invited review r eaction pathways and mechanisms of ......cmax-planck institut fur...

Journal of Photochemistry and Photobiology B: Biology 67 (2002) 71–108www.elsevier.com/ locate / jphotobiol

Invited Review

R eaction pathways and mechanisms of photodegradation of pesticidesa b , b c*H.D. Burrows , M. Canle L , J.A. Santaballa , S. Steenken

a ´Departamento de Quımica, Universidade de Coimbra, 3004-535 Coimbra, Portugalb ˜ ˜ ˜´ ´ ´ ´ ´Departamento de Quımica Fısica e Enxenerıa Quımica I, Universidade da Coruna, Rua Alejandro de la Sota 1, E-15008 A Coruna, Galicia, Spain

c ¨ ¨Max-Planck Institut f ur Strahlenchemie, D-45413 Mulheim an der Ruhr, Germany

Received 26 March 2001; accepted 25 February 2002

Abstract

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processesinvolved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methodsfor water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide,photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradationby reaction with intermediates generated by photolytic or radiolytic means. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Pesticides; Photodegradation; Photoionization; Photocatalysis; Photosensitization; Photolysis; Hydroxyl radicals

1 . Introduction worsen with the appearance of new and more sophisticatedsubstances.

Due to the world-wide general application of intensive The critical nature of this environmental problem hasagricultural methods during the last few decades and to the prompted the development of faster and more accuratelarge-scale development of the agrochemical industry, the methods for the characterisation and quantification of thevariety and quantities of agrochemicals present in con- pesticides dispersed in the environment. These have gener-tinental and marine natural waters has dramatically in- ally been very successful. However, until now no com-creased. Most pesticides are resistant to chemical and/or pletely efficient methods have been developed for remedia-photochemical degradation under typical environmental tion of contaminated waters [16].conditions [1]. In recent years, the scientific community Advanced oxidation processes (AOPs) are at presenthas shown a great concern about the possible adverse considered to have considerable potential in this area and aeffects that the presence of these pesticides in water and general survey of these can be found in the review byfood [2] may have for human health and for the equilib- Legrini et al. [17] The most efficient of these usedrium of ecosystems [3–7]. Such concern, which has nowadays involve the use of UV irradiation with light ofrecently been highlighted [8,9], is supported by results an appropriate wavelength [18–21]. The method is basedfrom major monitoring studies already performed over 20 on the ability of UV radiation to attack and damage theyears ago [10], and confirmed by more recent inves- DNA of undesirable microorganisms. As a consequence,tigations. Among the possible chronic effects of these photochemical processes may take place, in which differ-

2compounds are carcinogenesis [11,12], neurotoxicity [13], ent transient species are generated: e (photoionization),aq

effects on reproduction [14] and cell development effects, radicals generated by bond homolysis or bond heterolysis,particularly in the early stages of life [15]. With increasing etc, as well as a number of photophysical processesglobal demand for vegetables, the situation does not look (fluorescence, phosphorescence, etc.) Alternatively, UV-likely to improve. In fact, the current situation might sensitive materials may be added to give rise to photo-

sensitized oxidation, that allow the use of wavelengths thatare not absorbed by the pesticides [17]. Hydroxyl radical

?*Corresponding author. Tel.: 134-981-167-000x2150; fax: 134-981- (HO ) might also be generated by various different ways,167-065.

giving rise to induced photodegradation.E-mail addresses: [email protected] (H.D. Burrows), [email protected] regulations for the control of the quality of water(M. Canle L), [email protected] (J.A. Santaballa), steenken@mpi-

muelheim.mpg.de (S. Steenken). for human use in the European Union clearly establishes

1011-1344/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S1011-1344( 02 )00277-4

mailto:[email protected]





72 H.D. Burrows et al. / Journal of Photochemistry and Photobiology B: Biology 67 (2002) 71 –108

the maximum admissible values of different substances in evaluate or authoritatively criticise the cited literature, butsolution [22]. Among these, pesticides are identified as hope that this review will stimulate more detailed analysestoxic, with very low concentrations of them in general of the data in each of the particular areas discussed.being allowed (a maximum permissible concentration for aparticular pesticide of 0.1 ppb and of 0.5 ppb for the totalload of all plaguicides). In addition, these regulations also 2 . Direct photodegradationestablish maximum concentrations for the products ofdegradation of pesticides [23,22]. No maximum concen- Most pesticides show UV–Vis absorption bands attration levels have been established in the USA, the relatively short UV wavelengths. Since sunlight reachingregulations depending on the particular compounds, based the Earth’s surface (mainly UV-A, with varying amounts ofon the toxicological evidences available for them [24]. UV-B) contains only a very small amount of short wave-

The literature reports on pesticide photodegradation length UV radiation [32,33], the direct photodegradation ofproducts is relatively abundant. However, little information pesticides by sunlight is expected to be, in general, of onlyis available on the reaction mechanisms involved in the limited importance. Abundant studies are available, how-photolysis of pesticides under typical environmental con- ever, with steady state and/or laser-pulsed UV radiation.ditions [1]. The environmental photochemistry of her- Direct irradiation will lead to the promotion of thebicides was reviewed several years ago [25]. Similarly, the pesticides to their excited singlet states, which may thenkinetics and mechanisms of photodegradation of chloro- intersystem cross to produce triplet states. Such excitedphenol pesticides have recently been discussed [26]. states can then undergo, among other processes: (i)

Considering the environmental relevance and impor- homolysis, (ii) heterolysis or (iii) photoionization, astance of this problem, we review the state-of-the-art depicted in Scheme 1.situation on the photodegradation of pesticides. These will Because of the particular relevance to the photoreactivi-be classified in terms of the main structural groups, ty of these pesticides under sunlight, relevant references tofollowing a chronological order within each group. We product identification, kinetics, mechanisms and toxicity ofshall concentrate on the behaviour in solution and on solid degradation products are summarised in Table 1. Moresupports. The photochemical decomposition of pesticides detailed information of photoreactivity under both solarand herbicides on plants, on model plant systems and on and ultraviolet irradiation is given in the following sec-soil is of fundamental importance to the situation in vivo. tions.Some studies in this area, notably on model plant cuticles,are presented in Section 2. However, much less detailed 2 .1. Amidinohydrazone insecticidesmechanistic information is available in this case, and wefeel, that as a first approximation, it is not unreasonable to Adityachaudhury et al. [34] studied the sunlight photo-extrapolate the behaviour from solution or solid supports to degradation of hydramethylnon h5,5-dimethylperhydro-these systems. Most papers reviewed are from the last pyrimidin - 2 - one - 4 - trifluoromethyl - a - (4 - trifluorome-decade, although some previous articles have also been thylstyryl) cinnamylidenehydrazonej and found three prod-included to facilitate understanding of mechanistic path- ucts. Transformation took place within 10 h, with the rateways. Although the production and use of some of the of disappearance being only marginally affected by thepesticides discussed has been discontinued, we have still acidity of the medium.considered them, because of their possible persistence inthe environment. In identifying products, we report a 2 .2. Anilide herbicidessystematic name in all cases, as some of the commoncommercial names have changed through the years (CAS Tanaka et al. [35] studied the formation of biphenylsnames are given in those cases where no systematic name upon sunlight photolysis of propanil h39,49-dichloro-has been found). In addition, we note that several excellent propionanilidej. Coupling of two herbicide molecules tookcompendia are available compiling the different names and place, leading to formation of a chlorinated biphenyl, withproperties of the various different compounds [27–31]. Wehave chosen to classify the photodegradation studies intofour broad categories: direct photodegradation, photosen-sitized degradation, photocatalyzed degradation and degra-dation by reaction with hydroxyl radical. Within eachgroup these are discussed in terms of general classes ofcompounds. However, in some cases these categoriesoverlap. In such situations, we have tried to fit the work tothe field that is the authors’ main target area. Finally, ouraim in this review is to compile and annotate the mainreaction types and mechanisms. We do not attempt to Scheme 1. Possible chemical events taking place upon direct photolysis.

H.D. Burrows et al. / Journal of Photochemistry and Photobiology B: Biology 67 (2002) 71 –108 73

Table 1Studies on pesticides photodegradation under sunlight or simulated sunlight

Compounds Product Kinetics Mechanistic Toxicityidentification studies measurements

Amidinohydrazones [34] [34]Anilides [35] [36–38] [39]Avermectins [40]Carbamates [35,41,42,34,43–46,37] [47,48] [49]Carboxamides [50]Chloronicotinoids [51,52]Chlorophenols [53] [53,54]Dicarboximides [55]Dichloroanilines [53]Dinitroanilines [56] [57]Diphenylethers [58]Halobenzonitriles [59]Mercapto-pyridine-N-oxides [60]Methylisocyanate [61]Organophosphorousderivatives [62–65] [66,57] [67]Oxadiazoles [58] [36]Phenols [68] [68]Phosphorothioates [69]Pyrimidines [70]Sulfonylureas [71,72] [73] [74]Thiocarbamates [75–77] [78,38]Thiones [79]Triazines [80] [81,82,38] [83]Triazinones [84]Triazoles [85]Ureas [86–88,35,89,90]

very low yield. Probably, the photolability of such a reduced, possibly through photodegradation. The observedbiphenyl would lead to fast degradation, so that they would reduction of toxicity was higher in bay water than in freshaccumulate only to a very low extent. water.

The major photoproducts found upon irradiation of Recently, Mabury and Wilson [37] have investigated thealachlor [2-chloro-29,69-diethyl-N-methoxymethylacet- photodegradation of alachlor (vide supra), butachlor hN-anilide] with a medium pressure mercury lamp were (butoxymethyl)-2-chloro-29,69-diethyl-acetanilidej, metola-hydroxyalachlor and a lactam, plus another three products: chlor (vide supra) and the model compound 2-chloro-N-norchloralachlor, 29,69-diethylacetanilide and 2-hydroxy- methylacetanilide in a solar simulator, with the aim of29,69-diethyl-N-methylacetanilide [91]. The author sug- determining the extent of monochloroacetic acid product-gests that the mechanism proceeds via homolytic scission ion. The photodegradation followed pseudo-first orderof the C–Cl bond, followed by trapping of the radical kinetics, which was independent of the pH of the medium.so-formed by water or intramolecular hydrogen abstrac- Photolysis in synthetic field water reduced the time oftion. photolysis.

Albanis and Konstantinou [92] studied the photodegra- Hogenboom et al. [93] used on-line solid-phase ex-dation of propachlor h2-chloro-N-isopropylacetanilidej and traction liquid chromatography combined with mass spec-propanil h39,49-dichloropropionanilidej in water and soil, trometry to screen the products of photodegradation ofand found the half-lives of the compounds to be ¯1–2 alachlor (vide supra) and related structures. The structuresmonths in water and ¯1 month in soils. The presence of of ten photodegradation products were proposed, based onhumic substances reduced the photodegradation rate in the most likely fragmentation MS patterns. Comparisonwater but led to an enhancement in soil. The major with the results of Hapeman-Somich [91] indicates a ratherphotoproducts found were the hydroxy and dechlorinated complicated reaction scheme. However, it is not clear howderivatives. many of the products resulted from secondary photolysis

Lin et al. [39] looked into the effect of simulated pathways.sunlight on metolachlor h2-chloro-69-ethyl-N-(2-methoxy- The aqueous photodegradation of butachlor (vide supra)1-methylethyl)acet-o-toluidinej toxicity in surface waters. by sunlight takes place with a pseudo-first order rate

21They concluded that the toxicity of the herbicide is constant 0.012 h (t 558.2 h) [36].1 / 2


2 .3. Bipyridinium herbicides and photo-Fries rearrangement products, with a low quan-tum efficiency. In contrast to previous findings, these

Different authors [94,95] have shown that photolysis of authors did not observe any effect of the concentration ofparaquat [1,1-dimethyl-4,49-bipyridinium dichloride] yields starting carbamate [100].4-carboxy-1-methypyridium ion and methylamine, with the Tanaka et al. [35] investigated the generation of bi-mechanism involving a transient ketone or aldehyde. More phenyls upon sunlight photolysis of chlorpropham hiso-recently [96], it has been found that irradiation of paraquat propyl 3-chlorocarbanilatej and PPG-124 h4-chlorophenylin the presence of oxygen, led to formation of 4,49- methylcarbamatej. The photoreactions took place via cou-bipyridyl and 4-picolinic acid. In addition, sequential loss pling of the two insecticide molecules, leading with veryof methyl was found to lead to monoquat [1-methyl-4,49- low yield to formation of a chlorinated biphenyl. The highbipyridinium dichloride] and 4,49-bipyridyl. The mecha- photolability of these biphenyls would lead to their fastnism proposed for formation of 4-picolinic acid involves degradation, so that they would not accumulate signifi-an oxidative ring cleavage of 4,49-bipyridyl or demethyla- cantly.tion of 4-carboxy-1-methypyridium ion. If paraquat is A report has been presented of the photochemicalirradiated for longer times oxalate, succinate, malate and degradation of carbaryl (vide supra) and carbofuran h2,3-N-formyl glycine are formed. dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamatej by

the action of sunlight and UV (l.290 nm) in natural2 .4. Carbamate insecticides waters of Northern Greece [76]. The major photoproduct

observed from degradation of carbofuran was carbofuran–Aly and El-Dib [97] investigated the photodecomposi- phenol, which further photodegraded. The major photo-

tion of sevin (carbaryl) h1-naphthyl methylcarbamatej, product observed from degradation of carbaryl was a-baygon (propoxur) h2-(1-methylethoxy)phenyl methyl- naphthol. Continuous irradiation led to almost completecarbamatej, pyrolan h3-methyl-1-phenylpyrazol-5-yl dime- degradation of the pesticide.thylcarbamatej and dimetilane hpyrazole-1-carboxamide, The photodegradation of carbetamide h(R)-1-(ethylcar-3-hydroxy-N,N,5-trimethyl-, dimethylcarbamatej. In the bamoyl)ethyl carbanilatej by 290 nm light has beencase of sevin and baygon the rate of photodegradation studied in water, ethanol, methanol and a water /1%increase with the pH of the medium, while no effect was acetone mixture [41]. The highest quantum yield ofobserved for pyrolan and dimetilane. The main photo- photodegradation (¯9%) was observed in aqueous ace-chemical event observed was the bond scission of the ester. tone. The presence of formulation additives produced, ina-Naphtol was identified as the main photoproduct derived all cases, a decrease in the photodegradation quantumfrom sevin. The order of photostability found was yield. Aniline and hydroxy phenyl carbamoyloxy 2-N-ethylpyrolan.dimetilane.baygon.sevin. propionamide were identified as the main photoproducts of

The photodegradation of zectran h4-dimethylamino-3,5- aqueous photodegradation.xylyl-N-methyl carbamatej was studied in aerated and With relevance to the design and synthesis of light-degassed cyclohexane and ethanol [98]. A photo-Fries sensitive urethanes as photoprecursors of amines, Cameron

´rearrangement was proposed as the main reaction pathway. and Frechet [102] proposed a heterolysis mechanism forKumar et al. [99] studied the 265 nm photodegradation the UV photocleavage of carbamates. They subsequently,

of azak h2,6-di-tert-butyl-4-methylphenyl-N-methylcarba- while studying the photogeneration of organic bases frommatesj in aerated or degassed ethanol. A radical mecha- carbamate derivatives, reported the existence of steric andnism taking place via a photo-Fries rearrangement involv- electronic effects on the quantum efficiency of 254 nming a cyclohexadienone intermediate was proposed. photolysis [103].

While studying the photoinduced transformation of The photodegradation of aldicarb h2-methyl-2-ethyl-N-phenylcarbamate to ethyl o-aminobenzoate, ethyl (methylthio)propionaldehyde O-methylcarbamoyloximej,p-amino-benzoate and aniline, Masilamani and Hutchins carbaryl (vide supra) and carbofuran (vide supra) has been[100] found an effect of the concentration of the starting studied using the UV radiation from a high pressurematerial. According to these authors, at high concen- mercury lamp and from a suntest apparatus equipped withtrations of carbamate ethyl o-aminobenzoate is predomi- a Xe arc lamp in distilled, pond and artificial water, in thenant, while at low concentrations aniline is the only absence and presence of humic acids [42]. Aldicarbproduct observed. sulfoxide and 1-naphthol, respectively, were identified as

A more recent and very detailed study of the UV the major photodegradation products from aldicarb andphotodegradation of various alkyl N-arylcarbamates in carbaryl. An unidentified major product was producedethyl propionate and cyclohexane has been carried out by upon photolysis of carbofuran. Photosensitizing effects,Herweh and Hoyle [101]. These authors propose a mecha- leading to an enhancement of photodegradation werenism involving an excited singlet state that undergoes observed with carbaryl and carbofuran when using waterhomolytic cleavage of the C–N bond to a radical pair in a containing humic acids.solvent cage. This radical pair eventually leads to amines Adityachaudhury et al. [34] studied the sunlight induced


photodegradation of carbofuran (vide supra). The authors methylpyrimidin-4-yl dimethylcarbamate and 2-claim they characterized more than twenty photoproducts. (formylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarba-Only two of these were, however, were confirmed by mate. In the solid-phase 2-[(methylformyl)amino]-5,6-di-comparison with authentic samples. methylpyrimidin-4-yl dimethylcarbamate and 2-

´Benıtez et al. [104] studied the photooxidation of (methylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarba-aqueous solutions of propoxur (vide supra) using the mate were the observed photoproducts. The differentradiation supplied by a low-pressure mercury lamp. This wavelengths used do not seem to change the reactionstudy included the effects of temperature, pH and pesticide pathways, but only affect the degradation rate. The mecha-concentration, as well as of added O [104]. As a result, a nism does not seem to be radical-mediated.3

general rate equation for the direct UV photolysis was A detailed GC–MS study of the products resulting fromproposed, with the quantum yield being effectively insensi- photolysis of different carbamates by the UV radiationtive to the pH. The process in the presence of O can be produced by a medium-pressure mercury lamp has been3

divided into a direct reaction with O , a direct photochemi- carried out by Climent and Miranda [105,106]. The3

cal process and a combination of both. An appropriate compounds studied were bendiocarb h2,3-iso-reaction scheme has been proposed. propylidenedioxyphenyl methylcarbamatej, isoprocarb ho-

The 355 nm laser flash photolysis of N-hydroxy- cumenyl methylcarbamatej, promecarb h3-isopropyl 5-pyridine-2-thione carbamates yielded dialkylaminyl radi- methylphenyl methylcarbamatej, ethiofencarb (vide supra),cals which, upon protonation, produced the corresponding furathiocarb hbutyl 2,3-dihydro-2,2-dimethylbenzofuran-7-dialkylaminium radical cations [49]. The kinetics of the yl N,N9-dimethyl-N,N9-thiodicarbamatej, fenoxycarb hethylreactions undergone by these aminium radical cations have 2-(4-phenoxyphenoxy)ethylcarbamatej and pirimicarb h2-been studied and reported, as well as their acidity con- dimethylamino-5,6-dimethylpyrimidin-4-yl dimethylcarba-stants. matej. In general, photodegradation led to the corre-

The photodegradation of ethiofencarb h2- sponding phenols as main photoproducts. Photoprocesses[(ethylthio)methyl]phenyl methylcarbamatej by UV light such as photo-Fries rearrangements, C–S or N–S bond(l.280 nm) and natural sunlight has been studied in the cleavage or oxidative demethylation were observed, de-presence of cyclohexane, cyclohexene and isopropanol, in pending on the structures of the compounds.an attempt to model its photolysis on plant surfaces [44]. Mansour et al. [46] studied the photolysis of carbofuranThe efficiency of the photodegradation was higher in less (vide supra) in water and water / soil suspensions using UVpolar solvents, with the observed half-lives ranging from light of different wavelengths. The photodegradation was75 min to more than 20 h. Different products were very slow when carbofuran was irradiated with l.290obtained depending on the solvent and the light source: nm, but was substantially enhanced in the presence ofethiofencarb sulfoxide was the main product of irradiation TiO , H O or O .2 2 2 3

in cyclohexane, while photooxidation, hydrolysis and Albanis and Konstantinou [92] have studied the photo-addition product formation was observed when the photodegradation of molinate hS-ethyl hexahydro-1H-azepine-1lysis took place in isopropanol. carbothioatej in water and soil. The half-life of the

´Benıtez et al. [43] studied the photodegradation of compounds was ¯1–2 months in water and ¯1 month inaqueous solutions of carbofuran (vide supra) by the soils. The presence of humic substances reduced thepolychromatic light supplied by a high-pressure mercury photodegradation rate in water, but an enhancement waslamp (emitting from 185 nm through to the visible region). observed in soil. The major photoproduct found was theThe authors claim that the contribution of hydroxyl keto derivative.radicals to the degradation is negligible, since no effect of The photodecomposition of carbofuran (vide supra) bytert-butyl alcohol was observed on the rate of degradation 254 nm UV light in water follows first-order reactionupon direct photolysis. A simple rate equation is proposed kinetics [107]. The presence of dissolved organic matterto describe the process and the quantum yield obtained and was found to inhibit the photodegradation of carbofuran.

