Water Managem. News, April 18th and May 22nd. vanLoon GW, Duffy SL (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford.

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2 Why UV and Oxidation/Disinfection? Vogl K (1937) Lichtquellen für photochemische Arbeitsverfahren, Die Chem. Fabrik 10, No. 27/28: 296–299. WEDECO (2000) Water Technology, Annual Report, Düsseldorf, http://www.wedeco.de. Wenzel A, Gahr, A, Niessner R (1999) TOC-Removal and Degradation of Pollutants in Leachate Using a Thin-Film Photoreactor, Wat. Res. 33, No. 4: 937–946. Wickert J (1985) Isaac Newton, Ansichten eines universalen Geistes, 2nd ed., R. Piper & Co, München.

Zaleska A, Hupka J, Silowiecki A, Wiergowski M, Biziuk M (1999) Destruction of Chlorinated Pesticides in TiO2-Enhanced Photochemical Processes, Int. J. Photoenergy 1: 79–84. Zaleska A, Hupka J, Wiergowski M, Biziuk M (2000) Photocatalytic Degradation of Lindane, p,p’-DDT and Methoxychlor in an Aqueous Environment, J. Photochem. Photobiol. A: Chem. 135: 213–2220.

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Interaction of UV/VIS Radiation with Matter 3.1

Photoscience in Research and Development

Photoscience covers a broad spectrum of interdisciplinary and interrelated subjects and it may be subdivided into photomedicine, photobiology, photochemistry and photophysics (Fig. 3-1). Photochemistry, in general, studies the reactions that occur through electronically excited states of molecules. Specifically, photochemistry studies the change of substance quality and characteristics by the influence of UV/VIS radiation. The mechanistic interpretation of the formation of photoproducts and their characterization and identification are typical domains of photochemistry. This research concept is strictly based on photophysics, which investigates the primary event of photon absorption by a molecule, the properties of electronically excited states and their deactivation mechanisms, such as for example fluorescence, phosphorescence and energy or electron transfer reactions, and non-

Modern photoscience and its subdomains, and the major tasks of photophysics.

Fig. 3.1

38

3 Interaction of UV/VIS Radiation with Matter

Fig. 3.2

Some technological applications of UV/VIS radiation.

radiative deactivation processes. During recent decades a multitude of industrial applications of photoscience have been developed using sophisticated photochemical engineering based on specialized process technology (Braun, 1997, Braun et al., 1993, Böttcher, 1991). The fundamental uses of UV radiation and light can be classified according to the functional behavior as an energy or information carrier or as a specific and ecologically clean reagent in the form of photons (Böttcher, 1991) (Fig. 3-2). The term “photochemical technology” includes preparative techniques for the synthesis of valuable fine chemicals (Pfoertner, 1998), solar photochemistry (Mattay, 2002, Esser et al., 1994) and AOPs or AOTs acting destructively for water and air remediation, or surface treatment and disinfection. Further challenging applications of photochemical technology comprise UV radiation-induced chemical vapor deposition (photo-CVD), stereo-lithographic computer aided design and manufacturing (CAD, CAM) which apply polymer-based technologies using laser irradiation techniques, and many others (Sahyun, 1996), such as optical information recording and solar energy conversion (Böttcher, 1991). A major industrial application of UV radiation is UV curing of inks and varnishes. In addition, physical and chemical analysis using the broad spectrum of optical sensors and biosensors (Göpel et al., 1995) and the quantification of UV/VIS radiation via analysis of calibrated photochemical reactions, so called actinometric procedures (Braun et al., 1991, Kuhn et al., 1989), are of vital importance to photoscientific research and technical developments. It is evident that photochemical engineering involves specific problems that are related to the generation of an optimal UV/VIS radiation distribution within a photochemical reactor system. In contrast to well established chemical engineer-

3.1 Photoscience in Research and Development

Fig. 3.3 Influence of interactive domains on photochemical engineering (modified according to Braun, 1997).

ing concepts (Levenspiel, 1996) that describe mass and energy balances in conventional process development, the major tasks of photochemical engineering include technology for the tuning of the lamp and of the reaction system (Fig. 3-3) (Braun, 1997, Braun et al., 1993). The absorbance spectrum of the reactant dictates the choice of effective UV/VIS radiation sources: the emission spectrum of the lamp used must match the absorbance spectrum of the molecule to a maximal extent. Additionally, the lamps must have sufficient power and appropriate geometry for successful photoreactor design (Braun et al., 1991). Owing to absorption phenomena the photoreaction will only take place in a restricted volume of the photoreactor where UV radiation (or light) is actually absorbed, the so-called photoreaction zone (Pfoertner, 1986). This specificity of photochemical processes leads to an inherent inhomogeneity of the product distribution within a photoreactor and hence a variety of different photoreactor concepts has developed. Again, in drastic contrast to conventional chemical engineering, it is in no way (through use of mixing) possible to achieve a uniform concentration profile of electronically excited molecules within the photoreactor due to their extremely short lifetimes. Furthermore, the optical properties of transparent materials (glass, natural quartz or synthetic fused silica quartz of high purity and transmittance, i.e. Suprasil quartz) used for the construction of the photochemical reactors are of primary importance in photochemical engineering (Braun et al., 1991) and are often a limiting cost factor.

39

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3 Interaction of UV/VIS Radiation with Matter

3.2

Physical Constants and Standard Values Used in Photochemistry

The most important physical constants of interest in photochemistry and their units as used in this book are collected in Tab. 3-1. They are taken from Kurzweil (2000) and from Lide (2000). In some cases, these values differ slightly from those reported by Mills et al. (1993).

3.3

The Electromagnetic Wave

According to McCluney (1994) “radiation is the propagation of energy in a variety of forms through space”. One possibility of energy propagation is depicted by a perpendicularly coupled oscillating electric field (E) and magnetic field (H) that travels as a transverse wave with the constant speed of light c0 and that is characterized by the wavelength k. The wavelength k is inversely proportional to the frequency m of the oscillating electromagnetic field and is described by the classical relation k = c0/m. The sinusoidal E-wave and the perpendicular H-wave are in phase at a large distance from an oscillating dipole due to the non-static nature of the latter (Shortley and Williams, 1961) (Fig. 3-4). The interaction of UV/VIS radiation with matter, i.e. atoms or molecules, takes place within their electron shell, i.e. between the atomic or molecular orbitals (AOs or MOs) by promoting one electron from its ground to an electronically excited state. By this process (i.e. energy absorption), the electron is transferred to

Schematic drawing of an electromagnetic wave with the dependence on propagation time t and propagation direction x at a large distance from the generating source.

Fig. 3.4

3.4 The Photon Stream and Planck’s Equation Schematic representation of the coupling of two electronic states by an oscillating electric field (q represents an electron in an excited state and q' an electron in its ground state).

Fig. 3.5

an AO or to an antibonding MO of higher energy, respectively. The electron transition is mainly triggered by resonance of the oscillating electric field of an electromagnetic wave with the electron representing an oscillating electric dipole (Turro, 1978, 1991). Consequently, an electron in its excited state is able to emit energy in the form of an electromagnetic wave. This fact is used to produce UV/ VIS radiation. The corresponding process is called emission of electromagnetic radiation. Thus, two electrons q and q' in different energetic states are coupled by an oscillating electric field (Fig. 3-5).

3.4

The Photon Stream and Planck’s Equation

According to the wave/particle duality (see Fig. 2-6) electromagnetic radiation may be considered as a stream of photons that has a constant velocity c0 and travels in a straight line through space. Each photon carries a discrete package of radiant energy Qk that has a defined frequency and wavelength k (Planck’s equation, Eq. 3-1). Therefore, a photon represents a “quantum of electromagnetic energy at a given frequency” (Verhoeven, 1996). This means that a photon of low frequency and hence with long wavelength possesses little radiant energy in contrast to a high frequency (short wavelength) photon. Qk =J hm hc0 =k hc0m

(3-1)

Example 3-1: The frequency m of an electromagnetic wave is 4.4970 × 1014 Hz. Calculate the corresponding wavelength k in nm and the radiant energy Qk of this photon! k c0 m

1

299792458 m s 4:4970 1014 s

Qk hm 6:62606876 10

34

1 1

6:666 10 7 m 666:6 nm

J s 4:4970 1014 s

1

2:980 10

19

J per photon

41

42

3 Interaction of UV/VIS Radiation with Matter Tab. 3.1 Most important quantities, units, symbols and standard values used in photochemistry

(Kurzweil, 2000, Lide, 2000) a) Quantity

Symbol or definition

Value

Unit

Atomic mass constant

mu = 1/12 m (12C)

1.660 538 73(13) × 10–27

kg

Avogadro constant

NA

6.022 141 99(47) × 1023

mol–1

Boltzmann constant

k = R/NA

1.380 650 3(24) × 10–23 8.617 342(15) × 10–5

J K–1 eV K–1

Electronvolt

eV (= e × V)

1.602 176 462(63) × 10–19

J

Elementary charge

e

1.602 176 462(63) × 10

C

Molar gas constant

R

8.314 472(15)

J mol–1 K–1

Molar mass

M

10–3

kg mol–1

Molar Planck constant

NAh NAhc0

3.990 312 689(30) × 10–10 0.119 626 564 92(91)

J s mol–1 J m mol–1

Molar volume, ideal gas, p = 101325 Pa (1 atm), T = 273.15 K

Vm = RT/p

22.413 996(39) × 10–3

m3 mol–1

One mol of photons

Np

6.022 141 99(47) × 1023

einsteinb)

Planck constant

h

6.626 068 76(52) × 10–34 4.135 667 27(16) × 10–15

Js eV s

Speed of light in vacuum

c0

299 792 458

m s–1

Standard atmosphere

p0

101 325

Pa

Standard temperature

T0

273.15 (ideal gas, T0 = 0 8C) 298.15 (chemical kinetics, T0 = 25 8C

K

–19

a) The values in parentheses represent the standard deviation uncertainty in the least significant digits. Example: R = 8.314472(15) J mol–1K–1 reads R = 8.314472 ± 0.000015 J mol–1 K–1; b) this is not an IUPAC sanctioned unit and therefore its use is discouraged. It is, however, widely used throughout the photochemical literature.

The energy of the 666.6 nm photon is extremely low, compared with the energy content of particular materials. For example, one barrel of petroleum (1 bbl = 158.9873 L, US petroleum) is equivalent to 5.8 × 109 J, and the annual commercial energy consumption of the world in 1996 was estimated to be about 327 × 1018 J (vanLoon and Duffy, 2000). Using the molar Planck constant NAh (Tab. 3-1) it is possible to calculate the radiant energy of one mol of photons, i.e. of NA photons, according to Eq. 3-2: Qk =J mol

1

NA hm NA h c0 =k

(3-2)

3.4 The Photon Stream and Planck’s Equation The radiant energy Qk of 1 mol of photons as a function of the wavelength in the range between k of 100 nm and 1000 nm. Fig. 3.6

This inverse proportional relationship between wavelength k and radiant energy Qk of one mol of photons (Eq. 3-2) is graphically illustrated by Fig. 3-6. The radiant energy of one mol of photons is sometimes called the molar photon energy. Example 3-2: Calculate the radiant energy Qk of 1 mol of 172 nm photons. Q172 nm

6:02214199 1023 mol

1

6:62606876 10 172 10 9 m

34

Js 299792458 m s

1

= 695503.3 J mol–1 = 695.503 kJ mol–1 The energy unit of photochemical reaction kinetics is one mol of photons (i.e. NA photons). In the photochemical literature it is often designated as “one einstein” (Bolton, 1999), but this is not an IUPAC sanctioned unit. Therefore, the use of the unit “einstein” is discouraged. The concept of electromagnetic radiation represented by a particle (i.e. photon) stream allows a photon to be treated as a reagent, that “collides” and “reacts” with a molecule or an atom by promoting an electron of a ground state molecule (or from the electron shell of an atom) to an excited state (denoted by *), i.e. to a

Fig. 3.7 Photoexcitation of a molecule by absorption of a photon: the primary process in photophysics followed by secondary processes.

43

44

3 Interaction of UV/VIS Radiation with Matter

higher energy level (orbital). This “reaction” is the primary step in a photochemical (or photophysical) event and it is called absorption (Fig. 3-7). A ground state molecule represents a molecule with an electronic configuration of the lowest energy, and electronically excited states represent electronic configurations of a molecule with electrons in antibonding molecular orbitals (Turro, 1978, 1991). Photon absorption is the fastest process observed in nature. It occurs within a time scale of 10–15 s, i.e. within one femto second! The electronically excited molecule M* can get rid of its energy either by photophysical deactivation processes or by photochemical or photobiological pathways.

3.5

Electromagnetic Spectral Ranges of Interest in Photochemistry

The wavelength range generally utilized in photochemistry lies between k of 170 nm and 1000 nm. This is mainly due to the fact that electronically excited states M* of organic or inorganic molecules M are usually generated by photoexcitation within this wavelength range. This bandwidth is determined by the absorption characteristics of inorganic and organic molecules in a liquid or gaseous phase. The photochemically active region of the electromagnetic spectrum has been divided into five sub bands: the vacuum-UV (VUV), UV-C, UV-B, UV-A and VIS (Fig. 3-8). The subdivision of the UV spectral domain is mainly due to phenomenological reasons (Fig. 3-9) that are related to physical, biological or medicinal effects. The UV-B region is usually defined between k of 280 nm and 315 nm (van-

Classification of electromagnetic radiation in the wavelength range below k of 1200 nm as bands with specific names, and the interaction with molecules M (VUV: vacuumUV).

Fig. 3.8

3.5 Electromagnetic Spectral Ranges of Interest in Photochemistry

Loon and Duffy, 2000, McCluney, 1994), or sometimes between k of 280 nm and 320 nm (Sonnemann, 1992). Other authors define it between k of 290 nm and 320 nm because no solar radiation between 280 nm and 290 nm can penetrate through the atmosphere to reach the ground level (Moore, 1996). Ionizing electromagnetic radiation, such as deep VUV or X-ray and c-radiation, is of such high energy, that electrons are ejected from molecules, ultimately leading to photoionization (cf. Fig. 3-8). VUV radiation is absorbed by almost all substances including water, carbon dioxide and oxygen. Therefore, spectroscopic measurements below k of 200 nm have to be performed under vacuum or at least under non-absorbing gas conditions (cf. Hind, 2001). This special physical condition led to the nomenclature of this wavelength band. The absorption of VUV radiation by organic molecules in solution produces selective rearrangements of the carbon skeleton, mainly initiated by Rydberg state photochemistry (cf. Oppenländer et al., 1992, Adam and Oppenländer, 1986). However, the importance of VUV radiation to AOPs is related to the fact that it is efficiently absorbed by water in a liquid or gaseous phase. This causes the homolysis of water molecules with the formation of hydrogen atoms and hydroxyl radicals. This effect is used in the VUV oxidation process for the mineralization of organic water pollutants (cf. Oppenländer and Gliese, 2000) applying novel incoherent xenon excimer lamps at k of 172 nm (cf. Chapter 7.1.3). Absorption of UV-C radiation by cellular constituents of organisms may lead to severe cell mutations and cell death. Therefore, this wavelength band is often called the germicidal range. It is used in UV disinfection processes to inactivate bacteria, viruses and other microorganisms (cf. Chapter 9). At the Earth’s surface sunlight contains no VUV and UV-C radiation because of their efficient absorp-

Fig. 3.9 Phenomenological subdivision of ultraviolet radiation (according to DIN 5031-7) into four sub-bands and their characteristic effects.

45

46

3 Interaction of UV/VIS Radiation with Matter

tion by oxygen (absorption at k below 200 nm) and by ozone (absorption at k below 330 nm) in the upper atmosphere, respectively (Finlayson-Pitts and Pitts, 1986). On the other hand, solar UV-B and UV-A radiation reach the Earth’s surface and may induce sunburn, skin cancer and sun tanning. The VIS range is biologically active in photosynthesis. It is absorbed by dyes, drugs and many other photosensitizers that are used for example in photomedicine for photodynamic cancer therapy (PDT). The energy of photons with a wavelength k greater than 1000 nm is too small to reach electronic states of atoms or molecules. By absorption of radiation in the infrared (IR) region of the electromagnetic spectrum molecules M are vibrationally excited (Mvib). The atoms connected by chemical bonds in a polyatomic molecule behave like mass centers and acquire vibrational motion by IR absorption, thus generating different vibrational energy levels. These play a most important role in the photophysical processes of electronically excited molecules because every electronic state of a molecule couples with vibrational levels (Suppan, 1994). This interaction between electronic and vibrational levels is called vibronic coupling (Verhoeven, 1996).

3.6

Conversion of Energy Units and Other Useful Conversions

The conversion of energy units is based on the relationships presented in Eq. 3-3 (Kurzweil, 2000). 1 Ep h m h c0 m m c02 mkm2 k T m v2 e U 2

(3-3)

This set of equations connects Planck’s photon energy Ep with Einstein’s mass/energy equivalence, with Boltzmann’s kinetic energy, with the kinetic energy of a particle and with the kinetic energy of an electron in an electric field of a voltage U of 1 V. The most important conversion factors used in photochemistry and photophysics are collected in Tab. 3-2. The spectral domains of UV radiation discussed in Fig. 3-9 and the corresponding energy conversions according to Eq. 3-3 are summarized in Tab. 3-3. In addition, this table contains information about the emission lines of several types of lamp that are of interest to photochemical AOPs. Photochemists usually use the energy unit kJ mol–1, whereas physicists prefer the unit eV (electron volt), and spectroscopists are used to wavenumber r = 1/k in cm–1 instead of wavelength. Example 3-3: Convert the radiant energy Qk of 1 eV of a photon into the unit J and calculate the corresponding wavelength of the photon! Multiplication of the elementary charge e (Tab. 3-1) with the voltage U of 1 V yields:

3.6 Conversion of Energy Units and Other Useful Conversions

Qk e U 1:602176462 10

19

C 1V 1:602176462 10

19

J

The wavelength k that corresponds to this photon energy calculates as follows: k h c0 =Qk

6:62606876 1034 J s 299792458 m s 1:602176462 10 19 J

1

1:2398 10 6 m 1239:8 10 9 m 1239:8 nm This corresponds to the wavelength k of a photon in the IR region of the electromagnetic spectrum. Several useful equations and conversions that are of general importance in photochemistry are collected in Tab. 3-4. Some illustrative examples are calculated below to demonstrate the usage of the simplified and practical conversion equations of Tab. 3-4. Example 3-4: Calculate the radiant energy of 265.4 nm photons in kcal mol–1 using the appropriate equation from Tab. 3-4. Qk

m 2:8592 10 5 kcal mol 107:73 kcal mol 9 265:4 10 m

1

Example 3-5: Calculate the wavelength k that corresponds to the energy of 436 kJ mol–1 by using the corresponding equation of Tab. 3-4. k

m 1:1963 10 4 kJ mol 2:744 10 7 m 274:4 nm 1 436 kJ mol

Example 3-6: a) Calculate the photon flow Up in mol h–1 at k = 253.7 nm of a lowpressure mercury lamp (type NN 50/81, Heraeus) that has an electric input power in × 100 = 40% at this wavePel of 50 W and a radiant power efficiency g = Pk/Pel –1 length (Pk: spectral radiant power in W nm ); b) compare this value with Up at k = 253.7 nm of a medium-pressure mercury lamp with a spectral radiant power P253.7 nm of 1.035 kW (refers to a 20 kW lamp). –9 a) Up = 30094 Wmol m = 0.153 mol photons per hour m h × 50 W × 0.4 × 253.7 × 10 –9 mol b)Up = 30094 W m h ×1035 W × 253.7 × 10 m = 7.90 mol h–1

These specific calculations yield the amount of 253.7 nm photons (mol photons) that are produced (under optimal conditions) during 1 h of irradiation with common industrial low-pressure or medium-pressure mercury lamps, respectively. Also, Up may be expressed in mol photons per second which is equivalent to the older unit einstein s–1, or the number of photons per second emitted by the lamp can be calculated (Tab. 3-4).

47

L mol L mol–1 cm–1

–

Molar (decadic) absorption coefficient

Molar absorption coefficient a) ek, liquid phase at 298 K, (logarithmic base 10)

Napierian logarithm (ln)

cm

–1

kW h

Kilowatt hour –1

gal

eV

Electronvolt

in

eV

Electronvolt

Gallon (U.S.)

–

Decadic logarithm (log)

Inch

cal

Calorie

atm

Atmosphere (standard) atm

Å

Ångstrom

bar

Å

Ångstrom

Bar

cm2

Absorption cross section, r (logarithmic base e)

Atmosphere (standard)

Unit

To convert from

Tab. 3.2 Selection of useful conversion factors

2

m mol

J

m

m

3

–1

–1

kJ mol

J

–

J

kPa

kPa

Pa

nm

m

L mol–1cm–1

Unit

Decadic logarithm (log)

–

Absorption coefficientb), ak gas cm–1 atm–1 phase at 298 K, (logarithmic base e)

Molar (decadic) absorption coefficient

Joule

Meter

Cubic meter

Energy per mol

Joule

Napierian logarithm (ln)

Joule

Kilopascal

Kilopascal

Pascal

Nanometer

meter

Molar absorption coefficient, ek (logarithmic base 10)

To

Lide (2000)

2.3026

10.619

0.1

3.6 × 10

6

–

Bolton and Cater (1994) c)

–

Lide (2000)

Lide (2000)

Lide (2000)

3.785412 × 10 2.54 × 10–2

Lide (2000)

96.4853 –3

– 1.602177 × 10–19

Lide (2000)

Kurzweil (2000)

Kurzweil (2000)

Lide (2000)

0.43429

4.184

100

101.325

101325

Lide (2000)

Lide (2000)

1 × 10–10 0.1

Finlayson-Pitts and Pitts (1986)

Reference

3.82 × 10–21

Multiply the latter by

48

3 Interaction of UV/VIS Radiation with Matter

ppmv ppm ppm Torr Ws

Parts per million (gas phase)

Parts per million (gas phase, 1013 hPa, 25 8C)

Parts per million d) (gas phase, 1013 hPa, 298 K)

Torr

Watt second

Joule

Pascal

Mass density

number of molecules, atoms or radicals per cm3

Volume-%

To

40.9 × M e)

lg m–3

J

1

133.3224

Finlayson-Pitts and Pitts (1986)

2.46 × 1013

cm–3

Pa

vanLoon and Duffy (2000)

10–4

%

Lide (2000)

Kurzweil (2000)

Finlayson-Pitts and Pitts (1986)

Reference

Multiply the latter by

Unit

a) Absorbance A in the liquid phase is defined as A = log(P0k/Ptrans ) = ek ci l. k b) absorbance A in the gas phase is defined as A = ln(P0k/Ptrans ) = akpi l with P0k: incident spectral radiant power at wavelength k, Ptrans : transmitted spectral radiant k k power at wavelength k (Verhoeven, 1996). c) several other useful conversion factors for changing absorption coefficients from one set of units to another (either logarithmic base e or base 10) can be found in Finlayson-Pitts and Pitts (1986), and conversion factors for chemical kinetics (second order rate constants) are collected in Lide (2000). d) in aqueous solutions the unit ppm equals “grams of solute per million milliliters of solution“: ppm = lg ml–1 or mg L–1, implying the density of water is 1.00 kg L–1 (vanLoon and Duffy, 2000). e) M: molecular mass in g mol–1.

Unit

To convert from

Tab. 3.2 (continued)

3.6 Conversion of Energy Units and Other Useful Conversions 49

50

3 Interaction of UV/VIS Radiation with Matter Tab. 3.3 Spectral domains of electromagnetic radiation (VUV, UV, VIS and IR), energy conversions,

principle emission lines of mercury arc lamps, and of some novel incoherent excimer lamps Domain

Wavelength a) k/nm

Wavenumber m/cm–1

Frequency m/1014 Hz

Molar photon energy Photon energy Qk/eV Qk/kJ mol–1

VUV

100

100000

29.98

1196.27

12.40

172 b) (Xe2*) 184.9 (LP-S) 194.2 (LP-S)

58140 54083 51493

17.43 16.21 15.44

695.50 646.98 616.00

7.21 6.71 6.38

200

50000

14.99

598.13

6.20

222 c) (KrCl*) 248 (MP) 253.7 (LP, MP) 265.2 (MP) 265.4 (MP) 270 (MP) 275 (MP)

45045 40323 39417

13.50 12.09 11.82

538.86 482.37 471.53

5.58 5.00 4.89

37707 37679 37037 36364

11.30 11.30 11.10 10.90

451.08 450.74 443.06 435.01

4.68 4.67 4.59 4.51

280

35714

10.71

427.24

4.43

280.4 (MP) 289 (MP) 292 (MP) 296.7 (MP) 302.1 (MP) 308 d) (XeCl *) 312.6 (MP) 313.1 (MP)

35663 34602 34247 33704 33102 32468 31990 31939

10.69 10.37 10.27 10.10 9.92 9.73 9.59 9.57

426.63 413.93 409.68 403.19 395.98 388.40 382.68 382.07

4.42 4.29 4.25 4.18 4.10 4.03 3.97 3.96

315

31746

9.52

379.77

3.94

334.1 (MP) 365 (MP) 366.3 (MP)

29931 27397 27300

8.97 8.21 8.18

358.06 327.74 326.58

3.71 3.40 3.38

380

26316

7.89

314.81

3.26

391 (MP)

25575

7.67

305.95

3.17

400

25000

7.49

299.07

3.10

404.6 (MP) 407.8 (MP) 435.8 (MP) 492 (MP) 546.1 (MP) 577 (MP) 579.1 (MP)

24716 24522 22946 20325 18312 17331 17268

7.41 7.35 6.88 6.09 5.49 5.20 5.18

295.67 293.35 274.50 243.14 219.06 207.33 206.57

3.06 3.04 2.84 2.52 2.27 2.15 2.14

t 780

12821

3.84

153.37

1.59

800

12500

3.75

149.53

1.55

UV-C t

UV-B t

UV-A t

VIS t

IRe)

t

3.7 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset a) Spectral domains are designated according to German national norm (DIN 5031-7), see Figures 38 and 3-9. All other wavelengths correspond to emission lines of mercury arc lamps, with the exception of: b) emission maximum kmax of a xenon excimer (Xe2*) lamp. c) kmax of a krypton chloride excimer (KrCl*) lamp. d) kmax of a xenon chloride excimer (XeCl*) lamp (Kogelschatz, 1992). Mercury lamp types: LP-S, low-pressure mercury lamp with Suprasil quartz envelope; LP, low-pressure mercury lamp with ordinary quartz envelope; MP, medium-pressure mercury lamp with quartz envelope (cf. Pfoertner, 1998, Braun et al., 1991). e) according to DIN 5031-7 infrared radiation is subdivided into near IR (NIR, IR-A: 780–1400 nm, IR-B: 1400–3000 nm), medium IR (MIR, IR-C: 3000–50 000 nm) and far IR (FIR: 50 000–106 nm).

Example 3-7: Calculate the amount of hydrogen peroxide that is theoretically decomposed by irradiation of its aqueous solution (V = 1 m3) within 1 h by a 10 kW medium-pressure mercury lamp that has a radiant power Plamp of 682 W at an ac = (248 + 253.7 + 265)/3 of 256 nm. The quantum yield of tive average wavelength k H2O2 decomposition UH2O2 equals 0.5 at 253.7 nm (Bolton and Cater, 1994). DH2 O2 =molL

1

8:3593

mol 0:5 3600 s 682 W 256 10 9 m Wms 1000 L

DH2 O2 2:63 10 3 mol L

1

2:63 mmol L

1

Under conditions of total absorbance of the UV radiation by H2O2, a theoretical maximum amount of 2.63 mmol hydrogen peroxide per liter can be decomposed by irradiation with the specified lamp by consuming an energy Eel of 10 kW h. Example 3-7 convincingly demonstrates the relationship of photons to stoichiometric calculations of photochemical reactions (Tab. 3-4, section D). Additional examples of calculations concerning photons and energy of lamps can be found in Wöhrle et al. (1998). More important for further discussions is the fact, that the achievable concentration change D[M] of a photochemically active substrate M under conditions of total absorbance is directly proportional to the quantum yield U of the photoreaction, to the irradiation time t, to the radiant power Plamp, to the wavelength k of the UV radiation applied and reciprocally proportional to the irradiated volume V (see Tab. 3-4, section D).

3.7

Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset of Molecules

For many applications and for the understanding of the photochemical reactivity of molecules of environmental significance it is necessary to compare the homolytic bond dissociation enthalpies D0298 and the corresponding photon energies kTHW (threshold wavelengths) in relation to the absorption onset of these molecules (Bolton and Cater, 1994). D0298 is identical with the bond strength of a de-

51

52

3 Interaction of UV/VIS Radiation with Matter Tab. 3.4 Simplified, practical and easy to use conversions in photochemistry and photochemical

engineering Description A) Conversion of radiant energy Qk of one mol of photons b) of wavelength k to different energy units

Simplified conversion equations a) Qk

1:1963 10 k=m

2:8592 10 k=m

4 kJ m mol

5 kcal m mol

kJ mol

1

kcal mol

1

1:2399 10 6 eV m eV k=m

m m B) Conversion of energy E 1:1963 10 4 kJ 2:8592 10 5 kcal mol mol kJ kcal with different units to the en- k=m E= mol E= mol ergy equivalent of wavelength k in m 1:2399 10 6 eV m E=eV

C) Conversion of radiant power Plamp in watts emitted by a lamp to the photon flow Up c) in mol photons per hour d), mol photons per second, or photons per second e) at a defined wavelength k

UP d)/mol h–1 = 30094

mol Plamp =W k=m Wmh

UP/mol s–1 = 8.3593 ×

mol Plamp =W k=m Wms

1 Plamp =W k=m UP e)/s–1 = 5.0341 × 1024 Wms

D) Calculation of the variation D[M] = [M]t = 0 – [M]t of the concentration (D[M]) of mol UM t=s –1 Plamp =W k=m a reactant M as a function of D[M]/mol L = 8.3593 Wms V=L f) irradiation time under conditions of total absorption of UV radiation at wavelength k a) Derived by numerical calculation using the physical constants of Table 3-1 and conversion factors of Table 3-2. For generality the wavelength k is used with the unit m. b) sometimes designated as molar photon energy Qk /kJ mol–1 (cf. Table 3-3). c) the photon flow Up is defined as the amount of photons Nk per unit time, Up = dNk /dt, the SI unit is mol s–1, its equivalent is einstein s–1 (Verhoeven, 1996). d) according to Pfoertner (1998), derived from Qk = Nk × NA × h × c0/k with Qk /W s = Plamp × t and solving this equation to UP = dNk/dt = Plamp × k × 3600/(NAhc0). e) Up = Plamp × k/(h c0). f) according to Braun et al. (1991, Chap. 4.2, p. 158), cf. Table 3-8, derived from [M]t=0 – [M]t = EP,abs × UM × t/l, with EP,abs = photon irradiance expressed in mol s–1 cm–2, UM = quantum yield of the photochemical reaction at wavelength k, t = irradiation time in s, l = optical trajectory of solution traversed by the light in cm. According to definition EP,abs = l × UP/NA ×V, with V = volume in cm3 of solution irradiated, and using Up in photons s–1 representing the absorbed photon flow as defined above by C).

3.7 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset Tab. 3.5 Relationship of homolytic bond dissociation enthalpy D0298, threshold wavelength kTHW

and absorption onset konset of selected molecules in the gas phase Molecule

Primary products a)

D0298/kJ mol–1

Hydrogen peroxide, H2O2

2 •OH

213 c)

562

310 d)

305c)

392

400 e)

e)

127

126 e)

206.3 c)

580

400 e)

498.4 e)

240

190 d)

e)

1123

330 d)

Nitrogen dioxide, NO2 Nitrogen, N2 Nitrous acid, HNO2

NO + •O•(3P)

•

4

2

N( S) + N( D) NO + •OH

•

945

Threshold wavelength b) kTHW/nm

Absorption onset konset/nm

Molecular oxygen, 3O2

2 •O•(3P)

Ozone, O3

3

O2 + O ( P)

106.5

Ozone, O3

O2(1Dg) + (1D)

387 e, f)

309

330 d)

•

305 c)

392

290 e)

H• + •OH

498 c)

240

190 d)

Trichlorofluoromethane, CCl3F (CFC-11) Water, H2O

•

• 3

CCl2F + •Cl

a) The notations in parentheses are term symbols that denote the electronic energy states of atoms or molecules (cf. Huheey, 1988). b) calculated using kTHW/nm = 1.1963 × 105/D0298/kJ mol–1 (cf. Table 3-4, section B). c) Lide (2000). d) Bolton and Cater (1994). e) vanLoon and Duffy (2000). f) this value takes into account the excitation energies (Eex) to form 1O2 and O(1D) which are roughly 90 kJ mol–1 and 190 kJ mol–1, respectively: D0298 (O3)gas = DH0f (O)gas + Eex (O) + DH0f (O2)gas + Eex (O2) – DH0f (O3)gas = 249.2 + 190 + 0 + 90 – 142.7 (vanLoon and Duffy, 2000).

fined chemical bond in a polyatomic molecule. It is also related to the threshold wavelength kTHW (conversion according to Tab. 3-4) that is defined as “the maximum wavelength for which the photon energy matches the bond energy of the bond that breaks in the photolysis reaction” (Bolton and Cater, 1994). On the other hand, the absorption onset konset is defined as that specific wavelength k where absorption of UV/VIS radiation of a molecule begins. Specifically, it represents the wavelength k where the absorption coefficient a (gas phase) is about 0.1 cm–1 atm–1 or the molar absorption coefficient e (liquid phase) reaches 1 L mol–1 cm–1 (Bolton and Cater, 1994). Several bond dissociation enthalpies D0298 in kJ mol–1 and threshold wavelengths kTHW in nm of environmentally important molecules together with their absorption onset konset are collected in Tab. 3-5. The discussion of these data is made easier via their graphical representation as shown in Fig. 3-10. As can be deduced from the examples presented in Fig. 3-10 the threshold wavelength kTHW calculated from the bond dissociation energy D0298 lies slightly below the absorption onset konset only for the compounds nitrogen dioxide and ozone (see Tab. 3-5). The photodissociation of NO2 leads to the forma-

53

54

3 Interaction of UV/VIS Radiation with Matter

Fig. 3.10 Bond dissociation enthalpies D0298/ kJ mol–1 versus threshold wavelengths kTHW/ nm and the absorption onset konset/nm of some environmentally significant molecules,

which is represented by a dot (•), cf. Table 35. kTHW/nm = 1.1963 × 105/D0298/kJ mol–1 (cf. Table 3-4, section B).

tion of oxygen atoms in the ground state (3P). This photoreaction is in part responsible for the initiation of the oxygen atom chemistry of the atmosphere, especially in photochemical smog events (vanLoon and Duffy, 2000). According to the spin conservation theory ground state oxygen atoms (3P) react rapidly with ground state oxygen molecules O2 (3R) to form ozone, finally leading to high atmospheric ozone concentrations in polluted area. For ozone, the threshold wavelength kTHW calculates to 309 nm, this being slightly below the absorption onset (Tab. 3-5). Therefore, radiation of this wavelength or less is absorbed by ozone and initiates the bond dissociation with formation of electronically excited cleavage products, i.e. singlet molecular oxygen (1O2) and singlet oxygen atoms (1D) (Braun et al., 1991). The latter are responsible for the formation of hydroxyl radicals in the atmosphere by spin-allowed reaction with water vapor (vanLoon and Duffy, 2000). Because ozone absorbs strongly in the UV-B and UV-C band of the electromagnetic spectrum it protects the Earth from harmful solar radiation in this wavelength region. In the upper atmosphere of the Earth (above 50 km) ozone is produced by reaction of triplet molecular oxygen (3O2) with ground state oxygen atoms (3P) that are formed by photolysis of 3 O2 via absorption of solar VUV radiation (cf. vanLoon and Duffy, 2000). The situation is completely different for the other molecules depicted in Fig. 310 and Tab. 3-5 with respect to their threshold wavelength and absorption onset. All the substrates depicted in Fig. 3-10 having an absorption onset konset below the D0298/kTHW equivalence curve form photoproducts that have excess energy, which is usually either of vibrational or translational nature. For example, the absorption onset of hydrogen peroxide (konset = 310 nm) is considerably lower than the calculated threshold wavelength kTHW (562 nm), which corresponds to the energy necessary for homolysis of the central O–O bond of the molecule. This means, that irradiation of H2O2 with green light at k of 562 nm has absolutely no effect, because H2O2 is transparent at this wavelength. Another important consequence of this high-energetic absorption onset of H2O2 is the fact that irradiation

3.8 Absorption of UV Radiation by Molecules

of H2O2 at wavelengths k below 310 nm provides excess energy to the molecule. This inevitably leads to the formation of hydroxyl radicals with excess vibrational (or translational) energy. This fact is assumed to be one possible explanation for the formation of “free” hydroxyl radicals in water by escaping the water (solvent) cage (Prousek, 1996). The phenomenon of cage recombination of hydroxyl radicals in water will be discussed in Chapter 6.6.

3.8

Absorption of UV Radiation by Molecules

The principles of photochemistry and photophysics related to the interaction of UV/VIS radiation with matter are discussed extensively in several excellent monographs (refer to Chapter 10). However, some comments that are necessary for the understanding of the processes involved in photochemical advanced oxidation will be discussed briefly. Fundamentally, physical phenomena (reflection, refraction and scattering) have to be distinguished from photophysical (absorption, luminescence, etc.) and photochemical processes (formation of photoproducts) as is demonstrated schematically by Fig. 3-11. The physical phenomena are of considerable interest in sophisticated photoreactor modeling, because many photoreactors used for water treatment possess an air/quartz/water interface that influences the fluence rate distribution within a photoreactor. This is especially important for drinking water applications where the optical transmittance of water is usually very high (Bolton, 2000 a). The absorbed radiation is used to produce electronically excited states of the sample molecules initiating photophysical deactivation processes or the formation of photoproducts. The intensities of the incident and of the transmitted beam are correlated by the Beer–Lambert law (see Eq. 3-4), which quantitatively describes the attenuation of UV/VIS radiation by transmitting any absorbing medium.

Fig. 3.11 Schematic representation of physical, photophysical and photochemical phenomena and processes observed during interaction of UV/VIS radiation with matter.

55

56

3 Interaction of UV/VIS Radiation with Matter

3.9

The Beer–Lambert Relationship

To assess the photochemical potential of a reaction system it is essential to know its absorbance spectrum. The strength of the absorption of UV/VIS radiation as a function of the wavelength k is given by the well known Beer–Lambert law of absorbing molecules in solution (Eq. 3-4) or of absorbing gas phase components (Eq. 3-6). The relationship A = f(k) is called the absorbance spectrum. Liquid phase (solution): A10 log

Pk0 1 log Pktrans T

log T ek c l

(3-4)

Where, A10 is the (decadic) absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium (Verhoeven, 1996); P0k is the incident spectral radiant power; Ptrans is the transmitted spectral radiant power; k ek is the molar (decadic) absorption coefficient in L mol–1 cm–1 (SI unit: m2 mol–1, for the conversion factor see Tab. 3-2); c is the concentration of the substrate in mol L–1; l is the thickness of the solution traversed by the UV/VIS radiation (also called pathlength of irradiation). The transmittance T is the ratio of transmitted spectral radiant power Ptrans to k that incident on the sample P0k. Attenuance D is defined as the negative decadic logarithm of the transmittance T, i.e. D = –log T. D reduces to absorbance if reflection and scattering of the incident beam are neglected. Traditionally, many authors use the spectral radiant intensity Ik instead of the spectral radiant power Pk (Verhoeven, 1996). In addition, Eq. 3-4 may be expressed in its exponential form (Eq. 3-5): Pktrans Pk0 10

ecl

and Pkabs Pk0

Pktrans Pk0 1

10

ecl

(3-5)

Where Pabs k is the absorbed spectral radiant power. For the sake of simplicity, in many cases it is adequate to express P0k as 100%. For example, absorption of 99% of an incident beam means that the absorbance Pk0 = –log T = log 100% becomes A10 = log Ptrans 1% = 2. k In photochemistry and photochemical technology, conditions with A10 5 2 are usually designated as total absorbance, or in the past as total absorption (Braun et al., 1991) of UV/VIS radiation. Thus, for A10 5 2 the transmittance T is less than 0.01. In the gas phase the (napierian) absorbance Ae is proportional to the partial pressure p of the absorbing species at a specified reference temperature (usually T = 298 K) and to the pathlength l of the radiation absorbing column (Eq. 3-6).

3.9 The Beer–Lambert Relationship

Gas phase : Ae ln

Pk0 ak p l Pktrans

(3-6)

With ak being the absorption coefficient with the common unit cm–1 atm–1 (Finlayson-Pitts and Pitts, 1986). On the other hand, physicists usually use the number of absorbing molecules N per cm3 in gas phase photochemistry instead of the pressure p of the absorbing gas component leading to Eq. 3-7. Gas phase : Ae ln

Pk0 rN l Pktrans

(3-7)

With r representing the absorption cross section in cm2 per molecule (for the conversion to ek see Tab. 3-2). To avoid any misinterpretations it is very important to state the origin of the absorption coefficients ek and ak with respect to their logarithmic base as decadic or as Napierian (for conversions see Tab. 3-2). Many useful conversion factors for changing absorption coefficients from one set of units to another have been summarized by Finlayson-Pitts and Pitts (1986). To get a feeling for the magnitude of these values, several absorption coefficients and absorption cross sections of some molecules of environmental importance are presented in Tab. 3-6. Especially, for AOPs it is essential to note that the absorbance A is an additive property (cf. Braun et al., 1991), with the consequence that the individual compounds of a wastewater or a gas mixture may compete for the absorption of the incident spectral radiant power. Hence, the concentrations ci of any radiation absorbing species i present in water or air must be considered as well as their individual molar absorption coefficients ek,i. Therefore, the Beer–Lambert law changes to Eq. 3-8, which describes the absorbance A of a multi-component mixture at a specified wavelength k. A10 log

ni X Pk0 ek;n cn trans ek;1 c1 ek;2 c2 :::::ek;i ci l l Pk n1

(3-8)

According to Pfoertner (1998) the absorbed spectral radiant power Pabs k,i of a specific absorbed by all other component i is related to the spectral radiant power Pabs k,n components of a mixture according to the relationships of Eqs. 3-9 and 3-10. abs Pk;i abs Pk;n

ek;i ci l Pn l n1 ek;n cn

0 with Pabs k,n = Pk (1–10

l

(3-9) Pn n1

ek;i ci Pk0 1 n1 ek;n cn

abs Pk;i Pn

ek;n cn

), leads to Eq. 3-10:

10

l

Pn n1

ek;n cn

(3-10)

57

58

3 Interaction of UV/VIS Radiation with Matter Tab. 3.6 Absorption coefficients ek and absorption cross sections rk of some simple molecules of environmental importance (subscript k/nm)

Molecule

Molar absorption coefficient a)/ Reference absorption cross section b)

Br2 (gas phase)

e420 = 200 L mol–1cm–1

Pfoertner ( 1998)

C6H6 (solution)

log(e262) = 2.41

Hesse et al. (1997)

CH2Cl2 (gas phase)

e222 = 0.15 L mol cm

Perkampus (1992)

CH3–CHO (solution)

e293 = 12 L mol cm

Hesse et al. (1997)

CH3OH (neat)

e172 = 162 L mol–1cm–1

Heit et al. (1998)

Cl2 (gas phase)

e330 = 66 L mol cm

Pfoertner ( 1998)

–1

–1

–1

–1

–1

–1

Cl2C=CCl2 (gas phase)

–1

e222 = 5500 L mol cm

Perkampus (1992)

Cl2C=CHCl (gas phase)

e222 = 4000 L mol–1cm–1

Perkampus (1992)

ClNO (gas phase, 298 K)

r340 = 13.7 × 10

Finlayson-Pitts and Pitts (1986)

–1

–20

trans-ClHC=CClH (gas phase) e222 = 1200 L mol

2

cm –1

–1

cm

Perkampus (1992)

F2 (gas phase)

e284.5 = 6 L mol–1 cm–1

H2C=CH2 (solution)

e165 = 16 000 L mol cm

Hesse et al. (1997)

r290 = 1.13 × 10 cm r350 = 0.03 × 10–20 cm2

Finlayson-Pitts and Pitts (1986)

H2O2 (gas phase, 298 K)

–1

c)

–20

Pfoertner ( 1998) –1

2

I2 (gas phase)

e520 = 950 L mol–1cm–1

Pfoertner ( 1998)

NH3 (solution)

e194 = 5700 L mol cm

Hesse et al. (1997)

NO2 (gas phase, 298 K)

r215 = 40.2 × 10–20 cm2 r410 = 57.7 × 10–20 cm2

Finlayson-Pitts and Pitts (1986)

O3 (aqueous solution)

r290 = 3.45 ×10–18 cm2

Finlayson-Pitts and Pitts (1986)

O3 (gas phase, 298 K)

r254 = 1.41 × 10–17 cm2

Finlayson-Pitts and Pitts (1986)

cis-ClHC=CClH (gas phase)

e222 = 500 L mol cm

Perkampus (1992)

–1

–1

–1

–1

a) e, logarithmic base 10. b) r, logarithmic base e. c) cf. Table 6-2 for data in aqueous phase.

Pn The exponential term of Eq. 3-10 becomes negligible for 10 l n1 ek;n cn 1, i.e. if Pn the absorbance A l n1 ek;n cn of the additional components n is large compared with the absorbance of the component i. This is true in many cases for AOPs using hydrogen peroxide as an auxiliary oxidant to purify highly contaminated water, such as wastewater from industrial origin or from landfill leachates. The term a/cm–1 = ek,ncn is called the (decadic) absorption coefficient of a compound. This term a determines the thickness l of the photoreaction zone (i.e. the penetration depth l). High values of the molar absorption coefficient e (which is substrate specific and wavelength dependent) and high concentrations of a sub-

3.9 The Beer–Lambert Relationship

strate and hence a high value of a, reduce the thickness of this reaction zone considerably; a–1 corresponds to the penetration depth l if the absorbance A = 1.00. Example 3-8: Calculate the spectral radiant power Ptrans (in %) transmitted by a k wastewater with an absorbance A10 of 3.4 at k of 253.7 nm (A10 was determined by UV spectrophotometry).

Pktrans

Pk0 Pktrans

+

A10 log

Pk0 103:4 with Pk0 100% Pktrans

100% 0:04% 103:4

Example 3-9: The (decadic) absorption coefficient aH2O = eH2O × cH2O of pure water at the wavelength k of 172 nm is ca. 550 cm–1 (Weeks et al., 1963). Calculate the penetration depth of the 172 nm radiation into water under conditions of total absorbance (A10 = 2). A10 ek c l aH2 0 l l

A10 2 aH2 O 550 cm

1

0:00364 cm 36:4 10

6

m 36:4 lm

This calculation demonstrates that 99% of the incident 172 nm radiation (VUV radiation) is absorbed by water within a zone of a thickness l of 36.4 lm. This value represents the reaction zone of VUV-initiated photochemistry in water (see Fig. 7-12). Example 3-10: From the UV/VIS spectrum of a wastewater its absorbance A10 is determined to be 2.4 at the wavelength k of 253.7 nm (pathlength l = 1 cm). Calculate the necessary concentration of hydrogen peroxide in mol L–1 that guarantees an absorption of UV radiation at this wavelength of 90% by H2O2 [e253.7(H2O2) = 19.6 L mol–1 cm–1]. ek;H O cH O abs Pk;H Pn 2 2 2 2 Pk0 1 2 O2 n1 ek;n cn

10

l

Pn n1

en cn

Inserting the numerical values yields the following expression: 90% = to yield:

19:6cH2 O2 2:4

cH2 O2

× 100% × (1–10–2.4), where the exponential term can be neglected

90 2:4 0:110 mol L 100 19:6

1

This simple calculation clearly demonstrates the large concentration of hydrogen peroxide that must be used in highly UV absorbing wastewater to exploit effec-

59

60

3 Interaction of UV/VIS Radiation with Matter

tively the incident UV radiation for hydrogen peroxide photolysis. Unfortunately, this is due to the low molar decadic absorption coefficient of hydrogen peroxide at k of 253.7 nm in aqueous solution, which is exceptionally low for a primary absorber in photochemical AOPs (see Tab. 6-2). Example 3-11 (adapted from Bolton et al., 1998): A medium-pressure mercury lamp of 1 kW electric input power was used in the H2O2-UV process. The lamp delivers 4 × 10–4 einstein per second in the active wavelength range between k of 200 nm and 300 nm with a radiant power efficiency g of 25%. Calculate the amount of hydroxyl radicals formed from the photolysis of hydrogen peroxide during 1 h of irradiation under conditions of total absorbance by H2O2. The quantum yield of •OH radical formation is U (•OH) = 0.98 (cf. Legrini et al., 1993). This value implies that only one •OH radical is formed via absorption of one (to be exact 0.98) photon by hydrogen peroxide. Photon flow of the lamp: Up = 4 × 10–4 einstein s–1 × 3600 = 1.440 mol (photons) h–1. Amount of hydroxyl radicals formed: n (•OH) = 1.440 mol h–1 × 0.98 = 1.411 mol h–1. Therefore, per kWh about 1.4 mol of •OH radicals are produced.

3.10

The Nature of Electronically Excited States

The energetic situation of electronic excitation in comparison with common thermal or so-called “dark” reactions (ground state reactions) is shown in Fig. 3-12. By excitation of a ground state molecule M0 higher excited states are usually reached that deactivate very rapidly to the lowest electronically excited state M* of the molecule. In addition to a number of other deactivation mechanisms (luminescence emission or non-radiative deactivation), M* can react via a transition state TS* to the photoproduct P'. As in the ground state reaction M0 ? P an activation barrier must be overcome in the excited state reaction M* ? P' (EA and EA*, respectively). However, the activation energy EA* is much smaller than the EA of the ground state reaction. As a consequence of this scheme the photochemical reaction is in fact the thermal reaction of the excited electronic state M* (Suppan, 1994). The most obvious advantage of photochemical reactions is that photoproducts P' can be reached which are thermodynamically unfavorable in ground state reactions (Kopecky, 1992). Two successive approaches to the understanding of photophysical and photochemical processes and of the nature and properties of electronically excited states in molecules have to be considered (Turro, 1978, 1991): a) Specification of the origin of the electrons in an electronically excited molecule and in its ground state with respect to the theory of chemical bonds, and the electron transitions between molecular orbitals. The electron shell of molecules is constructed by linear combination of atomic orbitals (LCAO method) of the participating atoms to form bonding r and p molecular orbitals (MOs), non-bonding n (lone pairs of electrons) and anti-bonding

3.10 The Nature of Electronically Excited States

Fig. 3.12 Energy (E) diagram of ground state (“dark”) reactions and UV/VIS radiation-induced reactions of a molecule M. The values stated for the activation energies (EA) and the reaction enthalpies (DH0) are rough estimates and vary considerably with the nature of the substrate M (TS: transition state).

r* and p* MOs, respectively (Turro, 1978, 1991). Electrons in bonding orbitals stabilize a molecule, whereas anti-bonding electrons destabilize a chemical bond. Almost all stable ground state molecules have antiparallel spin orientations of their bond electrons designated as the S0 state, i.e. all electrons are “spin-paired”. One of the most important exceptions to this rule is the oxygen molecule that has two electrons in two degenerated antibonding orbitals (p*) which, according to Hund’s rule, have parallel spin orientation (::). Therefore, the ground state of molecular oxygen is a triplet state T0, denoted as 3O2. This results directly from construction of the molecular orbital (MO) diagram by linear combination of oxygen atomic orbitals (AOs) as is demonstrated by Fig. 3-13. The electronic transition of 3O2 to 1 O2 involves spin conversion of the electron from the ppy* orbital and its incorporation into the ppx* orbital. Thus, a spin-paired situation is generated, this being the first excited singlet state of oxygen. This process is spin-forbidden according to the rule of Wigner (Wigner and Witmer, 1928). Nevertheless, several possibilities exist for the generation of singlet molecular oxygen, e.g. triplet–triplet energy transfer by photosensitization. As a major consequence, the reactivity of

61

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3 Interaction of UV/VIS Radiation with Matter

Fig. 3.13 Molecular orbital (MO) diagram of

an oxygen molecule illustrating the electronic configuration of the triplet ground state (3O2) and of the first excited singlet state (singlet

3

molecular oxygen, 1O2), neglecting the 1s electrons (modified according to Braun et al., 1991). The energy difference DE between 3O2 and 1O2 is 94.2 kJ mol–1.

O2 (ground state oxygen) with organic molecules in their singlet ground state (S0) is very slow at ambient temperatures, i.e. it is spin-forbidden, thus enabling oxygen-based life on Earth without oxidative destruction of organic matter. The Pauli exclusion principle demands that the spins of two bonding electrons in each molecular orbital of the oxidation products are paired (:;). This, however, is only the case (without additional energy for spin conversion) in reactions of singlet molecular oxygen (1O2) with organic substrates, which is utilized industrially in photooxidation reactions. For a detailed description of 1O2 production and of its reactivity, see Braun et al. (1991). An electronically excited state of a molecule is formed by absorption of a photon promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO– LUMO promotion of an electron is the lowest possible energy transition that occurs between the frontier orbitals of a molecule. This situation is schematically presented in Fig. 3-14.

3.10 The Nature of Electronically Excited States

Fig. 3.14 Generalized molecular orbital diagram: possible electronic transitions in organic molecules, associated absorption regions and examples of common chromophores (modified according to Böttcher, 1991); n, non-bonding MO; antibonding MOs are indicated by *.

The concept of frontier orbitals (Fleming, 1976) is very helpful and illustrative for the understanding of many photochemical reactions. b) Definition of electronic states of a molecule by description of the electronic configuration with respect to the characteristic electron spin orientation and the characteristic molecular orbitals that are involved. This can be achieved by construction of a generalized state diagram, as is demonstrated by Fig. 3-15. Quantum mechanical theory includes the spin concept of electrons (Gilbert and Bagott, 1991). Each electron in a molecule is represented by a spin quantum number of s = ± 1/2. A multi-electron atom or molecule is characterized by the total spin quantum number S that is the sum of all individual electron spins s. The spin multiplicity M of an electronic state of a molecule is determined by the total spin quantum number S and it can be calculated according to M = 2S + 1. Atoms and molecules with an even number of electrons lead to S = 0, 1, 2, 3, resulting in multiplicities of M = 1 (singlet), 3 (triplet), 5 (quintet) and 7 (septet), whereas an odd number of electrons yields S = 1/2, 3/2, 5/2, 7/2 and leads to M = 2 (doublet), 4 (quartet), 6 (sextet) and 8 (octet). Hence, an electronic configuration with two electrons with antiparallel spins (:;) is called a singlet state (S). In a triplet configuration (T), the electrons are arranged with parallel spins (::). The spin multiplicity of the hydroxyl radical (:•OH equivalent to ;•OH) calculates to M = 2 × 1/2 + 1 = 2, i.e. it is a doublet.

63

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3 Interaction of UV/VIS Radiation with Matter

Fig. 3.15 Electronic states of molecules characterized by the orbitals involved and by the spin configuration of the electrons P (spin multiplicity M = 2S + 1, S = si and si = ±1/2, spin of individual electrons).

3.11

The Jablonski Diagram

The concepts mentioned above are clearly illustrated by the generalized Jablonski diagram (Fig. 3-16). It demonstrates the various electronic states of a molecule that are grouped in multiplicity manifolds, including vibrational and rotational sublevels (Suppan, 1994). The latter can usually be neglected because their energy is very low compared with electronically excited state energies. This famous diagram is used to visualize photophysical processes that allow an electronically excited molecule to dissipate its energy by non-radiative and by radiative deactivation phenomena (Braun et al., 1991). For a comprehensive historical assessment of Jablonski’s work, refer to the reference given in Tab. 2-2. Thus, absorption of a photon of sufficient energy by an organic ground state molecule results in promotion of one bonding or non-bonding electron to a vibrational level of the first (S1) or second (S2) excited singlet state, depending on the

3.11 The Jablonski Diagram

Fig. 3.16 Modified Jablonski diagram visualiz-

ing radiative, non-radiative and photoproduct forming processes (adopted from Braun et al., 1991 and Pfoertner, 1998). The lifetimes of the individual processes are given in parentheses. 1, Absorption (excitation) S0 ? S2 and 2, absorption S0 ? S1 (10–15 s); 3, fluorescence S1 ? S0 + hmFl (10–12–10–6 s); 4, phosphorescence T1 ? S0 + hmPh (10–3–102 s); IC: internal conversion S2 ? S1 + heat (10–13–10–12 s) and

IC S1 ? S0 + heat (10–12–10-6 s); ISC: inter system crossing T1 ? S0 + heat (10–7–10 s); ISC S1 ? T1 + heat (10–11–10–6 s). P1: Reaction product from the singlet state (intramolecular); P2: (intermolecular) reaction product from the triplet state. The reactions from S1 and T1 may also include electron or energy transfer reactions. The arrows in the boxes represent the spin orientation of the electrons in the participating MOs.

photon energy, respectively. Internal conversion (IC) from S2 and/or very fast relaxation (10–13–10–12 s) of vibrationally excited levels of the S1 state leads immediately to the occupation of the S1 state in its lowest vibrational level. According to Kasha’s rule (see Kopecky, 1992), all subsequent processes occur from the S1 excited state of a molecule. Either the S1 state deactivates via internal conversion and by vibrational relaxation to the ground state S0, ultimately transforming electronic excitation energy into vibrational energy (i.e. heat), or the S1 state deactivates by fluorescence emission. The short lifetime of the S1 state (10–12–10–6 s) usually enables only intramolecular photoreactions that may involve rearrangements of the molec-

65

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3 Interaction of UV/VIS Radiation with Matter

ular structure or bond cleavage processes leading to reaction product P1. The change of multiplicity by inter system crossing (ISC) is spin-forbidden according to the rule of Wigner (Wigner and Witmer, 1928), but it may occur through mechanisms that are called spin–orbit interactions. Then the relatively long-lived (10–7– 102 s) first excited triplet state (T1) of the molecule is occupied. Again, vibrational deactivation of the T1 state competes with radiative deactivation, called phosphorescence emission. Typical singlet energies E(S1) are in the range from 180 to 500 kJ mol–1 and triplet energies E(T1) are between 125 and 360 kJ mol–1 (Turro, 1978, 1991). It is obvious from the Jablonski diagram that the energy of fluorescence emission is greater than that of phosphorescence emission (schematically illustrated by the length of the solid arrows S1 ? S0 and T1 ? S0, respectively). This indicates the well-known phenomenon that the wavelength of fluorescence emission is shorter than that of phosphorescence. Hence, phosphorescence is shifted bathochromically (i.e. it is red shifted toward a longer wavelength) relative to fluorescence emission. Nearly all intermolecular reactions of electronically excited molecules occur from the T1 state (Braun et al., 1991), generating different molecular structures (reaction product P2). This is mainly due to the long lifetime of the T1 state compared with other competing deactivation processes enabling bimolecular reactions. The reaction products P1 and P2 are usually referred to as photoproducts. The lifetimes of T1 and of S1 are strongly dependent on the molecular structure and on the experimental conditions (Braun et al., 1991). Further, typical reactions of singlet or triplet excited states of many molecules are electron or energy transfer reactions to ground state molecules. The latter process is utilized in so-called photosensitization reactions where UV/VIS radiation is not absorbed by the substrate itself, but by an added sensitizer (photosensitizer). Subsequently, the electronically excited sensitizer molecule transfers its electronic energy to the substrate molecule (which is called a quencher) with formation of an excited substrate molecule and a ground state of the sensitizer molecule. Usually, the sensitizer is deactivated without chemical alteration during the photosensitization process (Pfoertner, 1998, Braun et al., 1991).

3.12

Absorption of UV/VIS Radiation by Solids

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Böttcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction

3.12 Absorption of UV/VIS Radiation by Solids Fig. 3.17 Schematic representation of some photophysical and photochemical processes in and on a semiconductor (SC) particle (for example TiO2). Ebg: Band gap energy; VB: valence band; CB: conduction band; h+: electron “hole” (“defect electron”) in the valence band; e–: photoelectron in the conduction band; LT: lattice trap; ST: surface trap; Aads, Dads: chemical species adsorbed on the surface of the SC particle with A being an electron acceptor and D an electron donor. Formation of an electron–hole pair (exciton) by irradiation: SC + hm ? e–cb + h+vb (modified according to Serpone, 1996 and Böttcher 1991).

band (in analogy with HOMO–LUMO transitions) require at least UV/VIS radiation with an energy equivalent to the band-gap energy (Ebg) (Fig. 3-17). Band-gap excitation initiates several physical and chemical processes within the semiconductor particle and on its surface, respectively. By absorption of a photon with the energy greater than or equal to Ebg an electron is promoted from the valence band to the empty conduction band of the SC particle, creating an electron deficiency in the valence band. This is called a defect electron or a positive hole (h+vb). The electron/hole pair thus formed is called an exciton (Serpone and Salinaro, 1999). The charge carriers (photoelectron e–cb and hole h+vb) can recombine by different mechanisms and decay channels (Hoffmann et al., 1995). In competition with non-radiative (lattice or surface trapping) and radiative deactivation mechanisms they can migrate to the surface of the SC particle. At the interface to the contacting medium, an adsorbed acceptor substrate Aads is reduced by transfer of the photoelectron e–cb to an unoccupied MO of the acceptor molecule and an adsorbed donor substrate Dads is oxidized by transfer of a donor electron to h+vb of the SC particle. Thus, the illumination of an SC particle with UV/VIS radiation ultimately initiates redox chemistry at its surface (Fig. 3-17). The photochemistry of semiconductors like titanium dioxide (TiO2) plays an important role in research concerning heterogeneous photocatalysis (Fujishima et al., 2000, Serpone, 1996) and solar synthetic chemistry (Kisch and Lindner, 2001). For example, the band-gap energy Ebg of TiO2 (crystal form anatase) is about 3.2 eV (Hoffmann et al., 1995). This energy is equivalent to the wavelength k of 387.5 nm (cf. Tab. 3-4). Therefore, TiO2 is able to absorb parts of the solar UV-A and the solar UV-B radiation, making it to one of the most investigated SC materials for solar driven detoxification and disinfection of aqueous or gaseous systems (Rajeshwar, 1995). However, aqueous heterogeneous photocatalysis involves the use of semiconductor suspensions. Therefore, it is accompanied by light scatter-

67

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3 Interaction of UV/VIS Radiation with Matter

ing, low penetration depth of UV radiation into the suspension, adsorption phenomena and by the mechanical problem of catalyst separation after treatment.

3.13

UV/VIS Radiation as a Specific Reagent: Quantum Yield, Quantum Efficiency, Actinometry and Photoreaction Kinetics

The quantum yield U of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express U in units of mol einstein–1, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of U are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct. Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield U over a defined wavelength range. Such chemical systems can be

Tab. 3.7 Definitions of the quantum yield a) Uk and of the quantum efficiency b) g of a photo-

physical process c) or of a photochemical reaction of a reactant M to yield a photoproduct P' (modified according to Verhoeven, 1996 and Braun et al., 1991) Mathematical expression

Definition

Uk

dnevent=dt Uabs p

Universally valid: Number n of events per unit time divided by the number of photons absorbed during this period

Uk

dnM=dt Uabs p

Number n of reactant molecules M consumed per unit time divided by the number of photons absorbed during this period d)

Uk

dnP0 =dt Uabs p

Number n of photoproduct molecules P‘ formed per unit time divided by the number of photons absorbed during this period d)

g

dmP0 =dt 1 Uabsk p

k2

6 Uk

Ratio of the number m of photoproduct molecules formed per unit time to the total number of photons absorbed in the spectral region k1–k2 during this period e)

a) Based on the number of absorbed photons Uabs P , i.e. on the absorbed photon flow (not on the incident photons!) at a stated wavelength k. b) based on the number of absorbed photons of polychromatic UV/VIS radiation in a defined spectral region k1–k2. c) for example, quantum yield of fluorescence UFl, phosphorescence UPh, internal conversion UIC, intersystem crossing UISC. d) if no photophysical processes or photochemical side reactions occur, the rate of reactant diminution equals the rate of photoproduct formation: –dn(M)/dt = dn(P')/dt. e) accordingly, this definition holds for the diminution of reactant molecules. If the quantum yield U of a photochemical system is independent of the wavelength range k1–k2, then g = Uk

3.13 UV/VIS Radiation as a Specific Reagent

easily used for the measurement of the absorbed photon flow Uabs p . These photochemical reaction systems, with well-known quantum yields, are called actinometers. Chemical actinometry is suitable for the measurement of the absorbed photon flow Uabs p in photoreactors of specific geometry and in a well-defined spectral domain. A comprehensive list of versatile actinometers and quality marks for solid phase, gas and liquid phase photolysis has been published by Kuhn et al. (1989). Braun et al. (1991) gave further examples of the most common and practical chemical actinometric procedures for monochromatic and polychromatic UV/ VIS photon flow determinations. Recently, several interesting applications of actinometers were published. For example, the fluence rate distribution E0 in mW cm–2 at k of 253.7 nm in UV reactors can be determined by the iodide/iodate system (Stefan et al., 2001, Rahn, 1997), this being important for UV disinfection development. In connection with the development of novel incoherent excimer UV lamps several actinometric procedures have been applied to establish their radiant power efficiency, e.g. using 3,4-dimethoxynitrobenzene at k of 308 nm (Zhang et al., 1999), uridine at 222 nm (Zhang et al., 1997), hydrogen bromide (Pfeifer Roland et al., 1998), oxygen (Laszlo et al., 1998) and methanol (Heit et al., 1998) at k of 172 nm. In photochemical AOP practice, however, polychromatic radiant sources (medium-pressure Hg lamps or sunlight) are usually used. Thus, the quantum efficiency g was defined, covering the wavelength range of interest from k1 to k2 (Tab. 3-7). The quantum efficiency can be determined for a given radiant source, if the chemical actinometer used absorbs all of the photons in the spectral region from k1 to k2 during the irradiation period (Braun et al., 1991). Alternatively, the photon emission weighted average of some quantity (e.g. relative photon flow Up of a lamp, the data are usually provided by the manufacturer) over a spectral range k1 to k2 may be used to calculate the relative spectral photon flow of a radiant source over this wavelength band (Bolton, 1999). However, (photo)chemists do not usually think in quantities such as the number of molecules consumed or formed by a reaction (cf. Tab. 3-7), but in concentrations given in mol L–1. Therefore, the quantum yield definition must be converted into a concentration dependency, as is demonstrated by Eqs. 3-11 to 3-13.

Quantum yield Uk

dnM=dt Uabs p

(3-11)

and the number of molecules nM M=

mol NA =mol L

1

V=L

(3-12)

With [M] the concentration of the reactant M; NA, Avogadro’s constant; and V the by its volume containing n molecules of M. Substitution of the photon flow Uabs p –2 –1 s times irradiated area A equivalent Uabs p = Ep × A (photon irradiance EP in m in m2) leads to Eq. 3-13, implying the concentration of M:

69

70

3 Interaction of UV/VIS Radiation with Matter

Uk

dM=dtNA V : Ep;abs A

(3-13)

Eq. 3-13 can be used for the actinometric determination of the absorbed photon irradiance Ep,abs, if Uk of the specific photoreaction system is known. The measurement and control of UV/VIS radiation and/or the quantification of the photon flow Up of an industrial lamp is of vital importance to AOP developments. It can also be achieved by using radiometers or UV sensors (McCluney, 1994). Moreover, it is essential to determine the UV/VIS absorbance spectrum of the water or air to be treated by UV spectroscopy prior to further research and development efforts. On the other hand, Eq. 3-13 may be rearranged to Eq. 3-14, expressing a rate equation, and thus raising the question of photochemical reaction kinetics. dM A cf : Tab: 3-8 Uk EP;abs dt NA V

3-14

It is obvious from Eq. 3-14, that the rate of decrease of M is proportional to the quantum yield Uk , the absorbed photon irradiance Ep,abs and to the area A which is irradiated at constant photon irradiance Ep,0, and it is inversely proportional to the irradiated volume V. In contrast to ground state (dark) reactions that are characterized by a reaction rate constant k, a photochemical reaction does not have a specific reaction order (Logan, 1997). This is demonstrated in Tab. 3-8. The kinetic behavior of a photoreaction is strongly dependent on the absorption conditions. Under total absorbance, i.e. with a substrate exhibiting a high absorption coefficient a(M) = eM k [M] (i.e. concentrated solutions of M), the observed decrease of the concentration of the reactant M by irradiation is linear. In very dilute solutions of M or under conditions with a low a(M) only a small fraction Fk of the incident photon irradiance is absorbed by M leading ultimately to an observed exponential decrease of [M]. Thus, photochemical reactions may be classified within two different regimes, depending on the absorption characteristics of the irradiated mixture and thus on the reaction conditions. There must be a stage of transition between the linear and exponential regimes, where the fraction of the absorbed radiation Fk plays an important role. This stage of change of regimes is dependent on the concentration of the absorber M. Consequently, there is no basis of rate constants in photochemical reactions and hence, this term is meaningless. In sharp contrast to thermal reactions, the only parameter that is important for the quantification of photochemical reactions is the quantum yield U.

k1 s–1

First approximation e): if total absorbance, Ak = eM k [M] l >2, M then 10–ek [M] l is negligible and Ep,abs = Ep,0.

M?P Reaction order: 1

M + hm ? M* ? P' Photochemical reaction c) General rate expression d):

UM EP;0 1

10

eM Ml k

A NA V

k0 mol L–1 s–1

M?P Reaction order: 0

dM A UM EP;abs dt NA V

Rate constant, unit

Reaction of substrate M

UM EP;0 for EP,0 = constant;

Mt Mt0

dM A UM EP;0 dt NA V

[M]t = [M]t = 0 e–k1t

dM k1 [M] dt

[M]t = [M]t = 0 – k0 t

dM k0 dt

A t NA V

Reaction rate, integrated form

Tab. 3.8 Comparison of the reaction rate expressions of simple thermal a) and photochemical reactions b) in solution

Graphical representation

3.13 UV/VIS Radiation as a Specific Reagent 71

Reaction rate, integrated form dM A M UM EP;0 2:303 eM k l dt NA V A t Mt Mt0 exp UM EP;0 2:303 eM k l NA V for Ep,0 = constant.

Rate constant, unit

Second approximation f): If Ak = eM k [M] l << 1, then M Fk = 1–10–ek [M] l % 2.303 eM k [M] l

Graphical representation

a) Kinetics of the ground state reactions of M to the product P with the rate constants k. b) irradiation of a cell with monochromatic UV/VIS radiation having an exposed area A, a volume V and a path length l of UV/VIS penetration. c) UM: quantum yield of the photochemical conversion of M to the photoproduct P' at wavelength k. The substrate M has a molar extinction coefficientat eM k at a defined wavelength k. –2 –1 d) cf. Eq. 3-14 and Table 3-4, section D, NA: Avogadro’s constant; EP,abs = Uabs s , refer to Eq. 3-5; EP,0: incident photon P /A: total absorbed photon irradiance in m irradiance. e) Ak: absorbance of the solution at wavelength k; concentrated solution of M or eM k l very large. For simplification reflectance, refraction and scattering are neglected. f) very diluted solution or eM k l very small, so that only a small fraction Fk of the incident photon irradiance EP,0 is absorbed by M; the original expression for Fk is simplified by exponential Taylor development and neglecting all terms after the second.

Reaction of substrate M

Tab. 3.8 (continued)

72

3 Interaction of UV/VIS Radiation with Matter

3.14 Terms Associated with the Emission and Receipt of Electromagnetic Radiation

Fig. 3.18 Terminology related to the measurement of electromagnetic

radiation.

3.14

Terms Associated with the Emission and Receipt of Electromagnetic Radiation

The various terms that are used for the description of the emission of electromagnetic radiation from a radiant source or for the receipt of electromagnetic radiation by a specified surface element are summarized in Tab. 3-9. The terminology of electromagnetic radiation measurement is divided into radiometry and the subset of photometry (Fig. 3-18). The former is the science that involves the energy measurement of electromagnetic radiation in general. The latter is applied for the same purpose when visible radiation is to be described or measured in relation to the human eye’s response. Important photometric quantities are for example luminous flux, luminous intensity, illuminance and luminance (McCluney, 1994). Every photometric quantity has its counterpart in radiometry, and vice versa. Radiometric quantities are important to describe and measure UV and VUV radiation. They are usually subdivided into energetic, spectral and photonic terms. Energetic terms (Tab. 3-9) are based on the energy of the radiation and they refer to all relevant wavelengths. For each of these terms a spectral derivative can be defined (Bolton, 1999) which is correlated with a specific wavelength k. For example, the extraterrestrial solar spectrum incident on the upper atmosphere is represented by the mean spectral irradiance E0 in W m–2 nm–1 over a defined wavelength interval Dk in nm (CIE, 1989). Further, each of the spectral units can easily be transferred to photon-based units, which themselves are related to radiometric units (cf. Braun et al., 1991).

73

74

3 Interaction of UV/VIS Radiation with Matter Tab. 3.9 Classification of terms used to describe and measure electromagnetic radiation either

received by a defined surface element or emitted by a surface element of a radiant source (for verbatim definitions refer to Serpone and Emeline, 2002, Bolton, 2000, 1999, Verhoeven, 1996, McCluney, 1994, Mills et al., 1993, Braun et al., 1991, and the glossary in this book) Name of quantity

Symbol

Definition

SI unit

Classification a)

Fluence b)

H0

H0 = E0 × t c)

J m–2

receipt

E0

E0 =

4P A

E

E=

Fluence rate

b, d)

Irradiance e) Radiance

f)

L

L=

P A P

Asource cos

Radiant energy

Q

Q=P × t

Radiant exitance

M

M=

Radiant exposure

H

H=E×t

g)

c)

P Asource

I

I=

P, U

P=U=

Photon exitance h)

Mp

MP =

Photon exposure

HP

HP = EP × t c)

UP

UP =

HP,0

HP,0 = EP,0 × t c)

EP,0

EP,0 =

4Up A

Photon irradiance

EP

EP =

Photon radiance j)

LP

LP =

Photon flow

i)

Photon fluence b) Photon fluence rate

b)

Wm

receipt

W m–2

receipt

–2

Wm

emission, receipt

J

receipt, emission

W m–2

emission

J m–2

receipt

–1

P 4p

Radiant power, radiant (energy) flux

Radiant intensity

–2

Q t

Up Asource

NP t

W sr

emission

J s–1, W

receipt, emission

s–1 m–2 mol s–1 m–2

emission

m–2, mol m–2

receipt

–1

–1

s , mol s

receipt, emission

m–2, mol m–2

receipt

–1

–2

s m mol s–1 m–2

receipt

Up A

s–1 m–2 mol s–1 m–2

receipt

Up Asource cos

s–1 m–2 sr–1 mol s–1 m–2 sr–1

emission, receipt

Spectral photon exitance MP,k

MP,k =

Up;k Asource

s–1 m–3 s–1 m–2 nm–1 mol s–1 m–2 nm–1

emission

Spectral photon flow

UP,k

UP,k =

Np;k t

s–1 m–1, s–1 nm–1 mol s–1 nm–1

receipt, emission

Spectral photon irradiance

EP,k

EP,k =

P A

s–1 m–3 s–1 m–2 nm–1 mol s–1 m–3 mol s–1 m–2 nm–1

receipt

Spectral photon radiance LP,k

LP,k =

Pp;k Asource cos

s–1 m–3 sr–1 s–1 m–2 nm–1 sr–1

emission, receipt

Spectral radiance

Lk =

W m–3 sr–1 W m–2 nm–1 sr–1

emission, receipt

Lk

Pk Asource cos

3.15 Safety Precautions for Radiant Sources Tab. 3.9 (continued)

Name of quantity

Symbol

Definition

Spectral radiant excitance

Mk

Mk =

k Asource

Pk 4p

Spectral radiant intensity Ik

Ik =

Spectral radiant power

Pk = Uk =

Pk

Qk t

SI unit

Classification a)

W m–3 W m–2 nm–1

emission

W m–1 sr–1 W nm–1 sr–1

emission

W m–1 W nm–1

receipt, emission

a) According to receipt of electromagnetic radiation by a surface or emission by a surface element of a radiant source. b) the IUPAC Working Party on UV Disinfection is recommending the symbol H' for fluence and E' for fluence rate, rather than H0 and E0, since the latter are easily confused with “initial” or “incident” values. Similarily, they recommend HP' for photon fluence instead of HP,0, and EP' for photon fluence rate instead of EP,0 (Bolton, 2000 b). c) these equations are valid only if the first variable is constant in time. d) radiant power P of all wavelengths incident from all directions on an infinitesimal small sphere of cross-sectional area A. e) counterpart to radiant exitance (also called emittance) concerning the emission of light by a defined surface element of a source. f) defined for a parallel beam of electromagnetic radiation of all wavelengths. g) the SI supplementary unit steradian (sr) is defined for a solid angle X = A/r2, the solid angle formed by a sphere is X = 4p. h) photon flow emitted by an element of the surface of the source point divided by the area A of that element. i) number of photons NP per unit time t. j) taking into consideration the direction of the beam leaving a surface element of the source.

3.15

Safety Precautions for Radiant Sources

The impact of electromagnetic radiation, especially of UV radiation, on the human skin and on the eye may lead to severe adverse effects (see Fig. 2-4) including skin cancer, severe skin burning, conjunctivitis or cataract formation. Hence, appropriate protective measures must be considered while working with artificial radiant sources. This includes UV-blocking eyewear and face wear. Never look directly and unprotected at an operating UV or VUV lamp! However, in AOTs the intensive radiating lamps are usually housed within stainless steel or other non-transparent reactor materials. If glass reactors are applied, efficient protection of the eyes is necessary. This can be achieved either by shielding the photoreactor or by wearing appropriate goggles. Even if the glass material of a photoreactor absorbs the UV radiation efficiently, most medium-pressure mercury lamps produce intensive and extremely bright visible radiation that may lead to eye damage on prolonged fixation. Further safety precautions for the operation of photochemical equipment include protection against electrocution risks and efficient ventilation to prevent ozone exposure. Certain lamps may produce significant amounts of ozone de-

75

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3 Interaction of UV/VIS Radiation with Matter

pending on their emission spectrum. High-pressure mercury lamps bear the risk of explosion and they should be handled carefully (Braun et al., 1991).

References Adam W, Oppenländer T (1986) 185-nm Photochemistry of Olefins, Strained Hydrocarbons, and Azoalkanes in Solution, Angew. Chem. Int. Ed. Engl. 25: 661–672. Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photosciences Inc., Ayr, Ontario, Canada, ISBN 0-9685432-0-0; also published in: Europ. Photochem. Assoc. Newsl. Nr. 66: 9–36. Bolton JR (2000 a) Calculation of Ultraviolet Fluence Rate Distributions in an Annular Reactor: Significance of Refraction and Reflection, Wat. Res. 34, No. 13: 3315–3324. Bolton JR (2000 b) Terms and Definitions in Ultraviolet Disinfection, in: Proceedings, Disinfection 2000, Disinfection of Wastes in the New Millennium, 15–18 March 2000, New Orleans, LA, Water Environment Federation, 601 Wythe St., Alexandria, VA, 22314. Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Helz GR (ed.) Aquatic and Surface Photochemistry, Lewis Publishers, Boca Raton: 467–490. Bolton JR, Valladares JE, Zanin JP, Cooper WJ, Nickelson MG, Kajdi DC, Waite TD, Kurucz CN (1998) Fig.s-of-Merit for Advanced Oxidation Technologies: A Comparison of Homogeneous UV/H2O2, Heterogeneous UV/TiO2 and Electron Beam Processes, J Adv. Oxid. Technol. 3, No. 2: 174–181. Böttcher H (ed.) (1991) Technical Applications of Photochemistry, Deutscher Verlag für Grundstoffindustrie, Leipzig. Braun AM (1997) New Potentials for Photochemical Technology, Spec. Chem. 17: 2122, 24–25. Braun AM, Jakob L, Oliveros E, Oller do Nascimento CA (1993) Up-Scaling Photochemical Reactions, in: Volman D, Hammond GS, Neckers DC (eds.) Adv. Photochem. 18: 235–313.

Braun AM, Maurette M-T, Oliveros E (1991) Photochemical Technology, John Wiley & Sons, New York. CIE (1989) Solar Spectral Irradiance, Kok C J, Kasten F, Justus C G (eds.), CIE Technic. Rep. No. 85, 1st ed. Esser P, Pohlmann B, Scharf H-D (1994) Die photochemische Synthese von Feinchemikalien mit Sonnenlicht, Angew. Chem. 106: 2093–2108. Finlayson-Pitts BJ, Pitts Jr JN (1986) Atmospheric Chemistry: Fundamentals and Experimental Techniques, John Wiley & Sons, New York. Fleming I (1976) Frontier Orbitals and Organic Chemical Reactions, Wiley-Interscience, Chichester. Fujishima A, Rao TN, Tryk DA (2000) Titanium Dioxide Photocatalysis, J. Photochem. Photobiol. C: Photochem. Rev. 1: 1–21. Gilbert A, Baggott J (1991) Essentials of Molecular Photochemistry, Blackwell Scientific Publications, Oxford. Göpel W, Hesse J, Zemel JN (eds.) (1995) Sensors, A Comprehensive Survey, Vol. 2, 3 and 6, VCH, Weinheim. Heit G, Neuner A, Saugy P-Y, Braun AM (1998) Vacuum-UV (172 nm) Actinometry. The Quantum Yield of the Photolysis of Water, J. Phys. Chem. A 102, No. 28: 5551– 5561. Hesse M, Meier H, Zeeh B (1997) Spectroscopic Methods in Organic Chemistry, Georg Thieme, Stuttgart. Hind A (2001) Spectrophotometry Takes Measure of Deep-UV Lithography, Photonics Spectra, December: 82–86. Hoffmann MR, Martin ST, Choi W, Bahnemann DW (1995) Environmental Applications of Semiconductor Photocatalysis, Chem. Rev. 95, No. 1: 69–96. Huheey JE (1988) Anorganische Chemie, Walter de Gruyter, Berlin. Kisch H, Lindner W (2001) Synthesen durch Halbleiter-Photokatalyse, Chem. Unserer Zeit 35, No. 4: 250–257

References Kogelschatz U (1992) Silent-discharge driven excimer UV sources and their applications, Appl. Surf. Sci. 54: 410–423. Kopecky J (1992) Organic Photochemistry: A Visual Approach, VCH, Weinheim. Kuhn HJ, Braslavsky SE, Schmidt R (1989) Chemical Actinometry, Pure Appl. Chem. 61, No. 2: 187–210. Kurzweil P (2000) Das Vieweg EinheitenLexikon, 2nd ed., Vieweg, Braunschweig. Laszlo Zs, Ilisz I, Peintler G, Dombi A (1998) VUV Intensity Measurement of a 172 nm Xe Excimer Lamp by Means of Oxygen Actinometry, Ozone Sci. Eng. 20: 421–432. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93: 671–698. Levenspiel O (1996) The Chemical Reactor Omnibook, OSU Book Stores, Corvallis, Oregon, USA. Lide DR (ed.) (2000) CRC Handbook of Chemistry and Physics, 81st ed., CRC Press, Boca Raton. Logan, SR (1997) Does a Photochemical Reaction have a Reaction Order?, J. Chem. Educ. 74, No. 11: 1303. Mattay J (2002) Von der Laborsynthese zur Solarchemie, Chem. Unserer Zeit 36, No. 2: 98–106. McCluney WR (1994) Introduction to Radiometry and Photometry, Artech House, Boston. Mills I, Cvitas T, Homann K, Kallay N, Kuchitsu K (1993) Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Scientific Publications, Oxford. Moore DE (1996) Photophysical and Photochemical Aspects of Drug Stability, in: Tonnesen H H (ed.) Photostability of Drugs and Drug Formulations, Taylor & Francis, London. Oppenländer T, Gliese S (2000) Mineralization of Organic Micropollutants (Homologous Alcohols and Phenols) in Water by Vacuum-UV-Oxidation (H2O-VUV) with an Incoherent Xenon-Excimer Lamp at 172 nm, Chemosphere 40: 15–21. Oppenländer T, Zang, G, Adam W (1992) Rearrangements and Photochemical Reactions Involving Alkanes and Cycloalkanes, in: Patai S, Rappoport Z(eds.) The Chemis-

try of Alkanes and Cycloalkanes, John Wiley & Sons, Chap. 15. Perkampus H-H (1992) UV-VIS Atlas of Organic Compounds, VCH, Weinheim. Pfeifer Roland R, Bolle M, Anderson RW (1998) Hydrogen Bromide Photochemistry: Actinometry for Determination of Absolute Power Outputs of Xenon Excimer and other UV/VUV Light Sources, J. Phys. D: Appl. Phys. 31: 1336–1342. Pfoertner KH (1986) Practical Photochemistry: Scale-up, in: Coyle J D (ed.) Photochemistry in Organic Synthesis, Chap. 13, The Royal Society of Chemistry, Burlington House, London. Pfoertner K-H (1998) Photochemistry, Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed., electronic release, Wiley-VCH, Weinheim. Prousek J (1996) Advanced Oxidation Processes for Water Treatment. Photochemical Processes, Chem. Listy 90: 307–315. Rahn RO (1997) Potassium Iodide as a Chemical Actinometer for 254 nm Radiation: Use of Iodate as an Electron Scavenger, Photochem. Photobiol. 66: 450–455. Rajeshewar K (1995) Photoelectrochemistry and the Environment, J. Appl. Electrochem. 25: 1067–1082. Sahyun MRV (1996) Photochemical Technology, in: Kirk-Othmer, Encyclopedia of Chemical Technology, 4th ed., John Wiley & Sons, New York, Vol. 18: 799–820. Serpone N (1996) Photocatalysis, in: Encyclopedia of Chemical Technology, 4th ed., John Wiley & Sons, New York, Vol. 18: 820–837. Serpone N, Emeline AV (2002) Suggested Terms and Definitions in Photocatalysis and Radiocatalysis, Int. J. Photoenergy 4, No. 3: 91–131; free of charge available at http://www.photoenergy.org/ijp. Serpone N, Salinaro A (1999) Terminology, Relative Photonic Efficiencies and Quantum Yields in Heterogeneous Photocatalysis. Part I: Suggested Protocol, Pure Appl. Chem. 71, No. 2: 303–320. Shortley G, Williams D (1961) Elements of Physics, 3rd ed., Prentice-Hall, Englewood Cliffs, N. J. Sonnemann G (1992) Ozon, Akademie Verlag, Berlin.

