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Environmental Degradation and Transformation of Organic Chemicals
Alasdair H. Neilson and Ann-Sofie Allard
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Library of Congress Cataloging-in-Publication Data
Neilson, Alasdair H. Environmental degradation and transformation of organic chemicals / Alasdair H. Neilson and
Ann-Sofie Allard. p. cm.
Rev. ed. of: Organic chemicals. c2000. Includes bibliographical references and index. ISBN 978-0-8493-7241-4 (alk. paper) 1. Aquatic organisms--Effect of water pollution on. 2. Organic water pollutants--Environmental
aspects. I. Allard, Ann-Sofie. II. Neilson, Alasdair H. Organic chemicals. III. Title.
QH545.W3N45 2008 577’.14--dc22 2007012219
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Chapter 1 Abiotic Reactions .........................................................................................................3
Hydroxyl Radicals in the Destruction of Contaminants ....................................................................9 Fenton’s Reagent .....................................................................................................................9 Polyoxometalates (Heteropolyacids) ..................................................................................... 10 Photolytic Degradation on TiO2 ........................................................................................... 10
Survey of Reactions .............................................................................................................. 15 Survey of Reactants .............................................................................................................. 17
Aliphatic Hydrocarbons ............................................................................................. 17 Aromatic Hydrocarbons............................................................................................. 17 Biogenic Terpenes ...................................................................................................... 17
Chemically Mediated Transformations ...........................................................................................22 Hydrolysis .............................................................................................................................22 Reductive Displacement: Dehalogenation and Desulfurization ...........................................25
vi Contents
Bacteria in Their Natural Habitats ............................................................................. 57 Marine and Oligotrophic Bacteria ............................................................................. 58 Lithotrophic Bacteria .................................................................................................60 Phototrophic Organisms ............................................................................................ 61 Aerobic and Facultatively Anaerobic Bacteria .......................................................... 62
Bacterial Metabolism of C1 Compounds: Methanotrophs, Methylotrophs, and Related Organisms ..............................................................................................69 Methane Monooxygenase and Related Systems ........................................................69
Anaerobic Bacteria ............................................................................................................... 72 Clostridia .................................................................................................................... 73 Anaerobic Sulfate-Reducing Bacteria ....................................................................... 73 Other Anaerobic Bacteria .......................................................................................... 73
Organisms from Extreme Environments .............................................................................. 74 Psychrophiles ............................................................................................................. 74 Thermophiles ............................................................................................................. 75 Alkaliphiles ................................................................................................................ 75 Halophiles .................................................................................................................. 75
Eukaryotic Microorganisms: Fungi and Yeasts ................................................................... 75 Metabolism by Fungi ................................................................................................. 75 Metabolism by Yeasts ................................................................................................77
References ............................................................................................................................. 78 Part 2 Reactions Mediated by Higher Organisms ........................................................................... 91
Introduction .......................................................................................................................... 91 Metabolism by Fish ..............................................................................................................93 Metabolism by Other Organisms..........................................................................................95
Contents vii
Part 1 Introduction of Oxygen ....................................................................................................... 103 Monooxygenation ............................................................................................................... 103
Hydroxylation of Alkanes ........................................................................................ 103 Epoxidation of Alkenes ........................................................................................... 104 Cycloalkanone Monooxygenases ............................................................................. 105 Monooxygenation of Aromatic Compounds ............................................................ 106 Reactions of Aromatic Hydrocarbons ..................................................................... 107 Reactions of Hydroxybenzoates and Related Compounds ...................................... 108 Monooxygenation of Phenols ................................................................................... 110
Cytochrome P450 Systems ................................................................................................. 113 Prokaryotic Organisms ............................................................................................ 114 Eukaryotic Organisms ............................................................................................. 116
Dioxygenation ..................................................................................................................... 118 Dioxygenases Involved in Dihydroxylation of Arenes ............................................ 118 Ring-Fission Dioxygenases ...................................................................................... 123
Incorporation of Oxygen from Water: Oxidoreductases and Hydratases ........................... 129 Oxidoreductases ....................................................................................................... 129 Hydratases ................................................................................................................ 130
Oxidases, Peroxidases, and Haloperoxidases ..................................................................... 131 Oxidases ................................................................................................................... 131 Peroxidases .............................................................................................................. 133 Haloperoxidases ....................................................................................................... 134
References ........................................................................................................................... 135 Part 2 Electron Acceptors Other than Oxygen .............................................................................. 147
Introduction ........................................................................................................................ 147 Oxyanions: Nitrate, Sulfate, Chlorate, Selenate, and Arsenate .......................................... 148
Nitrate and Related Compounds .............................................................................. 148 Sulfate and Related Compounds .............................................................................. 150 Chlorate and Perchlorate .......................................................................................... 150 Selenate and Arsenate .............................................................................................. 151
Metal Cations and Oxyanions ............................................................................................ 152 V(V), Mn(IV), Fe(III), Tc(VII), and U(VI) ............................................................. 152
Humic Acid and Anthraquinone-2,6-Disulfonate in Redox Systems ..................................................................................................... 154
Dehalorespiration................................................................................................................ 156 Alkane Sulfonates as Terminal Electron Acceptors ........................................................... 156 Nitroalkanes as Electron Acceptor ..................................................................................... 156 References ........................................................................................................................... 157
Part 3 Reductases and Related Enzymes ....................................................................................... 162 Reduction of Nitroarenes .................................................................................................... 162 Nitrate Ester Reductase ...................................................................................................... 162 Enones (α, β-Unsaturated Ketones) and Related Reductases ............................................. 163
Aerobic Conditions .................................................................................................. 163 Anaerobic Conditions .............................................................................................. 163
Azo Reductase .................................................................................................................... 163
viii Contents
Reduction of Carbocyclic Rings .............................................................................. 164 Dehydroxylation ....................................................................................................... 165 Reduction of Azaarene Rings .................................................................................. 165
Metal Cations and Oxyanions ............................................................................................ 165 References ........................................................................................................................... 166
Part 4 Microbial Reactions to Chemical Stress ............................................................................. 168 Introduction ........................................................................................................................ 168
Hydrocarbon Tolerance ............................................................................................ 168 Antibiotic Resistance ............................................................................................... 170
Resistance to Metals and Metalloids .................................................................................. 172 Reduction ................................................................................................................. 172 Methylation .............................................................................................................. 173 Effl ux Systems ......................................................................................................... 175 Chlorophenol Tolerance ........................................................................................... 176 Resistance to High Acidity ...................................................................................... 176
References ........................................................................................................................... 177 Part 5 Enzymes Containing Manganese, Iron, Nickel, Copper, Molybdenum,
Tungsten, and Vanadium ..................................................................................................... 181 Manganese .......................................................................................................................... 181 Iron .................................................................................................................................. 181 Nickel .................................................................................................................................. 182 Copper................................................................................................................................. 184 Molybdenum ....................................................................................................................... 185