1correlated by an Arrhenius-type expression. The photoproducts were identified by GC–MS and HPirisi et al. [45] have studied the photolysis of NMR. A tentative mechanism was proposed for the

pirimicarb h2-dimethylamino-5,6-dimethylpyrimidin-4-yl photodegradation. According to the authors, the carbamatedimethylcarbamatej in water and in the solid-phase, using group is cleaved from carbofuran via C–O heterolysis,three different light sources, a low pressure mercury lamp, yielding a phenoxide anion and an acylium cation. Thehigh pressure mercury lamps and sunlight, for irradiation. furan moiety of the phenoxide anion then undergoes ringThree photoproducts were observed in solution: 2- opening in a second step, leading to a substituted catechol[(methylformyl)amino]-5,6-dimethylpyrimidin-4-yl dime- moiety with a tert-butyl alcohol substituent. This catecholthylcarbamate, 2-(dimethylamino)-5,6-dimethyl-1-hydro- dehydrates, yielding an alkene.xypyrimidine and 2-(methylamino)-5,6-dimethylpyri- The photodegradation of the systemic insecticidemidin-4-yl dimethylcarbamate. This later product under- ethiofencarb (vide supra) has been studied in water,went decomposition to yield 2-amino-5,6-di- methanol and hexane using a solar simulation unit [47].


The kinetics of photodegradation can be fitted to a first- photodegradation, and in some cases may even be theorder rate equation, independent of the nature of the dominant pathway.medium, with the reaction rate increasing with the polarityof the medium. The photoproducts depend on the solvents. 2 .5. Carboxamide fungicidesIn water, photocleavage of the C–S bond yields 2-(methyl)phenyl-N-methylcarbamate. In the presence of humic acid the half-life of photo-

Vialaton and Richard [108] carried out a 266-nm laser degradation of carboxin h5,6-dihydro-2-methyl-N-phenyl-flash photolysis study of the photolysis of ethiofencarb 1,4-oxathiin-3-carboxamidej was reduced by 25%. Oxy-(vide supra). A quantum yield of 12% was estimated for carboxin h5,6-dihydro-2-methyl-N-phenyl-1,4-oxathiin-3-the photoconversion of ethiofencarb into its sulfoxide, carboxamide 4,4,-dioxidej photodegradation after 8 h2-methylphenyl-methylcarbamate and to a further two increased from 10 to 25% [111,112].unidentified products. The homolytic cleavage of the C–S UV irradiation (l.290 nm) of prochloraz hN-propyl-N-bond was confirmed by spectroscopic detection of benzyl [2 - (2, 4, 6 - trichlorophenoxy) - ethyl]imidazole - 1 - carbox-radicals. The authors suggest that the mechanism of amidej in soil led to prochloraz–formylurea, which sub-generation of the sulfoxide may result from an electron sequently hydrolysed to prochloraz–urea [50].transfer from the triplet excited state to oxygen, yielding aradical cation which then recombines with the superoxide 2 .6. Chloronicotinoid insecticidesion, to produce an intermediate peroxysulfoxide, eventual-ly leading to the sulfoxide. Irradiation of imidacloprid h1-(6-chloro-3-pyridin-3-yl-

A study of the direct photolysis of propoxur (vide supra) methyl)-N-nitroimidazolidin-2-ylidenaminej in aqueousin water, using 266 nm laser flash photolysis showed solution with 290 nm light resulted in 90% degradationalmost complete disappearance of propoxur, with forma- after 4 h [51]. The main photoproducts were identified astion of the corresponding photo-Fries rearrangement prod- 6-chloronicotinaldehyde, N-methylnicotinamide, 1-(6-chlo-ucts [109]. According to the authors, the process can be ronicotinyl)imidazolidone and 6-chloro-3-pyridylmethyl-rationalized on the basis of an initial homolytic –C(O)–O– ethylendiamine. Some minor products were also formed,scission of the carbamate moiety from the singlet excited but not identified.state of propoxur, with generation of 2-isopropoxyphenox- The sunlight induced photolysis of the insecticideyl-carbamoyl radical pair. Radical recombination or escape imidacloprid (vide supra) in aqueous solution was studiedout of the cage would produce the observed photoproducts. on the surface of tomato leaves [52]. The photodegradationNo oxygen quenching was observed. was rapid, and five photoproducts were found, four of

Ibarz et al. [48] studied the aqueous photodegradation of which could be identified.carbendazim hmethyl-2-benzimidazole carbamatej at vari- Kole et al. [56] studied the UV photolysis (240–260 nm)ous pH values, using the radiation supplied by a mercury of imidacloprid (vide supra) in 10% acetonitrile /water.lamp emitting between 250 and 750 nm. The process Three photoproducts were observed, and were suggested tofollowed a first-order rate equation, and the rate of be formed by N–NO bond cleavage, hydroxylation of the2

photodegradation increased with pH and O concentra- imidazolidine ring and oxidative cleavage of the methylene2

tions. The quantum yields for photodegradation at different bridge. The authors sketch a possible mechanism for thepH values were in all cases very low (less than 1%). process.

Detailed studies of the initial stages of photodegradation An additional study of the photodegradation of im-of a variety of carbamates using laser flash photolysis idacloprid (vide supra) has been reported in HPLC gradecombined with pulse radiolysis showed the photodegrada- water and as the formulated product Confidor in tap watertion is a biphotonic process when 308, 266 and 248 nm [113]. Several degradation products have been identified,light is used for excitation, and monophotonic if 193 nm with 1-6-chloro-3-pyridinyl-methyl-2-imidazolidinone aslight is used. Photoionization is observed in all cases, with the main one. The photodegradation followed a first-ordervery low quantum yields, as well as formation of the rate equation.corresponding phenoxyl radicals. The observed photo-products, detected using GC–MS and HPLC, and checked 2 .7. Chlorophenol pesticidesin some cases against authentic commercial samples, werethe corresponding phenols and photo-Fries products. The Miille and Crosby [53] studied the photodegradation ofrate constants for one-electron oxidation of the carbamates pentachlorophenol by simulated sunlight in seawater. The

?2 2 ?by SO , one-electron reduction by e , and for HO photolysis reaction was slower in seawater, which was4 aq

addition were obtained. Structure–reactivity correlations attributed to the photonucleophilic interaction of thehave been established in order to predict the reactivity of substrate with chloride ions. Photooxidation, photonu-other carbamates upon direct photolysis [110]. cleophilic substitution and photoreduction reactions were

However, it is worth noting that in aqueous solutions, observed. The amount of photoreduction products washydrolysis of carbamates may efficiently compete with higher than in distilled water, and the unstable tetrachloro-


muconic acid was isolated and identified as an intermediate Recently, Moza et al. [118] have revisited the photolysisring-fission photoproduct. of vinclozolin in water and methanol–water. Irradiation for

The photodegradation of MCPA h4-chloro-2- 10 min at 254 nm resulted in ¯90–95% degradation,methylphenoxy acetic acidj was studied in distilled water while photolysis for 8 h at 290 nm resulted in 10%and river (Thames) water, using low intensity 300–450 nm degradation, and irradiation for 8 h under artificial sunlightUV light and sunlight [54]. Upon UV irradiation, MCPA led to 55% degradation. The main photoproducts werephotodegradation was almost negligible in distilled water, 3,5-dichlorophenyl isocyanate and 3,5-dichloroaniline,while in river water it accelerated. Early autumn sunlight generated by ring opening of the 2,4-oxazolidine-dione.photolysis took place with a pseudo first-order rate con-

21stant k 5 0.009 h (t 5 168 h). 2 .10. Dinitroaniline pesticidesexp 1 / 2

192 .8. Cyclodiene insecticides F NMR was suggested as a possible analytical tool formonitoring the sunlight photodegradation of trifluralin

Mukerjee et al. [114] reported an amine-induced ha,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidinej wit-stereoselective method of photodehalogenation of aldrin hout extraction, cleanup, concentration or chromatographich(1R,4S,4aS,5S,8R,8aR)-1,2,3,4,10,10-hexachloro-1,4,4a,5, separation procedures [119]. The major photodegradation8,8a-hexahydro-1,4:5,8-dimethanonaphtalenej, in which products, identified by comparison with authentic samplesthey obtain a mixture of two photoproducts in the propor- were a,a,a-trifluoro-2,6-dinitro-N-propyl-p-toluidine,tion 9:1 upon irradiation with a high pressure Hg lamp. a,a,a-trifluoro-2,6-dinitro-p-toluidine and 2-ethyl-7-nitro-

5-(trifluoromethyl)benzimidazole. Different unidentified2 .9. Dicarboximide fungicides NMR peaks were attributed to labile intermediates not

generally observed with other analytical techniques.Schwack et al. [115] examined the photodegradation of Adityachaudhury et al. [34] investigated the transforma-

vinclozolin h3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3- tion of the herbicide pendimethalin hN-(1-ethylpropyl)-3,4-oxazolidine-2,4-dionej in various organic solvents, in an dimethyl-2,6-dinitrobenzenamine using UV light of l$

attempt to simulate the plant cuticle environment, using a 250 nm. They found that the main processes taking placehigh-pressure Hg lamp. Different photoproducts were are N-dealkylation, arylmethyl oxidation, nitro groupobserved depending on the solvent, indicating that the elimination and cyclization.formation of bound residues in plant cuticles is possible, Mansour et al. [46] studying the photolysis of pen-with the solvent molecules in the model reactions being dimethalin (vide supra) by UV light of different wave-replaced by constituents of plant waxes and the cutin lengths in water and water / soil suspensions, found variouspolymer. A similar study was carried out with iprodione conversion products, depending on the wavelength usedh3-(3,5-dichlorphenyl)-N-(1-methylethyl)-2,4-dioxo-1-im- for photolysis.idazolidinecarboxamidej [116]. UV and sunlight induced photolysis of fluchloralin hN-

Schwack et al. [55] analysed the potential of (2-chloroethyl)-2,6-dinitro-N -propyl-4-(trifluoromethyl)-procymidone h3-(3,5-dichlorophenyl)-1,5-dimethyl-3- anilinej in aqueous methanol (80%) yielded four photo-azabicyclo[3.1.0]hexane-2,4-dionej to undergo photoin- products, that the authors attribute to result from N-duced processes in the presence of different organic dealkylation, nitroreduction and cyclization pathways [56].groups. Their objectives were to seek explanations for the A theoretical framework has been presented to under-possible formation of bound residues in plant cuticles stand the combined effects of direct photolysis, layerwhere solvent, waxes and cutin polymer molecules are thickness and transport processes on the photodegradationpresent. A high pressure mercury lamp was used for of organic pollutants on soil surfaces under environmentalirradiation (l.280 nm). Photodegradation in isopropanol conditions, using a xenon long arc lamp, equipped withor cyclohexane led to photodehalogenation and substitution filters cutting out wavelengths below 280 nm and the IRby solvent molecules. Photodegradation in cyclohexene led [120]. The model has been used to explain the behaviourto substitution of Cl by a solvent molecule, the monode- of trifluralin (vide supra) on kaolinite layers of variablehalogenated procymidone resulting as photoproduct. thickness. The use of the appropriate experimental setup

Hustert and Moza [117] studied the photochemical and a mathematical model allowed the authors to de-degradation of procymidone (vide supra) and vinclozolin termine the rate constant for direct photolysis of trifluralin

21(vide supra) using a mercury lamp in the presence of soil as k 5 1.6 h . From this, it was shown that 20% of theconstituents in water. The photodegradation rate in the initial amount of trifluralin was degraded without transport.presence of humic and fulvic acids increased relative to the Miller et al. [57] developed a method based on asituation in which they are absent. Fe O and TiO solid-phase microextraction technique to determine the rate2 3 2

photocatalyzed the disappearance of both compounds. of sunlight (simulated) photodegradation of trifluralin (videDechlorination and isomerization are the major photo- supra) in the gas phase at high temperatures. The gas phasedegradation pathways. photolysis of trifluralin took place with half-lives of 22–24


min. The photodegradation rate was not affected by hO-aqueous solutions, thin films and n-hexane have beenchanging the temperature from 60 to 80 8C. known for some time [62]. Photoisomerization by light of

l.350 nm was observed in the S-benzyl esters IBP and2 .11. Diphenylether herbicides inezin, but not for the S-phenyl ester edifenphos. The

photoisomerization of IBP led to a reverse P=O↔P=SIn a study of the photodegradation of methanolic isomerization. The P=O derivatives of IBP and inezin

solutions of oxyfluorfen h2-chloro-1-(3-ethoxy-4-nitro- underwent photooxidation to yield the corresponding O-phenoxy)-4-(trifluoromethyl)benzenej, Adityachaudhury et benzyl esters. Cleavage of the P–S bond was observed,al. [34] found 13 photoproducts. leading to formation of phenylthio radicals, which sub-

Ying and Williams [58] studied the sunlight photodegra- sequently yielded benzoic acid or sulphuric acid viadation of the oxyfluorfen hvide supraj in detail on soil and disulphides and sulfonic acids. Reductive cleavage, photo-in water. The photoproducts detected showed that loss of hydrolysis and transesterification via radicals were alsothe nitro group, dechlorination and cyclisation were the observed.main processes taking place. The photodegradation was Mansour et al. [67] studied the sunlight induced photo-faster in water than in soil. degradation of parathion hO,O-diethyl O-4-nitrophenyl

phosphorothioatej in deaerated water, methanol, 2-pro-2 .12. Halobenzonitrile pesticides panol and aqueous mixtures of these. The authors suggest

that two main processes take place: a free-radical mecha-Kochany [59] studied the photodegradation of bromox- nism in apolar solvents and a ionic breakdown in aqueous

ynil h3,5-dibromo-4-hydroxyphenyl cyanidejin aqueous solvents. The quantum yield for generation of photo-solutions in the presence of carbonate and bicarbonate ions products was ¯0.1%. The half-life time for photodegrada-using from a mercury–xenon lamp (l.300 nm). A tion in water was ¯65 h at pH 7.8.quenching effect by carbonate was observed, as well as a Wamhoff et al. [122] studied the photodegradation ofsmaller quenching by bicarbonate. The main photoproducts azinphos-ethyl hO,O-diethyl-S-3,4-dihydro-4-oxobenzo[d]found were 3-bromo-4-hydroxybenzonitrile and 4-hy- [1,2,3]triazine-3-yl-methyl)phosphorodithioatej in chloro-droxybenzonitrile. Small amounts of other unstable or form and methanol solution using the radiation supplied byunidentified products were detected. The results suggest a high-pressure mercury lamp. 3,4-dihydro-3-methyl-4-that carbonate may be involved in the photodegradation of oxobenzo[d][1,2,3]triazine-3,4-dihydro-4-oxo-benzo[d]-bromoxynil, although the nature of the processes involved [1,2,3]triazine, O,O-diethyl-o(3-methylbenzo[d][1,2,3]-is not clear. triazine-4-yl)phosphate, N-methyl-anthranilic acid and sul-

The photodegradation has been studied of bromoxynil fur were found as photoproducts. The authors propose a(vide supra), chlorothalonil htetrachloroisophthalonitrilej, reaction mechanism to explain the formation of the ob-chloroxynil h3,5-dichloro-4-hydroxybenzonitrilej, dichlo- served products, starting either with a C–S or C–N bondbenil h2,6-dichlorobenzonitrilej and ioxynil h4-hydroxy- homolysis or with desulfuration by simultaneous C–S and3,5-diodobenzonitrilej in aqueous, pH-buffered and or- P–S bond cleavages. The intermediate radicals or otherganic solutions, using the radiation supplied by a xenon species have, however, not been detected.lamp [121]. The quantum yields for photolysis were, Photolysis of coumaphos ho,o-diethyl-o-(3-chloro-4-respectively, 0.0093, 0.0001, 0.0060, 0.0000 and 0.0024. methyl-coumarin-7-yl)-thiophosphatej by radiation of l.

Dichlobenil did not undergo photoreactions under the 313 nm in chloroform led to head-to-tail [212] regioselec-conditions used. Results indicate that the photoreactivity is tive photodimerization to yield the anti-dimer. The reactioninfluenced by the presence of both substituted halogen was not affected by singlet oxygen [63]. The authorsgroups and hydroxy substituents. established the structure of the compound by single crystal

The main processes involved in the UV photolysis (l$ X-ray diffraction, and concluded that the photoreactive250 nm) of the fungicide chlorothalonil (vide supra) in part of the molecule is the coumarin ring, with theethanolic and methanolic aqueous solutions seems to be thiophosphoryl group being relatively stable toward UVthe homolytic cleavage of 4-C–Cl bond followed by irradiation.alkylation of the a-radicals derived from the alcohols [56]. Esplugas et al. [123] studied the aqueous photodegrada-4,5,7-trichloro-6-cyano-3-methyl-1(3H )-isobenzofuranone tion of parathion (vide supra) using the UV radiationwas identified as photoproduct in ethanolic solution and supplied by a mercury lamp. The rate of the process4,5,7-trichloro-6-cyano-3,3-dihydro-1-isobenzofuranone in followed first-order kinetics, with the rate increasing withmethanolic solution. the pH of the medium. Very low quantum yields of less

than 1% were obtained, depending on the pH. Coloured2 .13. Organophosphorus pesticides compounds were obtained. The authors recommend carry-

ing out the photodegradation in alkaline medium, in orderThe UV-induced photoreactions of the fungicides IBP to improve the degradation yield.

hS-benzyl O,O-di-isopropyl phosphorothioatej, edifenphos The photochemistry of the herbicide butamifos hO-ethyl


O-5-methyl-2-nitrophenyl sec-butylphosphoramidothioatej possibly by one-electron abstraction on the sulfur atom tohas been studied using a 500-W xenon arc lamp [65]. An form a radical cation which would subsequently react withintramolecular oxygen transfer from the nitro group to the oxygen. 3-Methyl 4-methyl thiophenol was also formed byP=S moiety was observed. The nitroso-oxon derivative P–O bond cleavage. The photodegradation in carbonate /formed was photodegraded to various polar compounds. bicarbonate radical matrixes took place faster than byThe photodegradation of butamifos and its photoproducts direct photolysis.was found to be rapid and efficient. Miller et al. [57] developed a method based on the

The photodegradation of chlorpyrifos hO,O-diethyl O- analytical technique of solid-phase microextraction to3,5,6-trichloro-2-pyridyl phosphorothioatej and fena- determine the rate of sunlight (simulated) photodegradationmiphos hethyl 4-methylthio-m-tolyl isopropyl- of chlorpyrifos (vide supra) in gas phase at high tempera-phosphoramidatej by simulated sunlight in water /methanol tures.(2–4%) has been studied [64]. Fenamiphos degraded more The effect of visible and UV(A–C) light on fenthionreadily than chlorpyrifos. The major photoproducts were (vide supra) and disulfoton ho,o-diethyl S-2-ethylthioethyl3,5,6-trichloro-2-pyridinol and fenamiphos sulphoxide, re- phosphorodithioatej was studied. Visible light did notspectively. Under the same conditions, the extent of induced photodegradation, that took place with UV light.photodegradation of vamidothion hO,O-dimethyl S-2-(1- The photodegradation process was faster in aqueousmethylcarbamoylethylthio)ethyl phosphorothioatej after 6 solution than in solid-phase, probably due to the presenceh of irradiation was only 2%. the dissolved oxygen in the former. Under the same