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3 Interaction of UV/VIS Radiation with Matter Stefan MI, Bolton JR, Rahn RO (2001) Use of the Iodide/Iodate Actinometer Together with Spherical Cells for the Determination of the Fluence Rate Distribution in UV Reactors: Validation of the Mathematical Model, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release; available via IUVA, P. O. Box 1110, Ayr, ON, Canada N0B 1E0. Suppan P (1994) Chemistry and Light, The Royal Society of Chemistry, Cambridge. Turro NJ (1978) Modern Molecular Photochemistry, The Benjamin/Cummings Publishing Co., Menlo Park. Turro NJ (1991) Modern Molecular Photochemistry, University Science Books, MillValley, California. vanLoon GW, Duffy SJ (2000) Environmental Chemistry, University Press, Oxford.

Verhoeven JW (1996) Glossary of Terms Used in Photochemistry, 2nd ed., Pure Appl. Chem. 68: 2223–2286. Weeks JL, Meaburn GMAC, Gordon S (1963) Absorption Coefficients of Liquid Water and Aqueous Solutions in the Far Ultraviolet, Rad. Res. 19: 559–567. Wigner E, Witmer EE (1928) Über die Struktur der zweiatomigen Molekülspektren nach der Quantenmechanik, Z. Physik 51: 859–886. Wöhrle D, Tausch MW, Stohrer W-D (1998) Photochemie, Konzepte, Methoden Experimente, Wiley-VCH, Weinheim. Zhang J-Y, Boyd IW, Esrom H (1997) UV Intensity Measurement for a Novel 222 nm Excimer Lamp Using Chemical Actinometer, Appl. Surf. Sci. 109/110: 482–486. Zhang J-Y, Esrom H, Boyd IW (1999) UV Intensity Measurement of 308 nm Excimer Lamp Using Chemical Actinometer, Appl. Surf. Sci. 138-139: 315–319.

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4

VUV and UV Radiant Sources and their Characteristics 4.1

Types of Lamps used in AOP Research and Development

Much useful background information about UV/VIS and VUV lamp technology currently used in photochemical, AOP and UV disinfection applications can be gained from the specific literature (Braun et al., 1991, Böttcher, 1991, Scaiano, 1989, Kiefer, 1977, Murov, 1973). Although UV lamp technology and the production and application of UV/VIS radiation has been quite well established over the past century, many advances in this field are a result of intensive research over the last 10 to 15 years. The main technological improvements are related to the enhancement of the UV-C radiation output, the lamp compactness and the lifetime of the lamps (Rivera, 2001, Altena et al., 2001). Further new developments worth mentioning include the replacement of conventional electromagnetic device ballasts by transistorized electronic power supplies. The latter have several advantages over conventional ones: they are lightweight, run cool, much smaller than electromagnetic devices and their electrical efficiency gel is greater than 90%. Their output power is infinitely variable, and they are able to operate the lamps at their optimum with respect to the UV-C output and their lifetime. In addition, a novel generation of lamps with promising features for photochemical applications has been developed to industrial maturity over the last decade, the so-called incoherent excimer radiation sources (Eliasson et al., 1988). Note that these lamps are not laser sources. In contrast to well-known excimer lasers, excimer lamps are operated under different physical conditions and they emit incoherent electromagnetic radiation. Whereas pulsed laser radiation can reach very high irradiances, E up to 100 MW m–2, the irradiance E of excimer lamps is only in the range of 1000 W m–2. In photochemistry, and especially in photo-initiated AOP applications, it is obvious that the appropriate selection of the lamp type largely determines the effectiveness of the desired process. The evaluation has to take into account technical and economic considerations. First, the radiation source must emit radiation that is absorbed by the reactant, i.e. the emission spectrum of the lamp must match the absorbance spectrum of the reactant to a maximum extent. Secondly, the elecin tric input power Pel /W of a radiant source determines the radiant power P/W of

80

4 VUV and UV Radiant Sources and their Characteristics

Fig. 4.1

Types of VUV/UV radiant sources related to AOP research and development.

the lamp within a specified wavelength range Dk, with the radiant power effiin being the important criterion for the assessment of the quality of ciency g = P/Pel a lamp. The radiant power P differs considerably from the electrical input power in in , because a major part of Pel of a lamp is dissipated as heat or as undesired waPel velengths during the operation of the radiant source. Thirdly, the fixed geometry (usually line, tube or coil forms) of most of the lamps available limits the freedom in reactor design, because the reactor geometry must be adapted to the lamp’s appearance. However, this is not the case with the novel incoherent excimer VUV/ UV sources that are readily available with various geometries: cylindrical, planar or special shapes (cf. Chapter 4.3). Their design is independent of the configurations of the electrodes (Kogelschatz, 1992). The most important lamp types used in AOP research and development are collected in Fig. 4-1. Two main categories have to be distinguished: the lamps emitting line spectra and the sources that emit a continuous spectrum. Lasers are special sources of very intensive, coherent and monochromatic radiation with a halfwidth of the emission lines (i.e. full width at half maximum, FWHM) down to less than 10–12 m (FWHM <1 pm) (Sansonetti and Reader, 2001). They are not used in AOT applications because of the high costs of laser photons compared with mercury arc lamp (or excimer lamp) photons. Nevertheless, lasers are valuable tools in wavelength-selective and high intensity (multiphoton) photochemistry and they are used extensively for investigations of time-selective photoreactions, e. g. for the study of AOP reaction mechanisms. Furthermore, they are used in isotope separation, substance and surface cleaning, optical information recording and in micro- and stereo-lithography (Böttcher, 1991).

4.1 Types of Lamps used in AOP Research and Development

Excimer lamps (see also Chapter 4.3) are relatively new on the market. They are sources of incoherent VUV or UV radiation with a FWHM of 4 to 14 nm. The wavelength of emission mainly depends on the excimer gas or gas mixture that is applied. Because of their narrow band emissions they are usually called “almost monochromatic” or “quasi-monochromatic” radiant sources. These lamps are mercury free systems and therefore they are predestined for use in the lighting industry (Pease, 2000) and other promising advanced applications, such as VUV/UV-induced metal and dielectric thin film deposition (material deposition), UV curing of paints, varnishes and adhesives (Roth, 1995), VUV-initiated surface modification and photo-initiated degradation of various pollutants in water and air (Boyd and Zhang, 1997, Kogelschatz 1997). One of the most fascinating features of incoherent excimer lamps is that both large area irradiation units and miniaturized lamp geometries are possible. The latter led, in 1996, to a multi-billion dollar investment in production facilities (in Japan and South Korea) for large-area flat television screens, so called ac plasma display panels (PDPs). The PDPs consist of miniaturized Xe2* excimer lamps coated with layers of red, green and blue light-emitting phosphors (Kogelschatz, 1997). Many other interesting applications of incoherent excimer lamps evolved during the last decade (cf. Chapter 7.2.3), including research related to photochemical syntheses in aqueous phase (“green chemistry”) using XeCl* excimer lamps (Oelgemöller, 2000, Griesbeck et al., 1999). Incoherent excimer lamps are commercially available from Heraeus Noblelight, Ushio and Osram. The most common mercury arc lamp is the low-pressure (LP) type, being extensively used in the field of UV disinfection. This lamp type is an efficient converter of electrical into radiant energy (gel ~ 40–60%), usually with a UV-C output in the range of 30 to 50 W (Altena et al., 2001). They provide almost monochromatic UV radiation at k of 253.7 nm (usually referred to as 254 nm radiation in the technical literature) with an ordinary quartz envelope. Using envelopes that consist of synthetic quartz of high purity (so-called Suprasil quartz), the 184.9 nm line of the mercury fluorescence can be utilized with a relative radiant power efficiency of about 10% compared with the 253.7 nm emission. However, the maximum in of LP mercury lamps is usually restricted to less than 300 electric power input Pel W. Higher intensity LP mercury lamps (Braun et al., 1991) with an electric input in power Pel of greater than 1 kW and a UV-C output at 253.7 nm of higher than 450 W are difficult to operate with constant output, owing to the strong temperature dependence of the mercury vapor pressure that must be controlled by a heating system of the incorporated Hg reservoir. On the other hand, medium-pressure (MP) Hg lamps can be operated with much higher electrical input power up to 30 kW, but with a reduced UV radiant power efficiency of 30 to 40%. Their dimensions are more compact than the dimensions of the LP Hg lamps, but their polychromatic emission ranges from the UV (UV-C ~ 15–23%, UV-B ~ 6–7 %, UV-A ~ 8%) over the VIS (~15%) to the IR (~47–55%) region of the electromagnetic spectrum (e. g. McClean, 2001). Lambrecht (1999) reported the power balance of electromagnetic radiation of an industrially used high-pressure mercury lamp with a power density of 200 W cm–1 as follows: 13.2% UV-C, 7.15% UV-B, 7.15% UV-A, 21% VIS, and 14% IR

81

82

4 VUV and UV Radiant Sources and their Characteristics

Radiant power efficiency g = P/Pin el × 100 of mediumpressure mercury lamps as a function of the electric input power Pin el in the wavelength range between k of 200 nm and 300 nm (data taken from a product information brochure, Heraeus, Hanau, Germany, 1983). Fig. 4.2

(<4000 nm). This results in a total radiant density of 125 W cm–1 (62.5%). The remaining 75 W cm–1 is mainly due to heat losses at the wall of the lamp. Using chemical actinometry (Braun et al., 1991) it is possible to determine the actual radiant power P of mercury lamps within a defined spectral range Dk. As an example, the radiant power of a standard medium-pressure mercury lamp for laboratory use (Heraeus, TQ 718, Pel = 500 W) in the wavelength range of 220 nm < k < 600 nm was determined to be P = 155 ± 4.0 W (Bossmann et al., 1998). The typical emission lines of mercury lamps are summarized in Tab. 3-3. There are MP Hg lamps on the market with even higher electrical power input in of up to 100 kW (high-pressure mercury lamps). However, their use in AOP Pel applications is not recommended for two crucial reasons: the enormous heat production by the high power lamps must be carried off by an efficient cooling system during operation, and the radiant power PUV-C does not scale proportionally in with the electrical input power Pel , so that the radiant power efficiency g reaches its maximum at 10 to 20 kW of electrical input power (Fig. 4-2). As a compromise in situation, in industry MP Hg lamps up to a maximum input power Pel of 30 kW are usually applied. At high mercury pressures (above 101 kPa) the resonance line at k of 253.7 nm is auto-absorbed by the mercury atoms themselves. This generates a narrow dark zone separated by two lines centered at the original wavelength (Braun et al., 1991). By using Suprasil quartz envelopes, the radiant power efficiency in the UV-C region of MP mercury lamps can be shifted down to the near VUV region with an increase in efficiency of up to a total of 20% (Fig. 4-3). This effect is dependent on the specific operating conditions of the lamp, i.e. on the transmission characteristics of Suprasil quartz at elevated temperatures and on the vapor pressure of mercury (Lambrecht, 1999). Some companies claim that they have advanced MP Hg lamps available with electric-to-photon energy efficiencies of about 30% in the wavelength range Dk of 200 to 300 nm and with power densities of up to 250 W cm–1 (cf. Bolton et al., 1995).

4.2 Specific Properties of Mercury Arc Lamps

Fig. 4.3 Comparison of the emission spectra of medium-pressure mercury lamps with standard quartz or Suprasil quartz envelope in the wavelength region below k of 250 nm with a power density of 120 W cm–1 (reproduced by permission of eta plus electronic GmbH & Co KG, Nürtingen, Germany).

The artificial radiant sources emitting a continuum (xenon arc lamps) are of minor importance to AOP applications. Xenon flash lamps provide a broad continuum ranging from the VUV over the UV, the VIS and to the IR range of the electromagnetic spectrum (Wekhof et al., 1992). Pulsed xenon lamps have a radiant power efficiency g of about 19% below k of 300 nm and they are operated for exin of 2.7 kW or 3.7 kW (Haag et al., 1996). They usually have very high ample at Pel current densities, thus reducing their lifetime considerably to less than 500 h (Bolton and Cater, 1994). In sharp contrast, the emission continuum of the sun (cf. Figs. 4-18 and 4-19) represents a cheap energy source that can be exploited by photocatalytic AOPs and by its conversion into other forms of energy (Böttcher, 1991).

4.2

Specific Properties of Mercury Arc Lamps

Artificial radiant sources are characterized by their power density (i.e. the input power per unit length in W cm–1), the efficiency of conversion of electrical energy into photon energy (radiant power efficiency g PPin ) in a defined wavelength el

range Dk, their lifetime and the temperature stability of their radiant power P (Tab. 4-1). A representative drawing of an MP mercury lamp is shown schematically in Fig. 4-4. The sealed quartz body contains mercury vapor of defined pressure (cf. Tab. 4-1). Mercury atoms in the gas phase are electronically excited by an electrical discharge between two electrodes. These electrodes are in direct contact with the Hg vapor phase. The electronically excited Hg atoms deactivate to their

83

84

4 VUV and UV Radiant Sources and their Characteristics Schematic drawing of a MP mercury lamp DQ 1023 (modified according to the Technical Data Sheet, Heraeus Noblelight, Kleinostheim, Germany); additional characteristic data are available in Tab. 4-1.

Fig. 4.4

Emission spectra of a typical 50 W low-pressure mercury lamp and of a 120 W high-intensity low-pressure mercury lamp (Ek values were made available by Heraeus Noblelight, Kleinostheim, Germany). Fig. 4.5

ground state by emission of radiation according to the energy level diagram (Murov, 1973), thus generating an intense radiating arc within the quartz envelope. Typically, the geometry of Hg lamps is restricted to tubular forms of varying length and diameter, depending on the electrical input power. However, spiral LP and MP Hg lamps (cf. Fig. 8-4) are also in use (Meyer and Fischer, 1994). Typical emission spectra of two different LP and a MP mercury lamp are presented in Figs. 4-5 and 4-6. The 50 W LP mercury lamp (Heraeus, NN 50/81, Fig.

4.2 Specific Properties of Mercury Arc Lamps Tab. 4.1 Specific properties of mercury arc lamps used in UV disinfection and in AOTs a)

Lamp type:

LP Hg lamps

MP Hg lamp

Characteristic feature

NN50/81

NNI 120/84 b)

DQ1023

Spectrum appearance Wavelength k (UV-C) c) Surface temperature Temperature influence Arc length l Diameter of discharge tube in Electric power Pel Power densityd) Radiant power PUV-C e) Radiant power efficiency g f) Radiant density (UV-C)g) Irradiance E (k = 253.7 nm) h) Useful lifetime

line source 253.7 nm <50 8C significant 800 mm 15 mm 45 W 0.6 W cm–1 18 W 40% 0.2 W cm–1 0.17 mW cm–2 8000 h

line source 253.7 nm <150 8C significant 800 mm 15 mm 120 W 1.5 W cm–1 33 W 28% 0.4 W cm–1 0.27 mW cm–2 8000 h

broad band 200–280 nm 650–950 8C negligible 108 mm 22 mm 1000 W 93 W cm–1 140 W 14% 13 W cm–1 – 1500 h

Power range i) Mercury vapor pressure j)

5–50 W

50–300 W 0.001–13 mbar

1–30 kW ~1333 mbar

a) Data modified according to Rivera (2001), additional information received from Heraeus Noblelight Inc. (Germany). b) hybrid lamp: amalgam filling. c) spectral range of interest for AOPs and UV-disinfection, UV-C defined as 200 nm< k <280 nm. in d) defined as the ratio of electric input power Pel to the arc length l. e) measured in air at room temperature. in × 100. f) g = PUV-C/Pel g) defined as the ratio of radiant power PUV-C to the arc length l. h) measured at a distance of 100 cm from a new lamp. i) available power systems. j) ranges of Hg vapor pressure applied, data from Braun et al. (1991).

Typical emission spectrum of a 1 kW medium-pressure mercury lamp (Ek values were made available by Heraeus Noblelight, Kleinostheim, Germany). Fig. 4.6

85

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4 VUV and UV Radiant Sources and their Characteristics

Enhanced UV-C emission of a special 1 kW mediumpressure mercury lamp containing xenon as buffer gas (Ek values were made available by Heraeus Noblelight, Kleinostheim, Germany). Fig. 4.7

Irradiance E of a low-pressure mercury lamp as a function of the ambient temperature, measured in water at different water temperatures (relative E values were made available by Heraeus Noblelight, Kleinostheim, Germany).

Fig. 4.8

4-5) is an almost monochromatic source of 253.7 nm radiation. Unfortunately, the enhancement of the power density of LP mercury lamps leads to an increase in other emissions, as is exemplified by the emission spectrum of a high-intensity LP mercury lamp, type Heraeus, NNI 120/84 (Fig. 4-5). The emission spectrum of the MP mercury lamp (DQ 1023) (Fig. 4-6) demonstrates the typical line emissions of these lamp types over a broad wavelength range. An increase in UV-C emission intensity of MP Hg lamps can be achieved by adding the rare gas xenon in variable proportions. This leads to the formation of the mercury–xenon excimer (HgXe*) (cf. Gellert and Kogelschatz, 1991), which emits broad bands centered at k of 210 nm and 270 nm, and thus generating a “hump“-shaped emission structure below k of 220 nm (Stockwald, 1991), see Fig. 4-7. Other mechanisms to explain the “mysterious hump”, such as Hg resonance radiation, electron–ion-re-

4.3 Development of Incoherent Excimer Lamps

Fig. 4.9 Relative irradiance E of low-pressure mercury lamps as a function of their operation time (relative E values were made available by Heraeus Noblelight, Kleinostheim, Germany).

combination and molecular radiation, are discussed intensively by Lambrecht (1999). In contrast to MP mercury lamps, the radiant power P of LP mercury lamps is very sensitive to ambient temperature changes. This is shown by Fig. 4-8, demonin decreases drastically from strating that the optimum electric input power Pel about 140 W at 10 8C to about 100 W at 60 8C of ambient temperature. In fact, this temperature dependence of the optimal radiant output has to be taken into account in engineering LP mercury lamp systems for UV disinfection and treatment processes of water and air. The electrical power of the LP mercury lamps must be adjusted to the temperature of the medium. LP mercury lamps have a considerably longer lifetime than MP mercury lamps. The irradiance of the former drops slowly to a value of about 60% after 10000 h of operation (Fig. 4-9), depending on the lamp type. This fact is of central importance to the safe operation of UV disinfection units because a sufficient germicidal fluence H0 must be guaranteed at each time of operation. The major reason for the drop of the irradiance of LP Hg lamps with time is due to the reaction of mercury with various lamp parts during use (Altena et al., 2001). In MP mercury lamps, the mercury consumption is enhanced due to their more drastic operating conditions, hence, lowering the lamp lifetime considerably (cf. Tab. 4-1).

4.3

Development of Incoherent Excimer Lamps

In contrast to coherent radiation emitted by laser sources, incoherent or non-coherent radiation is characterized by elementary electromagnetic waves that do not have any phase relation in space and time. The production of incoherent excimer radiation in the UV or VUV region of the electromagnetic spectrum is made pos-

87

4 VUV and UV Radiant Sources and their Characteristics

sible for example by microwave discharges (Kitamura et al., 1994, Ametepe et al., 1999, He et al., 1998) or by silent electrical discharges in gas atmospheres (Eliasson et al., 1988, Kogelschatz, 1992, Ivanov et al., 1996). The latter operation mode is also referred to as a dielectric-barrier discharge (DBE) (Kogelschatz, 1993, Eliasson and Kogelschatz, 1988). DBEs are efficient in the treatment of rare-gases (Boyd et al., 1995), halogen gas atmospheres (Gellert and Kogelschatz, 1991) or rare-gas/halogen mixtures (Falkenstein and Coogan, 1997, Mezyk et al., 1997, Zhang and Boyd, 1998) leading to formation of short-lived electronically excited dimers (excimers: dimers consisting of two atoms or molecules of the same structure) or excited complexes (exciplexes: dimers consisting of two atoms or molecules of different structures). For example, a xenon atom can be electronically excited by electron impact according to Eq. 4-1. Subsequently, the short-lived xenon excimer (Xe2*) is formed in a three-body reaction by collision of the electronically excited xenon atom (Xe*) with ground state xenon atoms (Xe0) (Eq. 4-2). Under suitable conditions of the plasma (Eliasson and Kogelschatz, 1991), the xenon excimer decays within nano seconds with emission of radiation peaking at 172 nm and formation of two ground state xenon atoms (Eq. 4-3). Xe0 ehigh energy

! Xe elow energy

(4-1)

Xe Xe0 Xe0

! Xe:::Xe excimer Xe0

(4-2)

Xe2

! 2Xe0 VUV radiation kmax 172 nm

(4-3)

This principle is used to convert kinetic energy of electrons efficiently into almost monochromatic UV or VUV radiation by simple variation of the excimer gas or gas mixture. The radiant power efficiencies of the first generation of excimer lamps lies in the region of 5–10% (Eliasson and Kogelschatz, 1991). However, higher efficiencies in converting electrical to photon energy of over 60% have been reported (Ivanov et al., 1996, Vollkommer and Hitzschke, 1997). Finally, this research led to the development of a small Xe*2 excimer lamp with g of about 40%. This lamp is commercially available with the trade name XERADEX (Osram, 2000). The emission maxima of important excimers and exciplexes are

Tab. 4.2 Matrix of excimers (and exciplexes) obtained from halogens and rare gases and their

corresponding emission maxima (Kogelschatz et al., 2000, Esrom and Kogelschatz, 1992, Gellert and Kogelschatz, 1991)

Halogen

88

Rare Gas

–

Ne

– F Cl Br I

158 nm 259 nm 289 nm 342 nm

108 nm

Ar

Kr

Xe

126 nm 193 nm 175 nm 165 nm

146 nm 249 nm 222 nm 207 nm 190 nm

172 nm 354 nm 308 nm 283 nm 253 nm

4.3 Development of Incoherent Excimer Lamps

Fig. 4.10 Emission spectra of the incoherent xenon (Xe*2) and kryptonchloride (KrCl*) excimer lamps (see also Kogelschatz, 1992), FWHM: full width at half maximum, half-width; the spectra were provided by U. Kogelschatz (ABB, Baden, Switzerland).

collected in Tab. 4-2 (Kogelschatz et al., 2000, Esrom and Kogelschatz, 1992). For the sake of simplicity, in the following text all VUV and UV sources of this type are designated as excimer lamps. Typical emission spectra of a Xe2* and a KrCl* excimer radiant source are presented in Fig. 4-10. The xenon excimer emission shows a relatively broad band in the VUV region ranging from k of 160 nm to 200 nm peaking at k of 172 nm with a full width at half maximum (FWHM, half-width) of 14 nm. The KrCl* excimer lamp exhibits a narrow band emission at k of 222 nm with a half width (FWHM) of only 4 nm (Esrom and Kogelschatz, 1992). In both cases, almost no other emissions are present. In particular, these lamp types do not emit significant amounts of IR radiation, therefore being called “cold lamps”. Recently, the production of a multiwave excimer lamp was reported that combines the emission maxima of the KrCl*, KrF*, XeCl* and of the XeF* bands (Shuaibov, 1998). The experimental configuration to produce a silent discharge plasma is characterized by the presence of at least one dielectric (usually UV transparent quartz or VUV transparent Suprasil quartz) and a discharge gap between two electrodes (Kogelschatz, 1992). Several discharge configurations are schematically presented in Fig. 4-11. In Fig. 4-11 A the VUV/UV transparent electrode is in direct contact with the medium to be treated, whereas the configuration of Fig. 4-11 B allows irradiation of a medium without any contact to an electrode. The electrode arrange-

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Fig. 4.11 Various electrode configurations of incoherent excimer radiant sources; a) flow channel of water or air to be irradiated; b) discharge gap containing the excimer gas or excimer gas mixture.

ment of Fig. 4-11 C uses the electrically conductive fluid (in this case polluted water) that is in contact with the dielectric of the discharge gap and the metallic housing of the reactor as the ground electrode. Every arrangement of Fig. 4-11 has been realized at laboratory experimental scales (e. g. A: Oppenländer et al., 1995; B: Heit et al., 1998; C: Metzger and Falkenstein, 2001). Additionally, Fig. 4-11 demonstrates that the possible electrode configurations are different from those of mercury arc lamps (cf. Fig. 4-4): the electrodes are arranged outside the discharge gap and therefore they are not in contact with the radiating plasma. This special arrangement is called “electrode less” discharge. It should enhance the lifetime of the lamps considerably compared with mercury arc lamps. Further, this type of lamp needs no warm-up phase like Hg lamps. Hence, the radiation energy is available instantaneously after ignition, allowing a pulsed operation mode. The Xe2* excimer lamp shows no self absorption of its radiation (in contrast to Hg vapor lamps) because Xe0 atoms in their electronic ground state are transparent to radiation between k of 160 nm to 200 nm. The electrical discharge is operated by an alternating high voltage with amplitudes of several kV and in the kHz to MHz frequency range. Commercially available excimer lamps (Xe2*, KrCl* and XeCl*) are usually powered by plasmagenerators with an electric power of 20 W, 150 W or 1.5 kW. Electronic ballasts of higher power are under active development. The geometric variability of silent discharge configurations leads to an enormous flexibility in excimer lamp design (Fig. 4-12). Different tubular radiation

4.3 Development of Incoherent Excimer Lamps Fig. 4.12 Different possible geometrical configurations of incoherent excimer lamps (according to Kogelschatz, 1992).

Fig. 4.13 Schematic representation of an annular incoherent excimer VUV/UV source with inward directed (coaxial) radiation geometry, i.e. in flow-through operation mode (Oppenländer et al., 1996).

geometries may be designed, i.e. inward directed radiation or outward radiating lamps (e. g. Oppenländer, 1997 a). Even the construction of flat radiation panels is possible (Kogelschatz, 1992) which was recently commercially realized (Pease, 2000). However, even more important is the fact that for the first time in photochemical engineering, the geometry of the UV source can be adapted to the optimum conditions of a desired photoprocess (cf. Braun et al., 1993). Hence, it is possible to construct tubular excimer lamps with a radiation length l of 60 cm or more (e. g. Oppenländer et al., 1997a) for use in AOPs (pilot scale applications) or to apply miniaturized lamps for analytical purposes (Wörner et al., 1998), having a total length l of 80 mm and an inner diameter of the flow channel of only 1 mm. For photochemical reactions, usually the lamp configuration with an annular discharge gap is preferred. Consequently, a typical experimental setup is demonstrated schematically by Fig. 4-13 (Oppenländer, 1997 a, Oppenländer et al., 1996, 1995, Oppenländer and Baum, 1994, Braun et al., 1993). It consists of a cylindri-

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Fig. 4.14 Cross-sectional view of the excimer lamp of Fig. 4-13 and typical dimensions.

Fig. 4.15 Photograph of an incoherent excimer flow-through lamp (Pin el = 150 W)

with coaxial radiation geometry (configurations see Figs. 4-13 and 4-14).

cal inner tube of Suprasil quartz (Xe2* excimer lamp) or quartz (KrCl* excimer lamp) that is surrounded by an outer tube with a slightly greater diameter. Thus, the hermetically sealed discharge gap is produced that contains the excimer gas or excimer gas mixture. The inner electrode usually consists of a metal wire or optionally of a metallic static mixer (cf. Oppenländer et al., 1997 a, b). The construc-

4.3 Development of Incoherent Excimer Lamps

Fig. 4.16 Schematic representation of an excimer flow-through photoreactor. Two excimer lamps are coupled via flanges. They are used with inward directed radiation geometry (Oppenländer et al., 1995).

tion of the outer electrode can be adapted to the desired radiation mode of the lamp by use of UV-reflecting aluminum for inward directed radiation (coaxial geometry) or by use of a metal wire net (or checkered gold structures on the quartz surface) for outward radiating lamps. Typical dimensions of the quartz tubes and the electrode configurations are demonstrated in Fig. 4-14. The photograph in Fig. 4-15 shows a xenon excimer lamp with coaxial radiation geometry according to the schematic drawing of Fig. 4-13 and with the dimensions depicted in Fig. 4-14. This lamp was used as a flow-through photoreactor (cf. Fig. 4-16) to treat water or air with VUV radiation at k of 172 nm (Oppenländer and Fradl, 1999, Oppenländer, 1997 b, Baum and Oppenländer, 1995). Incoherent VUV/UV excimer lamps may be easily connected to generate irradiation units as is demonstrated by Fig. 4-16. Two or more lamps with different emission maxima can be coupled to treat aqueous or gaseous phases in flow-through type arrangements (Oppenländer et al., 1995). Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e. g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for “microwave photochemistry” experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f. Klan et al., 2001, 1999).

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4 VUV and UV Radiant Sources and their Characteristics Fig. 4.17 Electrode-less UV lamp based on high-frequency excitation (Pin el = 25 W). The available wavelengths include the LP and MP Hg emissions. The inner diameter l of the flow channel is 15 mm and the outer diameter l of the lamp is 45 mm; reproduced by permission of UMEX GmbH (Dresden, Germany).

4.4

Typical Photon Flow of VUV or UV Lamps

To get an idea of the amount of photons (i.e. the photon flow Up) that are emitted by different lamps at defined wavelengths in the VUV or UV region of the electromagnetic spectrum several examples are collected in Tab. 4-3. Note, that the same amount of photons, which is produced per period by a lamp at shorter wavelength consumes more of the lamp power, i.e. the photon flow Up is directly proportional to the wavelength k (cf. Tab. 3-4). For example, a LP mercury lamp emits 0.344 mol h–1 photons at k of 253.7 nm (Tab. 4-3). This is equivalent to a spectral power Pk of the lamp of 45 W at this wavelength. On the other hand, the same amount of 172 nm photons would correspond to a spectral power Pk of 66 W (cf. Tab. 3-4).

4.5

The Sun as Radiation Source

The sun is an intense source of corpuscular and electromagnetic radiation. The latter is called solar radiation. The sun is a spherical UV/VIS source with an average radius rsun of 6.9585 × 108 m, an average Earth–sun distance dE-s of 1.4957 × 1011 m and a radiant power Psun of 3.842 × 1026 W. Therefore, the radiant exitance M of the sun calculates to Msun = Psun/4p(rsun)2 = 6.314 × 107 W m–2 (cf. Bolton, 1999).

4.5 The Sun as Radiation Source Tab. 4.3 Photon flow Up/mol h–1 of several VUV or UV lamps at selected wavelengths k a) Pin el /W

Lamp type

MP Hg b)

LP Hg c) Xe2* Excimer e) XeCl* Excimer f)

2 000 10 000 20 000 150 130 150

Photon flow Up/mol h–1 at wavelength k/nm 172

222

253.7

308

313

366

– – – – 0.024 –

– – – – – –

0.28 3.06 7.90 0.344d) – –

– –

0.43 3.72 6.30

0.73 7.66 12.70

– –

– –

0.096

a) b) c) d) e)

Electric input power of the lamp as stated in the literature. according to Pfoertner (1998). TNN 150/150 Heraeus (Hanau, Germany). calculated from manufacturer’s data being equivalent to P254 = 45 W. according to Heit et al. (1998), the lamp was mounted in the axis of a Suprasil quartz tube and immersed in the actinometer solution. f) according to Benoit-Marquie et al., (2000).

Fig. 4.18 Solar spectral irradiance E outside of the atmosphere. A new solar spectral irradiance curve for the aerospace community; data taken from http:// rredc.nrel.gov/solar/standards/am0/newam0.html.

Extraterrestrial solar radiation has different radiant intensity and different spectral distribution compared with that portion of solar radiation that reaches the Earth’s surface (i.e. global radiation) (see Fig.s 4-18 and 4-19). This is due to attenuation of solar radiation by scattering and absorption by atmospheric components (Finlayson-Pitts and Pitts, 1986). The Commission Internationale De L’Éclairage (CIE, 1989) has published detailed data of extraterrestrial and global solar spectral irradiances. Additional information about solar spectra are readily available through the Internet (cf. Tab. 1-1). The energy flux Esolar to the surface of the Earth was estimated to be 3 × 1024 J per year (Böttcher, 1991). The distribution of global radiation depends strongly on atmospheric conditions, e. g. on the attenuation by clouds and fog, etc. Average values can be calcu-

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Fig. 4.19 Spectral solar irradiance E at sea level for a zenith angle

of 0o; data taken from CIE (1989).

lated for standard conditions. Only about 4.5% of the solar irradiance, reaching the Earth’s surface is UV radiation above k of 300 nm, about 43% is visible and 53% represents infrared radiation (Bolton et al., 1995). The average extraterrestrial irradiance Eex corresponds to the integral of the spectral irradiance Eex,k of the sun over the total wavelength range of solar emission (Kaase, 1992). This value can be radiometrically determined via instruments aboard satellites. It is called the solar constant and it can be calculated according to the inverse square law that defines the relationship between the irradiance and the distance from the source (point source): Eex = Psun/4pd2E-s = 1367 ± 7 W m–2 (Ryer, 1997, CIE, 1989, Minder et al., 1988). This is equivalent to 1367 J m–2 s–1. With clear skies, the solar irradiance at the Earth’s surface (Esurface) can reach values of about 1000 W m–2. Higher values up to 1200 W m–2 are possible for short periods. However, the average solar irradiance, for example, in the Swiss plateau reaches only 120 W m–2 at a horizontal surface (Minder et al., 1988). The average annual irradiance on a horizontal surface can vary from 100 W m–2 in the polar regions to about 300 W m–2 in the desert regions (Bolton, 1989). The spectral radiant exitance Mk/W m–2 nm–1 of the sun (outside of the atmosphere) can be approximated by the Planck Blackbody Law (Bolton, 1999, Finlayson-Pitts and Pitts, 1986). Usually, the solar emission spectrum is displayed in the literature in the wavelength range between k of 200 nm and 3000 nm (= 3 lm): about 98% of the total solar radiation is emitted in the wavelength interval from 250 nm to 3 lm (Kaase, 1992). However, the far UV-C and the VUV portion of the solar radiation are of significant importance to photochemical and photophysical phenomena in the upper atmosphere. VUV radiation is absorbed by molecular oxygen and by ozone molecules initiating the oxygen atom based radical chemistry. Various data concerning the relative distribution of the extraterrestrial solar irradiance in photochemically relevant spectral ranges below k of 400 nm are summarized in Tab. 4-4.

References Tab. 4.4 Relative spectral distribution of the extraterrestrial solar irradiance Eex in the photochemical relevant wavelength range below k of 400 nm

Wavelength range Dk/nm

Solar spectral irradiance a) Eex,Dk/W m–2

Relative values b) with respect to Eex = 1367 W m–2

315–400 (UV-A) 280–315 (UV-B) <280 180–400 (VUV/UV) <180 100–150 10–100 1–10 (X-rays) 0.1–1e)

– – – 118 0.023 0.0075 0.002 5 × 10–5 1 × 10–4

7% 1.5% c) 0.5%d) – – – – – –

a) Average values given by European Cooperation for Space Standardization (ECSS, 2001). b) by vanLoon and Duffy (2000). c) about 33% of this extraterrestrial solar irradiance reaches the surface of the Earth as global UV-B radiation (Sonnemann, 1992): for example, in Sydney (latitude of 33.5o S, summer 1992–1993) the midday global irradiance in the UV-B range (290 nm < k < 320 nm) was measured with a radiometer to be Eglobal = 6.08 ± 0.31 W m–2 (Moore, 1996). d) not present at the Earth’s surface. e) flare X-rays.

References Altena FW, van Overveld JBJ, Giller H (2001) Technological Advances in Disinfection Lamps Leading to More Compact UV Sources, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14-16, Washington, DC, electronic release. Ametepe JD, Diggs J, Manos DM, Kelley MJ (1999) Characterization and Modeling of a Microwave Driven Xenon Excimer Lamp, J. Appl. Phys. 85, No. 11: 7505– 7510. Baum G, Oppenländer T (1995) VacuumUV-Oxidation of Chloroorganic Compounds in an Excimer Flow Through Photoreactor, Chemosphere 30, No. 9: 1781– 1790. Benoit-Marquie F, Wilkenhörner U, Simon V, Braun AM, Oliveros E, Maurette M-T (2000) VOC Photodegradation at the Gas-Solid Interface of a TiO2 Photocatalyst Part I: 1-Butanol and 1-Butylamine, J Photochem. Photobiol. A: Chem. 132: 225–232.

Bolton JR (1989) Economics of Solar Energy Conversion Using a Product-Value Analysis, Chimia 43, No. 7–8: 226–227. Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photosciences Inc., Ayr, Ontario, Canada, ISBN 0-9685432-0-0; also published in: Europ. Photochem. Assoc. Newsl.. No. 66: 9–36. Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Helz GR (ed.) Aquatic and Surface Chemistry, Lewis Publishers, Boca Raton: 467– 490. Bolton JR, Safarzadeh-Amiri A, Cater SR (1995) The Detoxification of Waste Water Streams Using Solar and Artificial UV Light Sources, in: Sterret FS (ed.) Alternative Fuels and the Environment, Lewis Publishers, Boca Raton, Chap. 11: 187– 192. Bossmann SH, Oliveros E, Göb S, Siegwart S, Dahlen EP, Payawan L Jr, Straub M, Wörner M, Braun AM (1998) New Evidence against Hydroxyl Radicals as

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4 VUV and UV Radiant Sources and their Characteristics Reactive Intermediates in the Thermal and Photochemically Enhanced Fenton Reactions, J. Phys. Chem. A, 102: 5542–5550. Böttcher H (ed.) (1991) Technical Applications of Photochemistry, Deutscher Verlag für Grundstoffindustrie, Leipzig. Boyd IW, Zhang JY (1997) New Large Area Ultraviolet Lamp Sources and their Applications, Nucl. Instr. Meth. Phys. Res. B 121: 349–356. Boyd IW, Zhang J-Y, Bergonzo P (1995) New excimer ultraviolet sources for photoassisted deposition of thin films: an alternative to excimer laser-induced deposition, Proc. SPIE-Int. Soc. Opt. Eng. 2403: 290– 301. Braun AM, Jakob L, Oliveros E (1993) Advanced Oxidation Processes-Concepts of Reactor Design, J. Water SRT-Aqua 42: 166-173. Braun AM, Maurette M-T, Oliveros E (1991) Photochemical Technology, John Wiley & Sons, Chichester. CIE (1989) Solar Spectral Irradiance, Kok C J, Kasten F, Justus CG (eds.), CIE Technic. Rep. No. 85, 1st ed. ECSS (2001) Solar and Earth Electromagnetic Radiation and Indices, Chap. 6, European Cooperation for Space Standardization; Internet: www.estec.esa.nl/ecss. Eliasson B, Kogelschatz U (1988) UV Excimer Radiation from Dielectric-Barrier Discharges, Appl. Phys. B 46: 299–303. Eliasson B, Kogelschatz U (1991) Modeling and Applications of Silent Discharge Plasmas, IEEE Trans. Plasma Sci. 19, No. 2: 309–323. Eliasson B, Kogelschatz U, Stein HJ (1988) New Trends in High Intensity UV Generation, Europ. Photochem. Assoc. Newsl. No. 32: 29–40. Esrom H, Kogelschatz U (1992) Modification of Surfaces with New Excimer UV Sources, Thin Solid Films, 218: 231–246. Falkenstein Z, Coogan JJ (1997) The Development of a Silent Discharge-driven XeBr* Excimer UV Light Source, J. Phys. D: Appl. Phys. 30: 2704–2710. Fassler D, Aurich J, Guenther K, Meyer A, Johne S (2001) New Effective Microwave Excited Lamps – Design and Applications, Conference Proceedings of the First International Congress on Ultraviolet

Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Finlayson-Pitts BJ, Pitts Jr JN (1986) Atmospheric Chemistry: Fundamentals and Experimental Techniques, John Wiley & Sons, New York. Gellert B, Kogelschatz U (1991) Generation of Excimer Emission in Dielectric Barrier Discharges, Appl. Phys. B 52: 14–21. Griesbeck AG, Kramer W, Oelgemöller M (1999) Photoinduced Decarboxylation Reactions, Green Chem. August: 205–207. Haag WR, Johnson MD, Scofield R (1996) Direct Photolysis of Trichloroethene in Air: Effect of Cocontaminants, Toxicity of Products, and Hydrothermal Treatment of Products, Environ. Sci. Technol. 30, 414– 421. He Z, Prelas MA, Meese JM, Lin L-T (1998) Microwave Excitation and Applications of an Elliptical Excimer Lamp, Laser Part. Beams 16, No. 3: 509–524. Heit G, Neuner A, Saugy P-Y, Braun AM (1998) Vacuum-UV (172 nm) Actinometry. The Quantum Yield of the Photolysis of Water, J. Phys. Chem. A 102: 5551–5561. Ivanov VV, Klopovskii KS, Mankelevich YA, Rakhimov AT, Rakhimova TV, Rulev GB, Saenko VB (1996) Experimental and Theoretical Study of the Efficiency of an Excimer Lamp Pumped by a Pulse Distributed Discharge in Xenon, Laser. Phys. 6, No. 4: 654–659. Kaase H (1992) Fundamentals and Limitations of Optical Radiation Measurements, in: Göpel W, Hesse J, Zemel JN (eds.) Sensors, Vol. 6, VCH, Weinheim. Kiefer J (ed.) (1977) Ultraviolette Strahlen, Walter de Gruyter, Berlin. Kitamura M, Mitsuka K, Sato H (1994) A Practical High-Power Excimer Lamp Excited by a Microwave Discharge, Appl. Surf. Sci. 79/80: 507–513. Klan P, Hajek M, Cirkva V (2001) The Electrodeless Discharge Lamp: A Prospective Tool for Photochemistry Part 3. The Microwave Photochemistry Reactor, J. Photochem. Photobiol. A: Chem. 140: 185–189. Klan P, Literak J, Hajek M (1999) The Electrodeless Discharge Lamp: A Prospective Tool for Photochemistry, J. Photochem. Photobiol. A: Chem. 128: 145–149.

References Kogelschatz U (1992) Silent-Discharge Driven Excimer UV Sources and their Applications, Appl. Surf. Sci. 54: 410–423. Kogelschatz U (1993) UV Production in Dielectric Barrier Discharges for Pollution Control, in: Penetrante BM , Schultheis S E (eds.) Non-thermal Plasma Techniques for Pollution Control, NATO ASI Ser. G 34, Part B: 339–353. Kogelschatz U (1997) Dielectric-Barrier Discharges, Principle and Applications, Proceedings of the International Conference on Phenomena in Ionized Gases (ICPIG XXIII), July 17–22, Toulouse, France. Kogelschatz U, Esrom H, Zhang J-Y, Boyd IW (2000) High-Intensity Sources of Incoherent UV and VUV Excimer Radiation for Low-Temperature Materials Processing, Internat. Conf. Electronic Mat. & Europ. Mat. Res. Soc., Strasbourg, France, May 30– June 2, symposium D: Photon-induced Material Processing, paper D-II.1. Lambrecht M (1999) Untersuchungen an Quecksilberhochdrucklampen zur effizienten Erzeugung ultravioletter Strahlung, Shaker, Aachen. McClean JC (2001) Ultraviolet Photolysis of Pesticides in Potable Water Supplies; 5 Years Operating Experience, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Metzger W, Falkenstein Z (2001) Development of an Excimer UV Light Source System for Water Treatment, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Meyer A, Fischer R (1994) Moderne apparative Entwicklungen zur UV-Oxidationsmittelbehandlung, Gewässerschutz-Wasser-Abwasser 143: 261–274. Mezyk SP, Cooper R, Young JG (1997) Absolute Fluorescence Yields from ElectronIrradiated Gases. 3. XeCl* and XeI*, J. Phys. Chem. A 101: 2429–2435. Minder R, Wolf M, Leidner, J-R (1988) The Sun as Source of Radiation, Chimia 42, No. 4: 124–128. Moore DE (1996) Standardization of Photodegradation Studies and Kinetic Treatment

of Photochemical Reactions, in: Tonnesen HH (ed.) Photostability of Drugs and Drug Formulations, Taylor & Francis, London. Murov SL (1973) Handbook of Photochemistry, Marcel Dekker, New York. Oelgemöller M (2000) Photochemistry of Phthalimides: Photodecarboxylative Addition of Carboxylates and PET-Induced Macrocyclizations, Europ. Photochem. Assoc. Newsl. 69: 11–23. Oppenländer T (1997 a) Entwicklung von inkohärenten Vakuum-UV- und UV-Excimerstrahlern für lichtinduzierte Wasser- und Abwasserbehandlungsverfahren, Chem. Ing. Tech. 69: 1245. Oppenländer T (1997 b) Vakuum-UV-Photolyse von gasförmigem Trichlormethan im Xenon-Excimer-Durchflussphotoreaktor, Chem. Ing. Tech. 69, No. 1+2: 134–138. Oppenländer T, Baum G (1994) Ein modularer Excimer-Durchflussphotoreaktor zur Reinigung belasteter Abwässer durch Vakuum-UV/UV-Doppelbestrahlung ohne Oxidationsmittelzusatz, Chem. Ing. Tech. 66: 1523–1527. Oppenländer T, Baum G, Egle W, Hennig T (1995) Novel Vacuum-UV-(VUV) and UV-Excimer Flow-through Photoreactors for Waste Water Treatment and for Wavelength-selective Photochemistry, Proc. Indian Acad. Sci. (Chem. Sci.) 107: 621–636. Oppenländer T, Baum G, Streiff F, Mathys P, Schneider G (1996) Einsatz von Statischen Mischern in ExcimerDurchflussphotoreaktoren zur Abwasserbehandlung durch Vakuum-UV-Oxidation, Chem. Ing. Tech. 68, No. 1+2: 155–158. Oppenländer T, Fradl M (1999) TOC Destruction of a Phenol/Water Azeotrope by “Photoreactive Distillation” Trough an Incoherent Vacuum-UV Excimer Lamp, Chem. Eng. Technol. 22, No. 11: 951–954. Oppenländer T, Mathys P, Schneider G (1997 b) Apparatus for Treating Fluids with Ultraviolet Radiation, United States Patent, No. 5,614,723, March 25. Oppenländer T, Waizenegger K, Baum G (1997 a) Development and Engineering of Excimer Flow-through Photoreactors for Waste Water Treatment, Proceedings of the International Conference on Ozonation and Related Oxidation Processes in Water and Liquid Waste Treatment, IOA-Internat.

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4 VUV and UV Radiant Sources and their Characteristics Ozone Assoc., Berlin April 21–23, III.6.1– III.6.21. Osram (2000) XERADEX Xenon-ExcimerLampen, http://www.osram.de. Pease AF (2000) Lichtblicke für die Zukunft, Siemens-Forschung und Innovation, No. 1: 42–46. Pfoertner K-H (1998) Photochemistry, Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed., electronic release, Wiley-VCH, Weinheim. Rivera RL (2001) Mercury Amalgam Lamps: Characteristics, Design Parameters and Comparison to other UV Technologies, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Roth A (1995) UV-Härtung mit einer neuartigen monochromatischen UV-LampenGeneration, Coating 11: 440-442. Ryer AD (1997) Light Measurement Handbook, Technical Publications Dept., International Light, Inc., Newburyport, MA, USA; Internet: www.intl-light.com/handbook. Sansonetti C, Reader J (2001) Wavelength Characterization of a Molecular F2 Laser at 157 nm for DUV Lithography, Lamda Highlights No. 58: 4–5.

Scaiano JC (ed.) (1989) CRC Handbook of Organic Photochemistry, Vol. 1, CRC Press, Boca Raton. Shuaibov AK (1998) Multiwave Excimer Lamps Using XeF/XeCl/KrF/KrCl Molecules, Tech. Phys. 43: 1459–1462. Sonnemann G (1992) Ozon, Akademie Verlag, Berlin. Stockwald K (1991) Neuartige Xenon- und Xenon/Quecksilber-Lampen im VUV/UVSpektralbereich, Dissertation, University Fredericiana Karlsruhe (Germany). vanLoon GW, Duffy SL (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford. Vollkommer F, Hitzschke L (1997) Durchbruch bei der effizienten Erzeugung von Excimer-Strahlung, Phys. Bl. 53, No. 9: 887–889. Wekhof A, Folsom Jr EN, Halpern Y (1992) Treatment of Ground Water with UV Flashlamps, Hazard. Mat. Contr. Nov./ Dec.: 48–54. Wörner M, Schnabel C, Hashem TM, Zegenhagen F, Gieser J, Braun AM (1998) Dehalogenation of Halogenated Organic Compounds in Aqueous Solution by Different Photochemical Methods, J. Inf. Recording 24: 455–460. Zhang J-Y, Boyd IW (1998) Efficient XeI* Excimer Ultraviolet Sources from Dielectric Barrier Discharge, J. Appl. Phys. 84: 1174–1178.

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Photochemical Processes of Water Treatment 5.1

Description of Aqueous Systems 5.1.1

Classification of Water Constituents

The various types of water are classified according to their origin as surface, ground, rain or waste water, or to their industrial use as potable water, water for special use with drinking water quality (households, food and pharmaceutical industries), process water or pure and ultra pure water for special applications (cf. Fig.s 2-1 and 2-3). Community wastewater combines domestic sewage and industrial waste streams. It is treated in large sewage plants before it is discharged into natural water bodies such as rivers, lakes or the ocean. However, industrial effluents often need special treatment prior to combination with public waste streams, because of the complex nature of industrial water pollutants (e. g. dyes, pharmaceuticals, inorganic and organic chemicals, etc.). In order to define criteria for the water quality the nature of possible water constituents must be known (Fig. 5-1). These are subdivided into undissolved and dissolved matter. Undissolved matter usually consists of precipitable components, for example soil residues, solid food wastes and human excrements, of floating components such as plant residues, oil, grease and foams, and of suspended solids or colloidal material, such as clay minerals (cf. vanLoon and Duffy, 2000). In addition, many types of microorganisms are found in natural water and wastewater, such as phytoplankton, zooplankton, and virus, bacteria, dangerous parasites (e. g. Giardia lamblia, Cryptosporidium oocysts) and fungi. Unicellular organisms (actinomycetes), chlorophyll-containing algae and simple animal forms (protozoa) are forms of life that are widely distributed in water and soil. A general and comprehensive classification of microorganisms can be found in Atkinson and Mavituna (1991). On the other hand, dissolved organic matter (Fig. 5-1) consists of organic or inorganic substances that are either biodegradable or biologically non-degradable (non-biodegradable). Solutions of liquids, solids or gases may produce anions and cations by dissociation, metal-ion complexes or solvated covalent species. Several

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5 Photochemical Processes of Water Treatment

Fig. 5.1

Classification of water constituents.

gases of environmental importance, such as ammonia (NH3), hydrogen sulfide (H2S), sulfur dioxide (SO2), nitrogen dioxide (NO2) or carbon dioxide (CO2) react with water with formation of equilibria between the corresponding Brönsted acids and bases. Such reactions are strongly pH dependent. Many gases, such as radon, nitrogen, oxygen and ozone do not react with water itself. The versatile and complex solution chemistry of water is comprehensively presented by Worch (1997) and Sigg and Stumm (1996). Other topics relevant to water chemistry have been described by DVGW (1993). For a better understanding of the relations between the different types of organic water contents, the organic matter (OM) is divided into dissolved organic and particulate organic matter (DOM and POM), which can be of either natural or of anthropogenic (i. e. “man made”) origin (Fig. 5-2). The oxidation of DOM by ozonation or H2O2–UV treatment improves the biodegradability considerably, facilitating subsequent water treatment processes (Win et al., 2000). For example, the concentration of DOM in North American surface waters in terms of dissolved organic carbon (DOC) is in the range of 1.4 to 23 mg L–1 (Zuo and Jones, 1997). Natural organic matter (NOM) is derived from plant or microbial residues, or it is produced in situ in water by life cycles and by a variety of decomposition pathways. Humic material (HM) is found in surface water. Its concentration is typically about 50% of the DOM. HM itself consists of the fulvic acid (FA) fraction that is soluble in water of all pH values and has an average molar mass M of 500– 2000 g mol–1. In contrast, the humic acid (HA) fraction is only soluble under acidic conditions. HA has an average molar mass M of 2000–5000 g mol–1 and higher (Sigg and Stumm, 1996). The composition and structure of HM or of hu-

5.1 Description of Aqueous Systems

Fig. 5.2

Classification of organic matter (OM) in water.

mic substances (HS) is very complex and is under active investigation. HM consists of organic macro molecules of heterogeneous structure exhibiting many different functional groups that have been identified by UV/VIS, IR and NMR spectroscopy (vanLoon and Duffy, 2000, Abbt-Braun, 1993). The complex composition of HM and its indistinct chemical and biochemical reactivity is responsible for the difficult analytical characterization of this material. Humic substances are refractory to microbial oxidation and they represent important UV/VIS absorbing structures in aquatic systems. Hence, they can undergo a variety of photochemical reactions under solar irradiation. Because these refractory organic substances (ROS) are ubiquitous in aquatic systems, they are responsible for many interactions during water treatment processes (Hesse et al., 1999). In addition, they are responsible for the formation of disinfection by-products (DBP) during chlorination of potable water (Richardson, 1998). Further, they seem to be a major precursor for the natural photo-induced production of carbon monoxide (CO) (Zuo and Jones, 1997). However, it should be mentioned that the distinction between natural sources of organic matter and of anthropogenic origin is not always clear. This is especially true for chlorinated hydrocarbons (CHCs) that are not only produced by industrial processes but also synthesized by nature in large amounts (vanLoon and Duffy, 2000, Naumann, 1993). Similarly, nitrate ions (NO–3) are naturally occurring constituents of water (Hütter, 1994), but its man made entry into natural water cycles is significant, especially in agricultural regions, by washing out of fertilizers, dung or agricultural waste. This leads to an increase in the nitrate concentration in ground water. Excessive nitrate concentrations cause undesirable changes in water and are a factor that is responsible for the proliferation of harmful algal blooms. In addition, they can have an adverse impact on public health. Nitrate ab-

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Fig. 5.3

Natural and anthropogenic chemicals identified in water samples.

sorbs in the UV-C range (cf. Fig. 6-7) and thus interferes with UV disinfection procedures. The diversity and multitude of individual inorganic and organic compounds that may be present in water is demonstrated by Fig. 5-3. In fact, the chemical composition of the water sample depends strongly on its origin. The margin of different chemicals ranges from naturally occurring cationic, anionic and neutral species (including humic material) to anthropogenic substances such as veterinary and human drugs. For example, several human drugs (carbamazepine, anticonvulsant drug; clofibric acid, anticholesteremic drug; diatrizoate and iopamidol, radiopaque materials) could be identified in Lake Constance at a depth of 250 m in concentration ranges of several ng L–1 (Roßknecht et al., 2001). They represent an ecological and toxicological risk that has to be studied in detail. The pollution of groundwater systems with organic compounds represents a severe problem for drinking water production. The most important point sources of organic groundwater contamination are municipal landfills used for hazardous waste disposal. Further sources of anthropogenic organic and of inorganic chemicals are industrial wastewaters that enter the environment. However, modern analytical techniques, for example capillary gas-chromatography coupled with mass-selective detectors (GC-MS) allow the characterization of

5.1 Description of Aqueous Systems

individual organic water contents (Öman and Hynning, 1993). Leachates of municipal, chemical or sanitary landfills appertain to the most contaminated waters that are known. This is attributed to many different and interrelated biological, chemical and physical processes taking place within a landfill. These processes affect the chemical composition of the leachate and the production of gas. The diversity of organic chemicals in landfill leachates is amazing (Fig. 5-3). For example, in municipal landfill leachates over 200 individual organic compounds have been identified including cyclic and bicyclic compounds, aromatic hydrocarbons and many others (Fig. 5-3). Their concentration ranges from less than 1 lg L–1 (ppb) to several hundred lg L–1 (Paxeus, 2000). Several persistent organic pollutants (POPs) could even be identified in municipal landfill leachates (Murray and Beck, 1990). Also, colloidal heavy metals (Me = Cd, Ni, Zn, Cu, Cr, Pb) could be detected at varyious concentrations in landfill leachates (Jensen and Christensen, 1999). Several of them were complexed with dissolved organic matter (Me–DOM complexes). 5.1.2

Analytical Parameters of Water Quality

The complexity of water chemistry and the diversity of its constituents results in a system of water quality criteria that is based on the relationship between pollutant concentrations and environmental and human health effects. The compilation of the Environmental Protection Agency currently covers 157 priority pollutants (EPA, 1999). It lists estimations of maximum and continuous concentrations of pollutants in surface water according to the latest scientific information. Accordingly, the analytical-chemical requirements of water analyses and the correct assessments of their results are enormous (cf. Kölle, 2001). From the chemists point of view water quality criteria include chemical, physical and biological or microbiological properties (Fig. 5-4). Many of these parameters are important especially for the production and distribution of safe drinking water (Spellman and Drinan, 2000). Furthermore, several water parameters are of crucial importance to the optimal operation of AOPs and AOTs. For example, the pH, the alkalinity, the hardness and the absorbance or transmittance of the water play important roles in photochemical water treatment processes. High concentrations of dissolved molecular oxygen usually enhance photo-initiated oxidative degradation processes of organic water contaminants, whereas dissolved carbon dioxide reacts with water to produce the carbonate species HCO–3 and CO2– 3 , which act as efficient hydroxyl radical scavengers (cf. Fig. 6-16). The most important analytical parameters for the description of water quality and some randomly selected examples are presented in Tab. 5-1. These parameters are usually subdivided into global, group and specific parameters and into bacteriological and toxicological qualities. Global parameters (or sum parameters) of water quality are not substrate specific. Group parameters are related to defined classes of compounds such as chlorinated hydrocarbons, PAHs, phenols, whereas specific parameters indicate the concentrations of single chemicals present in

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Fig. 5.4

Important criteria of water quality. For specification refer to Table 5-1.

water samples. Detailed protocols for the analytical determination of all relevant parameters are standardized and published by DIN (2001), APHA (1998) and Keith (1996). Automatic analyzers can record many of these parameters and therefore on-line water quality control is made possible by appropriate instrumentation (Guhl and Werner, 1997). For the characterization of “real” wastewaters, the global parameters chemical and biological oxygen demand (COD, BOD) and total organic carbon (TOC) are of special interest because they are related to the organic chemical load of the water. For COD determination this is only true in the absence of easily oxidizable inorganic chemicals, for example ferrous (Fe2+) salts, ammonium ions (NH+4 ), nitrites (NO–2), sulfides (HS–, S2–) and sulfites (SO2– 3 ). The COD is defined as a measure of the oxygen equivalent (given in mg O2 L–1) of the organic (and inorganic) matter of a water sample that can be oxidized using strong oxidizing agents, such as potassium dichromate (K2Cr2O7), in an acidic medium at elevated temperatures. However, specific classes of compounds (e. g. alkyl amines) exhibit large deviations between measured and calculated theoretical COD values (for details refer to Baker et al., 1999). Therefore, this water parameter has to be used carefully. On the other hand, any material that is present in water and that is able to react with dissolved molecular oxygen contributes to the BOD. High concentrations of this oxidizable matter usually lead to anoxic conditions of the water body. The five-day BOD5 is defined as the amount of oxygen that is consumed by aerobic heterotrophic bacteria during metabolization of organic water constituents at 20 8C over a period of five days. Therefore, BOD is an important analytical param-

5.1 Description of Aqueous Systems Tab. 5.1 Important analytical parameters for the characterization of water contamination and

selected examples Analytical parameter

Abbreviation

Numerical examples a) or typical compounds

Global parameters Biological oxygen demand Chemical oxygen demand Dissolved organic carbon Nonpurgeable organic carbon Suspended organic carbon (or particulate organic carbon) Suspended solids Total carbon Total inorganic carbon Total organic carbon Total solids Total suspended solids Volatile organic carbon (or purgeable organic carbon)

BOD COD DOC NPOC SOC (POC) SS TC TIC TOC TS TSS e) VOC (POC)

Sewage: 250 mg L–1 b) Sewage: 500 mg L–1 b) Rhine/Koblenz: 3.8 mg L–1 c) Acetate, formate, urea – – Sewage: 220 mg L–1 b) Sewage: 243 mg L–1 d), TC = TIC + TOC Sewage: 67 mg L–1 d) Sewage: 176 mg L–1 d) 720 mg L–1 b) – Tetrachloroethene, trichloromethane f) Hazardous waste landfill: 49 mg L–1 g)

Group parameters Adsorbable organic halogen

AOX

Dissolved inorganic nitrogen Dissolved nitrogen Dissolved organic nitrogen Extractable organic halogen Purgeable organic halogen

DIN DN DON EOX POX

Total nitrogen Total organic halogen Total phosphorus Volatile organic halogen

TN TOX TP VOX

1,2,3-Trichlorobenzene, p-dichlorobenzene f) Hazardous waste landfill: 360 mg L–1 g) NH+4 , NO–2, NO–3, CN–, OCN– DN = DON + DIN Ethylenediaminetetraacetic acid (EDTA) – Vinyl chloride, chlorobenzenef) Hazardous waste landfill: 55 mg L–1 g) 40 mg L–1 b) – 8 mg L–1 b) Tetrachloroethene, hexachlorobenzene f)

Specific parameters Chlorinated hydrocarbons Dissolved oxygen Humic substances Hydrocarbons Phenol index Policyclic aromatic hydrocarbons

CHC DO HS HC – PAH

Polychlorinated Polychlorinated Polychlorinated Polychlorinated

PCB PCDF PCDD PCP

biphenyls dibenzofurans dibenzo-p-dioxins phenols

Bacteriology Colony forming units

CFU j)

Trichloroethene, dichloromethane Urban wastewater: 2.0 mg L–1 h) – Nonane, benzene Tar water: 960 lg L–1 i) Benzo[a]pyrene, chrysene, fluorene Tar water: 44 lg L–1 i) Structure see Figure 2-2 Structure see Figure 2-2 Structure see Figure 2-2 Pentachlorophenol, 2,4-dichlorophenol Greywater: psychrophiles = 337 000 ml–1 k) mesophiles = 3100 ml–1 k)

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5 Photochemical Processes of Water Treatment Tab. 5.1 (continued)

Analytical parameter

Abbreviation

Numerical examples a) or typical compounds

Toxicity Effective concentration

EC50 l)

Lethal concentration

LC50 o)

Lethal Dose

LD50

[Pentachlorophenol] = 4–10 lmol L–1 m) = 16.55 lmol L–1 n) [Pentachlorophenol] = 1.0 lmol L–1 (24 h goldfish) m) 0.6 lmol L–1 (96 h fathead minnow) m) 13 lmol L–1 (Saccharomyces cerevisiae) p)

a) Typical values taken from cited literature. The common unit of most parameters is mg L–1. Alternatively the unit ppm may be used. This unit implies that the density of water is exactly 1.00 kg L–1 being usually a good assumption, but not for ocean water with a density of 1.025 kg L–1 at 15 oC (vanLoon and Duffy, 2000). b) Data of a typical untreated sewage (vanLoon and Duffy, 2000, p. 353). c) Average value in 1990 (Hütter, 1994, p. 67). d) Untreated sewage of dilute load (Hütter, 1994, p. 107). e) Characterization of heterogeneous fractions of water is performed according to standard procedures by settling, microfiltration, ultrafiltration, centrifugation and cross flow filtration (e.g. Vaillant et al., 1999). f) Many other examples were collected by Chemviron Carbon (1997). g) Value taken from Först et al. (1989). h) Value taken from Beltran et al. (1997). i) Value taken from Thomanetz (1999). The water sample was obtained by stirring over hard coal tar. j) CFU: number of colonies per unit volume. Classification by temperature preferences of microorganisms: psychrophiles Toptimum <20 8C, mesophiles Toptimum = 20–45 8C, thermophiles Toptimum >45 8C. k) Value taken from Oppenländer and Baum (1996). The greywater sample was taken directly after a shower bath. l) Determined by a bioluminescence inhibition assay using Vibrio fischeri, formerly known as Photobacterium phosphoreum (cf. Chapter 7.1.5). This assay (Microtox Test) is based on the viability of the bioluminescent bacteria (Kaiser and Palabrica, 1991). m) Value taken from Ho and Bolton (1998). n) Value taken from Froehner et al. (2000). o) This value denotes the concentration of a toxic chemical that leads to 50% mortality of test organisms. p) Value taken from Iwahashi et al. (2000) based on the viability of yeast cells.

eter to establish the quality of natural waters and of industrial wastewater effluents or to control the water quality of communal sewage plants. An index of water quality classification includes the following BOD values (vanLoon and Duffy, 2000, p. 336): very clean water; BOD <1 mg L–1 O2: fairly clean water; BOD ~ 1–3 mg L–1 O2: water with doubtful purity; BOD ~ 3–5 mg L–1 O2: contaminated water. BOD >5 mg L–1 O2: The ratio of COD to BOD allows the estimation of the biodegradability of organic water pollutants (Huber et al., 1993). This ratio provides useful information for water monitoring concerning its self-purification process:

5.1 Description of Aqueous Systems

Fig. 5.5

COD/BOD <1.7: COD/BOD ~ 1.7–10: COD/BOD >10 :

Composition of the total carbon (TC) analytical parameter.

easily and entirely biodegradable organic content; incomplete biodegradation; persistent organic pollutants not biodegradable.

CODs are often higher than BOD values (by a factor of two or more) because many biologically resistant compounds are easily oxidized under the harsh conditions of potassium dichromate oxidation applied for COD measurement. Compounds that are usually refractory to aerobic biodegradability are for example dodecane (C12H26), dichloromethane (CH2Cl2), trichloromethane (CHCl3), chlorobenzene (C6H5Cl), hexachlorobenzene (C6Cl6) and many others (Simmler, 1995). COD and BOD determinations are very sensitive to residual traces of hydrogen peroxide used for example in H2O2-based AOPs. Hence, H2O2 in water samples must be efficiently destroyed prior to COD or BOD analyses. A convenient and easy to use scavenger of aqueous H2O2 is bovine catalase. In contrast to well known inorganic H2O2 scavengers (e. g. sodium thiosulfate, Na2S2O3, sodium sulfite, Na2SO3, sodium hypochlorite, NaOCl) low doses of 0.2 mg L–1 bovine catalase can effectively destroy H2O2 concentrations of as high as 145 mg L–1 in less than 10 min (Liu et al., 2001). TOC is the most relevant parameter for the global determination of organic matter in water and wastewater (Fig. 5-5). It is defined as the sum of all organically bound carbon atoms in dissolved and non-dissolved organic compounds that are present in a water sample. The TOC of a water sample may be composed of DOC, VOC (sometimes referred to as purgeable organic carbon, POC), SOC (sometimes called particulate organic carbon, POC) and NPOC (Fig. 5-5). DOC is

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defined as the fraction of TOC that passes through a filter with a pore diameter l of 0.45 lm. DOC diminution characterizes the mineralization of organic water contents by chemical or biochemical processes. Therefore, this parameter is best suited for the quantification of pollutant removal in wastewater and other applications. Similarly, the SOC is the fraction of TOC that is retained by this filtration step (Vogel et al., 2000). The ratio COD/DOC is called specific oxygen demand (Hütter, 1994). Measurements of the total carbon content (TC) of water, which consists of IC and TOC (TC = IC+ TOC), are usually based on the catalytic combustion of the water sample in the presence of oxygen to yield the TC value. Alternatively, the determination of TOC is possible by a photochemical method, i. e. the VUV-initiated oxidation of organic matter in the low lg L–1 concentration range with formation of CO2 (Huber and Frimmel, 1991). The CO2 resulting from both procedures is analyzed quantitatively in a nondispersive infrared analyzer (NDIR). Because of the complex nature of the TC parameter (Fig. 5-5) different strategies of its determination have to be considered: the purging and the differential method (Vogel et al., 2000). If the IC of a water sample consists only of carbonate species, the sample may be simply acidified with a carbon-free acid (e. g. phosphoric acid) followed by purging and CO2 analysis to yield the correct IC value of the sample. Subsequently, the catalytic combustion of the IC-free sample then results in the TOC if no volatile organic carbon (VOC) species were present. However, the situation gets complicated if other carbon containing inorganic species, such as cyanate, thiocyanates and cyanide are present, because these substrates are not transformed into CO2 by acidification. Instead, they are stripped off as HOCN, HSCN and HCN during the purging step. Consequently, these species are lost for IC determination, because they are not detected by NDIR measurement. If the TC of the sample that contains IC in the form of OCN–, SCN– and CN– has been determined first, the TOC value obtained by the differential method will be too high, according to TOC = TC – IC. These inadequacies of TC and TOC determination have to be considered in order to interpret the analytical data correctly (Vogel et al., 2000). Since 1999, new regulations for the discharge of wastewater from the chemical and pharmaceutical industries in Germany include requirements for tributary wastewater streams with high contents of refractory TOC loads (Kaltenmeier, 1999). Example 5-1: Calculate the concentration of DOC of an aqueous solution that contains 79.5 mg L–1 of 2,4-dichlorophenol (2,4-DCP). 2,4-DCP is a water-soluble industrial chemical of environmental relevance.

5.1 Description of Aqueous Systems

Molecular formula: C6H4Cl2O, molar mass M2,4-DCP = 163.00 g mol–1; molar mass of carbon MC = 12.011 g mol–1, number of carbon atoms n = 6; DOC 2; 4-DCP

nC MC 79:5 mg L M2;4-DCP

1

6 12:011 g mol 163:00 g mol 1

1

35:15 mg L

1

Chlorinated aromatic compounds, such as 2,4-dichlorophenol, pentachlorophenol (C6HCl5O, insecticide and general herbicide), DDT (C14H9Cl5, contact insecticide), atrazine (C8H14ClN5, selective herbizide) and aliphatic chlorohydrocarbons, such as trichloromethane (CHCl3), dichloromethane (CH2Cl2), 1,1,1-trichloroethane (C2H3Cl3) are typical chemicals that contribute to AOX (adsorbable organic halogen). Therefore, AOX constitutes a group parameter that exclusively implies compounds of environmental relevance and of substantial risk to human and animal health. The combination of TOC and COD measurements was used to introduce the MOC-concept of the mean oxidation number of carbon (Vogel et al., 2000). This concept may be useful to distinguish between oxidative, reductive and physical processes that are related to the diminution of the organic carbon content during water treatment. The MOC value is defined according to Equation 5-1. MOC 4

1:5

COD TOC

(5-1)

Advanced examination of water and wastewater also includes UV spectrophotometric techniques to gain additional information on the water quality and for the classification of wastewater (Muret et al., 2000, Thomas et al., 1996). Several research groups have tried to correlate UV/VIS absorbance data with global water parameters, such as BOD, TOC and total nitrogen (TN) (Muzio et al., 2001, Brookman, 1997, Thomas et al., 1999, Ferree and Shannon, 2001). The basic assumption of this approach is that, for example, the DOC concentration should be proportional to the UV absorbance of the water sample. A good correlation between DOC and ultraviolet absorbance at a specified wavelength (k = 253.7 nm) of sediment porewaters was established by Deflandre and Gagne (2001) allowing the quantification of DOC by UV spectroscopy. This methodology, however, is complicated by the inhomogeneous nature of the compounds that are responsible for the global parameters and it is greatly restricted to water samples without interfering substances, such as UV-C absorbing nitrate ions. One of the most important quality characteristics of advanced oxidation processes is their ability to reduce the toxicity of an industrial wastewater and to enhance its biodegradability (see Chapter 7.1.5). This criterion can be established by several standardized procedures using different test organisms ranging from microbes to intact animals (cf. Tab. 5-1). An easy to perform variant is the bioluminescence assay that uses the inhibition of the bioluminescence intensity of the test organism Vibrio fischeri in the presence of toxic substrates. This is a bioassay used worldwide for the evaluation of toxicity data of individual chemicals or of industrial wastewaters (Froehner et al., 2000, DIN, 1991). Commercial systems are

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available that include the test organism and the luminometer for the quantification of the bioluminescence, for example the Microtox® Analyzer System (Beckmann Instruments). Consequently, Kaiser and Palabrica (1991) published a comprehensive toxicity data index based on this methodology. Examples of toxicological studies related to AOTs are discussed in Chapter 7.1.5. Novel developments make use of mammalian cell assays that can provide toxicological information including cytotoxicity and genotoxicity, which is relevant to human risk assessment (Plewa et al., 2000). 5.1.3

Synopsis: Water and Wastewater Treatment Technologies

According to the various applications and uses of water, to its different sources and to the heterogeneous chemical or biological contents of water or wastewater, the treatment technologies are highly diversified. Many descriptions of modern water treatment processes and related topics can be found in standard technological reference books, such as Kirk-Othmer (1998) or Ullmann’s (1996) or in the technical literature presented in Chapter 1 of this book. The book of Drinan (2001) gives an introductory overview of current technologies. Rott and Minke (1999) presented a brief summary of wastewater treatment and recycling strategies in the textile processing industries. The most important water treatment technologies are summarized in Fig. 5-6. Depending on the source and on the water quality, either mechanical, biological, physical, thermal, or chemical processes or their combinations may be applied. Photochemical AOPs and AOTs are subordinated to chemical processes, mainly because the current technological versions of photochemical wastewater remediation are dependent on the addition of auxiliary oxidants, such as hydrogen peroxide, ozone or special catalysts such as titanium dioxide. Photochemical AOPs are attractive alternatives to non-destructive physical water treatment processes, for example adsorption, air stripping or desorption and membrane processes. The last merely transport contaminants from one phase to another, whereas the former are able to mineralize organic water contaminants (cf. Chapter 1). Amongst all the purification processes, water disinfection (see Chapter 9) is undoubtedly of utmost importance to prevent the spread of infectious waterborne diseases (Bersillon, 1999). However, it remains a challenge with respect to many aspects concerning its mechanisms, the formation of undesirable disinfection byproducts (DBP) and the development of new techniques. Disinfection by heat is applied to relatively small amounts of water. Chlorine (Cl2), chloramine (NH2Cl), chlorine dioxide (ClO2) and ozone (O3) represent the core of chemicals that are widely used for disinfection purposes. However, chlorine based water disinfection leads to the formation of potentially toxic or carcinogenic DBPs, which mostly consist of chlorinated organic compounds produced by reactions with dissolved organic matter. Therefore, alternative methods include physical processes (microfiltration or ultrafiltration) or UV-C irradiation, which do not produce halogenated DBPs. Only during the last two decades, has UV disinfection become an accepted

5.1 Description of Aqueous Systems

Fig. 5.6

Overview of the most common technologies for wastewater treatment.

Fig. 5.7

Elements of water treatment facilities.

alternative to chlorine-based methods and to physical processes for efficient disinfection of secondary effluents from communal sewage treatment plants (White et al., 1986), cf. Chapter 9. A typical wastewater treatment facility consists of primary, secondary and tertiary systems (Fig. 5-7). The primary processes are mainly physical ones. They

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serve to remove precipitable, floating and suspended matter. Secondary treatment of the wastewater stream is usually realized by biological processes under aerobic and/or anaerobic conditions. These microbiological processes are used to diminish global, group or specific parameters of the wastewater (cf. Tab. 5-1). Additionally, biological treatment strategies for phosphorus or nitrogen elimination (a twostep microbial nitrification–denitrification process) are widely applied in sewage treatment plants (vanLoon and Duffy, 2000). The effects of nitrate and nitrite on human health and new methods of nitrate removal from water have been reviewed by Shrimali and Singh (2001). The secondary effluent is neutralized and disinfected by a tertiary procedure before discharging into surface waters. 5.1.4

Synopsis: Oxidative Water Treatment Technologies and Methods of Hydroxyl Radical Production

The vast variety of oxidative water treatment processes and technologies are summarized in Fig. 5-8 (according to FIGAWA, 1997) and the underlying general and

Examples of oxidative water treatment technologies used in industry, research and development [adapted from FIGAWA (1997), and supplemented by novel methods]. The numbers 1 to 9 refer to the generalized reaction sequences presented in Figure 5-9. a) Oxidation at elevated temperatures between 220 8C < T <300 8C or supercritical water oxidation at DT >374 8C, Dp >221 bar (221000 kPa) (cf. Chapter 1); b) oxidation in the presence of bimetallics Fe0/Ni0 or Zn0/Ni0 (Cheng and Wu, 2001) or heterogeneous oxidation in supercritical water catalyzed by metals Me = Cu, Ag, Au/Ag-alloy; c) Fenton reaction at pH <5; d) photo-assisted Fenton reaction, irradiation in the UV-B/VIS range; e) the mixture of oxidants O3/H2O2 is called PEROXONE; f) ozonation using solid-bed catalysts with conditioned activated carbon (AC); g) vacuum-UV photolysis of water. Fig. 5.8

5.1 Description of Aqueous Systems

simplified reaction schemes are presented in Fig. 5-9. The most widely applied industrial oxidants for wet oxidation are oxygen, hydrogen peroxide and ozone. Rajenden (1991) described the aeration of wastewater for improvement of the water quality, to maintain aerobic conditions, and related problems. The oxidative water purification using hydrogen peroxide without activation has gained importance in relation to odor abatement (Schwarzer, 1988). Full-scale applications of ozone in drinking water and wastewater treatment were described comprehensively by Gottschalk et al. (2000). Hoigné (1987) studied the chemistry of ozone in water extensively. Rice (1997) reviewed novel developments and successful applications of ozone as a powerful oxidizing agent for the treatment of various industrial wastewaters, and many examples of wastewater purifications. The oxidants mentioned above are used either without initiation (activation) or in biologically, photolytically, catalytically and thermally initiated processes. Thermal processes are studied at elevated temperatures and pressures (e. g. the supercritical water oxidation, SUWOX process) or metallic catalysts are explored for the heterogeneous partial oxidation of organic substrates under supercritical water conditions (e. g. Bröll et al., 2000). Although AOPs make use of different reacting systems, they are almost all characterized by the efficient production of hydroxyl radicals (•OH). A short critical review of recent developments and the state of the art of hydrogen peroxide and/or ozone based AOPs with reference to research and industrial applications has been presented by Andreozzi et al. (1999). The prototype of a heterogeneous photocatalytic reaction is based on the irradiation of particles of the semiconductor titanium dioxide (TiO2) in the presence of

Fig. 5.9 Simplified primary reaction sequences of photochemical and/or H2O2/O3 based oxidative water treatment processes (presented in Figure 5-8).

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dissolved molecular oxygen (Fig. 5-9, reaction 1). The TiO2 particles are either suspended in water or immobilized on an appropriate support material. The underlying photophysical processes are in accord with Fig. 3-17. The generally accepted mechanism of TiO2 photocatalysis includes redox reactions of adsorbed water and oxygen molecules or of other substrates (Hoffmann et al., 1995). The formation of the electron–hole pair (h+vb – e–cb) after excitation is followed by the oxidation of adsorbed water molecules by electron transfer to h+vb with formation of hydroxyl radicals and H+ (Fig. 5-9, reaction 1). Oxygen molecules that are adsorbed at the reductive side of the TiO2 particle serve as electron acceptors to form superoxide radical anions according to Eq. 5-2. These are protonated to yield hydroperoxyl radicals (Eq. 5-3). Hydrogen peroxide and oxygen molecules are then formed by disproportionation of the intermediary hydroperoxyl radicals (Eq. 5-4). Alternatively, adsorbed substrate molecules (R–X) may be oxidized directly by electron transfer at the h+vb side of the semiconductor particle (Eq. 5-5) with formation of radical cations of the substrate. This mechanism depends strongly on the nature of R–X and on its electron donating properties. TiO2(e–cb) + O2 ? TiO2 + O•– 2

(reduction)

(5-2)

• + O•– 2 + H ? HO2

(protonation)

(5-3)

2HO•2? O2 + H2O2

(disproportionation)

(5-4)

TiO2(h+vb) + R–X ? TiO2 + R–X•+ (oxidation)

(5-5)

The photo-initiated oxidation of organic substrates in water or air by direct photolysis (Fig. 5-9, reaction 2) in the presence of molecular oxygen is not usually based on hydroxyl radical formation. These photoreactions rely on the electronic excitation of the corresponding substrates (Eq. 5-6). Subsequently, the electronically excited state of the substrate may form radicals via single bond homolysis. Carbon centered radicals are efficiently trapped by molecular oxygen to yield peroxyl radicals (R–O–O•) that can decompose finally to produce the corresponding oxidation products (Fig. 5-9, reaction 2). Other mechanisms that have been observed during direct photolysis of organic substrates in water include electron transfer to an oxygen molecule with formation of a super oxide radical anion and a substrate radical cation (Eq. 5-7). The radical cations can recombine or rearrange, or they can be hydrolyzed to yield the final reaction products. The electronically excited state of R–X may also be quenched by molecular oxygen with formation of singlet molecular oxygen (Eq. 5-8). Quenching denotes the process of deactivation of an electronically excited state of a molecule by an external influence (e. g. via contact with a quencher) either by energy transfer, electron transfer or by another chemical mechanism. Sensitized photooxidations (Eq. 5-8) are limited to substrates R–X with conjugated p-systems having triplet energies ET higher than that of singlet molecular oxygen [ET(1O2) = 94.2 kJ mol–1] and to sulfides and amines (Braun et al., 1991).