Molybdopterin Oxidoreductases .............................................................................. 185 Tungsten .............................................................................................................................. 187 Vanadium ............................................................................................................................ 188 References ........................................................................................................................... 188
Chapter 4 Determinants and Interactions ................................................................................. 193
Single Substrates: Several Organisms ........................................................................................... 193 Cometabolism and Related Phenomena......................................................................................... 195 Induction of Catabolic Enzymes .................................................................................................... 195
Pre-Exposure to an Analog Substrate ................................................................................ 195 Enzyme Induction by Growth on Structurally Unrelated Compounds .............................. 197
Role of Readily Degraded Substrates ............................................................................................ 198 Physical Parameters .......................................................................................................................200
Temperature ........................................................................................................................200 Oxygen Concentration ........................................................................................................ 201
Contents ix
Substrate Concentration, Transport into Cells, and Toxicity ......................................................... 210 Utilization of Low Substrate Concentrations ..................................................................... 211 Existence of Threshold Concentrations .............................................................................. 212 Strategies Used by Cells for Substrates with Low or Negligible Water Solubility ............ 213 Transport Mechanisms ....................................................................................................... 214
Pre-Exposure: Pristine and Contaminated Environments ............................................................. 215 Rates of Metabolic Reaction .......................................................................................................... 218
Kinetic Aspects ................................................................................................................... 218 Metabolic Aspects: Nutrients......................................................................................................... 219 Regulation and Toxic Metabolites .................................................................................................220
Introduction ....................................................................................................................................245 Abiotic Reactions ................................................................................................................245 Microbial Reactions ............................................................................................................246 Storage of Samples ............................................................................................................. 247 Determination of Ready Biodegradability ......................................................................... 247
Solid Media ......................................................................................................................... 257 Growth at the Expense of Alternative Substrates ............................................................... 258 Techniques for Anaerobic Bacteria .................................................................................... 258 Design of Experiments on Biodegradation and Biotransformation ................................... 259 Pure Cultures and Stable Consortia .................................................................................... 259 Cell Growth at the Expense of the Xenobiotic ...................................................................260
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and Association of the Substrate with Microbial Cells ...........................................268 References ......................................................................................................................................269
Chapter 6 Elucidation of Metabolic Pathways .......................................................................... 277
Introduction .................................................................................................................................... 277 Part 1 Application of Natural and Synthetic Isotopes ................................................................... 277
Carbon (14C and 13C) ........................................................................................................... 277 Sulfur (35S) and Chlorine (36Cl) .......................................................................................... 278 Hydrogen (2H) and Oxygen (18O) ........................................................................................ 278 Other Isotopes .....................................................................................................................280 Isotope Effects and Stable Isotope Fractionation ...............................................................280
Experimental Procedures .........................................................................................282 References ...........................................................................................................................282
Hydrogen 1H ............................................................................................................285 Carbon 13C ...............................................................................................................285 Nitrogen 15N .............................................................................................................286 Oxygen 17O ...............................................................................................................287 Fluorine 19F ..............................................................................................................287 Phosphorus 31P .........................................................................................................288 Silicon 29Si ...............................................................................................................288
H2O2 to Bacteria .........................................................................................289 Elucidation of the Mechanism of Pyruvate Formate
Lyase ........................................................................................................... 289 The Function of Humic Acids in Reactions Catalyzed by Geobacter
metallireducens .......................................................................................... 289 The Mechanism of Anaerobic Activation of Toluene ..............................................289 Dioxygenation with Elimination of Halide ..............................................................289 The Anaerobic Degradation of Benzoate ................................................................290 Manganese-Containing Enzymes ............................................................................290 Turnover of Naphthalene Dioxygenase ....................................................................290
Contents xi
Chapter 7 Aliphatic Compounds ...............................................................................................297
Aerobic Conditions .................................................................................................. 311 Anaerobic Conditions .............................................................................................. 316
Alkanoic Acids ................................................................................................................... 317 Aerobic Conditions .................................................................................................. 317 Anaerobic Conditions .............................................................................................. 318 Anaerobic Conditions .............................................................................................. 318
Amides and Related Compounds ....................................................................................... 321 Amides .................................................................................................................... 321
Nitriles ................................................................................................................................ 322 Isonitriles ............................................................................................................................ 322 Sulfonylureas and Thiocarbamates .................................................................................... 322 Carbon Monoxide ............................................................................................................... 323 Cyanide ............................................................................................................................... 323 Thiocyanate ........................................................................................................................324 References ........................................................................................................................... 324
Part 2 Cycloalkanes ....................................................................................................................... 336 Monoterpenes ..................................................................................................................... 339 Steroids ............................................................................................................................... 341
Hydroxylation........................................................................................................... 341 Other Aerobic Transformations and Degradation ................................................... 341 Anaerobic Transformation ....................................................................................... 343
Part 3 Alkanes, Cycloalkanes and Related Compounds with Chlorine, Bromine, or Iodine Substituents ......................................................................................................................... 349 Chlorinated, Brominated, and Iodinated Alkanes, Alkenes, and Alkanoates ................... 349
Elimination Reactions .............................................................................................. 350 Corrinoid Pathways .................................................................................................. 355 Nucleophilic Substitution: Hydrolytic Reactions of
Halogenated Alkanes and Alkanoates ....................................................... 358
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Monooxygenation ..................................................................................................... 363 Reductive Reactions: Dehalogenation Including Dehalorespiration .......................366
References ........................................................................................................................... 370 Part 4 Fluorinated Aliphatic Compounds ...................................................................................... 378
Alkanes and Alkenes .......................................................................................................... 378 Fluorohydrocarbons ................................................................................................. 378 Chlorofl uorocarbons and Hydrochlorofl uorocarbons .............................................. 379 Carboxylic Acids......................................................................................................380 Perfl uoroalkyl Carboxylates and Sulfonates ............................................................ 382
References ........................................................................................................................... 382
Part 1 Monocyclic Aromatic Hydrocarbons .................................................................................. 385 Introduction ........................................................................................................................ 385 Monocyclic Arenes ............................................................................................................. 385
Aerobic Conditions .................................................................................................. 385 Anaerobic Conditions .............................................................................................. 389 Synthetic Applications ............................................................................................. 392
References ........................................................................................................................... 394 Part 2 Polycyclic Aromatic Hydrocarbons (PAHs) ........................................................................ 398
Introduction ........................................................................................................................ 398 Aerobic Reactions Carried Out by Bacteria ............................................................ 399 PAHs with Three or More Rings .............................................................................402 Anaerobic Reactions Carried Out by Bacteria ........................................................408 Fungal Transformations ...........................................................................................409 White-Rot Fungi ...................................................................................................... 413