´Barcelo et al. [83] studied the photodegradation of the conditions, fenthion (vide supra) is more readily degradedinsecticide fenitrothion hphosphorothioic acid O,O-di- than disulfoton (vide supra). The corresponding sulfoxidesmethyl-O-(3-methyl-4-nitro-phenyl) esterj in a 20% were identified as photolysis products, which are photo-methanolic solution, using a high-pressure mercury lamp. stable in environmental conditions [128].Attempts were made to use a suntest apparatus for the Recently a general review on the photochemical trans-photolysis, but the photodegradation rate was too slow. formations of organophosphorus insecticides has beenOxidation of the P=S group to trimethyl phosphate was the published, focusing on environmental problems and cover-main photolytic pathway, although products of solvolysis ing from the late 1960s to 2000 [129].and isomerization (O,O,S-trimethyl phosphorothioate andthe S-methyl isomer of fenitrothion) were also observed. A 2 .14. Oxadiazole herbicidesfurther tentative photodegradation mechanism was pro-posed. These authors subsequently revisited the photo- Ying and Williams [58] studied the sunlight photodegra-degradation of fenitrothion under similar conditions [124], dation of the oxadiazone h3-[2,4-dichloro-5-(1-methyl-and identified 21 photoproducts, arising from oxidation, ethoxy)phenyl] - 5 - (1, 1 - dimethylethyl) - 1, 3, 4-oxadiazol-isomerization, denitration and solvolysis. A tentative 2-(3H )-onej on soil and water. Three photoproducts werephotodegradation scheme was proposed. isolated and identified, showing that loss of chlorine was

Mok et al. [66] carried out a comparative study of the the main process taking place. The photodegradation wasquantum yields of direct photolysis of 16 different organo- faster in water than in soil.phosphorus pesticides and their phenolic derivatives in The aqueous photodegradation of oxadiazone (ronstar)aqueous solution. The solutions were irradiated at 254 and h5-tert-butyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-313 nm, and a significant correlation was found between oxadiazol-2(3H )-onej by sunlight takes place with a

21both sets of quantum yields. Similarly, a significant pseudo-first order rate constant 0.0245 h (t 528.5 h)1 / 2

correlation was found between the quantum yields of the [36].studied organophosphorus pesticides and those of theirphenolic derivatives. 2 .15. Phenol-based pesticides

Niessner et al. [125] studied the aqueous photodegrada-tion of parathion-methyl hO,O-dimethyl O-(4-nitro- The aqueous photodegradation of mecoprop h(R,S)-2-(4-phenyl)phosphorothioatej by UV light at a pilot plant for chloro-o-tolyloxy)propionic acidj by 254 nm and poly-drinking water treatment in the presence of O and using a chromatic light was studied. The main products were2

combination of O /O . The photodegradation rate in- found to be 2-methyl-phenol, 2-methyl-cyclohexadie-2,5-2 3

creased when using this latter combination with respect to ene-1,4-dione, 2-methyl-methoxybenzene, 2-methyl-1,4-O . dihydroxybenzene and 2-methyl-4-chlrophenol [130], sug-2

In two separate papers, Mabury and co-workers gesting a radical-mediated mechanism.?2[126,127] have recently described the role of the CO A fast photodegradation of aqueous solutions of 2,4-D3

radical anion in the sunlight induced degradation of h2,4-dichlorophenoxyacetic acidj by UV radiation has beenfenthion hO,O-dimethyl O-[3-methyl-4-(methylthio)- observed, with more than 99% of the starting materialphenyl] phosphorothioatej in natural waters. The major decomposing within the first hour of treatment [131]. Thedegradation pathway was through fenthion sulfoxide, authors suggest that the main processes taking place


involve C–Cl and C–O bond homolysis, with a subsequent light. The effect of added humic acids was lower than thatcascade of various free radical reactions. However, none of of river water components.these radical intermediates were detected. The photoreactions of potasan hO,O-diethyl-O-(4-

Climent and Miranda [132] studied by GC–MS and methyl-2-oxo-2H -1-benzopyran-7-yl)-phosphorothioatejGC–FTIR the aqueous photodegradation of dichlorprop have been studied upon UV irradiation by light of l.313h2-(2,4-dichlorophenoxy)propionic acidj and 2-naphtoxy- nm in chloroform and/or methanol alone and underacetic acid, using a medium-pressure mercury lamp. conditions where singlet oxygen may be present [69]. ThePhotolysis of dichlorprop under oxygen led to 2-chloro- main photoproducts found were 2-oxo-2H-1-benzopyran-phenol, 2,4-dichlorophenol, 4-chlorophenol, 2,4-dich- phosphate, 7-ethoxy-4-methyl-2-oxo-sH-1-benzopyran andlorophenyl acetate, 2-(4-chloro-2-hydroxyphenoxy)- a 3,39-bipotasan dehydrodimer. The authors propose apropionic acid and 2-(2-chlorophenoxy)propionic acid. mechanism involving homolytic cleavage of the 7-O–PUnder Ar, the first three of these products and the last one bond to generate intermediate radicals which further react.were also obtained, plus 2,4-dichlorophenyl ethyl ether. Inthe case of 2-naphtoxyacetic acid under oxygen atmos- 2 .18. Pyrazole insecticidesphere, b-naphtol and minor amounts of 2-hydroxy-1-nap-htaldehyde and naphtho-[2,1-b]furan-2(1H )-one were the The photodegradation of fipronil h(6)-5-amino-1-(2,6-observed products, while only the first and last were dichloro-a,a,a - trifluoro-p - tolyl) -4 - trifluoromethyl - sul-observed under Ar atmosphere. finylpyrazole-3-carbonitrilej has been studied in acidic

A mechanistic study of the photodegradation of the aqueous solution and on the surface of Niger and Mediter-lampricide (3-trifluoromethyl-4-nitrophenol) was carried ranean soils by the radiation supplied by a xenon lampout, with irradiation at 365 nm [68]. The main photo- [133]. The degradation in aqueous solution followed first-product found was trifluoroacetic acid. The mechanistic order kinetics, and the observed photoproducts were 5-study was extended to various trifluoromethylated phenols, amino-4-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-and it was shown that the nature of the substituents, the 4-phenyl)pyrazole and 5-amino-3-cyano-1-(2,6-dichloro-4-substitution pattern and the acidity of the medium strongly (trifluoromethyl)-phenyl)pyrazole-4-sulfonic acid, corre-affected the photodegradation process. The half-life times sponding to desulfinylation and oxidation processes, re-found for photodegradation were 22 and 91.7 h, at pH spectively. The authors suggest that the mechanism isvalues of 9 and 7, respectively, with corresponding yields probably radical in nature, the primary photoproduct is theof trifluoroacetic acid of 5.1 and 17.8%. Photodegradation corresponding sulfone and from there those products areof trifluoromethylquinone led quantitatively to trifluoro- formed by simultaneous reactions. The mechanism was notacetic acid. elucidated, however. The photodegradation in soils led to

the formation of 4-(trifluoromethyl)pyrazole, with the2 .16. Phenylamide pesticides degree of photodegradation being inversely proportional to

fipronil adsorption.The photolysis of the fungicides metalaxyl (vide supra), Crosby et al. [134] studied the photolysis of fipronil

benalaxyl hmethyl N-phenylacetyl-N-(2,6-xylyl)-DL-alani- (vide supra) in aqueous solution to produce desthio-fi-natej and furalaxyl hmethyl N-(2-furoyl)-N-(2,6-xylyl)-DL- pronil. The need for a sulfone intermediate has not beenalaninatej in water was studied with different UV lamps clarified, and the products of photodegradation not iden-emitting between 254 and 290 nm [45]. The authors tified. Desthio-fipronil was formed directly and underwentpropose a relatively complex mechanism involving two photodechlorination, substitution of chlorine by trifluoro-C–N bond scissions taking place via consecutive or methyl, and pyrazole ring cleavage.parallel /consecutive processes. The bond cleavage is pro-posed to take place homolytically, leading to neutral 2 .19. Pyrimidine pesticidesnitrogen-centered radicals, that have not been detected.

The metalaxyl hmethyl N-(2-methoxyacetyl)-N-(2,6- Mateus et al. [135] investigated the photophysics andxylyl)-DL-alaninatej photodegradation after 8 h was ob- photochemistry of the fungicide fenarimol ha-(2-chloro-served to increase by 6–20% [111,112]. phenyl)-a-(4-chlorophenyl)-5-pyrimidine methanolj. The

photophysical studies suggest that the lowest excited*2 .17. Phosphorothioate insecticides singlet state has predominantly n,p character, and a small

singlet–triplet splitting. Halides were found to quenchMansour et al. [46] studied the photolysis of diazinon fenarimol fluorescence. The photodegradation of fenarimol

hO,O-diethyl O-[6-methyl-2-(1-methylethyl)-4-pyrimi- seems not to involve dechlorination. More recently, thedinyl] phosphorothioatej by UV light of different wave- same authors studied the kinetics and mechanism oflengths in water and water / soil suspensions. Diazinon photodegradation of fenarimol in natural water and thedegraded faster in river water than in distilled water, and effect of salt solutions, using both sunlight and light of 313this rate enhancement was higher upon exposure to sun- nm [70]. The rate of photodegradation was found to


decrease with increasing salinity. Fluorescence quenching gated [137]. Photodegradation products were clearly iden-studies with halide and non-halide salts were carried out, tified, although hydrolysis was faster than photolysis.and the correlation of the so-obtained rates with halide ion Chowdhury and co-workers [56,138] studied the UV-oxidation potentials suggests involvement of an electron induced photodegradation of metsulfuron h2-(4-methoxy-6-transfer mechanism. This may be used to improve/ reduce methyl-1,3,5-triazin-2-yl-carbamoylsulfamoyl)benzoic acidthe fungicide photodegradability. in water. They found that ¯50% of the herbicide was

Adityachaudhury et al. [56] studied the UV (240–260 degraded within 15 h. They detected three photoproductsnm) and sunlight photolysis of fenarimol (vide supra) in and proposed a mechanism involving hydrolytic cleavageaqueous methanol and isopropanol solutions. Upon 24 h of of the sulfonylurea bridge to form the correspondingUV photolysis of methanolic solution, 2,49-dichlorobenzil phenyl sulfonyl carbamic acid and s-triazine, with theand p-chlorobenzoic acid were identified as photoproducts, carbamic acid subsequently decarboxylating to form awhile p-chlorobenzoic acid, 2,49-dichlorobenzophenone phenyl sulfonamide and a cyclic derivative.and o-chlorobenzoic acid were the photoproducts found in The aqueous photodegradation of tribenuron-methylaqueous isopropanol. Two additional unidentified photo- hmethyl 2 - [4 - methoxy - 6 - methyl - 1, 3, 5 - triazin - 2 - yl-products were formed upon sunlight photolysis. The (methyl-amino)carbonyl]benzoatej both by sunlight and byauthors suggest a mechanism for the process. the radiation supplied by a medium-pressure mercury lamp

Recently, Bhattacharyya et al. [72] revisited the aqueous was found to take place readily [72]. The main photo-alcoholic photodegradation of fenarimol (vide supra) by products detected were N-methyl-4-methoxy-6-methyl-light with l$250 nm. Irradiation in an aqueous metha- 1,3,5-triazine-2-amine, methyl 2-(aminosulfonyl) benzoate,nolic solution for a day yielded 2,49-dichlorobenzil and o-benzoic sulfimide, N-(4-methoxy-6-methyl-1,3,5-triazin-p-chlorobenzoic acid, while in an aqueous isopropanol 2-yl)-N-methyl urea and N-(2-carbomethoxyphenyl)-N-(4-solution p-chlorobenzoic acid, 2,49-dichlorobenzophenone methoxy-6-methyl-1,3,5-triazin-2-yl)-N9-methyl urea. Theand o-chlorobenzoic acid were found. The authors hypoth- photodegradation process followed first-order kinetics andesize possible mechanisms for these transformations, in- the reaction rate increased with pH and with the con-volving different C–C bond homolysis processes. centration of dissolved inorganic substances. The authors

claim that the main processes taking place are cleavage ofthe sulfonylurea bridge, scission of the SO –NH bond and22 .20. Sulfone herbicidesde-esterification and contraction of the sulfonylurea bridge.

Bufo et al. [73] studied the photolysis of rimsulfuronMing and Wamhoff [136] studied the photolysis of

h1-(4,6-dimethoxypyrimidin-2-yl)-3-(3-ethylsulfonyl-2-dimethipin h2,3-dihydro-5,6-dimethyl-1,4-dithi-ine 1,1,4,4-

pyridylfulfonyl)ureaj and of its commercial formulationtetraoxidej by 254 nm UV radiation in acetonitrile and

both in water and in organic solvents, using a high-methanol containing a small amount of water (¯0.5%).

pressure mercury lamp and solar simulator. The photolysisThe disodium salt of ethane-1,2-disulfonic acid was iden-

of the commercial formulation in organic solvents wastified as photoproduct resulting from a,a9-cleavage follow-

faster than for pure rimsulfuron. The half-life for sunlight*ing n → p excitation of both sulfonyl groups. 2-Butyne,photodegradation in water ranged from 1 day at pH 5 to 9

the other fragment which should be produced from suchdays at pH 9. The authors report that the rate of hydrolysis

fragmentation could not be identified, possibly, accordingis comparable to that of photolysis. The observed reaction

to the authors, due to its volatility and/or rapid poly-products are, therefore, a mixture of hydrolysis and

merization. Photolysis in acetone with longer wavelengthsphotolysis products.

(l.313 nm) led to the same photoproduct.Aqueous photodegradation of triasulfuron h1-[2-(2-

chloroethoxy)phenylsulfonyl] - 3 - (4 - methoxy - 6 - methyl-2 .21. Sulfonylurea herbicides 1,3,5-triazin-2-yl)ureaj by 254 nm radiation followed first

order kinetics [74]. The observed photoproducts were 2-The photodegradation of chlorimuron-ethyl hethyl 2-(4- chloroethoxybenzene and (4-methoxy-6-methyl-1,3,5-tri-

chloro-6-methoxy-pyrimidin-2-yl -carbamoylsulfamoyl)- azin-2-yl)urea. Upon sunlight photolysis the reaction wasbenzoatej by sunlight and UV light has been studied on a slower, yielding the same photoproducts plus 2-amino-4-soil surface [71]. The major photoproducts arise from methoxy-6-methyltriazine and 2-(2-chloroethoxy)ben-cleavage of the sulfonylurea bridge, but some others are zenesulfonamide. The authors suggest a mechanism toformed by dechorination, hydrolysis and cyclization. The account for the observed photoproducts.photodegradation follows first-order kinetics, with theprocess being much faster upon UV irradiation than upon 2 .22. Thiocarbamate herbicidessunlight irradiation.

The environmental fate of flupyrsulfuron-methyl hmethyl The photodegradation of EPTC hS-ethyl dipro-2-(4,6-dimethoxy-pyrimidin-2-yl-carbomoylsulfamoyl)-6- pylthiocarbamatej, PEBC hS-propyl butyl(ethyl)thiocar-trifluoro-methylnicotinate monosodium saltj was investi- bamatej and cycloate hS-ethyl cyclohexyl(ethyl)-


thiocarbamatej by UV light supplied by a medium pressure thioamides and thioureas. Mechanisms involving radicalsmercury lamp in hexane has been studied [139]. The derived from C–S, S–S– and S–H bond homolysis haveobserved photoproducts were the corresponding form- been put forward.amide, dialkylamine, mercaptan and disulfide. The authors Thiobencarb (vide supra) photodegrades readily uponhypothesise a mechanism taking place via C–S bond sunlight or UV (254 nm) photolysis [78]. The photodegra-homolysis, with subsequent recombination and H abstrac- dation by UV light takes places much faster. It has alsotion from the solvent to yield, respectively, a disulfide and been observed that the photodegradation is faster ina formamide. The observed disulfides could result from the aqueous solution (t 51.5 h with UV light, 3 days under1 / 2

combination of two sulfur-centered radicals. The sunlight) or on glass (t 51 h) or silica gel surfaces1 / 2

dialkylamines found are probably due to the photolysis of (t 52 h) than on a soil surface (t 520 h). The major1 / 2 1 / 2

the formamides, with loss of CO, and not to the direct photoproducts found were 4-chlorobenzyl alcohol, 4-chlo-cleavage of the thiocarbamates. No effects of the con- robenzaldehyde and 4-chlorobenzoic acid. A tentativecentration of herbicide or temperature were observed. The mechanistic scheme is proposed, including a C–S bondauthors note the increased biotoxicity of the photoproducts, cleavage process to form a 4-chlorobenzyl radical thatrelative to the starting herbicides. further oxidizes successively to 4-chlorobenzyl alcohol,

Photolysis of thiobencarb hS-4-chlorobenzyl dieth- 4-chlorobenzaldehyde and 4-chlorobenzoic acid. Oxidationylthiocarbamatej at 300 nm in aqueous solution or in the at the S atom would yield thiobencarb sulfoxide andform of a thin film led to 30 photoproducts, the most thiobencarb sulfone, which further decompose to theabundant of which were thiobencarb sulfoxide, N-desethyl corresponding sulfinic and sulfonic acids, leading eventual-thiobencarb, N-acetyl thiobencarb, 4-chlorobenzyl dieth- ly to 4-chlorobenzoic acid.ylcarbamate, N-(4-chlorobenzyl)diethylamine and 4-chlo- Vialaton and Richard [108] carried out a 266-nm laserrobenzaldehyde [75]. The authors propose a mechanism flash photolysis study of the photolysis of thiobencarbinvolving photocleavage of the C–S bond to generate the (vide supra). A quantum yield of 14% was estimated for4-chlorobenzyl radical, which leads to 4-chlorotoluene in the photoconversion of thiobencarb into 4-chloroben-the presence of hydrogen donors. These authors also zaldehyde, 4-chlorobenzylalcohol and 2-methylphenyl-studied the photodegradation of diallate hS-2,3-dimethylcarbamate, with the first one being formed bychloroallyl diisopropyl thiocarbamatej, finding it to be homolytic C–S bond cleavage, and the last two bymore photostable than thiobencarb, reacting mainly by heterolytic C–S bond cleavage. The authors suggest thecis–trans isomerization and via radicals formed at the mechanisms involved electron transfer from a tripletN-alkyl and S-dichloroallyl moieties. excited state to oxygen, yielding a radical cation, which

The photochemical degradation of thiram htetra- then cleaves to yield the benzaldehyde derivative.methylthiuram disulfidej by the action of sunlight and UV(l.290 nm) was studied in natural waters of Northern

2 .23. Triazine pesticidesGreece [76]. Two photoproducts were detected, but couldnot be identified. However, and in contrast with a previous

Pape and Zabik [142] studied the 254-nm photolysis ofreport [140], it was shown that these were not ethyl-2 4enethiourea or ethylenethioammonium. atrazine h6-chloro-N -ethyl,N -isopropyl-1,3,5-triazine-

2 4The photodegradation of captan hN-(trichloro- 2,4-diaminej, propazine h6-chloro-N ,N -di-isopropyl-methylthio)cyclohex-4-ene-1,2-dicarboximidej by the UV 1,3,5-triazine-2,4-diaminej, simazine h6-chloro-N,N9-dieth-radiation (l.280 nm) supplied by a high-pressure mer- yl-1,3,5-triazine-2,4-diaminej, atraton h2-ethylamino-4-iso-

2 4cury lamp was shown to be more efficient in cyclohexene propylamino-6-methoxy-s-triazinej, prometon hN ,N -di-and isopropanol than in cyclohexane [77]. The photolysis isopropyl-6-methoxy-1,3,5-triazine-2,4-diaminej, simetonprocess takes place via C–Cl bond homolysis. If sufficient hN,N9-diethyl-6-methoxy-1,3,5-triazine-2,4-diaminej, ame-hydrogen donors are present N-(dichloromethylthio)-cis-4- tryn h2-ethylamino-4-isopropylamino-6-methythio-s-tri-