5.1 Description of Aqueous Systems

R–X + hm ? R–X*

(electronic excitation)

(5-6)

•+ R–X* + 3O2 ? O•– 2 + R–X

(electron transfer)

(5-7)

R–X* + 3O2 ? R–X + 1O2

(quenching)

(5-8)

Direct irradiation can be used to oxidize chlorinated hydrocarbons in diluted aqueous solutions, but its technical applications are limited. Legrini et al. (1993) reviewed several laboratory studies concerning the direct photolysis of CHCs. During photolysis, the organic bound chlorine atoms are transformed into inorganic chloride ions (Cl–). Typical domains of absorption of C–Cl bonds are in the wavelength range Dk of 210 nm to 230 nm. Hence, the krypton chloride excimer lamp with its emission maximum at k of 222 ± 4 nm should be destined for the specific photoexcitation of saturated chlorinated aliphatic compounds in aqueous solution (Legrini et al., 1993). However, the chloride ion mass balance of the direct photolysis of 1,1,1-trichloroethane (Cl3C–CH3) in water at k of 222 nm was unsatisfactory, even after prolonged irradiation (Oppenländer and Baum, 1995). This effect of unbalanced chloride ion formation (Loraine and Glaze, 1999) and the fate of chlorine atoms in water are still under active investigation (Alegre et al., 2000). For a detailed discussion, refer to Chapter 7.1.3. Interestingly, the photodegradation rates (at k of 184.9/253.7 nm) of several saturated chlorohydrocarbons in water were shown to be higher in the absence of dissolved molecular oxygen than in its presence (Shirayama et al., 2001). This was attributed by the authors to an “inner filter” effect exerted by oxygen molecules reducing the UV irradiance within the photoreactor by efficient absorption of VUV radiation. The photolysis of aqueous hydrogen peroxide (Fig. 5-9, reaction 3) in the UV-C range of the electromagnetic spectrum represents one of the easiest ways of producing sufficient amounts of hydroxyl radicals. However, the fate of •OH radicals in aqueous solution is complex due to their high reactivity and short lifetime, and it will be discussed later (cf. Chapter 6.7). The production of hydroxyl radicals can also be achieved by the classic Fenton reaction (Fig. 5-9, reaction 4) without photolytic initiation or by the photo-assisted Fenton reaction (Fig. 5-9, reaction 5), which involves the regeneration of ferrous ions (Fe2+) by photoreduction of ferric ions (Fe3+) (Zepp et al., 1992). These reactions are classified as catalytic with respect to Fe2+. The combined use of ozone and hydrogen peroxide (O3/H2O2) without photolytic initiation is known as PEROXONE, this being a potent oxidative mixture. It is a typical process for the indirect formation of hydroxyl radicals via a complex series of acid/base equilibria at high pH-values (Fig. 5-9, reaction 6) (Glaze et al., 1987). Deprotonation of hydrogen peroxide at a pH of 10 yields 2% of the conjugated base (hydroperoxide anion) present in the equilibrium. Electron transfer from the hydroperoxide anion to ozone then leads to the formation of an ozonide radical anion (O•– 3 ). The latter is protonated instantaneously to yield the transient species HO•3 (hydrogen trioxide), which immediately decomposes with the formation of oxygen and a hydroxyl radical (von Sonntag, 1995 a). The combination of UV irra-

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diation with an aqueous hydrogen peroxide/ozone mixture (Fig. 5-9, reaction 7) is based on the well-established photolysis of aqueous solutions of hydrogen peroxide and of ozone, respectively, and combines the complex reaction schemes of reactions 3, 6 and 8 of Fig. 5-9. The photochemical decomposition of ozone in water (Fig. 5-9, reaction 8) at wavelengths shorter than k of 330 nm is well established (Glaze et al., 1987). It yields molecular oxygen and oxygen atoms both in their singlet state according to the spin conservation rule (vanLoon and Duffy, 2000). A singlet oxygen atom immediately inserts into an O–H bond of water with formation of hydrogen peroxide being the primary product of ozone photolysis according to the overall reaction of Equation 5-9 (see also Fig. 6-12). Under constant irradiation the reaction sequences of reactions 3, 6 and 8 (Fig. 5-9) are activated and finally lead to the production of reactive hydroxyl radicals. O3 + H2O + hm (<330 nm) ? O2 + H2O2

(5-9)

The vacuum-UV photolysis of water (Fig. 5-9, reaction 9) with incoherent xenon excimer lamps (k = 172 nm) (cf. Chapter 7.1.3) or with low-pressure Hg lamps (Suprasil envelope, k = 184.9 nm) is an example of a process used for hydroxyl radical production (and of other reactive species) without the addition of any auxiliary oxidant.

5.2

Nomenclature of Photochemical AOPs

The diversity of photo-initiated AOPs as described in Chapter 5.1.4 led to an enormous confusion in naming these processes and thus producing a quagmire of terms, especially in the secondary technical literature. Almost all imaginable combinations of terms exist, such as UV/H2O2 peroxidation, UV photo-induced oxidation, UV radiation-induced oxidation, wet chemical UV oxidation, photochemical wet-oxidation, UV photooxidation, UV-activated hydrogen peroxide, TiO2-mediated photodegradation, semiconductor photocatalysis, photocatalytic degradation, and many others. It seems as if no limits exist to the inventive talents of many authors in generating new names for photochemical AOPs. The phrase “UV-light catalyzed oxidation” undoubtedly represents the worst case. This is because UV and light are both electromagnetic radiation, but light is only related to the visible part of the electromagnetic spectrum (see Fig. 3-8, Chapter 3.5). Further, photons can never act as catalysts. They are absorbed by a substrate and their energy is consumed for photophysical and/or photochemical secondary reactions (see Fig. 3-16, Jablonski diagram, Chapter 3-11). Hence, it is legitimate to define a correct set of terms that may be used for the description of photochemical AOPs. Photochemical processes of water treatment can be classified into three principal categories: photooxidation, photoreduction and direct photodegradation (Bolton and Cater, 1994). Common processes of reduction reactions induced by UV/

5.2 Nomenclature of Photochemical AOPs

VIS radiation include the addition of one or more electrons to an electronically excited species or the photochemical hydrogenation of a substance. Photo-initiated reductions are reactions in which the substrate itself is not electronically excited (Verhoeven, 1996). 5.2.1

Photooxidation Reactions

Oxidation processes that are driven by UV/VIS radiation are called photooxidation reactions. According to Verhoeven (1996), three classes of photooxidation reactions may be distinguished (Fig. 5-10): • photo-induced oxidations, such as photoionization of molecules M induced by absorption of electromagnetic radiation (Fig. 5-10, reactions 1). Ejection of an electron from the electronically excited substrate molecule M* into the surrounding medium leads to the formation of a radical cation (M•+). In general, photo-induced oxidation reactions include reactions of a substance M with oxygen under the influence of UV/VIS radiation (Fig. 5-10, reactions 2). In this case, the photooxidation is induced by electronic excitation of the substrate M. These reactions mostly imply electron transfer from the excited state (M*) to ground state molecular oxygen (cf. Eq. 5-7). Additionally, the VUV-initiated photochemistry of molecular oxygen via intermediary oxygen atoms belongs to the class of photo-induced oxidation reactions (Fig. 5-10, reactions 3);

Fig. 5.10 Nomenclature of photooxidation reactions (Verhoeven, 1996). The term “photo” denotes the wavelength ranges of VUV, UV and VIS. M and N represent substrate molecules, Sens = photosensitizer.

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• photo-initiated oxidations (Fig. 5-10, reactions 4). Neither the substrate M nor the oxygen molecule is electronically exited. M reacts with a transient and reactive species (usually a radical) that is formed from an electronically excited precursor molecule N*. Advanced oxidation processes are based on this type of photooxidation with N being an auxiliary oxidant (a-Ox) such as H2O2 and/or O3. In the special case of VUV oxidation water itself is electronically excited; • photooxygenation reactions (Fig. 5-10, reactions 5 to 7). These reactions are often very complex and many competitive reaction pathways may lead to different oxygen-containing products (Braun et al., 1991). Type I products are formed by reactions of photochemically produced radicals or radical ions with ground state molecular oxygen (3O2) via intermediary peroxyl radicals (M–O–O•). Their molecular structures are usually different from type II products obtained by sensitized photooxidations via singlet molecular oxygen (1O2). Singlet molecular oxygen has electrophilic character and its specific reactions differ from that of triplet molecular oxygen (3O2), which is a typical biradical. The reactivity of 1O2 with organic substrates includes cycloaddition to activated double bonds with formation of 1,2-dioxetanes, formation of hydroperoxides by ene-reactions involving allylic hydrogen atoms and Diels–Alder type [2 + 4]cycloaddition reactions with formation of endoperoxides. Many compounds are used as photosensitizers (Sens) to produce singlet molecular oxygen in solution according to Eq. 5-8. The most common sensitizers are organic dyes that absorb light. A photosensitizer is an electron or energy transfer agent that is usually not consumed in the photoreaction. Type III photooxidation products are produced by reaction of superoxide radical anions (O•– 2 ) with a suitable substrate molecule M. The chemical reactivity of the superoxide radical anion is characterized by its basicity, its oxidizing/reducing properties and by its nucleophilic properties (cf. Braun et al., 1991). As already mentioned above, AOPs mainly rely on photo-initiated oxidations (Fig. 5-10, reactions 4). However, depending on the particular situation of water or air contamination, photo-induced oxidation and photooxygenation reactions may compete effectively with photo-initiated oxidations generating a complex mechanistic scheme (cf. Zepp, 1988). 5.2.2

Photocatalytic Reactions

A broad definition of photocatalysis involves two distinct categories (Fig. 5-11): photogenerated catalysis and catalyzed photolysis (Serpone and Emeline, 2002, Serpone and Salinaro, 1999, Kisch and Hennig, 1983). The former implies that catalyst and substrate molecules are in their electronic ground state (Fig. 5-11, situation A). The catalytic step is exergonic, i. e. thermodynamically spontaneous. The catalyst is produced by a specified photoreaction involving a precursor molecule M. After photoproduction of the catalyst, the reaction does not require any further irradiation. The catalyst accelerates the kinetically unfavored reaction of a

5.2 Nomenclature of Photochemical AOPs

Fig. 5.11 Nomenclature used in photocatalysis (adopted from Kisch and Hennig, 1983).

substrate to form a specific product with the catalyst molecule being fully regenerated after the catalytic cycle. Therefore, this type of photocatalysis is “catalytic in the number of incident photons”, i. e. absorption of photons by precursor molecules M produces catalyst molecules with a specific quantum yield. The catalyst then starts the catalytic cycle having a distinct “turn over number” (TON). The traditional TON is a measure of the number of turnovers of a catalytic cycle (Serpone et al., 2000). In photogenerated catalysis, the quantum yield of product formation UProduct is greater than one. This, however, is also true for photo-induced radical chain reactions. In catalyzed photolysis either the catalyst molecule (Fig. 5-11, situation B) or the substrate molecule (Fig. 5-11, situation C), or both, are in an electronically excited state during the catalytic step. The electronically excited catalyst molecule is produced via photon absorption by a nominal catalyst (Fig. 5-11, situation B). The reaction of substrate to product is catalytic with respect to the concentration of the electronically excited catalyst species. It is non-catalytic in photons and therefore, continuous irradiation is required to maintain the catalytic cycle. The quantum yield of product formation UProduct is equal to or less than unity. Titanium dioxide photocatalysis is the most widely applied example of this type, with TiO2 representing the nominal catalyst that must be electronically excited by photon absorption with formation of the electron hole pair TiO2 (h+vb + e–cb), being the active catalytic species (cf. Fig. 3-17 and Fig. 5-9, reaction 1). The oxidation of substrates by the combination of UV/VIS radiation and an appropriate photocatalyst is often called photocatalytic oxidation (PCO).

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5.3

General Reaction Schemes 5.3.1

Photo-Initiated Oxidations

An idealized situation of photo-initiated oxidation reactions that is the basis of all AOPs in the presence of an auxiliary oxidant, such as H2O2 or O3, is presented in Fig. 5-12. The auxiliary oxidant (a-Ox) that contains oxygen atoms must absorb electromagnetic radiation with the formation of an electronically excited molecule (a-Ox*). Subsequently, homolytic cleavage of a chemical bond leads to the formation of primary reactive oxygen species (Ox•) which are mainly hydroxyl radicals, or oxygen atoms in their ground (3P) or excited (1D) states. Owing to the great reactivity of these transient species, their concentration is usually very low. Immediate reaction with substrate molecules M in the presence of oxygen predominantly initiates a complex series of radical reactions including peroxyl radical chemistry (von Sonntag and Schuchmann, 1991, Alfassi, 1997). In water or air, the primary reactive oxygen species can be transformed by radical, electron transfer or acid/ base reactions to secondary oxygen species (cf. Fig. 5-9) which are again able to oxidize substrate molecules. Further, the reaction of hydroxyl radicals with carbo•– nate or bicarbonate ions leads efficiently to the formation of CO•– 3 /HCO3 radical anions, which are known as selective tertiary oxidants. This scheme of reactions is substantially complicated by the fact that the auxiliary oxidants are able to oxidize certain water contaminants by direct oxidation related to their reduction potential (Simmler, 1995). This is especially the case for ozone based AOPs because O3 has

Fig. 5.12 Idealized scheme of photo-initiated oxidation reactions based on the presence of auxiliary oxidants and corresponding nomenclature.

5.3 General Reaction Schemes

a high reduction potential E of 2.07 V for the couple O3, H+/O2, H2O (Bard et al., 1985). Furthermore, the oxidation products accumulate during the course of the oxidation and they are therefore introduced into the oxidation cycle (Fig. 5-12) until finally all organic carbon atoms are transformed into carbon dioxide (mineralization). 5.3.2

Heterogeneous Photocatalysis

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron–hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions O•– 3 are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination TiO2–O3–UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of trichloroethene in the gas phase (Shen and Kub, 2002). Owing to fast competitive adsorption/desorption equilibria, the oxidation products intervene with oxidative surface reactions and thus are further oxidized, ultimately leading to complete mineralization. The Langmuir–Hinshelwood kinetic

Fig. 5.13 Idealized scheme of photocatalytically initiated oxidation reactions. M and N represent substrate molecules with different reduction potentials.

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model (cf. Hoffmann et al., 1995) can usually approximate the reaction kinetics of these heterogeneous surface reactions. Almquist and Biswas (2001) gave a summary of photocatalytic reactions that are of importance to irradiated suspensions of TiO2 in water. These authors developed a kinetic model that describes the influence of dissolved molecular oxygen on the degradation rate of organic substrates over a range of conditions in an aqueous phase. The model was validated by using the photocatalytic oxidation of aqueous solutions of phenol in the presence of TiO2 (Almquist and Biswas, 2001). In addition, the oxidation of an adsorbed substrate molecule M may occur via electron transfer to photogenerated holes h+vb (Ishibashi et al., 2000) leading to a radical cation M•+ that undergoes further reactions with water or dissolved molecular oxygen. On the other hand, suitable electron acceptors N, such as H2O2 or peroxodisulfate anions (S2O2– 8 ) accelerate the oxidative degradation of organic material considerably via enhanced •OH radical formation (Yamazaki et al., 2001) by suppressing the electron–hole recombination mechanism (cf. Eqs. 5-10 and 5-11 and Fig. 5-13). H2O2 + e–cb ? HO• + OH–

(5-10)

– •– 2– S2O2– 8 + ecb ? SO4 + SO4

(5-11)

However, the detailed mechanism of photocatalytic oxidations is still a controversial issue. 5.3.3

Homogeneous Photocatalysis

Homogeneous photocatalytic AOPs in aqueous phase are mainly based on the photo-Fenton reaction (Fig. 5-9, reaction 5) using Fe2+ salts and hydrogen peroxide in mildly acidic solutions (pH between 2.5 and 5). These processes utilize the photoreduction of produced ferric ions (Fe3+) and ferric complexes. In contrast to the Fenton reaction (without photo-activation), ferrous ion (Fe2+) is recycled continuously by irradiation of Fe3+–H2O and therefore it is not depleted during the course of the oxidation reaction. Therefore, the production of hydroxyl radicals is only limited by the availability of UV/VIS radiation and H2O2. The dominant iron(III)-hydroxo complex, Fe(OH)2+, absorbs in the UV/VIS range below k of 400 nm (Hislop and Bolton, 1999). It photolyzes with formation of Fe2+ exhibiting a relatively low quantum yield UFe(2+) of 0.14 ± 0.04 at 313 nm (Faust and Hoigné, 1990). However, the quantum yield of Fe2+ photoproduction can be drastically enhanced by photolysis of the highly photosensitive ferrioxalate complex [Fe(C2O4)3]3–, e. g. UFe(2+) = 1.24 at k of 300 nm (Murov et al., 1993). Potassium ferrioxalate (K3[Fe(C2O4)3] × 3 H2O) is well known as one of the most practical chemical actinometers (Hatchard–Parker actinometer, for a detailed description refer to Kuhn et al., 1989). Its absorption extends into the visible domain to k below 500 nm depending on its concentration. The quantum yield for the production of

5.3 General Reaction Schemes

ferrous ions from potassium ferrioxalate at k of 509 nm is still sufficiently high, being UFe(2+) = 0.86 (Braun et al., 1991). The photo-induced reduction of ferric ions to ferrous ions in the presence of oxalate follows the overall reaction of Eq. 5-12. 2+ 2Fe3+ + C2O2– + 2CO2 4 + hm ? 2Fe

(5-12)

Hislop and Bolton (1999) elucidated the complex mechanism of this reduction process. Presumably, it includes a ligand to metal electron transfer in the iron–oxalate complex with formation of an oxalyl radical anion as shown in Eq. 5-13. •– 2+ FeIII(C2O4)3– + 2C2O2– 3 + hm ? Fe 4 + C2O4

(5-13)

•– C2O•– 4 ? CO2 + CO2

(5-14)

•– 2+ + CO2 + 3C2O2– FeIII(C2O4)3– 3 + CO2 ? Fe 4

(5-15)

The oxalyl radical anion decomposes to a carbon dioxide radical anion and to carbon dioxide according to Eq. 5-14. Again, the CO•– 2 radical anion is able to reduce ferrioxalate with formation of ferrous ions (Eq. 5-15). The correct reaction stoichiometry of the photolysis of ferrioxalate in aqueous solution is given by Eq. 5-16 (Hislop and Bolton, 1999). 2+ + CO2 + 2.5 C2O2– FeIII(C2O4)3– 3 + hm ? Fe 4

(5-16)

The production of hydroxyl radicals by the Fenton reaction in the presence of ferrioxalate occurs according to the general Eq. 5-17 (Hislop and Bolton, 1999). 3– III – • Fe2+ + H2O2 + 3C2O2– 4 ? Fe (C2O4)3 + HO + OH

(5-17)

A reaction scheme of the overall photocatalytic cycle of the ferrioxalate-based photo-Fenton reaction is presented in Fig. 5-14. The principal mechanism outlined in Fig. 5-14 is generally accepted throughout the scientific community. The hydroxyl radicals initiate the oxidative destruction of organic substrates in water and in the presence of dissolved molecular oxygen the total mineralization may be achieved. Experimental evidence exists for the involvement of free hydroxyl radicals as reactive intermediates. This fact, however, was questioned in the case of thermal Fenton reactions. During the thermal reaction of Fe2+ aq and H2O2, a cationic iron intermediate was postulated that possesses an unusual charge. This species is most probably the ferryl ion, Fe4+ aq (Bossmann et al., 1998). It may also intervene in the photo-Fenton reaction. This was concluded from intensive product studies using 2,4-dimethylaniline (2,4-xylidine) as a model substrate. These studies revealed that the intermediate products generated during the Fenton reactions were different from those generated during the H2O2-UV process. Further, the simplified scheme presented in Fig. 5-14 does not take into account the coordination chemistry of iron ions, e. g. the formation of hydroxo and aquo complexes.

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Fig. 5.14 Schematic representation of the catalytic cycle in the photo-Fenton reaction using ferrioxalate (following Hislop and Bolton, 1999).

Their solubility in water is strongly dependent on the pH of the solution. For a detailed discussion of the aqueous solution chemistry of Fe(II) and Fe(III) the reader should refer to Sigg and Stumm (1996). Kim and Vogelpohl (1998) described the experimental realization of the ferrioxalate photo-Fenton process in detail. 5.3.4

Photolysis of Water

The homolysis of water molecules (Fig. 5-9, reaction 9) by VUV irradiation leads to the formation of hydrogen atoms and hydroxyl radicals as major primary species (Heit et al., 1998). In addition, small amounts of hydrated electrons (e–aq) are generated which are immediately trapped by dissolved molecular oxygen with the formation of superoxide radical anions (O•– 2 ) according to Eq. 5-18 (cf. Getoff, 1993). The pKA value of 4.8 (Bielski et al., 1985) of the conjugated acid HO•2 (Eq. 5-19) implies that the superoxide radical anion is a weak base in water (Braun et al., 1991). However, HO•2 radicals can disproportionate according to Eq. 5-4 with formation of molecular oxygen and of hydrogen peroxide. Thus, the acid/base equilibrium of Eq. 5-19 is shifted to the right and therefore the effective basicity of O•– 2 is significantly increased. Similarly, hydrogen atoms that are formed by VUV photolysis of water are immediately transformed into hydroperoxyl radicals by fast reaction with dissolved molecular oxygen (Eq. 5-20). Therefore, it seems justified to designate water in the case of VUV-initiated oxidation reactions as an auxiliary oxidant (see Fig. 5-12) because the primary reductive species (e–aq and H•) are efficiently introduced into the oxidative cycle (Eqs. 5-18 and 5-20). The details

5.4 Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies

of the VUV oxidation process (H2O-VUV) and generalized reaction schemes (see Fig.s 7-3, 7-4 and 7-16) are discussed in Chapter 7.1.3. e–aq + (O2)aq ? (O•– 2 )aq + • (O•– 2 )aq + H HO2

H• + (O2)aq ? HO•2

(5-18) (pKA = 4.8)

(5-19) (5-20)

The simplified schemes of photo-initiated AOPs presented in Fig.s 5-12, 5-13 and 5-14 already demonstrate one of the main problems related to this technology: the system must be configured so that the auxiliary oxidant or the nominal catalyst (homogeneous or heterogeneous) absorbs most of the incident photons of sufficient energy and that competing absorption of radiation by other water components is minimized. This requirement, however, can often be achieved only by high concentrations of the auxiliary oxidant, which worsens the economic situation significantly, or by special photoreactor design in the case of photocatalysis, e. g. immobilization of the catalyst on surfaces to guarantee absorption of UV/VIS radiation within thin layers. In many cases, high concentrations of hydrogen peroxide are necessary because of its low absorption coefficient a in the UV-C range. This is in contrast to O3 applications (Gottschalk et al., 2000), due to the high absorption coefficient of O3 in the UV-C region of the electromagnetic spectrum. However, in contrast to H2O2-UV, the O3-UV AOP is governed by the solubility of O3 in water, its temperature dependence and mass transfer processes (Gottschalk et al., 2000).

5.4

Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies

In general, the state of knowledge of photo-initiated AOPs is still incomplete and many questions concerning the complex and interrelated photochemical, photophysical, physical and radical mechanisms have to be answered. In addition, the problems that are related to up-scaling of photoreactors are of substantial concern and are certainly not trivial (Braun et al., 1993). Further, the implementation of VUV/UV-initiated oxidation processes requires an efficient lamp technology providing a maximum of high-energy photons at low cost. Comparative aspects concerning the effectiveness of a given treatment process and economic considerations are of utmost importance to the future progress in practical operation of photo-initiated AOPs (Rajeshwar, 1996). For the sake of simplicity, the paraphrases for the description of the different photooxidation reactions should include first the auxiliary oxidant or oxidant combinations and secondly the shorthand expression of the electromagnetic wavelength range. For example, “H2O2-O3-UV” designates all photochemical AOPs

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Fig. 5.15 State of realization of photo-initiated AOPs for water and air treatment. LP: lowpressure, MP: medium-pressure.

that are operated in the presence of both, hydrogen peroxide and ozone, with varying relative concentrations, under irradiation with the appropriate UV wavelength range. Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukherjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. The solar photocatalytic decontamination of trichloroethene (TCE) containing groundwater was tested successfully with large-scale outdoor equipment. Several other field tests and pilot studies of solar photocatalysis using different photoreactor concepts were reported by Zhang et al. (1994) and by Vidal (1998). Oliveros et al. (1997) reported the efficient large scale (V = 500 L) treatment of an industrial wastewater (TOC = 2700 mg L–1) contaminated with a toxic aromatic amine [3,4-xylidine, C8H11N: (CH3)2C6H3NH2] by a photo-enhanced Fenton reaction. Research and innovations in photoreactor design and in catalyst preparation and optimization are important future goals for the further development of photocatalysis.

5.4 Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies

The technology of UV disinfection of water and air is based on the direct irradiation of the media, mostly in homogeneous aqueous or gaseous phases. It is well established in industry, and monochromatic LP or polychromatic MP mercury lamps as sources of UV-C radiation are applied (cf. Schneider et al., 2001, Kalisvaart, 2001). UV oxidation of organic water pollutants in the presence of hydrogen peroxide, ozone or both with powerful medium-pressure mercury lamps is performed on industrial scales (Gottschalk et al., 2000). The synergistic combination of ozone and UV is especially suited for water sanitation, i. e. treatment of swimming pool and spa water (Rice, 1997). Many full-scale remediation applications of photo-initiated AOPs using hydrogen peroxide or ozone are already in operation (Chemviron Carbon, 1997, Freeman and Harris, 1995). Although the primary products of water photolysis at k of 184.9 nm using LP mercury lamps with Suprasil quartz envelopes have been well known for a long time (e. g. Getoff and Schenck, 1968) the investigation of VUV oxidation of organic matter (Oppenländer and Gliese, 2000, Heit et al., 1998) has gained new interest in recent years (cf. Chapter 7.1.3). This is related to the development of incoherent xenon excimer lamps with their emission maximum at k of 172 nm (cf. Chapter 4.3). The production of water with low organic carbon content for ultra pure water supplies is an important application of Suprasil LP mercury lamps and possibly for the future of Xe2* excimer lamps. Larson et al. (1991) reviewed other strategies for photochemical water treatment with marginal industrial importance: for example, the use of organic photosensitizers or electron transfer agents, and photoreductive dehalogenation reactions applying sodium borohydride as a potent photoreductive agent. Acher and Rosenthal (1977) reported the efficient solar treatment of an aerated sewage effluent using methylene blue as a photosensitizer on a laboratory scale. This process led to a significant reduction of fecal coliforms, of COD and of other species. Methylene blue is a well-known photosensitizer for singlet molecular oxygen production. The organic dye had to be removed from the sample after treatment by flocculation and precipitation with bentonite clay. Other AOPs under active investigation include anodic oxidation, electro-Fenton and photoelectron-Fenton reactions (e. g. Brillas et al., 1998). The developments in the field of photo-initiated AOPs over the last decade undoubtedly indicate their large-scale feasibility with some restrictions concerning the costs of these processes as stand-alone systems, the long-term reliability of the system components (lamps etc.) and the possible formation of undesirable byproducts in some cases. However, photo-initiated AOPs have many similarities with nature’s own self-repair mechanisms of natural pollution. Many of them can be adapted, in particular solar AOPs, for use in developing countries. Owing to the different and distinct absorption properties of the individual auxiliary oxidants or photocatalysts, the photo-initiated AOPs presented in Fig. 5-15 must be utilized at specific spectral bands covering the VUV, UV-C, UV-B, UV-A and parts of the visible range of the electromagnetic spectrum. This is outlined in Fig. 5-16. The photo-Fenton process using Fe(III) oxalate is probably the most favorable for solar photochemistry, since the quantum yield U is high (cf. Tab. 6-4), and ferrioxalate absorbs up to k of 500 nm.

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Fig. 5.16 Classification of photo-initiated AOPs according to active wavelength ranges designates modified titanium dioxide, for and spectral domains of excitation. TiOmod 2 example TiO2 doped by transition metals.

Fig. 5.17 The variety of chemicals with known photoreactivity in water (cf. Hal-

mann, 1995, Legrini et al., 1993).

5.5

Photodegradation of Water Pollutants

The enormous number of chemical substrates that have already been subjected to photodegradation experiments in water with or without auxiliary oxidants or photocatalysts (Fig. 5-17) includes inorganic ions and molecules, e. g. free and

5.5 Photodegradation of Water Pollutants

complexed cyanides, nitrite (NO–2) and nitrate (NO–3) anions (Mack and Bolton, 1999), heavy metal ions such as CrVI, Hg0,II, a wide spectrum of aliphatic and aromatic compounds and derivatives, saturated and unsaturated halogenated hydrocarbons (halocarbons), organic nitrogen, phosphorous and sulfur containing substrates and natural water constituents such as humic acid or other naturally occurring organic substances (von Sonntag, 1995 b). In his book, Halmann (1995) collected and interpreted most of the modern literature concerning the photodegradation of xenobiotics and of many other obnoxious and toxic chemicals. This publication is enriched enormously by the review of Legrini et al. (1993) who concentrated on the special photoreaction conditions applied in many laboratory scale investigations concerning AOPs related to organic substrates. The authors presented extensive lists of organic compounds that have been treated according to H2O2UV, O3-UV and TiO2-UV procedures. Richard and Grabner (1999) reviewed mechanistic work concerning the photodegradation of phenol and of its derivatives in aqueous solution. Phenolic compounds are systematically introduced into the environment via several domestic and industrial activities. Many fungicides, insecticides and herbicides are based on this structural element. Hence, this class of organic compounds represents a severe toxicological risk because most of them are water soluble and highly toxic. They were detected in ground water reservoirs. Photochemical degradation of phenol derivatives represents an alternative method for biological detoxification. Tab. 5-2 presents several examples of recent research concerning photo-initiated AOPs that are related to the photooxidation or photomineralization of specific compounds or to the diminution of global water parameters. It is a long way from being a comprehensive list (this would inevitably go beyond the scope of this book), and it was randomly selected from the current literature. Nevertheless, it demonstrates the enormous amount of research activity in the field of photoinitiated AOPs in aqueous systems that elucidate reaction mechanisms and that establish the broad application potential of these processes. Examples of O3-UV AOPs are not included. They were reviewed by Gottschalk et al. (2000).

131

–[Substrate], –TOC, +[Ox] –[Substrate], +[Ox] –Absorbance, –COD –[Substrate], –TOC –Absorbance –[Substrate], –TOC –Absorbance, –TOC –[Substrate] –[Substrate] –[Substrate] –[Substrate], +[Ox] –[Substrate], –TOC +[Ox] +[Cl–]

Fe3+-H2O2-VIS Fe3+-UV TiO2-VIS H2O2-UV H2O2-UV TiO2-UV/VIS TiO2-UV/VIS Fe0-UV UV/VIS Fe2+-H2O2-UV Fe3+-UV Fe2+-H2O2-UV/VIS H2O2-UV TiO2-UV/VIS

Acridine orange × HCl Alizarin violet B Alcohol ethoxylates b) Alizarin red Atrazine Azo dyes Azo dyes Azo dyes (As and P derivatives) C.I. Reactive red 2 Carbendazim (fungicide) Chlorophenol (p-) Chlorophenol (m-) Chlorophenol (p-) Chlorophenols Chlorophenols

–[Substrate], –TOC, +[Ox] –TOC, +[Cl–] –Absorbance, –DOC –[Substrate], +[Ox], toxicity +[Ox] –[Substrate], +[CO2], +[Ox] –[Substrate], -TOC –[Substrate] –[Substrate], –TOC, +[Ox]

H2O2-UV Fe[(C2O4)3]3–-H2O2-UV TiO2-VIS Fe2+-H2O2-UV TiO2-UV, VIS, Solar TiO2-UV Fe2+-H2O2-UV TiO2-UV H2O2-UV

1,4-Dioxane 2,4-Dichlorophenol 2,4-Dihydroxybenzoic acid 2-4-Dichlorophenol 2-Methylthiophene 3,4-Dichlorobut-1-ene 3,4-Xylidine a) in Wastewater 4-Chlorobenzoic acid Acetone

Change of analytical parameter

AOP

Substrate/source of water

Brand et al. (2000) Liu et al. (1999) Arantegui et al. (1995) Galindo and Kalt (1999) Tanaka et al. (2000) Kalil et al. (1999) Deng et al (2000) Panades et al. (2000) Yoon et al. (2000) Mazellier and Bolte (2001) Krutzler and Bauer (1999) Hirvonen et al. (2000) Ahmed et al. (2001)

Stefan and Bolton (1998) Nogueira and Guimaraes (2000) Benoit-Marquie et al. (1997) Ormad et al. (2001) Sokmen et al. (2001) Hamill et al. (2001) Oliveros et al. (1997) Dionysiou et al. (2000) Stefan et al. (1996), Stefan and Bolton (1999) Xie et al. (2000)

Reference

ment and characterized. Research examples concerning VUV oxidation in aqueous phase and UV oxidation in the gas phase are presented in Chapter 7 (Table 7-2 and Table 7-4, respectively). For O3-UV based AOPs refer to Gottschalk et al. (2000)

Tab. 5.2 Current examples of research related to photo-initiated AOPs in aqueous phase. Ox: intermediary oxidation products formed, detected during treat-

132

5 Photochemical Processes of Water Treatment

–[Substrate] –[Substrate], +[CO2] –Absorbance, –TOC –Absorbance (600 nm), –DOC –Absorbance, –TOC; +[Cl–] –[Substrate], –DOC, –DON, +[Ox] –Enzyme activity –CFU –[Substrate], –TOC, +[Ox] –[Substrate], +[F–] –[Substrate] –[Substrate] –Absorbance, +[Ox], toxicity –AOX, –COD, –TOC, –[Substrate], –DOC –TOC, –COD –TOC, –COD –[Substrate], –DOC, +[CO2] –[Substrate], +[Ox] –[Substrate], +[Cl–], +[CO2] –[Substrate] –[Substrate]

TiO2-UV TiO2-UV (photoelectrochemical) TiO2-UV/VIS Fe2+-H2O2-UV TiO2-Solar H2O2-UV TiO2-UV TiO2-UV/VIS Fe3+-H2O2-UV TiO2-H2O2-UV TiO2-UV TiO2-UV UV H2O2-UV H2O2-UV TiO2-UV Fe2+, Fe[(C2O4)3]3–-H2O2-UV TiO2-UV TiO2-UV PW12O3– 40-UV NO–3-UV/VIS TiO2-UV

Formaldehyde Herbicide g) Herbicides h) Hospital wastewater i) Humic acids Industrial wastewater Landfill leachate j) Lead-EDTA Lindane Lindane Mercury (Hg0) and methylmercury (CH3Hg+Cl–) Metal ionsk)

–Absorbance, +[Ox] –[Substrate], +[Ox]

UV TiO2-UV/VIS

Chlorophenols, chloroanilines Cryomazine (insect growth regulator, s-triazine) Cylindrospermopsin (cyanotoxin) c) Dodecylbenzenesulfonate Dyehouse effluent d) Dyehouse effluent e) Dyes (commercial samples) EDTA Enzyme f) Escherichia coli Ethylene glycol Fluorophenols

Change of analytical parameter

AOP

Substrate/source of water

Tab. 5.2 (continued)

Chen and Ray (2001)

Senogles et al. (2001) Hidaka et al. (2001) Arslan et al. (2000) Arslan et al. (1999) Wang (2000) Sörensen et al. (1996, 1998) Hancock-Chen and Scaiano (2000) Dillert et al. (1998) McGinnis et al. (2000) Dubnack and Kreisel (1996), Kreisel and Schmidt (1999) Shin et al. (1996) Topalov et al. (2001) Tixier et al. (2000) Sprehe et al. (1999) Wang et al. (2001) Dillert et al. (1999) Kim and Vogelpohl (1998) Vohra and Davis (2000) Guillard et al. (1996) Hiskia et al. (1997) Gardfeldt et al. (2001)

Boule et al. (1999) Goutailler et al. (2001)

Reference

5.5 Photodegradation of Water Pollutants 133

–[Substrate] –[Substrate], +[Ox] –[Substrate], –DOC, +[Ox] –Absorbance, photobleaching –Absorbance, –DOC, +[Ox] –[Substrate] –[Substrate], –DOC, –[Substrate], +[Ox] –[Substrate] –[Substrate]

Fe3+-H2O2-UV/VIS H2O2-UV TiO2-UV TiO2-UV TiO2-UV H2O2-UV TiO2-O3-UV Fe3+-H2O2-UV Fe2+-H2O2-UV H2O2-UV, UV Fe3+-H2O2-UV/VIS H2O2-UV TiO2-UV/VIS TiO2-UV UV/VIS Fe2+-H2O2-UV/VIS, solar UV, TiO2-UV TiO2-UV/VIS TiO2-UV TiO2-VIS, solar TiO2-solar H2O2-UV TiO2-VIS H2O2-UV TiO2-UV

Methyl orange Methyltert-butyl ether (MTBE)

Methylene blue Methylene blue Methylene blue Metol l) Monochlorocaetic acid, pyridine Nitrobenzene Nitrotoluenes m) N-Nitrosodimethylamine n)

Orange II o)

PAHs p)

Pentachlorophenol Pesticides, chlorinated q) Pesticides, organophosphorus Pesticides r) Phenol + NO–3 Phenol and derivatives \ Poly(vinyl pyrrolidone) Proteins s) Reactive red II (textile dye) Remazol Black-B t) Rhodamine B Sulfonates u) Trichloroethene

–[Substrate] –[Substrate] –[Substrate] –TOC, –COD +[Nitrophenols] –[Substrate], –TOC, +[Ox] +[CO2], +[NO–3], +[NH+4 ], +[Ox] Amino acid analysis –Absorbance –Absorbance, -TOC, toxicity –Absorbance, –COD –[Substrate] –[Substrate], +[Cl-]

–[Substrate], –absorbance, –TOC, +[CO2] –[Substrate]

Change of analytical parameter

AOP

Substrate/source of water

Tab. 5.2 (continued)

Ledakowicz et al. (1999), Miller and Olejnik (2001) Pecchi et al. (2001) Zaleska et al. (1999) Kamiya et al. (2001) Fallmann et al. (1999) Vione et al. (2001) Peiro et al. (2001) Horikoshi et al. (2001) Muszkat et al. (2001) Neppolian et al. (1999) Ince et al. (1997) Wu et al. (1998) Sörensen and Frimmel (1996) Yamazaki et al. (2001)

Chen et al. (2001) Stefan et al. (2000), Chang and Young (2000) Houas et al. (2001) Mills and Wang (1999) Zhang et al. (2001) Andreozzi et al. (2000) Kopf et al. (2000) Rodriguez et al. (2000) Schmidt and Butte (1999) Bolton et al. (2001), Sharpless and Linden (2001) Bandara et al. (1996)

Reference

134

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3,4-Dimethyl aniline. general molecular formula: H2m+1Cm-(OCH2CH2)n-OH. the cyanobacterium cylindrospermopsis raciborskii produces this cyanotoxin in drinking water reservoirs. exhausted dyebaths were simulated including dyestuffs and assisting chemicals. mixture of dyestuffs: Remazol Black SB, Remazol Red 3B, Remazol Brilliant Yellow GL, Drimarene Orange K-GL, Drimarene Rubine R/K-5BL, Drimarene Blue K-2RL. f) horseradish peroxidase was used. g) MCPA: (4-chloro-2-methylphenoxy)acetic acid, C9H9ClO3. h) 1-methyl-2-propinyl-(3-chlorophenyl) carbamate (chlorbufam), 3-(3-chloro-4-methylphenyl)-1,1-dimethylurea (chlorotoluron), 3-(4-isopropylphenyl)-1,1-dimethylurea (isoproturon). i) wastewater containing X-ray contrast media with organic bound iodine. j) biologically pretreated water containing biorefractory organic pollutants. k) toxic metal ions Ag+, Hg2+, Cr6+, Pb2+, and Cu2+ were used. l) N-methyl-p-aminophenol (Metol) is an important developing agent. m) biotic and abiotic transformation products of 2,4,6-trinitrotoluene (TNT): 2-amino-4,6-dinitrotoluene, 4-amino-2,6-dinitrotoluene, 2,4-diamino-6-nitrotoluene, 2,6diamino-4-nitrotoluene. n) NDMA: (CH3)2N-NO. o) non-biodegradable azo dye, 4-[(2-hydroxy-1-naphthalenyl)]azo-benzenesulfonic acid monosodium salt, C6H11N2NaO4S. In addition, NO–3, NO–2, NH+4 and SO2– 4 evolution were quantitatively analyzed. p) polycyclic aromatic hydrocarbons: benzo[a]pyrene, chrysene and fluorene. q) p,p'-DDT, lindane and methoxychlor. r) commercial pesticides and their active ingredients used for this study: acrinatrin, abamectin, endosulfan, formetanate, imidacloprid, lufenuron, methamidophos, oxamyl, pyrimethanil, propamocarb. s) proteins used: bovine plasma gamma globulin, bovine serum albumin and chicken ovalbumin. t) textile azo dye, molecular formula C26H21N5Na4O19S6. u) aromatic sulfonates used as model compounds: 4,4'-diaminostilbene-2,2'-disulfonate, diphenyl-4-sulfonate, 2-amino-1-naphthalenesulfonate.

a) b) c) d) e)

5.5 Photodegradation of Water Pollutants 135

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6

Properties, Reactivity and Photochemistry of Auxiliary Chemicals 6.1

Electronic Structures of Oxygen Species Involved

To understand the chemical reactivity of oxygen and of reactive oxygen species (Ox, HOx, x = 1, 2, 3, 4) it is important to memorize their electronic structures. The methodology used to describe covalent bonds in molecular systems is based on so-called Lewis structures. Lewis structures provide simple and convenient representations of ions and molecules that include accurate accounting of all valence electrons. In such structures, each element of the second period of the periodic system should have (if possible) a complete octet of electrons. Hydrogen has a duet shell and elements of the third period (S, P etc.) are able to expand their valence shell due to participation of empty d orbitals. Lines around the corresponding element symbolize bonding and non-bonding electron pairs (spin paired). This methodology is a standard concept in inorganic and organic chemistry. It is extremely valuable for the mechanistic interpretation of chemical reactions in AOPs. The Lewis structures of several auxiliary oxidants and of some reactive oxygen species are presented in Fig. 6-1. The unusual Lewis structure of triplet molecular oxygen is directly derived from interpretation of the MO diagram of molecular oxygen (cf. Fig. 3-13) leading to one r and two p bonds between the oxygen atoms. Although in this case the octet rule is violated, (the oxygen atoms possess nine electrons each in their outer shell) this structure best represents the paramagnetic and chemical characteristics of the oxygen molecule and explains its biradical character. The two anti-bonding electrons with parallel spin orientation “neutralize” or compensate for one of the three bonds between the two oxygen atoms, thus leading to a bond order of two. A similar MO situation exists for singlet molecular oxygen having an electron pair in an anti-bonding MO (cf. Fig. 3-13). Physical measurements have shown that the ozone molecule is best represented by a resonance hybrid consisting of the electronic structures shown in Fig. 6-1. Ozone is an electrophilic reagent that reacts with olefinic double bonds to form unstable ozonides. The arrangement of the valence electrons of the other neutral, anionic or radical oxygen species that are of importance to AOPs is straightforward (Fig. 6-1). The special electronic configuration of triplet molecular oxygen (see Fig. 3-13) is responsible for the radical character and for the relative “inertness” of this extraor-

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Fig. 6.1 Lewis structures of auxiliary oxidants and of reactive oxygen species.

Fig. 6.2 Activation of triplet molecular oxygen (3O2) by electronic excitation or by consecutive oneelectron reductions and the corresponding electron configurations of the resulting reactive oxygen species.

6.1 Electronic Structures of Oxygen Species Involved

Fig. 6.3

Oxygen derived reactive species in AOPs.

dinary molecule. Cacace et al. (2001) recently demonstrated that the chemical and physical behavior of molecular oxygen is still of active interest with relevance to atmospheric chemistry. Through experiment they detected metastable tetraoxygen (O4) and other species, such as hydrogen trioxide (HO•3) and the [H2O+. . .O•– 2 ] charge transfer complex. The paramagnetic triplet biradical :•O•2: can be activated by several mechanisms (Fig. 6-2): these include electronic excitation, consecutive one-electron reductions, or complexation with transition metals (Müller, 1988). Two singlet excited states of molecular oxygen are known. The higher excited state (1R+g) rapidly deactivates in solution to form the spin-paired species 1Dg, which is responsible for the chemical reactivity of singlet molecular oxygen (cf. Braun et al., 1991). The one-electron reductions by consecutive addition of two electrons into the singly occupied antibonding p* MOs of triplet molecular oxygen (see also Fig. 3-13) yield the superoxide radical anion and the diamagnetic peroxide anion, respectively. Oxygen derived primary and secondary reactive species that are important in promoting photo-initiated (and other) AOPs are summarized schematically in Fig. 6-3. The role of oxygen derived short-lived radical species in AOPs is still a field of active research. However, extensive knowledge has been accumulated on the mechanism of ozone aqueous chemistry (Rice, 2001, Gottschalk, 2000, Hoigné, 1998). The decomposition of molecular ozone in water by a radical-type chain reaction involves species such as hydrogen trioxide (HO•3) with a lifetime of about 10 ls and HO•4 radicals. The latter equilibrate with their dissociation products in about 30 ls, i. e. O3 and •OH (Hoigné, 1998) or they decompose into O2 and HO•2 promoting the chain reaction.

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Tab. 6.1 Selection of reduction potentials that are of interest in AOPs (data taken from Ward-

man, 1989) Couple

E/V a)

Remarks b)

aq/e–aq Br•/Br– – Br•– 2 /2Br •– Br2/Br2 CN•/CN– CO2/CO•– 2 2– CO•– 3 /CO3 + , H /HCO–2 CO•– 2 • – Cl /Cl ClO•2/ClO–2 ClO•2, H+/HClO2 – Cl•– 2 /2Cl Cl2/Cl•– 2 F•/F– I•/I– – I•– 2 /2I •– I2/I2 N•3/N–3 • NO2/NO–2 • NO3/NO–3 • OH, H+/H2O

–2.870 2.000 1.660 0.300 1.900 –1.900 1.500 c) 1.070 2.200–2.600 0.934 1.277 2.300 0.420–0.600 3.600 1.270–1.420 1.000–1.130 0.060–0.300 1.330 0.870–1.040 2.300–2.600 2.730 2.590–2.850 1.800–2.180 1.900 1.780–1.870 –0.330 –0.037 0.650 1.000 0.940 1.77 d) 1.420 0.790 0.800 2.07 d) 1.190–1.600 1.800 1.150 1.620 0.630 0.840 –2.470 2.430

– – rec. value – – rec. value – pH = 7 – rec. value, pH = 4–6 pH = 0 – – – – – – rec. value – – rec. value pH = 0 pH = 7 rec. value – rec. value pH = 0 pH = 7 – pH = 7 – pH = 0 – pH = 7 – pH 5 11 pH = 7 – pH = 1.3 pH 5 8 pH = 3.6 – –

•

OH/OH– O•–, H2O/2OH– O2/O•– 2 O2, H+/HO•2 O2(1Dg)/O•– 2 + – O•– 2 , H /HO2 + •– O2 , 2H /H2O2 H2O2, H+/2H2O HO•2, H+/H2O2 HO•2/HO–2 H2O2, H+/H2O, •OH O3, H+/O2, H2O O3/O•– 3 O3, H+/HO•3 HS•/HS– SCN•/SCN– • SO–3/SO2– 3 • SO–3/HSO–3 • – – SO2– 4 , H2O/ SO3, 2OH •– 2– SO4 /SO4

a) Reduction potentials referring to one-electron reductions versus standard hydrogen electrode. The dispersion of several E values is due to different methods of determination or calculation. b) rec. = recommended. In the case of proton involvement E values may be used for estimation of the standard potentials E0 as defined by the Nernst relationship. A general approach to describe the pH dependence of reduction potentials is discussed by Wardman (1989). c) Lilie et al., (1978); d) values taken from Cotton and Wilkinson (1974).

6.3 UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates

6.2

Reduction Potentials

In this connection, it is essential to discuss the reduction potentials E of oxygen derived reactive species and of several other intermediates that are important in AOPs (Tab. 6-1). For a viable definition of the term “reduction potential”, which should be used instead of the obsolete “oxidation potential” or of the ambiguous “redox potential” the reader is referred to Wardman’s (1989) comprehensive review and data collection. The reduction potentials E of couples M/M•– refer to reactions described by Eq. 6-1 or Eq. 6-2. oxidant + e– ? reductant

(Example: Cl•/Cl–)

oxidant + nH+ + e– ? reductant (Example: •OH, H+/H2O)

(6-1) (6-2)

These equations refer to one-electron reductions versus the standard hydrogen electrode. Substrates M with more positive reduction potentials for the couple M/ M•– are stronger oxidants than substrates with lower or negative E. Therefore, in this case, M is easier to reduce. For example, the couple Cl•/Cl– has a reduction potential E of 2.200 to 2.600 V, and therefore chloride ions can theoretically be oxidized in water to chlorine atoms by hydroxyl radicals with E(•OH, H+/ H2O) = 2.730 V, according to Eq. 6-3: Cl– + •OH + H+ ? Cl• + H2O

(6-3)

Similarly, hydroxyl radicals oxidize carbonate anions in water according to Eq. 6-4 generating carbonate radical anions. • + •– CO2– 3 + OH + H ? CO3 + H2O

(6-4)

Thus, carbonate and bicarbonate ions are efficient scavengers of hydroxyl radicals in water due to their relatively low reduction potentials.

6.3

UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates

For the evaluation of AOPs in an aqueous phase, it is essential to know the absorption properties of the auxiliary oxidants and of the most important intermediate reactive species. These data are collected in Tab. 6-2. Carbon-centered peroxyl radicals play an important role in AOPs, since free carbon radicals RCH•2, which are formed for example via hydrogen abstraction by hydroxyl radicals react rapidly with dissolved molecular oxygen with formation of the corresponding peroxyl radicals RCH2O•2. Two examples of transient absorption characteristics are included in

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Tab. 6.2 UV/VIS absorption properties of auxiliary oxidants, photocatalysts and of primary and

secondary transient oxygen species a) in aqueous solution Compound

Molecular formula

Butyl-peroxyl radical C4H9O•2 Carbon dioxide radical CO•– 2 anion Ferric hydroxy complexes FeIII(OH)2+ Hydrogen atom H• Hydrogen peroxide H2O2

Hydroperoxide ion Hydroperoxyl radical

HO–2 HO•2

Hydroxyl radical Nitrate ion

HO• NO–3

Nitrite ion

NO–2

Oxide radical ion Ozone

O•– O3

Absorption characteristics k and kmax/nm, ek/L mol–1 cm–1

References

kmax = 258: ek = 1480 kmax = 235: ek = 3000

Getoff (1991) Neta et al. (1969)

k <400 kmax <200, k = 188: ek = 1620 k = 295–299: ek = 0.88 k = 254: ek = 19.6 k = 254: ek = 18.7 k = 250–254: ek =19.02 k = 200–204: ek = 179.21 k = 253.7: ek = 240 k = 220: r = 3.4 × 10–18 b), c) kmax = 225: ek = 1050 c) kmax ~ 225; k = 188: ek = 540 kmax = 310: ek = 7.4 k = 254: ek = 4 kmax ~ 200: ek = 9900 kmax = 360: ek = 22.5 kmax = 205: ek = 5500 kmax = 240: ek = 240 kmax = 260: ek = 3314 kmax = 258: ek = 3150 k = 254: ek = 3300 k = 290: ek = 905

kmax = 430: ek = 2000 kmax = 350: ek = 300 k < 550 nm

Hislop and Bolton (1999) Buxton et al. (1988) Bolton (1999) Glaze et al. (1987) Weeks and Matheson (1956) Bolton (1999) Bolton (1999) Legrini et al. (1993) Crowley et al. (1991) Lightfoot et al. (1992) Buxton et al. (1988) Mack and Bolton (1999) Mark et al. (1996) Mack and Bolton (1999) Mack and Bolton (1999) Mack and Bolton (1999) Buxton et al. (1988) Forni et al. (1982) Hoigné (1998) Glaze et al. (1987) Finlayson-Pitts and Pitts (1986) Finlayson-Pitts and Pitts (1986) Bühler et al. (1984) Bühler et al. (1984) Hislop and Bolton (1999)

kmax = 255, ek = 1250 kmax = 720: ek = 18500 kmax = 245: ek = 2350 k < 500 nm k < 400 nm

Getoff (1991) Hart and Anbar (1970) Bolton and Cater (1994) Anpo (2000) Salinaro et al. (1999)

k = 172: ek = 10 d)

Weeks et al. (1963)

k = 313: ek = 60 O•– 3 HO•3 Potassium ferrioxalate K3[FeIII (C2O4)3] Propyl-peroxyl radical C3H7O•2 Solvated electron e–aq Superoxide radical anion O•– 2 TiO2-Crn+ TiO2-Cr-ion doped Titanium dioxide TiO2 (anatase) Water H2O Ozonide ion

a) Procedures for the generation and measurement of transient UV/VIS absorbance spectra were reviewed by Bonneau et al. (1997). b) absorption cross-section r/cm2 molecule–1. c) measured in the gas phase. d) the (decadic) absorption coefficient a of water at k of 172 nm (a = ek × cH2O, cH2O = 55.49 mol L–1 at a density of q = 1 kg L–1) was reported to be 550 cm–1.

6.3 UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates VUV/UV absorbance spectra of oxygen, ozone, water and hydrogen peroxide in the gas phase [redrawn from Bolton and Cater (1994), p. 471, Figure 1].

Fig. 6.4

Tab. 6-2, the propyl-peroxyl radical and the butyl-peroxyl radical (Getoff, 1991). Finlayson-Pitts and Pitts (1986) comprehensively discussed gas phase absorption data of many atmospheric components, such as nitrogen dioxide (NO2), sulfur dioxide (SO2), nitrosyl chloride (ClNO), etc. Carbon dioxide (CO2) dissociates at wavelength k <227.4 nm with formation of CO(1R+g ) and O(1D) (Sonnemann, 1992). The gas phase absorbance spectra of oxygen, ozone, water and hydrogen peroxide are presented in Fig. 6-4 (Bolton and Cater, 1994). Gaseous molecular oxygen starts to absorb electromagnetic radiation below k of 200 nm. The absorption region between 130 nm and 200 nm is known as the Schumann–Runge system. A weak and banded absorption structure is observed from k of 175 nm to 200 nm exhibiting total absorbance between k of 175 nm and 180 nm. The absorption becomes continuous down to k of 130 nm and is then again banded. The photodissociation of O2 below k of 220 nm is important in the stratosphere. Subsequent to the weak Herzberg photodissociation continuum at k of 200 nm to 240 nm a weak and structured part of the absorbance spectrum of O2 extends to k of 280 nm (Zelikina et al., 1998). Additionally, gaseous molecular oxygen exhibits very weak absorption bands in the visible (k = 762 nm) and in the infrared region (k = 1270 nm and 1070 nm), known as the atmospheric oxygen bands (FinlaysonPitts and Pitts, 1986). Photolysis of oxygen in the VUV region of the electromagnetic spectrum results in formation of oxygen atoms with triplet and singlet spin multiplicity (Eq. 6-5). O2 + hm (k <200 nm) ? O(3P) + O(1D)

(6-5)

151

152

6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Fig. 6.5 UV absorbance spectra of hydrogen peroxide in aqueous solution and in the gas phase [redrawn from Bolton and Cater (1994), p. 472, Figure 3].

In contrast, gaseous ozone absorbs strongly between k of 200 nm and 300 nm (Hartley bands). This absorption by O3 in the stratosphere controls the short-wavelength limit of UV radiation that reaches the troposphere. The UV absorption of O3 extents to k of 360 nm, which is known as the Huggins band (Finlayson-Pitts and Pitts, 1986). In addition, the weakly absorbing Chappuis bands appear between k of 440 nm and 850 nm (Sonnemann, 1992). The photolysis of gaseous ozone with UV radiation below k of 330 nm produces singlet molecular oxygen and singlet oxygen atoms according to Eq. 8 in Fig. 5-9. Water vapor begins to absorb VUV radiation below k of 190 nm (Weeks et al., 1963), whereas hydrogen peroxide in the gas phase and in aqueous solution just starts to absorb significantly below k of 310 nm (Fig. 6-5). The aqueous solution spectra of hydrogen peroxide and of ozone are quite similar compared with the corresponding gas phase absorbance spectra (cf. Figs. 6-5 and 6-6, respectively). However, they differ slightly with respect to the molar absorption coefficients, these being generally larger in the aqueous phase (Bolton and Cater, 1994). By comparison of the molar absorption coefficients of hydrogen peroxide and of ozone at k of 253.7 nm (Tab. 6-2) it is evident that in contrast to ozone a rather high concentration of hydrogen peroxide in the medium is necessary to create conditions of maximum absorption by the latter. However, the H2O2-UV process is the most important commercial AOT. To compensate for the low molar absorption coefficient of hydrogen peroxide in the UV-C region (e <180 L mol–1 cm–1) an adequate concentration of H2O2 must be used. On the other hand, the reactor design must guarantee a sufficient irradiation pathlength so that a maximum of the incident UV-C radiation is absorbed by H2O2. Among other sources of hydroxyl radicals, especially in natural aqueous systems, of special interest are the anions nitrite (NO–2) and nitrate (NO–3). They are

6.3 UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates UV absorbance spectra of ozone in aqueous solution and in the gas phase [redrawn from Bolton and Cater (1994), p. 471, Figure 2].

Fig. 6.6

Fig. 6.7 UV/VIS absorbance spectra of aqueous solutions of nitrite (NO–2) and nitrate (NO–3) ions in comparison with the UV absorbance spectrum of hydrogen peroxide [redrawn from Mack and Bolton (1999), p. 2, Figure 1]. The spectra between 200 nm and 250 nm scale with the left ordinate (0 ≤ e ≤ 10 000 L mol–1 cm–1), those between 250 nm and 400 nm with the right ordinate (0 ≤ e ≤ 30 L mol–1 cm–1).

153

154

6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

precursors responsible for the photo-initiated oxidation of trace organic compounds in natural water (Zepp et al., 1987). Their UV/VIS absorbance spectra are shown in Fig. 6-7. The research related to the photolysis of nitrite and nitrate has several other practical aspects. Firstly, the UV-C irradiation of drinking water is being used increasingly for disinfection and almost all natural waters contain significant concentrations of nitrate ions. According to European drinking water quality standards, the nitrate concentration [NO–3] must be below 50 mg L–1 and the nitrite limit [NO–2] is defined as being lower than 0.1 mg L–1. Nitrate photolysis leads, in an overall reaction, to nitrite and oxygen (Eq. 6-6). The photolysis proceeds via complex reaction sequences with the formation of intermediary hydroxyl radicals (Eqs. 6-7 and 6-8). For recent reviews of the UV photochemistry (k >200 nm) of nitrate and nitrite see Mack and Bolton (1999) and Mark et al. (1996), and for the effect of water photolysis at 172 nm on aqueous NO–3/NO–2 see Gonzalez and Braun (1995). NO–3 + hm ? NO–2 +

1 2

O2

(6-6)

NO–3 + hm ? (NO–3)* ? •NO2 + O•–

(6-7)

O•– + H2O ? •OH + HO–

(6-8)

The mechanistic situation is complicated by the competing photolysis of nitrite, which results in the formation of nitrogen monoxide and oxide radical anions (Eq. 6-9). The latter are immediately protonated in water at pH < 12 to yield hydroxide and hydroxyl radicals according to Eq. 6-8. NO–2 + hm ? (NO–2)* ? •NO + O•–

(6-9)

Secondly, the formation of nitrite and of nitrate ions during H2O2-UV AOP applications, for example via mineralization of nitrogen containing organic molecules, may have serious consequences concerning the continuous decrease of UV-C absorption by H2O2 as a result of an “inner filter” effect exerted by these ions. This is due to their high molar absorption coefficients in comparison with the low value of of hydrogen peroxide (see Fig. 6-7). Therefore, the rate of the photo-initiated oxidation may decrease significantly. The UV absorbance spectra of the transient species H• (konset ~ 250 nm), •OH (kmax ~ 230 nm), HO•2 (kmax ~ 231 nm) and O•– 2 (kmax ~ 248 nm) were reported by Getoff (1996). These data suggest that the direct photolysis of the intermediary species must also be considered for the evaluation of photo-initiated AOPs. However, this is not important for the •OH radical because its concentration is always so low (cf. Fig. 6-13) that its photolysis is negligible.

6.4 Physical-chemical Properties of Ozone and Hydrogen Peroxide

6.4

Physical-chemical Properties of Ozone and Hydrogen Peroxide 6.4.1

Ozone

Gottschalk et al. (2000) have reviewed the physical, chemical and toxicological properties of ozone, which is an allotropic modification of oxygen. Ozone is a highly toxic gas and a powerful oxidant. Its reactivity with inorganic and organic substances is quite well understood. However, the ozone molecule is only moderately stable. In the absence of catalysts and of oxidizable substrates, ozone decomposes within several days with the formation of the stable O2 modification. The decay is accelerated drastically in the presence of catalysts, by elevated temperatures or by UV-B/UV-C irradiation. Owing to the instability of the ozone molecule, ozone can neither be shipped nor stored in gas tanks. It must be produced on site by special non-equilibrium electrical gas discharges (Kogelschatz, 1987). Large ozone generators produce up to 100 kg ozone per hour by applying dielectric-barrier discharges (DBD) (Kogelschatz, 1997). The specific energy consumption of ozone generation depends on the type of ozone generator. For full-scale DBDdriven ozone generators the energy consumption usually lies between 12 and 18 kWh kg–1 O3 using oxygen as feed-gas (Gottschalk et al., 2000). Ozonation is widely used in organic synthesis, disinfection of potable water, in wastewater treatment and in many other applications (cf. Ullmann’s, 1991). The photochemistry of ozone in water has already been presented in Fig. 5-9 (Eq. 8) and was comprehensively discussed by Hoigné (1998). 6.4.2

Hydrogen Peroxide

Hydrogen peroxide is a ubiquitous compound present in natural waters, (for example: in seawater [H2O2] = 0.5 to 14 lg L–1; in fresh water [H2O2] = 1 to 30 lg L–1) and in air: [H2O2] = 0.1 to 1 ppb; in aerobic living cells (Offermanns et al., 2000) and in space on planetary surfaces (e. g. Carlson et al., 1999). Commercial aqueous solutions of hydrogen peroxide usually contain stabilizing agents (ca. 100– 1000 ppm), e. g. sodium pyrophosphate (Na2H2P2O7) or sodium stannate trihydrate (Na2SnO3 × 3 H2O), to suppress the metal ion catalyzed decomposition of H2O2 (Kirk-Othmer, 1995). Aqueous solutions of H2O2 are easy to handle and are mainly used for oxidation reactions, including bleaching processes, chemical syntheses, and AOPs for water and wastewater treatment (Offermanns et al., 2000, Köppke and Hagel, 1991). In drinking water purification, H2O2 is used to pre-oxidize organic water constituents and to eliminate iron and manganese ions. In Germany, the residual concentration of H2O2 in drinking water must lie below 0.1 mg L–1 (Aurand et al., 1991). Some important physical properties of aqueous solutions of hydrogen peroxide are presented in Tab. 6-3. The toxicological properties and occupational health risks related to H2O2 are briefly summarized below (Ullmann’s, 1989):

155

156

6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Tab. 6.3 Physical-chemical properties of commercially available concentrations of aqueous hy-

drogen peroxide a) Property

Density, q/g cm–3 T = 0 oC T = 20 oC T = 25 oC Viscosity, l/mPa s T = 0 oC T = 20 oC Refractive index, n20 D Melting point, mp/oC Boiling point, bp/oC b) pH H2O2 partial pressure, pB/ kPa c) H2O2 weight-% in vapor phase b) Concentration of aqueous solutions: [H2O2]/mol L–1 at T = 25 oC d) [H2O2]/g L–1 at T = 25 oC e) a) b) c) d)

H2O2 concentration/weight-% 30

35

50

70

90

– – 1.1081

1.1441 1.1312 1.1282

1.2110 1.1953 1.1914

1.3071 1.2886 1.2839

1.4136 1.3920 1.3867

– – – –25.7 106.2 – –

1.82 1.11 1.3563 –33 107.9 4.6 0.05

1.87 1.17 1.3672 –52.2 113.8 4.3 0.11

1.93 1.23 1.3827 –40.3 125.5 4.4 0.17

1.88 1.26 1.3995 –11.9 141.3 5.1 0.29

4.2

–

13.0

33.4

75.0

9.77

11.61

17.51

26.42

36.69

332.43

394.87

595.70

898.73

1248.03

Data taken from Ullmann’s (1989) and Kirk-Othmer (1995). at 101.3 kPa (1 atm). at T = 30 oC. 1000q w [H2O2]/mol L–1 = MH , with density q/g cm–3, weight percent factor w (1% = w = 0.01), molar 2 O2 mass M(H2O2) = 34.016 g mol–1.

e) [H2O2]/g L–1 = 1000 q w.

• • • •

contact leads to skin irritation and whitening (cutaneous emphysema); higher concentrations can lead to severe skin burns; eye contact leads to serious injury; vapor or aerosol causes damage of the upper respiratory tract and serious lung injuries; • accidental ingestion: chest pain, respiratory depression, foaming at the mouth, loss of consciousness; muscle and nerve disturbances and fever. Tab. 6-3 includes the molar and mass concentrations of aqueous H2O2 solutions of specified weight percentage. These data are necessary to calculate the actual concentration of H2O2 in the aqueous phase of a specific reactor system according to Eq. 6-10 and Example 6-1.

6.4 Physical-chemical Properties of Ozone and Hydrogen Peroxide

H2 O2 =mol L

1

100qw VH2 O2 MH2 O2 Vtotal

(6-10)

With: q/g cm–3 the density of aqueous H2O2 at specified temperature T, w the weight percent factor (1% = 0.01), M(H2O2)/g mol–1 the molar mass, and Vtotal/ L = V(aqueous H2O2) + V(water). Example 6-1: The reservoir of a photoreactor is filled with 0.9 m3 of wastewater, and 15 L of 35% H2O2 solution are added. Calculate the initial concentration of hydrogen peroxide at a temperature T of 25 8C. H2 O2 =molL

1

1000 1:1282 cmg 3 0:35 15 L 0:1903 g 915 L 34:016 mol

The water contains 0.1903 mol L–1 hydrogen peroxide, i. e. 6.47 g L–1 H2O2. Monitoring of hydrogen peroxide concentrations in aqueous systems is possible on- and off-line. Standard methods of H2O2 measurement include titrimetric, gasometric, electrochemical and colorimetric methods, and several others. For a short overview of H2O2 analyses, see Guwy et al. (2000).

6.4.3

Photochemistry of Hydrogen Peroxide

The photochemistry of H2O2 in aqueous solution was reviewed extensively by Lunak and Sedlak (1992) emphasizing the complexity of this photochemical reaction. It is still of actual scientific interest (e. g. Pehkonen et al., 1998). Although the decomposition of hydrogen peroxide by solar radiation in aqueous solution had already been observed in 1879 (cf. Tab. 2-3) many aspects of H2O2 photochemistry lead to confusions in modern scientific literature, especially concerning the quantum yield of this photoreaction (cf. Fig. 6-12). The thermal and the photo-induced decomposition of pure aqueous solutions of H2O2 lead to the formation of water and oxygen. The effect of electromagnetic radiation on the decomposition of H2O2 can be explained by the so-called Haber–Weiss mechanism (Fig. 6-8). This radical chain mechanism suggests that the hydroxyl radicals formed by homolytic cleavage of the central HO–OH bond initiate a propagation cycle with the net reaction leading to water and molecular oxygen. Wang et al. (1999) presented theoretical evidence (by ab initio molecular orbital calculations) for the existence of a fivemembered complex between a hydroxyl radical and hydrogen peroxide (HO•... H2O2), which may be formed in the HO• + H2O2 reaction. Similarly, Einschlag et al. (1997) presented reaction kinetic evidence for the formation of an intermediate six-membered cyclic complex between a hydroperoxyl radical and hydrogen peroxide in the liquid phase. This hydrogen-bonded complex (HOO•. . .H2O2, Fig. 6-8) rearranges by one-electron shifts to yield the final products water, molecular oxygen and a hydroxyl radical. The last again enters the propagation cycle. The most

157

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

Radical chain mechanism of the photo-induced decomposition of hydrogen peroxide in pure water according to Haber and Weiss (reference see Tab. 2-3).

Fig. 6.8

probable termination reactions of this radical chain include the radical–radical recombination of two hydroxyl radicals with formation of H2O2, the reaction of a hydroxyl radical with a hydroperoxyl radical with formation of water and oxygen, and the formation of H2O2 and O2 by disproportionation of two hydroperoxyl radicals (see Fig. 6-8). In this connection it is important to recall the dependency of the degradation rate of hydrogen peroxide, r(-H2O2), on the absorbed photon flow Up (Gmelin, 1966). In addition, the quantum yield of H2O2 photolysis is a discrete function of Up (Lunak and Sedlak, 1992, Gmelin, 1966). It was demonstrated that the rate of hydrogen peroxide decomposition at low incident photon flows (Up ≤ 1017 photons s–1) can be described by Eq. 6-11 and at Up above this value Eq. 6-12 is valid. r H2 O2

p dH2 O2 const: Up H2 O2 UP 1017 photons s 1 dt

(6-11)

r H2 O2

dH2 O2 const: Up dt

(6-12)

Up > 1017 photons s 1

High quantum yields of H2O2 degradation (U(-H2O2)1) are only found at low incident photon flow where the reaction HO•2 + H2O2 ? H2O + O2 + •OH becomes important and propagates the chain (cf. Fig. 6-8). This phenomenon was explained by the existence of two distinct mechanisms of H2O2 photodegradation: a chain mechanism at low incident photon flow and a non-chain mechanism at higher photon flows. The suppression of high quantum yields at high incident Up is possibly due to high concentrations of initial hydroxyl and hydroperoxyl radicals, which effectively terminate the radical chain reaction of the Haber–Weiss cycle (see Fig. 6-8).

6.5 Photocatalysts

The photochemical degradation rate of H2O2 is variable under certain pH conditions (cf. Legrini et al., 1993 and literature cited therein). Under alkaline conditions hydrogen peroxide deprotonates with formation of the H2O2/HO–2 equilibrium (von Sonntag, 1995), see Fig. 6-3 and Chapter 5.1.4. The HO–2 species has a significantly higher absorption coefficient than H2O2 (cf. Tab. 6-2) and thus, it efficiently competes for UV radiation. Hence its photochemical dissociation with formation of •OH radicals and oxygen anion radicals (O•–) prevails. Subsequently, O•– radical anion chemistry according to Eq. 6-8 is initiated (cf. Lunak and Sedlak, 1992). Again, the pH dependence of the rate of photo-induced H2O2 decomposition seems to be influenced by the absorbed photon flow Up. Alkaline conditions exhibit a marked inhibitory effect on r(-H2O2) at low Up and at low H2O2 concentrations. On the other hand, no effect of pH on r(-H2O2) was found at high Up (Gmelin, 1966). Nevertheless, most H2O2-UV AOPs are conducted in neutral to acidic aqueous solutions (see Legrini et al., 1993: Tab. IV) in order to avoid the •OH radical scavenging effect of the carbonate/bicarbonate system (refer to Fig. 6-15).

6.5

Photocatalysts

Titanium dioxide (TiO2) is one of the most widely used semiconductors for heterogeneous photocatalysis. This is mainly due to its activity, photostability, non-toxicity and commercial availability. It is found in nature and can exist in three crystal modifications: rutile, anatase and brookite (Kirk-Othmer, 1996). Its composition is temperature dependent: at calcination temperatures above 900 K, the anatase modification is transformed into rutile. TiO2 is insoluble in water and in diluted acids, but it dissolves slowly in hot sulfuric acid (Remy, 1973). It has a high surface activity and corrosion stability. The commercial production of this white pigment has been known since the early 1900s. The photocatalytic activity of TiO2 is strongly affected by its crystallinity and particle size (Pecchi et al., 2001). Interestingly, only the anatase modification is sufficiently active in photocatalysis having a band gap energy Ebg of 3.2 eV. Band gap energies of TiO2 aqueous suspensions can be measured by diffuse reflectance spectrometry (e. g. Riegel and Bolton, 1995). The effects of the nature and of the source of various TiO2 specimens have been explored extensively. Hombikat UV100 TiO2 (Sachtleben Chemie GmbH, Duisburg, Germany) consists of pure anatase modification and its particles have a BET surface area of about 186 m2 g–1 (by applying the Brunauer–Emmett–Teller theory of gas adsorption for the determination of the adsorption isotherm). However, the majority of investigations have been performed using Degussa P-25 TiO2. This material consists of about 80% anatase and 20% rutile and has a BET specific surface area of ca. 55 m2 g–1. The diameter of its particles usually lies between 25 nm and 35 nm. Hence, the separation of TiO2 nano-particles from its aqueous suspensions represents a serious problem for practical engineering. Therefore, a key technique for simple applications seems to be the preparation of immobilized TiO2 coatings on different sub-

159

160

6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Extinction spectrum (absorption + scattering) of polydispersed TiO2 aqueous solutions, with [P-25 TiO2] = 150 mg L–1 and pH ~ 2 [redrawn from Salinaro et al. (1999), p. 329, Figure 6b].

Fig. 6.9

Fig. 6.10 UV/VIS spectra of chromium ion-implanted TiO2; [Cr-ion] = a) 2.2 ×10–7 mol g–1; b) 6.6 × 10–7 mol g–1 and 13 × 10–7 mol g–1 [redrawn from Anpo (2000), p. 161, Figure 3].

strates (glass sheets, tiles etc.) without loss of photocatalytic activity (Kominami et al., 2001). The commercial material (P-25 TiO2) seems to be superior over synthetic sol-gel TiO2 with respect to the photocatalytic activity for the degradation of organic substrates. In fact, the adsorption capacity of the sol-gel material is lower than that of P-25 TiO2 leading to a higher degradation rate using the P-25 TiO2 (Piscopo et al., 2001). For the determination of the UV extinction spectrum (i. e. absorption and scattering), colloidal aqueous solutions of TiO2 were used (Salinaro et al., 1999). This spectrum is presented in Fig. 6-9. It clearly demonstrates that TiO2 begins to ab-

6.5 Photocatalysts

sorb electromagnetic radiation significantly below k of 400 nm, as is expected from its band gap energy. Many research activities are devoted to the design and development of secondgeneration TiO2 photocatalysts with the aim of enhancing the photocatalytic activity, or to extend the absorption band to the visible part of the electromagnetic spectrum. The latter can be achieved, for example, by metal ion-implantation techniques (Anpo and Takeuchi, 2001). UV/VIS spectra of chromium ion-implanted TiO2 samples are shown in Fig. 6-10. This method leads to an enormous red shift of the absorption band of TiO2 beginning below k of 600 nm (Anpo, 2000). Thus, a much better utilization of solar photons is achieved than by using pure TiO2. Other examples of second-generation photocatalysts are lanthanide oxide-doped TiO2 photocatalysts (Eu2O3/TiO2, Pr2O3/TiO2, Yb2O3/TiO2) that exhibit enhanced photodegradation activity compared with non-modified material using p-chlorophenoxyacetic acid as the model substrate (Ranjit et al., 2001). Other approaches to modifying TiO2 photocatalysts for the enhanced photodegradation of organic substrates include, for example, the specific modification of the TiO2 particle surface via attachment of N,N'-bipyridinium or iron(III)phthalocyanine units (Ranjit et al., 1999). Many other heterogeneous photocatalysts are of current interest, for example Fe0, a-Fe2O3, Fe-zeolites and CuO aerogel (Pulgarin and Kiwi, 1996), and polyoxometalates (e. g. Guo et al., 2001). Despite the fact that an enormous amount of scientific literature on the photocatalytic degradation of organic compounds in water and air has been published (Blake, 1999), very little information is available concerning the problems of catalyst inactivation and poisoning. For example, Vidal and Luengo (2001) reported on the inactivation of TiO2 by deposition of elementary sulfur on the surface of the catalyst during the photocatalytic treatment of monoalkyl dithiocarbamate [Vapam®, H3C–NH–C=S(SH)] which is used as a fungicide. According to the authors, this finding has significant effects on the commercialization of solar photocatalytic detoxification technology since in this case the catalyst costs dominate the economics of this process. The most interesting homogeneous catalyst for photochemical applications undoubtedly is the ferrioxalate system (Hislop and Bolton, 1999, refer to Fig. 5-14). The UV/VIS absorption properties of a ferrioxalate aqueous solution are shown by Fig. 6-11. This figure clearly proves the extraordinary absorption characteristics of this photo-Fenton material by exhibiting total absorbance of all incident photons below k of 400 nm at relatively low molar concentrations. The absorption extends well into the visible range of the electromagnetic spectrum and decreases to zero at about k of 550 nm (Hislop and Bolton, 1999). Thus, the UV/VIS/ferrioxalate/ H2O2 process seems to have great potentials for solar driven AOP applications (see Chapter 5.3.3). On the other hand, this system guarantees the absorption of additional UV and VIS emissions of medium-pressure mercury lamps (cf. Tab. 3-3) compared with the narrow band absorption range of hydrogen peroxide in the UV-C, thus making it to an attractive competitor to other AOTs including O3-UV. It should be mentioned, that solar photolysis of ferrioxalate is of importance within atmospheric water droplets (cf. Sigg and Stumm, 1996). Potassium ferrioxalate

161

162

6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Fig. 6.11 Comparison of the UV/ VIS absorption properties of hydrogen peroxide and of ferrioxalate in aqueous solution (redrawn from Bolton et al., 1995).

(K3[FeIII(C2O4)3] × 3 H2O) can be prepared by mixing solutions of ferric chloride (FeCl3) and potassium oxalate (K2C2O4) and by subsequent recrystallization of the light sensitive reaction product (Braun et al., 1991). For comparison, the UV/VIS absorbance spectra of FeIII aquo and hydroxo complexes ([Fe(H2O)6]3+, [Fe(OH)(H2O)5]2+ and [Fe(OH)2(H2O)4]+) and the dependence of their individual concentrations in aqueous solution on pH have been published by SafarzadehAmiri et al. (1996).

6.6

Quantum Yields

The quantum yield U of a photochemical process is an absolute measure of the efficiency of this process (cf. Chapter 3-13). Tab. 6-4 summarizes several quantum yields of primary photo-induced processes that are related to AOPs. The data were collected from original literature and from review articles. The stated values refer either to the degradation of a substrate [U(–)] or to the formation of a specified product [U(+)]. The difficulty of quantum yield determination in heterogeneous photocatalysis is related to reflection and scattering of UV/VIS radiation by suspended catalyst grains. Consequently, Serpone and Salinaro (1999) proposed a detailed protocol for the determination of photonic efficiencies (quantum yields) or relative photonic efficiencies, suggesting the photocatalyzed degradation of phenol as a standard reaction (using TiO2 as photocatalyst). However, the comparison of the data collected in Tab. 6-4 offers a realistic estimate of process efficiencies. The quantum yield of •OH radical formation on TiO2 is only 4 to 10% at best. This is the major reason why TiO2 processes have, in general, not been commercially successful. Pure economic analysis (favored by Bolton) results in an estimate that hydrogen peroxide based photo-initiated AOPs are 50 to 100 times more efficient in their use of electricity than TiO2 photocata-

Primary products

O (1D) O (1D) O (1D) 2•OH

Fe2+ Fe2+ Fe2+ HO– + •OH 2•OH 2•OH 2•OH 2•OH 2•OH 2•OH Iron(III)-hydroxy complex, [FeIII(OH)]2+/ Fe2+ + •OH H2O2 Fe2+ + •OH O3 Oxygen, O2 Oxygen, O2 2•O• (3P) a) O2 + O (1D) Ozone, O3 O2(1D) + O (1D) O2 + O (1D) H2O2 + O2

Ferric ion/hydrogen peroxide, Fe3+/H2O2 Ferric ion/oxalic acid, Fe3+/H2C2O4 Ferric ion/water, Fe3+/H2O Ferrioxalate, [FeIII(C2O4)3]3–/H2O2 Hydrogen peroxide, H2O2

Absorber

References

Bossmann et al. (1998) Bossmann et al. (1998) Bossmann et al. (1998) Hislop and Bolton (1999) Bolton and Cater (1994) Bolton and Cater (1994) Bolton and Cater (1994) Legrini et al. (1993) Pehkonen et al. (1998) Pehkonen et al. (1998) Hislop and Bolton (1999) Hislop and Bolton (1999) Baulch et al. (1982) Bolton and Cater (1994) Bolton and Cater (1994) Finlayson-Pitts and Pitts (1986) Taube (1957) Gurol and Akata (1996) Finlayson-Pitts and Pitts (1986) Finlayson-Pitts and Pitts (1986) Finlayson-Pitts and Pitts (1986) Legrini et al. (1993)

U(–H2O2) = 1.32 U(+Fe2+) = 1.14 U(+Fe2+) = 0.21 U(+•OH) = 2.23–3.65 U(–H2O2) = 1.0 U(–H2O2) = 0.50 U(–H2O2) 0.30 U(+•OH) = 0.98 U(+•OH) = 1.22 U(+•OH) = 1.58 U(+Fe2+) = 0.14 U(+Fe2+) = 0.195 U(+O3) = 2.0 U(–O2) = 1.0 U(–O3) = 0.99 U(–O3) = 0.23 U(–O3) = 0.61 U(–O3) = 0.48 ± 0.06 b) U[+O (1D)] = 0.95 U[+O (1D)] = 0.94 U[+O (1D)] = 1.0 U(+•OH) = 2.00 c)

220–600, in water 220–600, in water 220–600, in water 300, in water 200–350, gas phase 254, in water 313, in water 254, in water 193, gas phase 248, gas phase 313, in water 310, in water 172 nm, gas phase <190, gas phase k not stated, gas phase 310, in water 254, T = 14 oC, in water 254, T = 20 oC, in aqueous solution 290, gas phase 248, gas phase 230–280, gas phase 254, in water

Irradiation wavelength k/nm, phase

Quantum yield U

Tab. 6.4 Selected quantum yields of substrate degradation or of product formation in photochemical processes related to AOPs

6.6 Quantum Yields 163

Murov (1973) Murov (1973) Braun et al. (1991) Riegel and Bolton (1995)d) Valladares and Bolton (1993) Porada et al. (1999) Serpone and Salinaro (1999) Heit et al. (1998) Hart and Anbar (1970) Bolton and Cater (1994)

U(+Fe2+) = 1.25 U(+Fe2+) = 1.07 U(+Fe2+) = 0.86 U(+•OH) = 0.04 U(–MB) = 0.056 e) U(–AO7) = 0.007 U(–C6H6O) = 0.14 h) U(–H2O) = 0.42 U(+ e–aq) = 0.05 U(–H2O) = 1.0

254, in water 436, in water 509, in water <387, in water 320, in water

a) b) c) d) e) f) g) h)

546 f), in water 365, in water 172, liquid phase 170–200, liquid phase k not stated, gas phase

Below k of 176 nm O atoms may be formed in the singlet state. the rate of ozone photolysis increased with increasing UV-C intensity, ozone concentration and pH. the overall stoichiometry of this reaction is 3O3 ? 2•OH. see also Sun and Bolton (1996). the bleaching of the dye was measured by UV/VIS spectroscopy at k of 664 nm. only the dye absorbs light. phenol was proposed as a standard secondary actinometer and Degussa P-25 TiO2 as the standard photocatalyst. this value represents the calculated quantum yield of initial photooxidative degradation of phenol applying the method of relative photonic efficiencies.

