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Environmental Pollution 157 (2009) 3308–3313

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Environmental Pollution

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Photolytic degradation of methyl-parathion and fenitrothion in ice and water:Implications for cold environments

Jan Weber a, Romana Kurkova b, Jana Klanova b, Petr Klan c,*, Crispin J. Halsall a,*

a Lancaster Environment Centre, Centre for Chemicals Management, Lancaster University, Lancaster LA1 4YQ, UKb RECETOX, Faculty of Science, Masaryk University, Kamenice 3, 625 00 Brno, Czech Republicc Dept of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 625 00 Brno, Czech Republic

Photodegradation of methyl-parathion and fenitrothion in water and

ice under environmentally-relevant conditions is described.

a r t i c l e i n f o

Article history:Received 15 May 2009Received in revised form19 May 2009Accepted 21 May 2009

Keywords:PhotochemistryPesticidesSolarIceMountainsArctic

* Corresponding authors.E-mail addresses: klan@sci.muni.cz (P. Klan),

Halsall).

0269-7491/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.envpol.2009.05.045

a b s t r a c t

Here we investigate the photodegradation of structurally similar organophosphorus pesticides; methyl-parathion and fenitrothion in water (20 �C) and ice (�15 �C) under environmentally-relevant conditionswith the aim of comparing these laboratory findings to limited field observations. Both compounds werefound to be photolyzed more efficiently in ice than in aqueous solutions, with quantum yields ofdegradation being higher in ice than in water (fenitrothion > methyl-parathion). This rather surprisingobservation was attributed to the concentration effect caused by freezing the aqueous solutions. Themajor phototransformation products included the corresponding oxons (methyl-paraoxon and feni-troxon) and the nitrophenols (3-methyl-nitrophenol and nitrophenol) in both irradiated water and icesamples. The presence of oxons in ice following irradiation, demonstrates an additional formationmechanism of these toxicologically relevant compounds in cold environments, although further pho-todegradation of oxons in ice indicates that photochemistry of OPs might be an environmentallyimportant sink in cold environments.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Methyl-parathion (O,O-dimethyl-O-(4-nitrophenyl)phosphorothioate; 1) (see Scheme 1) and fenitrothion (O,O-dimethyl-O-(4-nitro-m-tolyl)phosphorothioate; 2) are current-use contact insec-ticides that were introduced in the 1950s (INCHEM, 1992, 1993).Methyl-parathion was amongst the 10 most used organophos-phorus pesticides (OPs) in the USA and was extensively used inRussia in the late 1990s and in 2001 (Kiely et al., 2004; Orme andKegley, 2004), and is currently registered in the EU (Orme andKegley, 2004). Fenitrothion, on the other hand, was not as widelyused and has now been banned in a number of European countries,but is registered in the United States, Russia, and the U.K. (Kielyet al., 2004; Orme and Kegley, 2004). There is growing evidencethat OPs are subject to regional and long-range atmospherictransport with subsequent deposition and accumulation in alpineand arctic environments (Chernyak et al., 1996; LeNoir et al., 1999;Muir et al., 2004; Hermanson et al., 2005; Hageman et al., 2006).Both methyl-parathion and fenitrothion have been detected in

c.halsall@lancaster.ac.uk (C.J.

All rights reserved.

dated snow cores taken in the Norwegian Arctic, with concentra-tions ranging between 7 and 33 ng L�1 (meltwater), comparable to,or greater than, those of organochlorine pesticides such as a- and g-HCH (Hermanson et al., 2005). These snow cores revealed historicalatmospheric accumulation, with both OPs showing an increase inconcentrations from the 1960s to the late 1990s, suggestingongoing and increasing use of these chemicals over this timeperiod. Despite their occurrence in remote environments, OPs aregenerally considered to be more susceptible to abiotic degradationthan organochlorine insecticides, with numerous studies investi-gating their degradation in aqueous systems (e.g Wolfe et al., 1977;Lartiges and Garrigues, 1995).