2 4cyclohexene-1,2-dicarboximide is formed, and then photo- azinej, prometyn hN ,N -di-isopropyl-6-methylthio)-1,3,5-decomposes, losing thiophosgene to yield tetrahydro- triazine-2,4-diaminej and symetryn hN,N9-diethyl-6-phtalimide. (methylthio)-1,3,5-triazine-2,4-diaminej in water and in the

The UV photolysis of sodium diethyldithiocarbamate, solvents: methanol, ethanol, n-butanol and benzene. Thethiram (vide supra), captan (vide supra), captafol hN- aqueous photolysis led to the corresponding 2-hydroxy-(1,1,2,3-tetrachloroethylthio)cyclohex-4-ene-1,2-dicarbox- triazines, the photolysis in methanol to the 2-methoxy-imidej and folpet hN-[(trichloromethyl)thio]phtalimidej has triazines, etc. The yields observed were high (¯85–90%).been studied under N atmosphere, photo-oxidation in the Exceptions to this were atraton, prometon and simeton,2

presence of oxygen, and by photosensitized oxidation which did not undergo photolysis in methanol, ethanol orusing a 100-W tungsten lamp, also under oxygen atmos- water. No photochemical reactions were observed atphere [141]. Various photoproducts have been identified, wavelengths longer than 300 nm. In a subsequent paper,

22including SH , S , SO , SO , CS , amines, hydrazines, these authors studied the photolysis of 2-fluoro- and, 2-2 8 2 4 2


bromo-4,6-bis(ethylamino)-s-triazine, 2-iodo-triazine ana- discussed [149]. The efficiency of atrazine removal de-logues of atrazine, propazine and simazine and 2-azido-4- pended only on the UV radiation input. Addition of H O2 2

ethylamino-6-methylthio-s-triazine in water, methanol, improved the efficiency of photodegradation, but requiredethanol and n-butanol [143]. From considerations of higher UV doses. The main photoproduct identified wasthermal, photochemical and spectroscopic data, the authors hydroxyatrazine.suggest the involvement of a Chugaev-type cyclic transi- Niessner et al. [125] studied the aqueous photodegrada-tion state in the dealkylation of s-triazines. tion of atrazine (vide supra) by UV light in the presence of

Atrazine (vide supra) has been showed to photo- O and using a combination of O /O at a pilot plant for2 2 3

dehalogenate in aqueous solution, with subsequent photo- drinking water treatment. The photodegradation rate wasdealkylation of the hydroxylated compounds to the mono- not enhanced by the last combination with respect to O .2

and nonalkylated species [144]. However, in experiments carried out at large scale in fieldMamaev et al. [145] found that nitro-1,3,5-triazines may conditions the degradation of atrazine and desethylatrazine

be obtained by photooxidation of azido-1,3,5-triazines by improved with increasing O dose and increasing radiant3

atmospheric oxygen. power.A laser microprobe analysis, using 266 nm laser light, Similarly, Lai et al. [150] studied the photooxidation of

was applied to study the photodegradation of atrazine (vide simazine (vide supra) by UV radiation in the presence andsupra), propazine (vide supra) and simazine (vide supra) absence of O . Simazine disappearance increased with UV3

[146]. The photoproducts were detected using time-of- intensity, yielding dechlorination byproducts. In contrast,flight mass spectrometry, and the results showed that the the combination of UV radiation with O led to the3

main process is ring-opening, with or without chlorine generation of dealkylation, deamination and dechlorinationexpulsion. byproducts.

´Dore et al. [130] studied the aqueous phototransfor- The UV photolysis of two products of photodegradationmations of atrazine by 254 nm and polychromatic light. of atrazine (vide supra), namely deethylatrazine and de-The main photoproducts found upon UV photodegradation, isopropylatrazine was studied [151]. Significant reductionsidentified by their mass spectra, were hydroxy atrazine, in the rate of photodegradation were observed in surface6-chloro-N-isopropyl-2,4-diamino-1,3,5-triazine, 6-chloro- waters, which the authors attribute to the presence ofN-ethyl-2,4-diamino-1,3,5-triazine, 6-hydroxy-amino- natural radical scavengers. No attempts were made to1,3,5-triazine, ammeline, (N,N9)-isopropyl-2,4-diamino- identify by-products of these photolysis reactions.1,3,5-triazine, 6-chloro-2,4-diamino-1,3,5-triazine and 6- The photodegradation of ametryn (vide supra), atratonhydroxy-N-ethyl-2,4-diamino-1,3,5-triazine. The authors (vide supra), atrazine (vide supra), propazine (vide supra),propose a complex mechanism including oxidations, simazine (vide supra) and terbuthylazine h2-chloro,4-tert-dealkylations and hydrolyses. The authors also suggest a butylamino,6-ethylamino,(1,3,5)-triazinej by the radiationsignificant involvement of radicals in these processes. supplied by a xenon lamp were investigated in aqueous

Recently, using a medium pressure mercury lamp, and buffered solutions (pH 7–9) containing a small´Hequet et al. [147] estimated the half-live of atrazine percentage of acetonitrile [152]. The quantum yields for

around 5 min, the corresponding 2-hydroxy-atrazine being photodegradation were measured, and comparable valuesthe major photoproduct. (¯5%) found. A temperature dependence was observed

´Barcelo et al. [83] studied the photodegradation of the for the photodegradation of terbuthylazine, which allows21herbicide propazine (vide supra) in aqueous solution, using the estimation of an activation energy ¯13 kJ mol .

a suntest apparatus. According to the authors, the main Atraton showed a very low photodegradation quantumphotodegradation pathways correspond to dealkylation, yield, ¯0.2%, which shows that it can be considered not tohydroxylation and dehalogenation, leading to deiso- be significantly photodegradable under the conditions used.propylpropazine (deethylatrazine), hydroxypropazine and The photodegradation quantum yields in acetonitrile and2-H analogues. A tentative photodegradation mechanism is hexane for terbuthylazine are about one half of the valuesproposed. observed in water. From the observed data, half-lives for

The photodegradation of cyanazine h2-(4-chloro-6- these pesticides under sunlight can be estimated, rangingethylamino-1,3,5-triazin-2-yl-amino)-2-methylpropionitrilej from 39 h to 3 months.was studied in an aqueous solution bubbled with an The photodegradation of terbuthylazine (vide supra) atoxygen stream, using a low pressure mercury lamp emit- 254 nm and with simulated sunlight (l.290 nm) has beenting monochromatically at 254 nm [148]. The observed studied [153]. The corresponding 2-hydroxylated deriva-photodegradation quantum yields ranged from 0.05 to tive is generated in a few minutes, and then undergoes a0.10%, at temperatures between 10 and 40 8C. The authors slow dealkylation to ameline h2-hydroxy,(4,6)-diamino,hypothesize the existence of a free-radical chain reaction (1,3,5)-triazinej.mechanism for the photodegradation process. Mansour et al. [46] studied the photolysis of metamitron

The use of an industrial-scale UV chamber to reduce the h4-amino-3-methyl-6-phenyl-1,2,4-triazine-5(4H )-onej andamount of atrazine in a drinking water supply has been terbuthylazine (vide supra) by UV light of different


wavelengths in water and water / soil suspensions. supra) and simazine (vide supra) in river water at differentMetamitron degraded rapidly, yielding desaminometamit- pH values and contents of dissolved organic carbon, alsoron, which was much more stable. In the case of ter- with buffered samples, under the artificial light supplied bybuthylazine, an important enhancing effect was observed fluorescent tubes (300,l,400 nm), and upon sunlightfor humic acids, leading to products of oxidation of side photolysis. Photodegradation was initiated by a wide rangechains, such as deethylterbuthylazine. or wavelengths at pH 4, but only wavelengths lower than

The photodegradation of five non-commercial unsatu- 300 nm were able to initiate it in near-neutral media. Onrated triazines with pesticide activity was investigated in this basis, and considering the rates of hydrolysis of thesewater, using a mercury lamp [154]. It was observed that pesticides at different pH values, the author estimates thethe photodegradation efficiency was enhanced by the half-life of the studied pesticides in acidic upland waters aspresence of unsaturated substituents. nearly a week, some months for lowland river and years

Albanis and Konstatinou [92], when studying the photo- for groundwaters.degradation of atrazine (vide supra), propazine (vide supra) The antifouling agent Irgarol h2-methylthio-4-tert-and prometryn hN,N9-bis(1-methylethyl)-6-(methylthio)- butylamino-6-cyclopropylamino-s-triazinej is rather stable1,3,5-triazine-2,4-diaminej, found the half-life of the com- toward hydrolysis, but undergoes sunlight photodegrada-pounds to be ¯1–2 months in water and ¯1 month in tion in aqueous solution [80], the process being muchsoils. The presence of humic substances produced a faster in natural waters. Three main photoproducts werereduction of the photodegradation rate in water and an observed, with two of them being stable even after 6enhancement in soil. The major photoproducts found were months irradiation.the hydroxy and dealkylated derivatives. Lin et al. [39] studied the effect of simulated sunlight on

The photodegradation of metribuzin h4-amino-6-tert- atrazine (vide supra) toxicity of surface waters, concludingbutyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-onej in that the toxicity of the herbicide is reduced, possiblywater was studied using a low pressure mercury lamp through photodegradation. The observed reduction of[155]. Desaminometribuzin, diketometribuzin and de- toxicity was higher in bay water than in fresh water.saminodiketometribuzin were identified as photoproducts, Breithaupt and Schwack [158] studied the photoinducedplus six other unknown metabolites resulting from side- addition of the fungicide anilazine h4,6-dichloro-N-(2-chlo-chain degradation, some of which were very stable toward rophenyl)-1,3,5-triazin-2-aminej to cyclohexene andphotolysis. Inorganic and organic ions, such as mono- methyl oleate upon irradiation with a metal halogen lamp.carboxylic, dicarboxylic and ketocarboxylic acids were Both olefinic compounds were used as models for plantalso formed by side-chain degradation and ring cleavage. cuticle components. Extensive reaction of anilazine with

Mansour et al. [81] studied the photodegradation of the cis-double bond of both olefins was observed. Theterbuthylazine (vide supra) in distilled water and in soil addition is considered to be radical-mediated, the initialusing simulated sunlight (l$290 nm), in the presence and step being suggested as C–Cl bond cleavage. The conclu-absence of humic acids. The half-life time in water, sion is drawn that photochemical reactions within the plantwithout and with humic acids added were, respectively, cuticle can lead to the formation of bound-residues of¯23 and 8 h. The photodegradation process followed anilazine with unsaturated biomolecules.pseudo-first kinetics. Dealkylation was the main degra- Detailed studies of the first stages of photodegradationdation pathway, the main photoproducts being desethylter- of a variety of triazines using laser flash photolysisbuthylazine and desisobutylterbuthylazine. combined with pulse radiolysis showed the photodegrada-

The photodecomposition of 4-amino-1,2,4-triazin-3,5- tion is monophotonic when 193 nm light is used fordiones and 4-amino-1,2,4-triazin-thiones in oxygenated excitation. It was not possible to observe the generation ofaqueous solutions at different pH values were studied by intermediates in the nanosecond time-scale using higherRaschke et al. [156]. Deamination, decarboxylation and wavelengths (222, 248, 266 nm). The photodegradation,dealkylation reactions were observed. Dealkylation was a however, certainly takes place and has been observed whenslow and non-selective process. The details of the pro- samples were exposed to 254 nm steady-state photolysis,posed mechanisms were discussed. and the some major photoproducts identified by HPLC,

A laser flash photolysis study of the photodegradation of comparing them with authentic samples. Photoionizations-triazine, ametryn (vide supra), desmetryn h2-iso- was observed for all compounds when photolysis waspropylamino-4-methylamino-6-methylthio-s-triazinej, pro- carried out at 193 nm, with very low quantum yields. Themetryn (vide supra) and terbutryn h2-tert-butylamino-4- rate constants for the one-electron oxidation of the tri-

?2 2 ?ethylamino-6-methylthio-s-triazinej by 193 nm UV light azines with SO , one-electron reduction with e and HO4 aq

showed all the compounds underwent monophotonic addition were obtained, using both the laser flash photo-?photoionization, with yields less than 10%. No significant lysis and the pulse radiolysis techniques. The fact that HO

photoionization was observed when using longer wave- undergoes addition to the aromatic ring, and not one-lenghts [157]. electron oxidation, has been proved by experiments per-

2Comber [82] studied the photolysis of atrazine (vide formed with conductance detection. The one-e reduction


2with e has also been studied, and the corresponding rate phenoxymethyl)-1,2,4-triazole, 1-(4-chlrophenoxy)-2,2-di-aq

constants obtained. Triazines have been shown to quench methyl-1-(1,2,4-triazol-1-yl)propane, 1-(1,2,4-triazol-1-21the fluorescence of excited UO . The rate of decay of the yl)-3,3-dimethylbutan-2-one and 1-(4-chlorophenoxy-3,3-2

laser flash photolysis generated uranyl in the presence and dimethyl-1-(1,2,4-triazol-1-yl)butan-2-ol [163]. The ob-absence of triazines has been measured. All these results served reactions followed first order kinetics. The authorsallow to confine the one-electron reduction potential of suggest the main processes taking place upon photolysis

?1triazines to the interval 2.4 , E8(T /T ) , 2.7 V vs. NHE. are C1 to C2 and C1 to triazole bond cleavage, hydrolysis,The observed photoproducts, detected using HPLC, have reduction and extrusion of C=O.been studied upon irradiation at 254 nm, obtaining mainly Murthy et al. [85] studied the photodegradation ofthe 2-hydroxy derivative and, in minor amounts, photo- triadimefon (vide supra) and epoxiconazol h(2RS,3SR)-1-products dealkylated in the side chains bind to the amino [3-(2-chlorophenyl)-2,3-epoxy-2-(4-fluorophenyl)-propyl]-groups. Structure–reactivity correlations have been estab- 1H-1,2,4-triazolej under simulated sunlight, using a xenonlished in order to predict the reactivity of other triazines suntest lamp. Triadimefon degraded to 21% in 72 h, andupon direct photolysis [157,110,159–161]. epoxiconazol to 12% in 120 h. When the same pesticides

were photolyzed in soil containing 6% moisture theobserved levels of photodegradation were, respectively, 37

2 .24. Triazinone herbicidesand 56% in 72 h. p-Chlorophenol was identified asphotoproduct upon photodegradation of triadimefon.

The aqueous photodegradation of metamitron h4-amino-The photophysics and photochemistry of triadimefon

3-methyl-6-phenyl-1,2,4-triazin-5(4H )-onej by UV radia-h1-(4-chlorophenoxy)-3, 3-dimethyl-1-(1H-1, 2, 4-triazol-1-

tion from a xenon lamp and by sunlight has been studiedyl)butan-2-onej and triadimenol h1-(4-chlorophenoxy)-3,3-

[84]. The study included UV and phosphorescence spec-dimethyl-1-(1H-1,2,4-triazol-1-yl)butan-2-olj in solution

troscopy, determination of quantum yields at different pHhave been studied. 4-Chlorophenol and 1,2,4-triazole were

values and determination of the photoproducts. No reactionidentified in both cases as major photodegradation prod-

was observed in methanol, acetonitrile and hexane, or inucts. The photodegradation quantum yield of triadimefon

water in the dark. The photodegradation products found inin cyclohexane at 313 nm is 0.022, whereas triadimenol is

aqueous solution were deamino metamitron and an un-reported as photostable under the same conditions. Flash

identified compound. The triplet energy was obtained byphotolysis experiments showed the presence of 4-chloro-21phosphorescence spectroscopy (E ¯245 kJ mol , to beT phenoxyl radicals, pointing to a mechanism via homolytic21compared with E ¯325 kJ mol ). This does not allow toS cleavage of the C–O bond of the asymentric carbon [164].

discard a possible reaction involving the triplet state. Inview of its pK and the fact that the photodegradationa

quantum yields at pH 4 and 9 were comparable (¯1.6%), 2 .26. Urea-based herbicides?the participation of the hydroperoxyl radical (HO ) cannot2

explain the photodegradation of metamitron. The photo-Metobromuron h3-p-bromophenyl)-1-methoxy-1-methyl-

degradation quantum yield at pH 7 was a factor of 2ureaj was used as a model compound to study the sunlight

higher, which the authors attributed to the influence of thephotodegradation of urea-based herbicides [86]. Upon

phosphate buffer used. The photodegradation of metamit-exposure to sunlight for 17 days, the major photoproduct

ron is strongly dependent on the presence of oxygen. Thefound was 3-( p-hydroxyphenyl)-1-methoxy-1-methylurea.

mechanism of generation of the photoproducts is unclear.Minor products were 3-( p-bromophenyl)-1-methyurea andp-bromophenylurea, with experimental evidence for the

2 .25. Triazole fungicides existence of a dimerization product.The sunlight photolysis of linuron h3-(3,4-dich-

In the presence of humic acid the photodegradation of lorophenyl)-1-methoxy-1-methylureaj and monuron h3-(4-triadimefon h1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H- chlorophenyl)-1,1-dimethylureaj was studied [87]. After 21,2,4-triazol-1-yl)butan-2-onej changed only from 87 to months exposure, linuron yielded 13% 3-(3-chloro-4-hy-91% after 4 h [111,112]. droxyphenyl)-1-methoxy-1-methylurea, 10% 3,4-di-

The photodegradation of folicur h1-(4-chlorophenyl-4,4- chlorophenylurea and 2% 3-(3,4-dichlorophenyl)-1-dimethyl-3-(1H-1,2,4-triazole-1-yl)methyl)-pentan-3-olj methylurea. Upon photolysis of monuron, 3-( p-hydroxy-by UV radiation has been studied in benzene, ether, phenyl)-1,1-dimethylurea was identified as photoproduct.methylene chloride, methanol [162]. Two photoproducts Kotzias and Korte [88] reviewed the photochemistry ofhave been observed and identified by chromatography/ phenylurea herbicides and their environmental reactions.spectroscopy and direct comparison with authentic sam- The authors conclude that substituted phenylurea her-ples. bicides are degraded by UV and sunlight, the degradation

The photodegradation of triadimefon (vide supra) by 254 being influenced by the presence of oxygen. H abstractionnm light on glass and soil surfaces led to 1-(4-chloro- is the main process in methanol, phenols are the main


photoproducts in water, while C–C bond scission takes replacement of halogen substituents by OH groups, (iii)place in benzene. replacement of halogen substituents by H atoms.

´Tanaka et al. [35] studied the generation of biphenyls Jirkovsky et al. [90] studied the photolysis of diuronupon sunlight photolysis of monuron (vide supra), diuron h3-(3,4-dichlorophenyl)-1,1-dimethylureaj using varioush3-(3,4-dichlorophenyl)-1,1-dimethylureaj, linuron (vide different wavelengths between 254 and 350 nm. Accordingsupra) and metobromuron h3-(4-bromophenyl)-1-methoxy- to their results, the major photoproducts resulted from a1-methylureaj. The photoreactions proceeded via coupling photoheterolysis process followed by substitution of Cl byof two herbicide molecules, leading to a chlorinated OH. Different photoproducts were observed depending onbiphenyl. The yields of biphenyls observed were very low, the wavelength used during the photolysis: 3-(4-chloro-3-suggesting a high photolability that would lead to their fast hydroxyphenyl)-1,1-dimethylurea was the major productdegradation, such that they would only accumulate to a when photolysing with 254 nm, while 3-(3-chloro-4-hy-very low extent. droxyphenyl)-1,1-dimethylurea was the main one when

The photodegradation of 1,3-dimethyl-1-(2-(3-fluoro- using 365 nm light.benzylthio)-1,3,4-thiadiazol-5-yl)urea has been studied in Mansour et al. [46] investigated the photolysis ofmethylene chloride using the UV radiation provided by a isoproturon (vide supra) by UV light of different wave-mercury lamp has been studied in a thin film and in lengths in water and water / soil suspensions. Irradiation ofsolution [165]. Two photoproducts were found and char- isoproturon in the presence of organic matter shows thatacterized. The rearrangements involved an S to N benzyl photolysis in inert atmosphere is faster, suggesting thatmigration, followed by a sulfur–oxygen substitution. molecular oxygen might act as a quencher.