Titanium dioxide/water, TiO2/H2O Titanium dioxide/Methylene Blue (MB), TiO2/C16H18ClN3S Titanium dioxide/Acid Orange 7 bleaching TiO2/C16H11N2O4SNa Titanium dioxide/phenol, TiO2/C6H6O g) – H• + •OH Water, H2O e–aq + H+ + •OH H• + •OH

Fe2+ + CO2 Fe2+ + CO2 Fe2+ + CO2 • O•– 2 + OH bleaching

Potassium ferrioxalate, K3[FeIII(C2O4)3]

References

Quantum yield U

Irradiation wavelength k/nm, phase

Primary products

Absorber

Tab. 6.4 (continued)

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

6.6 Quantum Yields

Fig. 6.12 Comparison of photochemistry and quantum yields of hydrogen peroxide

and of ozone; gas phase photochemistry of O3 see text.

lytic systems. On the other hand, Serpone argues that other parameters including the cost of chemicals (e. g. H2O2, Fe2+/Fe3+-salts), maintenance and labor have to be considered. For a discussion of these contrary points of view, refer to Wilson (1996). It will be very interesting to observe the future development of the competing advanced oxidation technologies with respect to their commercial feasibility. Another feature of interest is related to the quantum yields of hydrogen peroxide and of ozone photochemistry as is outlined in Fig. 6-12. Theoretically, the photolysis of 1 mol of H2O2 should produce 2 mol of hydroxyl radicals with a calculated maximum quantum 0yield of two, according to the definition of Uk pre=dt ). However, the experimental situation is much sented in Tab. 3-7 (Uk = dnP Uabs p more complex (cf. Chapter 6.4.3). Photodissociation of H2O2 in the gas phase at k of 248 nm and 193 nm yields largely vibrationless hydroxyl radicals, with about 90% of the excess energy being converted into translational energy of the •OH radicals (cf. Pehkonen et al., 1998). The quantum yield of •OH radical formation U(+•OH) of less than two (Fig. 6-12) in the gas phase photolysis of H2O2 indicates the existence of other (not yet identified) photophysical and/or photochemical reaction channels. Perhaps, in aqueous solution the so-called “cage effect” lowers the quantum yield of H2O2 decomposition to U(–H2O2) = 0.5. This is commonly explained by the fact that only 50% of the •OH radicals seem to be able to escape from the aqueous solvent cage, as a result of their excess energy Eexcess = Q (konset)– Q (kTHW) = 173 kJ mol–1 (cf. Tab. 3-5 and Tab. 3-4, section A). The excess energy Eexcess of the hydroxyl radicals exceeds this value by irradiation of H2O2 with shorter wavelengths k < konset (e. g. with UV-C radiation). The remaining 50% of

165

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

hydroxyl radicals within the aqueous cage immediately recombine to yield H2O2. Therefore, the quantum yield of •OH radical formation in aqueous solution U(+•OH) is found to be only half of the theoretically possible value. Photolysis of ozone in wet air is a source of free hydroxyl radicals according to Eqs. 6-13 and 6-14. The yield and wavelength dependencies of these reactions are the most important aspect of ozone photochemistry in the troposphere (Finlayson-Pitts and Pitts, 1986). Gas phase: O3 + hm(k <330 nm) ? 1O2 + O(1D); U(–O3) = 0.99, U[+O(1D)] = 1 (6-13) O(1D) + H2O ? 2•OH

(6-14)

On the other hand, the photolysis of ozone in aqueous solution [U(–O3) = 0.61 at k of 254 nm, cf. Tab. 6-4] is very different from its gas phase photoreactivity. It leads to the formation of hydrogen peroxide as the primary photoproduct (Glaze et al., 1987). The sequence of reactions may possibly include direct insertion of primarily formed singlet oxygen atoms into an O–H bond of water (von Sonntag, 1995; Fig. 6-12). The singlet molecular oxygen (1O2) formed in this photoreaction is deactivated very rapidly. In aqueous solution, the lifetime s of 1O2 is in the range of 3.3 to 7.42 ls (Braun et al., 1991). Its chemical reactivity toward organic chemicals is much lower than that of singlet oxygen atoms and of •OH radicals. Thus, this scheme (Fig. 6-12) suggests that the O3-UV and the H2O2-UV process are comparable with respect to •OH radical formation: in the former process, H2O2 is produced in situ by photolysis of aqueous O3. In the latter case, H2O2 is added from an external source as an auxiliary oxidant. In both cases, however, •OH radicals are produced via photolysis of H2O2. Still, there exist striking differences between the H2O2- and the O3-UV processes that are related to the nature of the auxiliary oxidants (Glaze et al., 1987), e. g. their absorption characteristics (cf. Tab. 6-2), their reduction potentials (cf. Tab. 6-1), and their water solubilities.

6.7

Primary and Secondary Reactive Species

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (e–aq), hydrogen atoms (H•) and hydroxyl radicals (•OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of e–aq and of H• were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [Fe(CN)6]4– or of I– with formation of [Fe(CN)6]3– and 12 I2, respectively (cf. Buxton et al., 1988).

6.7 Primary and Secondary Reactive Species

6.7.1

Hydrated Electrons

The hydrated electron may be described as an electron surrounded by a small number of oriented water molecules. It reacts rapidly with many substrates M having more positive reduction potentials by one-electron transfer processes according to the general Eq. 6-15 where n represents the positive charge on the substrate. e–aq + Mn ? Mn–1

(6-15)

The alkali metal cations Li+, Na+, K+, Rb+ and Cs+ cannot be reduced by hydrated electrons because they have a more negative reduction potential. In addition, the hydrated electron acts as a nucleophile, especially with organic molecules that contain halogen atoms (Eq. 6-16). This reaction results in rapid elimination of a halide ion from the initially formed negatively charged organic species. The reaction of Eq. 6-16 is of special interest for the degradation of perhalogenated saturated hydrocarbons that are usually not affected by hydroxyl radicals (Sun et al., 2000). e–aq + R–X ? (R–X)– ? R• + X–

(6-16)

Representative examples of the most common reactions of hydrated electrons are presented in Tab. 6-5.

Tab. 6.5 Representative examples of reactions of the hydrated electron (e–aq) (Hart and Anbar,

1970) e–aq + H+ ? H• e–aq + H3O+ ? H• + H2O e–aq + •OH ? HO– e–aq + H• ? H– ; H– + H2O ? H2 + HO– • – •– e–aq + CO2 ? CO•– 2 ; CO2 + H2O ? CO2H + HO • – • – • – eaq + N2O ? N2O ; N2O + H2O ? N2OH + HO– ; •N2OH ? N2 + •OH e–aq + I2 ? I–2 ; I–2 + I–2 ? I2 + 2I– e–aq+ (O2)aq ? (O•– 2 )aq e–aq + H2S ? H• + HS– e–aq + H2O2 ? HO– + •OH e–aq + NO–3 ? (NO2)aq + 2HO– • 2– – e–aq + S2O2– 8 ? SO4 + SO4

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

6.7.2

Hydrogen Atoms

The hydrogen atom is the conjugated acid of e–aq and has a slightly lower reduction potential E of –2.3 V (Buxton et al., 1988). Its chemical reactivity is often quite different from that of e–aq. The hydrogen atom reacts with organic substrates by hydrogen abstraction or by addition to double bonds with formation of the corresponding carbon-centered radicals (Eqs. 6-17 and 6-18, respectively). H• + CH3OH ? H2 + •CH2OH

(6-17)

H• + H2C=CH2 ? •CH2CH3

(6-18)

The different reactivity of hydrogen atoms and of hydrated electrons with organic substrates is best demonstrated by their reaction with chloroacetic acid (Eqs. 6-19 and 6-20). H• + ClCH2COOH ? •CHClCOOH + H2 (hydrogen abstraction)

(6-19)

e–aq + ClCH2COOH ? •CH2COOH + Cl– (chloride ion elimination)

(6-20)

Additional information can be gathered from original research studies cited in compilations of corresponding rate data (cf. Tab. 1-1, entry: radical reactions and rate constants).

6.7.3

Hydroxyl Radicals

Hydroxyl radicals are extremely reactive, short lived and unselective transient species. The mean lifetime of •OH radicals depends on their chemical environment and was estimated to be in the order of 10 ls in the presence of dissolved natural organic matter, bicarbonate and carbonate (Hoigné, 1998). Pryor (1986) estimated the half-life of hydroxyl radicals in the presence of linoleate (C18H31O–2, the conjugate base of linoleic acid) at T = 37 8C to be in the order of nano seconds. The high reactivity and the short lifetime of •OH radicals lead ultimately to a very low steady- state concentration of hydroxyl radicals [•OH]SS in water that contains dissolved natural organic matter (Fig. 6-13). This was demonstrated by Hoigné (1998) who calculated the steady-state concentration of •OH radicals during an ozonation process in the presence of dissolved natural organic matter (DNOM) to be in the region of 10–12 mol L–1. Lindsey and Tarr (2000) obtained similar data. These authors used the chemical probes benzoic acid and n-propanol for the quantification of •OH radical concentrations by measuring the product concentrations (ortho-hydroxybenzoic acid and propionaldehyde, respectively) in the Fenton-reaction (H2O2-Fe2+).

6.7 Primary and Secondary Reactive Species Fig. 6.13 Steady-state (SS) concentration of hydroxyl radicals (•OH) in water as a function of dissolved organic carbon (DOC) caused by dissolved natural organic matter (DNOM) [redrawn from Hoigné (1998), p. 117, Figure 15].

Fig. 6.14 Experimental evidence for the formation of •OH radicals generated in photo-in-

itiated AOPs: mechanism of the DMPO spin trapping technique.

The existence of free hydroxyl radicals in photo-initiated AOPs can be proven by applying a well-established method, the so-called spin trapping technique. The diamagnetic spin trap 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) forms a stable paramagnetic spin-adduct with •OH radicals. Its formation can be detected by electron paramagnetic resonance (EPR) spectroscopy. The underlying chemistry of

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

the reaction of •OH radicals with DMPO producing the stable spin adduct DMPO-OH is outlined in Fig. 6-14. For example, this method was applied to quantify the •OH radical concentration and to determine the generation efficiency of •OH radicals in H2O2-UV and TiO2-UV systems (Wolfrum et al., 1994, Sun et al., 1996, Riegel and Bolton, 1995). Also, second order rate constants of •OH radicals with organic substrates can be quantitatively measured by using this technique (Kochany and Bolton, 1991). Schwarz et al. (1997) described a similar method but used a paramagnetic spin trap [3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (3-carboxy-proxyl)] and they observed the disappearance of the EPR signal. Another method to detect and quantify •OH radicals in water is based on their reaction with dimethyl sulfoxide (DMSO) to produce CH3S(O)OH quantitatively and a methyl radical (•CH3) that is subsequently trapped by a sequence of specific reactions. The final reaction product can be separated by reversed-phase high-performance liquid chromatography (HPLC) and it is quantified fluorimetrically (Vaughan and Blough, 1998). However, several other methods to detect trace concentrations of hydroxyl radicals exist that are sensitive even in biological systems (Li et al., 1997). Hydroxyl radicals can oxidize organic and inorganic substrates (M, R–H ...) by different types of reactions (Eqs. 6-21 to 6-24) (Legrini et al., 1993, Hoigné, 1998): •

OH + Mn ? Mn+1 + (HO–)aq

(Electron transfer)

(6-21)

(Hydrogen abstraction)

(6-22)

OH + R2C=CR2 ? •CR2– C(OH)R2 (Electrophilic addition)

(6-23)

•

OH + R–H ? R• + H2O

•

Throughout the literature, many authors argue for the high reduction potential of hydroxyl radicals being responsible for the oxidative reactions observed in AOPs. However, simple electron transfer reactions such as those of Eq. 6-21 seem to be unlikely because of the large solvent reorganization energy involved in the formation of the hydrated hydroxide ion (Buxton et al., 1988). Instead, in the case of halide ions X– or pseudo-halide ions, the formation of intermediate adducts with hydroxyl radicals is observed (Eq. 6-24). •

OH + X– ? HOX• –

(6-24)

The scavenging reactions of •OH radicals by carbonate or bicarbonate ions (Eqs. 6-25 and 6-26) are usually referred to as electron transfer reactions. The carbonate radical is the conjugated base of the bicarbonate radical: the acid/base system HCO•3/CO•– 3 exhibits a pKA of between 7 and 9.6 (Hoigné, 1998). •

– •– OH + CO2– 3 ? HO + CO3

k•OH,M = 3.9 × 108 L mol–1 s–1

(6-25)

OH + HCO–3 ? HO– + HCO•3

k•OH,M = 8.5 × 106 L mol–1 s–1

(6-26)

•

6.7 Primary and Secondary Reactive Species

Fig. 6.15 Composition of the CO2/(H2CO3)/HCO–3/CO2– 3 /H2O system as a

function of pH at a temperature T of ca. 20 8C [redrawn from vanLoon and Duffy (2000), p. 11, Figure 1.2].

Due to the marked differences in second order rate constants k•OH,M the reaction of Eq. 6-25 is far more important for •OH radical scavenging than the reaction of Eq. 6-26. However, the bicarbonate and carbonate radicals (HCO•3, CO•– 3 ) may act as very selective oxidants. For example, the photodegradation of sulfur containing organic substrates (thioanisole C7H8S, fenthion C10H15O3PS2, dibenzothiophene C12H8S) was accelerated considerably in the presence of carbonate radicals. The latter were produced by photolysis of Na2CO3/H2O2 aqueous solutions (Huang and Mabury, 2000). Also, the rate of degradation of 4,4'-diamonostilbene-2,2'-disulfonate increased considerably in the presence of high concentrations of HCO–3 (up to 6000 mg L–1) during the H2O2-UV AOP (Sörensen and Frimmel, 1996). These reactions undoubtedly verify the special importance of the CO2/H2O system for the efficiency of AOPs. The omnipresent bicarbonate or carbonate ions can compete successfully for •OH radicals, especially at low loads of organic matter. The composition of the CO2 (H2CO3)/HCO–3/CO2– 3 system in water is strongly dependent on the pH of the solution (Fig. 6-15). When carbon dioxide is dissolved in water, only a small amount (ca. 0.1%) reacts to form carbonic acid (H2CO3). In fact, most of the undissociated acid is actually present as CO2(aq) at pH values lower than 4.3. At a pH of 8.2 the system consists solely of bicarbonate ions and at higher pH values above 12 the carbonate ion is mainly present. For a detailed discussion of the CO2/water system, see Sigg and Stumm (1996).

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

Fig. 6.16 Reactions of hydroxyl radicals with an organic substrate M in the presence of bicarbonate leading to a diversity of oxidized products Mox i (following Hoigné, 1998). The subscript i denotes different product molecules and the question mark denotes the complex reactivity of intermediary tetroxides (see Figures 617 and 6-18), and other competing decomposition pathways.

6.8

Oxidative Degradation of Organic Matter by Hydroxyl Radicals 6.8.1

Aliphatic and Aromatic Hydrocarbons

The above-mentioned reactions of hydroxyl radicals contribute to photo-initiated AOPs and lead to complex product mixtures according to the general scheme outlined in Fig. 6-16. Carbon-centered radicals M• that are formed via attack of •OH radicals on substrate molecules M immediately react in a bimolecular reaction with dissolved molecular oxygen to produce peroxyl radicals. These reactions are virtually diffusion controlled and usually irreversible (von Sonntag and Schuchmann, 1991). A large variety of rate constants kO2,M have been measured. They usually lie close to 2 × 10–9 L mol–1 s–1. There are however several cases known that are reversible, e. g. the reaction of cyclohexadienyl radicals with oxygen is an exception from this rule of thumb (Fang et al., 1995). The peroxyl radicals M–O–O• thus formed undergo a variety of molecular rearrangements and/or elimination reactions until the final oxidation products Mox i are formed. In reality, these oxidation products Mox i are interfering with hydroxyl radical attack on M and hence they are complicating the product spectrum considerably. Additionally, the bicarbonate and carbonate radicals may introduce selective oxidation reactions into the degradation cycle (Fig. 6-16).

6.8 Oxidative Degradation of Organic Matter by Hydroxyl Radicals

Fig. 6.17 Reactivity of aliphatic peroxyl radicals (following von Sonntag and

Schuchmann, 1991). R represents an aliphatic carbon skeleton.

Some general reactions of aliphatic peroxyl radicals R–O•2 are summarized in Fig. 6-17. It is assumed, that these radicals terminate with the formation of an intermediary and short-lived tetroxide R–O4–R (von Sonntag and Schuchmann, 1991). The tetroxide decomposes via a series of intramolecular reactions as shown in Fig. 6-17 with formation of a variety of oxidation products including ketones, alcohols, aldehydes, molecular oxygen, hydrogen peroxide and aliphatic peroxides, depending on the nature of the alkyl skeleton R. The decomposition with formation of hydrogen peroxide is interesting with respect to AOPs. This reaction may explain the fact that in many H2O2-UV applications H2O2 is still present in considerable amounts after the treatment is actually finished. The formation of the products from tetroxide decomposition can be rationalized by concerted processes involving six-membered structures of the transition state (Fig. 6-18) with or without water taking part. These reactions are known as Russell reactions, in which primary or secondary peroxyl radicals are transformed in electrocyclic reactions to give molecular oxygen, a product with a carbonyl group (C=O) and one with an OH group. The decomposition pathways are subject to the spin conservation rules: a singlet carbonyl compound, an alcohol and singlet molecular oxygen (1O2) or a triplet carbonyl compound, alcohol and ground state oxygen (3O2) may be formed. However, 1O2 was not detected as a product of peroxyl reactions in aqueous solutions (von Sonntag and Schuchmann, 1991). The incorporation of two water molecules seems to be of general importance in aqueous solution. Then, the electrocyclic process leads to the formation of hydro-

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

Fig. 6.18 Modes of decomposition of intermediary aliphatic tetroxides (Russell reactions) via electrocyclic rearrangements, with or without water taking part (von Sonntag and Schuchmann, 1991), or via one-electron shifts.

gen peroxide (H2O2) and of two carbonyl compounds (2R2C=O, Fig. 6-18). In addition, von Sonntag and Schuchmann (1991) discussed in detail several other potential pathways to these products. The recombination of oxyl radicals to give peroxides is of minor importance in aqueous solution. Instead, the water catalyzed rearrangement of oxyl radicals (R2HC–O•) to tautomeric a-hydroxyalky radicals (•CR2OH) increases in importance in aqueous solution (Fig. 6-17). Subsequently, the HO•2/O•– 2 system is formed in the presence of molecular oxygen, thus producing secondary oxidative species (Fig. 6-17). A further example of an electrocyclic fragmentation reaction is the acetic acid system (see Fig. 6-18) (von Sonntag and Schuchmann, 1991). The decomposition of the intermediary acetic acid tetroxide leads to the formation of formaldehyde, carbon dioxide, and again hydrogen peroxide. This reaction is one of the first steps toward the total mineralization of the organic material. Formaldehyde itself reacts efficiently with •OH radicals by way of hydrogen abstraction with formation of •CHO radicals or their hydrated form (Fig. 6-19). The latter are oxidized by dissolved molecular oxygen to yield formic acid and hydroperoxyl radicals. Again, formic acid reacts with •OH radicals to give formoyl radicals that react with molecular oxygen finally yielding carbon dioxide and hydroperoxyl radicals (cf. Stefan and Bolton, 1998, Heit et al., 1998, Gonzalez and Braun, 1996). Ogata et al. (1980) has already investigated the photo-initiated oxidation of simple organic acids (formic acid, acetic acid, and propionic acid) in the presence

6.8 Oxidative Degradation of Organic Matter by Hydroxyl Radicals

Fig. 6.19 Mechanism of the mineralization of formaldehyde by hydroxyl radicals.

of aqueous hydrogen peroxide. Thus, Fig. 6-19 completes the mechanistic scheme toward the oxidative degradation of DOC and hence toward the complete mineralization of organic matter dissolved in water. The degradation of aromatic compounds in water by hydroxyl radicals is even more complex (von Sonntag, 1996, von Sonntag and Schuchmann, 1991). For example, benzene and its derivatives react rapidly with •OH radicals to yield cyclohexadienyl-type radicals (Fig. 6-20), which immediately add dissolved molecular oxygen to the ortho or para position of the OH functional group (von Sonntag and Schuchmann, 1991). The reactions of the resulting para-peroxyl radicals with 1,4cyclohexadienyl structure are outlined in Fig. 6-21. The endoperoxide that is produced via intramolecular O2 radical addition to a double bond of the molecule is mainly responsible for the formation of open chain products with six or fewer carbon atoms. About 30 fragmentation products have been characterized during cradiolysis of benzene in aqueous solution (von Sonntag and Schuchmann, 1991). The main reaction product of •OH radical initiated oxidation of benzene is phenol, in addition to cyclohexa-2,5-diene-1,4-diol, hydroquinone, and catechol (1,2-dihydroxybenzene). The HO•2/O•– 2 radicals that are liberated during these decomposition reactions (Fig.s 6-20 and 6-21) again initiate chain reactions (see Fig. 7-1). For example, HO•2 radicals can add to phenol as shown in Fig. 6-22. Subsequent reaction with molecular oxygen and elimination of HO•2 finally leads to the formation of an open ox chain product Mox 1 (cf. Getoff, 1996). The reaction product M1 is again attacked • • •– by OH and/or HO2/O2 reactive species leading to the ultimate products CO2 and H2O. Von Sonntag and Schuchmann (1991) presented a survey of monomolecular and bimolecular radical reactions and Alfassi (1997) collected further information

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

Fig. 6.20 Oxidation of benzene by hydroxyl radicals in aerated water according to von Sonntag and Schuchmann (1991).

Fig. 6.21 Reactions of the 1,4-cyclohexadienyl-peroxyl radical formed by addition of O2 to the •OH radical adduct of benzene (Figure 6-20), according to von Sonntag and Schuchmann (1991).

6.8 Oxidative Degradation of Organic Matter by Hydroxyl Radicals

Fig. 6.22 Reaction of phenol with hydroperoxyl radicals (HO•2) in aerated aqueous solution leading to an open chain product (cf. Getoff, 1996).

about peroxyl radical chemistry. Specific examples of photo-initiated degradation reactions of aliphatic hydrocarbons are discussed in Chapters 7.1.1 and 7.1.2. 6.8.2

Chlorinated Hydrocarbons

Chlorinated hydrocarbons are of special interest in AOPs because the fully halogenated and saturated derivatives, for example tetrachloromethane (CCl4), do not react with •OH radicals. In contrast to the C–H bond, the C–Cl bond is inert against •OH radical attack. Trichloromethane (chloroform, CHCl3) and dichloromethane (methylenechloride, CH2Cl2) react with relatively low rate constants via hydrogen atom abstraction (von Sonntag et al., 1997), see also Chapter 7.1.3. However, unsaturated chlorinated hydrocarbons, such as tetrachloroethene (C2Cl4), react very fast with •OH radicals by electrophilic addition (Fig. 6-23). The OH-adduct radicals rapidly eliminate hydrogen chloride (HCl) with formation of the carbonyl functional group (C=O), and the remaining C-radicals add molecular oxygen. Two of the peroxyl radicals thus formed decompose with formation of oxygen, chlorine atoms and oxalylchloride. The last hydrolyses in water with formation of oxalic acid and HCl. Chlorine atoms add rapidly to tetrachlorethene with formation of pentachloroethyl radicals that are precursors of trichloroacetic acid (Cl3C–COOH) (von Sonntag, 1997). Mertens and von Sonntag (1995) investigated the direct photolysis of tetrachloroethene in aqueous solution at k of 253.7 nm with respect to the formation of trichloracetic acid. This compound is a registered herbicide and it falls under the German drinking water act (TVO: Trinkwasser Verordnung). However, the formation of the highly corrosive trichloroacetic acid can be suppressed effectively in the presence of moderate concentrations of bicarbonate or carbonate, especially in the case of low concentrations of dissolved organic matter (von Sonntag, 1997). This effect is due to the fast reaction of Cl• atoms with bicarbonate or

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

Fig. 6.23 Reaction of tetrachloroethene with •OH radicals in aerated aqueous solution (von Sonntag et al., 1997).

carbonate ions via electron transfer according to Eqs. 6-27 and 6-28, thus preventing the addition of Cl• atoms to the unsaturated chlorohydrocarbon. The values of the second order rate constants k•Cl,M have been determined by Mertens and von Sonntag (1995). Cl• + HCO–3 ? HCl + CO•– 3

(k•Cl,M = 0.22 × 109 L mol–1 s–1)

(6-27)

– •– Cl• + CO2– 3 ? Cl + CO3

(k•Cl,M = 0.5 × 109 L mol–1 s–1)

(6-28)

In conclusion, it is obvious from Fig.s 6-16 to 6-23 that the concentration of dissolved molecular oxygen [(O2)aq] during the application of photo-initiated AOPs (and others) must be maintained at a sufficiently high level to guarantee the formation of organic peroxyl radicals (R–O•2) on the way to mineralization of organic water contaminants.

6.9

Rate Law and Rate Constants of Hydroxyl Radical Reactions

The reaction of an •OH radical with a substrate molecule M to yield a molecule of an oxidation product Mox is a bimolecular reaction according to Eq. 6-29. •

OH + M ? Mox

(6-29)

6.9 Rate Law and Rate Constants of Hydroxyl Radical Reactions

The reaction kinetics can be described by the second order rate law of Eq. 6-30. r M

dMt kOH;M M OH dt

(6-30)

With r(–M) referring to the rate of diminution of the concentration of M and k•OH,M being the second order rate constant for the reaction of •OH with a substrate molecule M (cf. Hoigné, 1998). The unit of k•OH,M is usually given in L mol–1 s–1. Since the early 1960s, hundreds of rate constants k•OH,M have been measured, mainly by radiation chemical methods applying pulse radiolysis techniques (Buxton et al., 1988). However, the rate constants k•OH,M can also be determined by a competition kinetics method, in which the rate of decay of a target compound is compared with that of a reference compound with known k•OH,M. In these cases, the •OH radicals can be generated by photochemical methods via H2O2-UV or O3-UV AOPs in aqueous solution (cf. Onstein et al., 1999, De et al., 1999) or in the gas phase in the presence of water vapor (DeMore, 1996). Compilations of radical reactions and rate constants in connection with the original references are available via the Internet (cf. Tab. 1-1, entry: radical reactions and rate constants). Several important examples of relevant k•OH,M values are collected in Tab. 6-6. The following generalized observations can be extracted from these data (in analogy with Hoigné, 1998): • organic compounds of medium to high molecular mass react quickly with •OH radicals. Most rate constants of organic molecules k•OH,M in aqueous solution are in the range of 109 to 1010 L mol–1 s–1; • unsaturated and aromatic compounds react faster (via electrophilic addition) than saturated ones (via hydrogen abstraction); • with an increasing number of H atoms presented by a saturated organic molecule the k•OH,M increases considerably (statistical argument); • many values are larger than the diffusion controlled rate constant in aqueous solution, which has a value kdiff of 7 × 109 L mol–1 s–1 (Levine, 1988). Kochany and Bolton (1991) explain this fact by analogy with the transport of H3O+ and HO– ions through water. This takes place with little or no movement of atoms via the so called Grotthus mechanism, allowing extremely high mobility of the ions (or radicals) via hydrogen bridges and formation of clusters, e. g. H9O+4 ; • the deprotonated forms of low molecular weight organic acids (formate, acetate, oxalate) react faster with •OH radicals than their protonated forms; • saturated halogenated hydrocarbons usually react slowly with •OH radicals, and perhalogenated saturated organic compounds including chloro-fluoro carbons do not react with •OH radicals; • bicarbonate and carbonate ions react with •OH radicals via electron transfer with fairly low rate constants. However, usually these ions are omnipresent in polluted waters with high concentrations. Additionally, during progressive AOP treatment of water containing organic pollutants these ions can accumulate to relatively high concentrations, depending on the pH of the water (cf. Fig. 6-15). The scavenging effect of HCO–3/CO2– 3 decreases considerably with decreasing

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals Tab. 6.6 Representative examples of second order rate constants k•OH,M for reactions of •OH radicals with various substrates M in aqueous solution at T = 293 K (Buxton et al., 1988)

Substrate M

Formula of M

2,4-Dinitrophenol 2-Propanol Acetate ion Acetic acid Acetone Adenine Ammonia Benzene Bicarbonate ion Bromide ion Carbon dioxide Carbon monoxide Carbonate ion Chloride ion Cyanide ion Cytosine Dichloromethane Ethanol Formaldehyde Formate ion Formic acid Hydrazine Hydrogen Hydrogen atom Hydrogen peroxide Hydroperoxyl radical Hydroxide ion Hydroxyl radical Iodide ion Methanol Nitrite ion Oxalate dianion Oxalic acid Ozone Phosphate ion Phosphoric acid Sulfuric acid Superoxide radical anion Tert-butyl formate ester Thymine Trichloroethene Trichloromethane

C6H4N2O5 C3H8O CH3COO– CH3COOH CH3COCH3 C5H5N5 NH3 C6H6 HCO–3 Br– CO2 CO CO2– 3 Cl– CN– C4H5N3O CH2Cl2 C2H5OH HCHO HCOO– HCOOH HN2-NH2 H2 H• H2O2 HO•2 HO– HO• I– CH3OH NO–2 – OOC-COO– HOOC-COOH O3 PO3– 4 H3PO4 H2SO4 O•– 2 (CH3)3COCHO C5H6N2O2 Cl2C=CClH CHCl3

Products Mox of the reaction Rate constant a) k•OH,M/ 109 L mol–1 s–1 OH + M ? Mox

•

Products b) Products b) • CH2CO–2 + H2O • CH2COOH + H2O Products b) Adenine-OH-adduct • NH2 + H2O Hydroxycyclohexadienyl d) CO•– 3 + H2O • BrOH– CO•– 3 • CO2H – CO•– 3 + HO • – ClOH • C(OH)=N– Cytosine-OH-adduct • CHCl2 + H2O • CH3C HOH • CHO + H2O CO•– 2 + H2O • CO2H + H2O • NH-NH2 + H2O H• + H2O H2O HO•2 + H2O O2 + H2O O•– + H2O (pH = 11) H2O2 • IOH– • CH2OH + H2O • NO2 + HO– Products b) Products b) HO•2 + O2 Products b) H2PO•4 + SO•– 4 + H + H2O O2 + HO– (pH = 7.9) Products b) Thymine-OH-adduct • CCl2-CHCl(OH) • CCl3 + H2O

5.7 c) 1.9 0.067 0.017 0.13 5.8 0.097 7.8 0.0085 11.0 0.001 2.0 0.39 4.3 7.6 6.3 0.058 1.9 1.0 3.2 0.13 4.5 0.034 7 0.027 10 12 5.5 11.0 0.97 6.0 0.0077 0.0014 0.11 <0.01 0.000042 0.014 10 0.41 c) 6.4 4.0 0.05

a) The diffusion controlled rate constant in aqueous solution is kdiff = 7 × 109 L mol–1 s–1 (Levine, 1988). b) structures not specified. c) according to Onstein et al. (1999). d) structure see Figure 6-20.

6.9 Rate Law and Rate Constants of Hydroxyl Radical Reactions

Fig. 6.24 Comparison of several second order rate constants k•OH,M of the reaction of •OH radicals with different substrates M (•OH + M ? Mox) in aqueous solution (according to Hoigné, 1998), data taken from Table 6-6.

pH of the water because the k•OH,M of CO2– 3 is almost 46 times higher than that of HCO–3, the latter being the predominant species below pH 10. However, in general the pH of wastewaters is lowered significantly during AOP treatment due to the formation of organic or inorganic acids; • sulfate, phosphate and nitrate react extremely slowly with •OH radicals, thus their •OH scavenging effect can generally be neglected; • cyanide, chloride, bromide and iodide ions are effective •OH radical scavengers. Many industrial wastewaters have extraordinary high chloride ion concentrations due to extensive use of sodium chloride. High Cl– concentrations may cause serious corrosive problems with reactor components consisting of stainless steel. Dissolved chloride ions react rapidly with •OH radicals according to Eq. 6-24 with formation of •ClOH–. This species can decompose to yield chlorine atoms (Eq. 6-31). Chlorine atoms can add to C=C-double bonds of compounds present during AOP treatment, thus generating chlorinated hydrocar-

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6 Properties, Reactivity and Photochemistry of Auxiliary Chemicals

bons (Eq. 6-32 and Fig. 6-23). This was demonstrated by Hirvonen et al. (1996) who established the formation of trichloroacetic (TCA) and dichloroacetic (DCA) acids during H2O2-UV treatment of a groundwater containing trichloro- and tetrachloroethene. Hence, this process may lead to an undesirable increase of the global parameter AOX. Thus, high Cl– concentrations may decrease the efficiency of TOC diminution significantly: for an example see Seiss et al. (2001). •

ClOH– + H+ ? H2O + Cl•

Cl• + R2C=CR2 ? R2ClC•– CR2 ??? chlorohydrocarbons, DCA, TCA, formation of AOX.

(6-31)

(6-32)

For a general overview, some of the most important rate constants of •OH radicals with different target compounds are arranged in Fig. 6-24. Despite the fact that •OH radicals are very reactive and unselective oxidants (related to different substrates) significant selectivities of hydrogen abstraction reactions still exist with respect to the different types of hydrogen atoms bound in a specific substrate. This is mainly due to stabilizing or destabilizing effects of functional groups in the resulting radical structure. So, primary H atoms (e. g. in methyl groups) contribute a smaller fraction to the overall rate constant of hydrogen abstraction than secondary (methylene groups) or tertiary H atoms (Hoigné, 1998). An interesting example of selectivity with respect to hydrogen abstraction by hydroxyl radicals is demonstrated in Fig. 6-25. 2-Propanol is an effective •OH radical scavenger that is used in many kinetic studies (e. g. Hislop and Bolton, 1999). This is mainly due to the fact that the sole primary oxidation product acetone (2-propanone) has a much lower rate constant for •OH radical attack than 2-

Fig. 6.25 Selectivity of hydrogen abstraction in the reaction of •OH radicals with 2-propanol (Asmus et al., 1973).

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References tions in Aqueous Solution with the Help of Radiation-Chemical Methods, Angew. Chem. Int. Ed. Engl. 30: 1229–1253. Sörensen M, Frimmel FH (1996) Photochemical Degradation of Hydrophilic Xenobiotics in the UV/H2O2-process. Influence of Bicarbonate on the Degradation Rate of EDTA, 2-Amino-1-naphthalenesulfonate, Diphenyl-4-sulfonate, and 4,4'-Diaminostilbene-2,2'-disulfonate, Acta Hydrochim. Hydrobiol. 24, No. 4: 185–188. Stefan MI, Bolton JR (1998) Mechanism of the Degradation of 1,4-Dioxane in Dilute Aqueous Solution Using the UV/Hydrogen Peroxide Process, Environ. Sci. Technol. 32, No.: 11: 1588–1595. Stefan MI, Bolton JR (1999) Reinvestigation of the Acetone Degradation Mechanism in Dilute Aqueous Solution by the UV/H2O2 Process, Environ. Sci. Technol. 33, No. 6: 870–873. Sun G-R, He J-B, Pittman CU Jr (2000) Destruction of Halogenated Hydrocarbons with Solvated Electrons in the Presence of Water, Chemosphere 41: 907–916. Sun L, Bolton JR (1996) Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO2 Suspensions, J. Phys. Chem. 100: 4127– 4134. Sun L, Hoy AR, Bolton JR (1996) Generation Efficiency of the Hydroxyl Radical Adduct of the DMPO Spin Trap in Homogeneous and Heterogeneous Media, J Adv. Oxid. Technol. 1, No. 1: 44–52 Taube H (1957) Photochemical Reactions of Ozone in Solution, Trans. Faraday Soc. 53: 656–665. Ullmann’s (1989) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed., VCH, Weinheim, Vol. A 13: 444 foll. Ullmann’s (1991) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed., VCH, Weinheim, Vol. A 18: 349 foll. Valladares JE, Bolton JR (1993) A Method for the Determination of Quantum Yields in Heterogeneous Systems: the TiO2 Photocatalyzed Bleaching of Methylene Blue, in: Ollis DF, Al-Ekabi H (eds) Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam: 111–120.

vanLoon GW, Duffy SJ (2000) Environmental Chemistry – A Global Perspective, Oxford University Press, Oxford. Vaughan PP, Blough NV (1998) Photochemical Formation of Hydroxyl Radicals by Constituents of Natural Waters, Environ. Sci. Technol. 32: 2947–2953. Vidal A, Luengo MAM (2001) Inactivation of Titanium Dioxide by Sulfur: Photocatalytic Degradation of Vapam®, Appl. Catal. B: Environ. 32: 1–9. Wang B, Hou H, Gu Y (1999) Existence of Hydrogen Bonding between the Hydroxyl Radical and Hydrogen Peroxide: OH•H2O2, Chem Phys. Lett. 309: 274–278. Wardman P (1989) Reduction Potentials of One-Electron Couples Involving Free Radicals in Aqueous Solution, J. Phys. Chem. Ref. Data 18, No. 4: 1637–1755. Weeks JL, Meaburn GMAC, Gordon S (1963) Absorption Coefficients of Liquid Water and Aqueous Solutions in the Far Ultraviolet, Rad. Res. 19: 559–567. Weeks JL, Metheson MS (1956) The Primary Quantum Yield of Hydrogen Peroxide Decomposition, J. Am. Chem. Soc. 78: 1273, cited in: Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Aquatic and Surface Photochemistry, Helz GR (ed), Lewis Publishers, Boca Raton, 467–490. Wilson E (1996) TiO2 Appears Inefficient for Water Treatment, Chem. Eng. News 74, No. 27: 29. Wolfrum EJ, Ollis DF, Lim PK, Fox MA (1994) The UV-H2O2 Process: Quantitative EPR Determination of Radical Concentrations, J. Photochem. Photobiol. A: Chem. 78: 259–265. Zelikina GY, Kiseleva MB, Burtsev AP, Bertsev VV (1998) Spectrum of Induced Absorption of Oxygen in Mixtures with Various Gases in the Range of 190– 280 nm, Opt. Spectrosc. 85, No. 4: 520–524. Zepp RG, Hoigné J, Bader H (1987) NitrateInduced Photooxidation of Trace Organic Chemicals in Water, Environ. Sci. Technol. 21: 443–450.

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Photooxidation and Photomineralization of Organic Matter in Water and Air 7.1

Aqueous Systems 7.1.1

Substrate Oxidation and Substrate Mineralization

It is generally accepted that the mechanism of photo-initiated (and of other) AOPs includes chain reactions (cf. Chapter 6.8.1) that involve reactive and transient oxidative species (Gottschalk et al., 2000). Once initiated, the Haber–Weiss cycle (Fig. 6-8) or the O3/HO– cycle (Glaze et al., 1987) should produce hydroxyl radicals as long as the concentration of the auxiliary oxidants is sufficient (Fig. 7-1, chain I). Additionally, a reasonable assumption could also predict a radical chain propagation of the oxidation step (refer to Fig.s 6-19 to 6-22). Within this model a substrate molecule M would be introduced into chain I and thus, initiating the carbon/ peroxyl radical chain (Fig. 7-1, chain II) in the presence of dissolved molecular oxygen. Both radical chains are coupled directly with each other via exchange of HO•2 radicals. In competition with the oxidation of M by reactive oxygen species, direct oxidation of substrate molecules by the oxidative auxiliaries may occur with ozone being a stronger oxidant than hydrogen peroxide (cf. reduction potentials, Tab. 6-1). However, this concept simply implies that after activation of the radical chains I and II (Fig. 7-1) no further initiation via irradiation (or by other activations) should be necessary. Unfortunately, this situation is very rare because in most cases rapid terminations of the radical chains I and II occur by intervention of a multitude of radical scavengers S in chain I or by carbon–carbon radical recombination steps in chain II. The latter process may lead to the formation of higher molecular weight compounds, oligomers and polymers. Thus, the oxidation reaction usually stops immediately after the production of reactive species is finished, i. e. after shut down of the radiation source in photo-initiated AOPs. Nevertheless, an example of a delayed “dark reaction” is presented in Fig. 7-2. The diminution of DMSO during H2O2-UV treatment of water containing a mixture of DMSO and glycerol was considerably slower at discontinuous irradiation than was observed during continuous UV treatment allowing dark phases every 30 min. How-

190

7 Photooxidation and Photomineralization of Organic Matter in Water and Air Fig. 7.1 Generally accepted mechanism of advanced oxidation processes, modified according to Gottschalk et al. (2000), and extended by the author. M: Substrate moleox cule, S: scavenger, Mox i and Si are the oxidized forms.

ever, the dark reaction, i. e. the degradation of remaining DMSO without further irradiation of the water, could be established after 5 h of continuous UV treatment. The concentration of DMSO still declined from about 340 mg L–1 to the detection limit within a dark period of 2 h. Hence, a significant post-irradiation effect (dark reaction), if it has been established can improve the economics of photo-initiated AOPs considerably. To understand this “radical chain mystery” it has to be stressed again, that AOPs are based on two different but interrelated levels: the level of reactive intermediates (Fig. 7-3) and the level of oxidation products (Fig. 7-4). The former depends on the specific AOP that is applied (H2O-VUV, H2O2-UV etc.). The latter subdivides into simple oxidation reactions and substrate mineralization. Mineralization of organic matter in water (i. e. diminution of DOC or TOC of the water) usually competes with substrate oxidation, but with some time delay. However, the efficiency of DOC diminution is much slower than that of substrate consumption (depending on the specific reaction system). This is due to the fact, that the reaction of a substrate molecule with one •OH radical already leads to the disappearance of that specific molecule. This primary step is equivalent to the oxidation of the substrate, whereas the mineralization involves a complex series of oxidation and C–C-bond fragmentation reactions until all C-atoms of the organic molecule are transformed into their highest oxidation state in CO2. By accumulation of oxidation products and of radical recombination products, the progress of mineraliza-

7.1 Aqueous Systems

Influence of dark phases during the H2O2-UV treatment of a mixture of dimethyl sulfoxide (DMSO) and glycerol (CH2OHCHOHCH2OH) in aqueous solution. Solid line: 5 h of continuous irradiation: at t = 5 h the lamp was shut off. The black solid arrow indicates a dark phase of proceeding oxidation of DMSO after 5 h of UV treatment. Dotted line: discontinuous irradiation with Fig. 7.2

dark phases every 30 min. Reactor specification: Contact-free Batch Reactor (CFBR, cf. Figures 8-7 and 8-8), lamp type: MP Hg lamp, Pel = 15 kW, PUV-C ~3 kW (IST, Zizishausen, Germany), batch treatment of V = 1000 L, [DMSO]t=0 = 3400 mg L–1, [glycerol]t=0 = 2400 mg L–1, [H2O2]t=0 ~18 mmol L–1; redrawn from Thomanetz (1999): p. 67.

tion may be hindered considerably. As an example, the •OH radical induced destruction of ethanol (C2H6O) through acetaldehyde and acetic acid to the products of acetic acid destruction requires approximately 40 (!) distinct reactions, in addition to those needed for the generation of •OH radicals (Peyton, 1996). 7.1.2

Reaction and Kinetic Modeling of H2O2-UV AOPs

The implications of the versatile reaction mechanisms depicted in Figs. 7-1 to 7-4 are profound with respect to the complete understanding and hence to the kinetic modeling of AOPs. Despite the complexity of these photo-initiated reactions, it is possible to model AOPs with sufficient precision if all the rate constants of •OH radical reactions involved and those of all other elementary reactions are known (Crittenden et al., 1999). Most importantly, the structures and the concentrations of all intermediary reaction products must be known. In addition, photoreactor specific parameters have to be included, such as the incident photon flow Up and the dimensions of the irradiated volume. This task can be achieved for example

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

The level of reactive intermediates in •OH radical based AOPs. RH: Aliphatic or aromatic hydrocarbon substrate, ET: electron transfer to a suitable acceptor molecule. Fig. 7.3

by application of artificial neural networks for the modeling of homogeneous liquid phase photochemical systems (Do Nascimento et al., 1994). The identification and quantification of substrate [M] and product [Mox i ] concentrations is usually performed by standard analytical procedures using modern spectroscopic, spectrometric or chromatographic methods. For example, gas chromatography (GC) or high performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) are particularly suited to detect and identify intermediary oxidation products of AOPs. However, in water chemistry it is extremely difficult and laborious to characterize and quantify all by-products that may also include polar intermediates. This is especially true for the identification of trace amounts of organic acids or of highly hydroxylated intermediates. For the quantification of organic acids ion-exchange chromatography (IC) may be a suitable method. The difficulties related to the elaboration of a complete reaction scheme of an AOP were demonstrated by Stefan et al. (1996) who investigated the oxidative de-

7.1 Aqueous Systems

Fig. 7.4

The level of oxidation products in H2O-VUV and H2O2-UV AOPs.

On the way to complete mineralization of acetone (2-propanone): structures of the main and minor intermediate products during H2O2-UV treatment of a diluted aque-

Fig. 7.5

ous solution of acetone, according to Stefan and Bolton (1999). The abbreviations identify the intermediates presented in Figures 7-6 and 7-7.

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Decay of acetone and hydrogen peroxide and the time profiles of main intermediates during the H2O2-UV AOP of a diluted aqueous solution of acetone (2-propanone). AA: Acetic acid, PA: pyruvic acid, PAL: pyruvaldehyde, OA: oxalic acid. Reactor specification: Rayox photoreactor with a Solarchem Fig. 7.6

1 kW MP Hg lamp (photon flow Up = (2.93 ± 0.08) × 10–4 mol s–1 from k of 200 nm to 300 nm), total volume V = 28 L circulated at a flow rate of V˙ = 110 L min-1 between the batch tank and the photoreactor (V = 6.5 L); redrawn from Stefan and Bolton (1999): Figure 1.

gradation and mineralization of the simple organic compound acetone (2-propanone, C3H6O) in water using the H2O2-UV method (Fig. 7-5). In addition, the authors presented a comprehensive kinetic model that describes the complete oxidation and mineralization of acetone. They proposed mechanisms for the breakdown of the acetone structure including the formation and degradation pathways of all intermediary products. Perhaps, for the first time in AOP research, this excellent work led to a calculated concentration profile of reactants and intermediary products that was in good agreement with the experimental data, including TOC diminution toward total mineralization of all intermediary organic water components. However, a thorough reinvestigation of this system several years later revealed the previously undetected intermediates pyruvic acid (PA), pyruvaldehyde (PAL) and hydroxyacetone (Stefan and Bolton, 1999). Thus, after these findings had been made the kinetic model could be perfected considerably. Besides CO2 as the final product of mineralization, four main products (pyruvaldehyde, pyruvic acid, acetic acid and oxalic acid) and four minor products (formaldehyde, formic acid, glyoxylic acid and hydroxyacetone) were identified and quantified during the course of the H2O2-UV AOP of a diluted solution of acetone in water (Fig. 7-5). The time–concentration profiles of the main reaction products are shown in Fig. 7-6 and those of the minor products in Fig. 7-7. These profiles clearly demon-

7.1 Aqueous Systems Time profiles of minor intermediates during the H2O2-UV AOP of a diluted aqueous solution of acetone (2-propanone). GA: Glyoxylic acid, FAL: formaldehyde, FA: formic acid, HA: hydroxyacetone. Reactor specification is identical to that of Figure 7-6; redrawn from Stefan and Bolton (1999): Figure 2.

Fig. 7.7

strate the intermediary build-up of significant concentrations of oxidation and degradation products in accord with Figs. 6-16 and 7-4. The H2O2 photolysis follows zero-order kinetics over the first 30 min of irradiation, whereas the degradation kinetics of acetone can be described by first-order reaction kinetics (Stefan et al., 1996). Oxalic acid seems to be generated at a later stage of the photo-initiated oxidation. Its formation is slow and its maximum concentration is reached after acetone has already been oxidized completely (at t ~ 55 min). On the other hand, the fast generation of minor degradation products during the early beginning of the process and their subsequent oxidation (Fig. 7-7) demonstrate that the by-products compete efficiently with acetone for hydroxyl radicals. This effect would explain the slowness of acetone treatment by H2O2-UV (Stefan et al., 1996). Owing to the formation of intermediary organic acids the pH of the solution dropped rapidly from 6.0 (t = 0 min) to 3.6 (t = 30 min) and then the pH increased slowly to its final value of 5.0. This indicates the complete mineralization of all organic compounds with formation of carbon dioxide and water as the sole reaction products (cf. Fig. 6-15 for the carbon dioxide/water system). Typically, the TOC diminution does not start immediately at the beginning of the process but after a delayed period of ca. 10 min (Fig. 7-8). This is due to initial • OH radical consumption for the production of intermediary oxidation products, which is shown by the immediate build-up of hydroxyacetone within the first 10 min of irradiation. Most interestingly, this pragmatic work revealed an almost perfect correlation between the experimentally determined diminution of TOC concentration and the calculated values. The TOC concentrations as a function of time were calculated by adding up the TOC contributions of acetone and of all individual intermediary products. The good TOC balance between calculated and experimental values (Fig. 7-8) indicates that no other major intermediates were formed during H2O2-UV treatment of the aqueous acetone solution. Thus, the

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air Fig. 7.8 Experimental (solid line) and calculated (dotted line) TOC profiles during the H2O2-UV treatment of a diluted aqueous solution of acetone (2-propanone); [TOC]calculated = R[TOCacetone] + [TOCintermediates]) at irradiation time t; redrawn from Stefan and Bolton (1999): Figure 3.

mechanistic considerations are in total agreement with experimental details. Despite of the fact that acetone is a relatively simple structured molecule its photo-initiated mineralization to yield CO2 and H2O needs over 50 (!) individual reactions, reactions of H2O2 photolysis not being included (see Stefan and Bolton, 1999). Further highlights of H2O2-UV AOP reaction modeling from this research group include thorough investigations of the oxidative degradation and mineralization of 1,4-dioxane (Stefan and Bolton, 1998) and of methyl-tert-butyl ether (MTBE) (Stefan et al., 2000). 1,4-Dioxane is a water-soluble organic compound and an EPA priority pollutant widely used as a solvent in industry. It forms an azeotropic mixture with water consisting of 81.6% 1,4-dioxane at a boiling point of 87.8 8C and it is often found in contaminated ground waters and industrial effluents. Common water purification techniques are not applicable to remove 1,4dioxane. Therefore, contaminated waters are purified by distillation (Stefan and Bolton, 1998). MTBE is a gasoline additive that was introduced to meet clean air standards. Unfortunately, MTBE concentrations up to 1 mg L–1 have been detected in shallow ground water located in several areas in the United States due to contamination by leaking underground fuel storage tanks. The intermediary products of the H2O2-UV treatment of 1,4-dioxane are listed in Fig. 7-9. A set of intermediary aldehydes, organic acids and esters could be identified. Again, the authors proposed a complete mechanistic scheme explaining the formation of the observed intermediate products and their time profiles toward total mineralization. A satisfactory TOC balance of calculated and experimental values was obtained during the 1,4-dioxane treatment indicating the complete mineralization of all intermediates after prolonged H2O2-UV treatment. In a similar way a series of C5 to C1 intermediate products were identified during the H2O2-UV treatment of MTBE in aqueous solution (Tab. 7-1). This list of compounds demonstrates again the extraordinary complexity of reactions underly-

7.1 Aqueous Systems

Fig. 7.9 Route to complete mineralization of 1,4-dioxane (C4H8O2) by H2O2-UV treatment: list of identified intermediary reaction products (Stefan and Bolton, 1998).

ing AOPs. Further, it demonstrates the importance of thorough product studies with respect to environmental and health risks related to degradation products of photo-initiated AOPs. A detailed reaction mechanism for the degradation of MTBE was proposed that includes the elucidation of the reaction pathways toward the formation of all by-products. Tert-butyl formate (TBF) is a major primary product. This was also shown by Chang and Young (2000) who emphasized this finding with respect to the unknown human health impacts of residual amounts of this compound. However, the •OH radical induced oxidation of MTBE finally led to the mineralization of all intermediary products (including TBF) after prolonged irradiation and a satisfactory TOC balance could be established (Stefan et al., 2000). Sörensen et al. (1998) elaborated another interesting example of reaction modeling by identification and balancing of intermediates during the H2O2-UV treatment of ethylendiaminetetraacetate (EDTA). The complex scheme of intermediary degradation products is presented in Fig. 7-10. The oxidative degradation of EDTA led to the total diminution of DOC via formation of the organic intermediates iminodiacetate (IMDA), glycinate, oxamate, glyoxylate, oxalate, and formate, and finally the inorganic products of mineralization carbon dioxide, ammonium ions, nitrate, nitrite and cyanate ions were formed. The organic degradation products ethylenediaminetriacetate (ED3A), ethylenediaminediacetate (EDDA) and ethylenediaminemonoacetate (EDMA) were only found in the presence of iron(III) ions. The oxidation state of Fe3+ changed during irradiation in the presence of hydro-

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air Tab. 7.1 Intermediate degradation products a) of methyl-tert-butyl ether (MTBE, (CH3)3C-O-CH3, C5H12O) during H2O2-UV treatment, finally leading to total mineralization b) (Stefan et al., 2000)

Characterization

Compound

Formula

C5 Products

tert-Butyl formate (TBF) 2-Methoxy-2-methyl-propionaldehyde (MMP) tert-Butyl alcohol (TBA) Hydroxy-iso-butyraldehyde Hydroxy-iso-butyric acid iso-Butyraldehyde Acetone Methylacetate Hydroxyacetone Pyruvaldehyde Pyruvic acid Acetic acid Oxalic acid Formaldehyde Formic acid

(CH3)3C-O-CHO H3CO(CH3)2C-CHO (CH3)3C-OH HO(CH3)2C-CHO HO(CH3)2C-COOH (CH3)2CHCHO H3CCOCH3 H3CCOOCH3 HOCH2COCH3 H3CCOCHO H3CCOCOOH CH3COOH HOOC-COOH HCHO HCOOH

C4 Products

C3 Products

C2 Products C1 Products

a) MTBE, TBF, TBA, acetone, methyl acetate and hydroxyacetone were quantitatively analyzed using capillary gas chromatography. Organic acids were analyzed by ion chromatography. Aldehydes and ketones were derivatized according to standard procedures and identified by GC-MS. No organic peroxides such as tert-butyl hydroperoxide [(CH3)3C-OOH)] or di-tert-butyl peroxide [(CH3)3C-OOC(CH3)3] were detected. b) the TOC balance confirmed that all organic material was mineralized after prolonged H2O2-UV treatment.

gen peroxide due to photoreduction to Fe2+ as has already been pointed out in the discussion of photo-Fenton reactions (cf. Fig. 5-9 and 5-14). Again, a good agreement between the calculated and the measured DOC values could be established. In addition, the nitrogen balance of this system was determined by taking into account the contributions of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) to the dissolved total nitrogen (DN) (Fig. 7-11). Ammonium, nitrate, nitrite and cyanate ions contribute to the DIN, and the DON represents the organic nitrogen containing compounds (EDTA, ED3A, EDDA, EDMA, IMDA, glycinate, oxamate and ethylene diamine). As is demonstrated in Fig. 7-11 the DON decreased during prolonged irradiation whereas the DIN increased proportionally. Over 90% of the nitrogen containing compounds of this complex photoreaction system could be assigned. The DOC and DN balances convincingly demonstrate that all major intermediates were detected quantitatively. So, reaction (and kinetic) modeling of AOPs is a laborious and difficult task that needs experience not only in water analytical techniques but also in photochemical mechanisms and the underlying photophysical principles. However, sophisticated and comprehensive investigations such as those discussed above undoubtedly contribute to an increase in our knowledge of AOP mechanisms. Crittenden et al. (1999) gave a short overview of kinetic modeling of AOPs. For the removal of a specific contaminant M, such a model usually involves a “com-

7.1 Aqueous Systems

Fig. 7.10 Scheme of the mineralization of ethylenediaminetetraacetate (EDTA) via a series of degradation products in the H2O2-UV AOP in the presence (left pathway) and absence (right pathway) of Fe3+/Fe2+. For simplicity, dissociation equilibria of corresponding

acids have been neglected. ED3A: ethylenediaminetriacetate, EDDA: ethylendiaminediacetate, EDMA: ethylenediaminemonoacetate, IMDA: iminodiacetate, GA: glyoxylate, OA: oxalate, FA: formate (according to Sörensen et al., 1998).

petition function” FM,ox (Peyton, 1996) that is given by the relation of the rate of reaction of •OH with M to the sum of all scavenging reaction rates (Si represents scavenger molecules, such as HCO–3, Mox i , etc.) (Equation 7-1). FM;ox

rate of reaction of OH with M k OH;M M P P rates of OH reactions i k OH;Si Si

(7-1)

Most kinetic models are based on the steady-state approximation that assumes the net formation rate of •OH radicals (and of other reactive species) to be zero and that their steady-state concentration is very low. Bolton et al. used (2001) this model to elaborate design parameters (figures-of-merit) which are very useful for the development and comparison of different AOPs (see Chapter 8.5).

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.11 Nitrogen balance during the H2O2-

UV treatment of EDTA (C10H16N2O8). DN: dissolved total nitrogen, DIN: dissolved inorganic nitrogen, DON: dissolved organic nitrogen. according to DN = Pn DN was calculated MN i1 ci nN Mi , with M(N) =

14.007 g mol–1: atomic mass of nitrogen, M(i): molar mass of compound i, c: concentration of compound i in mol L–1, n(N): number of N-atoms in compound i, i: index of compound; redrawn from Sörensen et al. (1998): Figure 6.

However, in some cases this concept does result in significant errors (Crittenden et al., 1999). Therefore, Crittenden et al. (1999) developed a dynamic kinetic model for the H2O2-UV AOP in a completely mixed batch reactor to predict the destruction of organic contaminants. The latter model includes known rate constants of all participating elementary radical reactions and it considers the change of pH during the AOP. This advanced oxidation model (ADOx) uses a powerful ordinary differential equation solver. It should be able to predict the degradation of organic pollutants and the consumption of H2O2. Furthermore, the model should help to optimize important process variables, such as dependencies on the initial H2O2 concentration and on the incident UV photon flow Up. The complete set of radical rate constants and of other parameters, such as pKA values, and of elementary reactions in the H2O2-UV AOP system is given by Crittenden et al. (1999). This model was validated and tested using the photo-initiated oxidation of the saturated halogenated aliphatic compound, 1,2-dibromo-3-chloropropane (DBCP) in very dilute aqueous solution over an initial concentration range of [DBCP] between 1.42 and 3.06 lmol L–1. 7.1.3

Vacuum-UV Oxidation: The H2O-VUV AOP

Unlike the H2O2-UV process the VUV oxidation of organic substrates in water (H2O-VUV) is initiated by the photochemical homolysis of water producing a series of reactive species (cf. Fig.s 5-9 and 7-3, and Equations 5-18 to 5-20). In many cases water itself is the prominent absorber of VUV radiation at 172 nm (cf. Ex-

7.1 Aqueous Systems

ample 7-1) as a major consequence of its high molar concentration of [H2O] = 55.39 mol L–1 (q = 0.9982 kg L–1 at T = 20 8C) and of its molar absorption coefficient ek ~10 L mol–1 cm-1 (Tab. 6-2) giving a high decadic absorption a of ca. 550 cm–1 (cf. Chapter 3-9). Thus, the penetration depth of 172 nm radiation into water is very low (cf. Example 3-9) and total absorbance (A172 10 ≥ 2) of radiation occurs within an aqueous layer with a thickness of less than 0.1 mm. Example 7-1: The (decadic) absorption coefficient of absolute methanol a(CH3OH) at k of 172 nm is about 4000 cm–1 (Weeks et al., 1963). Calculate the portion of 172 nm radiation that is absorbed by methanol in an aqueous solution containing 0.11 mol L–1 of methanol (with [CH3OH]pure = 24.56 mol L–1, M = 32.04 g mol–1, q = 0.787 kg –1 172 L–1, then e172 cm–1); refer to Example 3-9. CH3 OH = aCH3 OH /[CH3OH]pure = 163 L mol A172 CH3 OH

e172 CH3 OH CH3 OH 172 eCH3 OH CH3 OH aH2 O

100

163 L mol 1 cm 1 0:11 mol L 1 163 L mol 1 cm 1 0:11 mol L 1 550 cm

1

100 3:2%

In the H2O-VUV AOP no additional auxiliary oxidants must be used to produce sufficient amounts of •OH radicals. The technical development of incoherent excimer VUV and UV sources of electromagnetic radiation (see Chapter 4.3) led to a renaissance of research related to water photolysis. Hence, the experimental conditions and results of several recent investigations that deal with the applications of VUV and UV incoherent excimer lamps in the field of AOP research (mainly in aqueous media) are briefly summarized in Tab. 7-2. Owing to the relatively high quantum yield of water homolysis at k of 172 nm (U172 H2 O = 0.42, Tab. 6-4), to the low penetration depth of the 172 nm radiation in water (see Example calculation 3-9), and to the short lifetime of the hydroxyl radicals (Chapter 6.7.3), high local concentrations of hydroxyl radicals ([•OH] >10–4 mol L–1, Lopez et al., 2000) are produced at the quartz–water interface by 172 nm irradiation of water using xenon excimer lamps. This leads to a strongly marked heterogeneity of the H2O-VUV process as is demonstrated in Fig. 7-12. Heit and Braun (1997, 1996) verified this phenomenon experimentally. In a well-designed experiment, these authors measured the spatial resolution of dissolved molecular oxygen concentrations during VUV photolysis of aqueous systems using a miniaturized oxygen optode. The optode was coupled with a micrometer screw and was mounted perpendicularly to the surface of the xenon excimer lamp. Consecutive concentration measurements of dissolved molecular oxygen during the VUV AOP at distance intervals of 0.1 mm, established a distinct O2 concentration profile as a function of the radial distance from the lamp’s surface. The results of this interesting work confirm the formation of a very thin photoreaction zone at the Suprasil quartz/water interface. This zone is characterized by diffusion controlled reactions of short-lived primary radicals (mainly •OH) with organic substrates. Thus, relatively long-lived carbon centered radicals are produced. The trapping of the lat-

201

hm hm 100 142 155 hm hm hm hm 155 105 105 105 960 984 1020 190 hm

Xe2* Xe2* + KrCl* KrCl* Xe2* Xe2* Xe2*, KrCl* Xe2* Xe2* Xe2* Xe2* XeCl* + TiO2 Xe2* XeCl* + TiO2 Xe2* Xe2* KrCl* + H2O2 Cl2* + H2O2 Xe2* KrCl* + H2O2 KrCl* + H2O2 Xe2*, SME KrCl* + H2O2

1,1,1-Trichloroethane 1,1,1-Trichloroethane 1,1,1-Trichloroethane 1,1,2-Trichloroethane (ground water sample) 1,1,2-Trichlorotrifluoroethane 1,2,4-Trichlorodibenzo-p-dioxin 1,2-Dichloroethane 1,2-Dichloroethene (ground water sample) 1,2-Ethanediol 1-Butanol

2,4-Dichlorophenol 2-Amino-1-naphthalenesulfonate

2,4-Dichlorophenol

2,4-Dichlorophenol

2,4-Dichlorophenol 2,4-Dichlorophenol

1-Butylamine

150 300 150 142

Excimer light source a)

Substrate

Pel/W b)

5 n.s.

1 Gas Gas Gas Gas 1 1 1 1 20 20 20 phase phase phase phase

Gas phase 3–4 mL 3 3

2.95 2.95 2.95 3

Irradiated volume c) V/L

References

TOC GC, IR, p.s. GC, IR, p.s. GC GC TOC TOC TOC TOC TOC TOC TOC

GC HPLC, GC/MS, p.s. TOC, [Cl–] GC/MS, p.s.

(1995 a) (1995 a) (1995 a) (1995)

Oppenländer and Waizmann (1997) Oppenländer et al. (1996) Sörensen and Frimmel (1997), Sörensen et al. (1996)

Oppenländer (1998)

Oppenländer and Gliese (2000) Oppenländer (1997a), Baum and Oppenländer (1995)

Benoit-Marquie et al. (1999)

Oppenländer and Gliese (2000) Benoit-Marquie et al. (1999)

Loraine (1993) Nohr et al. (1994) Wörner et al. (1998) Oppenländer and Baum (1995 a)

TOC, GC/MS, IC: [Cl–] Oppenländer and Baum GC/MS, IC: [Cl–] Oppenländer and Baum GC/MS, IC: [Cl–] Oppenländer and Baum GC/MS, p.s. Baum and Oppenländer

Analytical method e)

95 TOC, HPLC 0.01 mmol L–1 HPLC

– 5 5 5 5 – 100 100 100 – – ~100

– – n.s. 20

24.9 17.7 48.0 1

[co] d)/mg L–1

Tab. 7.2 Selected examples of applications of incoherent excimer lamps in photo-initiated AOPs in aqueous and gaseous media

202

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Pel/W b)

hm 52 hm 150 hm 155 150 150 100 2000 155 155 80

150 150 n.s. 150 1000 hm 200 hm hm 105

Excimer light source a) Xe2*, KrCl* Xe2* XeCl* Xe2* KrCl* + H2O2 Xe2* Xe2* Xe2* + O3 Xe2* Xe2* Xe2* Xe2* KrCl*

Xe2* Xe2* KrCl* KrCl* Xe2* KrCl* + H2O2 Xe2* KrCl* + H2O2 KrCl* + H2O2 Xe2*

Substrate

2-Chlorodibenzo-p-dioxin 2-Propanol 3,4-Dimethoxynitrobenzene 3-Amino-5-methylisoxazole 4,4'-Diaminostilbene-2,2'-disulfonate 4-Chlorophenol

4-Chlorophenol 4-Chlorophenol 4-Chlorophenol 4-Chlorophenol Acetic acid Alcohols f) (C1–C8) Atrazine

Atrazine Azo dyesg) Bromate ion (BrO–3) Chlorinated ethenes h) Cyclopropane Diphenyl-4-sulfonate Dissolved molecular oxygen EDTA EDTA FeIII-EDTA

Tab. 7.2 (continued)

0.220 0.8 n.s. Gas phase Gas phase n.s. 1 n.s. n.s. 5

0.750 3 3 n.s. 1 1 5

3–4 mL Gas phase 3–4 mL 0.220 n.s. 1

Irradiated volume c) V/L

– – n.s. 175 n.s. 0.01 mmol L–1 – 0.034 mmol L–1 0.034 mmol L–1 75

64 64,3 n.s. 30 – – 10

– 400 25 49 0.01 mmol L–1 –

[co] d)/mg L–1 References

Nohr et al. (1994) Falkenstein (1997) Zhang et al. (1999) Gonzalez et al. (1995) Sörensen and Frimmel (1997) Oppenländer and Gliese (2000), Jakob et al. (1993) HPLC, TOC, IC, p.s. Gonzalez et al. (1995) HPLC, TOC, UV/VIS Hashem et al. (1997) [Cl–] Wörner et al. (1998) TOC, COD Kreysig et al. (1996) TOC Oppenländer and Gliese (2000) TOC Oppenländer and Gliese (2000) GC/MS, HPLC, p.s. Rehberg et al. (1994) [NO–3], [NO–2], [NH+4 ], [Cl–] TOC, HPLC, p.s. Gonzalez et al. (1994) TOC, UV/VIS Oppenländer et al. (1995) n.s. Metzger and Falkenstein (2001) FT-IR, p.s. Prager and Hartmann (2001) GC Gassiot et al. (1998) HPLC Sörensen and Frimmel (1997) OSO Heit and Braun (1996), (1997) HPLC Sörensen and Frimmel (1997) HPLC Sörensen et al. (1996) TOC, pH, [O2], ICP: Oppenländer (1997 a) [Fen+]

HPLC, GC/MS, p.s. GC/MS, p.s. UV/VIS HPLC, IC, DOC, p.s. HPLC TOC

Analytical method e)

7.1 Aqueous Systems 203

KrCl* + H2O2 Xe2* KrCl* Xe2* + KrCl* Xe2* Xe2* Xe2* KrCl* Xe2* Xe2* Xe2* Xe2* Xe2* XeCl* Xe2* + KrCl* Xe2* Xe2* Xe2* Xe2* Xe2* Xe2* Xe2* XeCl* + TiO2 Xe2*

Fe-EDTA Formaldehyde

Phenol (+ NO–3)

Ground water: PAH Cyanide (CN–) Hexacyanoferrate(II) i) Hexacyanoferrate(III) i) Humic acid Hydrogen bromide Landfill leachate Methanol Methanol (+ NO–3) Oxygen Oxygen Phenol Phenol Phenol

Formic acid Galvanic waste water Greywater (disinfection)

Excimer light source a)

Substrate

Tab. 7.2 (continued)

hm 150 150 300 155 150 150 150 1500 1500 hm hm 150 8.7 300 200, hm 150 hm 150 155 150 hm hm 150

Pel/W b)

n.s. 0.8 0.8 0.8 1 4 4 4 4000 4000 0.17 0.17 0.350 Gas phase 4 1 0.220 n.s. Gas phase 1 Gas phase Gas phase Gas phase 0.220

Irradiated volume c) V/L

Analytical method e)

HPLC [HCHO] TOC TOC TOC TOC, [Ni2+] UV/VIS, TOC, COD CFU, COD, TOC [PAH] [CN–], pH UV/VIS UV/VIS TOC, ASV UV/VIS TOC GC, HPLC, DOC, p.s. GC, IC, DOC, p.s. UV/VIS UV/VIS TOC TOC GC GC HPLC, IC, DOC, GC, UV/VIS, p.s.

[co] d)/mg L–1

0.034 mmol L–1 30 – – – – – – 0.09 ppb 1.744 5 mmol L–1 5 mmol L–1 500 n.s. – ~8000 135 n.s. 0.8 bar – 1 1 71

Gonzalez and Braun (1996 b)

Laszlo and Dombi (2002) Laszlo and Dombi (2002) Grichetschkina et al. (1996) Pfeifer Roland et al. (1998) Oppenländer and Baum (1994) Heit et al. (1998) Gonzalez and Braun (1996 a) Laszlo et al. (1998) Hashem et al. (1997) Oppenländer and Gliese (2000) Oppenländer and Fradl (1999) Benoit-Marquie et al. (1999)

Oppenländer et al. (1997)

Oppenländer and Gliese (2000) Oppenländer and Baum (1995 b) Oppenländer and Baum (1996)

Sörensen and Frimmel (1997) Oppenländer et al. (1995)

References

204

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

4 4 20 5 0.220 2.95 3 gas phase gas phase n.s. Gas phase 4 3 2.95 3

150 300 1500 190 150 150 142 52 hm hm 1000 150 100 150 150

Xe2* Xe2* + KrCl* Rhodamine B Xe2*, SME Rhodamine B Xe2*, SME Sodium nitrite (NaNO2): NO–2 Xe2* Tetrachloroethene Xe2* Tetrachloroethene (ground water sam- Xe2* ple) Tetrachloromethane Xe2* Tetrachloromethane Xe2* Tetrachloromethane Xe2* Tetrahydrofurane (THF) Xe2* Textile waste water Xe2* Trichloroacetic acid Xe2* Trichloroethene Xe2* Trichloroethene (ground water sample) Xe2*

Rhodamine B

Rhodamine B

Rhodamine B

0.220 0.220 1 1 1 1 20 20 20

150 150 155 105 105 105 960 984 1020

Xe2* Xe2* Xe2* Xe2*, KrCl* Cl2*, XeCl* I2* (+ TiO2) Xe2* KrCl* + H2O2 KrCl* + H2O2

Irradiated volume c) V/L

Potassium cyanide (KCN): CN– Potassium nitrate (KNO3): NO–3 Propanone (acetone) Rhodamine B

Pel/W b)

Excimer light source a)

Substrate

Tab. 7.2 (continued)

326 – 23 2000 – n.s. 21 103

11 11 10 8 17 8 0.4

33 20 – 10 10 10 10 10 10

[co] d)/mg L–1

GC/MS, p.s. GC GC TOC, [CO2], p.s. TOC, UV/VIS [Cl–] TOC, GC/MS, IC GC/MS, [Cl–], p.s.

UV/VIS TOC UV/VIS UV/VIS IC, GC, UV/VIS, p.s. TOC, GC/MS, IC GC/MS, p.s.

IC, p.s. IC, GC, UV/VIS, p.s. TOC UV/VIS UV/VIS UV/VIS UV/VIS UV/VIS UV/VIS

Analytical method e)

Falkenstein (1997) Loraine (1993) Loraine (1993) Gassiot et al. (1998) Oppenländer and Baum (1995 b) Wörner et al. (1998) Oppenländer and Baum (1995 a) Baum and Oppenländer (1995)

Oppenländer et al. (1997) Oppenländer et al. (1996) Gonzalez and Braun (1995) Oppenländer and Baum (1995 a) Baum and Oppenländer (1995)

Oppenländer and Waizmann (1997) Oppenländer and Baum (1994)

Oppenländer (1998)

Gonzalez et al. (1995) Gonzalez and Braun (1995) Oppenländer and Gliese (2000) Oppenländer (1997 a)

References

7.1 Aqueous Systems 205

52 hm 150 hm 150 hm

Xe2* KrCl* Xe2* Xe2* Xe2* KrCl*

Trichloroethene Trichloroethene Trichloromethane Trichloromethane Trichloromethane Uridine (1-b-D-ribofuranosyl-uracil)

Gas phase Gas phase 2.95 Gas phase Gas phase 3–4 mL

Irradiated volume c) V/L 190 0.375 26.1 – 1000 0.03

[co] d)/mg L–1

GC/MS, p.s. GC/MS, p.s. GC/MS, IC GC IR, GC, GC/MS, p.s. UV/VIS

Analytical method e)

Falkenstein (1997) Scheytt et al. (1992) Oppenländer and Baum (1995 a) Loraine (1993) Oppenländer (1997 b) Zhang et al. (1997)

References

a) Incoherent excimer lamps applied (cf. Table 4-2): Xe2* (kmax = 172 nm), KrCl* (kmax = 222 nm), Cl2* (kmax = 259 nm), XeCl* (kmax = 308 nm), I2* (kmax = 342 nm). b) electrical input power of the excimer lamps. c) irradiated volume of aqueous phase. d) initial concentration of substrate in mg L–1 if not stated in mmol L–1 or ppb. e) abbreviations, ASV: anodic stripping voltammetry; CFU: colony forming units, microbiological determination; COD: chemical oxygen demand; DOC: dissolved organic carbon; FT-IR: Fourier-transform IR spectroscopy; GC/MS: GC/mass spectromertry; GC: gas-chromatography; HPLC: high performance liquid chromatography; hm: actinometry or photon flow stated; IC: ion chromatography; ICP: inductively coupled plasma; IR: infrared spectroscopy; n.s.: not stated; OSO: oxygen sensitive optode; p.s.: product study; PAH: polycyclic aromatic hydrocarbons; SME: static mixer electrode; TOC: total organic carbon; UV/VIS: ultraviolet/visible spectroscopy. f) primary alcohols: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol; secondary alcohols: 2-propanol, 2-butanol, 2-hexanol, cyclohexanol; tertiary alcohol: 2-methyl-2-propanol (t-butanol). g) azo dyes: naphthole blue black B, orange 1, orange G. h) perchloroethene (PCE), trichloroethene (TCE), 1,2-dichloroethenes (DCE), vinyl chloride (VC) and saturated chlorinated hydrocarbons (CHC). i) dissolved molecular oxygen strongly influences the redox properties of these systems due to scavenging of H• and eaq–.

Pel/W b)

Excimer light source a)

Substrate

Tab. 7.2 (continued)

206

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

7.1 Aqueous Systems

Fig. 7.12 Heterogeneity of the H2O-VUV pro-

cess: generation of different reaction zones during 172 nm irradiation of water containing dissolved organic matter. Total oxygen depletion at the Suprasil quartz/water interface was

confirmed by measurements using a very small oxygen optode (Heit and Braun, 1997). The thickness l of the photoreaction zone was calculated according to Example 3-9.

ter by dissolved molecular oxygen leads to a pronounced zone of oxygen consumption (Fig. 7-12) that extends to a distance l of about 0.5 mm from the lamp’s surface, i. e. deep into the “dark” zone, which is not reached by 172 nm radiation. Within the zone of O2 consumption the slower thermal chemistry of peroxyl radicals dominates (cf. Fig. 7-1: chain II). Further, a zone of oxygen saturation follows. Additionally, the spatially resolved oxygen measurements indicated that the oxygen depletion is of larger radial extension at lower flow rates V˙. Hence, an efficient radial mixing into the photoreaction zone leads to a significant reduction of the thickness of the zone of O2 consumption and therefore to an enhanced oxidation of organic substrates. This can be achieved either by applying higher flow rates V˙ generating turbulent flow conditions or by using static mixers (in the case of H2O-VUV) to transport dissolved molecular oxygen effectively from the zone of O2 saturation into the zone of O2 consumption (e. g. Oppenländer et al., 1996). Several examples selected from the current research literature will demonstrate the enormous potential of the H2O-VUV AOP for oxidizing and mineralizing organic water contaminants. The photo-initiated degradation of a mixture of saturated chlorohydrocarbons (trichloromethane, CHCl3, and 1,1,1-trichloroethane, CH3-CCl3) and of unsaturated chlorohydrocarbons (trichloroethene, Cl2C=CClH and tetrachloroethene, Cl2C=CCl2) in water by VUV oxidation with a Xe2* excimer flow-through lamp

207

208

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.13 172 nm irradiation of a mixture of trichloromethane ([CHCl3]t=0 = 26.1 mg L–1), 1,1,1-trichloroethane ([Cl3C-CH3]t=0 = 23.2 mg L–1), trichloroethene ([Cl2C=CClH]t=0 = 21.0 mg L–1) and tetrachloroethene ([Cl2C=CCl2]t=0 = 8.0 mg L–1) in water with a Xe2* excimer flow-through photoreactor. Conditions: photoreactor see Figure 4-16, one lamp was used with Pin el (lamp) = 150 W, irra-

diated volume V = 2.95 L, recycling flow rate V˙ = 14 L min–1 between batch tank and excimer flow-through lamp. The phase of stabilization of the mixture (before irradiation) established that the photoreactor system was totally sealed to avoid any evaporation of the volatile chlorohydrocarbons; redrawn from Oppenländer and Baum (1995 a).

(Fig. 7-13) led to a chloride ion mass balance of 80% (Oppenländer and Baum, 1995a). This work confirmed the slower VUV oxidation of saturated chlorohydrocarbons compared with the fast VUV oxidation of unsaturated derivatives. This difference in reaction behavior can be predicted from the higher second order reaction rate constants of the latter with hydroxyl radicals (c.f. Fig. 6-24 and Tab. 6-6). The 222 nm photolysis of the saturated compound 1,1,1-trichloroethane dissolved in water with a KrCl* excimer flow-through lamp led to an even worse Cl– yield (49% after 180 min of irradiation) and the elimination of the substrate was still incomplete (Oppenländer and Baum, 1995a). The incomplete Cl– mass balance at 222 nm irradiation of saturated chlorohydrocarbons in water seems to be related to the fate of the chlorine radical Cl• (Loraine and Glaze, 1999). In water, the chlorine atom rapidly equilibrates in the presence of Cl– with formation of the dichloride radical anion (Cl•– 2 ) according to Equation 7-2. The reaction manifold of the Cl•/Cl•– 2 system was studied by Alegre et al. (2000). However, many questions related to the significant deficit in the Cl– balance during H2O-VUV and H2O2-UV treatment of saturated chlorohydrocarbons in aqueous solution are not yet answered.

7.1 Aqueous Systems

Fig. 7.14 Oxidation and mineralization of 2,4dichlorophenol (2,4-DCP) in aqueous solution by the H2O-VUV AOP using a Xe2* excimer flow-through photoreactor: evolution of 2,4DCP, TOC and chloride ion concentrations. Conditions: photoreactor see Figure 4-16, Pin el (lamp) = 142 W, irradiated volume V = 2 L,

Cl• + Cl– Cl•– 2

recycling flow rate V˙ = 14 L min-1 between batch tank and excimer flow-through lamp, [2,4-DCP]t=0 = 79.5 mg L–1, [TOC]t=0 = 36.1 mg L–1; these are experimental values (cf. Example 5-1 for calculated TOC concentration); redrawn from Baum and Oppenländer (1995).

(7-2)

On the other hand, the VUV irradiation of a ground water sample that had been contaminated with the unsaturated chlorohydrocarbons cis-1,2-dichloroethene ([ClHC=CClH] = 20 mg L–1), trichloroethene ([Cl2C=CHCl] = 103 mg L–1) and tetrachloroethene ([Cl2C=CCl2] = 0.4 mg L–1) was investigated by Baum and Oppenländer (1995). In addition, this water sample contained a matrix of carbonate species ([C] = 600 mg L–1) and of Cl– ions ([Cl–] = 30 mg L–1). The chlorohydrocarbons were oxidized totally and the Cl– balance ([Cl–]max = 98 mg L–1) reached 93% after 180 min of irradiation. During the first 10 min of VUV irradiation of this water sample the appearance of 1,1,2-trichloroethane (Cl2HC–CClH2) as an intermediate reaction product was detected by gas chromatography/mass spectrometry (GCMS) analysis. Its concentration reached a maximum of about 1 mg L–1 after 30 min of VUV irradiation followed by its subsequent VUV-initiated degradation. The VUV-initiated oxidation of 2,4-dichlorophenol (2,4-DCP) led to an excellent Cl– mass balance of 98% and to the diminution of the total organic carbon content (TOC) of the solution (Fig. 7-14). Clearly, the TOC diminution kinetics was much slower than that of the substrate oxidation. It was shown that the observed kinetics of the TOC degradation of 2,4-DCP was strongly dependent on the electric input power of the Xe2* excimer lamps used (Baum and Oppenländer, 1995).

209

210

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.15 Comparison of the 172 nm radiation-initiated TOC diminution kinetics of a series of homologous n-alcohols (methanol, ethanol, n-propanol, n-butanol, n-pentanol, nhexanol, n-heptanol and n-octanol) at an initial concentration of TOC of 43 ± 5 mg L–1.

Conditions: photoreactor see Figure 4-16, one lamp was used with Pin el (lamp) = 155 W, irradiated volume V = 1 L, recycling flow rate V˙ = 7 L min–1 between batch tank and excimer flow-through lamp; according to Oppenländer and Gliese (2000).