Introduction ........................................................................................................................424 Benzoates ............................................................................................................................424
Aerobic Conditions ..................................................................................................424 Hydroxybenzoates and Related Compounds ...................................................................... 425
Mechanisms for Fission of Oxygenated Rings ........................................................ 428 Alternative Pathways for the Degradation of Benzoates
and Related Compounds............................................................................. 432 Aerobic Reduction of Arene Carboxylates ......................................................................... 433 Arenes with an Oxygenated C2 or C3 Side Chain ............................................................... 433
Anaerobic Metabolism ............................................................................................. 435 Aldehydes ........................................................................................................................... 439 References ........................................................................................................................... 439
Part 4 Nonhalogenated Phenols and Anilines ...............................................................................446 Phenols ................................................................................................................................446
Contents xiii
Chapter 9 Substituted Carbocyclic Aromatic Compounds ....................................................... 455
Part 1 Halogenated Arenes and Carboxylates with Chlorine, Bromine, or Iodine Substituents .......................................................................................................... 455 Introduction ........................................................................................................................ 455 Halogenated Arene Hydrocarbons ..................................................................................... 455
Aerobic Conditions .................................................................................................. 455 Monocyclic Chlorinated Arenes ......................................................................................... 456
Anaerobic Conditions .............................................................................................. 458 Polychlorinated Biphenyls .................................................................................................. 458
Aerobic Degradation ................................................................................................ 458 Degradation Is Initiated by Dioxygenation .............................................................. 459 Ring Fission by 2,3-Dihydroxybiphenyl Dioxygenase ............................................ 461 Metabolites ...............................................................................................................463
Fungal Dehalogenation .......................................................................................................465 Reductive Dehalogenation ..................................................................................................465
Halogenated Benzoates .......................................................................................................468 Dioxygenation ..........................................................................................................469 Hydrolytic Reactions ................................................................................................ 472
Mechanisms for the Ring Fission of Substituted Catechols ............................................... 472 Reductive Loss of Halogen ...................................................................................... 474
Halogenated Phenylacetates ............................................................................................... 475 Fungal Reactions ...................................................................................................... 476
References ........................................................................................................................... 476 Part 2 Halogenated (Chlorine, Bromine, and Iodine) Phenols
and Anilines ........................................................................................................................ 482 Phenols ................................................................................................................................ 482
Aerobic Conditions .................................................................................................. 482 Fungi and Yeasts ......................................................................................................486 Anaerobic Conditions ..............................................................................................487
References ........................................................................................................................... 491 Part 3 Fluorinated Hydrocarbons, Carboxylates, Phenols, and Anilines ...................................... 494
Fluorinated Aromatic Hydrocarbons .................................................................................. 494 Aerobic Conditions .................................................................................................. 494 Metabolism by Yeasts and Fungi ............................................................................. 495 Anaerobic Denitrifying Conditions ......................................................................... 495
Fluorobenzoates .................................................................................................................. 496 Aerobic Conditions .................................................................................................. 496 Difl uorobenzoates .................................................................................................... 498 Degradation under Denitrifying Conditions ............................................................ 499
Fluorinated Phenols ............................................................................................................500 Aerobic Conditions ..................................................................................................500 Anaerobic Conditions .............................................................................................. 501
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Part 4 Arene Sulfonates .................................................................................................................506 References ...........................................................................................................................508
Nitrobenzoates .................................................................................................................... 514 Nitrophenols ........................................................................................................................ 514 References ........................................................................................................................... 517
Part 6 Azoarenes ............................................................................................................................ 520 References ........................................................................................................................... 521
Chapter 10 Heterocyclic Aromatic Compounds ....................................................................... 523
Part 1 Azaarenes ............................................................................................................................ 523 Five-Membered Monocyclic Aza, Oxa, and Thiaarenes .................................................... 523
Aerobic Conditions .................................................................................................. 523 Indole and Carbazole .......................................................................................................... 524
Indole and 3-Alkylindoles ....................................................................................... 524 Carbazole ................................................................................................................. 527
Pyridine .............................................................................................................................. 527 Aerobic Conditions .................................................................................................. 527 Anaerobic Conditions .............................................................................................. 534
Quinoline and Isoquinoline ................................................................................................ 536 Bacterial Metabolism ............................................................................................... 536 Hydroxylation........................................................................................................... 537 Dioxygenation .......................................................................................................... 537
Triazines ............................................................................................................................. 545 1,3,5-Triazines .......................................................................................................... 545 1,2,4-Triazines..........................................................................................................546
References ........................................................................................................................... 562 Part 3 Thiaarenes: Benzothiophenes, Dibenzothiophenes, and Benzothiazole ............................ 565
Benzothiazole ..................................................................................................................... 567 References ........................................................................................................................... 568
Contents xv
Chapter 11 Miscellaneous Compounds ...................................................................................... 569
Part 1 Carboxylate, Sulfate, Phosphate, and Nitrate Esters ........................................................... 569 Carboxylates ....................................................................................................................... 569 Sulfates ............................................................................................................................... 569 Phosphates .......................................................................................................................... 570 Nitrates ................................................................................................................................ 571 References ........................................................................................................................... 572
Part 2 Ethers and Sulfi des .............................................................................................................. 573 Aliphatic and Benzylic Ethers ............................................................................................ 573 Aryl Ethers.......................................................................................................................... 576
Diaryl Ethers ............................................................................................................ 576 Aryl-Alkyl Ethers .................................................................................................... 576
Sulfi des, Disulfi des, and Related Compounds .................................................................... 578 References ........................................................................................................................... 581
Part 3 Aliphatic Nitramines and Nitroalkanes .............................................................................. 585 Nitramines .......................................................................................................................... 585 Nitroalkanes ........................................................................................................................ 585 References ........................................................................................................................... 586
Part 4 Aliphatic Phosphonates and Sulfonates .............................................................................. 588 Introduction ........................................................................................................................ 588 Phosphonates ...................................................................................................................... 588 Sulfonates ........................................................................................................................... 589 Boronates ............................................................................................................................ 591 References ........................................................................................................................... 591