Both methyl-parathion and fenitrothion absorb light at wave-lengths>290 nm in the solar UV range, making them susceptible todirect photolysis, with photodegradation considered to be animportant transformation process in both artificial and naturalwaters (Castillo et al., 2000; Derbalah et al., 2004; Draper andCrosby, 1983; Durand et al., 1994, 1992; Fukushima and Katagi,2006; Lacorte and Barcelo, 1994; Sakellarides et al., 2003). Irradi-ation of the structurally similar methyl-parathion (1) and feni-trothion (2) in aerated aqueous solutions, as illustrated in Scheme 1,leads to analogous photoproducts via three major pathways:oxidative replacement of sulphur (S]P) by oxygen (O]P) to giveparaoxon (3) (Castillo et al., 1997; Chukwudebe et al., 1989;

PH3COOCH3

S

NO2

R

PH3COOCH3

O

NO2

R

3: R = H4: R = CH3

1: R = H2: R = CH3

hν

H2O+

OH

NO2

R

5: R = H6: R = CH3

PH3COOCH3

S

R

7: R = H8: R = CH3

+

Scheme 1. Photochemcial reaction of methyl-parathion (1) and fenitrothion (2) withphotoproducts of methyl-paraoxon (3), fenitrooxon (4), 4-nitrophenol (5), 3-methyl-4-nitrophenol (6) and the denitrated forms of methyl-parathion (7) and fenitrothion (8).

J. Weber et al. / Environmental Pollution 157 (2009) 3308–3313 3309

Sakellarides et al., 2003) and fenitrooxon (4) (Castillo et al., 1997;Durand et al., 1994; Greenhalgh and Marshall, 1976; Lacorte andBarcelo, 1994; Sakellarides et al., 2003), formal hydrolysis yielding4-nitrophenol (5) (Castillo et al., 1997; Sakellarides et al., 2003) and3-methyl-4-nitrophenol (6) (Castillo et al., 1997; Durand et al.,1992; Lacorte and Barcelo, 1994; Sakellarides et al., 2003), ordenitration to give 7 (Sakellarides et al., 2003) and 8 (Durand et al.,1994, 1992; Sakellarides et al., 2003), respectively. Oxidation of themethyl group on the aromatic ring of 2 to give the correspondingcarboxylate 9 (Durand et al., 1994, 1992; Greenhalgh and Marshall,1976) or isomerization to its corresponding methylthio isomer 10(Castillo et al., 1997; Durand et al., 1994, 1992; Lacorte and Barcelo,1994; Sakellarides et al., 2003) have also been reported (Fig. 1). Thethrough photoproducts are relatively stable in aqueous solutions inthe dark (Gal et al., 1992), and the disappearance quantum yields of1 and 2 through photodegradation in aqueous solutions have beenreported and their photolysis rates in water under solar radiationwere estimated (Wan et al., 1994).

In cold environments, snow and ice can have a marked influenceon photochemical transformation of semi-volatile organiccontaminants (Klan and Holoubek, 2002; Grannas et al., 2007). Forexample, the photodegradation of chlorobenzenes or chlor-ophenols in ice can give rise to more persistent polychlorinatedbiphenyls (PCBs) or chlorohydroxybiphenyls through bimolecularreactions at the ice-crystals nodes (Klanova et al., 2003a,b; Klanet al., 2000, 2001). Hydrophobic or hydrophilic solute molecules areknown to become spontaneously segregated at grain boundaries ofice during the freezing process. Such a solute concentration-enhancing effect may cause the solute organic molecules to ‘self-organize’ and their chemical reactivity is significantly altered (Klanet al., 2000; Heger et al., 2005; Heger and Klan, 2007). The physico-chemical properties of the contaminants, their location and localconcentrations in the snow matrix, and the wavelength andintensity of radiation are the most important factors in controllingthe contaminants’ lifetimes (Matykiewiczova et al., 2007a).

While photochemical processes in aqueous media have beeninvestigated for many OPs, including methyl-parathion and feni-trothion, photochemistry in ice and snow is essentially unknown,yet may play an important role in the fate of these chemicals in both

PH3COSCH3

O

NO2

H3C

10

PH3COOCH3

S

NO2

H3CO2C

9

Fig. 1. Possible photoproducts formed through irradiation of 2 in aerated aqueoussolutions. Oxidation of the aromatic methyl substituent gives carbomethoxyfeni-trothion (9) and isomerisation may occur to yield the S-methyl isomer (10).

polar and mountainous environments. Furthermore, the formationof photoproducts, such as the oxons, may be of toxicological rele-vance if they are formed in ice and subsequently released withspring melt (Blaha et al., 2004). In this study, we report a laboratoryinvestigation into the photochemical fate of 1 and 2 in both waterand ice in an attempt to understand the fate of these photolabilecompounds in cold environments. Here the degradation photoki-netics of both compounds are evaluated in water and ice phases, aswell as the identification and fate of the main phototransformationproducts.