Again, high chloride ion yields were obtained by the VUV-initiated degradation of similar chlorinated organic compounds in aqueous solution (e. g. 1,2-dichloroethane, ClH2C–CH2Cl, Cl– yield = 93%; trichloroacetic acid, Cl3CCOOH, Cl– yield = 98%; and 4-chlorophenol, C6H5ClO, Cl– yield = 99%) using a small Xe2* exin = 20 W) (Wörner et al., 1998). As a consequence the dehalogenacimer lamp (Pel tion of organic halides in aqueous solution by VUV oxidation with miniaturized Xe2* excimer lamps may become an interesting analytical tool for the continuous monitoring of surface and drinking water quality via quantitative Cl– determination as was proposed by Wörner et al. (1998). The efficiency of VUV-initiated mineralization reactions in aqueous solution is strongly dependent on the nature of the organic material. This was demonstrated by a comparative study with a series of TOC degradation experiments using the homologous n-alcohols methanol (CH3OH) to 1-octanol (C8H17OH) as substrates (Oppenländer and Gliese, 2000). Under identical VUV irradiation conditions (using the Xe2* excimer flow-through photoreactor of Fig. 4-16) and starting from similar initial TOC contents of the C1 to C8 alcohol solutions in de-ionized water the substrates were mineralized in a descending order of efficiency from methanol to 1-octanol (Fig. 7-15). In this series, 1-octanol was mineralized with the lowest efficiency followed by 1-heptanol, the n-C3 to n-C6 alcohols, ethanol, and methanol. The last mineralized fastest. This order of mineralization seems to be due to the fact that 1-octanol and 1-heptanol exhibit the highest statistical probability of formation of intermediary oxidation and recombination products, which efficiently compete for •OH radicals. To get an impression of the complexity of the reaction manifold of simple molecules at VUV irradiation in aqueous solution the reaction system of methanol will be described briefly. Heit et al. (1998) comprehensively investigated this system. It was proposed as a new chemical actinometer in 172 nm photochemistry in an

7.1 Aqueous Systems

Fig. 7.16 Simplified reaction manifold of VUV (k = 172 nm) irradiated aqueous solutions of methanol (CH3OH) including direct photolysis of methanol and water photolysis. The second order rate constants are taken from

Heit et al. (1998) or they are listed in Table 6-6 (k2, k3, k4, k5): k1 = 9.7 × 108 L mol–1 s–1, k6 = 2 × 1010 L mol–1 s–1, k7 = 1 × 109 L mol–1 s–1, k8 = 2 × 108 L mol–1 s–1, k9 = 1.9 × 1010 L mol–1 s–1 for the reaction: e–aq + O2 ? O•– 2 .

aqueous phase. The main reactions that occur during VUV irradiation of aqueous methanol solutions are described in Fig. 7-16. Three primary photochemical reactions contribute to the overall mechanism of methanol degradation depending strongly on the special reaction conditions, i. e. on the initial concentration of methanol and on molecular oxygen saturation of the aqueous phase: • the direct 172 nm photolysis of methanol starts to contribute significantly to its degradation at concentrations above 110 mmol L–1 leading to about 4% of absorption by methanol at an initial concentration of 150 mmol L–1 (cf. Example calculation 7-1). Below this concentration of methanol, water molecules almost quantitatively absorb all 172 nm photons. The nature of the primary and secondary products and the quantum yields of photochemical homolysis of methanol at an excitation wavelength k of 184.9 nm are well known (von Sonntag, 1985, von Sonntag and Schuchmann, 1977); • the VUV-induced homolysis of water to produce hydroxyl radicals and hydrogen atoms proceeds with a quantum yield U of 0.42. In oxygen saturated solutions hydrogen atoms are immediately scavenged with formation of hydroperoxyl radicals (Fig. 7-16, reaction 6); • the production of hydrated electrons by VUV photolysis of water contributes only with a low quantum yield U of about 0.05 (cf. Tab. 6-3). Hydrated electrons

211

212

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

are scavenged very fast by oxygen molecules leading to superoxide radical anions that are subsequently protonated in neutral or acidic media to again give hydroperoxyl radicals (Fig. 7-16, reaction 9). This scheme of interrelated primary photochemical and subsequent radical reactions is complicated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH•2) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethanediole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). To use the hydroxyl radical-induced degradation of methanol as a reliable actinometer at k of 172 nm a special concentration window of methanol must be established experimentally (Heit et al., 1998). Thus, a concentration range for methanol must be found in which the degradation rate of methanol is independent of its initial concentration. This sector usually lies between 75 and 250 mmol L–1 of methanol. This domain of methanol concentration ensures that the efficiency gR of hydroxyl radical attack on methanol is 100% leading solely to the formation of hydroxymethyl radicals and water (Fig. 7-16, reaction 1). For methanol concentrations below 75 mmol L–1 gR is lower than 100% due to competing recombination and back reactions of the •OH radicals (Fig. 7-16, reactions 2 and 3). Thus, at low methanol concentrations the rate of methanol degradation increases with increasing initial methanol concentration until a plateau is reached. At this plateau gR is 100%. Under these conditions, the degradation rate of methanol is directly proportional to the incident photon flow Up. Heit et al. (1998) developed a mathematical model that quantitatively describes the degradation kinetics of aqueous methanol under conditions of water photolysis. This model considers the formation of methanol according to reaction 8 (Fig. 7-16) which can be quantified experimentally by analysis of the formaldehyde concentration. At methanol concentrations above the upper limit of the concentration window ([CH3OH] >250 mmol L–1) direct photolysis of methanol contributes significantly and a higher degradation rate of methanol is found. As in case of other actinometric procedures, the conversion of methanol must be kept very low to avoid disturbing photoreactions due to the formation of secondary photo- or radical products. During the 172 nm irradiation of aqueous methanol solutions the formation of the main products 1,2-ethanediole (ethylene glycol), formaldehyde, formic acid and trace amounts of glycolic acid could be established (Heit et al., 1998). Overall, this work demonstrates that it is possible to model such complex reaction systems with sufficient accuracy. Certainly, the efficiency of large-scale water treatment systems using incoherent Xe2* excimer lamps seems to be limited due to the extremely small reaction zone of VUV oxidation. However, VUV photolysis using efficient Xe2* excimer lamps has a great potential for the treatment of humidified polluted gaseous waste

7.1 Aqueous Systems

streams (Gassiot et al., 1998, Oppenländer, 1997 b). Furthermore, an enormous potential of this lamp technology for the VUV oxidative pretreatment of small (analytical) water samples lies in its geometric variability with respect to miniaturization. 7.1.4

Comparative Studies of Photo-Initiated AOPs

The direct comparison of the efficiency of different AOPs (including non-photochemical methods) is complicated by their various demands concerning pH, concentration of auxiliary oxidants or catalysts, specialties of the photoreactor systems, the excitation wavelength or spectral domain in the case of photo-initiated AOPs, and a multitude of other parameters. Furthermore, a major difficulty in the direct comparison of AOPs is related to the enormous variability that results from the nature of the auxiliary oxidants or photocatalysts, i. e. ozone-based homogeneous processes (O3, O3-Fe2+, O3-H2O2, O3-UV, O3-H2O2-UV, O3-H2O-VUV), hydrogen peroxide-based homogeneous systems (H2O2-UV, H2O2-H2O-VUV), homogeneous Fenton systems (H2O2-Fe2+, H2O2-Fe2+/3+-UV/VIS, O3-H2O2-Fe2+/3+-UV/ VIS), and TiO2-based heterogeneous systems (TiO2-UV/VIS, TiO2-H2O2-UV/VIS, TiO2-O3-UV/VIS). Each of these photoreaction systems has its optimum wavelength range or active spectral domain (see Fig. 5-16). This demand is of utmost importance and must be achieved by UV/VIS spectroscopic measurements or by actinometric determination of the active wavelength range of excitation. However, for many engineers this is a troublesome and laborious task, so that many research groups use their standard photoreactor equipped with standard MP or LP Hg lamps pragmatically for experimental comparison of various systems. Hence, comparative studies of different photo-initiated AOPs must be interpreted with necessary caution with respect to their generality. Nevertheless, many research groups try to evaluate the efficiency of various AOPs using model compounds or contaminated waters in this way. Several examples of comparative studies of photo-initiated AOPs are collected in Tab. 7-3. From this table the following conclusions may be drawn. Firstly, the effectiveness of photo-initiated AOPs is strongly dependent on the nature of the substrates; secondly, phenol and its halogenated derivatives are very sensitive to the photo-Fenton reaction; and thirdly, this table demonstrates that the photo-Fenton, the H2O2-UV and the O3-UV AOP seem to be very efficient in oxidative degradation of various organic substrates. However, it must be stated that the reaction pathways are different during the various treatment routes. This fact manifests itself in the different number and types of intermediate oxidation products that have been identified with identical substrates during different AOPs (cf. Rajeshwar, 1996). Further, the optimum conditions for a specific photo-initiated AOP treatment depend mainly on the nature of the waste or model water. For instance, H2O2-O3 treatment (without irradiation) can have advantages in the treatment of waters with high inherent UV ab-

213

++

++ ++++ ++ ++ ++

+++

+++

++

+++

+++ +++++ (Fe2+) ++++ (Fe3+)

+++

+++

+

+

++++

+ ++++

+

+++ (Fe2+) ++

++ ++

++++ (Fe2+)

+

+++

+++

+++

De Laat et al. (1999) Rajeshwar (1996) Mokrini et al. (1997) Doong et al. (2000) Beltran et al. (1996) Rajeshwar (1996) Beltran et al. (1996) Martyanov et al. (1997) Mokrini et al. (1997) Xu (2001)

+++ (Fe3+) + + ++ + ++ + + + (Fe2+)

+ +

Rajeshwar (1996) Bauer and Fallmann (1997), Ruppert et al. (1994) Bauer (1994) Beltran et al. (1996) Galindo et al. (2001)

Trapido et al. (1998)

Trapido et al. (1998)

References

+ + (Fe2+)

+

++++ (Fe2+)

H2O2-TiO2-UV UV c) +

TiO2-UV

++++ (Fe2+)

H2O2-Fe2+/3+-UV b)

++ ++ +++ +++

O3-H2O2-UV

a) Only photochemical methods have been selected from the literature. b) the oxidation state of iron ions used is given in parenthesis. c) direct UV irradiation without any auxiliary oxidants or in the presence of iron ions. Several other comparative studies that would go beyond the scope of this table have been performed for example by Arslan et al. (2002), Beltran-Heredia et al. (2001), Aplin and Waite (2000), Galindo et al. (2000), Benitez et al. (2000), Wenzel et al. (1999), Benitez et al. (1996), Hirvonen et al. (1996), Guittonneau et al. (1990).

4-Chlorophenol Acenaphthene (C12H10) Acid blue 74 (indigoid dye: C16H8N2Na2O8S2) Atrazine (C8H14ClN5) Benzamide (C6H5CONH2) Benzoic acid (C7H6O2) Chlorophenols Fluorene (PAH: C13H10) Formaldehyde (CH2O) Phenanthrene (C14H10) Phenol Phenol (C6H6O) Reactive brilliant Red X-3B (C19H10Cl2Na2N6O7S2)

+++

++

+++ (Fe2+ )

+++

++

2,4-Dichlorophenol (C6H4Cl2O) 2,4-Dimethylphenol (C8H10O) 4-Chlorophenol (C6H5ClO) 4-Chlorophenol

++++ ++

O3-UV

H2O2-UV

Substrate

Tab. 7.3 Comparison of efficiencies of oxidative degradation of various substrates with different AOPs in aqueous systems. The number of “+” signs designates the order of degradation efficiency from high to low within a row of this table a)

214

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

7.1 Aqueous Systems

sorbance because the •OH radical formation does not rely on UV irradiation. In addition, the photo-Fenton method can be superior over H2O2-UV treatment in cases where the wastewater strongly absorbs UV-C radiation. 7.1.5

Biodegradability and Toxicological Studies

Many water-soluble xenobiotics, for example pharmaceuticals (and particularly antibiotics) or organic textile dyestuffs of complex molecular structure including commercial brighteners, are recalcitrant or reluctant with respect to aerobic biodegradation. However, the pretreatment of industrial wastewaters or of other highly toxic effluents containing non-biodegradable pollutants by AOPs usually leads to less toxic intermediary oxidation products that are accessible to efficient biological treatment. Several studies established a positive synergistic effect of such abiotic– biotic process combinations (e. g. Ledakowicz and Solecka, 2000, Adams and Kuzhikannil, 2000, Pulgarin and Kiwi, 1996, and references cited therein). An example demonstrates the positive impact of photo-initiated AOPs (including nonphotochemical methods) on the subsequent biodegradation using the efficiency of COD elimination from a synthetic textile wastewater as an indicator (Fig. 7-17). In all cases (except with H2O2 pretreatment), the total amount of COD degradation after biological treatment could be enhanced significantly by AOP pretreatment. It is obvious that the O3-H2O2-UV AOP led to the best result. After the treatment sequence, it led to a COD residue of only about 15% of the original value (Fig. 717). This result perfectly demonstrates the efficiency of hybrid technologies. Therefore, photo-initiated AOPs are not usually used as stand-alone technologies

Fig. 7.17 Comparison of the efficiency of COD removal from a synthetic textile wastewater by AOP pretreatment followed by subsequent biological treatment and the relative contributions to the total diminution of COD (laboratory scale experiments). The first column represents the biological reference ex-

periment without AOP pretreatment. The textile wastewater had the following initial global parameter characteristics: intensive blue color, pH = 7.38, COD = 2154 mg O2 L–1, BOD5 = 1050 mg O2 L–1, TC = 442 mg L–1, DOC = 392 mg L–1; redrawn from Ledakowicz and Solecka (2000).

215

216

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.18 Direct irradiation of pentachloro-

phenol in aqueous solution without auxiliary oxidant: diminution of relative toxicity index (bacterial toxicity established by Toxichromo test using Escherichia coli), of organic chlorine in PCP ([Cl]organic = 5[PCP]t) and of total or-

– ganic chlorine ([Cl]total organic = 5[PCP]t=0 – [Cl ]t). –1 Conditions: [PCP] = 0.15 ± 0.02 mmol L , batch tank, and Rayox photoreactor CU01-1, MP Hg lamp Pel = 1 kW, V = 28 L, V˙ = 110 L min–1; redrawn from Ho and Bolton (1998): Figure 5 a.

for water remediation. They are, however, perfectly suited as powerful components in combinations with biological treatment (cf. Ollis, 2000, Bertanza et al., 2000), with activated carbon treatment (Ince and Apikyan, 2000) or air stripping, thus, providing the most economic solution to a specific treatment problem. For further information, refer to Chemviron Carbon (1997). Besides biodegradability, the toxicity of a wastewater is a crucial parameter for the evaluation of the effectiveness of an AOP treatment. It is of utmost importance with respect to the complex reaction schemes of AOPs producing a large variety of intermediary oxidation products (e. g. Figs. 7-4 to 7-10, Tab. 7-1). In most cases, the toxic potential of these intermediary products is not known and it should be evaluated by appropriate methods (cf. Chapter 5.1.2). Fortunately, the toxicity of wastewaters is usually reduced significantly during photo-initiated AOPs. This was, for example, demonstrated by several investigations including O3, TiO2-UV/VIS and O3-TiO2-UV/VIS treatment of a textile effluent (Gomes de Moraes et al., 2000) and the photo-Fenton treatment of 2,4-dichlorophenol (Ormad et al., 2001). Also, during treatment of pentachlorophenol (PCP) by direct UV irradiation (Fig. 7-18) and by H2O2-UV AOP (Fig. 7-19) a good correlation between degradation of the substrate and the diminution of toxicity could be established using a bacterial toxicity test (Toxichromo test using a special strain of Escherichia coli) and the Microtox technique using Vibrio fischeri (Ho and Bolton, 1998). In both cases the acute toxicity (expressed as relative toxicity factor) de-

7.1 Aqueous Systems

Fig. 7.19 H2O2-UV treatment of pentachlorophenol in aqueous solution: diminution of relative toxicity index (bacterial toxicity established by Toxichromo test using Escherichia coli), of organic chlorine in PCP ([Cl]organic =

5[PCP]t) and of total organic chlorine – ([Cl]total organic = 5[PCP]t=0 – [Cl ]t). Conditions: see Figure 7-18, [H2O2] = 3.0 mmol L–1; redrawn from Ho and Bolton (1998): Figure 5 b.

creased roughly proportionally to the total organic chlorine content of the solutions. As expected, the degradation rate during direct photolysis (Fig. 7-18) was significantly slower compared with the H2O2-UV treatment (Fig. 7-19). PCP is an important component of fungicides, bactericides, insecticides and wood preservatives and has many uses in industry and agriculture. This excellent study verifies the assumption that H2O2-UV treatment generally leads to a decrease in the acute toxicity of wastewaters. Furthermore, it suggests that the H2O2-UV AOP is suited for the safe oxidative degradation of PCP without formation of toxic intermediates. On the other hand, no significant change in toxicity (determined with the Microtox technique using Vibrio fischeri) during UV irradiation of an aqueous solution of the herbicide chlorbufam ([C11H10ClNO2] = 5.0 × 10–4 mol L–1) could be detected after 56% conversion (Tixier et al., 2000). Interestingly, the toxicity to Daphnia carinata of raw water samples from two Australian reservoirs increased considerably during direct UV-C irradiation and during H2O2-UV treatment. This result was attributed to the release of bioavailable and toxic copper ions due to photoinitiated oxidation and degradation of metal complexing natural organic matter (Parkinson et al., 2001). Sometimes an increase in toxicity is observed during the first few minutes of AOP treatment. Ince et al. (1997) demonstrated this effect by H2O2-UV treatment of a textile azo dye (Remazol Black-B) in aqueous solution (Fig. 7-20). After a sig-

217

218

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.20 H2O2-UV treatment of the reactive azo dye Remazol Black-B in aqueous solution: diminution of TOC, dye concentration and of relative toxicity index (established by Microtox technique). Conditions: [C26H21N5Na4O19S6] =

58.6 mg L–1, [H2O2] = 770 mg L–1, Rayox photoreactor CU01-1, MP Hg lamp Pel = 1 kW, V = 36 L, V˙ = 110 L min–1; redrawn from Ince et al., (1997): Figure 4.

nificant initial increase in the toxicity (measured by the Microtox technique) it decreased rapidly ending up at 68% reduction after 20 min of irradiation. The degradation of the dye followed pseudo-first order reaction kinetics and complete decolorization of the solution was achieved even within 15 min of treatment. In addition, 44% of TOC could be degraded after 30 min of irradiation. Total TOC removal needs significantly longer irradiation times or higher concentrations of H2O2. Unfortunately, excess H2O2 dosing resulted in a pronounced inhibitory effect on the efficiency of dye bleaching. This might be due to the •OH radical scavenging effect exerted by H2O2 itself that leads to the formation of hydroperoxyl radicals and water (see Fig. 6-8). Thus, in any case an optimum H2O2 concentration must be experimentally established (Ince et al., 1997). From the above study, the conclusion can be drawn that photo-initiated AOPs are suitable techniques for the remediation of toxic wastewaters on condition that a sufficient treatment time is guaranteed.

7.2 Gaseous Systems

7.2

Gaseous Systems 7.2.1

Applicability

The treatment of air-borne organic pollutants by homogeneous or heterogeneous photo-initiated AOPs offers several advantages over aqueous systems and over conventional air purification technologies (cf. Rajeshwar, 1996): • simple process technology and photoreactor design; • operation at ambient temperature and pressure; • small scale indoor and industrial scale systems are realizable; • higher penetration depth of VUV/UV radiation dependent on the vapor pressure of the absorbing substrates, especially in the case of VUV irradiation; • owing to low absorption cross-sections of gaseous substrates (cf. Tab. 3-6) the geometry of the photoreactor must ensure long optical pathways to guarantee total absorbance of electromagnetic radiation; • flow rates V˙ of polluted gas up to 100 000 m3 h–1 and more are possible; • contaminant levels of interest are often in the range of 0.1 to 1000 ppm ensuring an enormous excess of oxygen at any time of the VUV/UV treatment (Ollis, 1993); • oxygen starvation in the photoreaction zone is of minor importance compared with condensed media, especially with VUV oxidation of aqueous contaminants; • efficient mass transfer of substrate in the gas phase; • the extra addition of auxiliary oxidants is not necessary if, for example, ozone producing lamps are used, e. g. Suprasil quartz LP Hg lamps or incoherent Xe2* excimer lamps; • the influence of typical •OH radical scavengers (HCO–3/CO2– 3 ) is negligible and the influence of co-contaminants is usually lower than in the aqueous phase; • easy use of heterogeneous photocatalytic systems with immobilized photocatalysts (e. g. fixed powder or nano-particle layers). Thus, additional time consuming and expensive separation techniques, unlike in aqueous systems, are avoided. The fundamental principles of this process technology are based on the mechanisms of photooxidation of trace gases in the Earth’s atmosphere (Jenkin and Clemitshaw, 2000, Ehhalt, 1999). They involve complex free radical oxidation cycles that are mainly initiated by HOx (HO• and HO•2) radical species and direct photolysis of atmospheric trace pollutants. The hydroxyl radicals in the gas phase may be produced technically either by photolysis of ozone and subsequent reaction of singlet oxygen atoms with water vapor according to Eqs. 6-13 and 6-14, or by VUV photolysis of water vapor (Fig. 5-9, reaction 9). Numerous other secondary reaction pathways, for example, the reaction of hydroperoxyl radicals with nitrogen monoxide (Eq. 7-3) or H2O2 photolysis, may contribute to the production of hydroxyl radicals in gaseous systems.

219

220

7 Photooxidation and Photomineralization of Organic Matter in Water and Air

HO•2 + •NO ? •NO2 + HO•

(7-3)

In contrast to the water phase the HOx radicals can have a much longer lifetime in gaseous media, i. e. up to 1 s for the •OH and 60 s for the HO•2 radical, respectively (Fabian, 1989). Despite the low concentration of •OH radicals of about 107 molecules per cm–3 in the sunlit troposphere (Ehhalt, 1999) they play an important role in controlling the removal of many organic natural and manmade compounds from the atmosphere (Eisele et al., 1997, Eisele and Bradshaw, 1993). Even in indoor environments, the formation of hydroxyl radicals is possible by ozone/alkene reactions (Atkinson et al., 1995). Steady-state indoor hydroxyl radical concentrations of about 6.7 × 10–6 ppb equivalent to 1.7 × 105 molecules cm–3 were calculated at an ozone concentration of 20 ppb (Weschler and Shields, 1996). Interestingly, •OH radicals do not react with the most abundant atmospheric components N2, O2, Ar and CO2. However, the reaction of •OH radicals with carbon monoxide leads to the formation of carbon dioxide and H• atoms (Eq. 7-4). The latter are scavenged by triplet molecular oxygen in a three-body reaction (to dissipate excess energy, M: third partner molecule) to yield hydroperoxyl radicals (Eq. 7-5). Thus, the oxidative potential of the atmosphere is maintained (Comes, 1994). Ozone itself reacts with hydroperoxyl radicals in the gas phase according to Eq. 7-6 with formation of molecular oxygen and •OH radicals. The majority of secondary •OH radicals results from reactions of HO•2 and RO•2 with nitrogen monoxide (•NO). Loss of •OH radicals in the gas phase is caused by reactions with nitrogen dioxide with formation of nitric acid (HNO3). CO + •OH ? CO2 + H•

(7-4)

H• + •O•2 + M ? HO•2 + M

(7-5)

HO•2 + O3 ? 2O2 + •OH

(7-6)

Thus, in atmospheric chemistry a complex scheme of interrelated photo-induced and photo-initiated primary and secondary radical reactions in connection with direct photolysis of gaseous substrates (Fig. 7-21) is responsible for the oxidative degradation of volatile organic substrates. Industrial imitations of these atmospheric photoprocesses must utilize ozone producing LP Hg lamps emitting at k of 184.9 nm and 253.7 nm as radiation sources in order to enter hydroxyl radical chemistry efficiently via ozone production and its photolysis. Otherwise, the direct photolysis of gaseous organic (or inorganic) substrates prevails with formation of distinct photoproducts. It is important to realize that even with 100% humidity at 25 8C only about 10% of the O(1D) atoms from the photolysis of O3 (cf. Eq. 6-13) react with water vapor to form •OH radicals (cf. Eq. 6-14). 90% decay to O(3P) by collision induced energy transfer. This is one reason why the O3-UV process in air has had very limited success with respect to commercialization (Bolton, 2002). Nevertheless, the photochemical formation of organic radicals R• by direct photolysis of organic substrates RH (Fig. 7-21) may lead to type I photooxygenation

7.2 Gaseous Systems

Fig. 7.21 Simplified scheme of interrelated photo-initiated oxidation and secondary radical reactions in the gas phase (atmospheric

photochemistry): primary production, interconversion and loss of hydroxyl radicals (modified according to Eisele et al., 1997).

products (cf. Fig. 5-10, reactions 5) and/or the RO•2/HO•2 radical chain (cf. Fig. 71) may be entered. A compilation of kinetic data of reactions and photoreactions in the gas phase was presented by Atkinson et al. (1991) refering to publications of the IUPAC in the Journal of Physical and Chemical Reference Data. Further data on chemical kinetics in the gas phase have been made avaiable by Lide (2000). 7.2.2

Chemical and Physical-Chemical Aspects of Air Pollution Composition of the Atmosphere The UV/VIS absorbance spectra of O2, O3, H2O2 and H2O in the gas phase are described in Fig. 6-4 and the corresponding quantum yields of photodecomposition are presented in Tab. 6-4. The major components of the atmosphere are nitrogen, oxygen, argon and carbon dioxide with a mixing ration of 78.08%, 20.95%, 0.93% and 0.0365% (equals 365 ppmv), respectively (vanLoon and Duffy, 2000). Minor components are neon ([Ne] = 18 ppmv), helium ([He] = 5.2 ppmv), methane ([CH4] = 1.77 ppmv), hydrogen ([H2] = 0.53 ppmv) and nitrous oxide ([N2O] = 0.31 ppmv). Urban air-borne pollutants include a variety of gaseous inorganic and volatile organic compounds, such as carbon monoxide (CO), nitrogen dioxide (NO2) saturated hydrocarbons (CnH2n+2), ozone, peroxyacetic nitric anhydride [PAN, CH3C(O)OONO2], alkenes, alkynes, aromatic hydrocarbons, alcohols, ketones and aldehydes. A comprehensive collection of analytical methods for the quantitative determination of toxicants in the air is edited by Kettrup (2002).

221

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Concentrations of Gases It is obvious that a consistent system for the description of concentrations in the gas phase is necessary. Usually, fractional or percentage concentrations are used. Mixing ratios of low concentrated volatile compounds and gases are based on the “parts per...” unit system. This unit is obsolete but is still used in the current literature. It cannot be utilized for particle concentrations. Therefore, the WHO (1999) has adopted a mass per volume system with concentrations [C] in mg m–3. Other units frequently employed to express the concentration of gases include moles per volume or molecules per volume (vanLoon and Duffy, 2000, Kurzweil, 1999). To interconvert the units ppmv (parts per million by volume, Tab. 3-2) and mg m–3 for an ideal gas the following expression (Eq. 7-7) should be used (Kurzweil, 1999).

1 ppmv

M 10 6 g L Vm

1

M mg m Vm

3

(7-7)

With M/g mol–1 the molar mass of the volatile compound or gas, Vm/L mol–1 the molar volume at pressure p, and temperature T. For example, 1 ppmv of O2 32 equals 22:4 = 1.43 mg m–3 at p = 101.3 kPa and T = 0 8C. The molar volume Vm at p,T can be calculated according to Eq. 7-8 (related to standard conditions: 0 = 22.4 × 10–3 m3 mol–1 (Tab. 3-1) at T0 = 273.15 K, p0 = 101325 Pa (1 atm). Vm Vm =m3 mol

1

0 Vm

p0 T pT 0

(7-8)

Odor Measurement Special problems are related to odor measurement, its quantification and to odor policy (Frechen, 2000). Odors usually consist of many different compounds of very low concentrations that may exert synergistic or antagonistic effects. Four generally accepted dimensions of odor exist which include the concentration of odorants, the intensity of odor, its character, and its hedonic tone (i. e. its pleasantness or unpleasantness). It is extremely difficult to relate conventional analytical measurements (e. g. gas chromatographic determinations or sensor array measurements with an “electronic nose”) to the intensity, character, and hedonic tone of an odor perceived by the human nose. The standard measurement procedure for odor determination (VDI, 1994) is called olfactometry. It uses the human olfactory sense (Gostelow et al., 2001) for the determination of odor qualities. The human nose is an extremely sensitive odor detector and is used in subjective and objective sensory measurements. The latter expresses the strength of odor in terms of the number of dilutions of odorfree air required to reduce the sample odor to threshold concentration (Gostelow et al., 2001). The threshold concentration is reached when the human nose can just smell the odorous substance. Because every nose has different sensitivity, the standard test procedure involves four people at the same time. Prior to olfactory

7.2 Gaseous Systems

measurements the sensory system of these people is usually calibrated using test substances, for instance n-butanol (C4H10O) or hydrogen sulfide (H2S), to guarantee standardized (“standard nose”) conditions. The odorous air sample is diluted automatically with neutral air by use of an olfactometer. This procedure leads to the definition of an odor concentration according to Eq. 7-9 that is applied for the determination of mixing ratios of known flow rates V˙ (dynamic dilution method). The abbreviation “ou” stands for odor unit (Treitinger and Meyer-Pittroff, 1997). The odor concentration is inversely proportional to the dilution. Actually, odor concentrations are dimensionless (see Eq. 7-9) and they simply reflect a dilution to threshold ratio. However, it is common to express odor concentration in “odor units per cubic meter” ([odor]/ou m–3). Odor concentration=ou m

3

1 V_ neutral air 1 dilution V_ odorous air

(7-9)

Therefore, 1 ou m–3 is defined to be equivalent to the odor concentration at which 50% of the test people can recognize the difference between neutral air and the odorous air sample. If a waste gas was determined to have 90 ou m–3, this waste gas must be diluted 90 times with neutral air to reach the odor threshold. In other words, the larger the value of the odor unit per m–3 the more intensive is the odor. However, an increase of odor concentration from 100 ou m–3 to 1000 ou m–3 increases the sensory impression of the human nose only by a factor of about two, in accord with a logarithmic dependence. It is obvious that this method of odor measurement is a very time-consuming and expensive method. However, Stuetz et al. (2000) were able to correlate the global parameters BOD, COD and TOC of sewage samples by headspace measurements of odor using a commercial sensor array system. Thus, the “electronic nose” seems to have several interesting applications in wastewater treatment technologies. 7.2.3

Process Technologies of Air Treatment

Fig. 7-22 collects the most important process technologies for air treatment. They cover processes for the removal of gases or volatile compounds or the removal of particles. The removal of particulate matter in flew gases emitted from point sources can be performed by applying cyclones, fabric filters, electrostatic precipitators and scrubbers. The applicability of these different techniques depends on the particle size (cf. vanLoon and Duffy, 2000). The removal of gases and volatile compounds from waste streams is based on a variety of different technologies, such as thermal treatment, adsorption (on activated carbon or zeolites), membrane and biological processes, condensation, use of catalysts or absorption in water (Börger et al., 2000). Non-thermal plasma processing (e. g. Hsiao et al., 1996, Yamamoto et al., 1996) and photo-initiated AOPs are promising and innovative developments in this

223

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.22 Process technologies of air pollution control.

Fig. 7.23 Photo-initiated AOPs and their applications in air pollution control.

field. The latter have great potential in air pollution control (Fig. 7-23) with respect to odor abatement (e. g. Biey and Verstraete, 1999, Schneider, 1997/98), disinfection of air by control of air-borne bacteria, yeasts and mould spores (McClean, 2001), decontamination and mineralization of organic contaminants (Gassiot et al., 1998, Oppenländer, 1997 b). Photo-initiated AOPs of air treatment are usually not covered in standard textbooks dealing with the treatment of waste air and waste gas (cf. Ebling, 1999). Nevertheless, small and medium factories burdened with odor emission problems (e. g. meat and fish-processing factories, slaughterhouses, spice production or garbage collecting and recycling facilities) increasingly fall back on this clean and efficient technology of odor control (examples see Chapter 8.6.2).

7.2 Gaseous Systems

Fig. 7.24 Schematic representation of a photoelectric aerosol sensor (according to EcoChem, 2000): use of an incoherent kryptonchloride excimer lamp (kmax = 222 nm) as

photoionization chamber for real-time monitoring of particle-bound polycyclic aromatic hydrocarbons.

Perhaps one of the first industrial applications of incoherent kryptonchloride excimer lamps (KrCl*, kmax = 222 nm, see Fig. 4-10) is related to the analytical measurement of particle-bound polycyclic aromatic hydrocarbons (PAHs) in the gas phase (Matter et al., 1994). Hence, a portable real-time monitor based on a photoelectric aerosol sensor system was developed by EcoChem Technologies (West Hills, California, USA). This instrument can be used to examine and quantify combustion aerosols (from petrol engines, diesel engines or cigarette smoke) and for outdoor and indoor air quality monitoring and many other environmental applications including pollution detection, air conditioning and fire alarm systems. The measuring principle uses the photo-induced ionization of the particle bound PAHs (Fig. 7-24). Subsequently, the charged particles are separated and collected on a filter within an electrometer and the charge is measured. The resulting electrical signal is proportional to the concentration of the particle-bound PAHs. The use of the KrCl* excimer lamp has several advantages over commonly applied LP Hg lamps: it can be rapidly switched on and off and thus, measurements without time delay are guaranteed. The pulsed or chopped mode of the KrCl* excimer lamp is used to eliminate the background signal which is sometimes found during measurements very close to the combustion source. The miniaturization of the excimer lamp led to a lighter and more compact design of the instrument (EcoChem, 2000). Similarly, in time-of-flight mass spectrometry (TOFMS) novel VUV excimer lamps are potent alternatives to laser sources for the initiation of single-photon ionization processes (Ulrich et al, 2000). These examples again demonstrate the enormous application potential of incoherent excimer lamps, especially in analytical processes. Several recent research papers on photo-initiated AOPs in the gas phase are collected in Tab. 7-4. It is obvious from the table that the main activities of gas phase

225

TiO2-UV/VIS UV, solar UV/VIS d) TiO2-UV-B TiO2-UV/VIS e, f)

Cl2C=CH2 C6H4Cl2 C5H11OH CH3CH2CH2OH H3CCH(OH)CH3 H3CC(O)CH3 H3CC(O)CH3

H3CC(O)CH3 C12H8Cl6 CH2Cl2 C4H10NH

C2H5OH C2H5OH C2H5OH NO C3H7NH2

C5H5N

Cl2C=CCl2 Cl2C=CCl2 Cl2C=CCl2 C7H8 Cl2C=CClH

1,1-Dichloroethene 1,3-Dichlorobenzene 1-Pentanol 1-Propanol 2-Propanol Acetone Acetone

Acetone Aldrin Dichloromethane Diethylamine

Ethanol Ethanol Ethanol Nitrogen monoxide Propylamine

Pyridine

Tetrachloroethene Tetrachloroethene Tetrachloroethene Toluene Trichloroethene UV-C, O3-UV TiO2-UV-B TiO2-UV-B TiO2-UV/VIS UV-C

TiO2-UV/VIS e, f)

TiO2-UV TiO2-UV-B TiO2-UV-B TiO2-UV/VIS TiO2-UV/VIS e, f)

UV-C, O3-UV TiO2-UV-B Solar UV/VIS b) UV-B/CH3ONO TiO2-UV/VIS TiO2-UV UV-C, UV-B

Chemical formula

Substrate

AOP a) -[Substrate], +[Ox], +[CO2], +[Cl–] -[Substrate] -[Substrate], +[Ox]c) -[Substrate], +[Ox]c) -[Substrate], effect of humidity -[Substrate], +[CO2] -[Substrate]; effect of humidity; determination of U -[Substrate], effect of humidity -[Substrate], +[photoproducts] -[Substrate] -[Substrate], +[Ox], +[NH+4 ], +[NO–3], +[CO2], +[H2O] -[Substrate], +[Ox], +[CO], +[CO2] -[Substrate], +[Ox], +[CO2] -[Substrate], +[Ox], +[CO2] -[Substrate] -[Substrate], +[Ox], +[NH+4 ], +[NO–3], +[CO2], +[H2O] -[Substrate], +[Ox], +[NH+4 ], +[NO–3], +[CO2], +[H2O] -[Substrate], +[Ox], +[CO2], +[Cl–] -[Substrate] -[Substrate], +[Ox], +[HCl] -[Substrate], effect of humidity -[Substrate], +[Ox], +[HCl], +[CO2]

Shen and Ku (1998) Hager et al. (2000) Fukami et al. (2001) Hager and Bauer (1999) Haag et al. (1996)

Alberici et al. (2001)

Muggli et al. (1998) Sauer and Ollis (1996) Nimlos et al. (1996) Lim et al. (2000) Alberici et al. (2001)

Shen and Ku (1998) Hager et al. (2000) Cavalli et al. (2000) Azad and Andino (1999) Hager and Bauer (1999) Choi et al. (2001) Aloisio and Francisco (2000) Hager and Bauer (1999) Nojima and Isogami (1996) Hager et al. (2000) Alberici et al. (2001)

Change of analytical parameter, remarks References

excimer lamps are collected in Table 7-2. Ox: intermediary oxidation products formed and detected during treatment

Tab. 7.4 Examples of research related to homogeneous and heterogeneous photo-initiated AOPs in the gas phase. Examples of applications of incoherent

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

a) b) c) d) e) f) g) h) i)

-[Substrate], effect of humidity -[Substrate], +[Ox], +[CO2], +[Cl–] -[Substrate] -[Substrate], +[Cl2C=O], determination of U -[Substrate]

Wang et al. (1998) Shen and Ku (1998) Hager et al. (2000) Bhowmick and Semmens (1994) Shen and Ku (1999)

Change of analytical parameter, remarks References

Irradiations were usually performed in air unless stated otherwise. outdoor EUPHORE smog chamber facility in Valencia, Spain was used. in the presence of NOx. photolysis in the presence of benzaldehyde acting as a triplet photosensitizer. nitrogen mass balances in the presence and absence of oxygen were determined. deactivation of the photocatalyst by trapping of NH+4 and NO–3 ions on the TiO2 surface was observed. trichloromethane, tetrachloromethane, trichloroethene, tetrachloroethene and 1,1,2-trichloroethane were used as model compounds. ozone producing LP Hg lamps with the principal UV output at 253.7 nm and 5% VUV output at 184.9 nm were used. dichloromethane, trichloromethane, tetrachloromethane, 1,2-dichloroethane, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, benzene, toluene and o-xylene were used as model compounds.

i)

O3-UV-C

TiO2-UV-A UV-C, O3-UV TiO2-UV-B VUV/UV h)

Cl2C=CClH Cl2C=CClH Cl2C=CClH

Trichloroethene Trichloroethene Trichloroethene Volatile organic compounds (VOCs) Volatile organic compounds (VOCs)

g)

AOP a)

Chemical formula

Substrate

Tab. 7.4 (continued)

7.2 Gaseous Systems 227

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

photochemical AOP research are related to the investigation of the behavior of simple saturated alcohols and ketones and of volatile organic compounds, mainly chlorinated ones. These compounds are of major environmental concern because they are used excessively as industrial solvents. Further, heterogeneous systems based on TiO2 photocatalysis seem to prevail against homogeneous treatment options. One of the reasons for this development may be the possibility of using solar radiation in gas phase heterogeneous photocatalysis based on TiO2. Additionally, the TiO2-UV/VIS AOP is an air treatment and purification technology that is applicable to the most common and oxidizable air contaminants. Unfortunately, the treatment of gaseous saturated and unsaturated chlorohydrocarbons by photo-initiated AOPs is known to produce highly toxic by-products, such as acid chlorides and phosgene (e. g. Haag et al., 1996, Nimlos et al., 1993). The gas phase oxidation of halogenated unsaturated hydrocarbons has been reviewed comprehensively by Sanhueza et al. in 1976 emphasizing the formation of X2C=O products (X = F, Cl). As a serious consequence, special safety precautions and sensitive diagnostics are necessary in these cases. Phosgene (Cl2C=O) is a very toxic compound and a typical lung irritant gas. It was used as a combat gas in the First World War. Nowadays, phosgene is used on industrial scales mostly for the production of organic diisocyanates, which are precursor components of polyurethane plastics (Ullmann’s, 1991). However, acid chlorides and phosgene are sensitive to hydrolysis. In aqueous solution phosgene slowly hydrolyzes with formation of HCl and CO2 with a first order rate constant of khydrolysis = 9 s–1 (Mertens and von Sonntag, 1995). It can be easily decomposed by passing through scrubbers containing sodium hydroxide solution or through packed towers that are filled with activated carbon and fed with water (Ullmann’s, 1991). Several examples of gas phase photooxidation of chlorohydrocarbons will be discussed below to demonstrate the complexity of the photoinduced reaction mechanisms that are involved. For example, the direct photolysis of trichloroethene in air with UV-C radiation (using a xenon flash lamp) initially produced dichloroacetyl chloride ([Cl2HC– COCl] >90%) that subsequently was photooxidized to yield phosgene ([Cl2C=O] ≤ 25%), trichloroacetyl chloride (TCAC, [Cl3C–COCl] ≤ 2%), unidentified intermediates, and ultimately HCl and CO2 were produced (Haag et al., 1996). In this connection, it is relevant to stress that dichloroacetyl chloride is 40 times more toxic than trichloroethene (cf. Haag et al., 1996). Similar results were reported for the direct photolysis of tetrachloroethene and trichloroethene in air with an incoherent KrCl* excimer lamp at 222 nm (Prager and Hartmann, 2001). With this novel lamp, the substrates could be completely photodegraded, but the photolysis led to the formation of trichloroacetyl chloride, phosgene, CO and small amounts of HCl in the case of tetrachloroethene and to dichloroacetyl chloride, phosgene, formylchloride (HCOCl), CO and HCl using trichloroethene as substrate. Also, the VUV photolysis of trichloromethane in an air stream at k of 172 nm using a Xe2* excimer flow-through photoreactor (Oppenländer, 1997 b) led to a complex mixture of volatile products (Fig. 7-25). The process of mineralization

7.2 Gaseous Systems

Fig. 7.25 VUV photolysis (k = 172 nm) of trichloromethane in air using a Xe2* excimer flow-through photoreactor (cf. Figure 4-16),

and resulting products. Conditions: [CHCl3]t=0 ~ 1 g L–1 in the gas phase, Xe2* excimer 3 –1 ˙ lamp: Pin el = 150 W, V = 1 cm min .

was incomplete due to the high initial concentration of trichloromethane in the gas phase, i. e. [CHCl3]t=0 ca. 1 g L–1. The individual volatile components of the irradiated gas stream were identified by gas-chromatography/mass spectrometry (GC-MS) and by IR spectroscopy. With respect to the oxidation state of the initial carbon atom in CHCl3 (+II) the products are derived formally from mineralization (water, hydrogen chloride, carbon dioxide and carbon monoxide), oxidation (tetrachloromethane and phosgene), reduction (dichloromethane) and radical recombination (tetra-, penta- and hexachloroethane) processes, presumably accompanied by secondary photolysis of the primary photoproducts with formation of trichloro- and tetrachloroethene. The involvement of nitrogen by formation of traces of nitrotrichloromethane (CH3NO2) and nitrous oxide (N2O) is presumably due to electrical discharges within the irradiation chamber of the Xe2* excimer lamp that contains the “hot” inner electrode, being in direct contact with air and CHCl3 vapor (see Fig. 4-13). In order to demonstrate the complex radical reaction scheme of the direct photolysis of unsaturated chlorohydrocarbons in air, the start reaction and the chain propagation reactions are outlined in Fig. 7-26, using tetrachloroethene as a representative example. The initiation of the radical chain mechanism via C–Cl bond homolysis can be achieved by photolysis of tetrachloroethene at wavelength below k of 250 nm. The chlorine atom thus formed adds immediately to the double bond of another substrate molecule with formation of a pentachloroethyl radical. This is scavenged by molecular oxygen to yield a peroxyl radical. Subsequently, peroxyl radical chemistry leads either to the formation of TCAC and a chlorine atom, which starts the chain again, or to a trichloromethyl radical and phosgene. The trichloromethyl radical itself can react with O2 leading to phosgene, carbon monoxide and again a chlorine atom. The rate constants of each

229

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air

Fig. 7.26 Simplified mechanistic scheme of the photo-induced oxidation of tetrachloroethene in air leading to the formation of tri-

chloroacetyl chloride (TCAC), phosgene and carbon monoxide (CO), according to Prager and Hartmann (2001).

step are collected in the chemical kinetics database (Mallard et al., 1998). In contrast, the photolysis of air-saturated aqueous solutions of tetrachloroethene at k of 253.7 nm led to the formation of chloride ions, carbon dioxide, trichloroacetic acid, dichloroacetic acid, and hypochlorite (HOCl) as major products (Mertens and von Sonntag, 1995), see also Chapter 6.8.2. In conclusion, it should be stressed that the use of photo-initiated AOPs for the treatment of waste gases containing halogenated hydrocarbons requires an efficient analytical process control of the irradiated gas streams with respect to hazardous and toxic by-products. Another interesting feature of gas phase photolysis includes the possible generation of ozone by VUV irradiation of oxygen. Dohan and Masschelein previously presented the mechanistic and preparative aspects of this method in 1987. Unfortunately, the use of LP Hg lamps with a Suprasil quartz envelope for the irradiation of a gas stream containing oxygen inevitably leads to a photostationary equilibrium between ozone formation at k of 184.9 nm and ozone destruction at k of 253.7 nm. Thus, the efficiency of ozone production is relatively low due to the higher intensity of the radiation at k of 253.7 nm. However, the photochemical production of ozone on a small scale represents a potential application of the novel incoherent Xe2* excimer lamps. This was first demonstrated by Eliasson and Kogelschatz (1991), and on laboratory and preparative scales by Laszlo et al. (1998) and by Hashem et al. (1997). The latter research group realized the simultaneous generation of ozone in the gas phase and its transfer to the aqueous phase followed by the VUV irradiation of this water by

References

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air Oppenländer T, Baum G (1995 a) CKW-Eliminierung aus Wasser durch Vakuum-UVund UV-Photolyse in Excimer-Durchflußphotoreaktoren, Chem.-Ing. Tech. 67: 96–99. Oppenländer T, Baum G (1995 b) Industrieabwässer VUV/UV-oxidativ reinigen, Umwelt 25: 100–101. Oppenländer T, Baum G (1996) Wasseraufbereitung mit Vakuum-UV/UV-ExcimerDurchflußphotoreaktoren, gwf Wasser-Abwasser 137: 321–325. Oppenländer T, Baum G, Egle W (1995) Abwasserreinigung durch Vakuum-UV-Oxidation in Excimer-Durchflußphotoreaktoren, gwf Wasser-Abwasser 136: 311–316. Oppenländer T, Baum G, Streiff F, Mathys P, Schneider G (1996) Einsatz von statischen Mischern in Excimer-Durchflußphotoreaktoren zur Abwasserbehandlung durch Vakuum-UV-Oxidation, Chem.-Ing. Tech. 68:155–158. Oppenländer T, Fradl M (1999) TOC Destruction of a Phenol/Water Azeotrope by “Photoreactive Distillation” through an Incoherent Vacuum-UV Excimer Lamp, Chem. Eng. Technol. 22: 951–954. Oppenländer T, Gliese S (2000) Mineralization of Organic Micropollutants (Homologous Alcohols and Phenols) in Water by Vacuum-UV-Oxidation (H2O-VUV) with an Incoherent Xenon-Excimer Lamp at 172 nm, Chemosphere 40: 15–21. Oppenländer T, Waizenegger K, Baum G (1997) Development and Engineering of Excimer Flow-through Photoreactors for Waste Water Treatment, Proc. IOA-Internat. Ozone Assoc., International Conference on Ozonation and Related Oxidation Processes in Water and Liquid Waste Treatment, Berlin April 21–23, III.6.1-III.6.21. Oppenländer T, Waizmann J (1997) Photochemische Wasserbehandlung mit einem KrCl*-Excimer-UV-Rundrohrstrahler in einem Flachbettreaktor, Chem.-Ing. Tech. 69: 1470–1474. Ormad MP, Ovelleiro JL, Kiwi J (2001) Photocatalytic Degradation of Concentrated Solutions of 2,4-Dichlorophenol Using Low Energy Light. Identification of Intermediates, Appl. Catal. B: Environ. 32: 157– 166. Parkinson A, Barry MJ, Roddick FA, Hobday MD (2001) Preliminary Toxicity As-

sessment of Water after Treatment with UV-Irradiation and UVC/H2O2, Wat. Res. 35, No. 15: 3656–3664. Peyton GR (1996) Kinetic Modeling of FreeRadical Water Treatment Processes: Pitfalls, Practicality, and the Extension of the Hoigné/Bader/Staehelin Model, J. Adv. Oxid. Technol. 1, No. 2: 115–125. Pfeifer Roland R, Bolle M, Anderson RW (1998) Hydrogen Bromide Photochemistry: Actinometry for Determination of Absolute Power Outputs of Xenon Excimer and other UV/VUV Light Sources, J. Phys. D: Appl. Phys. 31: 1336–1342. Prager L, Hartmann E (2001) New Route for Degradation of Chlorinated Ethylenes in Exhaust Gases from Ground Water Remediation, J. Photochem. Photobiol. A: Chem. 138: 177–183. Pulgarin C, Kiwi J (1996) Overview on Photocatalytic and Electrocatalytic Pretreatment of Industrial Non-Biodegradable Pollutants and Pesticides, Chimia 50: 50–55. Rajeshwar K (1996) Photochemical Strategies for Abating Environmental Pollution, Chem. Ind. June: 454–458. Rehberg K, Mannschott P, Erdinger L, Sonntag HG, Schramm B (1994) Atrazinabbau durch UV-Oxidation und Identifizierung der Photoabbauprodukte, Vom Wasser 83: 139–156. Ruppert G, Bauer R, Heisler G (1994) UVO3, UV-H2O2, UV-TiO2 and the Photo-Fenton Reaction- Comparison of Advanced Oxidation Processes for Wastewater Treatment, Chemosphere 28, No. 8: 1447–1454. Sanhueza E, Hisatsune IC, Heicklen J (1976) Oxidation of Haloethylenes, Chem. Rev. 76, No. 6: 801–826. Sauer ML, Ollis DF (1996) Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air, J. Catal. 158: 570–582. Scheytt H, Esrom H, Prager L, Mehnert R, Sonntag C von (1992) Ultraviolet Light and Electron Beam Induced Degradation of Trichloroethene, NATO Adv. Res. Workshop on Non-thermal Plasma Techniques for Pollution Control, Cambridge University, UK, Sept. 21–25. Schneider H (1997/98) Licht frißt Gestank, Gerüche kostengünstig ohne Rückstände beseitigen, Chemie Umwelt Tech.: 16–17.

References Shen Y-S, Ku Y (1998) Decomposition of Gas-Phase Chloroethenes by UV/O3 Process, Wat. Res. 32, No. 9: 2669–2679. Shen Y-S, Ku Y (1999) Treatment of Gasphase Volatile Organic Compounds (VOCs) by UV/O3 Process, Chemosphere 38, No. 8: 1855–1866. Sonntag C von (1985) Photochemistry of Saturated Alcohols and Open-chain Ethers at 185 nm in the Liquid Phase, in: McGlynn SP, Findley GL, Huebner RH (eds.) Photophysics and Photochemistry in the Vacuum Ultraviolet, NATO ASI Series, D. Reidel, Dordrecht; cited in Heit et al. (1998). Sonntag C von, Schuchmann H-P (1977) Photolysis of Alcohols, Ethers, and Amines, in: Pitts JN Jr, Hammond GS, Gollnick K (eds.) Advances in Photochemistry, John Wiley & Sons, New York, Vol. 10: 59–145. Sörensen M, Frimmel F (1997) Photochemical Degradation of Hydrophilic Xenobiotics in the UV/H2O2 Process: Influence of Nitrate on the Degradation Rate of EDTA, 2Amino-1-naphthalenesulfonate, Diphenyl4-sulfonate and 4,4'-Diaminostilbene-2,2'disulfonate, Wat. Res. 31: 2885–2891. Sörensen M, Tanner U, Sagawe G, Frimmel FH (1996) Photochemical Degradation of Hydrophilic Xenobiotics in the UV/ H2O2-Process. Influence of Humic Matter on the Degradation Rate of 2-Amino-1Naphthalenesulfonate and Ethylenediaminetetraacetate, Acta Hydrochim. Hydrobiol. 24:132–136. Sörensen M, Zurell S, Frimmel FH (1998) Degradation Pathway of the Photochemical Oxidation of Ethylenediaminetetraacetate (EDTA) in the UV/H2O2-Process, Acta Hydrochim. Hydrobiol. 26, No. 2: 109–115. Stefan MI, Bolton JR (1998) Mechanism of the Degradation of 1,4-Dioxane in Dilute Aqueous Solution Using the UV/Hydrogen Peroxide Process, Environ. Sci. Technol. 32, No. 11: 1588–1595. Stefan MI, Bolton JR (1999) Reinvestigation of the Acetone Degradation Mechanism in Dilute Aqueous Solution by the UV/H2O2 Process, Environ. Sci. Technol. 33, No. 6: 870–873. Stefan MI, Hoy AR, Bolton JR (1996) Kinetics and Mechanism of the Degradation

and Mineralization of Acetone in Dilute Aqueous Solution Sensitized by the UV Photolysis of Hydrogen Peroxide, Environ. Sci. Technol. 30, No.: 7: 2382–2390. Stefan MI, Mack J, Bolton JR (2000) Degradation Pathways during the Treatment of Methyl tert-Butyl Ether by the UV/H2O2 Process, Environ. Sci. Technol. 34, No. 4: 650–658. Stuetz RM, Fenner RA, Hall SJ, Stratful I, Loke D (2000) Monitoring of Wastewater Odours using an Electronic Nose, Wat. Sci. Technol. 41, No. 6: 41–47. Thomanetz E (1999) Progress Report: Untersuchung und Optimierung der Reaktionsbedingungen bei der UV-aktivierten Naßoxidation von Industrieabwässern im Hinblick auf eine Steigerung der Eliminationswirkungsgrade bei minimierten Kosten, research project supported by the Willy-Hager-Foundation, Stuttgart (Germany). Tixier C, Meunier L, Bonnemoy F, Boule P (2000) Phototransformation of Three Herbicides: Chlorbufam, Isoproturon, and Chlorotoluron. Influence of Irradiation on Toxicity, Int. J. Photoenergy 2: 1–8. Trapido M, Veressinina Y, Munter R (1998) Advanced Oxidation Process for Degradation of 2,4-Dichloro- and 2,4-Dimethylphenol, J. Environ. Eng.124: 690– 694. Treitinger B, Meyer-Pittroff R (1997) Messung von Geruchsemissionen – Vergleich verschiedener Meßverfahren, Chem. Ing. Tech. 69, No. 9: 1120. Ullmann’s (1991) Phosgene, Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed., Vol. A 19: 411–420. Ulrich A, Wieser J, Salvermoser M, Murnick D (2000) Phys. Bl. 56(6): 46–52; cited in: Zimmermann R (2002) On-line-Prozess- und Produktanalytik mit Massenspektrometrie, Nachr. Chem. 50: 166–169. vanLoon GW, Duffy SJ (2000) Environmental Chemistry – A Global Perspective, Oxford University Press, Oxford. VDI (1994) Guidelines Olfactometry: 3882/2, 3940 (1993), 3882/1 (1992), 3883 (1991), 3881 (1986). Wang K-H, Tsai H-H, Hsieh Y-H (1998) The Kinetics of Photocatalytic Degradation of Trichloroethylene in Gas Phase over TiO2

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7 Photooxidation and Photomineralization of Organic Matter in Water and Air Supported Glass Bead, Appl. Catal. B: Environ. 17: 313–320. Weeks JL, Meaburn GMAC, Gordon S (1963) Absorption Coefficients of Liquid Water and Aqueous Solutions in the Far Ultraviolet, Rad. Res. 19: 559–567. Wenzel A, Gahr A, Niessner R (1999) TOC-Removal and Degradation of Pollutants in Leachate Using a Thin-Film Photoreactor, Wat. Res. 33, No. 4: 937–946. Weschler CJ, Shields HC (1996) Production of the Hydroxyl Radical in Indoor Air, Environ. Sci. Technol. 30, No. 11: 3250– 3258. WHO (1999) Air Quality Guidelines, Chap. 2.1.1. Wörner M, Schnabel C, Hashem TM, Zegenhagen F, Gieser J, Braun AM (1998) Dehalogenation of Halogenated Organic Compounds in Aqueous Solution by

Different Photochemical Methods, J. Inf. Recording 24: 455–460. Xu Y (2001) Comparative Studies of the Fe3+/2+-UV, H2O2-UV, TiO2-UV/Vis Systems for the Decolorization of a Textile Dye X-3B in Water, Chemosphere 43: 1103– 1107. Yamamoto T, Chang J-S, Berezin AA, Kohno H, Honda S, Shibuya A (1996) Decomposition of Toluene, o-Xylene, Trichloroethylene, and their Mixture Using a BaTiO3 Packed-Bed Plasma Reactor, J. Adv. Oxid. Technol. 1, No. 1: 67–78. Zhang J-Y, Boyd IW, Esrom H (1997) UV Intensity Measurement of a Novel 222 nm Excimer Lamp Using Chemical Actinometer, Appl. Surf. Sci. 109/110: 482–486. Zhang J-Y, Esrom H, Boyd IW (1999) UV Intensity Measurement of 308 nm Excimer Lamp Using Chemical Actinometer, Appl. Surf. Sci. 138/139: 315–319.

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Process Engineering and Reactor Concepts 8.1

Chemical Engineering

The complex field of photochemical engineering is governed by the reaction system, the lamp technology and by chemical engineering (see Fig. 3-3). Suitable photoreactor concepts can only be obtained by an appropriate combination of these three aspects. Nevertheless, many commercially available photoreactor systems are still based on empirical developments coupled with successive improvements, and on the expertise of the manufacturers. The specialties and problems related to the lamp technology and the reaction system have already been discussed in detail (Chapters 4, 6 and 7, respectively). The engineering of photochemical reactor systems for the application of AOPs refers to the basic principles of chemical reaction engineering (e. g. Levenspiel, 1996, 1972, Hagen, 1993, Hopp, 1992, Jakubith, 1991). However, it has to take into account the particular concepts of photochemical engineering (e. g. Bolton, 2000, Braun et al., 1993 a, b), which include the following substantial issues: • specific excitation of the auxiliary oxidant, oxidant mixture or photocatalyst must be achieved; • total absorbance of electromagnetic radiation must be ensured to avoid unnecessary losses of radiant energy, for example via absorption by the reactor walls; • radiation losses by reflection and scattering should be minimized; • strongly developed heterogeneity of photochemical reactions within the photoreactor volume has to be considered. Reflection of UV radiation at the reactor walls (e. g. by polished aluminum) can be advantageous, since it produces a more uniform fluence rate field and the reactor is less subject to mixing inefficiencies (Bolton, 2002); • optimal oxygen saturation of the aqueous phase is necessary in most cases; • convergence of lamp and reactor geometry and the spatial separation between the lamp and the reaction mixture are important; • efficient cooling of the lamp in the case of MP Hg lamps and isothermal conditions for LP Hg lamps must be maintained during operation of the photoreactor. Actually over 90% of the cooling of an MP Hg lamp occurs by visible and IR radiative heat transfer into the water. Active cooling (by air flow) at the top

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end needs to be imposed when the lamp is in a vertical orientation (Bolton, 2002), or in the case of CFBRs; • precise description of the radiation distribution (photon flow and fluence rate profiles) within the reactor volume considering the heterogeneity of the photochemical process is necessary; • combination of this spatially defined energy absorption with the classical mass and energy system balances allows the description of the photoreactor/photochemical system. Particular problems of photochemical engineering are related to the scaling-up of photoreactors. This is mainly due to problems of lamp technology related to the variations of the radiant exitance M with the increase of the lamp’s geometry and electrical input power. Thus, to carry out a reasonable scaling-up and optimization of photoreactors the radiant exitance M or the radiant density (expressed as the ratio of radiant power P to the arc length l of the lamp in W cm–1, see Tab. 4-1) of the lamps used must be fixed (Braun et al., 1993 a). This, however, is a challenge for the manufacture of lamps. Classical chemical reaction engineering provides mathematical concepts to describe the ideal (and real) mass balances and reaction kinetics of commonly used reactor types that include discontinuous batch, mixed flow, plug flow, batch recirculation systems and staged or cascade reactor configurations (Levenspiel, 1996). Mixed flow reactors are sometimes referred to as continuously stirred tank reactors (CSTRs). The different reactor types are shown schematically in Fig. 8-1. All these reactor types and configurations are amenable to photochemical reaction engineering. Many commercial photochemical reactor systems make use of the batch recirculation mode for the treatment of highly contaminated wastewaters of limited volume. On the other hand, cascades of photoreactor modules (in serial or parallel mode) allow the gradual treatment of contaminated water streams with a very high photon flow Up in total. Hence, there exist powerful photochemical waste-

Types of ideal chemical reactors (Levenspiel, 1996). The batch recirculation reactor system consists of a vessel of total volume V and a reactor of a specified volume VR; CSTR: continuously stirred tank reactor. Fig. 8.1

8.1 Chemical Engineering

water treatment units with 12 or more modules, each containing a 30 kW MP Hg lamp (Bircher et al., 1997). The advantages of such arrangements are obvious: in • MP Hg lamps are available with a maximum of Pel of about 100 kW. The in photon flow does not scale proportionally with Pel above 10 kW (cf. Chapter 4-1, Fig. 4-2). Thus, as a compromise, modern photoreactor cascades use MP Hg in of 30 kW (cf. Trageser, 1997/ lamps with a maximum electrical input power Pel 98); • the problems of cooling and heat transport per cascade module are manageable; • the photon flow Up per cascade module is designed appropriately so that oxygen starvation within the photoreaction zone is minimized. This prevents the accumulation of organic polymers by radical–radical recombination of carboncentered radicals. If formed, such polymers could deposit on the quartz sleeve and block the UV transmission to the aqueous phase considerably. Levenspiel earlier presented, in 1972, a qualitative discussion about the product distribution related to photochemical reactions comparing batch and batch recirculation photochemical reactors. The essentials of this discussion can be transferred to photo-initiated AOPs (at least to the H2O2-UV process), which at low concentrations of a pollutant M ([M] <100 mg L–1) usually follow an overall first order reaction kinetics (Bolton et al., 1996). Thus, a molecule M is oxidized by photochemically generated •OH radicals to yield a primary oxidation product Mox 1 according to Eq. 8-1. During further treatis gradually oxidized by successive oxidation reactions to Mox ment, Mox 1 2 . . . to ox Mi , until finally the organic carbon is completely mineralized (see Fig. 6-16). • ox M + •OH ? Mox 1 + OH ??? Mi

(8-1)

Assuming that the photo-induced production of •OH radicals and the oxidation reactions are very fast and take place only within the volume where UV/VIS radiation is absorbed, two different concentration profiles can be distinguished theoretically (Fig. 8-2). In the batch mode, M will be oxidized by •OH radicals to Mox 1 (Fig. 8-2, situation A). Because initially M is in a very large excess, the •OH radicals will preferentially be scavenged by M. Therefore, the concentration of Mox 1 rises and subsequently it competes efficiently with M for •OH radicals leading slowly to the forox mation of Mox i . If most of M is oxidized the concentration of M1 reaches its maxi• mum and it will preferably react with OH radicals leading to a decrease in its concentration. Finally, the concentration of Mox increases. Therefore, the batch i photoreactor configuration seems to be best suited for the partial oxidation of hazardous pollutants in water. The accumulation of oxidation products Mox 1 must be taken into account with respect to their toxicity and their environmental impact. On the other hand, the treatment of M in a batch recirculation reactor system will generate a completely different concentration profile (Fig. 8-2, situation B). Small fractions of fluid are continuously removed from the batch tank and pumped through the photoreactor module (PR). Here, the same amount of •OH

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Comparison of the concentration– time profiles in a batch (A) and a batch recirculation photoreactor (PR) system (B) (adopted from Levenspiel 1972, p. 174/175). In

Fig. 8.2

both cases an identical photon flow Up of the lamp (L), total absorbance of all incident photons and a sufficient concentration of H2O2 are assumed.

radicals is produced as in the batch reactor system due to the same absorbed photon flow Up. Depending on the flow rate V˙ and the residence time of the fluid within the photoreactor module the reaction of M with •OH radicals can produce high concentrations of Mox 1 within the photoreactor. Now both compounds M and • compete for OH radicals leading to high concentrations of Mox Mox 1 i . Finally, this fluid is returned to the batch tank and diluted by mixing. Therefore, the batch recirculation configuration seems to be best suited for the mineralization of hazardous pollutants in water, i. e. for the purpose of TOC or DOC diminution. The overall efficiency of this version of the process (Fig. 8-2, situation B) is controlled by the mixing characteristics of the batch tank. However, the efficiency of the batch recirculation reactor is adversely affected when the recirculation flow rate V˙ is too low, such that almost complete depletion of the contaminant M takes place in each pass through the UV reactor. The ideal case is when the depletion of M is less than 5% per pass (Bolton, 2002). The above considerations are made on a theoretical basis, so that many other factors (inefficient stirring, molecular diffusion, flow conditions, competing absorption of VUV/UV radiation by photoproducts, concentration of dissolved molecular oxygen, etc.) gain influences on the course of the concentration profiles (Andre et al., 1983).

8.2 Photoreactor Concepts

Cross-sectional view and top-view of various photoreactor types with different arrangement of the lamps. A: Continuous flow annular photoreactor with coaxial lamp position, B: external lamp position with reflector (R), C: perpendicular lamp position; D: contact-free photoreactor types. The possible ar-

Fig. 8.3

rangements of irradiated planes include the falling film photoreactor: vertical plane, the flat bed photoreactor: inclined plane and the contact-free batch photoreactor (CFBR): horizontal plane. L: Lamp = VUV, UV, UV/VIS source, RV: reactor vessel; Q: quartz tube.

8.2

Photoreactor Concepts

The various lamp configurations common in photo-initiated AOTs are shown in Fig. 8-3. For a comprehensive discussion of laboratory and industrial scale photoreactors refer to Braun et al. (1991). Two different arrangements of the lamp are possible, which lead to two fundamental classes of photoreactors: the tubular flow-through photoreactor types and the contact-free photoreactor systems. The first category can be subdivided into annular reactors, with the lamp(s) centered parallel to the axis of the reactor vessel (Fig. 8-3, situation A), and into cylindrical reactors with a coaxial radiation field that is generated by lamps which are placed outside of the quartz tube (Fig. 8-3, situation B). This arrangement of the lamps requires effective reflectors to make use of the outward directed radiation. Unfortunately, self-absorption by the lamp, in the case of a mercury vapor lamp, and reflection phenomena inevitably lead to a considerable loss of UV radiation. A special development uses spiral-shaped LP and MP Hg lamps, as is shown in Fig. 84. Again, this configuration requires effective reflectors to utilize UV radiation that is directed away from the reactor.

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Other photoreactors make use of lamps that are arranged perpendicularly to the flow of the medium (Fig. 8-3, situation C) (FIGAWA, 1990). For the treatment of aqueous media, the lamps must be housed in quartz jackets to permit immersion within the aqueous stream. Thus, the photoreactor categories A, B and C (Fig. 83) all have an interface at the surface of the quartz sleeve, providing direct contact with the water. At this interface, the VUV/UV/VIS radiation penetrates into the water generating a high photon irradiance Ep. Thus, high local concentrations of intermediate radicals may produce polymers. Therefore, at the quartz/water interface the formation of radiation absorbing films may diminish the performance of the radiation-based processes considerably. Hence, these reactor types may suffer from the “filming” problem. This is also the case during photo-initiated AOP treatment of highly contaminated wastewaters that contain, for example, oil or grease suspensions. The filming effect is strongly dependent on the quality of the wastewater. Fouling of quartz sleeves represents a special problem in UV disinfection technology. Extensive studies have been performed to understand the mechanisms of inorganic fouling on hydrophilic quartz surfaces (Lin et al., 1999) and to develop appropriate cleaning technologies, for instance in situ mechanical wiping mechanisms that use Teflon rings or metal brushes adapted to the surface of the quartz sleeves, or the use of ultrasonic devices. Organic fouling of UV lamp sleeves is in-

Continuous flow annular photoreactor with external spiral shaped lamp position; reproduced by permission of UMEX GmbH (Dresden, Germany).

Fig. 8.4

8.2 Photoreactor Concepts

fluenced by the bacterial content of the water and by natural organic matter. Karim and Ronald (2001) presented a summary of fouling studies. Even the formation of bio-films (bio fouling) is possible on quartz sleeves of UV lamps (e. g. Flemming et al., 2000, Flemming, 1991). Besides the mechanical wiping techniques several other efficient countermeasures against inorganic and organic fouling or film formation on quartz sleeves are possible: • the use of UV transparent and hydrophobic fluoropolymer sleeves instead of quartz material seems to have several advantages that include non-fouling and non-breaking properties. Hence, LP Hg lamps can be covered by fluoropolymers due to their heat-shrinkable properties providing an additional safety feature (Gobulukoglu, 2001); • the generation of a rotating water pillar within the irradiation chamber of the photoreactor by tangential water injection (VitaTec, Freigericht, Germany); • water flow rates V˙ with a Reynolds number Re of about 106 can generate highly turbulent conditions that result in a narrow residence time behavior VR =m3 s=h V=m 3 h 1 and a high abrasivity (a.c.k. aqua concept, Karlsruhe, Germany); _ • some installations use a chemical cleaning cycle for the detachment of acid soluble films from the quartz sleeve by acid rinsing;

Fig. 8.5 Falling film photoreactor [Pin el (max) = 15 kW, V˙(max) = 10 m3 h–1] used for AOX diminution of a highly contaminated production wastewater (Debus, 2000) and schematic representation of the interior view. L: LP or MP Hg lamps, Q: quartz protection tube,

R: reflector. Specifications for AOX elimination were: 30 LP Hg lamps with Pin el of 120 W each, UV irradiated area A ca. 4 m2, volume flow rate of wastewater V˙ = 8 m3 per day; reproduced by permission of E. Thomanetz (University of Stuttgart, Germany).

245

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8 Process Engineering and Reactor Concepts

Fig. 8.6 Flat bed photoreactor (UviKlar®) for the contact-free treatment of highly contaminated wastewater. L: MP Hg lamp, FB: flat bed; reproduced by permission of VitaTec UV-Systeme GmbH (Freigericht, Germany).

Schematic representation of the contact-free batch reactor (CFBR). L: MP Hg lamp (Pin el = 15 kW), R: reflector.

Fig. 8.7

• the use of contact-free photoreactors (Fig. 8-3, situation D) where the UV/VIS source is installed above the water phase to be treated. The contact-free photoreactors include the vertical falling film design (Fig. 8-5), the inclined flat bed system (Fig. 8-6) and the batch photoreactor (Figs. 8-7 and 88). These reactor configurations do not have a quartz/water interface (as in situations A, B, C, Fig. 8-3), so that the problems related to fouling and film formation are effectively circumvented. Such photoreactor types are even suited for the UV treatment of suspensions and cloudy wastewaters (cf. Kim and Thomanetz, 1995).

8.2 Photoreactor Concepts

The contact-free batch reactor (CFBR) and its main components: the MP Hg lamp and the reflector are installed within a safety cabinet and efficiently cooled by air ventilation. The batch tank optionally consists

Fig. 8.8

of stainless steel or polypropylene. The schematic drawing of the reactor is shown in Figure 8-7; reproduced by permission of E. Thomanetz (University of Stuttgart, Germany).

However, in some contact-free batch reactor (CFBR) installations the MP Hg lamp must be protected by a quartz pane from squirting water or from volatile organic compounds that evaporate from the water phase. The complexity of photo-initiated AOPs makes it very difficult to predict the efficiency of a distinct process related to special wastewaters. Therefore, almost every manufacturer uses mobile pilot UV irradiation systems to gain experience with the special treatment case directly on the spot. Such a system for the H2O2UV AOP is shown in Fig. 8-9. It has the all-important peripheral system components including pumps, flow meters and UV detectors mounted on a mobile rack allowing test experiments under real conditions. The Figs. 8-10 and 8-11 present examples of photoreactors, which are commonly used in industry to treat highly contaminated wastewaters by the H2O2-UV process. These systems usually consist of simple photoreactor modules that may be extended by serial connection, thus allowing the efficient treatment of wastewaters in cascades of photoreactors. Both systems of Figs 8-10 and 8-11 use the principle of a rotational water column within the irradiation chamber in order to minimize the formation of UV-blocking

247

248

8 Process Engineering and Reactor Concepts Fig. 8.9 A mobile photoreactor module designed to perform field trials. Reactor characterization: two spiral shaped MP Hg lamps (2 × Pin el = 6 kW) with an irradiation length of 120 cm each, maximum flow rate V˙max = 10 m3 h–1; reproduced by permission of UMEX GmbH (Dresden, Germany).

Fig. 8.10 The Enviolet® photoreactor module

consists of a borosilicate glass pipe with a MP Hg lamp (Pin el = 12 kW). Special flow regulators create a rotational flow of water with Reynolds numbers Re of up to 106. Thus, a highly turbulent and abrasive flow condition is maintained throughout the UV treatment that prevents deposits on the quartz sleeve; reproduced by permission of a.c.k. aqua concept GmbH (Karlsruhe, Germany).

8.2 Photoreactor Concepts ®

Fig. 8.11 Two UviTox photoreactor modules connected in series. Each module contains a MP Hg lamp of Pin el = 30 kW. The tangential injection technique (I) generates a rotating water column within the irradiation chamber of the photoreactor; reproduced by permission of VitaTec UV-Systeme GmbH (Freigericht, Germany).

Fig. 8.12 The Rayox® photoreactor tower for drinking water treatment. Characterization: the inner diameter d of the reactor is 122 cm and the stainless steel vessel has a volume V of about 3000 L. From 6 to 24 MP Hg lamps with Pin el of 20 kW each can be placed in a single tower [Pin el (max) = 480 kW]. The lamps are arranged horizontally (cf. Figure 8-3, situation C) in a criss-cross pattern. The typical range of the hydraulic flow rate V˙ through the tower is between 1.14 m3 min–1 and 11.4 m3 min–1; reproduced by permission of Calgon Carbon Corporation (Pittsburgh, PA, USA).

249

250

8 Process Engineering and Reactor Concepts

polymeric films on the quartz sleeves. They are typical representatives of flowthrough photoreactors with coaxial MP Hg lamps (Fig. 8-3, situation A). in A powerful photoreactor with a maximum electric input power Pel of 480 kW (Fig. 8-12) was designed by Calgon Carbon Corporation (Pittsburgh, PA, USA, product information). This mighty photoreactor was constructed to treat drinking water that is micro-contaminated with N-nitrosodimethyl amine [NDMA: (CH3)2N–N=O, see Tab. 5-2 for case studies]. This compound is a by-product in certain industries or it may be formed in the natural environment by nitrosation of dimethylamine [(CH3)2NH] with nitrite ions (NO–2). NDMA has been shown to be carcinogenic and mutagenic in animals. The current drinking water limits in the USA and in Canada range from 2 ppt to 14 ppt. In the UV treatment of drinking water in particular, it is very important to utilize a long pathlength in order to guarantee the absorption of all photons by the water constituents and not by the reactor wall. Thus, this photoreactor is optimally suited for the UV-C treatment of large volumes of drinking water. Many research activities in the field of AOPs and AOTs are related to photoreactor development, reactor modeling, optimization and scaling-up of photoreactors. Several recent examples are collected in Tab. 8-1.

8.3

Reactor Materials

For applications in process technology, the reactor vessels of UV photoreactors can be built of stainless steel, titanium, glass, or plastic material [e. g. polypropylene, poly(vinyl chloride)]. In the last case, care must be taken to avoid any short wavelength irradiation of the UV sensitive plastic material. Furthermore, plastic materials can absorb organic starting materials and intermediates, and then slowly leach them back into solution. Thus, the selection of the reactor material depends mainly on the corrosiveness of the wastewater to be treated, and on economic considerations. Wastewaters with a high content of chloride ions (Cl–) often cause severe corrosion damage of stainless steel material. The lamp jackets can be constructed of quartz glass (sometimes called fused silica), highly transparent Suprasil quartz material, or UV transparent fluoropolymer (Gobulukoglu, 2001). The last material is sensitive to heat and thus can only be used in connection with LP Hg lamps. The transmission spectra of several glasses are presented in Fig. 8-13 (Philips, 1993). In contrast to normal quartz material the transmittance of synthetic and highly pure Suprasil quartz (purity: 99.9% SiO2) extends down to k of about 165 nm. At 190 nm, it still has a transmittance T of over 80%, whereas the transmittance of normal quartz at this wavelength is insufficient for applications below k of 200 nm. Therefore, Suprasil quartz is best suited for the construction of VUV lamps or VUV photoreactors (see Fig.s 4-13 to 4-15). In addition, it enhances the VUV emission of MP Hg lamps when used as the envelope material (see Fig. 43). However, Suprasil quartz is expensive and very sensitive to mechanical strain.

Application a)

Solar TiO2-UV/VIS TiO2-UV-A TiO2-UV-A Solar-TiO2-UV/VIS TiO2-UV-A H2O2-AOP TiO2-UV-A H2O2-AOP TiO2-UV-A UV-C TiO2-UV-A Solar UV/VIS TiO2-UV/VIS TiO2-UV-A TiO2-UV/VIS Solar TiO2-UV/VIS TiO2-UV/VIS TiO2-UV-A Solar TiO2-UV/VIS H2O2/O3-UV H2O2-UV H2O2-AOP H2O2-UV H2O2-UV UV

Specification

Type of photoreactor b) Aerated cascade reactor Batch rotating disk reactor Coated glass beads reactor Concentrating and non-concentrating solar reactors Double-skin sheet reactor Falling film reactor Falling film reactor Flat bed reactor Fountain photocatalytic reactor Membrane UV reactor Multiple tube reactor, tube light reactor Outdoor simulation chamber EUPHORE Photocatalytic membrane reactor Rotating disk photocatalytic reactor Sonophotocatalytic reactor Solar photocatalytic plant Spinning disk reactor Thin-film cascade reactor Thin-film fixed-bed reactor Thin-film UV reactor Thin-layer rolling reactor Tube reactor system UV cascade reactor

Photoreactor modeling c) Annular photoreactor Annular photoreactor

Xi et al. (2000) Hamill et al. (2001) Franke and Franke (1999) Zhang et al. (1994) Dillert et al. (1999) Debus (2000) Sclafani et al. (1999) Trageser and Greunig (2000) Puma and Yue (2001) Li and Tan (2001) Mukherjee and Ray (1999) Brockmann et al. (1996) Molinari et al. (2001) Dionysiou et al. (2000), (2001), (2002) Davydov et al. (2001) Malato et al. (2002) Yatmaz et al. (2001) Chan et al. (2000) Freudenhammer et al. (1997) Wenzel et al. (1999) Kreisel and Schmidt (1999) Trageser and Greunig (2000) Sprehe et al. (2000)

References

UV fluence rate distribution Bolton (2000) Fluence rate distribution, effects Bolton (2000) of reflection and refraction

Pilot Laboratory Laboratory Pilot Pilot Industry Pilot Industry Pilot Laboratory Laboratory Atmospheric simulation Laboratory Laboratory Laboratory Pilot Laboratory Pilot Pilot Pilot Laboratory Industry Pilot

Development status/remarks

Tab. 8.1 Selected examples of photoreactor development, photoreactor modeling, scaling-up and optimization

8.3 Reactor Materials 251

UV UV UV UV UV Solar UV/VIS TiO2-UV/VIS TiO2-UV-A TiO2-UV/VIS H2O2-UV O3-UV, H2O2-UV UV

UV Fe2+-H2O2-UV UV TiO2-UV/VIS Fe2+/3+-H2O2-UV/VIS

Annular photoreactor Annular photoreactor Continuous photochemical reactor Cylindrical mirror photoreactor Cylindrical photoreactor Flat-plate solar photoreactor Fluidized-bed photocatalytic reactor

Fountain photocatalytic reactor Photocatalytic reactor

Photochemical batch stirred tank reactor

Photochemical tubular flow and stirred tank reactor

Photoreactors

Scaling-up and optimization Annular photochemical reactors Light-enhanced Fenton reaction Photochemical reactors

Photochemical reactors Photo-Fenton prototype reactor

Yue (1993)

Shimoda et al. (1997)

Prengle Jr et al. (1996)

Puma and Yue (2001) Periyathamby and Ray (1999)

Irazoqui et al. (1973) Suidan and Severin (1986) Do Nascimento et al. (1994) Findeling et al. (1993) Roger and Villermaux (1983) Rossetti et al. (1998) Chiovetta et al. (2001)

References

Effects of radiation characteristics Shirotsuka and Sudo (1979) Optimal experimental design Oliveros et al. (1997) Mass and radiation energy bal- Braun et al. (1993) ances Optimal experimental design Jakob et al. (1993) Reaction system Krutzler and Bauer (1999)

Radiation profiles Radiation intensity models Actinometry Optical system Absorption of radiation Radiation field and actinometry Radiation field, reaction simulation Parabolic profile Computational fluid dynamics, reacting flow, reaction simulation Molecular composition structure model Reaction simulation, reactor comparison Reaction system, energy absorption

Development status/remarks

a) Only some examples are stated. b) Naming of the photoreactors is adapted from the literature. c) Includes the modeling of the UV/VIS source, the energy absorption, the flow dynamics, and the reaction system in order to simulate the AOP.

Application a)

Specification

Tab. 8.1 (continued)

252

8 Process Engineering and Reactor Concepts

8.3 Reactor Materials Fig. 8.13 UV transmittance T of several optical materials at a thickness of 10 mm (cf. Murov, 1973). 1: Suprasil quartz, 2: quartz, 3: Ultrasil, 4: Infrasil, 5: Optosil; redrawn from Philips (1993): Figure 21.

Fig. 8.14 Reflectance R of several metals used in engineering; redrawn from Philips (1993): Figure 47.

The internal transmittance of quartz and Suprasil quartz is related to the thickness of the sample according to the Beer–Lambert law, decreasing with increasing thickness of the quartz layer. Flow-through photoreactors (cf. Fig. 8-3) require quartz sleeves for lamp protection and hence suffer from radiation losses due to reflectance at the air(gas)/quartz and quartz/water interfaces (cf. Fig. 3-11). Reflection losses depend on the magnitude of the refraction index n and are usually in the range of about 7 to 8% for n = 1.45 (Bolton, 1999, Braun et al., 1991, Scaiano, 1989). Thus, a maximum transmittance T of quartz materials of 93% can be reached. The application of the Fresnel Law was demonstrated by Bolton (2000).

253

254

8 Process Engineering and Reactor Concepts

The contact-free photoreactors (falling film, flat bed and batch photoreactors) need efficient reflectors to use the outward directed photon flow of the LP or MP Hg lamps. Fig. 8-14 demonstrates the importance of aluminum as a suitable reflector material in the UV and VUV region of the electromagnetic spectrum. Fortunately, there are aluminum-based reflectors commercially available that can be integrated into photoreactor concepts. Such reflector systems are used in typical lighting industries. Silver and palladium cannot be used as VUV/UV reflectors without considerable losses of radiant energy. Detailed information about materials in photoreactor design and about ozone handling for O3-UV-AOPs are available from Braun et al. (1991), Scaiano (1989) and Gottschalk et al. (2000), respectively.

8.4

Influences on the Process Performance and Design Criteria

The efficiency of the process of photo-initiated AOPs is strongly influenced by the water (or air) quality, by many different engineering parameters and by the analytical strategy (Fig. 8-15). The last indirectly influences the performance of the process. It is related to the differentiation of treatment goals that can include mineralization by elimination of global parameters (cf. Tab. 5-1), oxidation of a group of compounds (e. g. chlorinated hydrocarbons), partial oxidation of non-biodegradable pollutants to enhance their biodegradability (e. g. antibiotics), or the oxidation

Fig. 8.15 Summary of factors influencing the reaction kinetics and the process efficiency of photo-initiated AOPs.