Part 5 Degradation of Organic Compounds of Metals and Metalloids ......................................... 592 Tin .................................................................................................................................. 593 Lead .................................................................................................................................. 593 Mercury .............................................................................................................................. 594 Arsenic ................................................................................................................................ 594 References ........................................................................................................................... 594
SECTION IV Bioremediation
Introduction .................................................................................................................................... 599 Strategies............................................................................................................................. 599 Contaminants ......................................................................................................................600 Sites ..................................................................................................................................602 Chemical Procedures ..........................................................................................................602 Phytoremediation ................................................................................................................602
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Microbiological Aspects ................................................................................................................ 610 Regulation of Pathways ...................................................................................................... 610 Alternative Electron Acceptors .......................................................................................... 611 Aging .................................................................................................................................. 611 Biofi lms ............................................................................................................................... 611 Metabolites and Enzymes ................................................................................................... 611 Utilization of Nitrogen, Sulfur, and Phosphorus ................................................................ 612 Substrate Concentration ...................................................................................................... 612 Temperature ........................................................................................................................ 613
References ...................................................................................................................................... 613
Analysis of Degradative Populations .................................................................................. 622 Application to the Degradation of Specifi c Contaminants ................................................. 623
Hydrocarbons ........................................................................................................... 623 Trichloroethene ........................................................................................................624 Phenol ....................................................................................................................624 Chlorophenol ............................................................................................................ 625 Chlorobenzoate ........................................................................................................ 625 Phenylurea Herbicides ............................................................................................. 625 Dehalogenation of Chloroalkanoates ....................................................................... 626 Dechlorination of PCBs ........................................................................................... 626
Application to Specifi c Groups of Organisms .................................................................... 626 Nondirected Examination of Natural Populations ............................................................. 627
Application of Stable Isotopes ....................................................................................................... 627 Stable Isotope Probes .......................................................................................................... 627 Application of Stable Isotope Enrichment .......................................................................... 628
Application of δ13C .................................................................................................. 629 References ...................................................................................................................................... 633
Chapter 14 Applications of Bioremediation ............................................................................... 639
Introduction .................................................................................................................................... 639 Part 1 Petroleum Hydrocarbons: Refi nery Waste and Stranded Oil .............................................. 639
Terrestrial Habitats .............................................................................................................640 Alkanes ....................................................................................................................640 Naphthenates ............................................................................................................ 641
Marine Habitats .................................................................................................................. 641 The Baffi n Island Oil Spill Project .......................................................................... 641
Conclusion ..........................................................................................................................642 References ...........................................................................................................................642
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The Range of Substrates .......................................................................................... 647 The Range of Degradative Bacteria .........................................................................648 Bioavailability and the Use of Surfactants ..............................................................649
Supplementation with Fungi ............................................................................................... 650 Application of Higher Plants .............................................................................................. 652 Anoxic or Anaerobic Environments ................................................................................... 652
Hydrocarbons ........................................................................................................... 652 Phenols .................................................................................................................... 652 Heteroarenes ............................................................................................................ 653
Abiotic Transformations ..................................................................................................... 653 Conclusions .........................................................................................................................654 References ...........................................................................................................................654
Aerobic Conditions ..................................................................................................664 Anaerobic Conditions ..............................................................................................665
Part 5 Agrochemicals .................................................................................................................... 671 Phenoxyalkanoic Acids ....................................................................................................... 672 Chlorinated Anilines .......................................................................................................... 672 Triazines ............................................................................................................................. 673
1,3,5-Triazines .......................................................................................................... 673 1,2,4-Triazines.......................................................................................................... 673 2,4-Dintro-6-sec-Butylphenol (Dinoseb) ................................................................. 673
Explosives ........................................................................................................................... 675 Nitroarenes .......................................................................................................................... 675 Nitrodiphenylamines .......................................................................................................... 676 Nitrate Esters ...................................................................................................................... 676 Nitramines .......................................................................................................................... 676 Conclusions ......................................................................................................................... 677 Chemical Warfare Agents .................................................................................................. 677 References ........................................................................................................................... 677
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Halogenated Alkanes and Alkenes ..................................................................................... 682 Tetrachloromethane ................................................................................................. 682 Chloroethenes .......................................................................................................... 682 Aerobic Conditions .................................................................................................. 682 Application of Indigenous Bacteria ......................................................................... 683 Application of Exogenous Bacteria .........................................................................684 Anoxic and Anaerobic Conditions ...........................................................................684 Application of Higher Plants ................................................................................... 685
Conclusion ..........................................................................................................................686 Methyl tert-Butyl Ether.......................................................................................................686 References ...........................................................................................................................687
xix
Preface This volume is an updated and expanded version of a previous edition Organic Chemicals: An Envi- ronmental Perspective. This one deals, however, with only degradation and transformation in their widest senses, and the sections in the earlier volumes on analysis, distribution, and ecotoxicology have been omitted since these lie beyond my current competence. In addition, there are specialized volumes that already cover these topics.
I have been extremely fortunate in having as coauthor Ann-Sofi e Allard, who has been a research collaborator for many years. She has been both guide and illustrator, and without her constant help and encouragement, this volume would neither have begun nor would it have come to fruition.
Degradation and transformation occupy a central position in assessing the environmental impact of organic contaminants. We have avoided the term “pollutant” on account of its negative connotation. Although phase partition is not discussed, this is an important factor in determining biodegradability since a contaminant will seldom remain in the phase to which it is initially dis- charged. This must be considered as the following examples illustrate:
1. Compounds deposited as solids (including agrochemicals and contaminants) may reach groundwater and watercourses as a result of partition and leaching.
2. Substances with even marginal volatility will enter the atmosphere, and after transforma- tion may then reenter the aquatic and terrestrial environments through precipitation.
3. Aquatic biota may bring about transformation to metabolites that are then disseminated to a considerable distance from their source.
4. Particularly polar contaminants may associate with polymeric humic components of soil, water, and sediment. Their biodegradation then depends on the degree to which these processes are reversible and the contaminants become accessible to microorganisms (bioavailable). This is especially signifi cant after weathering (aging), even for nonpolar compounds.
Contaminants seldom consist of single substances. The pathways used for biodegradation of some components may be incompatible with those for others that are present, or the pathway for a single compound. Examples of this are given, though in less detail than they merit.
In this volume, emphasis is placed on the pathways by which degradation or transformation has taken place, and the approach is essentially chemical and mechanistic. There are several reasons for this.