Tanaka et al. [89] also studied the photolysis of diuron The photolysis of chlorotoluron h3-(3-chloro-p-tolyl)-h3-(3,4-dichlorophenyl)-1,1-dimethylureaj using UV lamps 1,1-dimethylureaj in acetonitrile, hexane and aqueousand natural sunlight. Seven photoproducts, with similar solutions buffered to pH 4, 7 and 9, and of isoproturonyields, were identified in both cases. (vide supra) in aqueous and buffered solutions by the

A study of the photodegradation of diuron h3-(3,4- radiation supplied by a Xe lamp were investigated [169].dichlorophenyl)-1,1-dimethylureaj has been reported using Both herbicides were degradable by l,240 nm, butsimulated sunlight in distilled water and artificial seawater photostable when l.240 nm. In the case of chlorotoluroncontaining humic acids [166]. The main photoproduct was in aqueous solution, the main photoproduct found wasmonuron h3-(4-chlorophenyl)-1,1-dimethylureaj, a product 3-(3-hydroxy-4-methyl-phenyl)-1,1-dimethylurea, whileof dechlorination. A quenching effect was observed by the the quantum yields for photodegradation were ¯0.07 inchloride ions present in seawater, leading to retardation of aqueous and buffered solutions, ¯0.035 in acetonitrile,the photodegradation. and 0.70 in hexane. In the case of isoproturon the quantum

The photoreduction of 1-(2-chlorobenzoyl)-3-(4-chloro- yield obtained for photodegradation was ¯0.004 in allphenyl)urea in methanol was studied using a Xe lamp solutions.[167]. The observed process followed in all cases first- The photodegradation of hexaflumuron h1-[3,5-dichloro-order kinetics, with rate constants in O atmosphere greater 4-(1,1,2,2,-tetrafluoroethoxy)phenyl]-3-(2,6-difluoroben-2

than in N atmosphere. The main photoproducts found zoyl)ureaj, teflubenzuron h1-(3,5-dichloro-2,4-difluoro-2

were 2-chlorobenzamide, N-phenylmethylcarbamate, N-(4- phenyl)-3-(2,6-difluorobenzoyl)ureaj and diflubenzuronchlorophenyl)methylcarbamate and 4-chlorophenyl urea. hN - [((4-chlorophenyl)amino)carbonyl] - 2, 6 - difluoroben-

´Dore et al. [130] studied the aqueous photodegradation zamidej in river water buffered to pH 9.0 was studiedof isoproturon h3-(4-isopropylphenyl)-1,1-dimethylureaj [170]. Hexaflumuron was the most rapidly degraded, withby 254 nm and polychromatic light, and found N-(4- a half-life of 8.6 and 5.0 h at pH 7 and 9, respectively.isopropyl-hydroxyphenyl)-N9-methylurea, N-(4-isopropyl- Diflubenzuron was the slowest to degrade, with half-lifephenyl)-N9-dimethylurea and N-(4-isopropyl-phenyl)-N9- times of 17 and 8 h at pH 7 and 9. Dark controls showedacetamide as main photoproducts. The authors propose a no loss of the parent compounds over similar time periods.complex mechanism involving hydroxylation, demethyla- The initial stages of photodegradation of various urea-tion and deamination reactions, possibly with participation based herbicides has been studied using laser-flash photo-or radical intermediates. lysis and pulse radiolysis. Radical cations are generated

The photochemical degradation of nine different urea- immediately after the laser pulse, and deprotonate to yieldbased herbicides in aqueous solution by UV light supplied neutral radicals. The quantum yield of 193 nm photoioni-

2by a low pressure mercury lamp was studied, and the main zation are ¯10%. Pulse-radiolytic one-e oxidation with?2photoproducts identified [168]. The quantum yields for SO also yields the radical cation and, after deprotona-4

photodegradation ranged from 0.5% for fenuron h3-phenyl- tion, the neutral radical. Semiempirical simulations of1,1-dimethylureaj to 13% for monuron (vide supra). The UV–Vis spectra support the experimental observations. Aobserved photoproducts were formed through three differ- bathochromic displacement of the bands of the radical

2ent pathways: (i) cleavage of the N–O bond in the case of cation is observed as the number of e -withdrawing ringpesticides containing an N-methoxy moiety, such as substituents increases. The extinction coefficients of thelinuron (vide supra), with formation of formaldehyde, (ii) radical cation and neutral radical are, respectively, ¯3000


21 21and ¯2000 M cm . The pK for deprotonation of the photolysis reaction took place at about the same rate ina

radical cation is ¯0. The reaction with hydroxyl radical seawater and in distilled water. Photooxidation, photonu-has also been studied, both by laser flash photolysis and cleophilic substitution and photoreduction reactions arepulse radiolysis, and the rate constants obtained. The observed. The amount of photoreduction products wasphotoproducts observed by GC–MS and HPLC, and in higher than in distilled water.some cases checked against authentic samples, are the The photolysis of the fungicide and bactericide 2-mer-corresponding anilines and the photo-Fries products. Fur- captobenzothiazole, and of the p-isoelectronic 2-mercap-thermore, the rate constants for one-electron reduction with tobenzimidazole by UV radiation in benzene, toluene,

2 ?e and for HO addition have been obtained. Structure– methanol, ethanol and acetonitrile by UV radiation hasaq

reactivity correlations have been established in order to been studied [176]. The thiazole derivative yielded bis-(2-predict the reactivity of other ureas upon direct photolysis benzothiazolyl)disulfide in benzene and toluene and bis-(2-[171,110,172,173]. benzothiazolyl)disulfone and eventually benzothiazole sul-

phate when methanol, ethanol and acetonitrile were the2 .27. Miscellaneous solvents. The imidazole led to bis-(2-benzimidazolyl)disul-

fone and benzimidazole sulfate when methanol, ethanolNeihof et al. [60] evaluated the sunlight photodegrada- and acetonitrile were used as solvents.

tion of the biocides mercaptopyridine-N-oxide, sodium-2- Jensen-Korte et al. [177] studied the aqueous photo-mercaptopyridine-N-oxide, 2-(tert-butylamine)-mercap- degradation of various types of pesticides: carbamates,topyridine-N-oxide and 2,29-dithio-bis-(pyridine-N-oxide) phosphates, pyrethroids, triazoles and ureas using a highin artificial seawater and in aged natural seawater sterilized pressure mercury lamp. In all cases the photodegradationby autoclaving. The biocide activity dramatically reduced rate increased in the presence of humic substances.after 5–10 h of exposure to sunlight. UV spectral measure- Irradiation by 313 nm light supplied by a high-pressurements showed that pyridine-N-oxide-2-sulfonic acid was mercury lamp of 1,3-dihydro-2H-benzimidazole-2-thione,an early photoproduct. An unidentified insoluble photo- 2(3H )-benzothiazolethione and 2-chlorobenzothiazole hasproduct was also observed. According to the authors, been studied in the presence of oxygen in 1% acetonitrile,temperature, pH and salinity were not relevant variables. ethanol or benzene solutions. The main photoproductsThe rates of loss of toxicity were lower in the presence of were disulfides, 2-oxo-derivatives, benzimidazole, ben-oxygen. zothiazole and elemental sulfur [178]. The authors discuss

The photodegradation of the fungicide isoprothiolane a possible mechanism for such processes, starting with ahdiisopropyl 1,3-dithiolan-2-ylidnemalonatej by 254 nm S–H bond homolysis and/or oxidation due to singletUV light on solid particles yielded oxalic acid, oxygen formation. The intermediates, however, have notdithiolanylidenemalonic acid, dithiolanylideneacetic acid, been detected.2,4-bis[bis(isopropoxycarbonyl)methylene]-1,3-dithietane, The UV photolysis of thioamides was studied using low3, 5 - bis[bis(isopropoxy - carbonyl)methylene]1, 2, 4 - tri- pressure mercury lamps under nitrogen and oxygen atmos-thiolane and sulfur [174]. Photolysis of the methyl and phere [179]. The photoproducts found under nitrogen wereethyl analogues yielded the corresponding photoproducts. nitriles and H S, while under oxygen, nitriles, amides,2

Based on product analysis, the authors propose the in- carboxylic acids, 3,5-disubstituted-1,2,4-thiadiazoles, SO222volvement of dithiolane ring cleavage, ester hydrolysis, and SO were formed. Photodegradation of secondary4

decarboxylation, dithiethane and trithiolane formation and and tertiary thioamides was negligible both in inert andsulfur liberation. A surface effect was observed, the oxidant atmosphere. The mechanism of photolysis underphotodegradation being much more rapid on sand than on nitrogen is proposed to take place through an intermediatea glass plate. The authors observed the same phenomena biradical, centered on the C and S, that would undergoalso with other sulfur-containing pesticides. hydrogen transfer followed by elimination of H S. The2

Badr et al. [175] studied the photolysis of photolysis under oxygen is explained assuming the forma-phenylacetamide, N-benzylphenylacetamide and tion of an intermediate sulphinic acid, that could eliminatephenylacetanilide in air using a mercury lamp. The photo- H SO to give a nitrile or hydrolyse to an amide or2 3

products obtained from phenylacetamide were benzal- carboxylic acid and ammonia.dehyde, toluene, bibenzyl, stilbene and phenanthrene. In A comparative study between fragmentation processesaddition to these, benzylamine was found upon photolysis taking place in mass spectrometry using an electronof N-benzylphenylacetamide, and aniline, N-benzylaniline, ionization source and photodegradation processes has beeno-aminodiphenylmethane and p-aminodiphenylmethane carried out for atrazine (vide supra), simazine (vide supra)

2 2 4following photolysis of phenylacetanilide. The authors and trietazine h6-chloro-N ,N ,N -triethyl-1,3,5-triazine-suggest the mechanism proceeds via C–N bond homolysis 2,4-diaminej [180]. Simazine showed the lowest tendencyto yield phenylacetyl and aminyl radicals, that would lead to fragmentation, and atrazine the highest. The same kindto the observed photoproducts. of fragmentations were observed for the three compounds:

Miille and Crosby [53] studied the photodegradation of C–N bond cleavage in the lateral chains, C–Cl bond3,4-dichloroaniline by simulated sunlight in seawater. The scission and heteroatomic ring cleavage.


Sharma and Chibber [181] studied the sunlight photo-degradation of diniconazole h(E)-(RS)-1-(2,4-dichloro-phenyl) - 4, 4 - dimethyl - 2 - (1H - 1, 2, 4 - triazol - 1 - yl)pent-1-en-3-olj as a thin film on glass surface. Thephotoproducts generated were separated and identified asthe (Z)-isomer of diniconazol, a cyclic alcohol and itscorresponding ketone, and an isoquinoline.

Mushtaq et al. [40] studied the photodegradation ofemamectin benzoate h40-deoxy-40-(epi-methylamino)aver-mectin B benzoatej in aqueous solution buffered to pH 71a

and natural pond water under sunlight. Avermectins are16-member lactones, used as insecticides. The sunlightphotodegradation in the mentioned media took place in 22and 7 days, respectively. Upon continuous exposure tolight from a Xe lamp, photodegradation of aqueoussolutions of the pesticide, buffered to pH 7 and containing Scheme 2. Chemical events taking place upon photosensitized photolysis

involving energy transfer.1% acetonitrile or ethanol as cosolvents took place in 64.5and 8.5 days, respectively. A photoisomer and unknownpolar residues were found as photoproducts. The amountof polar residues found increased with the time of irradia- pesticides, that can undergo different processes, as follow-tion. ing direct photodegradation (Scheme 2).

Miller et al. [61] reported a big difference in the rate of Photosensitization may also involve redox processes,photolysis of methylisocyanate which, while stable in such as the photo-Fenton reaction, where there is an initialwater, has a half-life time of less than 1 day under electron or atom transfer to produce free radicals, but themidsummer sunlight. oxidised or reduced sensitizer undergoes subsequent re-

The influence of soil and sediment composition on the actions to regenerate the initial species [184–186,54,187].photodegradation of napropamide h(RS)-N,N-diethyl-2-(1- Some examples of this will be given in Section 5.naphtyloxy)propionamidej has been studied using a photo- An important advantage of photosensitized photodegra-reactor emitting with a spectrum 300,l,450 nm, with a dation is the possibility of using light of wavelenghtsmaximum at 365 nm [182]. Soil or sediment reduced the longer than those corresponding to the absorption charac-rate of photolysis, although no dependence on its com- teristics of the pollutants.position was found.

Dureja et al. [183] studied the photodegradation of the 3 .1. Anilide herbicidesinsecticide azadirachtin hdimethyl[2aR-[2aa,3b,4b

* * * * * * *(1aR , 2S , 3aS , 6aS , 7S , 7aS ), 4ab, 5a, 7aS , 8b(E), The aqueous photodegradation of butachlor hN-(butox-10b, 10aa, 10bb]] - 10 - (acetyloxy) octahydro - 3, 5 - dihy- ymethyl)-2-chloro-29,69-diethyl-acetanilidej by sunlight indroxy - 4 - methyl - 8 - [2 - methyl - 1 - oxo-2-butenyl) oxy]-4- the presence of diethylamine increased significantly rela-(3a,6a,7,7a -tetrahydro-6a -hydroxy-7a -methyl-2,7-meth- tive to the process in its absence [36]. In this case the lightanofuro[2, 3 - b]oxireno[e]oxepin - 1a(2H ) - yl) - 1H,7H- is absorbed by the pesticide, and it is probable that, as withnaphtol[1,8-bc: 4,4a -c9]difuran-5,10a(8H ) -dicarboxylatej other reactions involving excited states of aromatic mole-upon 254 nm irradiation as a thin film on a glass surface. cules and aliphatic amines the reaction involves electronThe half-life of azadirachtin was ¯48 min, yielding an transfer [188].isomerised (Z)-2-methylbut-2-enoate product. Irradiationin the presence of saturated fatty acids and fatty oils led to 3 .2. Carbamate insecticidesan increase in the rate of photodegradation. Conversely,irradiation in the presence of unsaturated fatty acids such The degradation of mercaptodimethur h4-methylthio-3,5-as oleic, linoleic and elaidic acid reduced the rate of xylyl methylcarbamatej and ethiofencarb (vide supra)photodegradation. photosensitized by anthraquinone in an acetonitrile /water

mixture, has been studied, using a medium-pressure mer-cury lamp as radiation source [189]. The degradations werecomplete after irradiating for 0.5 and 0.8 h, respectively.

3 . Photosensitized degradation For mercaptodimethur, the observed photoproducts were3,5-dimethyl-4-methylsufinylphenol (50%) and 3,5-di-

Photosensitized photodegradation is based on the ab- methyl-4-methylthiophenol (49%) and 3,5-dimethyl-4-sorption of light by a molecule. In one possible scenario, methylthio-2-N-methylacetamidophenol (1%). Forthis can then transfer energy from its excited state to the ethiofencarb, the observed photoproducts were 2-hydroxy


benzaldehyde (55.5%), 3-methyl benzo[e] [1,3] oxazine- rates, by hydroxyl radicals produced by nitrate and nitrite,2,4-dione (33.3%) and o-hydroxybenzaldehyde (11.3%). and by photo-oxidants generated by humates, probablyThe authors hypothesise different mechanisms leading to singlet oxygen or peroxyl radicals. Tests performed usingthe observed products, taking place by electron transfer combinations of these photosensitizers led to first-order

21from the pesticide to the excited anthraquinone. rate constants ranging from 0.03 to 0.048 h . Although?HO radicals tend to enhance the photooxidation, their

effect is diminished due to scavenging by humates. The3 .3. Chloroaromatic pesticidesauthors point out that photo-oxidants generated by humatesappear to be of greater environmental significance thanJulliard et al. [79] investigated the N,N,N9,N9-tetra- ?HO in the photodegradation of MCPA.methylbenzidine photosensitized reductive dechlorination

of chloroneb h2,5-dichloro 1,4-dimethoxybenzenej, 4,49-3 .4. Imidazolinone herbicideDDT h1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethanej and

MCPA methyl ester h4-chloro-2-methylphenoxy methylThe photodegradation of imazapyr h2-(4-isopropyl-4-acetatej, in an ethanol /water mixture using sodium sulfite

methyl-5-oxo-2-imidazolin-2-yl)nicotinic acidj by simu-as sacrificial electron-donor. After 7 h of irradiation oflated sunlight was studied at pH 7 in the presence of humicchloroneb, 95% dechlorination was observed, with theacids [191]. The study was carried out at different ratios ofmain photoproducts being 2-chloro-1,4-dimethoxybenzeneimazapyr to humic acid (1:0.1, 1:0.5, 1:1), and the(80%) and 1,4-dimethoxybenzene (20%). 2 h of irradiationconcentration of remaining herbicide followed by HPLC.of 4,49-DDT led to quantitative degradation, the mainThe photolysis followed first-order kinetics, with thephotoproduct being 4,49-dichorodiphenyl dichloroethane,presence of humic substances slowing down the photo-that subsequently dehalogenated. Irradiation of MCPAdegradation rate.methyl ester for 2 h led to complete conversion, 2(2-

methylphenoxy)methyl ethanoate being formed. The re-3 .5. Organophosphorus pesticidessults show that the presence of an electron donor may

increase the rate of dehalogenation of haloaromatic pes-´Barcelo et al. [166] studied the photodegradation ofticides.

fenitrothion (vide supra) in distilled water and artificialThe application of tris-2,29-bipyridylruthenium(II) per-seawater using a suntest apparatus to simulate sunlight.oxydisulphate as a photosensitizer for the oxidative photo-Direct photolysis was very slow and no breakdown prod-degradation of 4-chlorophenol has been studied in detailucts were observed after 3 h of irradiation. When 20%[190]. Tris-2,29-bipyridylruthenium(II) is water soluble,acetone was added as photosensitizer, different metaboliteseasy to immobilize in polymers, absorbs strongly in the

4 21 21 were observed, which were tentatively assigned to fenit-visible (e(450 nm)¯1.4310 M cm ) and emits fluo-rooxon, carboxy-fenitrothion and 3-methyl,4-nitrophenol.rescence at 620 nm. Upon photoexcitation, the excited

22 The photodegradation of vamidothion (vide supra) bystate of the cation is quenched by S O to generate the2 831 ?2 simulated sunlight has been studied in water /methanolstrong oxidants Ru(bpy) and SO :3 4 (2–4%) with 5% acetone added as a photosensitizer [64].

21 21 The major photoproduct observed was vamidothion sulfox-*Ru(bpy) 1 hn → (Ru(bpy) )3 3ide.

21 22 31 22 ?2 Kamiya and Kameyama [192] studied the photochemical*(Ru(bpy) ) 1 S O → Ru(bpy) 1 SO 1 SO3 2 8 3 4 4effects of humic materials on the degradation of various

The photodegradation was carried out using a medium organophosphorus pesticides: bensulide hS-2-benzenesul-pressure mercury lamp with cut-off filters to suppress the fonamidoethyl O,O-di-isopropyl phosphorodithioatej,short wavelengths. 4-Chlorophenol undergoes rapid photo- chlorpyrifos (vide supra), diazinon (vide supra), EPN hO-oxidation. The main photoproducts found were benzo- ethyl O-4-nitrophenyl phenylphosphonothioatej, fenitro-quinone, 1,4-dihydroxybenzene, 4-chlorocatechol, chloride thion (vide supra), fenthion (vide supra), isofenphosand sulphate. Prolonged irradiation leads to generation of hO-ethyl O-2-isopropoxycarbonylphenyl isopropylphos-CO and HCl. The reaction mechanism probably involves phoramidothioatej, isoxathion hO,O-diethyl O-5-phenyl-2

one-electron oxidation yielding a radical cation (4-chloro- isoxazol-3-yl phosphorothioatej, malathion hdiethyl?1phenol) and or phenoxyl radicals. Further addition– (dimethoxythiophosphorylthio) succinatej, parathion (vide

elimination processes are proposed, eventually leading to supra) and tolchlofos-methyl hO-2,6-dichloro-p-tolyl O,O-4-chlorodihydroxycyclohexadienyl radicals. dimethyl phosphorothioatej. The authors suggest the sen-

The photodegradation of MCPA h4-chloro-2- sitization effects of humic substances depends on themethylphenoxy acetic acidj was studied in aqueous solu- binding affinity of pesticides to the radical source of thetions of naturally occurring photosensitizers, using low humic material. It was also found that metal ions havingintensity 300–450 nm UV light and sunlight [54]. Upon paramagnetic property inhibit the degradation of organo-UV irradiation, MCPA was photooxidized at significant phosphorus pesticides sensitized by humic acids [193].