8.4 Influences on the Process Performance and Design Criteria

of a specific substrate. The oxidation of specific substrates is usually fast, unlike the elimination of global and specific parameters (cf. Fig.s 7-14 and 7-20). Therefore, it is very important to determine the treatment objectives exactly (partial or complete oxidation, detoxification, or mineralization) before establishing a photoinitiated AOP. Furthermore, water analysis must establish the process-affecting constituents of the water matrix, such as for example, • bicarbonate/carbonate concentrations and alkalinity (alkalinity = [OH–] + [HCO–3] + + 2[CO2– 3 ] – [H3O ]); – • the acid neutralizing capacity (ANC = [OH–] + [HCO–3] + 2[CO2– 3 ] + [B(OH)4] + – 2– – – + 3+ [H3SiO4] + [HPO4 ] + [HS ] + [NOM ] + . . . – [H3O ] – 3[Al ] – ... (vanLoon and Duffy, 2000); • concentration of other inorganic interfering species (e. g. [Cl–], [NH+4 ], [NO–2], [NO–3], [CN–]); • concentration of heavy metals (e. g. [Fe3+], [Cu2+], [Crn+], [Mn2+], [Ni2+], [Zn2+]); • global and group parameters of organic contamination (e. g. COD, TOC, BOD, AOX, CHC, PAH); • oil, grease and suspended solids (SS), possibly CFU and other biological parameters of significant relevance. The outcome of the water analyses determines the method(s) of pretreatment, for example filtering if the concentration of suspended solids is too high ([SS] >50 mg L–1) or clarification using a clarifier for removal of SS, oil droplets or other suspended materials. For a first orientation, Chemviron Carbon (1997) presented a helpful checklist of contaminants that are most amenable to photo-initiated AOPs. However, many wastewaters contain complex mixtures of different pollutants (e. g. chemical leachates). As a consequence, these cases require detailed design tests with a representative water sample (on laboratory scales) prior to the installation of a photoreactor. Details concerning the H2O2-UV AOP can be found in Chemviron Carbon (1997). Weichgrebe and Vogelpohl (1995) published a comprehensive scheme of design experiments for the selection of H2O2-UV versus O3-UV and TiO2-UV/VIS AOPs. Rice (2001) has given additional information concerning tentative design rules for photo-initiated AOPs. Before conducting any design tests, the measurement of the UV absorbance spectrum (usually between 200 nm < k < 400 nm) of the water (or gas) to be treated is most important to gain valuable information on its UV transmittance Tk and UV absorbance Ak, possibly on the concentrations of substrates and/or pollutants and thus on the penetration depth of UV radiation. A generally accepted standard value for water characterization is the transmittance T (at l = 10 mm) at k of 253.7 nm. This value determines the optical pathlength of the photoreactor in order to guarantee total absorbance of the UV radiation by the water to be treated, and thus in part determines the photoreactor technology. Further, these UV absorbance spectra provide important information concerning the choice of the appropriate lamp technology and of the auxiliary oxidant to be applied.

255

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8 Process Engineering and Reactor Concepts

Fig. 8.16 Schematic representation of a pilot unit for the performance of design tests for the H2O2-UV, O3-UV and TiO2-UV/VIS AOPs (adopted from Weichgrebe and Vogelpohl, 1995).

In all cases proper design tests have to be performed by using a photochemical pilot unit (for an example see Weichgrebe und Vogelpohl, 1995 and Fig. 8-16) that is adapted to the development and variation of the key process parameters affecting the AOP, such as • variation of the concentration of auxiliary oxidants and/or catalysts ([H2O2], [O3], [Fe2+/3+], [TiO2], or of their mixtures); • influence of the concentration of dissolved molecular oxygen [O2]; • influence of pH variations. Common values of pH are as follows: H2O2-UV between 2 and 8, O3-UV between 2 and 8, H2O2-O3-UV around 7, TiO2-UV/VIS between 1 and 10 (cf. Legrini et al., 1993), Fen+-H2O2-UV below 4; • variation of the flow rate V˙ (in the case of plug flow or batch recirculation reactors, or cascade photoreactor systems) and influence of the residence time behavior; • influence of the photon fluence Hp,0 or in a qualitative manner the influence of in of the radiant source. the input power Pel During the design experiments, many observations regarding the future engineering concept can be gathered, for example, concerning the formation of inner filters or films on the quartz/water interface. The proper and thorough development of these parameters finally allows the process optimization by producing conditions for a scaling-up that results in con-

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs

struction of the photochemical plant. The heat production and resulting cooling measures of the lamp system have to be integrated into the final photoreactor design. Furthermore, the design experiments lead to a phenomenological description of the overall reaction kinetics. Upon completion of the design test series, a comprehensive report (or publication) should contain precise data (cf. Braun and Oliveros, 1997) concerning: • the results of analyses of the water matrix, including pH changes during the photo-initiated AOP treatment: pHt=0 ? pHt; • the results of photochemical experiments with respect to the concentrations and dosages of auxiliary oxidants, catalysts and substrate or water parameter variations: [C]t=0 ? [C]t in mol L–1 or g L–1; • the determination of the design parameter EEO (cf. Chapter 8.5); • thermodynamic quantities: temperature of the water treated at inlet and reactor outlet positions; temperature and pressure in the case of gas treatment; • the specification of optional gas supplies (air, O2, production of O3 or use of inert gases N2, Ar); • the total volume V of the water treated, the volume V/L of the irradiation chamber, and the irradiation pathlength. In the case of TiO2-UV or CFBR installations the thickness of the irradiated suspension or water-film, or the irradiated area; • in the case of continuous processes the flow rate V˙/m3 h–1 through the irradiation unit(s) (or the residence time s/s); • the detailed description of the VUV/UV/VIS source (type, manufacturer, irradiain /kW including or excluding the power supply, power density tion length, Pel –1 in W cm ), if possible the electrical efficiency gel of the power supply; • the emission spectrum of the lamp used, and if possible the spectral photon flow Up at wavelength k, per unit wavelength interval Dk or over the spectral range of interest; • the surface of the light source, its temperature control and the geometry of irradiation.

8.5

Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs

Which of the various photo-initiated AOPs is best suited for a specific treatment case depends on the water quality, the performance related to pollutant removal, to the diminution of toxicity, to the possible formation of by-products and the formation of secondary wastes. Further, capital and operating costs and other factors such as ease of operation and maintenance have to be considered (Bolton et al., 1998). Simple theoretical comparisons of relative efficiencies of AOPs may generate contradictory results, as is exemplified in Tab. 8-2, mainly depending on the basis of calculation (Bolton and Cater, 1994, Glaze et al., 1987). The method favored by Glaze et al. (1987) used the same molar concentrations of auxiliary oxidants

257

258

8 Process Engineering and Reactor Concepts Tab. 8.2 Theoretical comparison of H2O2-UV and O3-UV AOPs: formation of •OH radicals from photolysis of O3 and H2O2 (Glaze et al., 1987) and energy requirements of O3-H2O2 and H2O2UV (Bolton and Cater, 1994)

Auxiliary oxidant or AOP

Molar absorption Stoichiometry coefficient e253.7 nm/ L mol–1 cm–1

H2O2 O3 O3-H2O2 H2O2-UV TiO2-UV

20 3300 – – –

H2O2 ? 2 •OH 3O3 ? 2 •OH c) – – –

•

OH Radicals Amount of energy formed per inci- Eel/kW h required to produce 1 mol dent photon a) of •OH radicals b)

0.09 2.00 – – –

1.05 d) 0.54 e), 0.71 f) 11.49 g)

a) Based on a pathlength l of 10 cm, quantum yields as predicted from stoichiometry, and equal concentrations [O3] = [H2O2] = 1 × 10–4 mol L–1. b) Based on the following assumptions. c) The correct stoichiometry is: 3O3 + HO– + H+ ? 2•OH + 4O2 (Gottschalk et al., 2000). d) Efficiency of O3 generation g(O3) = 10 kW h per pound (0.4536 kg) of O3 from air. e) Efficiency of photon formation of the lamp used below k of 300 nm gp = 30%, and the concentration of H2O2 must be sufficient to absorb at least 90% of the incident photons below k of 280 nm 0 (absorbance of H2O2 A = log PPtrans >1). f) gp of the lamp used was 25% (Bolton et al., 1998). g) For the calculation the following parameters were used: the efficiency gp of the “tanning” lamps at k of 360 nm is about 20%, the quantum yield U•OH = 0.04 (Bolton et al., 1998).

[a-Ox] = 1 × 10–4 mol L–1, i. e. [O3] = 4.8 mg L–1 and [H2O2] = 3.4 mg L–1, at a defined pathelength l of 10 cm. Thus, this calculation leads to completely different absorption conditions in each case, i. e. total absorbance of radiation in the case of O3 (A253.7 = 3.3) and for H2O2 the absorbance A253.7 = ekc l calculates to be 0.02. Under real conditions, this is rarely the case because usually higher H2O2 concentrations are applied (cf. conditions of the experiment depicted by Fig. 7-2). On the other hand, Bolton and Cater (1994) based their comparison of the non-photochemical O3-H2O2 AOP with H2O2-UV on the energy consumption needed to produce the same amount of hydroxyl radicals in both cases, thus including the electrical energy that is necessary to generate O3 from air (Tab. 8-2). This seems to be a more realistic approach although the costs of H2O2 have not been taken into consideration. Although the diminution of the concentration of a substrate M or of a global parameter such as TOC (represented by M) by AOPs usually involves very complex schemes of radical reactions (cf. Fig.s 7-1, 7-3 and 7-4) the overall (i. e. the observed) reaction kinetics can often be described phenomenologically by simple rate expressions (Tab. 8-3). The underlying reaction mechanisms and rate expressions for AOPs are demonstrated in Fig. 8-17. The kinetic modeling of these complex processes according to Bolton et al. (2001a) involves the following equations with respect to the formation (Equation 8-2) and consumption of •OH radicals, namely their desired reaction to yield the

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs Tab. 8.3 Simple kinetic models for AOPs related to the concentration of the substrate (accord-

ing to Ollis, 2000) AOP

Substrate concentration [M]

Rate expression

O3-UV, H2O2-UV

Low a)

First order: r = –

O3 H2O2-UV

Moderate high a)

Zero order: r = –

O3

High

Half order: r = –

TiO2-UV/VIS

–

Saturation b): r =

dM dt dM dt

= k(1) [M]1.0 = k(0)

dM dt = – dM dt

k [M]0.5 =

kM KM

a) The demarcation between low and high substrate concentrations varies considerably with the specific reaction system but is often around [M] of 100 mg L–1 (Bolton et al., 1996). b) K represents the apparent adsorption/desorption equilibrium constant.

Fig. 8.17 Competition kinetic scheme of the reaction of hydroxyl radicals with a substrate M in the presence of scavengers Si and the corresponding second order rate expressions.

oxidation products (Equation 8-3) and their undesired reactions leading to sideproducts by scavenging reactions (Equation 8-4), as is outlined in Fig. 8-17: a-Ox ! OH

r1 =

mol d OH Pel const: UP U OH v=V V Ls dt

(8-2)

r2 =

L d OH mol s dt

(8-3)

OH M ! products

kOH;M OHM

259

260

8 Process Engineering and Reactor Concepts

OH Si ! side products

ri =

L d OH mol s dt

kOH;Si OHSi

(8-4)

Where r1, r2 . . . . ri describe the rates of the individual reactions of •OH radicals. The constant term of Eq. 8-2 depends on the system and technology (its usual unit is const./mol s–1 kW–1): Pel/kW is the electrical power input to the system (i. e. in the case of photo-initiated AOPs this is Pel of the lamp used including the power supply), V/L is the volume of the medium (water or air) treated, Up/mol s–1 is the total photon flow from the lamp, U•OH is the quantum yield of production of •OH radicals and v is the fraction of the photon flow that enters the water in the reactor. Thus, a steady-state (SS) analysis (Bolton et al., 2001 a) is based on Eq. 8-5 that describes the rate of substrate oxidation by •OH radicals under steady-state conditions, in which the concentration of •OH radicals is unknown. However, the total rate of •OH radical formation and consumption under steady-state conditions can be described by Eq. (8-6), which is derived by adding up the rate expressions of Eqs. 8-2 to 8-4. This term is set to zero with the assumptions of extremely low • OH radical concentration and a zero rate of •OH radical formation in the steady-state. dM dt

k OH;M OHSS M

d OH Pel const: V dt

(8-5)

k OH;M OHSS M

X

k OH;Si OHSS Si 0

(8-6)

i

Consequently, the steady-state concentration of hydroxyl radicals can be described by Eq. 8-7 via the solution to Eq. 8-6 for [•OH]SS. OHSS

const: Pel V 1 P k OH;M M i k OH;Si Si

(8-7)

By inserting this expression for [•OH]SS (Eq. 8-7) into Eq. 8-5 the following overall rate law of the diminution of the substrate M (or global parameter) is derived (Eq. 8-8): dM k OH;M M const: Pel V 1 P dt k OH;M M i k OH;Si Si

(8-8)

Two limits of Eq. 8-8 demonstrate why AOPs often follow a simple overall zero or first order reaction kinetics (Bolton et al., 2001 a): P • if the concentration of the substrate M is high, so that k•OH,M [M] >> i k OH;Si Si , then the oxidation reaction of M dominates over the scavenging reactions (cf. Fig. 8-17) and the overall rate expression of Eq. 8-8 simplifies to Eq. 8-9: dM Pel k0 const: V dt

overall zero order reaction rate

(8-9)

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs

• if the concentration of the substrate M is low, so that k•OH,M [M] P i k OH;Si Si , then the scavenging of •OH radicals prevails (cf. Fig. 8-17) and the overall rate expression of Eq. 8-8 changes to Eq. 8-10: dM k OH;M M const: Pel V P dt i k OH;Si Si

1

k01 M overall first order reaction rate (8-10)

k P const: Pel V With the pseudo first order rate constant k'(1) = OH;M (all parameters are k OH;Si Si i assumed to be constants). Thus, two rate equations (Eqs. 8-9 and 8-10) were derived by steady-state analysis, which describes the zero and the first order overall reaction rates that are common for many photo-initiated (and other) AOPs in water and air. Once having established the two kinetic regimes for photo-initiated AOPs two figures-of-merit were proposed that can be used as valuable design parameters: one for high concentrations of the substrate M (usually [M] >100 mg L–1) and one for low concentrations of M (often [M] <100 mg L–1). The connection between electric energy Eel and the overall reaction kinetics of photochemical AOPs is obvious: the electric energy Eel/kWh needed to eliminate a specific amount of substrate M (or of a global parameter) is directly proportional to the electric power Pel of the lamp used, inversely proportional to the treated volume of water or air, and inversely proportional to the observed overall rate constant koverall of the process, el (see Fig. 8-18). i. e. Eel ~ V kPoverall Therefore, Bolton and Cater (1994) defined the figures-of-merit “electric energy per mass” (EEM) for use in the zero order kinetic regime and “electric energy per order” (EEO) for use in the first order kinetic regime of AOPs. This concept was refined (Bolton et al., 1996) and finally accepted by the IUPAC (Bolton et al., 2001 a) as a technical report, and extended by the design parameters “collector area per mass” (ACM) and “collector area per order” (ACO) for solar-driven AOPs. The EEM/EEO concept should allow an objective comparison of different AOPs including photo-initiated ones. The corresponding definitions are as follows (taken from Bolton et al., 2001 a): • EEM/kWh kg–1: “Electric energy per mass (EEM) is the electric energy in kilowatt hours (kWh) required to bring about the degradation of a unit mass (e. g., one kilogram, kg) of a contaminant C in polluted water or air”. • EEO/kW h m–3 order–1: “Electric energy per order (EEO) is the electric energy in kilowatt hours (kWh) required to degrade a contaminant C by one order of magnitude in a unit volume [e. g., 1 m3 (1000 L)] of contaminated water or air”. • ACM/m2 kg–1: “Collector area per mass (ACM) is the collector area required to bring about the degradation of a unit mass (e. g., one kilogram, kg) of a contaminant C in polluted water or air in a time t0 (1 h) when the incident solar irradiance is 1000 W m–2”. 1

261

262

8 Process Engineering and Reactor Concepts

Fig. 8.18 Relation of the EEM and EEO values to fundamental photochemical parameters. Up/mol s-1 is the photon flow emitted by the lamp, v is the fraction of these photons that is absorbed by the auxiliary oxidant (e.g. H2O2), U•OH is the quantum yield of •OH radical production, V/L is the volume of water or air treated, koverall represents the observed rate 0 constant of substrate diminution (k(0) or k1 , MM is the molar mass of the substrate M

treated. The factor 0.4343 converts ln to log C C (log Ct0 = 0.4343 ln Ct0 ). According to Eq. t

0 8-10, k1

t

k OH;M P const: Pel V k OH;Si Si i

1

and with

const. Pel = UPvU•OH the overall first order 0 is again inversely proporrate constant k1 tional to the fundamental photochemical parameters (for more details refer to Bolton et al., 2001a and Bolton, 2001).

• ACO/m2 m–3 order–1: “Collector area per order (ACO) is the collector area required to reduce the concentration of a contaminant C in polluted water or air in a unit volume (e. g., 1 m3) by one order of magnitude in a time t0 (1 h) when the incident solar radiation is 1000 W m–2”. The electric energy Eel/kW h = Pel/kW × t/h is defined as the total amount of energy that is supplied to the AOT treatment system, i. e. “out-of-the-wall”. It includes the electric efficiencies gel of any power supplies or transformers and corresponds to the electric energy Eel that is billed by the power company (Bolton et al., 2001a). The expression “one order of magnitude” is related to the diminution of concentrations of substrates, global or other parameters about 90%, i. e. from C initially [C]t=0 = 100% to finally [C]t = 10%, i. e. log Ct0 = 1, for example: [C]t=0 = t –1 –1 100 mg L reduced to [C]t = 10 mg L . The mathematical appropriate equations proposed by Bolton et al. (2001 a) for idealized batch (perfectly mixed) or flow-

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs Tab. 8.4 Design parameters (“figures-of-merit”) based on electric energy consumption proposed

by Bolton et al. (2001 a) for the comparison and scaling-up of various homogeneous and heterogeneous AOPs for water or air treatment: EEM electric energy per mass; EEO electric energy per order; ACM collector area per mass; ACO collector area per order Design parameter

Mathematical expression a)

EEM/kW h kg–1

EEM c) =

EEM/kW h kg–1

EEM =

EEM/kW h kg–1 EEM/kW h kg–1

Pel V_ MM Ct0 Ct

EEM d) = EEM =

Pel t 103 V MM Ct0 Ct

t 106

Pel VCt0 Ct 3

Pel 10 V_ Ct0 Ct

EEO/kW h m–3 order-1 EEO e) =

t 103

Pel C V log Ct0

Mode of operation

Units of concentrations of substrate M, [C] b)

Batch

mol L–1

Flow-through

mol L–1

Batch

mg L–1

Flow-through

mg L–1

Batch

mol L–1 or mg L–1

Flow-through

mol L–1 or mg L–1

t

EEO/kW h m–3 order–1 EEO =

Pel C V_ log Ct0 t

s t 103 AE ES0 t0 MM V Ct0 Ct s AE V_ MM Ct0 Ct s t 103 AE VCt0 Ct

ACM/m2 kg–1

ACMc, f) =

Batch

mol L–1

ACM/m2 kg–1

ACM g) =

Flow-through

mol L–1

ACM/m2 kg–1

ACM g) =

Batch

mg L–1

ACM/m2 kg–1

ACM g) =

Flow-through

mg L–1

ACO/m2 m–3 order–1

ACO g) =

Batch

mol L–1 or mg L–1

ACO/m2 m–3 order–1

ACO g) =

Flow-through

mol L–1 or mg L–1

s 103 AE V_ Ct0 Ct s t AE V log

Ct0 Ct

s AE C V_ log Ct0 t

a) Pel/kW: electric energy supplied to the AOT “out-of-the-wall”, MM/g mol–1: molar mass of the substrate M, in the case of TOC: MM = 12.011 g mol–1, V/L: volume of water or air treated in the time t/h (V/1000 US gallons = 3785.412 L), V_ Vt /m3 h–1: volume flow rate of water or air, A/m2: actual collector area. b) [C]t=0 and [C]t are the initial (or influent) and the final (or effluent) concentrations of the substrate (or global or other parameter). c) The factor 103 converts g into kg. d) The factor 106 converts mg to kg. 3 e) The factor 103 converts L into Rt m . ES tdt

s t0 f) ES: solar irradiance, E t t0 : the average solar irradiance can be determined by an integrating radiometer or an actinometer (Murov et al., 1993, Braun et al., 1991, Kuhn et al., 1989), ES0 : 1000 W m–2 is the standardized irradiance based on the AM 1.5 standard solar spectrum on a horizontal surface. g) Note that ES0 t0 = 1000 Wh m–2.

through (pure plug-flow) situations are collected in Tab. 8-4. These design parameters are correlated with the efficiency of an AOT and thus, the lower an EE or AC value the more efficient is the corresponding AOT. The use of the design parameters is illustrated by the following example calculations.

263

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Example 8-1 (taken from Bolton et al., 2001 a): 2000 L of wastewater containing 500 mg L–1 of TOC as phenol (C6H6O) are treated in a batch reactor for 10 h with a 30 kW AOT to yield an effluent with TOC of 100 mg L–1. The rate of degradation followed zero order kinetics. Calculate the corresponding design parameter. E EM

30 kW 10 h 106 375 kWh kg 2000 L 500 mg L 1 100 mg L 1

1

Example 8-2 (taken from Bolton et al., 2001 a): A contaminated groundwater that contains 200 lg L–1 of trichloroethene (C2HCl3) is treated with a 25 kW flowthrough AOT at a flow rate V˙ of 20 m3 h–1. The effluent concentration of C2HCl3 had dropped to 5 lg L–1. Calculate the appropriate design parameter. EEO

25 kW 0:78 kWh m lg L 1 20 m3 h 1 log 200 5 lg L 1

3

order

1

Example 8-3 (taken from Bolton et al., 2001 a): A solar collector of an area of 4 m2 is used to treat a water containing 500 mg L–1 of 1,4-dioxane (C4H8O2) at an average solar irradiance E¯s of 850 W m–2. The total volume V of the batch system was 300 L and the substrate was degraded within 1.5 h by a zero order reaction kinetics to a final concentration of [C4H8O2] = 200 mg L–1. Calculate the appropriate design parameter. ACM

4 m2 850 W m 300 L 500 mg L 1

2

1:5 h 103 56:7 m2 kg 200 mg L 1

1

In the case of photo-initiated AOTs the EE values are inversely proportional to the fundamental parameters of the irradiation system, i. e. the photon flow Up of the lamp, the fraction v of these photons absorbed by the auxiliary oxidant and the quantum yield U•OH: the higher the photon flow Up of a lamp, the more photons that are absorbed by the auxiliary oxidant and the higher the quantum yield of hydroxyl radical formation, the lower are the EE values. These fundamental relationships are schematically worked out in Fig. 8-18. In this connection, it is again essential to recall the basic principles of photoreactions in an aqueous phase as exemplified in Fig. 8-19 for the H2O2-UV AOP. The efficiency of this AOP is determined by a yield factor that is composed of four basic parameters: the ratio of photons that penetrate into the water phase to the total lamp ), the ratio of these photons that are absorbed photon flow of the lamp (UAq p /Up by H2O2 (vH2O2), the quantum yield of •OH radical formation and the intermolecular competition function FM,ox (cf. Eq. 7-1). A strategy of optimization of the H2O2UV AOP, i. e. the minimization of the EE design parameters, must be aimed at an lamp improvement of UAq , vH2O2 and FM,ox. In non-optimized situations, the yield p /Up factor can be reduced drastically with respect to the treatment goal of oxidation of the substrate M. Furthermore, the radiant power efficiency g of the lamp used contributes substantially to the overall costs of photo-initiated AOPs.

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs

Fig. 8.19 Fundamental processes influencing the efficiency of the H2O2-UV AOP in aqueous phase. M: substrate or other parameter (TOC, AOX.... etc.), NAbs: absorbing molecule that competes with H2O2 for photons, Si: •OH radical scavengers, L: VUV/UV/VIS lamp, Q: : total quartz sleeve of the photoreactor, Ulamp p

photon flow emitted by the lamp, UAq p : that which enters the aqueous part of Ulamp p phase, vH2O2 : the fraction of photons absorbed by H2O2, U•OH: quantum yield of •OH radical formation and FM,ox: competition function (see Eq. 7-1).

The design parameters allow the calculation of the system costs and of the total power and/or capital investments that are necessary for a specific application. Thus, the EE/AC concept represents a valuable tool for the scaling-up of AOTs (Bolton et al., 2001 a) and it provides the basis for a cost analysis (cf. Chemviron Carbon, 1997). It has, however, to be stated that several important limitations of this concept must be considered in order to develop a consistent set of reliable EE or AC data: • the AOP must be optimized prior to the determination of EE or AC values with respect to the parameters outlined in Fig. 8-15; • any inefficiencies of the photoreactor system and in mixing of the fluid will lead to higher experimental values of EEO; • the EEO concept does not apply to CSTRs because the fundamental kinetic C 1 description is different (cf. Levenspiel, 1996): C t 1ks , with s being space t0 time); • the EE/AC concept strictly applies only to reactions with pure overall zero or first order diminution kinetics; • EEM values are inherently dependent on the initial concentration of the pollutant or parameter to be removed. This is because the half lifetime t1/2 of a zero Ct 0 . Therefore, the values are order reaction is dependent on [C]t=0: t1/2 = 1/2 k0

265

266

8 Process Engineering and Reactor Concepts

•

•

•

•

only reliable when stated together with the initial concentration of the pollutant: EEM([C]t = 0]); unfortunately, the EEO values may also depend on [C]t=0 (Cater et al., 2000). This can be attributed to the rapid formation of intermediates during AOP treatment. At higher initial concentrations [C]t=0 of a pollutant higher concentrations of intermediates may be formed that may compete efficiently for •OH radicals. In the case of methyl-tert-butyl ether (MTBE) AOP treatment (H2O2-UV) the EEO value increased by a factor of about 42 when increasing [MTBE]t=0 from 100 ppb to 100000 ppb (cf. Tab. 8-5 and Cater et al., 2000); during the AOP treatment, changes of reaction kinetics from zero to first order may occur (cf. Fig. 7-14: TOC diminution). This leads to mixed order or even more complex kinetic situations, and thus the EE/AC concept cannot be applied; the determination of “real” EE/AC values must take into account the costs of auxiliary oxidants or catalysts. Auxiliary oxidants such as H2O2 should be regarded as “stored electric energy” and added to the experimental values. For example, 1 kg of H2O2 (100%) is equivalent to 10 kW h of energy, based on the commercial prices of hydrogen peroxide in Germany (Müller et al., 2000). Thus, the amount of H2O2 used must be stated. Accordingly, the costs of TiO2 and the costs of TiO2 immobilization on appropriate supports, and the investment concerning real estate for the installation of the huge solar collectors must be included in the same way. Furthermore, additional costs of base (or acid) consumption to maintain the pH (especially in the H2O2-O3 AOP) should be included. In the case of O3 based AOPs additional costs arise from off-gas control and treatment (cf. Fig. 8-16). The costs of active UV photons (cf. Tab. 4-1) and of O3 production (cf. Tab. 8-2) are easily available via the electric power consumption of the lamp or of the ozone generator and their electric efficiencies, respectively. Finally, these corrections must be stated in a report or publication in order to classify correctly the various results; a thorough investigation and determination of EE or AC values should indicate the basic reactor variables, as was outlined for photoreactors (Chapter 8.2).

Several EEO and EEM values that have been published lately are collected in Tab. 8.5. Müller et al. (2000) recently published a literature survey of EEO values of O3UV, H2O2-UV and H2O2-O3 AOPs. In contrast to the theoretical calculations presented in Tab. 8-2, these authors demonstrated that the O3-H2O2 AOP seems to be superior in several cases over O3-UV and H2O2-UV treatment, respectively. This was accomplished by comparison of published EEO values and it was again verified experimentally by determination of the corresponding EEOs of atrazine degradation under optimized conditions and by taking into account the costs of hydrogen peroxide (see Tab. 8-5). In all probability, each of the treatment variations O3-H2O2, O3-UV and H2O2-UV will find their niches in water purification and/or reclamation. The evaluation of the AOP treatment costs represents a very critical aspect. The overall costs of an AOT are composed of capital costs, the operating costs (including auxiliary oxidants and all other supplementary chemicals) and the costs of

Stefan and Bolton (1998)

EEM c, d) ([TOC]t=0 ~ 3.7 mmol L–1) = 1496 kW h kg–1 EEM c, e) ([TOC]t=0 ~ 3.7 mmol L–1) = 418 kW h kg–1 EEO c) ([C]t=0 = 100 mg L–1) = 3.09 kW h m–3 order–1 EEM c) ([TOC]t=0 ~ 3 mmol L–1) = 1268 kW h kg–1 EEO ([C]t=0 = 10 lg L–1) = 2.81 kW h m–3 order–1 EEO ([C]t=0 = 10 lg L–1) = 0.90 kW h m–3 order–1 EEOi) ([C]t=0 = 10 lg L–1) = 1.67 kW h m–3 order–1 EEOi) ([C]t=0 = 10 lg L–1) = 0.102 kW h m–3 order–1 EEO ([C]t=0 = 10 lg L–1) = 0.182 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 2–10 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 20–100 kW h m–3 order–1 EEO c) ([C]t=0 = 0.08 mg L–1) = 0.18 kW h m–3 order–1 EEO c) ([C]t=0 = 85 mg L–1) = 7.5 kW h m–3 order–1 EEO c) ([C]t=0 ~ lg L–1) = 0.11 kW h m–3 order–1 EEO c) ([C]t=0 ~ 0.5 mmol L–1) = 3.6 kW h m–3 order–1 EEOc) ([C]t=0 ~ 0.5 mmol L–1) = 336 kW h m–3 order–1 EEO c) ([C]t=0 ~ 150 lg L–1) = 0.5 kW h m–3 order–1 EEO c) ([C]t=0 ?) >250 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 40–250 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 0.5–5 kW h m–3 order–1

H2O2-UV a,b) H2O2-UV b, f) H2O2-UV b, g) UV (k = 253.7 nm) h) O3-UV h) H2O2-UV h) H2O2-O3 h) O3 h) H2O2-UV H2O2-UV H2O2-UV b, j) H2O2-UV b, j) UV-C b) H2O2-UV b) TiO2-UV b) H2O2-UV k) H2O2-UV H2O2-UV H2O2-UV

1,4-Dioxane

1,4-Dioxane Acetone Atrazine Atrazine Atrazine Atrazine Atrazine Benzene and its derivatives Dichloromethane MTBE MTBE NDMA Phenol Phenol Tetrachloroethene Tetrachloromethane Trichloromethane Unsaturated chlorinated Alkenes

a) d) e) h) i)

[H2O2] = 15 mmol L–1. b) EE values determined under optimized conditions using a batch recirculation photoreactor. c) Costs of H2O2 are not included. A significant change of the reaction kinetics was observed: EEM determination during the first 5–7 min of H2O2-UV treatment and subsequently till 20 min of treatment. f) [H2O2] = 200 mg L–1. g) [H2O2] = 3–16 mmol L–1. Determined under optimized conditions in a flow-through reactor system. Including the “stored electric energy” costs of H2O2 [1 kg of H2O2 (100%) : 10 kW h]. In the case of H2O2-UV: [H2O2] = 17 mg L–1 and for H2O2-O3: [H2O2] = 1.7 mg L–1. j) MTBE: methyl-tert-butyl ether, [H2O2] = 30 mg L–1. k) [H2O2] = 88 mg L–1, field-scale pilot plant for treatment of groundwater contaminated with Cl2C=CCl2.

Bolton (2001) Stefan et al. (1996) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Bolton (2001) Bolton (2001) Cater et al. (2000) Cater et al. (2000) Bolton et al. (2001 b) Bolton et al. (1998) Bolton et al. (1998) Hirvonen et al. (1998) Bolton (2001) Bolton (2001) Bolton (2001)

References

Design parameter

AOP

Substrate

Tab. 8.5 Several examples of typical EEM and EEO values published in the literature

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs 267

268

8 Process Engineering and Reactor Concepts

continuous maintenance. For a pilot or full-scale system, these costs strongly depend on the nature and concentration of the specific pollutant(s), the water quality, the water matrix, their stability over the treatment time, the flow rate V˙ of the effluent, the rector configuration and on the treatment goal. Further, a thorough cost evaluation requires the preceding optimization of the AOP and AOT, at least on a pilot scale. In the literature few data have been reported for AOP or AOT overall costs (refer to references cited in Andreozzi et al., 1999, to Chemviron Carbon, 1997 and to Hand et al., 1995). However, owing to their complex composition and to dependencies on plant specifications these data are often meaningless. More important, several comparable studies have demonstrated that the costs of AOP treatment of aqueous systems are at least comparable to conventional technologies of pollutant abatement (Hirvonen et al., 1998).

8.6

Selected Industrial Applications 8.6.1

Aqueous Systems

Many industrial applications of photo-initiated AOPs, including UV disinfection technology, have been published all over the technical and scientific primary and secondary literature and in many special periodicals. To obtain an overview, several examples of industrial and environmental significance were reviewed by Rice (1997) (O3, O3-UV, H2O2-UV and O3-H2O2-UV) and by Leitzke et al. (1996) (O3H2O2, H2O2-UV, O3-UV). Chemviron Carbon (1997) collected together further case histories of the H2O2-UV AOT. Trageser and Greunig (2000) presented the treatment of wastewater from metal processing industries that contain AOX, cyanides and organic metal complexes. The photo-initiated oxidation of persistent organic compounds in nickel containing waters of low optical transmittance (l <1 mm) by the H2O2-UV AOP could be satisfactorily achieved by using the photoreactor presented in Fig. 8-10 (Sörensen and Weckenmann, 1998). An interesting application of the contact-free batch reactor (CFBR, Fig. 8-8) for the elimination of AOX from a highly contaminated wastewater containing halogenated organic disinfectants, aromatic compounds and a high initial COD of 18250 mg L–1 was published by Debus (2000). The AOX could be eliminated by direct UV irradiation without addition of H2O2 (Fig. 8-20). High concentrations of H2O2 were counterproductive with respect to the AOX elimination. Thus, this is a rare example of direct UV treatment of a wastewater leading to the desired effect of AOX diminution.

8.6 Selected Industrial Applications

Fig. 8.20 Successful AOX elimination from a wastewater by direct UV irradiation under aerobic conditions. High concentrations of H2O2 exhibited a negative effect on the treatment efficiency. Conditions: photoreactor type CFBR (Figure 8-8), V = 1000 L, 2 MP Hg lamps (Pin el = 3.8 kW each) and 9 LP Hg lamps (Pin el = 250 W each), contaminants are preser-

vatives useful in cosmetics and food industries: p-chloro-m-cresol (C7H7ClO), p-chlorom-xylenol (C8H9ClO), o-benzyl-p-chlorophenol (C12H9ClO), 2-bromo-2-nitro-1,3-propanediol (C3H6BrNO4), 3-iodo-2-propinyl butylcarbamate (C8H12INO2) (redrawn from Debus, 2000: Figure 4).

8.6.2

Gaseous Systems Odor Abatement Many industrial and environmental activities lead to substantial odor emissions. For example, the recycling industry of household wastes (plastics, metals, food cans, milk containers etc.) produces sweet-sour odors that bother the neighborhood impertinently. Many molecular odor components absorb UV radiation below k of 300 nm. They are converted into odorless substances by photooxidation reactions (cf. Fig. 7-21). Consequently, the direct irradiation of odorous waste air streams with LP Hg lamps at k of 253.7 nm leads to a significant reduction of the odor emitted from such a plant. Also, in food processing industries (meat, fish, cereals etc.) photo-induced odor treatment strategies are of interest. Two examples of photo-induced odor abatement in industry are collected in Tab. 8-6. The photoreactor technology for waste air treatment is relatively simple: the contaminated waste air is filtered to remove aerosols, dust, and other particles and subsequently the air stream is blown through an irradiation chamber that contains a series of LP Hg lamps, possibly in connection with mats or layers of immobilized TiO2 photocatalyst. The corresponding photoreactor installation is shown in Fig. 8-21. This photoreactor concept can be applied for the irradiation of volume flow rates V˙ of waste gas from about 100 m3 h–1 to over 100 000 m3 h–1. Large installations in contain up to 72 LP Hg lamps with a total electric input power Pel of 5.4 kW (personal communication from HDN-Technik, Germany).

269

270

8 Process Engineering and Reactor Concepts Tab. 8.6 Odor abatement (diminution of odor units, ou, c.f. Eq. 7-9) by photo-induced oxidation

of waste air streams. For specification of the photoreactor, see Figure 8-21 (examples received from HDN-Technik GmbH, Rednitzhembach, Germany) Source of odor

Volume flow rate _ 3 h–1 V/m

Olfactometry odor/ou m–3

% Odor reduction

Before treatment a) After treatment b) Waste recycling plant (DSD) c) Food industry (production of muesli) e)

16 200 d)

3620

2 800 f)

8540

200 ± 13 1230

94% 86%

a) b) c) d)

Raw gas. Gas stream irradiated by UV radiation at k of 253.7 nm. DSD: Duales System Deutschland. The dry air stream was filtered and hence dust-free before UV treatment. It had a temperature T of 22.9 8C. e) Processing and drying of cereals (production capacity of 1970 kg h–1). f) The humidified air stream had a temperature T of 90 8C. In addition to the LP Hg lamps the photoreactor contained TiO2 immobilized on titanium fabric.

Fig. 8.21 Photoreactor for the treatment of odorous waste air streams containing 24 LP Hg lamps (Heraeus NNI 120/84, Pin el = 120 W each, maximum flow rate of air V˙ = 20 000 m3 h–1),

dimensions of the reactor: length × width × height = 2.0 m × 2.0 m × 5.6 m; reproduced with permission of HDN-Technik GmbH (Rednitzhembach, Germany).

8.6 Selected Industrial Applications

Fig. 8.22 A mobile system for indoor air treatment: efficient combination of adsorption, particle filtering, photocatalysis and direct UV-C irradiation. Technical data of the AEROPUR®-QA-20 device: length × width × height: 54.6 cm × 20.3 cm × 45.7 cm, weight: ca. 10.4 kg, V˙ ca. 7.5 m3 min–1, LP Hg lamp:

Pin el = 6 W, k = 253.7 nm. The lamp is housed in a quartz tube of length l of 28.5 cm and a diameter l of 1.6 cm; reproduced with permission of C & C Consult (Schleiden/Eifel, Germany). AEROPUR® is the registered Trade Mark of the Company C & C Consult, Schleiden/Eifel (Germany).

Air Purifier System A mobile air purifier system for home, office and hospital usage is presented schematically in Fig. 8-22. This system efficiently combines different air treatment techniques including photocatalysis and direct UV-C irradiation of indoor air streams. The air purifier reduces the level of airborne particles down to a size range of 0.3 lm by using a special filter technology. The LP Hg lamp exhibited a radiant power P of 11.7 mW cm–2 at a distance l of 2.54 cm from its surface. Thus, the combination of photocatalysis and direct UV-C irradiation of air streams that are contaminated with volatile organic chemicals, bacteria, viruses or other microorganisms can achieve an efficient purification and/or disinfection. This was demonstrated by the successful reduction of mould spores and bacteria in a room under standardized conditions (TÜV, 2002 a). During a treatment time t of 21 h, the colony forming units (CFU) of airborne moulds could be reduced by 96% per m–3 (Fig. 8-23) using the mobile combination system of Fig. 8-22. In addition, the photocatalytic oxidation of organic pollutants, for example formaldehyde in air, could be demonstrated successfully (TÜV, 2002 b). Hence, this air treatment apparatus seems to be well suited for specific applications such as filtering of the air supply for critical surgery rooms, temporary filtering of buildings that undergo asbestos abatement procedures, filtering of the air supply to commercial jetliners, and the prevention of contamination during critical manufacturing processes, degermination of air in the food industry, and air purification in hotel rooms, restaurants, sport and fitness centers etc.

271

272

8 Process Engineering and Reactor Concepts

Fig. 8.23 Reduction of airborne moulds using the AEROPUR®-QA-20 device in a room under standardized conditions. The inner walls of the room were coated with Teflon foil. It had a total volume V of 58 m3, a relative humidity of the air of 50 ± 5%, and a tempera-

ture T of 25 ± 2 8C. The colony forming units were determined to be CFUt=0 400 m–3 and CFUt=21h = 15 CFU m–3 (TÜV, 2002 a). AEROPUR® is the registered Trade Mark of the Company C & C Consult, Schleiden/Eifel (Germany).

Fig. 8.24 Surface disinfection of bottle screw

line speed ~ 25 cm s–1. The lamp housing contains a highly polished aluminum reflector; reproduced with permission of Dr. Hönle AG (Planegg/München, Germany).

caps which are applied in beverage producing industries by UV-C irradiation using a MP Hg –1 lamp, Pin el = 1.5 kW, power density = 100 W cm , flow rate of cooling air V˙ = 105 m3 h–1,

References

Surface Disinfection The disinfection of surfaces can easily be achieved by irradiation with LP or powerful MP Hg lamps. The latter need efficient air-cooling. An example applied in food and beverage industries is presented in Fig. 8-24. A line of screw caps for sealing beverage bottles is transported at high speed in front of an MP Hg lamp irradiation system, thus guaranteeing the disinfection of the inner surface of the screw caps.

References Andre JC, Bouchy M, Kossany J (1983) Computer Analysis of Mixing Problems Arising in Consecutive Photochemical Reactions, J. Photochem. 22: 213–221. Andreozzi R, Caprio V, Insola A, Marotta R (1999) Advanced Oxidation Processes (AOP) for Water Purification and Recovery, Catal. Today 53: 51–59. Bircher KG, Lem W, Simms KM, Dussert BW (1997) Combination of UV Oxidation with other Treatment Technologies for the Remediation of Contaminated Water, J. Adv. Oxid. Technol. 2, No. 3: 435–441. Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photosciences, ISBN 09685432-0-0; also published in: Eur. Photochem. Assoc. Newslett. Nr. 66: 9–36. Bolton JR (2000) Calculation of Ultraviolet Fluence Rate Distributions in an Annular Reactor: Significance of Refraction and Reflection, Wat. Res. 34, No. 13: 3315–3324. Bolton JR (2001) Non-Disinfection Based UV Applications, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bolton JR (2002) personal communication. Bolton JR, Bircher KG, Tumas W, Tolman CA (1996) Figures-of-Merit for the Technical Development and Application of Advanced Oxidation Processes, J. Adv. Oxid. Technol. 1, No. 1: 13–17. Bolton JR, Bircher KG, Tumas W, Tolman CA (2001 a) Figures-of-Merit for the Technical Development and Application of Advanced Oxidation Technologies for Both Electric- and Solar-Driven Systems, Pure Appl. Chem. 73, No. 4: 627–637; also available on the Internet: www.iupac.org/publications/pac/2001.

Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Helz G R (ed.), Aquatic and Surface Photochemistry, Lewis Publishers, Boca Raton, 467– 490. Bolton JR, Fitzsimmons SR, Yoo LJ, Shen YF (2001 b) Ultraviolet Treatment and Potential Formation of N-Nitrosodimethylamine (NDMA) in a Water Reclamation Treatment Plant, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bolton JR, Valladares JE, Zanin JP, Cooper WJ, Nickelsen MG, Kajdi DC, Waite TD, Kurucz CN (1998) Figures-of-Merit for Advanced Oxidation Technologies: A Comparison of Homogeneous UV/H2O2, Heterogeneous UV/TiO2 and Electron Beam Processes, J. Adv. Oxid. Technol. 3, No. 2: 174–181. Braun AM, Jakob L, Oliveros E (1993 b) Advanced Oxidation Processes – Concepts of Reactor Design, J. Water SRT-Aqua 42, No. 3: 166–173. Braun AM, Jakob L, Oliveros E, Do Nascimento CAO (1993 a) Up-Scaling Photochemical Reactions, in: Volman D, Hammond G S, Neckers D C (eds.) Adv. Photochem. 18: 235–313. Braun AM, Maurette M-T, Oliveros E (1991) Photochemical Technology, John Wiley & Sons, Chichester. Braun AM, Oliveros E (1997) How to Evaluate Photochemical Methods for Water Treatment, Wat. Sci. Technol. 35, No. 4: 17– 23. Brockmann KJ, Kriesche V, Etzkorn T, Volkamer R, Wirtz K (1996) The Euro-

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8 Process Engineering and Reactor Concepts pean Photoreactor EUPHORE: A Technical Description, Proc. EUROTRAC Symp., Borrell P M, Borrell P, Cvitas T, Kelly K, Seiler W (eds.), Computational Mechanics Publications, Southampton: 457–461. Cater SR, Stefan MI, Bolton JR, Safarzadeh-Amiri A (2000) UV/H2O2 Treatment of Methyl tert-Butyl Ether in Contaminated Waters, Environ. Sci. Technol. 34: 659–662. Chan AHC, Porter JF, Barford JP, Chan CK (2000) Photocatalytic Thin Film Cascade Reactor for Treatment of Organic Compounds in Wastewater, Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, electronic release. Chemviron Carbon (1997) AOT Handbook, Advanced Oxidation Technologies, ref. AOT/E-03, European Operating Unit of Calgon Carbon Corporation, Brussels (Belgium). Chiovetta MG, Romero RL, Cassano AE (2001) Modeling of Fluidized-bed Photocatalytic Reactor for Water Pollution Abatement, Chem. Eng. Sci. 56: 1631–1638. Davydov L, Reddy EP, Fance P, Smirniotis PG (2001) Sonophotocatalytic Destruction of Organic Contaminants in Aqueous Systems on TiO2 Powders, Appl. Catal. B: Environ. 32: 95–105. Debus O (2000) Licht reinigt Wasser, Chemikalienfreie Oxidation chlororganischer Abwasserinhaltsstoffe, Chem. Tech. 29, No. 6: 40–42. Debus O (2000) Oxidieren mit Luft, Dechlorierung von Industrieabwasser im UV-Freispiegelreaktor, Verfahrenstechnik 34, No. 11: 28–31. Dillert R, Vollmer S, Schober M, Theurich J, Bahnemann D, Arntz H-J, Pahlmann K, Wienefeld J, Schmedding T, Sager G (1999) Photocatalytic Treatment of an Industrial Wastewater in the DoubleSkin Sheet Reactor, Chem. Eng. Technol. 22: 931–934. Dionysiou DD, Balasubramanian G, Suidan MT, Khodadoust AP, Baudin I, Laine J-M (2000) Rotating Disk Photocatalytic Reactor: Development, Characterization, and Evaluation for the Destruction of Organic Pollutants in Water, Wat. Res. 34, No. 11: 2927–2940.

Dionysiou DD, Suidan MT, Baudin I, Laine J-M (2001) Rotating Disk Photocatalytic Reactor (RDPR): Fundamental Applied Studies for its Development and Evaluation for the Destruction of Recalcitrant Organic Contaminants in Water, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Dionysiou DD, Suidan MT, Baudin I, Lane J-M (2002) Oxidation of Organic Contaminants in a Rotating Disk Photocatalytic Reactor: Reaction Kinetics in the Liquid Phase and the Role of Mass Transfer Based on the Dimensionless Damköhler Number, Appl. Catal. B: Environ. 38, No. 1: 1–16. Do Nascimento CAO, Oliveros E, Braun AM (1994) Neural Network Modelling for Photochemical Processes, Chem. Eng. Proc. 33: 319–324. FIGAWA (1990) UV-Bestrahlung in der Wasseraufbereitung, Part 4: Desinfektion von Wasser in der Getränkeindustrie, Technische Mitteilungen Nr. 11, Sonderdruck aus bbr Wasser Rohrbau No. 7. Findeling C, Viriot ML, Carre MC, Andre JC (1993) Industrial Photochemistry XXII: The Best Use of Photons in Water Purification, J. Photochem. Photobiol. 71: 191– 193. Flemming H-C (1991) Biofilme und Wassertechnologie, Teil 1: Entstehung, Aufbau, Zusammensetzung und Eigenschaften von Biofilmen, gwf-Wasser/Abwasser 132, No. 4: 197–207. Flemming H-C, Griebe T, Szewzyk U (eds) (2000) The Investigation of Biofilms, Technomic Publishers, Lancaster, PA. Franke R, Franke C (1999) Model Reactor for Photocatalytic Degradation of Persistent Chemicals in Ponds and Waste Water, Chemosphere 39, No. 15: 2651–2659. Freudenhammer H, Bahnemann D, Bousselmi L, Geissen S-U, Ghrabi A, Saleh F, Si-Salah A., Simon U, Vogelphol A (1997) Detoxification and Recycling of Wastewater by Solar-catalytic Treatment, Wat. Sci. Technol. 35, No. 4: 149–156. Glaze WH, Kang J-W, Chapin DH (1987) The Chemistry of Water Treatment Pro-

References cesses Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation, Ozone Sci. Eng. 9: 335–352. Gobulukoglu I (2001) A Cost Effective Ultraviolet (UV) Water Disinfection Chamber with a Unique Safety Feature, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Hagen J (1993) Chemische Reaktionstechnik, Eine Einführung mit Übungen, VCH, Weinheim. Hamill NA, Weatherley LR, Hardacre C (2001) Use of a Batch Rotating Photocatalytic Contactor for the Degradation of Organic Pollutants in Wastewater, Appl. Catal. B: Environ. 30: 49–60. Hand DW, Perram DL, Crittenden JC (1995) Destruction of DBP Precursors with Catalytic Oxidation, J. Am. Wat. Works Assoc. June: 84–96. Hirvonen A, Tuhkanen T, Ettala M, Korhonen S, Kalliokoski P (1998) Evaluation of a Field-Scale UV/H2O2-Oxidation System for the Purification of Groundwater Contaminated with PCE, Environ. Technol. 19: 821–828. Hopp V (1992) Chemische Reaktionstechnik, Eine Einführung mit Übungen, VCH, Weinheim. Irazoqui HA, Cerda J, Cassano AE (1973) Radiation Profiles in an Empty Annular Photoreactor with a Source of Finite Spatial Dimensions, AIChE J. 19, No. 3: 460–467. Jakob L, Oliveros E, Legrini O, Braun AM (1993) TiO2 Photocatalytic Treatment of Water. Reactor Design and Optimization Experiments, in: Ollis D F, Al-Ekabi H (eds) Photocatalytic Purification and Treatment of Water and Air, Elsevier: 511–532. Jakubith M (1991) Chemische Verfahrenstechnik, Einführung in Reaktionstechnik und Grundoperationen, VCH, Weinheim. Karim S, Ronald G (2001) Fouling of UV Lamp Sleeves: Exploring Inconsistencies in the Role of Iron, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultra-

violet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Kim Y-J, Thomanetz E (1995) Deponiesickerwasserbehandlung durch UV-katalysierte Naßoxidation mit Wasserstoffperoxid in einem Fallfilmreaktor, gwf-Wasser/Abwasser 136, No. 7: 341–345. Kreisel G, Schmidt M (1999) Wet Chemical UV Oxidation of 2-Fluorophenol Optimization with Chosen Reaction Parameters, Chem. Eng. Technol. 22, No. 12: 1001–1006. Krutzler T, Bauer R (1999) Optimization of a Photo-Fenton Prototype Reactor, Chemosphere 38, No. 11: 2517–2532. Kuhn H J, Braslavsky SE, Schmidt R (1989) Chemical Actinometry, Pure Appl. Chem. 61, No. 2: 187–210. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93: 671–698. Leitzke O, Schulte P, Gilbert E (1996) Erweiterte Oxidationsverfahren in der Trinkwasseraufbereitung, gwf-Wasser/Special 137, No. 14: S173–S177. Levenspiel O (1972) Chemical Reaction Engineering, 2nd ed., John Wiley & Sons, New York. Levenspiel O (1996) The Chemical Reactor Omnibook, OSU Book Stores, Inc., Corvallis, Oregon 97339, ISBN: 0-88246-170-2. Li K, Tan XZ (2001) Development of Membrane-UV Reactor for Dissolved Oxygen Removal from Water, Chem. Eng. Sci. 56: 5073–5083. Lin L-S, Johnston CT, Blatchley III ER (1999) Inorganic Fouling at Quartz:Water Interfaces in Ultraviolet Photoreactors – II. Temporal and Spatial Distributions, Wat. Res. 33, No. 15: 3330–3338. Lin L-S, Johnston CT, Blatchley III ER (1999) Inorganic Fouling at Quartz:Water Interfaces in Ultraviolet Photoreactors – I. Chemical Characterization, Wat. Res. 33, No. 15: 3321–3329. Malato S, Blanco J, Vidal A, Fernandez P, Caceres J, Trincado P, Oliveira JC, Vincent M (2002) New Large Solar Photocatalytic Plant: Set-up and Preliminary Results, Chemosphere 47, No. 3: 235–240. Molinari R, Grande C, Drioli E, Palmisano L, Schiavello M (2001) Photocatalytic Membrane Reactors for the Degradation of

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8 Process Engineering and Reactor Concepts Organic Pollutants in Water, Catal. Today 67: 273–279. Mukherjee PS, Ray AK (1999) Major Challenges in the Design of a Large-Scale Photocatalytic Reactor for Water Treatment, Chem. Eng. Technol. 22, No. 3: 253– 260. Müller J-P, Gottschalk C, Jekel M (2000) Comparison of Advanced Oxidation Processes in Flow-Through Pilot Plants, Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, electronic release. Murov SL (1973) Handbook of Photochemistry, Marcel Dekker, New York. Murov SL, Carmichael I, Hug GL (1993) Handbook of Photochemistry, Marcel Dekker, New York. Oliveros E, Legrini O, Hohl M, Müller T, Braun AM (1997) Industrial Waste Water Treatment: Large Scale Development of a Light-enhanced Fenton Reaction, Chem. Eng. Proc. 36: 397–405. Ollis DF (2000) On the Need for Engineering Models of Integrated Chemical and Biological Oxidation of Wastewaters, Conference Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, May 28–31 (2000), Clausthal-Zellerfeld (Germany); electronic release. Periyathamby U, Ray AK (1999) Computer Simulation of a Novel Photocatalytic Reactor Using Distributive Computing Environment, Chem. Eng. Technol. 22, No. 10: 881– 888. Philips (1993) Grundlagen der optischen Strahlung, Philips Lighting. Prengle Jr HW, Symons JM, Belhateche D (1996) H2O2/VisUV Process for Photo-Oxidation of Waterborne Hazardous Substances – C1-C6 Chlorinated Hydrocarbons, Waste Management 16, No. 4: 327–333. Puma GL, Yue PL (2001) A Novel Fountain Photocatalytic Reactor: Model Development and Experimental Validation, Chem. Eng. Sci. 56: 2733–2744. Puma GL, Yue PL (2001) The Modeling of a Fountain Photocatalytic Reactor with a Parabolic Profile, Chem. Eng. Sci. 56: 721– 726.

Rice RG (1997) Applications of Ozone for Industrial Wastewater Treatment – A Review, Ozone Sci. Eng. 18: 477–515. Rice RG (2001) Practical Applications of UV in Combination with Ozone, Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14-16, Washington, DC, electronic release. Roger M, Villermaux J (1983) Modelling of Light Absorption in Photoreactors Part II. Intensity Profile and Efficiency of Light Absorption in a Cylindrical Reactor. Experimental Comparison of Five Models, Chem. Eng. J. 26: 85–93. Rossetti GH, Albizzati ED, Alfano OM (1998) Modeling and Experimental Verification of a Flat-Plate Solar Photoreactor, Ind. Eng. Chem. Res. 37: 3592–3601. Scaiano JC (1989) CRC Handbook of Organic Photochemistry, Vol. 1, CRC Press, Boca Raton. Sclafani A, Sciascia A, Rizzuti L (1999) Pollutants Photocatalytic Degradation in a Falling Film Photoreactor, J. Adv. Oxid. Technol. 4, No. 1: 91–96. Shimoda S, Prengle Jr HW, Symons JM (1997) H2O2/VISUV Photo-Oxidation Process for Treatment of Waterborne Hazardous Substances – Reaction Mechanism, Rate Model, and Data for Tubular Flow and Flow Stirred Tank Reactors, Waste Management 17, No. 8: 507–515. Shirotsuka T, Sudo M (1979) Scale-up of Annular Photochemical Reactors, Int. Chem. Eng. 19, No. 2: 293–299. Sörensen M, Weckenmann J (1998) Galvanotechnik 89: 9, cited in: Sörensen M, Weckenmann J, CyanoMat® – Moderne Cyanidentgiftung mittels UV-Oxidation, Anwendungsbeispiele aus der Praxis, a.c.k. aqua concept GmbH, Karlsruhe (Germany). Sprehe M, Geissen S-U, Vogelpohl A (2000) UV-Kaskade zur strömungstechnischen Anpassung bei der Photooxidation mit H2O2, Chem.-Ing. Tech. 72, No. 7: 754– 759. Stefan MI, Bolton JR (1998) Mechanism of the Degradation of 1,4-Dioxane in Dilute Aqueous Solution Using the UV/Hydrogen Peroxide Process, Environ. Sci. Technol. 32, No. 11: 1588–1595.

References Stefan MI, Hoy AR, Bolton JR (1996) Kinetics and Mechanism of the Degradation and Mineralization of Acetone in Dilute Aqueous Solution Sensitized by the UV Photolysis of Hydrogen Peroxide, Environ. Sci. Technol. 30, No. 7: 2382–2390. Suidan MT, Severin BF (1986) Light Intensity Models for Annular UV Disinfection Reactors, AIChE J. 32, No. 11: 1902–1909. Trageser M (1997/98) Leistung aus dem Baukasten, UV-aktivierte Oxidation in der Abwasserbehandlung, Chem. Umwelt Techn., Special Issue: 38–41. Trageser M, Greunig H-J (2000) Destruction of Organic and Inorganic Complexes by UV-Activated Oxidation, Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, Wat. Sci. Technol. 44, issue 5; electronic release. TÜV (2002 a) Bericht-Nr. 425-251001 über die Messung der durch den AEROPUR-Luftreiniger verursachten Verminderung der Luftkeimkonzentration, TÜV Produkt und Umwelt GmbH, Unternehmensgruppe TÜV Rheinland/Berlin-Brandenburg; information received and published by permission of C & C Consult, Schleiden /Eifel (Germany). TÜV (2002 b) Bericht-Nr. 425-251001/03 über die Reduzierung von Formaldehyd in der Raumluft durch den Luftreiniger AEROPUR® im Auftrag der Firma C&C Consult, TÜV Produkt und Umwelt GmbH,

Unternehmensgruppe TÜV Rheinland/ Berlin-Brandenburg; information received and published by permission of C & C Consult, Schleiden/Eifel (Germany). vanLoon GW, Duffy SJ (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford. Weichgrebe D, Vogelpohl A (1995) Strategie zur Auswahl geeigneter Oxidationsverfahren, in: Vogelpohl A (ed.) CUTECSchriftenreihe, 2. Fachtagung Naßoxidative Abwasserbehandlung Wenzel A, Gahr A, Niessner R (1999) TOC-Removal and Degradation of Pollutants in Leachate using a Thin-Film Photoreactor, Wat. Res. 33, No. 4: 937–946. Xi W, Geissen S-U, Vogelpohl A (2000) Solar Detoxification of Wastewater in a Novel Aerated Cascade Photoreactor (ACP), Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, Wat. Sci. Technol. 44, No. 5; electronic release. Yatmaz HC, Wallis C, Howarth CR (2001) The Spinning Disk Reactor – Studies on a Novel TiO2 Photocatalytic Reactor, Chemosphere 42: 397–403. Yue PL (1993) Modelling of Kinetics and Reactor for Water Purification by Photo-Oxidation, Chem. Eng. Sci. 48, No. 1: 1–11. Zhang Y, Crittenden JC, Hand DW (1994) The Solar Photocatalytic Decontamination of Water, Chem. Ind. September: 714–717.

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UV Disinfection Ultraviolet radiation has been used for disinfection purposes since the early 20th century (cf. Tab. 2-3). Nowadays, wastewater (secondary and tertiary sewage effluents) and drinking water disinfection are important technical applications of direct UV irradiation in the wavelength range between 240 nm and 290 nm (Hoyer, 1998, Parrotta and Bekdash, 1998, Steuer, 1989). For current drinking water standards in the U.S. refer to EPA, Office of Water (Internet address: Tab. 1-1) and for terms and definitions used in UV disinfection refer to Bolton (2000 a). Other significant contributions to the field of UV disinfection include the photo-inactivation of surface bound or airborne microorganisms by using TiO2 based photocatalytic techniques or solar photocatalytic disinfection of water, which is of importance in tropical developing countries (e. g. Cooper et al., 1998). The bactericidal and virus inactivating activity of illuminated TiO2 has been demonstrated on laboratory scales (cf. Belhacova et al., 1999, Kersters et al., 1998, Lee et al., 1998, Wei et al., 1994) and by solar pilot plant studies (Vidal et al., 1999). Recently, Jacoby et al. (1998) demonstrated the total mineralization of Escherichia coli cells adsorbed on illuminated TiO2 in air. The mechanism of cell killing by photocatalysis is still under active investigation (Huang et al., 2000). Blake et al. (1999) published a comprehensive review including references of microbiological photocatalytic research. Hunter (2000) briefly reviewed the developments related to UV disinfection over the past 20 years with references to regulatory and process changes, system design, applicability, and the growth of industry during this period. The last is enormous: in 1984, there were only a small number of wastewater UV disinfection systems in operation in the U.S.A. and Canada (White et al., 1986). At the end of 1999, the number of UV systems installed for wastewater disinfection exceeded 500, these having a design capacity of an average of 20 million gallons per day. Many recent developments of this expanding technology can be found in IUVA (2001). Common definitions of the term disinfection usually aim at its medical aspects related to transmissible diseases and to the killing or irreversible inactivation of the disease-causing organisms by use of chemical agents or physical procedures. This includes the destruction of infectious or other undesirable bacteria, pathogenic fungi or viruses and protozoa on surfaces or any inanimate objects (Block, 1993, Harke, 1987). Thus, disinfection of water, air or surfaces should produce a

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condition under which no hazardous health effects for humans can occur. It is generally accepted, that this condition is achieved by reduction of hazardous microorganisms to 0.01% of their initial number. However, a clear distinction between disinfection and sterilization must be made. The latter refers to the status of a substance being “free from living microorganisms with a probability previously agreed to be acceptable” (Augurt and van Asten, 1997). Thus, the goal of sterilization is the elimination of all pathogen and apathogen microorganisms including their spores with physical processes (Roche Enzyclopedia, 1987). Hence, sterilization is more rigorous than disinfection. In contrast to sterilization, the aim of disinfection techniques is to reduce the number of pathogenic bacteria, viruses, protozoa and spores by inactivation, so that the infectivity of the material is reduced drastically. During disinfection processes, the inactivation of microorganisms follows an exponential decay according to Eq. 9-1. Nt e Nt0

kt

(9-1)

With Nt=0 the number of microorganisms before treatment, Nt: number of microorganisms at time t of treatment and k being the rate constant of microorganism inactivation. The number of microorganisms is usually stated as colony forming units (CFU) and it is determined by plate counts. This technique is limited to colony forming microorganisms. Thus, parasites must be isolated and identified visually and viruses are identified through their effects (Bersillon, 1999). For the assessment of the disinfection efficiency, the reduction factor Rf according to Eq. 9-2 was defined. Rf log

Nt0 Nt

(9-2)

Thus, a reduction factor Rf of 4 achieved by a disinfection technique means that the initial number of microorganisms (100%) was reduced by 99.99%, this being equivalent to a four “log-unit” of inactivation. Membrane techniques, such as microfiltration, ultrafiltration or reverse osmosis are being investigated for their water disinfecting potential (Lazarova et al., 1999). However, the most common methods of water disinfection are chlorination by use of molecular Cl2 (cf. Shang and Blatchley, 2001), hypochlorous acid (HOCl) or chlorine dioxide (ClO2), ozonation and use of UV-C radiation (Hoyer, 1996). Clearly every method has its advantages and disadvantages, which will be indicated below (Lazarova et al., 1999). For details concerning the disinfection of water with ozone, refer to Gottschalk et al. (2000). Advantages (:) and disadvantages (;) of chlorination are: • : very good killing of bacteria; • : long term effects due to low residual Cl2 concentrations ([Cl2] = 0.1 g m–3) in water supply lines;

9 UV Disinfection

• ; relatively long hydraulic contact time of about 15 to 30 min is necessary; • ; low efficiency towards inactivation of parasites, such as Giardia muris and Cryptosporidium oocysts (Bersillon, 1999), as is the case with ozonation (Rennecker et al., 2000); • ; at low dosages that are used for removal of coliforms, the efficiency of inactivation of spores, cysts and of some viruses is poor (Lazarova et al., 1999); • ; stringent safety regulations for the handling of chlorine and of its derivatives and investments for safety equipment are necessary; • ; negative impact of chlorine containing effluents on receiving water ecosystems; thus, effective dechlorination using sodium bisulfite (NaHSO3) solutions is increasing the salinity of effluents; • ; production of toxic disinfection by-products (DBPs), for example chlorite ions (ClO–2), haloacetic acids (Cl2CHCOOH, Cl3CCOOH), dichloroacetonitrile (Cl2CHCN), trihalomethanes (THMs: CHCl3, CHBrCl2, CHBr2Cl, CHBr3). The formation of halogenated DBPs constitutes a serious problem related to chlorination techniques because they are harmful to human health and the environment. Consequently, their concentrations in water are the object of stringent regulations. Thus, it is necessary to develop alternative strategies for water disinfection. Advantages (:) and disadvantages (;) of UV disinfection are: • : effective against a wide range of pathogens in water (Cosman and Wright, 2000) or air; • : protozoan parasites, such as infective Cryptosporidium parvum oocysts can be inactivated with a reduction factor of Rf >4 by irradiation with MP Hg lamps at a fluence H0 of 10.4 J m–2 accompanied by a slight formation of nitrite ions in a concentration range well below the European Drinking Water Regulation ([NO–2]max = 0.1 mg L–1) (Cervantes et al., 2001). The latest investigations demonstrate that LP Hg lamps are also effective for inactivation of Cryptosporidium parvum oocysts (Craik et al., 2001). The first demonstration that UV radiation is very effective against Cryptosporidium parvum was presented by Bolton et al. (1998); • : minimal space requirements and if necessary, repeated applications in the course of the water distribution systems are possible; • : UV disinfection is a pure physical process thus, no chemical disinfectant must be added, generated, handled, transported or stored; • : no influence on taste and smell of the treated water; • : minimal residual effects (if any) that could be harmful to humans or aquatic life; • : negligible formation of toxic by-products, if any are formed (Zheng et al., 1999 a, b); • : no mutagenic photoproducts could be isolated (Hoyer, 1998); • : no contribution to regrowth problems in distribution systems (Hengesbach et al., 1993); • : the technology is easily automated by UV monitoring with UV sensors and it is user friendly, safe and reliable to operate;

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Fig. 9.1

Various influences on the efficacy of UV disinfection; refer also to Figure 8-15.

• : very short contact time in the order of several seconds is necessary; • ; marked influence of the water quality (e. g. total suspended solids, UV transmittance T, turbidity) on the required UV fluence for a given reduction rate R of pathogen inactivation; • ; fouling of tubes containing the UV lamps must be controlled; • ; photoreactivation, and in the absence of UV/VIS radiation dark repair may lead to a considerable regrowth or recolonialization of several microorganisms depending strongly on the UV fluence applied; • ; in the case of high nitrate ion (NO–3) concentrations the formation of nitrite ions (NO–2) by extensive UV-C irradiation must be taken into account, especially with MP Hg lamps that emit significant amounts of UV radiation below k of 253.7 nm (Cervantes et al., 2001, Hoyer, 1998). The various influences on the efficacy of UV disinfection are compiled in Fig. 9-1 (cf. Malley Jr, 2000). Primarily, the efficacy of UV disinfection depends on the germicidal fluence H0 = E0 × t which is the product of fluence rate E0 and the duration time t of the irradiation (often called “UV dose”, Chapter 2.1) (see Sommer et al., 1998). Other key factors include the hydraulics and hydrodynamics of the UV reactor (Kreft et al., 1986), its geometry (FIGAWA, 1998, Hoyer, 1996), the number and type of UV lamps required (Loge et al., 1996), their temperature profiles with respect to a maximum fluence rate E0 (in the case of LP Hg lamps, cf. Fig. 4-8), the water quality and its variability such as UV absorbance/transmittance (Bolton et al., 2001, Sommer et al., 1997), the water matrix, e. g. nitrate concentration, its potential for quartz fouling by inorganic constituents particularly iron ions and hardness (cf. Chapter 8-2), the turbidity, the particle content (total sus-

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UV sensitivity of indicator germs E. coli, pathogens, facultative pathogens and viruses with and without photo-reactivation; redrawn from Hoyer (1998).

Fig. 9.2

pended solids), the particle size distribution and the corresponding number of particle-associated bacteria (cf. Emerick et al., 1999), the target organism(s) and the level of inactivation that is required (i. e. the desired reduction factor Rf). Furthermore, the biology of the microorganism(s) to be inactivated is of utmost interest: the reproduction rate is dependent on environmental factors (i. e. nutrient supply, oxygen demand etc.) and the UV sensibility is related to the action spectrum. The action spectrum represents a plot of a relative biological response against wavelength under the same fluence H0. However, the correct way to measure an action spectrum is to select a “standard biological response” (e. g. 2 logs inactivation) and at each wavelength determine the fluence H0 required to achieve that response. A plot of 1/H0 versus wavelength is then the relative action spectrum (Bolton, 2002). Several microorganisms have the ability to photo-reactivate on irradiation with longer wavelengths (in the UV-A range), so that two to four times higher UV-C radiant exposure H is necessary for comparable inactivation in these cases (Tosa and Hirata, 1999, Hoyer, 1998). This is demonstrated by the comparison of the UV sensitivity of indicator germs (E. coli), pathogens and facultative pathogens and viruses with and without photoreactivation (Fig. 9-2). Thus, a germicidal radiant exposure H of at least 400 J m–2 seems to be required for the safe UV disinfection of water containing germs and viruses of hygienic relevance (Hoyer, 1998). Photoreactivation testing can be easily performed by exposure of the microorganisms to daylight under standardized conditions. However, several spores (B. subtilis: H ~ 600 J m–2) and viruses (MS2 phage: H ~ 750 J m–2)

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seem to require much higher UV fluences to reach the same order of inactivation (Bolton, 1999). The design and optimization of UV disinfection processes has to take into account the biology of the microorganism(s), the water quality and concepts of hydraulic and photochemical engineering (see Fig. 9-1). Usually, UV disinfection installations for treatment of drinking water or of effluents from sewage treatment in = 40–100 W) that emit mainly at k of plants use large banks of LP Hg lamps (Pel 253.7 nm. Current UV disinfection systems use open channel horizontal, vertical, parallel-to-flow and perpendicular-to-flow lamp configurations or closed chamber arrangements (FIGAWA, 1998, ATV, 1993) (compare with Fig. 8-3). Cryptosporidium control has become a primary focus of regulatory agencies in the U.S.A. and U.K. Sources of drinking water contamination by Cryptosporidium oocysts are human and animal fecal waste. Cyryptosporidiosis causes gastrointestinal illness, such as watery diarrhea (over 5 to 22 days), vomiting, cramps, and a general feeling of illness with headaches for the duration of up to 6 weeks (Roche Encyclopedia, 1987). Outbreaks of cryptosporidiosis may lead to death in immunocompromized individuals (Clancy, 1999). Thus, over the past few years a great deal of research activity has been invested in investigating the effect of UV-C radiation on these parasites. in = 1–5 kW) seemed Originally, the use of a smaller number of MP Hg lamps (Pel to have several advantages over the LP Hg lamp systems, particularly with respect to inactivation of Cryptosporidium parvum oocysts at a low UV fluence H0 of 19 mJ cm–2 and a reduction factor Rf of 3.9 (Bukhari et al., 1999). However, recent investigations consistently established a 3 log-unit inactivation of C. parvum oocysts at a germicidal UV fluence H0 of 25 mJ cm–2 for both MP and LP Hg lamps. Both lamp types were found to be equally effective for the inactivation of oocysts when compared on a germicidal fluence basis (Craik et al., 2001). In contrast, previous studies with LP Hg lamps indicated that a very high UV fluence H0 of up to 8000 mJ cm–2 is required to inactivate Cryptosporidium. This observation was initially assigned to the much lower irradiance E of LP Hg lamps compared with that of MP Hg lamps or to wavelength effects (Bukhari et al., 1999). LP Hg lamps can be treated as being almost monochromatic UV-C sources (kmax = 253.7 nm) whereas MP Hg lamps emit polychromatic radiation in the germicidal range between 200 and 300 nm (cf. Fig.s 4-5 and 4-6). Another and more convincing explanation for this discrepancy of previous and recent studies related to the inactivation of Cryptosporidium refers to the methods of viability testing. So, it seems that in vitro viability assays (chemical excystation and vital stains) that have been used previously may have significantly underestimated the inactivation efficacy of UV-C irradiation of the parasites compared with in vivo infectivity assays applied in recent studies using neonatal mouse models (Craik et al., 2001). This was also demonstrated by UV inactivation of Giardia muris cysts using MP Hg lamps (Craik et al., 2000). In conclusion, the resent research by Clancy et al. (2000, 1998), Bukhari et al. (1999) and Craik et al. (2001) has convincingly demonstrated that UV-C is highly effective at inactivating Cryptosporidium parvum oocysts (and Giardia muris cysts)

9 UV Disinfection Comparison of the emission spectra of LP and MP Hg lamps in relation to the action spectrum of Escherichia coli photoinactivation; received from Heraeus Noblelight (Kleinostheim, Germany).

Fig. 9.3

by using mouse infectivity as a convenient measure of viability. The study of Craig et al. (2001) recommends that the choice of the lamp type should be based on economic considerations. LP Hg lamps usually have twice the germicidal efficiency compared with MP Hg lamps. Thus, the former will be the choice for small UV disinfection installations. On the other hand, MP Hg lamps have a much greater UV-C intensity and thus fewer lamps must be installed reducing the size of the UV reactor vessel and the expenditure on maintenance. Thus, MP Hg lamps are a convenient alternative to LP Hg lamps in large UV disinfection units. For comparison of both lamp types related to the action spectrum of photo-inactivation of Escherichia coli bacteria refer to Fig. 9-3. Thus, it is necessary to develop appropriate new models for the calculation of the total average fluence rate distribution in a photoreactor in the spectral range of 200 to 300 nm when using MP Hg lamps (Bolton, 2000 b). In UV disinfection technology, the germicidal UV fluence H0 in J m–2 (commonly designated as UV dose) is crucial for the efficacy of a photoreactor system. It can be determined using a collimated beam apparatus for the UV treatment of suspensions of microorganisms under defined irradiation conditions (Bolton, 1999, Leuker, 1999, Leuker, 1998, Sommer et al., 1999). This method is called “biodosimetry” (cf. Cabaj et al., 1996). However, it is more accurate and it would be more convenient to designate this procedure as bioactinometry! The results of bioactinometry (“biodosimetry”) can be related to chemical actinometry (cf. Chapter 3-13), e. g. by applying uridine as an appropriate actinometer for the determination of effective germicidal UV fluence from polychromatic UV sources (Jin et al., 2001, Linden and Darby, 1997). The testing of the disinfection performance of UV systems, especially in drinking water supplies, can be established by UV exposure of a harmless test organism (e. g. Bacillus subtilis spores or bacteriophage MS2) under controlled conditions within testing facilities, followed by the determination of the inactivation efficacy. For example, in Germany drinking water disinfection requires an inactivation efficacy of 99.99% of water borne pathogens. This is achieved by an effective UV fluence H0 of 400 J m–2 (= 40 mJ cm–2) at k of 253.7 nm (Hoyer, 2001). To guarantee a constant efficacy of UV disinfection plants their testing, monitoring and certification is of importance (Snozzi et al., 1999). The mechanism of UV disinfection is based on the absorption of UV radiation by proteins, RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) in a given micro-

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Fig. 9.4

UV absorbance spectrum of DNA; redrawn from von Sonntag

(1987).

The molecular structures of DNA and RNA components: the purine bases adenine and guanine, the pyrimidine bases cytosine, thymine and uracil and the structure of riboso-monophosphates.

Fig. 9.5

organism (von Sonntag, 1987). However, damage to the protein structure of a microorganism requires at least ten times the fluence H0 as does inactivation through absorption by DNA or RNA (Bolton, 2002). The absorption of UV radiation by the DNA of a microorganism induces photochemical reactions of DNA components and damage of proteins by destruction of sulfur combinations. Thus, the ability of the microorganism to replicate is disrupted. Hence, the microorganism can no longer infect.

9 UV Disinfection

Photochemical formation of the cyclobutane-type dimer by irradiation of thymine nucleotides.

Fig. 9.6

Biological effects are achieved in the wavelength range k of 190 to about 350 nm. This is in agreement with the absorbance spectrum of DNA (Fig. 9-4). It exhibits two distinct absorption maxima at 200 nm and around k of 260–265 nm, which extends to k of about 350 nm (von Sonntag, 1987). The absorption of DNA above k of 210 nm is due to the absorption of the nucleotide bases adenine (A), guanine (G), thymine (T), cytosine (C) and in the case of RNA to uracil (U). Their structures are shown in Fig. 9-5 together with the deoxyriboso- and riboso-5-phosphates. The nucleotides (i. e. the base-monophosphates) have their absorption maxima around k of 260 nm. The two DNA strands that are formed by base pairing (A-T, G-C) are held together by hydrogen bonds. Thus, a chiral, right-handed double helix (or spiral) is formed which contains the genetic code of living organisms (the Watson–Crick model of the DNA). By absorption of UV radiation, two thymine bases that are located adjacent to each other in the DNA strands react photochemically to produce a thymine dimer by a [2+2]cycloaddition reaction with formation of a new pair of covalent C–C single bonds (Fig. 9-6). After photochemical formation of sufficient thymine dimers, the replication of the DNA is blocked efficiently so that the cell mitosis is inhibited. In addition to the photochemistry of DNA, photoreactions of various cell components, such as membrane proteins, enzymes and other biomolecules may also be relevant to photo-induced cell killing in some cases (Kalisvaart, 2001). In drinking water UV treatment, usually only the photochemical reactions that are induced by direct irradiation are observed. However, if nitrate ion photochemistry (producing •OH radicals) is involved, the complex radical chemistry of DNA/RNA and of their fragments may prevail (Pogozelski and Tullius, 1998). Some microorganisms, especially bacteria, posses repair mechanisms that substitute or dissociate the thymine dimers. These reactivation mechanisms can take place under both dark and UV-A/VIS conditions. They are designated as either dark repair (cutting-repairing) or photoreactivation. However, the repair mechanisms are not universal to all microorganisms. Microorganisms that are unable to repair include the following species (Kalisvaart, 2001): Heamophilus influenzae, Diplococcus pneumo-

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Two-step reaction mechanism of photoreactivation (modified according to Lindenauer and Darby, 1994).

Fig. 9.7

niae, Bacillus subtilis, Micrococcus radiodurans, viruses, Cryptosporidium parvum, Staphylococcus aureus phage A994, Rotavirus SA-11, Poliovirus, MS2 Phage. Typical organisms that are capable of photoreactivation include (Kalisvaart, 2001): Streptomyces spp., Escherichia coli, Saccharomyces spp., Aerobacter spp., Micrococcus spp., Erwinia spp., Proteus spp., Penicillum spp., Neurospora spp., Enterobacter cloacae, Citrobacter freundii, Enterocolitica faecium, Klebsiella pneumoniae, Mycobacterium smegmatis, Pseudomonas aeruginosa, Salmonella typhi, Salmonella typhimurium, Seratia marcescena, Vibrio cholerea, Yersinia enterocolitica (spp. = plural of sp.: species, denotes two or more unspecified species or all species of the genus). Photoreactivation can influence the performance of UV disinfection and it must be taken into account when designing UV systems. Hence, the repair mechanisms can be inhibited by application of high UV-C fluence (see Fig. 9-2). Repair of virus species has never been reported in the literature because they do not possess an appropriate enzyme system (Baron, 1997). During evolution, the bacteria that are capable of photoreactivation developed a subtle mechanism to defend their cells against lethal DNA damage that may be caused by exposure to solar radiation. Postirradiation with UV-B, blue or white light may increase the survival rate of microorganisms significantly. Thus, the efficacy of UV disinfection decreases substantially when photoreactivation is facilitated. A two-step reaction sequence involving a photoreactivating enzyme (PRE) has previously been proposed by Harm in 1975 (Harm, 1975). It is outlined in Fig. 9-7. The pyrimidine dimers that are produced by an irreversible [2+2]photocycloaddition reaction form a PRE-dimer complex. This complex is able to absorb UV-B/VIS radiation, and the photocycloaddition reaction is reversed with formation of the monomers and PRE. The formation of repaired DNA occurs within a millisecond. All known DNA photolyases contain a flavin adenine dinucleotide entity, which is attributed to be the active chromophore (Kato et al., 1997). The magnitude of photoreactivation strongly depends on the fluence H0 of the UV-B/VIS radiation that is applied to the microorganisms after UV-C induced disinfection (Schoenen and Kolch, 1992). Thus, to evaluate the efficacy of UV disinfection it is essential to assess the degree of recovery of photoreactivating microorganisms following UV-B/VIS irradiation under defined conditions. Several large-scale studies concerning UV disinfection are collected in Tab. 9-1.

Type

UV unit

5200 250

LP Hg lamps MP Hg lamps

a) b) c) d)

Coliforms

Coliforms, virus Heterotrophic plate counts Giardia lamblia, Cryptosporidium parvum

E. coli

Kummer (1990) Chiu et al. (1999) Bolton (2000) Lyn et al. (1999)

Ho et al. (1998) Blomberg and Eriksson (2001) Schneider et al. (2001)

Wist et al. (2002)

Andreadakis et al. (1999) Chandrakanth et al. (2001) Schleypen and Loy (1991)

Whitby et al. (1984)

References

in High efficiency and high intensity LP Hg lamps were used in this pilot study at Pel = 1.620 kW. in Pel = 1300 W per lamp. Immediately after irradiation high inactivation yields of 99.98% were obtained. Depending on the water matrix, effective recolonization was observed. Spectrotherm lamps, Pelin = 170 W and P253.7 nm = 64 W.

Wastewater, secondary effluent Dose distribution function Fluence rate distribution Modeling of flow and disinfection

LP Hg lamps 24 Point-source summation method Significance of reflection/refraction Steady two-dimensional model of flow and disinfection

20 216

LP Hg lamps LP Hg lamps

Secondary effluent Surface water treatment: drinking water Unfiltered drinking water

Photocatalytic disinfection c) 40 W, TiO2 (P-25, Degussa) UV versus chlorination 13.824 kW d) at 253.7 nm UV + chlorination 1.716 MW 5 MW; comparison of reactor systems –

Coliforms

768 W at 253.7 nm

LP Hg lamps 48 MP Hg lamps 6 b) Black light lamp 1

Coliforms Coliforms

Lamp fouling 1.008 kW at 253.7 nm

? 2

Fecal coliforms

Microorganism

1.656 kW at 253.7 nm

Remarks

120

No. of UV lamps

Municipal wastewater effluents

Domestic wastewater, secondary LP Hg lamps effluent Municipal sewage, secondary effluent LP Hg lamps Municipal wastewater effluents LP Hg lamps a)

Description

Tab. 9.1 Selected studies related to UV disinfection and modeling of UV disinfection processes

9 UV Disinfection 289

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References Andreadakis A, Mamais D, Christoulas D, Kabylafka S (1999) Ultraviolet Disinfection of Secondary and Tertiary Effluent in the Mediterranean Region, Wat. Sci. Technol. 40, No. 4-5: 253–260. ATV (1993) Desinfektion von Abwasser, Ein Regelwerk der Abwassertechnischen Vereinigung e.V., (Germany), H 205. Augurt TA, van Asten JAAM (1997) Sterilization Techniques, in: Kirk-Othmer, Encyclopedia of Chemical Technology, John Wiley & Sons, New York, Vol. 22: 832–851. Baron J (1997) Repair of Wastewater Microorganisms after Ultraviolet Disinfection under Seminatural Conditions, Wat. Environ. Res. 69, No. 5: 992–998. Belhacova L, Krysa J, Geryk J, Jirkovsky J (1999) Inactivation of Microorganisms in a Flow-through Photoreactor with an Immobilized TiO2 Layer, J. Chem. Technol. Biotechnol. 74: 149–154. Bersillon J-L (1999) Water Purification and Disinfection Processes, Acta Hydrochim. Hydrobiol. 27, No. 2: 98–100. Blake DM (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, National Renewable Energy Laboratory (available on the web, see Tab. 1-1). Block SS (1993) Disinfectants and Antiseptics, in: Kirk-Othmer Vol. 8: 237–292, John Wiley & Sons, New York. Bloomberg J, Eriksson U (2001) From Pilot Plant Studies to a Full-Scale UV Installation at a Large Surface Water Treatment Plant in Sweden, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photoscience Inc., ISBN 0-9685432-0-0; also published in: Eur. Photochem. Assoc. Newslett. Nr. 66: 9–36. Bolton JR (2000 a) Terms and Definitions in Ultraviolet Disinfection, in: Proceedings, Disinfection 2000: Disinfection of Wastes in the New Millennium, 15–18 March, New Orleans, LA, Water Environment Federation, 601 Wythe St., Alexandria, VA, 22314.