1. Biochemical reactions parallel those in organic chemistry and, for both of them, a mecha- nistic approach has proved valuable. In addition, most of the principles that have emerged apply equally to the aquatic, the atmospheric, and the terrestrial environments.
2. Metabolites may be produced by biochemical transformation of the substrate rather than by degradation, or may result from partial abiotic reactions. These products may be (a) terminal and persistent or (b) toxic to other components of an ecosystem—including the microorganisms that produce them. Both of these represent important considerations that are illustrated by examples in this book.
3. Pathways can provide a guide to the probable reactions that other contaminants may undergo including those that bear only a partial—or even more remote structural resem- blance. This is especially important in view of the widening spectrum of contaminants that is the result of the impressive ingenuity of the organic chemist, and particularly for the complexity and novelty of pharmaceutical and agrochemical products.
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xx Preface
A word of caution is appropriate. The reactions that are used for illustration are very seldom specifi c for a single taxon—or even close relatives, and it is not generally possible to establish the range of organisms that will be able to carry out the reaction.
The contents of the chapters are interdependent. In summary they deal with the following:
Chapters 1 through 4 provide a broad perspective on abiotic and biotic reactions, including the signifi cance of a range of environmental determinants.
Chapters 5 and 6 attempt to provide a brief introduction to experimental procedures with emphasis on procedures for establishing the structure of metabolites using isotopes and physical methods.
Chapters 7 through 11 outline details of biochemical reactions involved in the biodegrada- tion of the major groups of aliphatic, carbocyclic aromatic, and heterocyclic compounds. Although emphasis is placed on the pathways, rather general accounts of the enzymes involved and the genetics are provided where they are available.
Chapters 12 through 14 deal with bioremediation that has attracted increasing concern with the realization of the hazard presented by the disposal of unwanted chemicals, or by- products from their manufacture. These chapters should be viewed within the wider con- text of metabolic details that have been presented in Chapters 7 through 11.
There are a number of inevitable omissions and limitations in the material that is covered.
1. Emphasis is placed exclusively on xenobiotics, although some of them are also naturally occurring metabolites. There are substantial groups of compounds that are not discussed. These include the following: a. Naturally occurring polymers—cellulose, chitin, lignin, and polyisoprenoids; synthetic
polyamides, polyurethanes, and polysiloxanes. b. Natural products including polypyrroles such as hemin and chlorophyll; plant and
microbial metabolites. c. Agrochemicals including veterinary chemotherapeutic agents that have come into use
in large-scale animal husbandry. d. Pharmaceuticals including hormone disrupters about which there has arisen serious
concern. e. Halogenated metabolites produced by marine biota, even though some such as halo-
methanes, polybrominated phenols, and polybrominated diphenyl ethers are also xeno- biotics. It is expected that the pharmaceutical industry that has hitherto been dominated by synthetic products—except for antibiotics—will come to rely increasingly on metab- olites produced by marine biota including microorganisms.
2. The number of references that are cited is necessarily restrictive and, although numerous, they represent merely an eclectic selection from a vast literature. Relevant references have no doubt been omitted, but the writers can assure the authors of these that there is no mal- ice in the selection. They invariably refer to the primary literature that has been subjected to the scrutiny of peer review. It is therefore assured that even when the interpretations of the authors should prove faulty—and this is inevitable—a solid and reproducible basis of fact is available to the critical reader. Some older work has been cited when this has led to lasting concepts, though other early work may be diffi cult to evaluate by the standards of today; and no doubt work at the cutting edge of current research will rightly require modifi cation and extension in the future.
3. Although some of the examples used for illustration in Chapter 14 impinge on biological wastewater treatment technology, a systematic account of these lies beyond the scope of this volume and the competence of the authors. It is worth noting, however, that anaerobic reactors that are developed to treat wastewater with the object of producing methane may
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Preface xxi
not necessarily degrade recalcitrant xenobiotics for which inoculation with specifi c micro- organisms may be necessary.
4. No discussion of models will be found in this volume, either for the analysis of degradation kinetics or for the prediction of biodegradability. For these, the interested reader should consult monographs by experts.
The nomenclature of bacteria has presented serious problems principally for two reasons: (a) the number of new taxa that are being described appears to be increasing exponentially and (b) it is seldom possible to determine the correct taxonomic assignment for important organisms that have been used in the historical literature. Although no consistent attempt has been made to provide the current assignment for all taxa, we have tried to use those that occur most frequently.
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xxiii
Acknowledgments It is a pleasure to thank Östen Ekengren, director of Environmental Technology and Toxicology for his generosity in extending access to library facilities. We thank Springer Verlag for kind permis- sion to use a number of fi gures that were used in chapters that we have contributed to Volume 3J (1998), Volume 3N (2002), and Volume 3R (2003) of The Handbook of Environmental Chemistry of which I was volume editor.
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xxv
Authors Alasdair H. Neilson was principal scientist until retirement from IVL Swedish Environmental Research Institute in Stockholm. He studied chemistry at the University of Glasgow and obtained his PhD in organic chemistry from Alexander Todd’s laboratory at Cambridge. He carried out research at Cambridge in organic chemistry, and in theoretical chemistry with Charles Coulson at Oxford. He held academic positions in the universities of Glasgow and Sussex, and obtained industrial experience in the pharmaceutical industry. He consolidated his experience by turning to research in microbiology during a prolonged stay with Roger Stanier and Mike Doudoroff in Berke- ley. His interests have ranged widely and included studies on nitrogen fi xation, carbon and nitrogen metabolism in algae, and various aspects of environmental science including biodegradation and biotransformation, chemical and microbiological reactions in contaminated sediments, and ecotoxi- cology. With his group of collaborators, these studies have resulted in publications in Applied and Environmental Microbiology, Journal of Chromatography, Environmental Science & Technology, and Ecotoxicology & Environmental Safety, and in chapters contributed to several volumes of The Handbook of Environmental Chemistry. He is a member of the American Chemical Society, the American Society for Microbiology, the American Society of Crystallography, and the AAAS.
Ann-Sofi e Allard was trained as a chemical microbiologist and is currently a senior microbiologist at IVL Swedish Environmental Research Institute in Stockholm. She has carried out research in a wide range of environmental issues including water quality, processes for the removal of hormone disrupters, biodegradation and biotransformation of organic contaminants in aquatic and terrestrial systems, and ecotoxicology. She has implemented studies on the uptake and metabolism of organic contaminants and metals in higher plants in the context of bioremediation. Her studies have been published in Applied and Environmental Microbiology, International Biodeterioration and Bio- degradation, Environmental Chemistry and Ecotoxicology, and Journal of Environmental Science and Health, and in chapters contributed to several volumes of The Handbook of Environmental Chemistry. She is a member of the American Chemical Society.