A photosensitizing process was suggested to explain the overall and singlet molecular oxygen quenching processes.enhanced degradation of iprobenfos hS-benzyl O,O-di-iso- Again, a charge-transfer mechanism, involving an initialpropyl phosphorothioatej when poly(3-octylthiophene-2,5- excited encounter complex is suggested.diyl) films were blended with perylene and N,N,N9,N9-tetramethylbenzidine (a cation scavenger) [194]. The effect

?2could be ascribed to an enhancement in O production. 3 .9. Triazine herbicides2

Cyclodextrins have been shown to have an importantenhancing effect on the rate of photodegradation of Acetone (1%) was used as a photosensitizer in a studyorganophosphorus pesticides in water containing humic of the photodegradation of atrazine (vide supra), atratonsubstances [193]. Such enhancing effects are attributed to (vide supra) and ametryn (vide supra) in water by lightthe inclusion effects of cyclodextrins to catalyze the with l.290 nm from a medium pressure mercury lampinteraction of pesticides with radicals generated by the [198]. The photostability increased in the order ametryn,

photosensitizing humic acids, that are trapped in the atrazine,atraton. The main photoproducts in all casescyclodextrins. were the corresponding de-N-alkyl and de-N,N9-dialkyl

triazines, as well as the hydroxytriazines. The observation3 .6. Oxadiazole herbicides of two different de-N-alkyl products suggests the de-N-

alkylation to take place in two steps. Higher yields ofThe aqueous photodegradation of oxadiazon (ronstar) de-N-ethyl triazines were observed than those of de-N-

h5-tert-butyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4- isopropyl triazines. Irradiation of ametryn yielded alsooxadiazol-2(3H )-onej by sunlight significantly increases in de(methylthio)ametryn. The reaction was strongly sensi-the presence of diethylamine, added as a photosensitizer, tized by acetone. No breakdown products were observedrelative to the process in the absence of sensitizer [36]. for control dark reactions over 96 h, showing that the

degradation was exclusively due to photolysis.3 .7. Phenylamide fungicides The products of sensitized photooxidation of ametryn

(vide supra), atraton (vide supra), atrazine (vide supra),Hustert and Moza [195] reported on the aqueous photo- propazine (vide supra) and simazine (vide supra) have been

degradation of carboxin h5,6-dihydro-2-methyl-1,4-oxathi- studied in water, and using sunlight irradiation [199].ine-3-carboxanilidej and oxycarboxin h2,3-dihydro-6- Methylene blue, Rose Bengal, riboflavin and flavin mono-methyl-5-phenylcarbamoyl-1,4-oxathi-ine 4,4-dioxidej in nucleotide were used as sensitizers. The primary photo-the presence of humic and fulvic substances and inorganic degradation products were dealkylated s-triazines andsoil components, using both a high-pressure mercury lamp oxidation products, with a yield of ¯60%.and a xenon lamp for irradiation. The presence of humic The photosensitized degradation of atrazine (vide supra)and fulvic materials enhanced photodegradation, possibly was investigated in distilled water and artificial seawaterby forming reactive oxygen species such as singlet oxygen containing humic acids, using simulated sunlight [166].and hydroxyl radicals. The main photoproduct observed The main photoproduct was hydroxyatrazine. The rate offor both was oxanilic acid. degradation was faster in seawater, with evidence for

photosensitized degradation of hydroxyatrazine.3 .8. Pyrimidine fungicides The photodegradation of five non-commercial unsatu-

rated triazines with pesticide activity was investigated in´Garcıa et al. [196] studied the Rose Bengal dye-sen- water containing a 5% acetone as photosensitizer, using a

sitized singlet oxygen-mediated photodegradation of 2- mercury lamp [154]. The observed photodegradation ef-hydroxypyridine, 3-hydroxypiridine, 4-hydroxypiridine, 4- ficiency was higher than in the absence of acetone.hydroxyquinoline, 8-hydroxyquinoline, 2-hydroxy- Bendig et al. [200] studied the photosensitized photo-pyrimidine and 4-hydroxypyrimidine in water and in the catalytic oxidation of triazine herbicides by natural sun-mixture MeOH/MeCN, aiming to use them as models for light. Tris(bipyridyl)ruthenium complexes adsorbed onpesticides. The photooxidation quantum yields ranged TiO were uses as sensitizers. The authors discuss the2

from 1 to 50%, with the higher values occurring in thermodynamic and kinetic requirements for the photo-aqueous media. The authors suggest a charge-transfer sensitized process. The efficiency of the sensitizationmechanism, involving an initial excited encounter com- seems to be related to the adsorption of the sensitizer andplex. More recently, the same authors studied the kinetics triazines on TiO .2

of Rose Bengal dye-sensitized photooxidation of the modelcompound 2-amino-4-hydroxy-6-methylpyrimidine inwater and in acetonitrile /water [197]. The photooxidation 3 .10. Urea-based pesticidesquantum yields ranged from 0.5 to 41%, with the highervalues observed in alkaline medium. Rate constants in the Crank and Mursyidi [201] studied the Rose Bengal

4 7 21 21range 9310 to 2.7310 M s are reported for the dye-sensitized singlet oxygen-mediated oxidation of thio-


ureas in ethanol, using low pressure mercury lamps 4 . Photocatalytic degradationemitting mainly at ¯254 nm. According to their results,and in contrast with a previous work in which the Although different definitions have been suggested forirradiation was carried out in pyridine [202], thiourea do photocatalysis [204–206], and a detailed discussion isnot undergo photolysis or photo-oxidation in the absence beyond the scope of this review, we use photocatalyticof sensitizers. Thiourea was shown to react with singlet degradation to mean cyclic photoprocesses in which theoxygen when using Rose Bengal, methylene blue and pesticides photodegrade, but spontaneous regeneration ofchlorophyll as sensitizers. The reaction was inhibited by the catalyst occurs to allow the sequence to continueb-carotene. The photoproducts found were cyanamides, indefinitely until all the substrate is destroyed. For aureas, products of heterocyclic condensation and sulfur- detailed description of this field, readers are referred to twocontaining fragments of the starting materials. Possible excellent multi-author volumes [207,208]. It is clear,reaction mechanisms are suggested. however, that there will be some overlap between this

A study of the photocatalytic degradation of meto- section and that describing photosensitized processes.bromuron h3-(4-bromophenyl)-1-methoxy-1-methylureajand isoproturon (vide supra) using UV/TiO showed the 4 .1. Amide herbicides2

rate of degradation was always lower than when usinghydroxyl radicals [203]. The solution resulting from photo- Pathirana and Maithreepala [209] examined the photo-degradation of metobromuron was bio-recalcitrant due to degradation of 3,4-dichloropropionamide in aqueous TiO2

accumulation of bromide intermediates, which was not the suspensions, using 365 nm light. They found acidiccase for the solution resulting from photodegradation of medium to be suitable for the degradation, that occurred

2isoproturon. quantitatively via dechlorination within 5 h, yielding Cl ,1 2H , NO and CO . The presence of O seems to be3 2 2

relevant. Degradation took place also under sunlight, with3 .11. Miscellaneous 30% yield after 7 h in the absence of TiO and with 70%2

yield in its presence. A mechanism for the completeThe anthraquinone photosensitized degradation of perth- photodegradation was tentatively proposed.

ane h1,1-dichloro-2,2-bis(4-ethylphenyl)ethanej has beenstudied in an acetonitrile /water mixture, using a medium- 4 .2. Bipyridinium herbicidespressure mercury lamp as radiation source [189]. Thedegradation was complete after irradiating for 2 h. The The photocatalytic degradation of paraquat (vide supra)observed photoproducts were 1,1-dichloro-2,2-bis(4- in water was studied using TiO and a batch of six long2

acetylphenyl)ethane (74%), 4-acetyl-49-ethylbenzophenone wavelength UV lamps (l5365 nm) [210]. Under these(21%), 1-chloro-2,2-bis-(4-acetylphenyl)ethene (3.5%), conditions, at high pH complete photodegradation ofand 4,49-diethylbenzophenone (1.5%). The authors indi- paraquat and the reaction intermediates took place in lesscate different possible mechanisms leading to the observed than 3 h, while direct photolysis led to 60% photodegrada-products, which are suggested to take place by electron tion.transfer from the pesticide to the excited anthraquinone.

Mushtaq et al. [40] studied the photodegradation of 4 .3. Carbamate insecticidesemamectin benzoate (vide supra) in aqueous solutionbuffered to pH 7 containing 1% acetone as sensitizer. The A comparison was made of the photocatalytic degra-sunlight photodegradation in this medium took place in 1 dation of carbetamide (vide supra) in water, using TiO2

day, much faster than in the absence of sensitizer (see and ZnO and irradiation at l.310 nm [211]. The mainabove). Upon continuous exposure to the radiation sup- photoproducts detected were hydroxylated compounds andplied by a Xe lamp the photodegradation took place in 0.5 products of cyclization of the side chain. After a longdays, also much faster than in the absence of sensitizer. A irradiation time carbetamide underwent complete miner-photoisomer, epoxide derivatives and unknown polar res- alization.idues were found as photoproducts. The amount of polar Tanaka et al. [212] studied the photocatalytic degra-residues found increased with the time of irradiation. dation of MCC hmethyl 3,4-dichlorocarbanilatej, MIPC

Different authors have described the ability of humic h2-(1-methylethyl)phenyl methylcarbamatej, MPMC h3,4-acids as photosensitizers to promote photooxidation of dimethylphenyl methylcarbamatej, MTMC hm-tolylcommon pollutants. Canonica et al. [187] proved the methylcarbamatej and XMC h3,5-xylyl methylcarbamatejenhanced photodegradation of electron-rich phenols in in water, using TiO and irradiating with a super-high2

presence of humic acids. Increased photodegradation of pressure mercury lamp. The rate of degradation is gov-monuron and fenuron photosensitised by soil extracted erned by the adsorbability of the pesticides to TiO . The2

humic substances has been reported by Aguer and co- degradation was proposed to take place by successiveworkers [184,182]. hydroxylation of the aromatic ring or polyhydroxylation,


leading to opening of the aromatic ring to yield oxygenated photolysis, which the authors attribute to result from aaliphatic intermediates. The degradation was fast, with its dimerization pathway [56].rate being dependent on the acidity of the medium.

The effect of ionic and non-ionic surfactants on the 4 .7. Halobenzonitrile pesticidesdegradation of carbaryl h1-naphthyl N-methylcarbamatejhas been studied using simulated solar light and catalysed The sunlight photodegradation of aqueous solutions ofby aqueous TiO suspensions [213]. An inhibition of the2 bromoxynil (vide supra) was studied in the presence ofdegradation rate was observed, depending on the surfactant TiO and Na W O [217]. When the pure pesticide was2 4 10 32and on the acidity of the medium. The kinetics of used, TiO was more efficient as a photocatalyst. When the2

42photodegradation followed a pseudo-first order kinetic law pesticide was used as a formulated solution, W O10 32below the critical micellar concentration, and a complex became more efficient. The authors suggest that thebehaviour above it. Carbaryl was mineralized to CO ,2 difference in reactivity can be accounted for by the

2 1NO and NH .3 4 different nature of the active species involved in the?The photolysis of propoxur h2-(1-methylethoxy)phenyl photodegradation: HO in the case of TiO and an excited2

42methylcarbamatej has been studied in water using 266 nm polyoxometalate in the case of W O .10 32laser flash photolysis in water, with 2,4,6-triphenylpyrylium encapsulated inside supercages of zeolite Y as

4 .8. Organochlorine insecticidesphotocatalyst under conditions in which propoxur is notdirectly altered [214]. According to the authors, the

Vidal [218] has tested the feasibility of photocatalyticprocess could be initiated by one-electron transfer betweenoxidation of lindane h1,2,3,4,5,6-hexachlorocyclohexanejthe excited 2,4,6-triphenylpyrylium cation and propoxur tousing TiO , a high pressure xenon arc lamp and a2form the corresponding 2,4,6-triphenylpyrylium radicalparabolic reflector, and found that it is possible to reduceand propoxur radical cation.99.9% of lindane levels in water in acceptable times.

Hupka et al. [219] investigated the photocatalytic degra-4 .4. Chloronicotinoid insecticidesdation of lindane in water, using a medium pressuremercury lamp and TiO /O . TiO was used in the form of2 2 2The TiO -catalyzed photodegradation of imidacloprid2 rutile, anatase and supported on glass hollow microspheres.(vide supra) has been followed as the formulated productThe lowest efficiency of catalysis was observed for rutileConfidor in tap water. In the presence of TiO a first-order2 and the highest for TiO supported on glass microspheres.2rate equation was followed, and the observed half-life was1,2,3-trichloro benzene, g-2,3,4,5,6-pentachloro-cyclohex-144 min, longer than in the absence of TiO (126 min, see2 1-ene and a hexachlorocyclohexane (lindane) isomer, a-Part 2) [113]HCH, were detected as reaction products after 150 minirradiation. Similarly, they studied the photocatalytic deg-4 .5. Chlorophenol pesticidesradation of p, p9-DDT h1,1,1-trichloro-2,2-bis(4-chloro-phenyl)ethanej under the same conditions, and with similarTseng and Huang [215] studied the removal of phenol;results in relation to the photocatalysts. Up to ten photo-2-, 3- and 4-chlorophenol; 2,3-, 2,4-, 2,5-, 2,6- and 3,4-products were identified in this case.dichlorophenol; 2,4,6-trichlorophenol and pentachloro-

phenol from water by photocatalytic oxidation using TiO /2

UV. The rate of photocatalyzed degradation follows first- 4 .9. Organophosphorus pesticidesorder kinetics, independent of the pH. The rate increaseswith increase in TiO concentration until a maximum of 3 The solar TiO photocatalyzed mineralization of malath-2 2

21g l , and then decreases. The influence of the degree of ion (vide supra) has been studied in rinse water ofchlorination was described. agricultural sprayers and in polluted well water [220]. The

The photodegradation of 2,4-dichlorophenol and penta- observed half-life was about 1 h in the presence of TiO2

chlorophenol was studied in O -free aqueous medium, in and absence of H O . Using polyethylene-film covered2 2 2

the presence of the hole scavengers polyethyleneimine, vessels it was observed that efficient photodegradationtriethylamine, 2-propanol and ethylenediaminetetraacetic occurred in sealed systems.acid [216]. The photodegradation of both chlorophenols The photocatalytic removal of fenitrothion (vide supra)was inhibited by the above mentioned hole scavengers. from TiO suspensions has been studied [221]. Several2

intermediates of the photocatalytic photodegradation have4 .6. Dinitroaniline pesticides been identified. These subsequently underwent mineralisa-

2 22 2tion into CO , H PO , SO and NO .2 2 4 4 3

UV and sunlight induced photolysis of fluchloralin (vide Doon and Chang [222] studied the photocatalytic degra-supra) in aqueous methanol (80%) in the presence of TiO dation of diazinon (vide supra), EPN (vide supra), malath-2

yielded an additional product to those found upon direct ion (vide supra), methamidophos hO,S-dimethyl


phophoramidothioatej and phorate hO,O-diethyl S- 4 .10. Phenol-based pesticides[(ethylthio)methyl] phosphorodithioatej, in water, assistedby TiO and using the UV radiation supplied by a medium The photocatalytic decomposition of 2,4-D (vide supra)2

pressure mercury lamp. The efficiency of degradation over TiO and ZnO in water has been shown to take place2

followed the order phorate.methamidophos.malathion¯ with complete mineralisation to CO and HCl [224]. Two2

diazinon.EPN. Apparently, methamidophos and phorate mechanisms were proposed for the oxidation: oxidation?photodegrade mainly through direct photolysis, while TiO with HO radicals and direct oxidation by holes created on2

enhances the photodegradation of diazinon, malathion and the semiconductor. The rate of photodegradation increasedEPN. The use of UV/H O was more effective than that of as the acidity of the medium decreased, and the presence2 2

2 2UV/TiO , with the combination UV/H O /TiO being the of Cl or HCO slowed down the photodegradation rate2 2 2 2 3?most promising. by scavenging HO radicals.

Ku and Jung [223] studied the photocatalytic degra- Herrmann et al. [225] studied also the photocatalyticdation of monocrotophos hdimethyl (E)-1-methyl-2- degradation of 2,4-D at a pilot scale under solar irradiation.(methylcarbamoyl)vinyl phosphatej in water by UV/TiO 2,4-D disappearance followed first-order kinetics and2

(80% anatase, 20% rutile) irradiation at different pH eventually yielded complete mineralization products withinvalues, TiO dosages, light intensities and dissolved a residence time of less than 1 h. The main photoproduct2

oxygen levels. The presence of oxygen inhibited the of degradation of 2,4-D was 2,4-dichlorophenol.recombination of electrons and holes, enhancing the photo- The photodegradation of 4-nitrophenol in TiO aqueous2

degradation of monocrotophos. An excess of oxygen had suspension has been studied as a function of the initialno effect. The degradation was more effective in acid concentration of 4-nitrophenol, light intensity, partialsolution than in alkaline medium. Increasing the light pressure of oxygen, concentration of catalyst, pH, chlorideintensity enhanced the decomposition. concentration and temperature [226]. A kinetic expression

The use of 2,4,6-triphenylpyrylium ion encapsulated was obtained incorporating the influence of the manywithin Y zeolite as a photocatalyst for the photodegrada- factors.tion of methyl parathion hO,O-dimethyl O-4-nitrophenyl The sunlight photodegradation of aqueous solutions ofphosphorothioatej in aqueous solution has recently been phenol, 4-chlorophenol and 2,4-dichlorophenol has beenstudied, using both sunlight and the radiation supplied by studied in the presence of TiO and Na W O [217].2 4 10 32

high-pressure mercury lamps [109]. It was found that the When pure pesticides were used, TiO was more efficient2

zeolite encapsulation stabilizes the 2,4,6-triphenylpyrylium as a photocatalyst. When the pesticides were used in42ion, leading to degradation efficiencies comparable to formulated solutions W O became more efficient. The10 32

those obtained using TiO . The authors rule out the authors suggest that the difference in reactivity can be2

intermediacy of singlet oxygen in the photodegradation accounted for by the different nature of the active species?process, as well as a direct photoinduced electron transfer involved in the photodegradation: with HO in the case of

process from 2,4,6-triphenylpyrylium ion to methyl para- TiO and an excited polyoxometalate in the case of242thion. They also suggest that radicals, resulting from W O .10 32

interaction of 2,4,6-triphenylpyrylium ion with water and The photodegradation of 4-chlorophenol using light ofoxygen, are involved in the process. l.290 nm was used to model the photocatalytic prop-

Recently, Albanis et al. [38] reported on the photo- erties of industrial TiO catalysts and to compare their2

catalytic degradation of methyl parathion (vide supra), efficiencies [227]. Physical characteristics such as theethyl parathion hO,O-diethyl O-4-nitrophenyl phosphoro- particle size or the structure of the active site seem ofthioatej, methyl bromophos hO-4-bromo-2,5-dich- importance for the photocatalytic activity.lorophenyl O,O-dimethyl phosphorothioatej, ethyl bromo- Mazellier and Bolte [228] studied the goethite (a-phos hO-4-bromo-2,5-dichlorophenyl O,O-dimethyl phos- FeOOH) light-induced transformation of aqueous 2,6-di-phorothioatej and dichlofenthion hO-(2,4-dichlorophenyl) methylphenol, using 365, 436 and 546 monochromaticO,O-diethyl phosphorothioatej in aqueous TiO suspen- light.The rate of disappearance of the phenol increased2

sions using simulated sunlight from a xenon lamp, sup- with the concentration of oxygen. The involvement ofpressing wavelengths below 290 nm with transmission hydroxyl radicals was discarded on the basis of the lack offilters. The observed half-life times varied from 10 to 40 influence of isopropanol. The authors suggest the phenolmin in the presence of TiO , one or two orders of may react with positive holes in goethite, the electron2

magnitude lower than in its absence. The major photo- being trapped by oxygen.products of photodegradation of ethyl bromophos and Topalov et al. [229] studied the photocatalytic miner-dichlofenthion were identified from the corresponding alization of mecoprop (vide supra) in water, using TiO as2

GC–MS (EI) fragmentation patterns, corresponding to photocatalyst and irradiating with a medium-pressureoxidation and photohydrolysis pathways. The authors mercury lamp. The rate of degradation was also followed

2hypothesize a mechanism for the photodegradation of using a Cl selective electrode. Complete mineralizationirgarol under the above mentioned conditions. was observed, and a possible scheme suggested on the


basis of NMR spectra taken at different times. The same Bellobono et al. [234] used photocatalytic compositemembranes immobilizing 30 wt% TiO to study photo-authors reported also on the photodegradation of MCPA 2

mineralization of atrazine (vide supra) solutions saturatedh4-chloro-2-methylphenoxy acetic acidj over TiO . After2

with ozone. Co-immobilization of m-peroxo-bis[N,N9-15 h of irradiation the mineralization was complete. As inethylene-bis(salicylideneiminato)dimethylformamide cobaltthe case of mecoprop, the proposed pathway of MCPA(III)] and of a mixture of tri-(t-butyl)- and tri-(i-pro-photodegradation in the presence of TiO also involves the2

pyl)vanadate (V) led to rate enhancements of, respectively,formation of radicals [230].1.5- and 10-fold.