Bolton JR (2000 b) Calculation of Ultraviolet Fluence Rate Distributions in an Annular Reactor: Significance of Refraction and Reflection, Wat. Res. 34, No. 13: 3315–3324. Bolton JR (2002) personal communication. Bolton JR, Dussert B, Bukhari Z, Hargy T, Clancy JL (1998) Inactivation of Cryptosporidium parvum by Medium-Pressure Ultraviolet Light in Finished Drinking Water, Proc. AWWA 1998 Annual Conference, Dallas TX, Vol. A: 389–403. Bolton JR, Stefan MI, Cushing RS, Mackey E (2001) The Importance of Water Absorbance/Transmittance on the Efficiency of Ultraviolet Disinfection Reactors, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bukhari Z, Hargy TM, Bolton JR, Dussert B, Clancy JL (1999) Medium-pressure UV for Oocyst Inactivation, J. Am. Water Works Assoc. 91, No. 3: 86–94. Cabaj A, Sommer R, Schoenen D (1996) Biodosimetry: Model Calculations for U.V. Water Disinfection Devices with Regard to Dose Distributions, Wat. Res. 30, No. 4: 1003–1009. Cervantes P, Vidal A, Luck F (2001) Microorganism Inactivation and By-products Formation by an UV-Medium Pressure System, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Chandrakanth M, Colak P, Laquidara M, Prevost M, Leparc J (2001) Disinfecting Treated Municipal Wastewater Effluents of Varying Qualities Using Low-Pressure High-Intensity UV Lamps, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Chiu K, Lyn DA, Savoye P, Blatchley III ER (1999) Integrated UV Disinfection Model Based on Particle Tracking, J. Environ. Eng. January: 7–16; Effect of UV System Modifications on Disinfection Performance, J. Environ. Eng. May: 459–469.

References Clancy JL (1999) Ultraviolet – A Solution to the Cryptosporidium Threat?, IUVA News 1, No. 1: 18–22. Clancy JL et al. (1998) Inactivation of Cryptosporidium Oocysts in Water using Ultraviolet Light, J. Am. Water Works Assoc. 90: 92–102; cited in: Clancy (1999). Clancy JL, Bukhari Z, Hargy TM, Bolton JR, Dussert B, Marshall MM (2000) Comparison of Medium- and Low-Pressure Ultraviolet Light for Inactivation of Cryptosporidium parvum Oocysts, J. Am. Water Works Assoc. 92, No. 9: 97–104. Cooper AT, Goswami DY, Block SS (1998) Solar Photochemical Detoxification and Disinfection for Water Treatment in Tropical Developing Countries, J. Adv. Oxid. Technol. 3, No. 2: 151–154. Cosman J, Wright H (2000) UV Disinfection of Drinking Water, IUVA News 2, No. 3: 14–18. Craik SA, Finch GR, Bolton JR, Belosevic M (2000) Inactivation of Giardia Muris Cysts Using Medium-Pressure Ultraviolet Radiation in Filtered Drinking Water, Wat. Res. 34, No. 18: 4325–4332. Craik SA, Weldon D, Finch GR, Bolton J R, Belosevic M (2001) Inactivation of Cryptosporidium Parvum Oocysts using Medium- and Low-Pressure Ultraviolet Radiation, Wat. Res. 35, No. 6: 1387–1398. Emerick RW, Loge FJ, Thompson D, Darby JL (1999) Factors Influencing Ultraviolet Disinfection Performance Part II: Association of Coliform Bacteria with Wastewater Particles, Wat. Environ. Res. 71, No. 6: 1178–1187. FIGAWA (1998) Technische Mitteilung der FIGAWA Nr. 20/98, UV-Bestrahlung in der Trinkwasser und Abwasserbehandlung zu Desinfektionszwecken, bbr, Wasser Rohrbau 49: 22–31. Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Harke H-P (1987) Disinfectants, in: Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A 8: 551–563, VCH, Weinheim. Harm W (1975) Molecular Mechanisms for Repair of DNA, Part A (Edited by Hanawalt P C and Setlow R B), Plenum Press, New York; cited in: Lindenauer and Darby (1994).

Hengesbach B, Schoenen D, Hoyer O, Bernhardt H, Mark G, Schuchmann H-P, Sonntag C von (1993) UV Disinfection of Drinking Water – The Question of Bacterial Regrowth and the Photolytic Degradation of Biogenic High-MolecularWeight Substances, J. Water SRT-Aqua 42, No. 1: 13–22. Ho C-FH, Pitt P, Mamais D, Chiu C, Jolis D (1998) Evaluation of UV Disinfection Systems for Large-scale Secondary Effluent, Wat. Environ. Res. 70, No. 6: 1142–1150. Hoyer O (1996) Anforderungen an UV-Anlagen zur Trinkwasserdesinfektion, bbr, Wasser Rohrbau 47, No. 12: 12–19. Hoyer O (1998) Testing Performance and Monitoring of UV Systems for Drinking Water Disinfection, Wat. Suppl. 16, No. 1/ 2: 424–429. Hoyer O (2001) Testing and Monitoring the Efficacy of UV-Disinfection Systems- The German DVGW Approach, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Huang Z, Maness P-C, Blake DM, Wolfrum EJ, Smolinski SL, Jacoby WA (2000) Bactericidal Mode of Titanium Dioxide Photocatalysis, J. Photochem. Photobiol. A: Chem. 130: 163–170. Hunter G (2000) The History of UV Disinfection in the Last 20 Years, IUVA News 2, No. 3: 5–7. IUVA (2001) Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Jacoby WA, Maness PC, Wolfrum EJ, Blake DM, Fennell JA (1998) Mineralization of Bacterial Cell Mass on a Photocatalytic Surface in Air, Environ. Sci. Technol. 32, No. 17: 2650–2653. Jin S, Chou CI, Mofidi AA, Linden KG (2001) Comparative Analysis of UV Fluence Measurement using Chemical Actinometry, Biodosimetry, and Mathematical Techniques, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release.

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Probabilistic Approach to Design, J. Environ. Eng. December: 1078–1084. Lyn DA, Chiu K, Blatchley III ER (1999) Numerical Modeling of Flow and Disinfection in UV Disinfection Channels, J. Environ. Eng. January: 17–26. Malley Jr JP (2000) Engineering of UV Disinfection Systems for Drinking Water, IUVA News 2, No. 3: 8–12. Parrotta MJ, Bekdash F (1998) UV Disinfection of Small Groundwater Supplies, J. Am. Water Works Assoc. 90, No. 2: 71–81. Pogozelski WK, Tullius TD (1998) Oxidative Strand Scission of Nucleic Acids: Routes Initiated by Hydrogen Abstraction from the Sugar Moiety, Chem. Rev. 98: 1089–1107. Rennecker JL, Driedger AM, Rubin SA, Marinas BJ (2000) Synergy in Sequential Inactivation of Cryptosporidium Parvum with Ozone/Free Chlorine and Ozone/ Monochloramine, Wat. Res. 34, No. 17: 4121–4130. Roche Encyclopedia (1987) Roche Lexikon Medizin, 2nd ed., Urban & Schwarzenberg, München. Schleypen P, Loy H (eds.) (1991) Untersuchungen zur Keimreduktion im gereinigten Abwasser durch UV-Bestrahlung, Informationberichte des Bayer. Landesamtes für Wasserwirtschaft, No. 3, ISSN 01764217. Schneider OD, Malanchuk D, Nickols D, Ashe CR (2001) Comparison of LargeScale UV Systems for Disinfection of an Unfiltered Drinking Water Supply, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Schoenen D, Kolch A (1992) Photoreactivation of E. coli Depending on Light Intensity after UV Irradiation, Zbl. Hyg. 192: 565–570. Shang C, Blatchley III ER (2001) Chlorination of Pure Bacterial Cultures in Aqueous Solution, Wat. Res. 35, No. 1: 244–254. Snozzi M, Haas R, Leuker G, Kolch A, Bergman RW (1999) Prüfung und Zertifizierung von UV-Anlagen, Gas, Wasser, Abwasser 79, No. 5: 380–385. Sommer R, Cabaj A, Pribil W, Haider T (1997) Influence of Lamp Intensity and

References Water Transmittance on the UV Disinfection of Water, Wat. Sci. Technol. 35, No. 11–12: 113–118. Sommer R, Cabaj A, Sandu T, Lhotsky M (1999) Measurement of UV Radiation using Suspensions of Microorganisms, J. Photochem. Photobiol. B: Biol. 53, 1–6. Sommer R, Haider T, Cabaj A, Pribil W, Lhotsky M (1998) Time Dose Reciprocity in UV Disinfection of Water, Wat. Sci. Technol. 38, No. 12: 145–150. Sonntag C von (1987) Disinfection with UVRadiation, in: Stucki S (ed.) Process Technologies for Water Treatment, Plenum Press, New York. Steuer W (1989) in: Bartz WJ, Wippler E (eds) (1989) Desinfektion von Trinkwasser durch UV-Bestrahlung, 2nd ed., Expert-Verlag, Ehningen: 1–22. Tosa K, Hirata T (1999) Photoreactivation of Enterohemorrhagic Escherichia Coli Following UV Disinfection, Wat. Res. 33, No. 2: 361–366. Vidal A, Diaz AI, El Hraiki A, Romero M, Muguruza I, Senhaji F, Gonzalez J (1999) Solar Photocatalysis for Detoxification and Disinfection of Contaminated Water: Pilot Plant Studies, Catal. Today 54: 283–290. Wei C, Lin W-Y, Zainal Z, Williams NE, Zhu K, Kruzic AP, Smith RL, Rajesh-

war K (1994) Bactericidal Activity of TiO2 Photocatalyst in Aqueous Media: Toward a Solar-Assisted Water Disinfection System, Environ. Sci. Technol. 28, No. 5: 934–938. Whitby GE, Palmateer G, Cook WG, Maarschalkerweerd J, Huber D, Flood K (1984) Ultraviolet Disinfection of Secondary Effluent, J. Wat. Pollut. Control Fed. 56, No. 7: 844–850. White SC, Jernigan EB, Venosa AD (1986) A Study of Operational Ultraviolet Disinfection Equipment at Secondary Treatment Plants, J. Wat. Poll. Contr. Fed. 58, No. 3: 181–192. Wist J, Sanabria J, Dierolf C, Torres W, Pulgarin C (2002) Evaluation of Photocatalytic Disinfection of Crude Water for Drinking-water Production, J. Photochem. Photobiol. A: Chem. 147: 241–246. Zheng MS, Andrews A, Bolton JR (1999 a) Impacts of Medium-Pressure UV on THM and HAA Formation in Pre-UV Chlorinated Drinking Water, Proc. Water Quality Technology Conference, Tampa FL. Zheng MS, Andrews A, Bolton JR (1999 b) Impacts of Medium-Pressure UV and UV/ H2O2 Treatments on Disinfection Byproduct Formation, Proc. AWWA 1999 Annual Conference, Chicago IL.

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Alternative Glossary The specialized language of photochemistry and of photochemical technology is not very common to engineers who are active in the expanding field of water and air treatment processes. Fortunately, three excellent and internationally accepted glossaries covering the terminology in physical chemistry (Mills et al., 1993), photocatalysis (Serpone and Emeline, 2002) and photochemistry (Verhoeven, 1996) have been published. The photochemical and photophysical terms and notations used in this book are adapted to the last, which was proposed by the International Union of Pure and Applied Chemistry (IUPAC). Additionally, the remarkable dictionary prepared by Kurzweil (2000) on units, equations, constants and conversions used in science, technology and medicine enables not only newcomers but also specialists to enter interdisciplinary research activities in photoscience. Furthermore, fundamental physical constants, energy equivalents and conversion factors can be found in the CRC Handbook of Chemistry and Physics edited by Lide (2000). Mattay and Griesbeck (1994) collected a versatile accumulation of introductory and sophisticated textbooks, journals and series for further reading on photochemistry, photophysics and photochemical technology. Of course, this reference list contains the standard photochemistry textbook by Turro (1978, 1991), which covers mechanistic and synthetic aspects of photochemical reactions. A landmark of technical literature in this research area represents the outstanding book presented by Braun et al. (1991). It covers the principles of photochemical technology comprehensively. It was specifically written to meet the sophisticated needs of users and engineers and it is a valuable reference desk book. However, this collection of literature has to be extended to include several important recent contributions to the fields of photochemistry, photochemical AOPs and AOTs. For example, the text book by Suppan (1994) covers the basic principles of various UV and light induced processes in different research areas and demonstrates in an excellent manner the interdisciplinary applications and potentials of photoscience. This is also emphasized by Böttcher et al. (1991) who concentrate on several important technical applications using UV/VIS radiation as a reagent, an information carrier and as an energy carrier. Unfortunately, these authors hardly mention the expanding field of photochemical AOTs for waste treatment. Concepts, methods and experiments of photochemistry are perfectly presented in the book by Wöhrle et al. (1998). McCluney (1994) collected a comprehensive

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system of language, mathematics and instrumentation used for the measurement of electromagnetic radiation in radiometry and photometry. Recently, Balzani (2001) edited a comprehensive five-volume handbook on electron transfer reactions in chemistry. It covers different and interrelated fields of active research and presents a current and broad overview of every aspect of electron transfer. This includes the discussion of AOP related topics, such as photocatalytic oxidations and reductions, radiation-chemical techniques and photocatalysis for waste remediation. Another important contribution by Halmann (1996) gives a comprehensive description of the photoreactions of inorganic and organic water pollutants and of xenobiotics that are recalcitrant to biodegradation. In addition, it includes a brief introduction to the mechanisms of AOPs. In this connection, the short introductory handbook on ultraviolet applications prepared by Bolton (2001, 1999) has to be mentioned as a quick desk reference. It has been written to provide information on UV technology related to AOPs. In addition, Bolton (2000) summarized a series of terms and definitions for use in UV disinfection. System sizing and cost calculations, specific treatment examples and several case histories are discussed synoptically in the AOT handbook prepared by Chemviron Carbon (1997). Gottschalk et al. (2000) presented a practical guide to the ozonation of water and wastewater. It also covers briefly some mechanistic aspects of photochemical AOPs using ozone and hydrogen peroxide. Two excellent reviews describe comprehensively the photochemical processes for water treatment (Legrini et al., 1993) and environmental applications of semiconductor photocatalysis (Hoffmann et al., 1995). In addition, Blake (1999) compiled a comprehensive bibliography of work on photocatalysis. The combined reports include patents and more than 3300 citations to work published between 1970 and 1999. They also include tables of organic and inorganic compounds, bacteria, algae and viruses that have been treated in an aqueous or gaseous phase providing the corresponding reference data. In addition to these different books and reviews on themes related to AOPs and AOTs the increasing importance of research and development of photo-initiated water and air treatment technologies is reflected by the “Journal of Advanced Oxidation Technologies”, which was first launched by Al-Ekabi (1996) and several internationally renowned associate editors. Photochemical and photobiological sciences are growing areas of interdisciplinary importance. Therefore, in 2002 the Royal Society of Chemistry launched a new journal, “Photochemical and Photobiological Sciences”, which covers aspects of the interaction of UV radiation and light with molecules, supramolecular systems or biological matter. Further information about the versatile subject coverage are available on the web via www.rsc.org/pps. Furthermore, in addition to many workshops and symposia around the world the yearly international conference on AOTs for water and air remediation has to be mentioned. It was organized successfully for the first time in 1995 in London (Ontario, Canada). In addition, the proceedings of the first international conference on TiO2 photocatalysis (Ollis and Al-Ekabi, 1993) provide a selection of methods and techniques applied in photocatalyzed water and air purification. Recent developments in the field of UV applications can be found in the conference pro-

References

ceedings of the first international congress on ultraviolet technologies (IUVA, 2001). The abstracts of these conferences present an excellent insight into actual research and development activities of the international “photonic community” related to environmental topics. In addition, the reader should refer to AOTs-6 (2000), or to the proceedings of the second international conference on oxidation technologies for water and wastewater treatment, held in Clausthal-Zellerfeld in Germany (Vogelpohl et al., 2001). Furthermore, the proceedings of the symposia on chemical oxidation edited by Eckenfelder et al. (1996) cover many aspects of AOPs written by leading international experts. In general, the diversified interests of photoscientists are represented by biennial symposia on photochemistry organized by IUPAC. General topics related to modern water research are documented comprehensively in the series “Vom Wasser” edited by the Society of German Chemists (GdCh). It also contains a list of international journals and periodicals that are published in the field of water science (e. g. Vom Wasser, 1998, 2001). vanLoon and Duffy (2000) and Spellman (1999) comprehensively covered many aspects of environmental chemistry and of environmental pollution, respectively. These books are suitable as introductory texts concerning the chemistry, physics and biochemistry of the atmosphere, hydrosphere and of the terrestrial environment. Verschueren (2001) presented the latest research in the field of environmental effects of organic chemicals including reference data on dangerous organic chemicals.

References Al-Ekabi H (ed.) (1996) J. Adv. Oxid. Technol. 1, No. 1. AOTs-6 (2000) Abstracts of the Sixth International Conference on Advanced Oxidation Technologies for Water and Air Remediation, London, Ontario (Canada), June 26– 30. Balzani V (ed.) (2001) Electron Transfer in Chemistry, Wiley-VCH, Weinheim. Blake DM (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, National Renewable Energy Laboratory (available on the web, see Tab. 1–1). Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photoscience Inc., ISBN 0-9685432-0-0; also published in: Eur.. Photochem. Assoc. Newslett. Nr. 66: 9–36. Bolton JR (2000) Terms and Definitions in Ultraviolet Disinfection, in: Proceedings, Disinfection 2000: Disinfection of Wastes in the New Millennium, 15–18 March, New

Orleans, LA, Water Environment Federation, 601 Wythe St., Alexandria, VA, 22314. Bolton JR (2001) Ultraviolet Applications Handbook, 2nd ed., Bolton Photosciences Inc., 628 Cheriton Cres., NW, Edmonton, AB, Canada T6R 2M5, ISBN 0-9685432-19. Böttcher H, Bendig J, Fox MA, Hopf G, Timpe H-J (1991) Technical Applications of Photochemistry, Deutscher Verlag für Grundstoffindustrie, Leipzig. Braun AM, Maurette M-T, Oliveros E (1991) Photochemical Technology, John Wiley & Sons, Chichester. Chemviron Carbon (1997) AOT Handbook, Advanced Oxidation Technologies, ref. AOT/E-03, European Operating Unit of Calgon Carbon Corporation, Brussels (Belgium). Eckenfelder Jr WW, Bowers AR, Roth JA (1996) Chemical Oxidation: Technologies for the Nineties, Vol. 1–6, Technomic Publishing, Lancaster.

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10 Alternative Glossary Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Halmann MM (1996) Photodegradation of Water Pollutants, CRC Press, Boca Raton. Hoffmann MR, Martin ST, Choi W, Bahnemann DW (1995) Environmental Applications of Semiconductor Photocatalysis, Chem. Rev. 95: 69–96. IUVA (2001) Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Kurzweil P (2000) Das Vieweg EinheitenLexikon, 2nd ed.,Vieweg, Braunschweig. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93: 671–698. Lide DR (ed.) (2000) CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton. Mattay J, Griesbeck AG (1994) Photochemical Key Steps in Organic Synthesis, An Experimental Course Book, VCH, Weinheim: 1–14. McCluney WR (1994) Introduction to Radiometry and Photometry, Artech House, Boston. Mills I, Cvitas T, Homann K, Kallay N, Kuchitsu K (1993) Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Scientific Publications, Oxford. Ollis DF, Al-Ekabi H (eds.) (1993) Photocatalytic Purification and Treatment of Water and Air, Proceedings of the 1st International Conference on TiO2 Photocatalytic Purification and Treatment of Water and Air, London, Ontario, Canada 8–13 November 1992, Elsevier, Amsterdam. Serpone N, Emeline AV (2002) Suggested Terms and Definitions in Photocatalysis and Radiocatalysis, Int. J. Photoenergy 4,

No. 3: 91–131; available free of charge at http://www.photoenergy.org.ijp. Spellman FR (1999) Science of Environmental Pollution, Technomic Publishing, Lancaster. Suppan P (1994) Chemistry and Light, Royal Society of Chemistry, Cambridge. Turro NJ (1978) Modern Molecular Photochemistry, The Benjamin/Cummings Publishing Co., Inc., Menlo Park, CA. Turro NJ (1991) Modern Molecular Photochemistry, University Science Books, MillValley, CA. vanLoon GW, Duffy SJ (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford. Verhoeven JW (1996) Glossary of Terms Used in Photochemistry, 2nd ed., Pure Appl. Chem. 68, No. 12: 2223–2286 (available on the web, see Tab. 1-1). Verschueren K (2001) Handbook of Environmental Data on Organic Chemicals, Wiley-VCH, Weinheim. Vogelpohl A, Geissen SU, Kragert B, Sievers M (issue eds.) (2001) Selected Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, Wat. Sci. Technol. 44, issue 5; electronic release. Vom Wasser (1998) Appendix: Abbreviations of Journals according to “International Serials Catalogue”, Fachgruppe Wasserchemie (ed.) in der Gesellschaft Deutscher Chemiker, 91: 361–363, Wiley-VCH, Weinheim. Vom Wasser (2001) Wasserchemische Gesellschaft, Fachgruppe in der GdCh (ed.), Vol. 97. Wöhrle D, Tausch MW, Stohrer W-D (1998) Photochemie – Konzepte, Methoden, Experimente, Wiley-VCH, Weinheim.

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Glossary of Terms Used in Photochemistry International Union of Pure and Applied Chemistry Organic Chemistry Division, Commission on Photochemistry

2nd Edition, March 1996 Prepared for publication by: S. E. Braslavsky, K. N. Houk (first edition: Pure & Appl. Chem. 60 (1988): 1055 foll.) and J.W. Verhoeven (2nd edition: Pure & Appl. Chem. 68 (1996) No. 12: 2223–2286), http://www.iupac.org/reports/1996/6812verhoeven/index.html

Alphabetic List of Terms and Definitions (relevant parts reprinted by permission of IUPAC and J. W. Verhoeven)

Absorbance (A): The logarithm to the base 10 of the ratio Rof the spectral radiant power of incident, essentially monochromatic, radiation (I = k Ik dk) to the radiant power of transmitted radiation (Pk): A logPk0 =Pk

log T

In practice, absorbance is the logarithm to the base 10 of the ratio of the spectral radiant power of light transmitted through the reference sample to that of the light transmitted through the solution, both observed in identical cells. T is the (internal) transmittance. This definition supposes that all the incident light is either transmitted or absorbed, reflection or scattering being negligible. Traditionally (spectral )radiant intensity, I, was used instead of spectral radiant power, Pk, which is now the accepted form. (The terms: absorbancy, extinction, and optical density should no longer be used.) See absorption coefficient, absorptance, attenuance, Beer-Lambert law, depth of penetration, internal transmittance, Lambert law, molar absorption coefficient. Absorptance: The fraction of light absorbed, equal to one minus the transmittance (T). See absorbance.

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Absorption (of electromagnetic radiation): The transfer of energy from an electromagnetic field to a molecular entity. Absorption coefficient (decadic-a or Napierian-a): Absorbance divided by the optical pathlength, l: a A=l 1=l logPk0 =Pk Physicists usually use natural logarithms. In this case: a a ln 10 1=l logPk0 =Pk where a is the Napierian absorption coefficient. Since absorbance is a dimensionless quantity, the coherent SI unit for a and a is m–1. Also cm–1 is often used. See also absorptivity, molar absorption coefficient. Absorption cross section (r): Operationally, it can be calculated as the absorption coefficient divided by the number of molecular entities contained in a unit volume of the absorbing medium along the light path: r a=N 1=Nl lnPk0 =Pk where N is the number of molecular entities per unit volume, l is the optical pathlength, and a is the Napierian absorption coefficient. The relation between the absorption cross section and the molar (decadic) absorption coefficient, e, (units M–1 cm–1) is r e=NA 0:1 ln 10 0:2303e=NA 3:825 10

24

e

where r is in m2 and NA is the Avogadro constant. See attenuance, Beer-Lambert law. Absorptivity: Absorptance divided by the optical pathlength. For very low attenuance it approximates the absorption coefficient (within the approximation (1–e–A) ~ A). The use of this term is not recommended. Actinometer: A chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected. Action spectrum: A plot of a relative biological or chemical photoresponse (= Dy) per number of incident photons, against wavelength or energy of radiation under

Alphabetic List of Terms and Definitions

the same radiant power of light. This form of presentation is frequently used in the studies of biological or solid state systems, where the nature of the absorbing species is unknown. The action spectrum is sometimes called spectral responsivity or sensitivity spectrum. The precise action spectrum is a plot of the spectral (photon or quantum) effectiveness. By contrast, a plot of the biological or chemical change or response per absorbed photon (quantum efficiency) versus wavelength is the efficiency spectrum. See also excitation spectrum, efficiency spectrum. Adiabatic electron transfer: Electron transfer process in which the reacting system remains on a single electronic surface in passing from reactants to products. For adiabatic electron transfer the electronic transmission factor is close to unity (see Marcus equation). See also diabatic electron transfer. Adiabatic photoreaction: Within the “Born-Oppenheimer approximation”, a reaction of an excited state species that occurs on a single “potential-energy surface”. Compare diabatic photoreaction. Alpha-cleavage (a-Cleavage): Homolytic cleavage of a bond connecting an atom or group to a specified group. Often applied to a bond connected to a carbonyl group, in which case it is called a Norrish type I photoreaction. This reaction should be distinguished from an alpha-(a-)expulsion. Alpha-expulsion (a-Expulsion): A general reaction by which a group attached to the alpha carbon of an excited chromophore is expelled either as an odd electron species or as an anionic species. This reaction should be distinguished from an alpha-(a-)cleavage. AM (0) sunlight: The solar irradiance in space just above the earth atmosphere (air mass, AM, zero). Also called extraterrestrial global irradiance. AM (1) sunlight: The solar irradiance traversing the atmosphere when the sun is in a position perpendicular to the earth surface. Also called terrestrial global irradiance. See also AM (0) sunlight. Annihilation: Two atoms or molecular entities both in an excited electronic state interact often (usually upon collision) to produce one atom or molecular entity in an excited electronic state and another in its ground electronic state. This phenomenon is sometimes referred to as energy pooling. See singlet-singlet annihilation, spin-conservation rule, triplet-triplet annihilation. Antimony-xenon lamp (Arc): An intense source of ultraviolet, visible, and near infra-red radiation produced by an electrical discharge in a mixture of antimony va-

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por and xenon under high pressure. Its output in the ultraviolet region is higher than that of the mercury-xenon arc. See lamp. Anti-stokes shift: See Stokes shift. Apparent lifetime: See lifetime. Argon ion laser: A CW or pulsed laser emitting lines from 334 to 529 nm from singly ionized argon. Principal emissions are at 488.0 and 514.5 nm. See laser, gas lasers. Attenuance (D): The logarithm to the base 10 of the ratio of the transmittance (T): D

log T logPk0 =Pk

Attenuance reduces to absorbance if the incident beam is only either transmitted or absorbed, but not reflected or scattered. See Beer-Lambert law, depth of penetration. Attenuance filter: An optical device (filter) which reduces the radiant power of a light beam by a constant factor over all wavelengths within its operating range. Sometimes called attenuator or neutral density filter. Auxochrome: An atom or group which, when added to or introduced into a chromophore, causes a bathochromic shift and/or a hyperchromic effect in a given band of the chromophore, usually in that of lowest frequency. This term is obsolete. Back electron transfer: A term often used to indicate thermal reversal of excited state electron transfer restoring the donor and acceptor in their original oxidation level. In using this term one should also specify the resulting electronic state of the donor and acceptor. Bandgap energy (Eg): The energy difference between the bottom of the conduction band and the top of the valence band in a semiconductor or an insulator. See conduction band, Fermi level. Bandpass filter: An optical device which permits the transmission of radiation within a specified wavelength range and does not permit transmission of radiation at higher or lower wavelengths. It can be an interference filter. See also cut-off filter. Bathochromic shift (Effect): Shift of a spectral band to lower frequencies (longer wavelengths) owing to the influence of substitution or a change in environment

Alphabetic List of Terms and Definitions

(e.g., solvent). It is informally referred to as a red shift and is opposite to hypsochromic shift. Beer-Lambert law (or Beer-Lambert-Bouguer Law): The absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium is proportional to the absorption pathlength, l, and to the concentration, c, or – in the gas phase – to the pressure of the absorbing species. This law holds only under the limitations of the Lambert law and for absorbing species exhibiting no concentration or pressure dependent aggregation. The law can be expressed as A logPk0 =Pk ecl or Pk Pk0 10

ecl

where the proportionality constant, e, is called the molar (decadic) absorption coefficient. For l in cm and c in mol dm–3 or M, e will result in dm3 mol–1 cm–1 or M–1 cm–1, which is a commonly used unit. The SI unit of e is m2 mol–1. Note that spectral radiant power must be used because the Beer-Lambert law holds only if the spectral bandwidth of the light is narrow compared to spectral linewidths in the spectrum. See absorbance, attenuance, extinction coefficient, Lambert law. Bioluminescence: Luminescence produced by living systems. See luminescence. Biphotonic excitation: Also called two-photon excitation. The simultaneous (coherent) absorption of two photons (either same or different wavelength) the energy of excitation being the sum of the energies of the two photons. Biphotonic process: A process resulting from biphotonic excitation. See multiphoton process. Biradical (Synonymous with diradical): An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other. Species in which the two radical centres interact significantly are often referred to as biradicaloids. If the two radical centres are located on the same atom, they always interact strongly, and such species are called carbenes, nitrenes, etc. The lowest-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kT, the product of the Boltzmann constant k and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets. Theoretical descriptions of low-energy states of a biradical display the presence of two unsaturated valences (biradicals contain one fewer

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Glossary of Terms Used in Photochemistry

bond than permitted by the rules of valence): the dominant valence bond structures have two dots, the low energy molecular orbital configurations have only two electrons in two approximately nonbonding molecular orbitals, two of the natural orbitals have occupancies close to one, etc. Biradicaloid: Biradical-like. Bleaching: In photochemistry this term refers to the loss of absorption or emission intensity. Blue shift: Informal expression for hypsochromic shift. Cadmium-helium laser: See Helium-Cadmium laser. Charge hopping: Electron or hole transport between equivalent sites. Charge recombination: Reverse of charge separation. In using this term it is important to specify the resulting electronic state of the donor and acceptor. Charge separation: A process in which, under the influence of a suitable driving force (e.g. provided by photoexcitation), electronic charge moves in a direction that increases the difference in local charges between donor and acceptor sites. Electron transfer between neutral species is one of the most important examples. Charge shift: A process in which under the influence of a suitable driving force (e.g. provided by photoexcitation) electronic charge moves without changing the difference in local charges between donor and acceptor sites. Electron transfer reversing the charges in a system composed of a neutral donor and a cationic acceptor or of a neutral acceptor and an anionic donor provide prominent examples. Charge-transfer (CT) state: A state related to the ground state by a charge transfer transition. Charge-transfer (CT) transition: An electronic transition in which a large fraction of an electronic charge is transferred from one region of a molecular entity, called the electron donor, to another, called the electron acceptor (intramolecular CT) or from one molecular entity to another (intermolecular CT). Typical for donor-acceptor complexes or multichromophoric molecular entities. In some cases the charge transfer absorption band may be obscured by the absorption of the partners. Charge-transfer (CT) complex: A ground-state complex which exhibits an observable charge transfer absorption band. See charge-transfer transition.

Alphabetic List of Terms and Definitions

Charge-transfer transition to solvent (CTTS): Electronic transition which is adequately described by single electron transfer between a solute and the solvent, as opposed to excitation followed by electron transfer to solvent. See also charge-transfer (CT) transition. Chemical laser: A CW or pulsed laser in which the excitation and population inversion of the emitting species results from a chemical reaction. Typical examples are HF and DF lasers emitting many lines in the IR region. Chemiexcitation: Generation, by a chemical reaction, of electronically excited molecular entities from reactants in their ground electronic states. Chemiluminescence: Luminescence arising from chemiexcitation. Chromophore: That part of a molecular entity consisting of an atom or group of atoms in which the electronic transition responsible for a given spectral band is approximately localized. CIDEP (Chemically Induced Dynamic Electron Polarization): Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscopy. CIEEL (Chemically Initiated Electron Exchange Luminescence): A type of luminescence resulting from a thermal electron-transfer reaction. Also called catalyzed chemiluminescence. Collision complex: An ensemble formed by two reaction partners for which the distance is the sum of their Van der Waals radii. As such it constitutes a subclass of the species indicated as encounter complex. See also encounter complex. CO2 laser: A continuous or pulsed source of coherent radiation normally tunable through the CO2 vibration-rotation band centered near 10.6 lm. See gas lasers, laser. Coherent radiation: A source is said to emit coherent radiation when all the elementary waves emitted have a phase difference constant in space and time. Conduction band: A vacant or only partially occupied set of many closely spaced electronic levels resulting from an array of a large number of atoms forming a system in which the electrons can move freely or nearly so. This term is usually used to describe the properties of metals and semiconductors. See bandgap energy, Fermi level, valence band.

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Glossary of Terms Used in Photochemistry

Configuration (electronic configuration): A distribution of the electrons of an atom or a molecular entity over a set of one-electron wavefunctions called orbitals, according to the Pauli principle. From one configuration several states with different multiplicities may result. For example, the ground electronic configuration of the P oxygen molecule (O2) is 1r2g, 1r2u, 2r2g, 2r2u, 1p4u, 3r2g, 1p2g resulting in the 3 g , P 1 Dg , and 1 g states of different energy. Configuration Interaction (CI): The mixing of wavefunctions representing different electronic configurations to obtain an improved wavefunction for a many-electron state. Contact ion pair: Pair of ions in direct contact and not separated by an intervening solvent or other neutral molecule. One mode of formation for a (geminate) contact ion pair is electron transfer between precursor species in an encounter complex (cf. collision complex). If one of the precursors in the encounter complex is electronically excited the contact ion pair formed by electron transfer is equivalent to a polar exciplex. Conversion spectrum: A plot of a quantity related to the absorption (absorbance, cross section, etc.) multiplied by the quantum yield for the considered process against a suitable measure of photon energy, such as frequency, m, wavenumber, r, or wavelength, k. E.g., the conversion cross section, rU, has the SI unit m2. See also action spectrum, efficiency spectrum, spectral effectiveness. Copper vapor laser: A pulsed source of coherent radiation emitting at 578.2 and 510.5 nm from excited copper atoms. See gas lasers, laser. Correlation diagram: A diagram which shows the relative energies of orbitals, configurations, valence bond structures, or states of reactants and products of a reaction, as a function of the molecular geometry, or another suitable parameter. An example involves the interpolation between the energies obtained for the united atoms and the values for the separated atoms limits. Correlation energy: The difference between the Hartree-Fock energy calculated for a system and the exact nonrelativistic energy of that system. The correlation energy arises from the approximate representation of the electron-electron repulsions in the Hartree-Fock method. Critical quenching radius (r0): See Förster excitation transfer. Crystal field splitting: The removal of a degeneracy of the energy levels of molecular entities or ions due to the lower site symmetry created by a crystalline environment. This term is sometimes incorrectly used synonymously with the term ligand field splitting.

Alphabetic List of Terms and Definitions

CT: Abbreviation for charge-transfer. Current yield: See photocurrent yield. Cut-off filter: An optical device which only permits the transmission of radiation of wavelengths that are longer than or shorter than a specified wavelength. Usually, the term refers to devices which transmit radiation of wavelengths longer than the specified wavelength. See filter. CW (Continuous Wave): Nonpulsed source of electromagnetic radiation. Dark photochemistry (photochemistry without light): Chemical reactions involving electronically excited molecular entities which are generated thermally rather than by absorption of electromagnetic radiation. The use of this term is discouraged. Deactivation: Any loss of energy by an excited molecular entity. See emission, energy transfer, internal conversion, radiationless deactivation and transition, radiative transition. Delayed fluorescence: See delayed luminescence. Delayed luminescence: Luminescence decaying more slowly than that expected from the rate of decay of the emitting state. The following mechanisms of luminescence provide examples: (1) triplet-triplet annihilation to form one molecular entity in its excited singlet state and another molecular entity in its electronic ground state (sometimes referred to as P type), (2) thermally activated delayed fluorescence involving reversible intersystem crossing (sometimes referred to as E type), and (3) combination of oppositely charged ions or of an electron and a cation. For emission to be referred to in this case as delayed luminescence at least one of the two reaction partners must be generated in a photochemical process. Depth of penetration (of light): The inverse of the absorption coefficient. The SI unit is m. If the decadic absorption coefficient, a, is used, the depth of penetration (1/a) is the distance at which the spectral radiant power, Pk decreases to one tenth of its incident value, Pk0 . If the Napierian absorption coefficient, a, is used, the depth of penetration (1/a = b in this case) is the distance at which the spectral radiant power decreases to 1/e of its incident value. See absorbance, attenuance. Dexter excitation transfer (Electron Exchange Excitation Transfer): Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap

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Glossary of Terms Used in Photochemistry

of the wavefunctions of the energy donor and the energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. The transfer rate constant, kET, is given by kET / h=2pP2 J exp 2r=L; where r is the distance between donor (D) and acceptor (A), L and P are constants not easily related to experimentally determinable quantities, and J is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed. See also radiative energy transfer. Diabatic electron transfer: Electron transfer process in which the reacting system has to cross over between different electronic surfaces in passing from reactants to products. For diabatic electron transfer the electronic transmission factor is 1 (see Marcus equation.) The term non-adiabatic electron transfer has also been used and is in fact more widespread, but should be discouraged because it contains a double negation. See also adiabatic electron transfer. Diabatic photoreaction: Within the Born-Oppenheimer approximation, a reaction beginning on one excited state “potential-energy surface” and ending, as a result of radiationless transition, on another surface, usually that of the ground state. Also called non-adiabatic. Compare with adiabatic photoreaction. Diode lasers: Sources of CW or pulsed coherent radiation in the visible and infrared regions. These lasers are semiconductor devices of small dimensions. Also called semiconductor lasers. Di-p-methane rearrangement: A photochemical reaction of a molecular entity comprising two p-systems, separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and “bond formation” between the lateral carbons of the nonmigrating moiety.

See also oxa-di-p-methane rearrangement. Dipolar mechanism (of energy transfer): Same as Förster excitation transfer. See also energy transfer.

Alphabetic List of Terms and Definitions

Dipole-dipole excitation transfer: Same as Förster excitation transfer. See also energy transfer. Diradical: This term, synonymous with biradical, is no longer recommended. Dose: The energy or amount of photons absorbed per unit area or unit volume by an irradiated object during a particular exposure time. In medicine and in some other research areas (e.g. photopolymerization and water handling through irradiation) dose is used in the sense of fluence, i.e. the energy or amount of photons per unit area or unit volume received by an irradiated object during a particular exposure time. The SI units are J m–2 or J m–3 and mol m–2 or mol m–3, respectively. See also UV-dose. Doublet state: A state having a total electron spin quantum number equal to 1/2. See multiplicity. Driving force (for electron transfer): Term widely used to indicate the negative of the standard Gibbs energy change (DG0) for (photoinduced) outer-sphere electron transfer. This quantity can often be calculated rather accurately from independently determined properties of the donor and acceptor species involved. Thus e.g. for photoinduced electron transfer between a neutral acceptor (A) and a neutral donor (D) (either one of them may be the electronically excited molecular entity) to form an ion pair, the driving force in a solvent with static dielectric constant es can be approximated as (see: A. Weller, Z. Phys. Chem. Neue Folge 133, 93–98 (1982)): DG0 eE 0 D=D

E 0 A=A

e2 =4pe0 es q

DE0;0

with E0(D/D+) the standard oxidation potential of the donor, E0(A/A–) the standard reduction potential of the acceptor, e2/(4pe0esq) the change in Gibbs energy for bringing the two radical ions to an encounter distance q, and DE0,0 the electronic excitation energy of the excited partner, all data referring to the same solvent. Dye laser: A CW or pulsed source of coherent radiation in which the active medium is usually a solution of a fluorescent organic molecule (the dye) pumped with a suitable pump laser or with a flash lamp. These lasers can be tuned over a large part of the fluorescence band of the dye. Dynamic quenching: See quenching. Effectiveness: See spectral effectiveness.

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Glossary of Terms Used in Photochemistry

Efficiency (of a step; g): The ratio between the useful energy delivered or bound and the energy supplied, i.e., energy output/energy input. It is also used in the sense of a quantitative measure of the relative rate of a given step involving a species with respect to the sum of the rates of all of the parallel steps which depopulate that species. See also quantum yield. Efficiency spectrum: A plot of the efficiency of a step (g) against wavelength or photon energy. See action spectrum, conversion spectrum. Compare spectral effectiveness. Einstein: One mole of photons. Although widely used, it is not an IUPAC sanctioned unit. It is sometimes defined as the energy of one mole of photons. This use is discouraged. Electrochemiluminescence: See electrogenerated chemiluminescence. Electrochromic effect: See Stark effect. Electrogenerated luminescence (ECL): Luminescence produced by electrode reactions. Also called electroluminescence or electrochemiluminescence. Electroluminescence: See electrogenerated chemiluminescence. Electron correlation: The adjustment of electron motion to the instantaneous (as opposed to time-averaged) positions of all the electrons in a molecular entity. See also correlation energy. Electron exchange excitation transfer: Same as Dexter excitation transfer. See also energy transfer. Electronic configuration: See configuration. Electronic energy migration (or Hopping): The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another of the same kind (e.g. excitation migration between the chromophores of an aromatic polymer). The migration can happen via: radiative or radiationless processes. Electronically excited state: A state of an atom or molecular entity which has greater electronic energy than the ground state of the same entity. Electron transfer: The transfer of an electron from one molecular entity to another or between two localized sites in the same molecular entity.

Alphabetic List of Terms and Definitions

See also inner-sphere electron transfer, outer-sphere electron transfer, Marcus equation. Electron transfer photosensitization: Photochemical process in which a reaction of a non-absorbing substrate is induced by electron transfer (not energy transfer) with an excited light-absorbing sensitizer. The overall process must be such that the sensitizer is recycled. Depending on the action of the excited sensitizer as electron donor or acceptor the sensitization is called reductive or oxidative. See also photosensitization. Electrophotography: Processes of photoimaging which are based on photo-induced changes of electric fields (photo-conductive or photo-electrostatic effects). El-Sayed rules: The rate of intersystem crossing, e.g. from the lowest singlet state to the triplet manifold, is relatively large if the radiationless transition involves a change of orbital type. E.g. 1p,p* ? 3n,p* is faster than 1p,p* ? 3p,p* and 1n,p* ? 3p,p* is faster than 1n,p* ? 3n,p*. See multiplicity. Emission: Radiative deactivation of an excited state; transfer of energy from a molecular entity to an electromagnetic field. See also fluorescence, luminescence, phosphorescence. Emission spectrum: Plot of the emitted spectral radiant power (spectral radiant exitance) or of the emitted spectral photon irradiance (spectral photon exitance) against a quantity related to photon energy, such as frequency, m, wavenumber, r, or wavelength, k. When corrected for wavelength dependent variations in the equipment response, it is called a corrected emission spectrum. Emittance: See radiant exitance. Encounter complex: An intermolecular ensemble formed by molecular entities in contact or separated by a distance small compared to the diameter of solvent molecules and surrounded by several shells of solvent molecules; the innermost shell is the solvent “cage”. If one of the species is excited, the excitation usually takes place prior to formation of the encounter complex. During the lifetime of the encounter complex the reactants can collide several times to form collision complexes, and then undergo structural and electronic changes. If the interaction between the reactants leads to a minimum in the potential energy and one of the entities is electronically excited, the encounter complex may represent an exciplex or excimer. See also contact ion pair and collision complex. Energy migration: See electronic energy migration.

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Glossary of Terms Used in Photochemistry

Energy pooling: See annihilation. Energy storage efficiency (g): The rate of the Gibbs energy storage in an endothermic photochemical reaction divided by the incident irradiance. See also efficiency. Energy transfer: From a phenomenological point of view, the term is used to describe the process by which a molecular entity absorbs light and a phenomenon originates from the excited state of another molecular entity. In mechanistic photochemistry the term has been reserved for the photophysical process in which an excited state of one molecular entity (the donor) is deactivated to a lower-lying state by transferring energy to a second molecular entity (the acceptor), which is thereby raised to a higher energy state. The excitation may be electronic, vibrational, rotational or translational. The donor and acceptor may be two parts of the same molecular entity, in which case the process is called intramolecular energy transfer. See also Dexter excitation transfer, Förster excitation transfer, radiative energy transfer, and spectral overlap. Energy transfer plot: A plot of the quenching rate constant of an excited molecular entity by a series of quenchers versus the excited state energy of the quenchers. Alternatively, a plot of the rate constant for the sensitization of a reaction versus the excited state energy of different sensitizers. This type of plot is used to estimate the energy of the excited molecular entity quenched (in the former case) or produced (in the latter case). Also known as Hammond-Herkstroeter plot. See also Stern-Volmer kinetic relationships. Enhancer: A fluorescent compound which accepts energy and thus enhances or promotes the emission from a sample containing a chemically or enzymatically generated excited molecular entity. ESCA: See photoelectron spectroscopy. Excimer: An electronically excited dimer, “nonbonding” in the ground state, a complex formed by the interaction of an excited molecular entity with a ground state partner of the same structure. See also exciplex. Excimer laser: A source of pulsed coherent radiation obtained from an exciplex. The proper name should be exciplex laser. Typical lasing species are noble gas halides (XeCl, KrF, etc.) emitting in the UV domain. See laser, gas lasers. Exciplex: An electronically excited complex, of definite stoichiometry, “nonbonding” in the ground state. For example, a complex formed by the interaction of an

Alphabetic List of Terms and Definitions

excited molecular entity with a ground state partner of a different structure. If the partners have pronounced electron-donor and -acceptor character their exciplex attains ionpair character. The terms compact exciplex and loose exciplex have sometimes been used to indicate that such polar exciplexes may have structures closely related to a contact ion pair or a solvent-separated ion pair. Excitation spectrum: Plot of the spectral radiant exitance or of the spectral photon exitance against the frequency (or wavenumber, or wavelength) of excitation. When corrected for wavelength dependent variations in the excitation radiant power this is called a corrected excitation spectrum. See also emission spectrum. Excitation transfer: Same as energy transfer. Excited state: A state of higher energy than the ground state of a chemical entity. In photochemistry an electronically excited state is usually meant. Exciton: In some applications it is useful to consider electronic excitation as if a quasi-particle capable of migrating, were involved. In organic materials two models are used: the band or wave model (low temperature, high crystalline order) and the hopping model (higher temperature, low crystalline order or amorphous state). Energy transfer in the hopping limit is identical with energy migration. See electronic energy migration. Exitance: See radiant exitance. External heavy atom effect: See heavy atom effect. Exterplex: Termolecular analogue of an exciplex. Use of this term is discouraged. See also: exciplex. Extinction: This term, equivalent to absorbance, is no longer recommended. Extinction coefficient: This term, equivalent to molar (decadic) absorption coefficient, is no longer recommended. See Beer-Lambert law. Fermi level (EF): The chemical potential of electrons in a solid (metals, semiconductors or insulators) or in an electrolyte solution. See bandgap energy, conduction band, valence band. Filter (optical): A device which reduces the spectral range (bandpass, cut-off, and interference filter) or radiant power of incident radiation (neutral density or attenuance filter) upon transmission of radiation.

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Flash photolysis: A technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce a sufficient concentration of a transient species suitable for spectroscopic observation. Fluence (H0): When applied to energy, it is the total radiant energy traversing a small transparent imaginary spherical target containing the point under consideration, divided by the cross sectionRof this target. The product of the fluence rate and the duration of the irradiation ( E0 dt, simplified expression: H0 = E0 t when the fluence rate is constant over the time considered). The SI unit is J m–2. Energy fluence is identical to spherical radiant exposure and reduces to radiant exposure (H) for a parallel and normally incident beam, not scattered or reflected by the target or its surroundings. See also dose, photon fluence. Fluence rate (E0): The rate of fluence, H0. Four times the ratio of the radiant power, P, incident on a small transparent imaginary spherical volume element containing R the point under consideration, divided by the surface area of that sphere, SK. ( 4p Ldx, simplified expression: E0 = 4 P/SK when the radiant power is constant over the solid angle considered). For energy fluence rate the SI unit is W m–2. It reduces to irradiance, E, for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings. See intensity, radiance. See also photon fluence rate. Fluorescence: Spontaneous emission of radiation (luminescence) from an excited molecular entity with the formation of a molecular entity of the same spin multiplicity. Flux (energy flux): see radiant energy flux, radiant power. f Number: See oscillator strength. Förster excitation transfer (Dipole-Dipole Excitation Transfer): A mechanism of excitation transfer which can occur between molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It is described in terms of an interaction between the transition dipole moments, (a dipolar mechanism). The transfer rate constant (kD?A) is given by kD!A

K 2 J8:8 10 n4 s0 r 6

28

mol

where K is an orientation factor, n the refractive index of the medium, s0 the radiative lifetime of the donor, r the distance (cm) between donor (D) and acceptor (A), and J the spectral overlap (in coherent units cm6 mol–1) between the absorp-

Alphabetic List of Terms and Definitions

tion spectrum of the acceptor and the fluorescence spectrum of the donor. The critical quenching radius, r0, is that distance at which kD?A is equal to the inverse of the radiative lifetime. See also Dexter excitation transfer, energy transfer, radiative energy transfer. Förster cycle: Indirect method of determination of excited state equilibria, such as pKa values, based on ground state thermodynamics and electronic transition energies. This cycle considers only the difference in molar enthalpy change (DDH) of reaction of ground and excited states, neglecting the difference in molar entropy change of reaction of those states (DDS). Fourier transform spectrometer: A scanning interferometer, containing no principal dispersive element, which first splits a beam into two or more components, then recombines these with a phase difference. The spectrum is obtained by a Fourier transformation of the output of the interferometer. Franck-condon principle: Classically, the Franck-Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The resulting state is called a Franck-Condon state, and the transition involved, a vertical transition. The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition. Franck-condon state: See Franck-Condon principle. Free electron laser: Source of coherent radiation in which the active medium is an electron beam moving at speeds close to the speed of light in the spatially periodic magnetic field produced by an array of magnets (the wiggler). The emitted wavelength, kL, is approximately given by kx/(4E2 ), with kx being the wiggler period and E the kinetic energy of the electrons in MeV. See laser. Free-running laser: It applies to a pulsed laser and means that the laser emission lasts as long as the pumping process is sufficient to sustain lasing conditions. Typical pulse durations are in the ls-ms range, depending on the pumping source. When the operation mode of a pulsed laser is not specified as Q-switched, mode-locked, or anything else, it must be considered as free-running. Frequency (m or x): The number of waveperiods per unit time. The linear frequency, m, is the number of cycles per unit time. The SI unit is Hz : s–1. For the angular frequency, the symbol x (= 2pm) is used, with rad s–1 as the SI unit. Frequency doubling: See harmonic frequency generation, nonlinear optical effects.

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Glossary of Terms Used in Photochemistry

FWHM (Full Width at Half Maximum): See half-(band)width. Gas lasers: CW or pulsed lasers in which the active medium is a gaseous mixture usually composed of a buffer gas (He for instance) and an active medium consisting of: – neutral atoms (e.g., Ne, Cu, Au, etc.) or molecules (e.g., N2, CO2, CO, I2, etc.), or – ionized atoms (e.g., Ar, Kr, Cd, etc.) These lasers are not tunable but most of them can emit several lines which in many cases may be selected from a single apparatus. Pulsed lasers may be free-running, Q-switched, or mode-locked. Some CW lasers may be mode-locked. See argon ion-, CO2-, excimer, copper vapor-, helium-neon-, krypton-, nitrogenlasers. Geminate ion pair: Ion pair, formed from a precursor that constitutes a single kinetic entity. I.e. by electron transfer or ion transfer in an encounter complex (cf. collision complex) or by ionic dissociation of a single molecular entity. Geminate pair: Pair of molecular (or atomic) species in close proximity in liquid solution with a solvent cage and resulting from reaction (e.g. bond scission, electron transfer, group transfer) of a precursor that constitutes a single kinetic entity. Geminate recombination: Recombination reaction of a geminate pair. The reaction can either be a back electron transfer that restores the donor and acceptor species in their ground-state, from which the pair was created via electron transfer, or a bond formation or bond reorganization. Ground state: The lowest energy state of a chemical entity. In photochemistry ground electronic state is usually meant. Half-width (of a band): The full width of a spectral band at a height equal to half of the height at the band maximum. Also known as full width at half maximum (FWHM). The dimension of band width should be either inverse length (wavenumbers) or inverse time (frequencies) so that the values give an indication of the energies. Note the hyphen in half-width. Half bandwidth has the meaning of halfwidth at half maximum. Hammond-Herkstroeter plot: See energy transfer plot. Harmonic frequency generation: Production of coherent radiation of frequency km (k = 2, 3, . . .) from coherent radiation of frequency m. In general, this effect is obtained through the interaction of laser light with a suitable optical medium with nonlinear polarizability. The case k = 2 is referred to as frequency doubling, k = 3 is frequency tripling, k = 4 is frequency quadrupling. Even higher integer values of k are possible.

Alphabetic List of Terms and Definitions

Harpoon mechanism: Reaction sequence (thermal or photoinduced) between neutral molecular or atomic entities in which long-range electron transfer is followed by a considerable reduction of the distance between donor and acceptor sites as a result of the electrostatic attraction in the ion pair created. Heavy atom effect: Enhancement of the rate of a spin-forbidden process by the presence of an atom of high atomic number, which is either part of, or external to, the excited molecular entity. Mechanistically, it corresponds to a spin-orbit coupling enhancement produced by a heavy atom. Helium-cadmium laser: A CW laser emitting mainly at 325.0 and 441.6 nm from singly ionized cadmium. See gas lasers. Helium-neon laser: A CW laser emitting mainly at 632.8, 1152.3, and 3391.3 nm from excited neutral Ne atoms. See gas lasers. Herkstroeter plot: See energy transfer plot. Heteroexcimer: Same as exciplex. High-pressure mercury lamp (arc): Radiation source containing mercury at a pressure of ca. 8 MPa (ca. 80 bar) or higher which emits lines over a background continuum between about 200 and 1400 nm. See lamp. Hole burning: The photobleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy. Hole transfer: Charge migration process in which the majority of carriers are positively charged. Hot ground state reaction: A hot state reaction of the ground electronic state. Hot quartz lamp: A term sometimes used to describe a high-pressure mercury lamp. The use of this term is not recommended. Hot state reaction: A reaction proceeding from an ensemble of molecular entities possessing a higher average vibrational, rotational or translational energy than they would at thermal equilibrium with the surrounding medium.

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Hund rules: (1) Of the different multiplets resulting from different configurations of electrons in degenerate orbitals of an atom those with greatest multiplicity have the lowest energy (multiplicity rule). (2) Among multiplets having the same multiplicity, the lowest-energy one is that with the largest total orbital angular momentum (angular momentum rule) (valid if the total orbital angular momentum is a constant of motion). (3) In configurations containing shells less than half full of electrons, the term having the lowest total angular momentum J lies lowest in energy, whereas in those with shells more than half filled, the term having the largest value of J lies lowest (fine structure rule). Hund rules apply if the “Russell-Saunders” coupling scheme is valid. Sometimes the first rule is applied to molecules. Hush model: See: Marcus-Hush relationship. Hyperchromic effect: Increase in the intensity of a spectral band due to substituents or interactions with the molecular environment. See also auxochrome. Hyperfine: Due to nuclear spin. Hypochromic effect: Opposite of hyperchromic effect. Hypsochromic shift: Shift of a spectral band to higher frequency or shorter wavelength upon substitution or change in medium (e.g., solvent). It is informally referred to as a blue shift, and is opposite to bathochromic shift. Imaging (photoimaging): The use of a photosensitive system for the capture, recording, and retrieval of information associated with an object using electromagnetic energy. Incoherent radiation: Not having the properties of the coherent radiation. Inner filter effect: This term is used in two different ways. In an emission experiment, it refers to an apparent decrease in emission quantum yield and/or distortion of bandshape as a result of reabsorption of emitted radiation. During a light irradiation experiment, absorption of incident radiation by a species other than the intended primary absorber is also described as an inner filter effect. Inner-sphere electron transfer: Historically an electron transfer between two metal centers sharing a ligand or atom in their respective coordination shells. The definition has more recently been extended to any situation in which the interaction between the donor and acceptor centers in the transition state is significant (>20 kJ mol–1). Compare outer-sphere electron transfer.

Alphabetic List of Terms and Definitions

Integrating sphere: A hollow sphere having a highly reflecting inside surface used as a device to collect, with very high efficiency, light scattered or emitted from a sample contained in it or located outside and near one of the ports. Small ports allow the entrance of light and access to a detector. Intensity: Traditional term for photon flux, fluence rate, irradiance or radiant power (radiant flux). In terms of an object exposed to radiation, the term should now be used only for qualitative descriptions. Intensity (I) (of a light source): Same as radiant intensity. Intensity (of a spectral feature): Describes the magnitude of the particular feature in the spectrum. Interference filter: See filter. Interferometer: See Fourier transform spectrometer. Internal conversion: A photophysical process. Isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation. Internal transmittance: See transmittance. Intersystem crossing: A photophysical process. Isoenergetic radiationless transition between two electronic states having different multiplicities. It often results in a vibrationally excited molecular entity in the lower electronic state, which then usually deactivates to its lowest vibrational level. Intervalence charge transfer: Electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centers. Intimate ion pair: See contact ion pair. Inverted region (for electron transfer): In plots relating rate constants to changes in standard Gibbs energy (DG0) for electron transfer a region may occur in which the rate constants decrease as the exergonicity of the reaction increases. This region is often referred to as the inverted region and its presence is predicted by the theory developed for outer sphere electron transfer for the case –DG0 > k in the Marcus equation, k being the reorganization energy.

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Note the similarity to the energy gap law for radiationless conversion of an excited state. Compare normal region. Irradiance (E): The radiant flux or radiant power, P, of all wavelengths incident on an infinitesimal element of surface containing the point under consideration divided by the area of the element (dP/dS, simplified expression: E = P/S when the –2 radiant power is R constant over the surface area considered). The SI unit is W m . Note that E = k Ek dk, where Ek is the spectral irradiance at wavelength k. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings fluence rate (E0) is an equivalent term. See also photon irradiance, spectral irradiance. Isoabsorption point: The use of this term, equivalent to isosbestic point, is not recommended. Isoclinic point: A wavelength, wavenumber, or frequency at which the first derivative of an absorption spectrum of a sample does not change upon a chemical reaction or physical change of the sample. Isoemissive point: Same as isostilbic point. Isooptoacoustic point: A wavelength, wavenumber, or frequency at which the total energy emitted by a sample as heat does not change upon a chemical reaction or physical change of the sample. Its position depends on the experimental conditions. The spectral differences between the isosbestic points and the isooptoacoustic points are the result of the nonlinear relationship between the molar absorption coefficient and the photoacoustic signal. See photoacoustic spectroscopy. Isosbestic point: A wavelength, wavenumber, or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The term derives from the Greek word for ‘same attenuance’. A simple example occurs when one molecular entity is converted into another which has the same molar absorption coefficient at a given wavelength. As long as the sum of the concentrations of the two molecular entities in the solution is held constant, there will be no change in absorbance at this wavelength as P the ratio of the two entities is varied. In general, A(k)l–1(= in1 eki ci ) must remain constant during the reaction or physical change in order to observe an isosbestic point. The use of the term isoabsorption point is not recommended. Isostilbic point: The wavelength at which the intensity of emission of a sample does not change during a chemical reaction or physical change. The term derives from the Greek word for ‘same luminescence’. The terms isoemissive and isolampsic are sometimes used. See isosbestic point.

Alphabetic List of Terms and Definitions

Jablonski diagram: Originally, a diagram showing that the fluorescent state of a molecular entity is the lowest excited state from which the transition to the ground state is allowed, whereas the phosphorescent state is a metastable state below the fluorescent state, which is reached by radiationless transition. In the most typical cases the fluorescent state is the lowest singlet excited state and the phosphorescent state the lowest triplet state, the ground state being a singlet. Presently, modified Jablonski diagrams are frequently used and are actually state diagrams in which molecular electronic states, represented by horizontal lines displaced vertically to indicate relative energies, are grouped according to multiplicity into horizontally displaced columns. Excitation and relaxation processes that interconvert states are indicated in the diagram by arrows. Radiative transitions are generally indicated with straight arrows, while radiationless transitions are generally indicated with wavy arrows. Jahn-Teller effect: For nonlinear molecular entities in a geometry described by a point symmetry group possessing degenerate irreducible representations there always exists at least one nontotally symmetric vibration that makes electronically degenerate states unstable at this geometry. The nuclei are displaced to new equilibrium positions of lower symmetry causing a splitting of the originally degenerate states. This effect is due to the odd terms in the vibronic perturbation expansion. See also Renner-Teller effect. Kasha rule: Polyatomic molecular entities luminesce with appreciable yield only from the lowest excited state of a given multiplicity. There are exceptions to this rule. Kasha-Vavilov rule: The quantum yield of luminescence is independent of the wavelength of exciting radiation. There are exceptions to this rule. Koopmans’ theorem: See photoelectron spectroscopy. Krypton ion laser: A CW or pulsed laser emitting lines from 337 to 859 nm from singly ionized krypton. Principal emissions are at 530.9, 568.2, 647.1, and 752.5 nm. See gas lasers. Lambert law: The fraction of light absorbed by a system is independent of the incident spectral radiant power (Pk0 ). This law holds only if Pk0 is small, scattering is negligible, and multiphoton processes, excited state populations, and photochemical reactions are negligible. See absorbance, Beer-Lambert law. Lamp: A source of incoherent radiation.

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Glossary of Terms Used in Photochemistry

See high-pressure, medium-pressure, and low-pressure mercury lamp (arc), and antimony-xenon, mercury-xenon, quartz-iodine, tungsten-halogen, resonance, and xenon lamp. Laporte rule: For monophotonic radiative transitions in centro-symmetric systems, the only nonvanishing electric-dipole transition moments are those which connect an even term (g) with an odd term (u). Laser: A source of ultraviolet, visible, or infrared radiation which produces light amplification by stimulated emission of radiation from which the acronym is derived. The light emitted is coherent except for superradiance emission. See argon ion, helium-cadmium, chemical, CO2 copper vapor, diode, dye, excimer, free electron, free-running, gas, helium-neon, krypton ion, mode-locked, neodymium, nitrogen, Q-switched, solid state, and ruby laser. See also lasing. Lasing: The process of light amplification by stimulated emission of radiation (laser). Latent image: The primary result of radiation absorption in a photo-imaging system which is susceptible to development. Lifetime (s): The lifetime of a molecular entity which decays in a first-order process is the time needed for a concentration of the entity to decrease to 1/e of its original value. Statistically, it represents the life expectation of the entity. It is equal to the reciprocal of the sum of the (pseudo)unimolecular rate constants of all processes which cause the decay. Lifetime is used sometimes for processes which are not first order. However, in such cases, the lifetime depends on the initial concentration of the entity, or of a quencher and therefore only an initial or a mean lifetime can be defined. In this case it should be called apparent lifetime, instead. Occasionally, the term half-life (s1/2) is used, representing the time needed for the concentration of an entity to decrease to one half of its original value. Ligand field splitting: The removal of a degeneracy of atomic or molecular levels in a molecule or ion with a given symmetry induced by the attachment or removal of ligands to produce reduced symmetries. See crystal field splitting. Ligand to ligand charge transfer (LLCT) transition: An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two ligands has occurred.

Alphabetic List of Terms and Definitions

Ligand to metal charge transfer (LMCT) transition: An electronic transition in a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from a ligand to a metal center has occurred. See also metal to ligand charge transfer transition. Light polarization: When the end point of the electric vector of a polarized light beam is viewed along the direction of light propagation, it moves along a straight line if the light is linearly polarized, along a circle if it is circularly polarized, and along an ellipse if it is elliptically polarized. Light source: See lamp, laser. Low-pressure mercury lamp (Arc): A type of resonance lamp which contains mercury vapor at pressures of about 0.1 Pa (0.75 × 10–3 Torr; 1 Torr = 133.3 Pa). At 25 8C, such a lamp emits mainly at 253.7 and 184.9 nm. Other terms used for such a lamp are germicidal, cold and hot cathode, Wood lamp. See lamp. Luminescence: Spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment. See also bioluminescence, chemiluminescence, electro-generated chemiluminescence, fluorescence, phosphorescence, photoluminescence, radioluminescence, sono luminescence, thermoluminescence, triboluminescence. Lumiphore (Luminophore): A part of a molecular entity (or atom or group of atoms) in which electronic excitation associated with a given emission band is approximately localized. (Analogous to chromophore for absorption spectra.) Marcus equation (for electron transfer): Equation proposed by R.A. Marcus to relate the rate of outer-sphere electron transfer with the thermodynamics of this process (see: R.A. Marcus, J. Chem. Phys. 24, 966–978 (1956)). Essentially the rate constant within the encounter complex (or the rate constant of intramolecular transfer) is given by the Eyring equation: ket jel kT=h exp DGz =RT where k represents Boltzmann’s constant and jel is the so called electronic transmission factor: (jel ~ 1 for adiabatic and 1 for diabatic electron transfer). It was shown by Marcus that for outer-sphere electron transfer the barrier height can then be expressed as: DGz k DG0 2 =4k where DG0 represents the standard Gibbs energy change accompanying the reaction and k the total reorganization energy.

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Glossary of Terms Used in Photochemistry

It should be noted that whereas the classical Marcus equation has been found to be quite adequate in the normal region, it is now generally accepted that in the inverted region a more elaborate formulation, taking into account explicitly the Franck-Condon factor due to quantum mechanical vibration modes, should be employed. Marcus inverted region (for electron transfer): See inverted region. Marcus-hush relationship: Relationship between the barrier (DGz) to thermal electron transfer, the energy of a corresponding optical charge transfer transition (DEop ), and the overall change in standard Gibbs energy accompanying thermal electron transfer (DG0). Assuming a quadratic relation between the energy of the system and its distortions from equilibrium (harmonic oscillator model) the expression obtained is: 2 =4DEop DGz DEop

DG0

The simplest form of this expression obtains for degenerate electron transfer (DG0) in e.g. symmetrical mixed valence systems: DGz DEop =4 Note that for this situation the Marcus equation reads DGz = k/4. See Marcus equation. Medium-pressure mercury lamp (Arc): Radiation source containing mercury vapor at pressures ranging from 100 to several hundred kPa (1 atm = 101.325 kPa). Emits mostly from 310 to 1000 nm with most intense lines at 300, 303, 313, 334, 366, 405, 436, 546, and 578 nm. See lamp. Mercury-xenon lamp (Arc): An intense source of ultraviolet, visible, and near infrared radiation produced by an electrical discharge in a mixture of mercury vapor and xenon under high pressure. See lamp. Merry-go-round reactor (Turntable Reactor): An apparatus in which several samples are rotated around a radiation source in order to expose each to equal amounts of radiation. Metal to ligand charge transfer (MLCT) Transition: An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from the metal to a ligand has occurred. Compare ligand to metal charge transfer transition.

Alphabetic List of Terms and Definitions

Metal to metal charge transfer (MMCT) transition: An electronic transition of a bior poly-nuclear metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two metal centers has occurred. See also intervalence charge transfer. MLCT: See metal to ligand charge transfer. Mode-locked laser: A laser in which many resonant modes are coupled in phase, to yield a train of very short pulses (e.g. ps pulses). The coupling of the modes is obtained by modulation of the gain in the resonator, and can be active (electro-optic modulation of the losses or of the pump intensity), or passive (with a saturable absorber). See also free-running laser. Molar absorption coefficient, molar decadic absorption coefficient: Absorbance divided by the absorption pathlength, l and the concentration, c: e 1=cl logPk0 =Pk A=cl In common usage for l in cm and c in mol dm–3 or M, e results in dm3 mol–1 cm–1 or M–1 cm–1, which equals 0.1 m2 mol–1 (coherent SI units) = 103 cm2 mol–1 = cm2 mmol–1 = dm3 cm–1 mol–1. The term molar absorptivity for molar absorption coefficient should be avoided. See absorbance, absorption coefficient, Beer-Lambert law. Multiphoton absorption: See multiphoton process. See also biphotonic excitation. Multiphoton process: A process involving interaction of two or more photons with a molecular entity. See biphotonic process, two-photon process. Multiplicity (Spin Multiplicity): The number of possible orientations, calculated as 2S + 1, of the spin angular momentum corresponding to a given total spin quantum number (S), for the same spatial electronic wavefunction. A state of singlet multiplicity has S = 0 and 2S + 1 = 1. A doublet state has S = 1/2, 2S + 1 = 2, etc. Note that when S > L (the total orbital angular momentum quantum number) there are only 2L + 1 orientations of total angular momentum possible. Non-adiabatic electron transfer: See diabatic electron transfer. Normal region (for electron transfer): In plots relating rate constants for electron transfer, or quantities related to it, with the standard Gibbs energy for the reaction (DG0), the region for which the rate constants increase with increasing exergoni-

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Glossary of Terms Used in Photochemistry

city of the reaction is called the normal region. This region is predicted by the Marcus theory for outer-sphere electron transfer for the case of DGo ≤ k in the Marcus equation. n ? p* transition: An electronic transition described approximately as promotion of an electron from a “nonbonding” (lone-pair) n orbital to an “antibonding” p orbital designated as p*. n–p* state: An excited state related to the ground state by a n ? p* transition. n ? r* transition: An electronic transition described approximately as promotion of an electron from a “nonbonding” (lone-pair) n orbital to an “antibonding” r orbital designated as r*. Such transitions generally involve high transition energies and appear close to or mixed with Rydberg transitions. Natural lifetime: Same as radiative lifetime. The use of this term is discouraged. Neodymium laser: A CW or pulsed laser emitting radiation from excited Nd+3 principally occurring around 1.06 lm (the precise position depends on the matrix). The Nd+3 is present as a dopant in suitable crystals (e.g., yttrium-aluminum garnet, YAG) or in suitable glasses (phosphate, silicate, etc.). See solid state lasers. Neutral-density filter: See attenuance filter. Nitrogen laser: A source of pulsed semi-coherent superradiance mainly around 337 nm. The lasing species is molecular nitrogen. See gas lasers. Non-adiabatic photoreaction: Same as diabatic photoreaction. Use of double negative is discouraged. Compare adiabatic photoreaction. Non-linear optical effect: An effect brought about by electromagnetic radiation the magnitude of which is not proportional to the irradiance. Non-linear optical effects of importance to photochemists are harmonic frequency generation, lasers, Raman shifting, upconversion, and others. Non-radiative decay: Disappearance of an excited species due to a radiationless transition. Non-vertical energy transfer: An energy transfer process which has a low FranckCondon factor. See Franck-Condon principle.

Alphabetic List of Terms and Definitions

Norrish type I photoreaction: a-Cleavage of an excited carbonyl compound leading to an acyl-alkyl radical pair (from an acyclic carbonyl compound) or an acyl-alkyl biradical (from a cyclic carbonyl compound) as a primary photoproduct; e.g.,

Norrish type II photoreaction: Intramolecular abstraction of a c-hydrogen by an excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct; e.g.,

Optical density: Synonymous with absorbance. The use of the term optical density is discouraged. Optoacoustic spectroscopy: Same as photoacoustic spectroscopy. Orbital (Atomic or Molecular): A wavefunction which depends explicitly on the spatial coordinates of only one electron. Oscillator strength (f Number): A measure of the intensity of a spectral band: a classical concept (giving the effective number of electrons taking part in a certain transition) adapted to wave mechanics. For a transition between state i and state j, fij

8p2 mcm GR2ij 3h

where m is the mass of the electron, c the velocity of light, m the frequency, h the Planck constant, G the degeneracy of the final state. Rij is the transition moment calculated omitting e, the electron charge, in the integral; better called the transition length. Experimentally, fij is determined by integration of the absorption band, using the equation Z fij k=n erdr; where e(r) is the molar absorption coefficient at wavenumber r and n the average refractive index of the medium. k = 4.32 × 10–9 L–1 mol cm (for e in the common units L mol–1 cm–1); k = 4.32 × 10–8 mol m–1 (for e in SI unit m2 mol–1). The oscillator strength, fij , is a dimensionless quantity. See also transition (dipole) moment.

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Outer-sphere electron transfer: Electron transfer between redox centers which do not share a common atom or group, i.e., the interaction between the relevant electronic orbitals of the two centers in the transition state is weak (<20 kJ mol–1). Compare inner-sphere electron transfer. Oxa-di–p-methane rearrangement: A photochemical reaction of a b,c-unsaturated ketone to form a saturated a-cyclopropyl ketone. The rearrangement formally amounts to a 1,2-acyl shift and “bond formation” between the former a and c carbon atoms.

See also di-p-methane rearrangement. Paterno-Büchi reaction: The photocycloaddition of an electronically excited carbonyl group to a ground state olefin yielding an oxetane.

PEC: Abbreviation for photoelectrochemistry. Penetration depth: See depth of penetration. PES: Abbreviation for photoelectron spectroscopy. Phonon: Elementary excitation in the quantum mechanical treatment of vibrations in a crystal lattice. Phosphorescence: From a phenomenological point of view, the term has been used to describe long-lived luminescence. In mechanistic photochemistry, the term designates luminescence involving change in spin multiplicity, typically from triplet to singlet or vice versa. The luminescence from a quartet state to a doublet state is also phosphorescence. Photoacoustic effect: Generation of heat after absorption of radiation, due to radiationless deactivation or chemical reaction. See also photoacoustic spectroscopy. Photoacoustic spectroscopy: A spectroscopic technique based on the photoacoustic effect. A photoacoustic spectrum consists of a plot of the intensity of the acoustic signal detected by a microphone or a “piezoelectric” detector, against the excita-

Alphabetic List of Terms and Definitions

tion wavelength or another quantity related to the photon energy of the modulated excitation. See also isooptoacoustic point. Photoaffinity labelling: A technique in which a photochemically reactive molecular entity, specifically associated with a biomolecule, is photoexcited in order to covalently attach a label to the biomolecule, usually via intermediates. Photo-assisted catalysis: Catalytic reaction involving production of a catalyst by absorption of light. See photocatalysis. Photocatalysis: Catalytic reaction involving light absorption by a catalyst or by a substrate. Photochemical hole burning: See hole burning. Photochemical reaction: This term is generally used to describe a chemical reaction caused by absorption of ultraviolet, visible, or infrared radiation. There are many ground state reactions which have photochemical counterparts. Among these are photoadditions, photocycloadditions, photoeliminations, photoenolizations, photo-Fries rearrangements, photoisomerizations, photooxidations, photoreductions, photosubstitutions, etc. Photochemical smog: Product of photochemical reactions caused by solar radiation and occurring in polluted air. Photochemistry: The branch of chemistry concerned with the chemical effects of light (far UV to IR). See photochemical reaction. Photochromism: A photoinduced transformation of a molecular structure (e.g. of a solution), photochemically and/or thermally reversible, that produces a spectral change, typically, but not necessarily, of visible color. Photoconductivity: Increase in electrical conductivity resulting from photoproduction of charge carriers. Photocrosslinking: Photoinduced formation of a covalent linkage between two macromolecules or between two different parts of one macromolecule. Photocuring: Technical expressions for the photoinduced hardening of a monomeric, oligomeric or polymeric substrate normally in the form of a film.

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Photocurrent yield: The quantum efficiency of charge photo-generation between the two electrodes of a photovoltaic cell or a photoelectrochemical cell. Photodegradation: The photochemical transformation of a molecule into lower molecular weight fragments, usually in an oxidation process. This term is widely used in the destruction (oxidation) of pollutants by UV-based processes. Photodetachment (of electrons): Ejection of an electron from a negative ion upon photoexcitation. Photodynamic effect: A term used in photobiology to refer to photoinduced damage requiring the simultaneous presence of light, photosensitizer and molecular oxygen. A sensitized photooxidation which involves molecular oxygen. Photoelectrical effect: The ejection of an electron from a solid or a liquid by a photon. Photoelectrochemical cell: An electrochemical cell in which current and a voltage are simultaneously produced upon absorption of light by one or more of the electrodes. Usually at least one of the electrodes is a semiconductor. Photoelectrochemical etching: The dissolution of a semiconductor in an electrolytic solution upon exposure to light. Used in the photopatterning of semiconductor surfaces. Photoelectrochemistry: A term applied to a hybrid field of chemistry employing techniques which combine photochemical and electrochemical methods for the study of the oxidation-reduction chemistry of the ground or excited states of molecules or ions. In general, it is the chemistry resulting from the interaction of light with electrochemical systems. See also photoelectrochemical, photogalvanic, photovoltaic cell. Photoelectron spectroscopy: A spectroscopic technique which measures the kinetic energy of electrons emitted upon the ionization of a substance by high energy monochromatic photons. A photoelectron spectrum is a plot of the number of electrons emitted versus their kinetic energy. The spectrum consists of bands due to transitions from the ground state of an atom or molecular entity to the ground and excited states of the corresponding radical cation. Approximate interpretations are usually based on “Koopmans’ theorem” relating ionization potentials to orbital energies. PES and UPS is the spectroscopy using vacuum ultraviolet sources, while ESCA (electron spectroscopy for chemical analysis) and XPS use X-ray sources. Photoexcitation: The production of an excited state by the absorption of ultraviolet, visible, or infrared radiation.

Alphabetic List of Terms and Definitions

Photogalvanic cell: An electrochemical cell in which current or voltage changes result from photochemically generated changes in the relative concentrations of reactants in a solution phase oxidation-reduction couple. Compare photovoltaic cell. Photoimaging: See imaging. Photoinduced electron transfer: An electron transfer resulting from an electronic state produced by the resonant interaction of electromagnetic radiation with matter. Photoinduced polymerization: Polymerization of a monomer by a free radical or ionic chain reaction initiated by photoexcitation. See photoinitiation. Photoinitiation: Photoproduction of a free radical or ion capable of initiating a chain reaction such as a polymerization. See photoinduced polymerization. Photoionization: Ejection of an electron into a surrounding medium induced by the absorption of electromagnetic radiation, from a neutral or positively charged molecular entity. See also photodetachment. Photoluminescence: Luminescence arising from photoexcitation. Photolysis: A light-induced bond cleavage. This term is often used incorrectly to describe irradiation of a sample, although in the combination flash photolysis this usage is accepted. Photon: The quantum of electromagnetic energy at a given frequency. This energy, E = hm is the product of the Planck constant (h) and the frequency of the radiation (m). See also quantum. Photon counting: Also called single photon counting. The recording of sequential single photon pulses counted by way of recording the electron emission events from a photosensitive layer (photocathode) and multiplied by means of a “dynode” arrangement (photomultiplier). This technique is used for two purposes: (1) the sensitive measurement of low levels of radiation and (2) the recording of emission decays. See time-correlated single photon counting. Photon emittance: See photon exitance.

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Photon exitance (Mp): The photon flow, Up, emitted by an element of the surface containing the source point under consideration divided by the area (S) of that element. (dUp/dS, simplified expression: Mp = Up/S when the photon flow is constant over the surface area considered). The SI unit is s–1 m–2. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1 m–2. Also called specific photon emission. Formerly called photon emittance. See spectral photon exitance. See also radiant exitance. Photon exposure (Hp): The photon irradiance, Ep, integrated over the time of irraR diation ( Ep dt, simplified expression: Hp = Ept when the photon irradiance is constant over the time considered). The SI unit is m–2. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol m–2. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings photon fluence (Hp0 ) is an equivalent term. See also fluence, radiant exposure. Photon flow (Up): The number of photons (quanta, N) per unit time. (dN/dt, simplified expression: Up = N/t when the number of photons is constant over the time considered). The SI unit is s–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1. See spectral photon flow. See also radiant power. Photon fluence (H0p): The integral of the amount of all photons (quanta) which traverse a small, transparent, imaginary spherical target, divided by the cross-sec0 over the duration tional area of this target. R 0 The photon fluence rate, Ep ,0 integrated of the irradiation ( Ep dt, simplified expression: Hp Ep0 t when Ep0 is constant over the time considered). Photons per unit area (quanta m–2). The SI unit is m–2. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol m–2. See also fluence. Photon fluence rate (E0p): The rate of photon fluence. Four times the ratio of the photon flow, Up, incident on a small, transparent, imaginary spherical volume element containing the point under consideration divided by the surface of that R sphere, SK. ( 4p Lp dx, simplified expression: Ep0 = 4 Up/SK when the photon flow is constant over the solid angle considered). The SI unit is m–2 s–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol m–2 s–1. It reduces to photon irradiance for a parallel and normally incident beam not scattered or reflected by the target or its surroundings. See photon radiance. See also fluence rate.

Alphabetic List of Terms and Definitions

Photon flux: Same as photon irradiance. Photon irradiance (Ep): The photon flow, Up, incident on an infinitesimal element of surface containing the point under consideration divided by the area (S) of that element (dUp/dS, simplified expression: Ep = Up/S when the photon flow is constant over the surface considered). The SI unit is m–2 s–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol m–2 s–1. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings photon fluence rate (Ep0 ) is an equivalent term. See spectral photon irradiance. See also irradiance. Photon radiance (Lp): For a parallel beam it is the photon flow, Up, leaving or passing through an infinitesimal transparent element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, h, [(dUp/dS)/cos h, simplified expression: Lp = Up/(S cos h) when the photon flow is constant over the surface area considered]. The SI unit is m–2 s–1. For a divergent beam propagating in an elementary cone of the solid angle dx containing the direction H, the photon radiance is d2Up/(dx dS cos h), with SI unit m–2 s–1 sr–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol m–2 s–1 and mol m–2 s–1 sr–1, respectively. See spectral photon radiance. See also radiance. Photooxidation: Oxidation reactions induced by light. Common processes are: (1) The loss of one or more electrons from a chemical species as a result of photoexcitation of that species; (2) The reaction of a substance with oxygen under the influence of light. When oxygen remains in the product this latter process is also called photooxygenation. Reactions in which neither the substrate nor the oxygen are electronically excited are sometimes called photoinitiated oxidations. Compare photoreduction. Photooxygenation: Incorporation of molecular oxygen into a molecular entity. There are three common mechanisms: Type I: the reaction of triplet molecular oxygen with radicals formed photochemically. Type II: the reaction of photochemically produced singlet molecular oxygen with molecular entities to give rise to oxygen containing molecular entities. The third mechanism proceeds by electron transfer producing superoxide anion as the reactive species. Compare photooxidation.