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Section I
3
INTRODUCTION
Virtually any of the plethora of reactions in organic chemistry may be exploited for the abiotic degradation of xenobiotics. These include nucleophilic displacement, oxidation, reduction, ther- mal reactions, and halogenation. Hydrolytic reactions may convert compounds such as esters, amides or nitriles into the corresponding carboxylic acids, or ureas and carbamides into the amines. These abiotic reactions may therefore be the fi rst step in the degradation of such com- pounds. The transformation products may, however, be resistant to further chemical transforma- tion so that their ultimate fate is dependent upon subsequent microbial reactions. For example, for some urea herbicides, the limiting factor is the rate of microbial degradation of the chlorinated anilines that are the initial products of hydrolysis. The role of abiotic reactions should therefore always be taken into consideration, and should be carefully evaluated in laboratory experiments on biodegradation and biotransformation. The results of experiments directed to microbial degradation are probably discarded if they show substantial interference from abiotic reactions. A good illustration of the complementary roles of abiotic and biotic processes is offered by the deg- radation of tributyltin compounds. Earlier experiments (Seligman et al. 1986) had demonstrated the degradation of tributyltin to dibutyltin primarily by microbial processes. It was subsequently shown, however, that an important abiotic reaction mediated by fi ne-grained sediments resulted in the for- mation of monobutyltin and inorganic tin also (Stang et al. 1992). It was therefore concluded that both processes were important in determining the fate of tributyltin in the marine environment.
A study of the carbamate biocides, carbaryl, and propham illustrates the care that should be exercised in determining the relative importance of chemical hydrolysis, photolysis, and bacterial degradation (Figure 1.1) (Wolfe et al. 1978). For carbaryl, the half-life for hydrolysis increased from 0.15 d at pH 9 to 1500 d at pH 5, while that for photolysis was 6.6 d: biodegradation was too slow to be signifi cant. In contrast, the half-lives of propham for hydrolysis and photolysis were >104 and 121 d—so greatly exceeding the half-life of 2.9 d for biodegradation that abiotic processes would be considered to be of subordinate signifi cance. Close attention to structural features of xenobiotics is therefore clearly imperative before making generalizations on the relative signifi cance of alternative degradative pathways.
PHOTOCHEMICAL REACTIONS IN AQUEOUS AND TERRESTRIAL ENVIRONMENTS
Photochemical reactions are important in atmospheric reactions, in terrestrial areas of high solar irradiation such as the surface of soils, and in aquatic systems containing ultraviolet (UV)- absorbing humic and fulvic acids (Zepp et al. 1981a,b). They may be relevant especially for other- wise recalcitrant compounds. It has also been shown (Zepp and Schlotzhauer 1983) that although the presence of algae may enhance photometabolism, this is subservient to direct photolysis at the cell densities likely to be encountered in rivers and lakes. It should be noted that different products may be produced in natural river water and in buffered medium. For example, photolysis of triclopyr (3,5,6- trichloro-2-pyridyloxyacetic acid) in sterile medium at pH 7 resulted in hydrolytic replace- ment of one chlorine atom, whereas in river water the ring was degraded to form oxamic acid as the principal product (Woodburn et al. 1993). Particular attention has therefore been understandably directed to the photolytic degradation of biocides—including agrochemicals—that are applied to terrestrial systems and enter the aquatic system through leaching. There has been increased interest
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4 Environmental Degradation and Transformation of Organic Chemicals
FIGURE 1.1 Carbaryl (A) and propham (B).
O−CO−NHCH3 NH−CO−O−CH(CH3)2
(B)(A)
in their phototoxicity toward a range of biota (references in Monson et al. 1999), and this may be attributed to some of the reactions and transformations that are discussed later in this chapter. It should be emphasized that photochemical reactions may produce molecules structurally more complex and less susceptible to degradation than their precursors, even though the deep-seated rearrangements induced in complex compounds such as the terpene santonin during UV irradiation (Figure 1.2) are not likely to be encountered in environmental situations.
THE DIVERSITY OF PHOTOCHEMICAL TRANSFORMATIONS
In broad terms, the following types of reactions are mediated by the homolytic fi ssion products of water (formally, hydrogen, and hydroxyl radicals), and by molecular oxygen including its excited states—hydrolysis, elimination, oxidation, reduction, and cyclization.
THE ROLE OF HYDROXYL RADICALS
The hydroxyl radical plays two essentially different roles: (a) as a reactant mediating the transfor- mations of xenobiotics and (b) as a toxicant that damages DNA. They are important in a number of environments: (1) in aquatic systems under irradiation, (2) in the troposphere, which is discussed later, and (3) in biological systems in the context of superoxide dismutase and the role of iron. Hydroxyl radicals in aqueous media can be generated by several mechanisms:
a. Photolysis of nitrite and nitrate (Brezonik and Fulkerson-Brekken 1998) b. Fenton reaction with H2O2 and Fe2+ in the absence or presence (Fukushima and Tatsumi
2001) of light c. Photolysis of fulvic acids under anaerobic conditions (Vaughan and Blough 1998) d. Reaction of Fe(III) or Cu(II) complexes of humic acids with hydrogen peroxide (Paciolla
et al. 1999).
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Abiotic Reactions 5
ILLUSTRATIVE EXAMPLES OF PHOTOCHEMICAL TRANSFORMATIONS IN AQUEOUS SOLUTIONS
1. Atrazine is successively transformed to 2,4,6-trihydroxy-1,3,5-triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine side chains and hydrolytic displacement of the ring chlorine and amino groups (Figure 1.3). A comparison has been made between direct photolysis and nitrate-mediated hydroxyl radical reactions (Torrents et al. 1997): the rates of the latter were much greater under the conditions of this experiment, and the major difference in the products was the absence of ring hydroxylation with loss of chloride.
2. Pentachlorophenol produces a wide variety of transformation products, including chlor- anilic acid (2,5-dichloro-3,6-dihydroxybenzo-1,4-quinone) by hydrolysis and oxidation, a dichlorocyclopentanedione by ring contraction, and dichloromaleic acid by cleavage of the aromatic ring (Figure 1.4) (Wong and Crosby 1981).