The photodegradation of prometryn (vide supra) and4 .11. Pyrimidine pesticidesprometon (vide supra) has been followed in aqueoussolution using membranes immobilizing TiO , and usingThe photocatalytic degradation by sunlight of 2

H O as an oxygen supplier [235]. The observed kineticspyrimethanil hN-(4,6-dimethylpyrimidin-2-yl)anilinej was 2 2

followed a first-order equation. The quantum yields forstudied in its pure form and as commercial formulation inphotodegradation were rather high, 40% for prometryn anda preindustrial pilot plant, using TiO as a photocatalyst2

66% for prometon. Cyanuric acid was found as photostable[231]. Complete photodegradation took 230 min, withoutphotoproduct.total mineralization. Twenty two photoproducts were de-

The UV photodegradation of acidified aqueous solutionstected, several of which were identified. Some of the major(pH52.4) of atrazine (vide supra) was studied in thephotoproducts characterized were aniline, formamide, 1,3-presence of TiO and Na W O [236]. GC–MS, HPLC,benzenediol and 4,6-dimethyl-2-pyrimidinamine. Two 2 4 10 32

14mechanistic routes are suggested for the photodegradation, radioactivity counting and TLC on C-labelled solutionsone involving reaction of hydroxyl radicals with the were used. Both catalysts are efficient, leading to substan-pyrimidine and benzene rings, with subsequent ring open- tial increases in the rate of photodegradation. The mecha-ings and the other involving photoinduced hydrolysis. nisms of photodegradation in the presence of each photo-

catalyst are different. The authors suggest that in the?4 .12. Thiocarbamate herbicides presence of TiO , HO radicals are the oxidant species,2

?while in the case of Na W O , H abstraction on the4 10 32

Albini et al. [232] studied the TiO photocatalyzed alkyl side chains of atrazine and dehalogenation by elec-2

degradation of molinate hS-ethyl azepane-1-carbothioatej, tron transfer on the dealkylated metabolites takes place.eptam hS-ethyl dipropylcarbamothioatej, tiocarbazil hS- Neither of these photocatalysts is able to mineralise thebenzyl di-sec-butylthiocarbamatej and thiobencarb (vide aromatic ring of atrazine.

´supra) in water by 340–380 nm light. They consider the Hequet et al. [237] showed that porphyrin and phthalo-process to be efficient, although the maximum quantum cyanine complexes, used as photocatalysts for the degra-yield is ¯12%. The process is rationalized as being dation of atrazine (vide supra) are able to cleave the

?initiated by hydrogen abstraction by HO from the N-alkyl triazine ring more efficiently than TiO . The best degra-2

chains, rather than from the S-alkyl ones, except in the dation half-life time found was ¯200 min, using ironcases where this is a benzyl group. phthalocyanine as photocatalyst.

Recently, Albanis et al. [38] studied the photocatalytic4 .13. Triazine herbicides degradation of atrazine (vide supra), propazine (vide

supra), cyanazine (vide supra), prometryn (vide supra) andPelizzetti et al. [233] studied the TiO photocatalyzed irgarol (vide supra) in aqueous TiO suspensions under2 2

degradation of atrazine (vide supra), simazine (vide supra), simulated sunlight, supplied by a xenon lamp from whichtrietazine (vide supra), prometon (vide supra) and pro- wavelengths below 290 nm had been suppressed usingmetryn (vide supra) by simulated sunlight in aqueous transmission filters. The observed half-life times variedsolution, using a Xe lamp. All the herbicides were rapidly from 10 to 40 min in the presence of TiO , one order of2

degraded. Full mineralization was not observed. Different magnitude lower than in its absence. Eleven major photo-intermediates have been identified, cyanuric acid being the products were detected from GC–MS (EI) fragmentationcommon final photoproduct of all herbicides. A mecha- patterns for irgarol, corresponding to oxidation, dealkyla-nism is proposed for the generation of the observed tion and dechlorination pathways. The authors hypothesizephotoproducts. a mechanism for the photodegradation of irgarol under the

The solar TiO photocatalyzed mineralization of at- above mentioned conditions.2

´razine (vide supra) has been studied in agricultural water Macounova et al. [238] studied the photodegradation ofand in polluted well water [220]. The observed half-life metamitron h4-amino-6-phenyl-3-methyl-1,2,4-triazin-time was less than 1 h in the presence of TiO and absence 5(4H )-onej on a plate photoreactor with immobilized TiO2 2

of H O . The use of polyethylene-film covered vessels or as Q-TiO particles. The main photoproducts found2 2 2

showed that efficient photodegradation takes place in were deaminometamitron h6-phenyl-3-methyl-1,2,4-triazin-sealed systems. 5(4H )-onej, deaminohydroymetamitron h6-(29-hydroxy-


phenyl)-3-methyl-1,2,4-triazin-5(4H )-onej and hydroxy- alization process was slow when using TiO alone. The222metamitron h4-amino-6-(29-hydroxyphenyl)-3-methyl- use of S O as electron scavenger enhanced the photo-2 8

1,2,4-triazin-5(4H )-onej. The proposed mechanism ac- mineralization process.counts for the formation of the mentioned products interms of the coexistence of two pathways: heterogeneouscatalysis and homogeneous direct photolysis. According to 5 . Degradation by reaction with the hydroxyl radical

?the authors, photocatalysis is the major route when the (HO )plate photoreactor is used, while the homogeneous directphotolysis has the same weight as photocatalysis in Some of the most frequently used AOPs involve mole-colloidal solutions of the Q-TiO particles. cules that upon photolysis generate the hydroxyl radical2

?(HO ). This can be achieved by different ways, such as:4 .14. Miscellaneous

Turchi and Ollis [239] proposed different mechanisms to (i) Addition of hydrogen peroxide that undergoes?explain the reactivity with HO radicals in photocatalytic homolysis upon photolysis:

systems. The cases of reaction on the surface, in the fluid?H O 1 hn → 2HO2 2and through a Rideal mechanism yielded rate equations

analogous to Langmuir–Hinshelwood expressions. The(ii) Photolysis of ozone, either with generation of atoms ofproposed model allows estimation of the photocatalytic

singlet oxygen that then react with water to generatedegradation rate from data on the physical properties of the?HO :photocatalyst, knowledge of the electron-hole recombina-

1tion and trapping rates and the values of the second order O 1 hn → O 1 O ( D)3 2?rate constants for HO radicals.1 ?The photocatalyzed degradation of the pesticides O ( D) 1 H O → 2HO2propyzamide h3,5-dichloro-N-(1,1-dimethyl-2-propynyl)-

or which react directly with water to produce hydrogenbenzamidej, iprobenfos (vide supra), simazine (vide supra)peroxide [17]:and fenobucarb h2-(1-methylpropyl)phenyl methylcarba-

matej irradiating poly(2,5-dihexoxy-p-phenylene) andO 1 H O 1 hn → H O 1 O3 2 2 2 2poly(3-oxylthiophene-2,5-diyl) films was studied with a

400-W high-pressure mercury lamp [240]. Photoexcitation followed by its homolysis to generate hydroxyl radi-of the polymer films leaves holes in the valence band and cals.

31electrons in the conduction band. These electrons are (iii)Aqueous photolysis of Fe , generated by oxidation of?2 ? 21trapped by O to form O , from which HO radicals can Fe by H O :2 2 2 2

be formed, that can react with the pesticides. Degradation21 31 2 ?H O 1 Fe → Fe 1 OH 1 HOcan be initiated also from the holes in the valence band. 2 2

The photocatalytic degradation of methoxychlor h1,1,1-31 21 ? 1Fe 1 H O 1 hn → Fe 1 HO 1 H ortrichloro-2,2-bis(4-methoxyphenyl)ethanej in water was 2

investigated by Hupka et al. [219], using a medium(iv) Radiolysis of water:pressure mercury lamp and TiO /O . The results were2 2

similar to those obtained for lindane and p, p9-DDT (vide 1 ?H O V H 1 HO 1 e 22 aqsupra).A comparison of the photodegradation of phenol by ?The HO radical can then react either by:direct photolysis and by TiO -assisted photocatalysis was2

? 2 ?1studied by Chun et al. [241] using various experimental electron transfer: HO 1 P → OH 1 P? ? ?conditions: l.200 nm in the presence of O or N or2 2 H abstraction: HO 1 PH → H O 1 P2

using O /TiO . Direct photolysis yielded a brownish- ? ?2 2 or addition to aromatic rings: HO 1 P → P–OHyellow polymer, both under O and in inert atmosphere.2

Complete mineralization takes place under UV irradiation 5 .1. Acetamide herbicidesin O atmosphere or in O /TiO . The polymer was found2 2 2

to be more difficult to degrade than the products of Brekken and Brezonik [243] studied the reaction be-hydroxylation from TiO -assisted photocatalysis. tween acetochlor h2–chloro–N–ethoxymethyl–69–2

?Malato et al. [242] described the use of a pre-industrial ethylaceto–o–toluididej and HO , assuming that the pri-?solar TiO photocatalytic treatment for the photodegrada- mary source of HO is nitrate photolysis. The measured2

? 9 21 21tion of high pesticide content waters from rinsing of rate constant was k(HO 1acetochlor)57.5310 M s ,pesticide containers. The authors used oxamyl as a model. a value that is close to the one previously reported forOxamyl was completely photodegraded, but the miner- alachlor, based on structure–activity relationships [244].


21The reported data do not eliminate the possibility that the dose was higher than 2 g TiO l decreased again.231direct photolysis takes place, but indicate that it would be The system Fe /H O was shown to be involved in the2 2

?much slower than HO -mediated degradation. degradation when Fe/H O were combined. Oxidant2 2

oxyanions were shown to act as electron scavengers,2 25 .2. Anilide herbicides enhancing the degradation in the sequence ClO .IO .2 3

2 2BrO .ClO . Oxygen had little effect on the photodegra-3 3

The photo-Fenton degradation of metolachlor (vide dation process. Diethylphosphoric acid, p-nitrophenol,supra) in water has been studied using 330–400 nm light diethylmonothiophosphoric acid, O,O-ethyl p-nitrophenyl[245]. Apparently, irradiation at these wavelengths did not monothiophosphoric acid and oxalate were identified asenhance the rate of degradation. intermediates, which were subsequently oxidized.

Miller et al. [57] developed a method based on the5 .3. Carbamate insecticides technique of solid-phase microextraction to determine the

?rate of reaction of chlorpyrifos (vide supra) with HO in´Benıtez et al. [43] studied the aqueous photolysis of the gas phase at high temperatures. The half-life observed

carbofuran (vide supra) by the polychromatic light supplied for chlorpyrifos was 1.4 h. Changing the temperature fromby a high-pressure mercury lamp (emitting from 185 nm to 60 to 80 8C did not affect the rate of photodegradation.far into the visible) in the presence of H O . Hydroxyl2 2

radicals accelerate the degradation relative to the case in 5 .6. Phenol-based pesticideswhich no H O is added. A simple rate equation is2 2

proposed, taking into account both the contribution of The aqueous photodegradation of 2,4-D h2,4-dichloro-?direct photolysis and that of the reaction with HO . phenoxyacetic acidj by UV radiation in the presence of

H O was faster than in its absence, with more than 98%2 2

5 .4. Chlorophenol pesticides of the starting material having disappeared within the first10 min of treatment [131]. The authors suggest the main

Hirvonen et al. [246] studied the degradation of 2- processes taking place involve substitution of Cl by OH atchlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6- the o- and p- positions of the aromatic ring, and elimina-

?trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachloro- tion of H from the a-methylene of phenoxyacetic acid.? ?phenol in aqueous solution by the HO radicals generated HO reactivity with bromoxynil (vide supra), adsorbed

by 254 nm photolysis of H O . Hydroxylated chloro- on highly dispersed thin layer of silicon dioxide powder,2 2

phenols and dimeric intermediates were found as transient used as unreactive carrier, was studied [84]. The degra-intermediates in the degradation. Eventually, a complicated dation was fast, and was mainly attributed to directmixture of products was obtained, and the authors suggest photolysis.the necessity of toxicity testing to assess the detoxification Mazellier and Bolte [250] studied the degradation of

?effects of the treatment. Furthermore, they consider the 3-chlorophenol by reaction with HO , generated via andechlorination degree to be insufficient. intramolecular photoredox process in iron(III) excited

21The degradation of pentachlorophenol by photo-Fenton hydroxy-complexes, specially Fe(OH) . Monochromaticsystems has been analysed by Fukishima and Tatsumi 365 nm radiation was used as excitation source. The[247], possible degradation pathways by such photo-Fen- quantum yields observed for 3-chlorophenol disappearanceton systems being proposed. were in all cases rather low, ¯5%. The main reaction

products observed were 2-chlorohydroquinone, 2-chloro-5 .5. Organophosphorus pesticides benzoquinone, 3-chlorocatechol and 4-chlorocatechol.

´ ´ ´Cernak and Cernakova [248] studied the application of a 5 .7. Phosphorothioate insecticidesH O /UV (254 nm) combination for removing parathion-2 2

methyl (vide supra) and phenitrotion hO,O-dimethyl O-4- Miller et al. [57] developed a method based on thenitro-m-tolyl phosphorothioatej. The maximum degrada- technique of solid-phase microextraction to determine the

?tion took place within the first 5 min, longer exposures did rate of reaction of diazinon (vide supra) with HO in thenot significantly increase the efficiency. gas phase at high temperatures. The half-life observed for

More recently, Doon and Chang [249] investigated the diazinon was 0.5 h. Changing the temperature from 60 tophotodegradation of parathion (vide supra) in water, and in 80 8C did not affect the rate of photodegradation.the presence of TiO , Fe compounds and H O , using a2 2 2

100-W medium pressure mercury lamp. The addition of 5 .8. Triazine herbicidesTiO or Fe in combination with H O was effective to2 2 2

degrade the pesticide. The authors measured the apparent Haag and Yao [244] measured the rate constants for?first-order rate constants for disappearance of parathion. reaction of HO radicals with different drinking water

These increased with increasing load of TiO , but when contaminants, including atrazine (vide supra) and simazine2


?(vide supra). A variety of HO generation methods were ised TiO . The rate constants so-obtained are k(triazine12? 10 10 10 10used, among which is the photo-Fenton method, that was HO )51.5310 , 0.95310 , 1.1310 and 1.3310

21 21used with both compounds. The rate constants found were, M s , respectively, for ametryn, atrazine, propazine and9 9 21 21respectively, 2.660.4310 and 2.860.2310 M s . prometryn. The authors point out that the presence of a

´ ´ ´Cernak and Cernakova [248] studied the application of a methylthio group in the 6-position (ametryn, prometryn)H O /UV combination for removing atrazine (vide supra), instead of a chlorine (atrazine, propazine) seems to induce2 2

prometryn (vide supra), terbuthylazine (vide supra), ter- a higher reactivity.?butryne (vide supra). They observed a maximum of HO reactivity has been studied with simazine (vide

degradation in the first 5 min, longer exposures being supra) and terbuthylazine (vide supra) adsorbed on highlyinefficient. dispersed thin layer of silicon dioxide powder, used as

?A comparison of the photochemical oxidation of at- unreactive carrier [84]. The HO radicals were produced byrazine by combined UV/H O and other advanced oxida- photolysis of hydrogen peroxide in the gas phase with2 2

tion methods was carried out [251]. A monochromatic sunlamps. In this way, the first-order degradation rateslight source emitting at 254 nm was used for irradiation. observed for simazine and terbuthylazine were similar

21 21 21Hydroxyatrazine, N and N,N9-dealkylated, deaminated and (k ¯1.1310 M s ).HO?

hydroxyderivatives of atrazine were the major photo- Von Gunten et al. [255] recently revisited the degra-?products found. dation of atrazine (vide supra) by reaction with HO during

UV/H O photolysis of two products of photodegrada- advanced oxidation processes, identifying the main degra-2 2

tion of atrazine (vide supra), namely deethylatrazine and dation products: 6-amino-2-chloro-4-isopropylamino-s-tri-deisopropylatrazine was studied [151]. Significant reduc- azine, 6-amino-2-chloro-4-ethylamino-s-triazine, 4-acet-tions in the rate of photodegradation were observed in amido-2-chloro-6-isopropylamino-s-triazine, 4-acetamido-surface waters, which the authors attribute to the presence 6-amino-2-chloro-s-triazine, chlorodiamino-s-triazine, 2-of natural radical scavengers. No attempts were made to chloro-4-ethylimino-6-isopropylamino-s-triazine, 6-amino-identify by-products of these photolysis reactions. 2-chloro-4-ethylimino-s-triazine. The rate constant for

? ?Hapeman et al. [252] examined the reaction of HO reaction between atrazine and HO was determined as? 9 21 21radical reaction with atrazine (vide supra). HO was 3310 M s , approximately three times higher than

?generated by irradiation of nitrate solutions with 290 nm those measured for the different degradation products. HO?light from a xenon lamp. The products found were 20% attack on the ethyl group was favoured relative to HO

6-amino-2-chloro-4-isopropylamino-s-triazine, 10% 6- attack on the isopropyl group.amino-2-chloro-4-ethylamino-s-triazine, 6% 4-acetamide-2-chloro-6-isopropylamino-s-triazine, 3% 4-acetamide-2-chloro-6-ethylamino-s-triazine, 16% chlorodiamino-s-tri- 5 .9. Uracil herbicidesazine and 3% hydroxy atrazine, at 87% atrazine conver-sion. The reaction was found to be much faster than direct Muszkat et al. [253] studied the reaction patterns ofphotolysis that, at 23% photolysis led to 14% hydroxy photooxidation of bromacil h5-bromo-3-sec-butyl-6-atrazine and ¯9% of chloroalkyloxydized and chloro- methyluracilj in water by UV light from a mercury lamp,dealkylated products. and showed a small effect of oxygen on the degradation

A study of the reaction patterns of photooxidation of rate, but a strong influence from the presence of H O .2 2?metribuzin (vide supra) in water by the UV radiation The authors suggest the reaction is initiated by HO

supplied by a mercury lamp showed a pronounced effect of radicals generated by H O photolysis.2 2

oxygen on the degradation rate, with a small influencefrom the presence of H O [253]. The authors suggest that2 2

the process starts via a reaction of excited metribuzin with 5 .10. Urea-based pesticidesH O or O .2 2 2

The photo-Fenton degradation of atrazine (vide supra) in From a study of the application of a H O /UV (254 nm)2 2

water, using 330–400 nm light has been studied [245]. combination for removing chlorbromuron h3-(4-bromo-3-According to the authors, irradiation at these wavelengths chlorophenyl)-1-methoxy-1-methylureaj, linuron (videdid not enhance the degradation of the herbicides. The supra) it was concluded that the maximum extent ofmain pathways of degradation of atrazine were N-dealkyla- degradation took place within 5 min, with longer expo-tion and dechlorination. sures being inefficient [248].