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Glossary of Terms Used in Photochemistry

Photophysical processes: Photoexcitation and subsequent events which lead from one to another state of a molecular entity through radiation and radiationless transitions. No chemical change results. Photopolymerization: Polymerization processes requiring a photon for the propagation step. See also photoinduced polymerization. Photoreaction: See photochemical reaction. Photoreduction: Reduction reactions induced by light. Common processes are: (1) Addition of one or more electrons to a photoexcited species; (2) The photochemical hydrogenation of a substance. Reactions in which the substrate is not electronically excited are sometimes called photoinitiated reductions. Compare photooxidation. Photoresist: A photoimaging material, generally applied as a thin film, whose local solubility properties can be altered photochemically. A subsequent development step produces an image which is useful for the fabrication of microelectronic devices (e.g., integrated circuits). Photosensitization: The process by which a photochemical or photophysical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called a photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is not consumed in the reaction. See energy transfer. Photosensitizer: See photosensitization. Photostationary state: A steady state reached by a reacting chemical system when light has been absorbed by at least one of the components. At this state the rates of formation and disappearance are equal for each of the transient molecular entities formed. Photothermal effect: An effect produced by photoexcitation resulting partially or totally in the production of heat. Photothermography: A process utilizing both light and heat, simultaneously or sequentially, for image recording. Photovoltaic cell: A solid state device, usually a semiconductor, such as silicon, which absorbs photons with energies higher than or equal to the bandgap energy and simultaneously produces electric power. Compare photogalvanic cell.

Alphabetic List of Terms and Definitions

Piezoluminescence: Luminescence observed when certain solids are subjected to a change in pressure. See triboluminescence. Polarization: See light polarization, transition polarization. Population inversion: A situation in which a higher energy state is more populated than a lower energy state. Precursor complex: May either indicate an encounter complex or a collision complex, but furthermore implies that this complex undergoes a reaction (e.g. electron transfer). Predissociation: Dissociation occurring by tunnelling from a “bound” to an “unbound” rovibronic state. In an absorption spectrum of a molecular entity, the appearance of a diffuse band region within a series of sharp bands, is called predissociation, since irradiation with frequencies within the diffuse region leads to effective dissociation. The energy of the band is smaller than that of the dissociation continuum of the bound state. Primary (photo)process: See primary photochemical process. The term primary (photo)process for photophysical processes is apt to lead to inconsistencies, and its use is therefore discouraged. Primary photochemical process (primary photoreaction): Any elementary chemical process undergone by an electronically excited molecular entity and yielding a primary photoproduct. See primary (photo)process. Primary (photo)product: The first observable chemical entity which is produced in the primary photochemical process and which is chemically different from the reactant. See primary (photo)process. Pump-probe technique: A flash photolysis technique in which the light beam (probe) used for spectral analysis is generated from a portion of the excitation (pump) beam. A time delay in the latter allows the obtention of kinetic data. p ? p* transition: An electronic transition described approximately as a promotion of an electron from a “bonding” p orbital to an “antibonding” p orbital designated as p*. p–p* state: An excited state related to the ground state by a p ? p* transition.

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p ? r* transition: An electronic transition described approximately as a promotion of an electron from a “bonding” p orbital to an “antibonding” r orbital designated as r*. Such transitions generally involve high transition energies and appear close to or mixed with Rydberg transitions. Q-switched laser: A laser in which the state of the device introducing important losses in the resonant cavity and preventing lasing operation is suddenly switched to a state where the device introduces very low losses. This increases rapidly the Quality factor of the cavity, allowing the build-up of a short and very intense laser pulse. Typical pulse durations are in the ns range. The Q-switching may be active (a rotating mirror or electro-optic device) or passive (a saturable absorber). See also free-running laser. Quantum (of radiation): An elementary particle of electromagnetic energy in the sense of the wave-particle duality. See photon. Quantum counter: A medium emitting with a quantum yield independent of the excitation energy over a defined spectral range (e.g., concentrated rhodamine 6G solutions between 300 and 600 nm). Also used for devices producing an electrical signal proportional to the photon flux absorbed in a medium. Quantum efficiency: See efficiency. For a primary photochemical process, quantum efficiency is identical to quantum yield. Quantum yield (U): The number of defined events which occur per photon absorbed by the system. The integral quantum yield is

number of events number of photons absorbed

For a photochemical reaction,

amount of reactant consumed or product formed amount of photons absorbed

The differential quantum yield is

dx=dt ; n

where d[x]/dt is the rate of change of a measurable quantity, and n the amount of photons (mol or its equivalent einstein) absorbed per unit time. U can be used for photophysical processes or photochemical reactions. See also efficiency.

Alphabetic List of Terms and Definitions

Quartet state: A state having a total electron spin quantum number equal to 3/2. See multiplicity. Quartz-iodine lamp: A tungsten filament high-intensity incandescent lamp which contains iodine in a quartz envelope. Used primarily as a source of visible light. Quencher: A molecular entity that deactivates (quenches) an excited state of another molecular entity, either by energy transfer, electron transfer, or by a chemical mechanism. See quenching, Stern-Volmer kinetic relationships. Quenching: The deactivation of an excited molecular entity intermolecularly by an external environmental influence (such as a quencher) or intramolecularly by a substituent through a nonradiative process. When the external environmental influence (quencher) interferes with the behavior of the excited state after its formation, the process is referred to as dynamic quenching. Common mechanisms include energy transfer, charge transfer, etc. When the environmental influence inhibits the excited state formation the process is referred to as static quenching. See Stern-Volmer kinetic relationships. Quenching constant: See quencher, quenching, Stern-Volmer kinetic relationships. Radiance (L): For a parallel beam it is the radiant power, P, of all wavelengths leaving or passing through an infinitesimal element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, h, [(dP/dS)/cos h, simplified expression: L = P/(S cos h) when the radiant power is R constant over the surface area considered]. The SI unit is W m–2. Note that L = k Lk dk, where Lk is the spectral radiance at wavelength k. For a divergent beam propagating in an elementary cone of the solid angle dx containing the given direction h, the radiance is d2P/(dxdS cos h), with SI units W m–2 sr–1. See also photon flow, photon radiance, spectral radiance, spherical radiance. Radiant emittance: See radiant exitance. Radiant energy (Q): The total energy emitted, transferred or received as radiation R of all wavelengths in a defined period of time (Q = Qk dk). It is the product of radiant power, P, and time, t: Q = P t when the radiant power is constant over the time considered. The SI unit is J. See also spectral radiant power. Radiant (energy) flux (P,U): Although flux is generally used in the sense of the “rate of transfer of fluid, particles or energy across a given surface”, the radiant energy flux has been adopted by IUPAC as equivalent to radiant power, P. (P = U

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Glossary of Terms Used in Photochemistry

= dQ/ds, simplified expression: P = U = Q/t when the radiant energy, Q, is constant over the time considered). In photochemistry U is reserved for quantum yield. See also photon flow, photon radiance, radiant energy, spectral radiant flux. Radiant exitance (M): The radiant power, P, emitted at all wavelengths by an element of the surface containing the source point under consideration divided by the surface area (S) of that element. (dP/dS, simplified expression: M = P/S when the radiant power is constant over the surface area considered). It is the integration of the radiant power leaving a source over the solid Rangle and over the whole wavelength range. The SI unit is W m–2. Note that M = k Mk dk, where Mk is the spectral radiant exitance at wavelength k. Formerly called radiant emittance. Same as spherical radiant exitance. See also photon exitance, spectral radiant exitance. Radiant exposure (H): The irradiance, E, integrated over the time of irradiation R ( Edt, simplified expression H = E t when the irradiance is constant over the time considered). The SI unit is J m–2. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings fluence (H0) is an equivalent term. Radiant intensity (I): Radiant (energy) flux or radiant power, P, at all wavelenghts per unit solid angle, x. The radiant power emitted in a given direction by a source or an element of the source in an infinitesimal cone containing the given direction divided by the solid angle of the cone (dP/dx, simplified expression: I = P/S when the radiant power is R constant over the surface area considered). The SI unit is W sr–1. Note that I = k Ik dk, where Ik is the spectral radiant intensity at wavelength k. See also spectral radiant intensity. Radiant power (P): Same as radiant (energy) flux U. Power emitted, transferred or received as radiation. The SI unit is J s–1 = W. See spectral radiant power. Radiationless deactivation (Decay): Loss of electronic excitation energy without photon emission or chemical change. See energy transfer, internal conversion, intersystem crossing. Radiationless transition: A transition between two states of a system without photon emission or absorption. Compare radiative transition. Radiative energy transfer: Transfer of excitation energy by radiative deactivation of a donor molecular entity and reabsorption of the emitted light by an acceptor molecular entity. The probability of transfer is given approximately by

Alphabetic List of Terms and Definitions

Prt / AvJ where J is the spectral overlap integral, [A] is the concentration of the acceptor, and v is the specimen thickness. This type of energy transfer depends on the shape and size of the vessel utilized. Same as trivial energy transfer. See also Dexter excitation transfer, energy transfer, Förster excitation transfer. Radiative lifetime (s0): The lifetime of an excited molecular entity in the absence of radiationless transitions. It is the reciprocal of the first-order rate constant for the radiative step, or of the sum of these rate constants if there is more than one such step. The equivalent term, natural lifetime, is discouraged. Approximate expressions exist relating s0 to the oscillator strength of the emitting transition. Radiative transition: A transition between two states of a molecular entity, the energy difference being emitted or absorbed as a photon. See luminescence. Compare radiationless deactivation, radiationless transition. Radical pair: Two radicals in close proximity, usually within a solvent “cage” or at least sufficiently close to allow spin correlation. The radicals may be formed simultaneously by some unimolecular process, e.g., photochemical bond breaking, or they may have come together by diffusion. A radical pair is called geminate radical pair provided that each radical partner is a descendant of the same parental pair. Radioluminescence: Luminescence arising from excitation by high energy particles or radiation. Radiolysis: Bond cleavage induced by high-energy radiation. The term is also more loosely used for any chemical process brought about by high-energy radiation. The term has also been used to refer to the irradiation technique itself (“pulse radiolysis”). Red shift: Informal term for bathochromic shift. Relative spectral responsivity (sk): See action spectrum. Relaxation: Passage of an excited or otherwise perturbed system towards or into thermal equilibrium with its environment. See radiationless deactivation, radiationless transition, radiative transition. Renner-Teller effect: Splittings in the vibrational levels of molecular entities due to even terms in the vibronic perturbation expansion. This is generally a minor effect for nonlinear molecular entities compared to the Jahn-Teller effect which is due to

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the odd terms. For linear molecular entities it is the only possible vibronic effect characteristic of degenerate electronic states. Reorganization energy (in electron transfer): Defined as the Gibbs energy dissipated when a system that has undergone “vertical” electron transfer (i.e. electron transfer obeying the Franck-Condon principle) relaxes to the equilibrium state for its new charge distribution. Commonly the total reorganization energy (k) is written as the sum of an inner contribution (kin) and an outer contribution (kout) attributed to nuclear reorganizations of the redox partners and their environment (solvent) respectively. Resonance absorption technique: The monitoring of atoms or radicals generated in the gas phase by observing the attenuation of the radiation from a lamp emitting the characteristic resonance radiation of the observed species. Resonance fluorescence: Fluorescence from the primary excited atomic or molecular species at the wavelength of the exciting radiation (no relaxation within the excited manifold). This term is also used to designate the radiation emitted by an atom of the same wavelength as the longest one capable of exciting its fluorescence, e.g. 122.6 nm in the case of the hydrogen atom, and 253.7 nm in the case of the mercury atom. See also resonance line. Resonance fluorescence technique: The monitoring of atoms or radicals generated in the gas phase by observing the intensity of fluorescence (exitance) emitted by the species after excitation with radiation of the same wavelength. Resonance lamp: A lamp emitting resonance radiation of atoms and their ions. Depending on the requirements the lamp is filled either with pure vapor of the element or with a mixture of it and other gases. E.g., Hg (253.7 nm), Cd (228.8 and 643.8 nm), Na (589.0 nm), Zn (213.8, 330.0, 334.5, and 636.2 nm), Kr (116.5 and 123.6 nm), Xe (129.6 and 147.0 nm). Resonance line: The longest wavelength capable of exciting fluorescence in an atom. See also resonance fluorescence. Resonance radiation: Same as resonance fluorescence. Rovibronic state: A state corresponding to a particular rotational sublevel of a particular vibrational level of a particular electronic state. Ruby laser: A pulsed source of coherent radiation emitting mainly at 694.3 nm from chromium ions (Cr+3) in aluminum oxide. See laser, solid state laser.

Alphabetic List of Terms and Definitions

Rydberg orbital: For an atom, an orbital with principal quantum number greater than that of any occupied orbital of the ground state. For a molecular entity, a molecular orbital which correlates with a Rydberg atomic orbital in an atomic fragment produced by dissociation. Typically, the extension of the Rydberg orbital is large compared to the size of the atom or molecular entity. Rydberg transition: An electronic transition described approximately as promotion of an electron from a “bonding” orbital to a Rydberg orbital. Spectral bands corresponding to Rydberg transitions approximately fit the Rydberg formula rI

R=n

D2 ;

where r is the wavenumber, I the ionization potential of the atom or molecular entity, n a principal quantum number, R the Rydberg constant, and D the quantum defect which differentiates between s, p, d, etc., orbitals. The notation used is, e.g., p ? ns. Sacrificial acceptor: Molecular entity that acts as the electron acceptor in a photoinduced electron transfer process and is not restored in a subsequent oxidation process but is destroyed by irreversible chemical conversion. Sacrificial donor: Molecular entity that acts as the electron donor in a photoinduced electron transfer process and is not restored in a subsequent reduction process but is destroyed by irreversible chemical conversion. Semiconductor laser: See diode laser. Schenck sensitization mechanism: The mechanism of chemical transformation of one molecular entity caused by photoexcitation of a sensitizer which undergoes temporary covalent bond formation with the molecular entity. Scintillators: Materials used for the measurement of radioactivity, by recording the radioluminescence. They contain compounds (chromophores) which combine a high fluorescence quantum efficiency, a short fluorescence lifetime, and a high solubility. These compounds are employed as solutes in aromatic liquids and polymers to form organic liquid and plastic scintillators, respectively. Selection rule: A selection rule states whether a given transition is allowed or forbidden, on the basis of the symmetry or spin of the wavefunctions of the initial and final states. Self-absorption: Absorption of part of the fluorescence from excited molecular entities by molecular entities of the same species in the ground state. The mechanism operating is a radiative energy transfer.

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Self-quenching: Quenching of an excited atom or molecular entity by interaction with another atom or molecular entity of the same species in the ground state. See also Stern-Volmer kinetic relationships. Sensitizer: See photosensitizer. Sensitization: See photosensitization. Simultaneous pair transitions: Simultaneous electronic transitions in two coupled absorbers or emitters. Because of the coupling, transitions which are spin-forbidden in one of the centres might become spin allowed (spin flip). Single photon counting: See photon counting. Single photon timing: See time-correlated single photon counting. Singlet molecular oxygen: The oxygen molecule (dioxygen), O2, in an excited sinP glet state. The ground state of O2 is a triplet 3 g . The two metastable singlet P states derived from the ground state configuration are 1Dg and 1 g . The term singlet oxygen alone, without mention of the chemical species is discouraged since it can also refer to an oxygen atom in a 1S or 1D excited state. While the oxygen atom ground state is a triplet 3P state, the 1S and 1D states are also derived from the ground state configuration. Singlet-singlet annihilation: See annihilation, spin conservation rule. Singlet-singlet energy transfer: Transfer of excitation from an electronically excited donor in a singlet state to produce an electronically excited acceptor in a singlet state. See electron exchange excitation transfer, Förster excitation transfer, radiative energy transfer. Singlet state: A state having a total electron spin quantum number equal to 0. See multiplicity. Singlet-triplet energy transfer: Transfer of excitation from an electronically excited donor in a singlet state to produce an electronically excited acceptor in a triplet state. See energy transfer, spin conservation rule. Solar conversion efficiency: The ratio of the Gibbs energy gain per unit time per m2 of surface exposed to the sun to the solar irradiance, E, integrated between k = 0 and k = ?.

Alphabetic List of Terms and Definitions

Solid state lasers: CW or pulsed lasers in which the active medium is a solid matrix (crystal or glass) doped with an ion (e. g., Nd3+, Cr3+, Er3+). The emitted wavelength depends on the active ion, the selected optical transition, and the matrix. Some of these lasers are tunable within a very broad range (e.g., from 700 to 1000 nm for Ti3+ doped sapphire). Pulsed lasers may be free-running, Q-switched, or mode-locked. Some CW lasers may be mode-locked. Solvatochromism: The (pronounced) change in position of an electronic absorption or emission band, accompanying a change in solvent polarity. Solvent-separated ion pair: Pair of ions separated by at least one solvent molecule. During electron-transfer processes between neutral molecular species, solvent separated ion pairs may form either directly or via solvation-induced separation of contact ion pairs. See also contact ion pair. Solvent shift: A shift in the frequency of a spectral band of a chemical species arising from interaction with its solvent environment. See bathochromic shift, hypsochromic shift, solvatochromism. Sonoluminescence: Luminescence induced by sound waves. See triboluminescence. Specific photon emission: Same as photon exitance. Spectral (photon) effectiveness: The reciprocal of the photon fluence rate, Ep0 , at wavelength k, causing identical photoresponse, Dy, per unit time (Dy/Dt). The effectiveness spectrum is directly proportional to the conversion spectrum of the sensory pigment, if spectral attenuance is negligible. Spectral irradiance (Ek): Irradiance, E, at wavelength k per unit wavelength interval. The SI unit is W m–3, but a commonly used unit is W m–2 nm–1. Spectral R 1 overlap: In the context of radiative energy transfer, it is the integral, J = 0 fD0 reA rdr, which measures the overlap of the emission spectrum of the 0 excited donor, D, and the absorption spectrum of the ground R 1 state acceptor, A. fD is 0 the measured normalized emission of D, fD fD r= 0 fD rdr; fD r is the photon exitance of the donor at wavenumber r, and eA r is the decadic molar absorption coefficient of A at wavenumber r. In the context of Förster excitation transfer J is given by: Z

1

J 0

fD0 reA r dr r4

343

344

Glossary of Terms Used in Photochemistry

In the context of Dexter excitation transfer J is given by Z

1

J

fD reA rdr

0

In this case fD and eA, the emission spectrum of donor and absorption spectrum of acceptor, respectively, are both normalized to unity, so that the rate constant for energy transfer, kET, is independent of the oscillator strength of both transitions (contrast to Förster mechanism). See energy transfer. Spectral photon exitance (Mpk): The photon exitance, Mp, at wavelength k per unit wavelength interval. The SI unit is s–1 m–3, but a commonly used unit is s–1 m–2 nm–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1 m–3 and the common unit mol s–1 m–2 nm–1. Spectral photon flow (Upk): The photon flow, Up, at wavelength k per unit wavelength interval. The SI unit is s–1 m–1, but a commonly used unit is s–1 nm–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1 m–1 and the common unit mol s–1 nm–1. Spectral photon flux (photon irradiance) (Epk): The photon irradiance, Ep, at wavelength k per unit wavelength interval. The SI unit is s–1 m–3, but a commonly used unit is s–1 m–2 nm–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1 m–3 and the common unit mol s–1 m–2 nm–1. Spectral photon radiance (Lpk): The photon radiance, Lpk, at wavelength k per unit wavelength interval. The SI unit is s–1 m–3 sr–1, but a commonly used unit is s–1 m–2 sr–1 nm–1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being mol s–1 m–3 sr–1 and the common unit mol s–1 m–2 sr–1 nm–1. Spectral radiance (Lk): The radiance, L, at wavelength k per unit wavelength interval. The SI unit is W m–3 sr–1, but a commonly used unit is W m–2 sr–1 nm–1. Spectral radiant exitance (Mk): The radiant exitance, M, at wavelength k per unit wavelength interval. The SI unit is W m–3, but a commonly used unit is W m–2 nm–1. Spectral radiant flux: Same as spectral radiant power.

Alphabetic List of Terms and Definitions

Spectral radiant intensity (Ik): The radiant intensity, I, at wavelength k per unit wavelength interval. The SI unit is W m–1 sr–1, but a commonly used unit is W nm–1 sr–1. Spectral radiant power (Pk): The radiant power at wavelength k per unit wavelength interval. The SI unit is W m–1, but a commonly used unit is W nm–1. Spectral responsivity: The spectral output quantity of a system such as a photomultiplier, diode array, photoimaging device, or biological unit divided by the spectral irradiance s(k) = dy(k)/dE(k), simplified expression: s(k) = Yk/Ek, where Yk is the magnitude of the output signal for irradiation at wavelength k and Ek is the spectral irradiance of parallel and perpendicular incident beam at the same wavelength. Spectral sensitivity: See spectral responsivity. Spectral sensitization: The process of increasing the spectral responsivity of a (photoimaging) system in a certain wavelength region. Spherical radiance: Same as radiant exitance, M. It is the integration of the radiant power, P, leaving a source over the solid angle and over the whole wavelength range. The SI unit is W m–2. Spherical radiant exposure: Same as fluence. Spin-allowed electronic transition: An electronic transition which does not involve a change in the spin part of the wavefunction. Spin conservation rule (Wigner rule): Upon transfer of electronic energy between an excited atom or molecular entity and other atom or molecular entity in its ground or excited state, the overall spin angular momentum of the system, a vector quantity, should not change. See annihilation. Spin flip: See simultaneous pair transitions. Spin-orbit coupling: The interaction of the electron spin magnetic moment with the magnetic moment due to the orbital motion of the electron. One consequence of spin-orbit coupling is the mixing of zero-order states of different multiplicity. This effect may result in fine structure called spin-orbit splitting. Spin-orbit splitting: Removal of state degeneracy by spin-orbit coupling.

345

346

Glossary of Terms Used in Photochemistry

Spin-spin coupling: The interaction between the spin magnetic moments of different electrons and/or nuclei. It causes, e.g. the multiplet pattern in nuclear magnetic resonance spectra. Spontaneous emission: That mode of emission which occurs even in the absence of a perturbing external electromagnetic field. The transition between states, n and m, is governed by the Einstein coefficient of spontaneous emission, Anm. See also stimulated emission. Stark effect: Splitting or shifts of spectral lines in an electric field. Also called electrochromic effect. State crossing: See avoided crossing, surface crossing. State diagram: See Jablonski diagram. Static quenching: See quenching. Stern-volmer kinetic relationships: This term applies broadly to variations of quantum yields of photophysical processes (e.g., fluorescence or phosphorescence) or photochemical reaction (usually reaction quantum yield) with the concentration of a given reagent which may be a substrate or a quencher. In the simplest case, a plot of U0/U (or M0/M for emission) vs. concentration of quencher, [Q ], is linear, obeying the equation 0 = or M0 =M 1 KSV Q:

1

In equation (1) Ksv is referred to as the Stern-Volmer constant. Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. U0 and M0 are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while U and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant Ksv is the product of the true quenching constant kq and the excited state lifetime, s0, in the absence of quencher. kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2) 0 = or M0 =M 1 kq s0 Q:

2

When an excited state undergoes a bimolecular reaction with rate constant kr to form a product, a double-reciprocal relationship is observed according to the equation 1=p 1 1=kr s0 S1=A B

3

Alphabetic List of Terms and Definitions

where Up is the quantum efficiency of product formation, A the efficiency of forming the reactive excited state, B the fraction of reactions of the excited state with substrate S which leads to product, and [S] is the concentration of reactive ground-state substrate. The intercept/slope ratio gives kr s0. If [S] = [Q ], and if a photophysical process is monitored, plots of equations (2) and (3) should provide independent determinations of the product-forming rate constant kr. When the lifetime of an excited state is observed as a function of the concentration of S or Q, a linear relationship should be observed according to the equation s0 =s 1 kq s0 Q;

4

where s0 is the lifetime of the excited state in the absence of the quencher Q. See also self-quenching. Stimulated emission: That part of the emission which is induced by a resonant perturbing electromagnetic field. The transition between states, n and m, is governed by the Einstein coefficient of stimulated emission, Bnm. CIDNP emission and lasing action are examples of processes which require stimulated emission. See also spontaneous emission. Stokes shift: The difference (usually in frequency units) between the spectral positions of the band maxima (or the band origin) of the absorption and luminescence arising from the same electronic transition. Generally, the luminescence occurring at a longer wavelength than the absorption is stronger than the opposite. The latter may be called an anti-Stokes shift. Superexchange interaction: Electronic interaction between two molecular entities mediated by one or more different molecules or ions. Superradiance: Spontaneous emission amplified by a single pass through a population inverted medium. It is distinguished from true laser action by its lack of coherence. The term superradiance is frequently used in laser technology. See coherent radiation. Surface crossing: In a diagram of electronic energy versus molecular geometry, the electronic energies of two states of different symmetry may be equal at certain geometrical parameters. At this point (unidimensional representation), line or surface (more than one dimension), the two potentialenergy surfaces are said to cross one another. See avoided crossing. r ? r* transition: An electronic transition described approximately as promotion of an electron from a “bonding” r orbital to an “antibonding” r orbital designated as r*. Such transitions generally involve high transition energies, and appear close to or mixed with Rydberg transitions.

347

348

Glossary of Terms Used in Photochemistry

Thermal lensing: A technique that determines the alteration in the refractive index of a medium as a result of the temperature rise in the path of a laser beam absorbed by the medium. The lens produced (usually divergent) causes a change (usually a decrease) in the irradiance measured along the laser beam axis. See also photothermal effects. Thermally activated delayed fluorescence: See delayed fluorescence. Thermochromism: A thermally induced transformation of a molecular structure or of a system (e.g. of a solution), thermally reversible, that produces a spectral change, typically, but not necessarily, of visible color. Thermoluminescence: Luminescence arising from a reaction between species trapped in a rigid matrix and released as a result of an increase in temperature. See luminescence. Through-bond electron transfer: Intramolecular electron transfer for which the relevant electronic interaction between the donor and acceptor sites is mediated by through-bond interaction, i.e. via the covalent bonds interconnecting these sites, as opposed to through-space interaction. See also through-space electron transfer. Through-space electron transfer: Electron transfer for which the relevant electronic interaction between the donor and acceptor sites is mediated either by direct orbital overlap or by superexchange interaction via intervening molecular entities not covalently bound to the donor or acceptor sites. See also through-bond electron transfer. TICT emission: Electronic emission emanating from a TICT state. See also TICT state, twisted internal charge transfer. TICT state: The acronym derives from Twisted Internal Charge Transfer State, proposed to be responsible for strongly Stokes-shifted fluorescence from certain aromatics, particularly in polar medium. See Twisted Internal Charge Transfer. Tight ion pair: See contact ion pair. Time-correlated single photon counting: A technique for the measurement of the time histogram of a sequence of photons with respect to a periodic event, e.g. a flash from a repetitive nanosecond lamp or a CW operated laser (mode-locked laser). The essential part is a time-to-amplitude-converter (TAC) which transforms the arrival time between a start and a stop pulse into a voltage. Sometimes called single photon timing.

Alphabetic List of Terms and Definitions

Time-resolved microwave conductivity (TRMC): Technique allowing the quantitative and qualitative detection of radiation-induced charge separation by time-resolved measurement of the changes in microwave absorption resulting from the production and decay of charged and dipolar molecular entities. Time-resolved spectroscopy: The recording of spectra at a series of time intervals after the excitation of the system with a light pulse (or other perturbation) of appropriately short duration. Transient spectroscopy: A technique for the spectroscopic observation of transient species (excited-state molecular entities or reactive intermediates) generated by a pulse of short duration. See also flash photolysis, time-resolved spectroscopy. Transition (dipole) moment (Mnm): An oscillating electric or magnetic moment can be induced in an atom or molecular entity by an electromagnetic wave. Its interaction with the electromagnetic field is resonant if the frequency of the latter corresponds to the energy difference between the initial and final states of a transition (DE = hm). The amplitude of this moment is referred to as the transition moment. It can be calculated from an integral taken over the product of the wavefunctions of the initial (m) and final (n) states of a spectral transition and the appropriate ~ of the electromagnetic radiation. dipole moment operator (D) Z Mnm e

Wn

X

ri Wm ds Zi~

i

where the summation is over the coordinates of all charged particles (electrons and nuclei). Its sign is arbitrary, its direction in the molecular framework defines the direction of transition polarization, and its square determines the strength of the transition. If e is omitted one obtains Rnm in the sense used in oscillator strength. The SI unit of the transition dipole moment is C m. The common unit is debye (D). Transition polarization: The direction of the transition moment in the molecular framework. Transmittance (T): The ratio of the transmitted spectral radiant power (Pk) to that incident on the sample (Pk0 ): T Pk =Pk0 Internal transmittance refers to energy loss by absorption, whereas the total transmittance is that due to absorption, reflection, scatter, etc. See absorbance, attenuance, Beer-Lambert law.

349

350

Glossary of Terms Used in Photochemistry

Triboluminescence: Luminescence resulting from the rubbing together of the surface of certain solids. It can be produced, for example, when solids are crushed. See sonoluminescence. Triplet state: A state having a total electron spin quantum number of 1. See multiplicity. Triplet-triplet annihilation: Two atoms or molecular entities both in a triplet state often interact (usually upon collision) to produce one atom or molecular entity in an excited singlet state and another in its ground singlet state. This is often, but not always, followed by delayed fluorescence. See also annihilation, spin conservation rule. Triplet-triplet energy transfer: Energy transfer from an electronically excited triplet donor to produce an electronically excited acceptor in its triplet state. See spin conservation rule. Triplet-triplet transitions: Electronic transitions in which both the initial and final states are triplet states. Trivial energy transfer: Same as radiative energy transfer. Tungsten-halogen lamp: See quartz-iodine lamp. Other halogens may fill the lamp. Tunnelling: The passage of a particle through a potential-energy barrier the height of which is larger than the energy of that particle. This effect is important for some processes involving the transfer of electrons and light atoms, particularly H atoms. Turntable reactor: See merry-go-round reactor. Twisted internal charge transfer (TICT): Intramolecular, photoinduced charge transfer between chromophores interconnected by a single bond leading to an excited state (a TICT state) in which the chromophores interact only weakly because of a considerable twist about the interconnecting bond. Two-photon excitation: Excitation resulting from successive or simultaneous absorption of two photons by an atom or molecular entity. This term is used for successive absorption only if some of the excitation energy of the first photon remains in the atom or molecular entity before absorption of the second photon. The simultaneous two-photon absorption can also be called biphotonic excitation. See two-photon process. Two-photon process: A photophysical or photochemical event triggered by a twophoton excitation.

Alphabetic List of Terms and Definitions

Upconversion: A nonlinear optical effect in which light frequency is increased. UPS: See photoelectron spectroscopy. UV dose: A dose of ultraviolet (UV) radiation. UV stabilizer: A substance added to a sample to prevent photodeterioration by ultraviolet (UV) light. See photochemical reaction. Valence band: The highest energy continuum of energy levels in a semiconductor that is fully occupied by electrons at 0 K. See bandgap, conduction band, Fermi level. Vavilov rule: See Kasha-Vavilov rule. Vertical transition: See Franck-Condon principle. Vibrational redistribution: Intramolecular redistribution of energy among the vibrational modes usually giving a statistical distribution of their populations, characterized by the “vibrational temperature”. For large molecules, this process does not require collisions. Vibrational relaxation: The loss of vibrational excitation energy by a molecular entity through energy transfer to the environment caused by collisions. The molecular entity relaxes into vibrational equilibrium with its environment. See relaxation. Vibronic coupling: Interaction between electronic and vibrational motions in a molecular entity. See Jahn-Teller and Renner-Teller effects. Vibronic transitions: A transition which involves a change in both the electronic and vibrational quantum numbers of a molecular entity, as opposed to purely electronic or purely vibrational transition. The transition occurs between two states, just as in a purely electronic transition, but involves a change in both electronic and vibrational energy. Wavelength (k): The distance, measured along the line of propagation, between two corresponding points on adjacent waves. The wavelength depends on the medium in which the wave propagates. Wavenumber (r; m): The reciprocal of the wavelength, k, or the number of waves per unit length along the direction of propagation. The SI unit is m–1, but a commonly used unit is cm–1.

351

352

Glossary of Terms Used in Photochemistry

Weller correlation: Empirical correlation for the energy of full charge-transfer exciplexes relative to the ground-state in n-hexane as a function of the electrochemical one electron oxidation and reduction potentials measured in a polar solvent for the donor (D) and the acceptor (A) involved (see e.g.: A. Weller, in “The Exciplex”, Gordon and Ware eds., Academic Press Inc., N.Y., 1975): DHD A hexane E 0 D=D E 0 A=A 0:15 0:10eV Wigner rule: See spin conservation rule. Wood horn: A mechanical device that acts by absorption as a perfect photon trap. Wood lamp: A term used to describe a low-pressure mercury arc. See lamp. Xenon lamp: An intense source of ultraviolet, visible and near-infrared light produced by electrical discharge in xenon under high pressure. See also antimony-xenon, lamp, mercury-xenon lamp (arc). XPS: See photoelectron spectroscopy. YAG: See neodymium laser. Zeeman effect: The splitting or shift of spectral lines due to the presence of external magnetic field. Zero field splitting: The separation of multiplet sublevels in the absence of external magnetic field. Zero-zero (0-0) absorption or emission: A purely electronic transition occurring between the lowest vibrational levels of two electronic states.

353

Index a Absorbance spectrum 56 correlation with global water parameters 111 of reactant 79 water quality 111 Absorption 43 (Fig.), 44, 64 ff absorbance (liquid, gas) 56 ff (decadic) absorption coefficient a 58 f absorption coefficient a (gas phase) 53, 56 f absorption cross section r (gas phase) 57, 58 (Tab.) examples 58 (Tab.) absorption onset 53 attenuance 56 Beer-Lambert law 55, 56 ff Jablonski diagram 64 ff, 65 (Fig.) molar absorption coefficient e (liquid phase) 53, 56, 58 (Tab.) examples 58 (Tab.) molecules 55 ff, 63 (Fig.), 64 (Fig.), 65 (Fig.) multi-component mixture 57 f solids 66 ff semiconductor particle 67 (Fig.) total absorbance 56 transmittance 56 vacuum-UV/UV radiation 45 Acetone 192 ff Actinometry, actinometer 69, 82, 124, 212 Activation energy 60 f, 61 (Fig.) Adsorbable organic halogen (AOX) 107, 111 (Tab.) formation during AOP 182 Advanced oxidation processes (AOPs) 5 ff absorbance 57 f advanced oxidation model (ADOx) 200 AOX formation 182

biodegradability 215 ff case studies 132 ff (Tab.) chain reaction 190 (Fig.) comparative studies 213 f, 214 (Tab.) comparison of H2O2/O3/TiO2-AOPs 258 (Tab.) CO2/H2O system 171, 171 (Fig.) pH dependence 171 (Fig.) dark reaction 189, 191 (Fig.) dissolved molecular oxygen 172 ff, 178 economical aspects 257 ff electric energy consumption 262 ff existing processes 128 (Fig.) general mechanism 189 f, 190 (Fig.) level of oxidation products 190, 193 (Fig.) level of reactive intermediates 190, 192 (Fig.) radical chain mechanism 190 (Fig.) radical chain termination 189 f hydrogen peroxide based 114 ff inner filter effect 117, 154, 256 kinetic models 259 (Tab.) competition kinetic scheme 259 (Fig.) mechanism 172 (Fig.), 190 (Fig.), 192 (Fig.), 193 (Fig.), 259 (Fig.) overall reaction kinetics 260 ff ozone based 114 f pH dependence 105, 171, 179 f, 256 f photocatalysis 120 f photochemical methods 12, 15, 113 (Fig.), 119 (Fig.), 122 ff, 128 (Fig.) competing absorption 127 problems, limitations 24, 127, 129 quantum yields 162 ff, 163 f (Tab.) primary oxygen species 122 (Fig.), 146 (Fig.), 147 (Fig.) process combination (abiotic-biotic)

354

Index synergistic effect 215 reaction schemes 122 ff heterogeneous photocatalysis 123 (Fig.) homogeneous photocatalysis 124 ff, 126 (Fig.) photo-initiated oxidation 122 (Fig.), 172 (Fig.), 190 (Fig.), 192 (Fig.), 193 (Fig.) secondary oxygen species 122 (Fig.), 147 (Fig.) spectral domains of excitation 130 (Fig.), 213 substrate mineralization 190 ff substrate oxidation 190 ff titanium dioxide based 115 f, 120 f, 124, 159 ff toxicological studies 215 ff UV oxidation 127 ff, 189 ff VUV oxidation (H2O-VUV) 200 ff Water photolysis 126 f, 192 (Fig.) Advanced oxidation technologies (AOTs) 5 ff applications 23 (Tab.) end-of-the-pipe technologies 13 industrial applications 268 ff lamp types 79 ff, 80 (Fig.) (see also Radiant sources) photochemical 112 f photon flow determination 69 f (see also Actinometry) toxicity diminution 111, 215 ff water and air remediation 12 (Fig.) water analytical parameters 105 ff water quality criteria 105 f, 106 (Fig.) Air pollution chemical, physical-chemical aspects 221 ff Air purification 271 f Air purification technologies 22, 223 f, 224 (Fig.) Air treatment (see Gaseous systems) Alcohols 210 (Fig.) Aqueous systems 189 ff Atmosphere air-borne pollutants 221 quantitative determination 221 chemical/physical-chemical properties 221 ff composition 221 photochemistry 220 VUV/UV absorption (O2, O3, H2O) 151 (Fig.) Atomic orbital (AO) 40, 60 f, 62 (Fig.) Auxiliary chemicals 145 ff (see also Auxiliary oxidants)

Auxiliary oxidants (a-Ox) 120, 122 (Fig.), 145 ff hydrogen peroxide 114 ff, 155 ff photochemistry 157 ff UV absorbance 149 ff, 150 (Tab.), 152 (Fig.) Lewis structures 146 (Fig.) oxygen 114 f VUV/UV absorbance 151 (Fig.) ozone 114 f VUV/UV absorbance 153 (Fig.) UV absorption properties 149 ff, 150 (Tab.) water 126 homolysis 126 f VUV/UV absorbance 151 (Fig.)

b Batch reactor 240 (Fig.) Batch recirculation reactor 240 (Fig.) Beer-Lambert law 56 ff Bicarbonate 170, 172 (Fig.), 181 (see Carbonate species) Biodegradability 215 ff (see also Water constituents) analysis 106, 111 f estimation, COD/BOD ratio 109 refractory compounds 109 Biodosimetry (bioactinometry) 285 Biological oxygen demand (BOD) 106 ff Bioluminescence assay (Vibrio fischeri) 111 f toxicity data index 111 Biphenyl, polychlorinated (PCB) 20 (Fig.), 21 Bond cleavage, photochemically 51 ff absorption onset 51 ff dissociation energies (enthalpy) 51, 53 (Tab.) threshold wavelength (photon energy) 51, 53

c Cage effect (see Hydroxyl radical, Solvent cage) Calculation of (examples) absorption (90%) of 254 nm radiation by H2O2 59 amount of •OH radicals produced 60 amount of H2O2 decomposed photochemically 51 collector area per mass (ACM/m2 kg–1) 264

Index DOC concentration, 2,4–dichlorophenol solution 110 f electric energy per mass (EEM/kWh kg–1) 264 electric energy per order (EEO/kWh m–3 order–1) 264 frequency, radiant energy, wavelength 41, 46, 47 H2O2 concentration in water 157 penetration depth of 172 nm photons in water 59 photon flow, LP/MP Hg lamps 47, 95 (Tab.) portion of 172 nm radiation absorbed by aqueous methanol 201 radiant energy of photons 43, 47 spectral radiant power transmitted by wastewater 59 Carbon dioxide 102, 105, 221 (see also Carbonate species) aqueous solution chemistry 171 (Fig.) photochemistry 151 product of mineralization 10 f Carbon dioxide radical anion (CO•– 125 f 2 ) Carbon radical (RCH•2) (see Reactive species) Carbonate (see Carbonate species) Carbonate species CO2/H2O system 171 pH dependence 171 (Fig.) inorganic carbon (IC) 109 (Fig.), 110 radical anions 122, 149 selective oxidants 171 reaction with Cl• 178 reaction with •OH 180 (Tab.), 181 (Fig.) scavenger 105, 149, 259 (Fig.) Case studies biodegradability, enhancement 215 comparative studies (aqueous AOPs) 214 (Tab.) excimer lamp applications (research) 202 ff (Tab.) photo-initiated AOPs aqueous phase 132 ff (Tab.) gas phase 226 f (Tab.) process combination (AOP-biology) 215 (Fig.) toxicological studies 215 ff pentachlorophenol (direct photolysis) 216 (Fig.) pentachlorophenol (H2O2-UV) 217 (Fig.) Remazol Black-B (H2O2-UV) 218 (Fig.) UV disinfection 289 (Tab.)

Chain reaction 190 (Fig.) Chemical engineering 239 ff reactor types 240 (Fig.) Chemical oxygen demand (COD) 106 ff Chlorinated hydrocarbons (CHC) 103, 111, 177 f, 208 (Fig.) absorption properties 117 direct photolysis 117, 208 f, 228, 229 f chloride ion mass balance 117, 208 phosgene 228 photo-initiated oxidation, gas phase formation of phosgene, acid chlorides 228 ff tetrachloroethene, mechanistic scheme 230 (Fig.) reaction with e–aq 167 reaction with •OH 177 ff VUV photolysis 228 f trichloromethane, photoproducts 229 (Fig.) Chlorine atom (Cl•) fate in water 117, 208 reaction with carbonate/bicarbonate 178 reaction with tetrachloroethene 177, 178 (Fig.) Continuously stirred tank reactor (CSTR) 240 (Fig.) Conversion factors 48 f (Tab.)

d DDT 20 (Fig.), 21 Decontamination air pollution control 224 (Fig.) ground and process water 15 photocatalytic 128 Definition of terms (see also Glossary, p. 299 ff) analytical parameters of water contamination 107 f (Tab.) disinfection 279 f electromagnetic radiation, classification 44 (Fig.), 45 (Fig.), 50 (Tab.) terminology 73 (Fig.), 74 f (Tab.) electronic states of molecules 64 (Fig.) figures-of-merit (design parameters) 261 f, 263 (Tab.) light 5, 44 (Fig.) odor unit (ou) 223 photocatalysis 121 (Fig.) photoreaction rate 71 f (Tab.) quantum yield 68 (Tab.) radiation 40 f reaction rate 71 f (Tab.)

355

356

Index reduction factor Rf (disinfection) 280 total absorbance (absorption) 56 total carbon (TC) 109 (Fig.) UV lamp characteristics 85 (Tab.) UV radiation 45 (Fig.) VUV radiation 45 (Fig.) Design parameters (see Figures-of-merit) Design tests 256 f Detoxification 10, 67, 131, 255 Dibenzofurans, polychlorinated (PCDF) 20 (Fig.), 21 208 f Dichloride radical anion (Cl•– 2 ) Dichlorophenol, 2,4- (2,4-DCP) aqueous solution: DOC 110 f toxicity diminution (photo-Fenton) 216 VUV oxidation (H2O-VUV) 209 Dioxane, 1,4- 196 f Dioxins, polychlorinated (PCDD) 20 (Fig.), 21, 203 (Tab.) Direct photolysis 116 f atmospheric trace pollutants 219 f, 221 (Fig.) pentachlorophenol (aqueous solution) 216 (Fig.) 1,1,1-trichloroethane, 222 nm (aqueous solution) 208 Cl– mass balance 208 Disinfection 10, 22 f, 279 ff (see also UV disinfection) chemical methods advantages/disadvantages (Cl2) 280 f chloramine (NH2Cl) 112 chlorination (Cl2) 14, 112, 280 f chlorine dioxide (ClO2) 112 ozone (O3) 112 disinfection by-products (DBP) 103, 112, 281 of surfaces 273 UV technology 13, 23 (Tab.), 87, 112 f, 128 (Fig.), 280 ff air stream 22, 271 f drinking water 13, 279 fluence H0 23 germicidal range 45 greywater 22, bacteriology 107 radiant sources 81 secondary sewage effluents 14, 113 (Fig.) spa water 129 special features 23 (Tab.) swimming pool water 129 waste water 13 f, 289 (Tab.) Disinfection by-products (DBP) 281

Dissolved organic carbon (DOC) 109 f calculation (example) 110 f Dissolved oxygen (DO) 106 f, 124, 127, 149, 172 ff, 178, 189 f concentration profile (H2O-VUV) 201 ff measurement, oxygen optode 201 DMSO 191 (Fig.)

e Economic aspects 257 ff Electric energy 261 ff EDTA 197 ff Electromagnetic radiation 37 ff classification, nomenclature 44 ff energy conversions 50 (Tab.) energy units 46 ff spectral domains 50 (Tab.) spectral ranges 44 (Fig.), 45 (Fig.) electromagnetic wave 40 ff, 40 (Fig.) photometry 73 radiant sources 79 ff radiometry 73 terminology (measurement) 73 (Fig.) classification of terms, units 74 f (Tab.) UV radiation 44 ff characteristic effects 25 (Fig.), 45 (Fig.) germicidal range 45 phenomenological subdivision 45 (Fig.) VIS, UV-A, UV-B, UV-C, VUV 44 (Fig.) UV/VIS radiation interaction with matter 37 ff, physical, photophysical, photochemical 55 ff production of 41 (Fig.) technological applications 38 (Fig.) total absorbance 56 vacuum-UV radiation 44 f, 50, 89 penetration depth, water 59 wave/particle duality 26 ff De Broglie equation 29 Einstein’s equation 29 experimental verification 29 (Fig.) Planck’s relationship 29, 41 ff Electronic states 63 ff electronic configuration 64 (Fig.) electronic transitions 63 (Fig.) singlet, triplet 64 (Fig.) spin orientation 61 Emission 41 (Fig.), 65 (Fig.) Energy diagram: ground state vs. photochemical reaction 61 (Fig.)

Index Energy units 46 ff conversion of 46, 50 (Tab.) Enhanced oxidation processes (EOPs) 11 Environmental chemistry 13 Environmental policy 24 air quality control 24 environmental legislation 24 waste management 24 Environmental technology 13 EPR spectroscopy 169 evidence for •OH radicals 169 f spin trap DMPO 169 f spin trapping technique 169 f European Environment Agency (EEA) 20 Excimer 88 Excimer lamps 87 ff (see also Radiant sources) actinometry 69 applications 81, 202 ff (Tab.) characteristic features 89 ff cold lamp 89 dielectric-barrier discharge (DBP) 88 discharge configuration 89, 90 (Fig.) electrode configuration 90 (Fig.) geometrical configurations 91 (Fig.), 92 annular excimer UV source 91 (Fig.) flow-through lamp 92 (Fig.) flow-through photoreactor 93 (Fig.), 243 miniaturization 91, 213, 225 typical dimensions 92 (Fig.) incoherent radiation 87 production 88 Kryptonchloride (KrCl*) excimer lamp applications 202 ff (Tab.), 225 (Fig.) emission spectrum 89 (Fig.) photoelectric aerosol sensor 225 tetrachloroethene, direct photolysis 228 matrix of excimers emission maxima 88 (Tab.) microwave excitation 93 photon flow Up 94 f radiant power efficiency 88 radiation mode (coaxial, outward) 93 xenon excimer (Xe*2), formation 88 f Xe*2 excimer lamp configuration 90 ff emission spectrum 89 (Fig.) O3-H2O-VUV AOP 231 ozone production 230 f vacuum-UV (VUV) source 89 VUV oxidation (H2O-VUV) 128, 129, 200 ff

Exciplex 88 Excitation energy 61 (Fig.) singlet energy 66 triplet energy 66 Excited electronic state 43 (Fig.), 60 ff characteristic orbital 64 (Fig.) deactivation 60 ff fluorescence emission 65 f non-radiative 65 phosphorescence 66 f electronic configuration 63 f, 64 (Fig.) electronic transitions 63 (Fig.) energy diagram 61 (Fig.) Jablonski diagram 65 (Fig.) lifetime 65 f molecular orbital theory (MO) 60 ff relaxation 65 singlet configuration (state) 63 f, 64 (Fig.) spin multiplicity 63 spin orientation 63 f, 64 (Fig.) spin quantum number 63 state diagram 63 f, 64 (Fig.) triplet configuration (state) 63 f vibrational relaxation 65 f

f Fenton reaction 117, 124, 125 Figures-of-merit (design parameters) collector area per mass (ACM) 261 ff collector area per order (ACO) 262 definition 261 ff electric energy per mass (EEM) 261 ff electric energy per order (EEO) 261 ff example calculations 264 examples 267 (Tab.) limitations 265 f Fluence H0 23, 74 (Tab.) Forest decline 26 Fouling 24 countermeasures 244 f UV absorbing films 24, 244 f, 282

g Gaseous systems 219 ff, 269 ff (see also Atmosphere) advantages vs. aqueous phase 219 air pollution 221 air pollution control 224 (Fig.) air treatment process technologies 223 f, 224 (Fig.) applicability 219 applications 269 ff case studies 226 f (Tab.)

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358

Index chemical kinetics 221 concentration of gases 222 decontamination 224 disinfection 224 odor abatement, control 224, 269 f odor measurement 222 f electronic nose 223 odor unit (ou) 223 olfactometry 222 f ozone photolysis 118, 166, 220 photo-initiated oxidation chlorohydrocarbons (see Chlorinated hydrocarbons) phosgene formation 228 ff reaction scheme 221 (Fig.) reactive intermediates 221 (Fig.) type I photooxygenation products 119 (Fig.), 220 f, 221 (Fig.) photooxidation of tetrachloroethene (222 nm) 228 trichloroethene (UV-C) 228 trichloromethane (172 nm) 228 f, 229 (Fig.) principles 219 f Glossary (IUPAC) terms used in photochemistry 299 ff Greywater 22 Ground water chlorinated hydrocarbons (H2O-VUV) 209 1,4-dioxane 196 MTBE 196 phenols 131 pollution of 104

h Hatchard-Parker actinometer 124 Heterogeneous photocatalysis 115 f, 123 f (see also Semiconductor photochemistry, Photocatalysis) band-gap energy 67 photoprocesses 67 quantum yield determination 162 reaction scheme 123 (Fig.) suspension, aqueous 67, 159 titanium dioxide (TiO2) 67, 159 Historical aspects AOPs and AOTs 29 ff chronology 30 f (Tab.) photochemistry 29 ff radiation concept 24 ff UV lamps 29 UV radiation/light 25

chronology 28 (Tab.) positive/negative effects 25 f Homogeneous photocatalysis (see also Semiconductor photochemistry, Photocatalysis) photo-Fenton reaction 117, 124 ff, 126 (Fig.) quantum yields 163 (Tab.) Homogeneous, heterogeneous AOPs 128 (Fig.) Homolysis of water 45, 126 f, 200 ff Hybrid technologies 215 Hydrated electron (see also Reactive species) reactivity 167 (Tab.) Hydrogen atom (see also Reactive species) reactivity 168 Hydrogen peroxide (H2O2) 155 ff (see also Auxiliary oxidants) absorption onset 53 f acid/base properties 117 amount decomposed by irradiation 51 aqueous solutions, concentration 156 (Tab.) calculation 157 bond dissociation enthalpy 53 concentration analyses 157 concentration in wastewater (calculation) 59 f, 152 destruction (by bovine catalase) 109 formation during AOPs 173 monitoring 157 occupational health risks 155 f photochemistry 157 ff gas phase photolysis 165 Haber-Weiss mechanism 157, 158 (Fig.) irradiation of 54 f pH dependence 159 quantum yield of decomposition 158 f, 163 (Tab.), 165 (Fig.) rate of decomposition 158 f photo-Fenton reaction 124 ff physical-chemical properties 155 ff aqueous solution 156 (Tab.) primary absorber 60 residual concentration, drinking water 155 scavengers 109 use in AOPs 114 ff UV absorbance spectrum (aqueous phase) 152 (Fig.) UV absorbance spectrum (gas phase) 151 (Fig.), 152 (Fig.)

Index •

Hydroxyl radical ( OH) (see also Reactive species) atmospheric detergent 26, 220 characteristic features 11 (Fig.) evidence, experimental 169 spin trapping technique (DMPO) 169 (Fig.) excess energy 55, 165 formation of (atmosphere) 54 calculation (H2O2-UV) 60 photolysis of hydrogen peroxide 157 ff, 158 (Fig.) quantum yield 163 f (Tab.), 165 (Fig.) Haber-Weiss mechanism 158 (Fig.) industrial chemical 15 lifetime 168 gaseous phase 220 Lewis structure 146 (Fig.) primary species in AOPs 122 ff production by AOTs 12, 60, 114 ff, 219 energy requirements 258 (Tab.) quantification 168 f reaction with aliphatic hydrocarbons 172 ff aromatic hydrocarbons 172 ff benzene 176 (Fig.) bicarbonate ions 170, 172 (Fig.), 181 carbonate ions 170 chlorinated hydrocarbons 177 f DMPO (5,5-dimethyl-1-pyrroline N-oxide) 169 (Fig.) formaldehyde 175 (Fig.) halide ions 170 2-propanol 182 (Fig.) saturated hydrocarbons 170 tetrachloroethene 178 (Fig.) unsaturated hydrocarbons 170 rate constants 178 ff, 180 (Tab.) rate law 178 ff, 259 ff reactivity 170 ff, 172 (Fig.), 179 ff electron transfer 170 electrophilic addition 170 gas phase 220 hydrogen abstraction 170 reaction kinetics 178 ff, 258 ff selectivity of hydrogen abstraction 182 (Fig.) reduction potential 148 (Tab.), 149 scavenger 105, 149, 170 f, 259 f second order rate constants k·OH,M 179 ff, 259 ff comparison 181 (Fig.) competition kinetics method 179

determination 170, 179 representative examples 180 (Tab.), 181 (Fig.) solvent cage (cage effect) 55, 165 (Fig.) steady-state concentration 169 (Fig.) gas phase 220 spin multiplicity 63

i Industrial applications 268 ff aqueous systems: AOX elimination, preservatives 268 f, 269 (Fig.) photo-initiated oxidation of POPs 268 gaseous systems: air purifier system (AEROPURR) 271 f formaldehyde, photocatalytic oxidation 271 mould reduction (CFU) 272 (Fig.) odor units, diminution 270 (Tab.) odor abatement, food industry, waste recycling plant 269 f photoreactor 270 (Fig.) surface disinfection 273 bottle screw caps 272 (Fig.) Inner filter effect 117, 154, 256 Inorganic carbon (IC) 109 International Union of Pure and Applied Chemistry (IUPAC) glossary of terms used in photochemistry 299 ff Internet, links to relevant topics 6 ff (Tab.)

j Jablonski diagram 64 ff, 65 (Fig.) excited state deactivation 65 (Fig.) internal conversion 65 inter system crossing 66

k Kasha’s rule 65 Kinetic modeling 124, 198 ff, 212, 258 ff Kinetic reaction control 12 Kinetic regimes 260 ff, 259 (Tab.), 262 (Fig.)

l Lambert-Beer’s law (see Beer-Lambert law) Lamps (see Radiant sources) Landfill leachate chemical composition 104 (Fig.), 105 POPs 105 Light (VIS) 5, 44 (Fig.) Literature, photoscience 295 ff

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360

Index

m Mass balance chloride ion (Cl–) 117, 208 ff, 210 dissolved total nitrogen (DN) 200 (Fig.) total organic carbon (TOC) 196 (Fig.) Material (see Photoreactor) Matrix (see Water matrix) Mean oxidation number (MOC) 111 Mercury arc lamps (see also Radiant sources) emission spectra 83 ff low-pressure mercury lamp 50 medium-pressure mercury lamp 50 properties 85 (Tab.) Methanol 210 ff Mineralization 10 f, 189 ff DOC diminution 110 TOC diminution 196 (Fig.), 209 (Fig.), 210 (Fig.), 218 (Fig.) Mixed flow reactor (CSTR) 240 (Fig.) Modeling (see Photo-initiated AOPs, Photoreaction, Photoreactor, Reaction) Molecular orbital (MO) 40, 60 ff bonding, non-bonding, antibonding 61 MO diagram (3O2, 1O2) 62 (Fig.) Molecular oxygen (3O2) activation, one-electron reductions 146 (Fig.), 147 bond dissociation enthalpy 53 f Lewis structure 145 f molecular orbital (MO) diagram 62 (Fig.) photodissociation (gas phase) 53 f, 119 (Fig.), 151 quantum yields 163 (Tab.) spin orientation (of electrons) 61 f triplet ground state 61 f VUV irradiation 230 f VUV/UV absorbance spectrum (gas phase) 151 (Fig.) MTBE 196 ff

n Nature, photochemical reactor 1 Nitrate ion 103, 152 f photolysis of 154 hydroxyl radical formation 154 UV/VIS absorbance spectrum (aqueous solution) 153 (Fig.) Nitrite ion 152 f photolysis of 154 UV/VIS absorbance spectrum (aqueous solution) 153 (Fig.) Nitrogen dioxide (NO2)

bond dissociation enthalpy 53 f photodissociation (gas phase) 53 f N-Nitrosodimethyl amine (NDMA) direct photolysis, photoreactor 250 photo-initiated AOP treatment 134 (Tab.)

o Odor abatement 10, 269 Odor measurement 222 f Odor unit (ou) 223 Overall reaction kinetics 241, 257, 258 ff Oxidants, industrial 115 Oxidative degradation general reaction scheme 172 (Fig.) of aliphatic hydrocarbons 172 ff of aromatic hydrocarbons 172 ff Oxygen (see Molecular oxygen) Oxygen atoms NO2 photolysis 53 f O2 photolysis 151 O3 photolysis 118, 166, 220 quantum yield 163 (Tab.) Oxygen species, reactive 146 (Fig.), 147 (Fig.) Ozonation water and wastewater 13, 115, 155 Ozone 155 absorption onset 53 f aqueous chemistry 147 bond dissociation enthalpy 53 decomposition by chlorinated fluorocarbons 25 formation 25, 54 generation 155 VUV irradiation of O2 230 f Lewis structure 146 (Fig.) ozone hole 25 ozone shield 25, 54 photochemical decomposition in water 118, 165 (Fig.), 166 photochemical production 230 f photodissociation (gas phase) 53 f, 115, 166, 220 physical-chemical properties 155 quantum yields 163 f (Tab.), 165 (Fig.) reaction with HO•2 220 use in AOPs 115, 155 O3-H2O-VUV 231 UV absorbance spectrum (gas/aqueous phase) 153 (Fig.) VUV/UV absorbance spectrum (gas phase) 151 (Fig.), 152

Index

p Pathlength, penetration depth, photoreaction zone 56, 58 f Pauli exclusion principle 62 Pentachlorophenol (PCP) 216 (Fig.), 217 (Fig.) Peroxone (O3/H2O2) 117 Peroxyl radical (RCH2O•2) (see Reactive species) Peroxyl radical chemistry 122 Persistent organic pollutants (POPs) 20 f, 105 AOT treatment 20 (Fig.) “dirty dozen”: chemical structures 20 (Fig.) photochemical degradation 21 POP convention 20 POPs sink 20 (Fig.) properties 21 use 21 Pesticides 21, 134 (Tab.) Phenol, photodegradation 131 Phosgene (see Chlorinated hydrocarbons) Photobiological processes 26 Photocatalysis (see also Semi conductor photochemistry) application, air purifier 271 f case studies aqueous phase 132 ff (Tab.) gaseous phase 226 f (Tab.) catalyzed photolysis 120 f definition 120 f, 121 (Fig.) Fe2+ based 114 f Fe3+ based 124 ff heterogeneous, mechanism 123 (Fig.) homogeneous, mechanism 124 ff nomenclature 121 (Fig.) nominal catalyst 123 (Fig.), 127 photocatalysts 159 ff photo-Fenton reaction 124 ff, 214 (Tab.) mechanism 125 f pH 124, 126, 256 potassium ferrioxalate 124 ff, 161 f quantum yield 124, 163 (Tab) reaction scheme 126 (Fig.) photogenerated catalysis 120 f turn over number (TON) 121 reactions 120 f reactor system 128 solar systems 128, 161 f design parameters (figures-of-merit) 261 ff TiO2 based 67 (Fig.), 115 f

acceleration 124 disinfection 279 kinetic model 124 mechanism 116 photoreactions in water 124 Photocatalysts 159 ff Fe based catalysts 161 ferrioxalate system 161 f preparation 162 UV/VIS absorption properties 161, 162 (Fig.) inactivation, poisoning 161 polyoxometallates 161 UV/VIS absorption properties 150 (Tab.) titanium dioxide (TiO2) 159 f band-gap energy, measurement 159 BET surface area 159 chemical properties 159 Degussa P-25 TiO2 159 immobilization 159 f metal-ion implantation 160 (Fig.), 161 modification 161 photocatalytic activity 159 ff quantum yields 162, 164 (Tab.) UV extinction spectrum 160 (Fig.) Photocatalytic oxidation (PCO) 121 Photocatalytic ozonation 123 Photocatalytic reactions 120 f Photochemical AOPs (see also Advanced oxidation processes (AOPs)) case studies 132 ff (Tab.), 226 ff (Tab.) design parameters (figures-of-merit) 257 ff economic factors 257 ff nomenclature 118 ff pH 256 ff photodegradation, water pollutants 130 f photoionization 119 Photooxidation reactions 119 f photo-induced oxidation 119 (Fig.) photo-initiated oxidation 119 (Fig.) photooxygenation 119 (Fig.), 120 photoreduction 118 f process combinations 215 f process engineering 239 ff spectral domains 130 (Fig.) strategies 129 Photochemical engineering 39 (Fig.), 239 ff (see also Photoreactor, Chemical engineering) characteristics 38 f concepts of 239 f

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362

Index design criteria 254 ff design tests 256 f excimer lamps 91 photoreactor concepts 243 ff process efficiency, influences on 254 (Fig.) process engineering 239 ff process performance 254 ff requirements 239 f Photochemical technology applications 38, 268 ff process efficiency, influences on 254 (Fig.) process engineering 239 ff radiant sources 79 ff water and air treatment objectives 10 (Fig.) oxidative processes 114 ff problems, limitations 24 special features 23 (Tab.) water, analytical parameters 105 ff Photochemistry (see also Jablonski diagram) absorption 43 f bond dissociation 53 (Tab.) calculation of concentration changes 51, 52 (Tab.), 70 of photon energy 50 (Tab.) conversion of energy units 52 (Tab.) definition 37 energy units 46 ff conversions 52 (Tab.) first law of (Grotthus-Draper) 29 historical aspects (Ciamician, Silber) 32 language of 2 microwave photochemistry 93 physical constants 42 (Tab.) primary step 43 (Fig.), 44 radiant sources 79 ff second law of (Stark-Einstein) 32 spectral ranges 44 Photodegradation of water pollutants 130 f Photoexcitation 44 (Fig.) Photo-Fenton reaction (see Homogeneous photocatalysis, Photocatalysis) Photo-initiated AOPs (see also Advanced oxidation processes) air pollution control 224 (Fig.) aqueous phase 189 ff case studies 132 ff (Tab.), 226 f (Tab.) comparative studies 213 f, 214 (Tab.) design experiments 255 f

comprehensive report of results 257 pilot unit 256 (Fig.) design parameters (figures-of-merit) 257 ff definition 261, 263 (Tab.) derivation 261 f electric energy/photochemical parameters 262 (Fig.) examples 267 (Tab.) limitations 265 system costs, calculation of 265, 266 ff economic factors 257 ff, 265 effect of dissolved molecular oxygen 178 electric energy consumption 261 ff energy requirements 258 (Tab.) full-scale applications 129 gaseous phase 219 ff H2O2-UV (aqueous phase) efficiency and yield factor 264 f, 265 (Fig.) industrial applications 268 ff kinetic modeling 198 ff, 258 ff competition function FM,Ox 199 competition kinetic scheme 259 (Fig.) dynamic model (ADOx) 200 overall reaction kinetics 260 ff pseudo first order reaction rate 261 rate equations 259 ff steady state approximation, analysis 199, 260 f zero order reaction rate 260 kinetic regimes 261 f, 262 (Fig.) nomenclature 118 ff, 127 f photo-initiated oxidation 119 f photon fluence Hp,0 23 special features 23 (Tab.) POPs degradation 20 f process combinations 215 f process efficiency, influences on 254 ff, 254 (Fig.) design tests 256 f water matrix 255 process engineering 239 ff quantum yields 162 ff, 163 f (Tab.) reaction modeling 191 ff analytical procedures 192 intermediary products, identification 193 f time-concentration profiles 194 (Fig.), 195 (Fig.) TOC balance experimental/calculated 196 (Fig.)

Index reaction scheme 122 (Fig.), 123 (Fig.), 172 (Fig.) reactive oxygen species (see Reactive species) reactor concepts 239 ff spectral domains 130 (Fig.) status of development 127 ff, 128 (Fig.) substrate mineralization 189 ff substrate oxidation 189 ff TOC diminution 195, 196 (Fig.) toxicity diminution 216 ff treatment goals 254 f, 257 water analysis 255 UV absorbance spectrum 255 water matrix 255 yield factor 264 f, 265 (Fig.) Photo-initiated oxidation, treatment of acetone (2-propanone) (H2O2-UV) 192 ff concentration profiles: main products 194 (Fig.) concentration profiles: minor products 195 (Fig.) structure of intermediary products 193 (Fig.) TOC profiles (experimental vs. calculated) 196 (Fig.) alcohols (H2O-VUV), TOC diminution 210 (Fig.) chlorinated hydrocarbons (H2O-VUV) concentration profiles 208 (Fig.) chloride ion mass balance 208 ff 2,4-dichlorophenol (H2O-VUV) 209 (Fig.) substrate oxidation vs. mineralization 209 dimethyl sulfoxide (DMSO) (H2O2UV) 191 (Fig.) 1,4-dioxane (H2O2-UV) 196 f structure of intermediary products 197 (Fig.) ethylenediaminetetraacetate (EDTA) (H2O2-UV) 197 ff nitrogen balance 200 (Fig.) scheme of mineralization 199 (Fig.) methanol (H2O-VUV) 210 ff mechanistic scheme 211 (Fig.) methyl-tert-butyl ether (MTBE) (H2O2-UV) 196 f structure of intermediary products 198 (Tab.) pentachlorophenol 216 (Fig.) (direct UV), 217 (Fig.) (H2O2-UV) Remazol Black-B 218 (Fig.) tetrachloroethene (gas phase) 230 (Fig.)

trichloromethane (VUV, gas phase) 228 f, 229 (Fig.) Photoionization 44 (Fig.) Photolysis, water 126 f Photomedicinal treatment 26 Photomineralization 10 f, 189 ff real pollutant sink 23 Photon absorption 43 (Fig.) amount of (mol) 42 f, 47 energy 50 (Tab.), 51 f flow Up 94, 95 (Tab.) calculation of 47 determination 69 and spectral power Pk 94 frequency 41 momentum 29 photon stream 41 f radiant energy Qk 41 ff of NA photons 43 (Fig.) reagent 2, 5, 23, 43, 68 ff stoichiometric calculation 23, 43, 51, 52 (Tab.), 71 (Tab.) Photon energy 46 ff Photon fluence Hp,0 23 Photooxidation 189 ff photo-induced oxidation 119 (Fig.) photo-initiated oxidation 116, 119 (Fig.) photooxygenation 119 (Fig.) sensitized 116 f Photophysics major tasks 37 Photoreaction concentration changes 70 ff heterogeneity, inhomogeneity 39, 239 intramolecular 65 intermolecular 66 kinetics 68 ff, 71 (Tab.) quantum yield 68 ff, 163 f (Tab.) reaction order 70 ff reaction rate 71 f (Tab.) Photoreaction zone 39 Photoreactor batch vs. batch recirculation 241 f, 242 (Fig.) development 251 f (Tab.) design criteria 254 ff drinking water treatment, photoreactor 249 (Fig.), 250 excimer flow-through 93 (Fig.) filming problem 244 fouling of quartz surfaces 244 bio-film formation 245

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364

Index cleaning technologies 244 f countermeasures 245 f mechanism 244 geometry 80, 243 (Fig.) lamp arrangement 243 f, 243 (Fig.) material 250 ff corrosion 181 metals, reflectance of 253 (Fig.) optical material, UV transmittance 253 (Fig.) quartz 250 f selection of 250 Suprasil quartz 250 f modeling 251 f (Tab.) optimization 251 f (Tab.) photoreactor modules Enviolet® 248 (Fig.) Rayox® 249 (Fig.) mobile photoreactor 247, 248 (Fig.) UviTox® 249 (Fig.) process performance 254 ff product distribution, inhomogeneity 39, 239 radiation distribution 240 reactor cascades 240 f reactor concepts 239 ff, 243 ff reflector 254 scaling-up 240, 252 (Tab.) UV absorbing film 244 various types 243 (Fig.), 251 f (Tab.) contact-free batch reactor (CFBR) 243, 246 f, 247 (Fig.) continuous flow annular reactor 243, 244 (Fig.) falling film reactor 243, 245 (Fig.) flat bed reactor 243, 246 (Fig.) Photoscience 37 (Fig.) Phototoxicity 26 Plug flow reactor 240 (Fig.) Polycyclic aromatic hydrocarbons (PAHs) measurement, gas phase 225 measuring principle 225 (Fig.) particle-bound PAHs 225 photoelectric aerosol sensor 225 (Fig.) photo-induced ionization 225 POPs (see Persistent organic pollutants) Primary oxygen species 122 (Fig.), 147 (Fig.) Primary reactive species (see Reactive species) Process combinations 215 f AOP-biology 215 f hybrid technologies 215 f synergistic effect 215 (Fig.)

Process engineering 239 ff (see also Photoreactor, Chemical engineering, Photochemical engineering) Production-integrated environmental protection (PIEP) 13 Purification 10, 22

q Quantum efficiency 68 f Quantum yield U 51, 68 ff, 162 ff, 163 f (Tab.) definition 68 (Tab.) product formation 163 f (Tab.) substrate degradation 163 f (Tab.) Quartz, Suprasil quartz (see Photoreactor, Material)

r Radiant sources 79 ff classification 80 (Fig.) 82 (Fig.) electrical input power Pin el electrode-less UV lamp 94 (Fig.) emission lines 50 (Tab.) excimer lamps 50 mercury arc lamps 50 excimer lamp 79, 87 ff high-pressure Hg lamp 76, 81, 82 laser 79 , 80 lifetime 83, 85 (Tab.) low-pressure (LP) Hg lamp 81 f emission characteristics 81 emission spectrum 84 (Fig.) lifetime 87 (Fig.) temperature sensitivity 86 (Fig.), 87 medium-pressure (MP) Hg lamp 81 f emission characteristics 81 emission spectrum 83 (Fig.), 84 (Fig.), 85 (Fig.), 86 (Fig.) geometry 84 (Fig.) radiant power efficiency 82 (Fig.), 85 (Tab.) photon flow Up 47, 95 (Tab.) calculation of 47 determination 69 f (see also Actinometry) power density 83, 85 (Tab.) power supply 79 properties 85 (Tab.) radiant power Plamp 85 (Tab.) calculation of 52 (Tab.) determination (MP Hg lamp) 82 radiant power efficiency 47, 80, 82 f, 85 (Tab.)

Index radiation measurement (terminology) 74 f (Tab.) safety precautions 75 spectral radiant power 47, 75 (Tab.) sun 94 (see Solar radiation) vacuum-UV (VUV) lamp 80 f, 89 ff xenon flash lamp 83 Radiation 40 Radical scavenger 189 f, 259 f Reaction electron transfer 66, 124 energy transfer 66 excited state 43 (Fig.), 61 (Fig.) general schemes 122 ff ground state 61 (Fig.) kinetic model TiO2 photocatalysis 124 kinetics heterogeneous surface 124 photo-initiated AOPs 258 ff photoproduct formation 60 f photosensitization 66 Reactive species 145 ff carbon radicals (RCH•2) 149 hydrated electron (e–aq) 167 formation 166 reactions of 167 (Tab.) hydrogen atom (H•), reactivity 168 hydrogen trioxide (HO•3) 117, 147 (Fig.) hydroperoxide anion (HO–2) 147 photochemical dissociation 159 hydroperoxyl radical (HO•2) 116, 126, 127, 157, 189 f formation during AOPs 173, 174, 175, 176 reaction with phenol 175, 177 (Fig.) hydroxyl radical (•OH) 168 ff oxygen species (Ox, HOx) 145 ff, 147 (Fig.) Lewis (electronic) structures 145 f, 146 (Fig.) 117, 123, 147 ozonide radical anion (O•– 3 ) (Fig.) 147, 147 (Fig.) peroxide anion (O2– 2 ) peroxyl radicals (RCH2O•2) 149, 172 ff, 190 (Fig.) aliphatic, reactivity 173 (Fig.), 174 (Fig.) cyclohexadienyl-type radicals 175, 176 (Fig.), 177 (Fig.) rate constants of formation 172 Russell reactions 173, 174 (Fig.) tetroxides, decomposition 173, 174 (Fig.)

primary species 122 (Fig.), 123 (Fig.), 166 reduction potentials 148 (Tab.), 149 secondary species 122 (Fig.), 123 (Fig.), 166 singlet molecular oxygen 116 f, 146 (Fig.) singlet oxygen atom 54, 118, 151, 165 (Fig.), 166 super oxide radical anion (O•– 116 f, 123, 2 ) 146 (Fig.), 147 (Fig.) basicity 126 reactivity 119 f, 126 tetraoxygen (O4) 147 triplet oxygen atom 54, 151, 220 UV absorption properties 149 ff, 150 (Tab.), 154 Reactor concepts 239 ff Reactor material (see Photoreactor) Reduction potential 148 (Tab.), 149 Remazol Black B 218 (Fig.) Remediation 10 toxic wastewater 218 Russell reaction 173 f

s Scavenger (see Carbonate species, Hydroxyl radical) Second order rate constants k·OH,M 180 (Tab.), 181 (Fig.) Secondary oxygen species 122 (Fig.), 123 (Fig.) Secondary reactive species (see Reactive species) Semi conductor photochemistry 66 f, 67 (Fig.) (see also Photocatalysis) band-gap energy (Ebg) 67, 159 conduction band 67 (Fig.) electron hole (h+vb) 67 exciton 67 photoelectron (e–cb) 67 redox chemistry 67 (Fig.) valence band 67 (Fig.) Semivolatile organic compounds (SOC) 21 Singlet molecular oxygen 54, 62 (see also Reactive species) formation 116 f, 119 (Fig.) Lewis structure 146 (Fig.) lifetime (aqueous solution) 166 molecular orbital (MO) diagram 62 (Fig.) photosensitizer 120 reactivity 120 type II products 119 f Singlet oxygen (see Singlet molecular oxygen) Smog 54

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Index Solar AOPs (see Photocatalysis) Solar photocatalysis (see Photocatalysis) Solar radiation 45 f, 73 Solar radiation (see also Radiant sources) extraterrestrial 95 (Fig.) spectral distribution 97 (Tab.) VUV photochemistry 96 global radiation 95, 96 (Fig.) solar irradiance 96 spectral distribution 96 solar constant 96 solar spectral irradiance 95 (Fig.) Spectral domain (of AOPs) 130 (Fig.), 213 Spectral radiant intensity 56 Spectral radiant power 56 Spin conservation 54, 61, 66 Steady-state analysis 260 ff Stratosphere 25 Super critical water oxidation (SUWOX) 12, 114, 115 Surface treatment 10, 273 Suspended organic carbon (SOC) 109

t Tetrachloroethene 230 (Fig.) direct photolysis 177 reaction with •OH 178 (Fig.) Titanium dioxide (see Semi conductor photochemistry) TOC diminution 196 (Fig.), 209 (Fig.), 210 (Fig.), 218 (Fig.) Total carbon (TC) composition 109 (Fig.) measurement 110 f catalytic combustion 110 interpretation of analyses 110 VUV-initiated oxidation 110 Total organic carbon 106 ff, 109 (Fig.) (see also Total carbon) measurement 110 refractory TOC 110 Toxicity, of water constituents analysis 108 (Tab.), 111 f, 216 ff bioluminescence assay (Vibrio fischeri) 111 f mammalian cell assay 112 toxichromo test (Escherichia coli) 216 case studies 216 ff effective concentration (EC50) 108 (Tab.) lethal concentration (LC50) 108 (Tab.) lethal dose (LD50) 108 (Tab.) microtoxR analyzer system 111, 216 relative toxicity index 216 ff

toxicity data index 111 Toxicological studies 215 ff relative toxicity index 216 ff Transient species (see Primary, Secondary oxygen species) Transmittance (see Absorbance) of wastewater (calculation) 59 Trichloroacetic acid 177 formation 178 (Fig.) Trichloromethane (VUV oxidation) 229 (Fig.) Triplet molecular oxygen (see Molecular oxygen)

u Ultra pure water 22, 129 United Nations Environment Program (UNEP) 20 UV disinfection 279 ff (see also Disinfection) action spectrum 283, 285 (Fig.) advantages/disadvantages 23 (Tab.), 281 f (see also Disinfection, chemical methods) alternative methods 280 applications, products 23 (Tab.) biodosimetry 285 case studies 289 (Tab.) collimated beam apparatus 285 colony forming units (CFU) 107 (Tab.), 280 Cryptosporidium parvum control 284 f low-pressure Hg lamp 284 f medium-pressure Hg lamp 284 f DNA, RNA components, molecular structures 286 DNA, UV absorbance spectrum 286 (Fig.) drinking water 279 efficacy, influences on 282 (Fig.) hydraulic engineering 282, 284 photochemical engineering 282 photoreactivation 288 (Fig.) water quality 282 (Fig.) efficiency 280 germicidal fluence H0 282 determination 285 germicidal radiant exposure H 283 (Fig.) Giardia muris control 284 f indicator germ 283 industrial installations 289 (Tab.) mechanism 285 ff absorption characteristics of DNA 286 (Fig.), 287 cyclobutane-type thymine dimer 287 (Fig.)

Index photocatalytic techniques (TiO2 based) 279 bactericidal activity 279 cell killing, mechanism 279 Escherichia coli mineralization 279 virus inactivation 279 photoreactivation 283, 283 (Fig.), 287 f mechanism 288 (Fig.) photoreactivation testing 283 photoreactor design 282 photoreactor modeling water flow 289 (Tab.) fluence rate distribution 285, 289 (Tab.) process efficacy, influences on 282 f, 282 (Fig.) rate law 280 reduction factor Rf 280 repair mechanisms 287 f test organism 285 UV sensitivity of indicator germs 283 (Fig.) UV system inactivation efficacy 285 performance test 285 viability assay 284 wastewater 279 UV dose 23 f UV oxidation 14 (Tab.), 127 ff, 189 ff (see also Photo-initiated AOPs) UV Research and development 10 (Fig.) UV sources (see Radiant sources) UV technology AOTs: applications and products 14 (Tab.) UV/VIS 5, 44

v Vacuum-UV sources (see Radiant sources) Vibrational excitation (Fig.) 44, 46 Vibronic coupling 46 Volatile organic carbon (VOC) 109 (Fig.) VUV oxidation 45, 128, 200 ff chloride ion mass balance 208 f, 210 effect on nitrate/nitrite 154 mineralization of alcohols (aqueous phase) 210 chlorohydrocarbons (aqueous phase) 207 f, 208 (Fig.) 2,4-dichlorophenol (aqueous phase) 209 (Fig.) methanol (aqueous phase) 210 f, 211 (Fig.)

172 nm photolysis 211 f. chemical actinometer (172 nm) 210, 212 kinetic modeling 212 reaction manifold 211 (Fig.) reactive species 126 f, 192 (Fig.), 211 (Fig.) water photolysis, mechanism 115 (Fig.), 192 (Fig.) dissolved molecular oxygen, concentration profile 201 heterogeneity 207 (Fig.) penetration depth, 172 nm radiation 59 photoreaction zone 207 (Fig.) quantum yield 164 (Tab.) radial mixing 207 static mixer 207 zone of oxygen consumption 207 (Fig.) zone of oxygen saturation 207 (Fig.) water: prominent absorber 200 f, 211 example calculation (H2O/ CH3OH) 201

w Waste gas/air purification 13, 269 ff Wastewater treatment 13, 22 biological processes 114 (Fig.) facility, elements of 113 (Fig.), 114 f primary, secondary, tertiary systems 113 methods of 112 ff nitrate removal 114 oxidative processes 114 ff examples 114 (Fig.) reaction sequences, mechanisms 115 (Fig.) photochemical AOPs 112 f, 114 ff mechanisms 115 (Fig.) processes 113 (Fig.) technologies 113 (Fig.) toxicity diminution 111, 216 ff Water (see also Water quality) absorption onset 53 analysis 104 ff analytical parameters 105 ff, 107 f (Tab.) dissolved total nitrogen 200 (Fig.) bond dissociation enthalpy 53 f constituents, classification of 101 ff anthropogenic chemicals 104 (Fig.) biodegradability 102, 108 f, 111, 215 dissolved matter 101, 102 (Fig.) dissolved organic matter (DOM) 102 f

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Index fulvic acid 102 human drugs 104 humic acid 102 humic material (HM) 102 f humic substances (HS) 103 landfill leachate 104 (Fig.) microorganisms 101 natural chemicals 104 (Fig.) natural organic matter (NOM) 102 f organic chemical load 106 organic matter 102, 103 (Fig.) particulate organic matter (POM) 102 f refractory organic substances (ROS) 103 toxcicity 215 ff undissolved matter 101, 102 (Fig.) veterinary drugs 104 photolysis 118, 200 ff quantum yields 164 (Tab.) vacuum-UV oxidation (H2O-VUV) 200 ff quality criteria 105 f, 106 (Fig.) priority pollutants (EPA) 105 solution chemistry 102 threshold wavelength 53 VUV absorbance spectrum (gas phase) 151 (Fig.) Water analysis 105 ff, 255 analytical procedures 192 Water constituents (see Water) Water market 21 (Fig.) drinking water 21 industrial process water 21 wastewater 21 Water matrix 255 Water photolysis (see VUV oxidation) Water quality (see also Water) analytical parameters 105 ff, 107 f (Tab.) examples 107 f (Tab.)

global, group, specific, bacteriological, toxicological 105 f, 107 f. (Tab.) UV spectrophotometry 111 UV/VIS absorbance 111 analytical protocol 106 bacteriology 107 (Tab.) biodegradability (see Water constituents) classification 108 criteria of 106 (Fig.) global parameters 107 (Tab.) AOX, BOD, COD, TOC 106 ff dissolved organic carbon (DOC) 109 f total carbon (TC) 109 (Fig.) group parameters 107 (Tab.) on-line control 106 specific oxygen demand (COD/DOC) 110 specific parameters 107 (Tab.) toxicity data 108 (Tab.), 111 f bioluminescence assay (Vibrio fischeri) 111 f mammalian cell assay 111 relative toxicity index 216 ff toxicological studies 215 ff Water resources, global 19 (Fig.) Water treatment (see also Wastewater treatment) photochemical processes 101 ff, 189 ff Wavenumber 46 Wavelength 40 (Fig.), 50 (Tab.) Wavelength range, optimum (AOPs) 130 (Fig.) Wigner, rule of (spin conservation) 61, 66

x Xenon excimer lamps (see Excimer lamp)