3. The main products of photolysis of 3-trifl uoromethyl-4-nitrophenol are 2,5 dihydroxy- benzoate produced by hydrolytic loss of the nitro group and oxidation of the trifl uoromethyl group, together with a compound identifi ed as a condensation product of the original compound and the dihydroxybenzoate (Figure 1.5) (Carey and Cox 1981).
N
N
N
Cl
Cl
Cl
Cl
Cl
Cl
OH
Cl
Cl
Cl
Cl
OH
OH
Cl
ClCl
Cl
O
O
Cl
ClCl
Cl
OH
Cl
Cl
OH
HO
O
O
Cl
Cl
CO2H
CO2H
OH
CF3
NO2
OH
CF3
OH
O
O
CF3
OH
OH
CO2H
6 Environmental Degradation and Transformation of Organic Chemicals
4. A potential insecticide that is a derivative of tetrahydro-1,3-thiazine undergoes a number of reactions resulting in some 43 products of which the dimeric azo compound is the principal one in aqueous solutions (Figure 1.6) (Kleier et al. 1985).
5. The herbicide trifl uralin undergoes a photochemical reaction in which the n-propyl side chain of the amine reacts with the vicinal nitro group to form the benzopyrazine (Figure 1.7) (Soderquist et al. 1975).
6. Heptachlor and cis-chlordane, both of which are chiral, produce caged or half-caged struc- tures (Figure 1.8) on irradiation, and these products have been identifi ed in biota from the Baltic, the Arctic, and the Antarctic (Buser and Müller 1993).
7. Methylcyclopentadienyl manganese tricarbonyl that has been suggested as a fuel additive is decomposed primarily by photolysis in aqueous medium. This resulted in the formation of methylcyclopentadiene that may plausibly be presumed to polymerize, and a manganese carbonyl that decomposed to Mn3O4 (Garrison et al. 1995).
8. Stilbenes that are used as fl uorescent whitening agents are photolytically degraded by reactions involving cis–trans isomerization followed by hydration of the double bond, or oxidative fi ssion of the double bond to yield aldehydes (Kramer et al. 1996).
9. The photolysis of chloroalkanes and chloroalkenes has received considerable attention and results in the formation of phosgene as one of the fi nal products. The photodegradation of 1,1,1-trichloroethane involves hydrogen abstraction and oxidation to trichloroacetaldehyde that is degraded by a complex series of reactions to phosgene (Platz et al. 1995; Nelson et al. 1990). Tetrachloroethene is degraded by reaction with chlorine radicals and oxidation to pentachloropropanol radical that also forms phosgene (Franklin 1994). Attention is drawn to these reactions in the context of the atmospheric dissemination of xenobiotics.
10. Although ethylenediaminetetraacetic acid (EDTA) is biodegradable under specifi c labora- tory conditions (Belly et al. 1975; Lauff et al. 1990; Nörtemann 1992; Witschel et al. 1997),
S
O2N NO2
N N
Cl
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Abiotic Reactions 7
the primary mode of degradation in the natural aquatic environment involves photolysis of the Fe complex (Lockhart and Blakeley 1975; Kari and Giger 1995). Its persistence is criti- cally determined not only by the degree of insolation but also on the concentration of Fe in the environment, since complexes with other metals including Ca and Zn are relatively resistant to photolysis (Kari et al. 1995). The available evidence suggests that in contrast to nitrotriacetic acid (NTA) that is more readily biodegradable, EDTA is likely to be persistent except in environments in which concentrations of Fe greatly exceed those of other cations.
11. The photolytic degradation of the fl uoroquinolone enrofl oxacin involves a number of reac- tions that produce 6-fl uoro-7-amino-1-cyclopropylquinolone 2-carboxylic acid that is then degraded to CO2 via reactions involving fi ssion of the benzenoid ring with loss of fl uoride, dealkylation, and decarboxylation (Burhenne et al. 1997a,b) (Figure 1.9).
12. Photolysis of the oxime group in the pyrazole miticide fenpyroximate resulted in the for- mation of two principal transformation products: the nitrile via an elimination reaction and the aldehyde by hydrolysis (Swanson et al. 1995).
13. Photochemical transformation of pyrene in aqueous media produced the 1,6- and 1,8- quinones as stable end products after initial formation of 1-hydroxypyrene (Sigman et al. 1998).
14. The transformation of isoquinoline has been studied both under photochemical conditions with hydrogen peroxide, and in the dark with hydroxyl radicals (Beitz et al. 1998). The for- mer resulted in fi ssion of the pyridine ring with the formation of phthalic dialdehyde and phthalimide, whereas the major product from the latter reaction involved oxidation of the benzene ring with formation of the isoquinoline-5,8-quinone and a hydroxylated quinone.
15. In the presence of both light and hydrogen peroxide, 2,4-dinitrotoluene is oxidized to the corresponding carboxylic acid; this is then decarboxylated to 1,3-dinitrobenzene, which is degraded further by hydroxylation and ring fi ssion (Figure 1.10) (Ho 1986). Analogous reaction products were formed from 2,4,6-trinitrotoluene and hydroxylated to various nitro- phenols and nitrocatechols before fi ssion of the aromatic rings, and included the dimeric 2,2′-carboxy-3,3′,5,5′-tetranitroazoxybenzene (Godejohann et al. 1998). Nitrobenzene, 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, and 4-nitrophenol were degraded with the formation of formate, oxalate, and nitrate (Einschlag et al. 2002).
16. It has been suggested that the photochemical reaction of pentachlorophenol in aqueous solu- tion to produce octachlorodibenzo[1,4] dioxin and some of the heptachloro congener could account for the discrepancy between values for the emission of chlorinated dioxins and their deposition, which is signifi cant for the octachloro congener (Baker and Hites 2000).
N
8 Environmental Degradation and Transformation of Organic Chemicals
CH3
NO2
NO2
NO2
NO2
CO2H
NO2
NO2
NO2
NO2
OH
NO2
OH
OH
CO2H
FIGURE 1.10 Photochemical transformation of 2,4-dinitrotoluene.
17. The psychopharmaceutical drug fl uoxetine (Prozac) is degraded both directly and by the faster reaction with OH radicals (Lam et al. 2005). In both reactions, the ring bearing the CF3 group was degraded in high yield to 4-(difl uoromethylene)-cyclohexa-2,5-diene-1-one.
18. Whereas photolysis of 2- and 4-chlorophenols in aqueous solution produced catechol and hydroquinone, in ice the more toxic dimeric chlorinated dihydroxybiphenyls were formed (Bláha et al. 2004).