Bellobono et al. [254] used competition kinetics to The degradation of diuron h3-(3,4-dichlorophenyl)-1,1-31 31obtain the rate constants for reaction of ametryn (vide dimethylureaj photoinduced by Fe , using different Fe

supra), atrazine (vide supra), propazine (vide supra) and species and exciting at 365 nm and under sunlight has been?prometryn (vide supra) with HO radicals at pH 7.0 on studied [256,257]. Apparently, the mechanism of photo-

?photocatalytic membranes, in the presence and absence of induced degradation involves only attack by HO radicals,trialkyl vanadates as photocatalytic promoters of immobil- yielding 3-(3,4-dichloro-phenyl)-1-formyl-1-methylurea as


the major degradation product, in a relatively slow process fometuron-methyl hmethyl 2-[4,6-dimethylpyrimidin-2-yl-31(t ¯1–2 h to a few days depending on [Fe ]). carbamoylsulfamoyl]benzoatej with photochemically gen-1 / 2

erated hydroxyl radical. The obtained values suggestindirect photolysis is most relevant in the photodegradation

5 .11. Miscellaneous of the studied compounds.Bauer et al. [264] proved the photo-Fenton method to be

Draper and Crosby [258] studied the sunlight and very powerful for treating mixtures of ten pesticides:near-UV induced photodegradation of different classes acrinathrin hcyano(3-phenoxyphenyl)methyl 2,2-dimethyl-of pesticides, such as thiocarbamates, chlorophenoxy 3-[3-oxo-3-[2,2,2-trifluoro-1-(trifluoromethyl)ethoxy]-1-acids, organophosphorus pesticides, N-methylcarbamate, propenyl]cyclopropanecarboxylatej, abamectin h(10E,14E,

?cyclodienes and others in the presence of dilute H O . HO 16E, 22Z)-(1R, 4S, 59S, 6S, 69R, 8R, 12S, 13S, 20R, 21R, 24S)-2 2

radicals were found to be the major reactants, and appro- 69[(S) - sec - butyl] - 21, 24 - dihydroxy - 59, 11, 13, 22-tetra4, 8 20, 24priate reaction mechanisms have been proposed in some methyl-2-oxo-3, 7, 19-trioxatetracyclo-[15.6.1.1 .0 ]-

cases. pentacosa-10, 14, 16, 22-tetraene-6-spiro-29-(59, 69-dihydro-A method has been developed for determining rates of 29H pyran)-12-yl-2, 6-dideoxy-4-O-(2, 6-dideoxy-3-O-

?generation and consumption of HO radicals in natural methyla - L - arabino-hexopyranosyl) - 3 - O - methyl - a - L -waters, based on measurement competition kinetics with a arabino-hexopyranosidej, endosulfan h(1,4,5,6,7,7-hexa-trace compound, using both a high pressure mercury lamp chloro - 8, 9, 10 - trinorborn - 5 - en - 2, 3 - ylenebismethylene)and sunlight irradiation [259]. Possible interference’s arise sulfitej, formetanate h3-dimethylaminomethyleneimino-from the presence of nitrate in natural waters. phenyl methylcarbamatej, imidacloprid (vide supra),

The rate of aqueous photodegradation of atrazine (vide lufenuron h(RS) - 1 - [2, 5 - dichloro - 4 - (1, 1, 2, 3, 3, 3 -supra), ametryn (vide supra), prometon and prometryn hexafluoropropoxy)-phenyl]-3-(2,6-difluorobenzoyl)ureaj,(vide supra) using the radiation supplied by a medium- methamidofos hO,S-dimethyl phosphoramidothioatej, ox-pressure mercury lamp was greatly enhanced by the amyl hN,N-dimethyl- 2-methylcarbamoyloxyimino-2-

31addition of Fe [260]. The rate enhancement is attributed (methylthio)acetamidej, propamocarb hpropyl 3-(di-? ?to the reaction with HO . Quenchers of HO reduced the methylamino)propylcarbamatej and pyrimethanil hN-(4,6-

rate of photodegradation. The photoproducts found were dimethylpyrimidin-2-yl)anilinej. The mixtures were usedthe ones expected for a Fenton reaction. The reaction rate as a model for a plant conceived for destroying pesticidesdecreased also in the absence of O and in natural water. in the wastewater of a pesticide bottle recycling plant. All2

?Mabury and Crosby [261] studied the reactivity of HO the pesticides underwent degradation, some of them rathertoward ten different pesticides: atrazine (vide supra), slowly. The degradation was incomplete, a residual amountcarbaryl (vide supra), carbofuran (vide supra), 2,4-D (vide remaining in solution. Addition of oxalate accelerated thesupra), hexazinone, MCPA (vide supra), picloram h4- degradation, but did not improve the level of degradation.amino-3,5,6-trichloropicolinic acidj, propanil (vide supra) The results seem to be better than those previouslyand quinclorac h3,7-dichloro-8-quinolinecarboxylic acidj. obtained by photocatalysis in the presence of TiO .2

The reaction rates were estimated by competition kinetics,using p-nitroso-N,N-dimethylaniline as a standard com-

?petitor. HO radicals were generated by photolysis of H O2 2

using a 40-W fluorescent UV lamp, with a maximum in its 6 . Other papers of interestUV emission at 310 nm. The most reactive compound was

9 21 21carbaryl (3.4310 M s ), and the least reactive hexa- A seminal detailed study of the photo-Fries reaction,9 21 21zinone (0.62310 M s ). However, the estimated including a comprehensive mechanistic scheme is due to

values are rather lower than those determined by Haag and Sandner et al. [265] These authors present evidence thatYao [244] by photo-Fenton reaction. The authors estimated the photo-Fries rearrangement takes place via a 1,3-sigmat-

?for the reaction of HO with benzoic acid a rate constant of ropic shift originating from an upper singlet state. The9 21 213.2310 M s , to be compared with the authentic process is of most interest in relation to the photodegrada-

9 21 21value of 5.9310 M s , obtained by Buxton et al. tion of pesticides such as carbamates and ureas.[262] by pulse radiolysis. Bent and Hayon [266] studied by 265-nm laser flash

Armbrust and Reilly [263] measured the rate constants photolysis the formation and the reactivity of the tripletfor reaction of methomyl h(Z)-S-methyl N-(methylcar- state of s-triazine in acetonitrile, cyclohexane and iso-bamoyloxy)thioacetimidatej, oxamyl hN,N-dimethyl-2- propanol. The triplet–triplet absorption spectrum showedmethylcarbamoyloxy-imino-2-(methylthio)acetamidej, ben- maximae at 245 and 303 nm, and shoulders at 345 and 450

6 21sulfuron-methyl hmethyl 2-[(4,6-dimethoxypyrimidin-2- nm. The rate of decay in acetonitrile was ¯1.1310 s .yl)carbamoylsulfamoylmethyl]benzoatej, azimsulfuron h1- The triplet state was efficiently quenched by H-atom

2(4,6-dimethoxypyrimidin-2-yl)-3-[1-methyl-4-(2-methyl- donors, and probably by e -donors.2H-tetrazol-5-yl)-pyrazole-5-yl]sulfonyljureaj and sul- Wells et al. [267] described the photochemistry of the


pyrimidinic fungicides dimethirimol h5-butyl-2-di- triplet radical pair in a solvent cage, that can then decay bymethylamino-6-methylpyrimidin-4-olj and ethirimol h5- three different pathways: (i) intersystem crossing to thebutyl-2-ethylamino-6-methylpyrimidin-4-olj, as well as the singlet radical pair that then forms the (2,5-dimethox-structurally related compound 2-dimethylamino-5,6-di- yphenyl)hydrazone of 9-fluorenone, (ii) separation of themethylpyrimidin-4-ol. The fluorescence, phosphorescence triplet radical pair, and (iii) disproportionation yieldingand other photophysical properties have also been de- 9-fluorenone ketimine and triplet (2,5-dimethoxy-scribed. phenyl)nitrene.

Bersohn et al. [268] studied the photodissociation of Schmidt et al. [275] carried out an NMR and moleculars-triazine at 248 and 193 nm, finding three HCN molecules mechanics study of alachlor (vide supra) conformation, andas photoproducts. Different considerations are made about of its implications on the mechanism of photodegradation.the dynamics of the system and of the kinetic energy The authors conclude that environmental factors affectingdistributions of the molecules at both wavelengths. the conformation of alachlor at the molecular level may

Getoff et al. [269] studied the pulse radiolysis of alter both the rate and pathway of alachlor degradation.32paraquat (vide supra) in air-free aqueous solution at The use of ferrioxalate (Fe(C O ) ) as mediator of2 4 3

various pHs. The very fast reaction of paraquat with the sunlight photodegradation of organic pollutants in water2solvated electron took place with k(paraquat1e )57.53 has been studied [276]. Irradiation of ferrioxalate proceedsaq

10 21 2110 mol M s , yielding a long-lived radical cation as:showing to absorption maximae at 392.5 and 600 nm,

32 22 ?2(Fe(C O ) ) 1 hná(Fe(C O ) ) 1 (C O )respectively. Other processes undergone by paraquat have 2 4 3 2 4 2 2 4

also been characterized.Wardman [270] measured by pulse-radiolysis the one- ?2 32 22(C O ) 1 (Fe(C O ) ) → (Fe(C O ) ) 12 4 2 4 3 2 4 2electron reduction potential for various bipyridinium com-

22pounds. Among them, the one-electron reduction potential (C O ) 1 2CO2 4 221for paraquat is E8(methyl viologen /methyl

?1 ?viologen )520.45 V vs. NHE. In the presence of H O , HO radicals are generated as in2 2

Katagi [271] has performed quantum chemical estima- the photo-Fenton method (see above). The efficiency oftions of the environmental fate of pesticides, expected to this process for photodegradation of organic pollutants inbe useful in making considerations about the photooxida- tap water has been shown to be at least 25 times highertion and photosensitized degradation of pesticides. The than using sunlight /TiO /H O .2 2 2

photoinduced decarboxylation and isomerization of pyre- The fate of peroxyl radicals in aqueous medium hasthroids were successfully explained using a photolysis been recently reviewed by von Sonntag et al. [277].index based on the change of electron population at each Peroxyl radicals are formed during drinking-water andbond upon excitation. wastewater processing by reaction of carbon-centered

Halmann et al. [272] studied the photodegradation of radicals with dissolved O , in a reaction that is irreversible2

xenobiotic contaminants in water samples from a heavily in most cases. The carbon-centered radicals from which the?polluted well by sunlight, using three different combina- reaction starts are generated by reaction of HO with

31tions of photocatalysts: TiO /H O , Fe /H O and dissolved organic compounds.2 2 2 2 231TiO /Fe /H O . Among these, were acetamides, ben- Heit and Braun [278] described different effects ob-2 2 2

zonitriles, ureas and triazines. The third combination led to served upon VUV-irradiation (172 nm) of aqueous solu-appreciable degradation within 3–4 h. More complete tions containing organic compounds. The observed pro-photodegradation required long exposures. cesses are dominated by the very short lifetimes of the

Fischer and Nwankwoala [273] studied the photolysis of intermediates generated by homolysis of water.1,2,3-triazine in hexane and methanol and in vapour phase. Braun and Oliveros [279] recently reviewed the state-of-They found decreasing photodissociation quantum yields the art of methods for water treatment, discussing how toas the wavelength increases, and independence of the evaluate them. The authors suggest appropriate solutionsquantum yields with the nature of the solvent and the may be found in the combination of photochemical andpresence of air in solution. Acetylene and an unidentified thermal (catalyzed) processes.compound were the major photoproducts, indicating that Wagner et al. [280] described the use of excimer lasersphotofragmentation is not the main photodegradation for wastewater treatment; 193 nm and 248 nm excimerpathway. This is not the case in vapour phase, were lasers were used, with the first showing higher efficiencyacetylene and HCN were they main products, suggesting a than the second.concerted mechanism. A detailed study that is related to the photodegradation

Scaiano et al. [274] studied the laser flash photolysis of of carbamates and ureas has been carried out by Tsen-carbamates derived from 9-fluorenone oxime. The authors talovich et al. [281], who investigated the photo-Friespropose a mechanism involving N–O and C–N bond rearrangement of 1- and 2-naphtyl acetates using steady-scission upon excitation and subsequent generation of a state photolysis and laser flash photolysis, and proposed a


general mechanism for the photolysis of both compounds. amine, isopropylamine, 2-isopropylamino-s-triazine, 2-Upon irradiation, naphthyl acetates are excited into the first ethylamino-s-triazine and 2-hydroxy-4-ethylamino-6-iso-excited singlet state, that is responsible for most of the propylamino-s-triazine were the observed products. Cleav-generated photoproducts. This singlet state can either age of the C–Cl bond was the main process, with halogendissociate to yield a radical pair or undergo intersystem substitution by a hydroxyl group.crossing to the T triplet state, which can then yield the Dionysiou and co-workers [289,290] recently developed,2

same radical pair as the singlet state or relax to the characterized and evaluated a rotating disk photocatalyticunreactive T triplet state within few nanoseconds. reactor for the photodegradation of organic pollutants.1

The UV–Vis absorption and emission spectra of the They tested the device using 4-chlorobenzoic acid andfungicide fenarimol in solution have been studied [282]. chlorophenols as a model compounds and a batch of lowComparison of the spectra with those of chlorotoluenes pressure mercury tubes. The reactor exhibited advantagesand pyrimidine, as well as the effect of solvent polarity on such as immobilization of the catalyst, reactions in a thinthe absorption spectrum allowed the authors to conclude liquid film and a mixing regime close to that of an idealthat the lowest excited singlet state is localized on the continuously stirred tank reactor. Although the photonic

*pyrimidine ring, having n,p character. Higher excited efficiency of the reactor was low in some cases, the authors*p,p states are localized on the chlorotoluene and py- expect to be able to optimize it.

*rimidine rings. A weak fluorescence from the n,p state is Feng and Nansheng [291] reviewed the photochemistryobserved, with its quantum yield being strongly dependent of hydrolytic iron(III) species and the photoinduced degra-on the solvent. Phosphorescence was observed at low dation of organic compounds in connection with these

31temperatures, and was attributed to a triplet state with inorganic species. They show how the pH, Fe con-*p,p character. centration, wavelength and energy of irradiation source,

The use of the photo-Fenton method for degradation of and the nature of the organic compounds influence thenitrogen-containing compounds was studied [283]. Aro- photodegradation processes. All the low-molecular weight

31 ? 21matic compounds degraded to a extent higher than the derivatives of Fe can generate HO , with Fe(OH)31aliphatic ones. In most cases nitrogen was released as being the most relevant. Hydrolytic Fe polymers are

ammonia upon photodegradation. presumed to also be photoactive. The mechanisms ofSolar et al. [284] have studied the degradation aqueous action of other environmentally relevant species are dis-

chlorophenol solutions by the g-radiation obtained from a cussed.60Co-g-source. The results showed that 50-mM solutions The UV absorption and fluorescence maximae of 2,4,6-were completely degraded using 600 and 1000 Gy, respec- triaryl-1,3,5-triazines show a marked bathochromic shift astively. the acidity of the medium increases Evidence for a simple

Leintner et al. [285] studied the degradation of aqueous protonation equilibrium came from the existence of clearlyatrazine (vide supra) from solutions containing humic defined isosbestic points. As the acidity of the medium

60substances, using g-radiation obtained from a Co-g- increases, the fluorescence of the unprotonated triazines issource. The studies were carried out with continuous replaced by the much weaker fluorescence of the proton-bubbling of N , O , N O and N O/O (80:20). Atrazine ated species, which shifts to longer wavelengths. As with2 2 2 2 2

? 2was degraded both by reaction with HO and with e . For many other aza-nitrogen containing heterocycles, the pKaq a? ?2the radiation dose applied (¯800 Gy), HO and O were increases from 6.8 to 9 units upon excitation from 21.7 to2

the most reactive species. 3.7 in the ground state to 7–11 in the lowest singlet excitedArmbrust [286] used a computerized model to compare state [292,293]. The excited state of the protonated form

21the relative importance of the photochemical processes was calculated to be 37–51 kJ mol lower than that of theinfluencing pesticide degradation in rice paddies. Among unprotonated species.these, indirect photodegradation processes: degradation by Lester et al. [294] reviewed the abiotic degradation ofhydroxyl and carbonate radicals, and by singlet oxygen, pyrethroids, triazines and ureas, concluding they arehave been shown to be far more important than direct susceptible of photodegradation, and that the potential forphotolysis. their photosensitised degradation is uncertain.

Esplugas et al. [287] studied the influence of H O and Recently, Cid et al. [295] carried out different bioassays2 221Fe in the photodegradation using UV radiation of with two freshwater microalgal species, Chlamydomonas

nitrobenzene in aqueous solution. Both the concentration moewusii and Chlorella vulgaris, in an attempt to assess21of H O and Fe showed great effect on the rate of the efficiency of photodegradation of paraquat (vide supra)2 2

photodegradation, with the first being more efficient. The and isoproturon (vide supra) in water. The evolution ofmajor photoproducts found were nitrophenol isomers. growth and of concentration of photosynthetic pigments of

Atrazine (vide supra) was shown to be completely the above microalgae upon a 96-h treatment with the puredestroyed in aqueous solution upon g-irradiation using herbicides, as well as with solutions of the same that had

60doses of 0.1–50 kGy from a Co source [288]. Ethyl- been photolysed with 248 nm light were measured. The


results show that the attempted photodegradation can effect of factors such as the acidity of the medium, salteffectively reduce the biotoxicity of herbicides. concentrations, presence of buffers, etc. are, in general, not

properly understood. It will be difficult to model theenvironmental fate of agrochemicals unless detailed mech-

7 . Concluding remarks anistic studies become available, and these should be takenas a major task and encouraged for the near future.

Photodegradation studies on pesticides have focussed ontwo distinct, but not unrelated, objectives. Firstly, it isimportant that for their efficacy they do not photodegrade A cknowledgementsduring the time they are exercising their biocidal activity.On the other hand, for environmental considerations it is The authors gratefully acknowledge the financial supportimportant that they can eventually be converted to innocu- ´of the Spanish Ministerio de Educacion y Cultura (projectous, and preferable mineral, photoproducts. Direct sunlight AMB99-0325), the Conselho de Reitores das Univer-induced photodegradation of most pesticides is a minor sidades Portuguesas (Portugal) and the Deutscherevent, which leads to the problem of their long-term Akademisher Austauschdienst (Deutschland Bundesrepub-persistence in the environment. Instead, these are more lik) through the Acciones Integradas bilateral programseffectively degraded by using UV light, despite the higher HP1999-0078 and HA1997-0128, as well as of the Minis-economic cost of the process. Alternatively, photosensit- ˜´terio de Ciencia y Tecnologıa (Espana) and of the Xuntaized and photocatalytic processes are also becoming ˜de Galicia (Galicia, Espana) through projects PPQ2000-popular for treating pesticide residues, although they do 0449-C02-01 and PGIDTOOMAM10303PR, respectively.have certain drawbacks, such as the high biotoxicity of During the editorial process, both the editor and refereessome of the sensitized compounds or metal catalysts used. made useful suggestions that helped improve this reviewIndirect degradation methods based on the reaction of the and must be acknowledged.pesticides with hydroxyl radicals are also effective andhave low cost, but the reaction products produced are insome cases as problematic or more so than the starting R eferencesmaterials. In this review, we have discussed examples ofthese processes with the most important groups of pes- [1] R. Grover, A.J. Cessna (Eds.), Environmental Chemistry of Her-ticides, herbicides, insecticides and fungicides. bicides, CRC Press, Boca Raton, FL, 1991.

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