19. The photodegradation of the contact herbicide paraquat yielded many degradation products, but the major pathway produced 1,2,3,4-tetrahydro-1-ketopyrido[1,2-a]-5-pyrazinium that was further degraded to pyridine-2-carboxamide and pyridine-2-carboxylate (Figure 1.11) (Smith and Grove 1969).
20. Photodegradation of the nonsteroidal anti-infl ammatory drug diclofenac produced carbazole-1-acetate as the major product (Figure 1.12) (Moore et al. 1990). In a lake under natural conditions, it was rapidly decomposed photochemically though none of the products produced in laboratory experiments could be detected (Buser et al. 1998).
21. Phototransformation of DDT in the presence of surfactants produced DDE by elimination and DDD by reductive dechlorination (Chu 1999).
N eMNeM N NeM NMe CO2 MeNH2 + CO2
FIGURE 1.11 Photochemical transformation of paraquat.
N
Cl
Cl
H
CH2
CO2H
N
CH2CO2H
H
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Abiotic Reactions 9
HYDROXYL RADICALS IN THE DESTRUCTION OF CONTAMINANTS
The destruction of contaminants has directed attention to the use of hydroxyl radical–mediated reactions. These reactions should be viewed against those with hydroxyl radicals that occur in the atmosphere.
FENTON’S REAGENT
Hydrogen peroxide in the presence of Fe2+ or Fe3+ (Fenton’s reagent) has been used in a range of confi gurations including irradiation, electrochemical, and both cathodic and anodic conditions (references in Wang et al. 2004). In all of these, the reaction involves hydroxyl radicals and has been studied particularly intensively for the destruction of agrochemicals including chlorinated phenoxyacetic acid (Sun and Pignatello 1993) and chloroacetanilide herbicides (Friedman et al. 2006). Systematic investigations have been carried out on the effect of pH, the molar ratio of H2O2/ substrate, and the possible complications resulting from the formation of iron complexes. Although this reaction may have limited environmental relevance except under rather special circumstances, it has been applied in combination with biological treatment of polycyclic aromatic hydrocarbons (PAHs) (Pradhan et al. 1997). Attention is drawn to it here since, under conditions where the con- centration of oxidant is limiting, intermediates may be formed that are stable and that may possibly exert adverse environmental effects. Some examples that illustrate the formation of intermediates are given, although it should be emphasized that total destruction of the relevant xenobiotics under optimal conditions can be successfully accomplished. The structures of the products that are pro- duced by the action of Fenton’s reagent on chlorobenzene are shown in Figure 1.13a (Sedlak and Andren 1991), on 2,4-dichlorophenoxyacetate in Figure 1.13b (Sun and Pignatello 1993), and on pentachlorophenol in Figure 1.13c (Fukushima and Tatsumi 2001). The UV-enhanced Fenton trans- formation of atrazine produced 2,4-diamino-6-hydroxy-1,3,5-triazine by a series of interacting reactions (Chan and Chu 2006). Whereas the degradation of azo dyes by Fenton’s reagent produced water- and CH2Cl2-soluble transformation products including nitrobenzene from Disperse Orange 3
Cl
OH
O
Cl
O
Cl
Cl
Cl
Cl
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10 Environmental Degradation and Transformation of Organic Chemicals
that contains a nitro group, benzene was tentatively identifi ed among volatile products from Solvent Yellow 14 (Spadaro et al. 1994).
POLYOXOMETALATES (HETEROPOLYACIDS)
These are complexes formed between tungstates and molybdates, and silicate or phosphate, and have been used to generate hydroxyl radicals photochemically. The tungstates PW12O40
3− and SiW12O40
4− have been used most frequently.
a. Degradation of 2,4,6-trichlorophenol by PW12O40 3− formed a number of products as inter-
mediates, including 2,6-dichlorohydroquinone followed by fi ssion of the ring to maleate, oxalate, acetate, and formate (Androulaki et al. 2000).
b. Nonafl uoropentanoic acid was decomposed in aqueous solution to fl uoride and CO2 cata- lyzed by H3PW12O40 under UV-visible light radiation. The reaction was initiated by decar- boxylation followed by a series of reactions involving oxidations (Hori et al. 2004b).
c. The polyoxometalate PW12O40 3− was immobilized on an anion-exchange resin, and
used to demonstrate the degradation in the presence of H2O2 of the phthalein dye rhodamine B to phthalate and a number of short-chain aliphatic mono- and dicarboxylates (Lei et al. 2005).
They have also been used to bring about photochemical reduction of Hg2+ via Hg2 2+ to Hg0
(Troupis et al. 2005).
PHOTOLYTIC DEGRADATION ON TiO2
The mechanism involves photochemical production of a free electron in the conduction band (ecb
−) and a corresponding hole (hvb +) in the valence band. Both of these produce H2O2 and thence
hydroxyl radicals.
a. In the presence of slurries of TiO2 that served as a photochemical sensitizer, methyl tert-butyl ether was photochemically decomposed at wavelengths <290 nm. The products were essentially the same as those produced by hydroxyl radicals under atmospheric con- ditions (Baretto et al. 1995): tert-butyl formate and tert-butanol were rapidly formed and further degraded to formate, acetone, acetate, and but-2-ene.
b. The photocatalytic oxidation of various EDTA complexes has been examined (Madden et al. 1997). The rates and effi ciencies were strongly dependent on the metal and the reactions are generally similar to those involved in electrochemical oxidation (Pakalapati et al. 1996).
c. A number of products are formed from trichloroethene including tetrachloromethane, hexachloroethane, pentachloroethane, and tetrachlororethene, although the last two were shown to be degradable in separate experiments (Hung and Marinas 1997). In TiO2 slur- ries, the photochemical degradation of chloroform, bromoform, and tetrachloromethane involves initial formation of the trihalomethyl radicals. In the absence of oxygen, these are further decomposed via dihalocarbenes to CO. Dichlorocarbene was found as an interme- diate in the degradation of trichloroacetate (Choi and Hoffmann 1997).
d. The degradation of the herbicide 3-amino-1H-1,2,4-triazole (amitrole) produced a number of transformation products after fi ssion of the triazole ring. These reacted together to form 2,4,6-trihydroxy-1,3,5-triazine that was a stable end product (Watanabe et al. 2005).
e. The oxidation of tetrachlorobiphenyl congeners (23-34, 25-34, 345-4) with only one ortho- chlorine substituent was examined using nanostructured TiO2 immobilized on quartz beads. Initial reactions were hydroxylations by OH radicals that were followed by fi ssion of a single ring to chlorobenzoates and low yields of succinate and gl