2. Experimental

2.1. Materials

The following chemicals, methyl-parathion (Pestanal 99.8%), fenitrothion (Pes-tanal 95.4%), anhydrous sodium sulfate (99%), 3-methyl-4-nitrophenol (98%), andvalerophenone (99.9%) (Sigma Aldrich), fenitrooxon (96.8%) and methyl-paraoxon(96.3%) (LGC Promochem), 4-nitrophenol (99%; Lachema), acetonitrile, cyclohexaneand dichloromethane (all 99.9%; LabScan), and hexadecane (99.8%; Fluka), wereused as received. Water was purified on an Osmonics 2 and a Millipore Simplicity185 purifier (res. 18 MU cm, TOC < 5 ppb).

2.2. Instrumentation

UV–vis spectra of the liquid aqueous solutions were measured against a Milli-pore water sample in 1 cm-quartz cuvettes on a Varian Cary 50 Bio UV–vis spec-trophotometer. HPLC analyses were performed on an HP 1100 Agilent (HewlettPackard) equipped with photodiode array detector (l ¼ 220 nm) and a reverse-phase column (Gemini C-18, 5 mm; 250 � 4.6 mm, Phenomenex; acetonitrile/water(75:25%); runtime ¼ 15 min at 1 mL min�1, 30 �C). A GC–MS system consisting of anHP 6890 gas chromatograph (Hewlett Packard) and a J&W Scientific DB-5MS fusedsilica column (60 m � 0.25 mm with 0.25 mm stationary-phase film (5% phenyl/95%methyl)polysiloxane) combined with an HP 5972 mass selective detector (HewlettPackard) was also used; the carrier gas was helium at a flow of 1.5 mL min�1. Injectorand transfer line temperature was 280 �C. Samples (1 mL) were injected in split mode(1:20) for a full-scan analysis and split-less in SIM mode. Initial temperature was80 �C, held for 1 min, the temperature was raised by 15 �C min�1–180 �C, then by5 �C min�1–310 �C, and the final temperature was held for 20 min. A GC-FIDShimadzu GC-2010 (fitted with a DB-5 column) was used during the quantum yieldmeasurements of valerophenone, using hexadecane as an internal standard.

2.3. Sample preparation and irradiation procedures

Photodegradation experiments were performed with methyl-parathion (1) andfenitrothion (2) in aqueous solutions (MilliQ water) prepared without the use oforganic co-solvents, in order to simulate environmental aquatic degradationprocesses more closely. The sample solutions were prepared by evaporating thesolvent (acetonitrile) of the stock solution under a stream of nitrogen. The residuewas re-dissolved in water and the solution was sonicated for 5 min. Formation ofa small amount (<5%) of 4-nitrophenol and paraoxon from 1 and 3-methyl-4-nitrophenol and fenitrooxon from 2 was observed during this procedure. Theconcentrations of the products in the irradiated samples were then corrected forthose in the sonicated (non-irradiated) sample.

Liquid (20 �C) or ice (�15 �C) samples (c w 3 � 10�5 mol L�1) in 13 � 100-mmquartz vessels (purged either by argon or air), sealed with septa, were irradiatedsimultaneously at >290 nm (Pyrex-filter) using a medium-pressure mercurydischarge lamp (125 W; Teslamp Co.) in a ‘‘merry-go-round’’ apparatus: the icesamples maintained at �15 �C by immersion in an ethanol cooling-bath. Each runwas conducted in triplicate and seven time points were sampled over each irradi-ation period lasting 3 h. The irradiated samples were analyzed for 1 and 2 directly onHPLC-DAD to obtain the degradation dependencies (Figs. 3 and 4). Identification andquantification of the photoproducts was carried out by comparison of their retentiontimes and MS spectra with those of the authentic compounds or the NIST massspectral library. The aqueous samples for both compounds (75 mL ofc w 3 � 10�5 mol L�1) were liquid extracted with dichloromethane (3 � 15 mL). Theextracts were concentrated under a nitrogen stream to 10 mL, dried with 5g Na2SO4,filtered with glass wool, and further concentrated to 0.1 mL under a nitrogen streamfor GC–MS analysis.

2.4. Quantum yield measurements

Quantum yield experiments were carried out on an optical bench consisting ofa high-pressure 350 W Hg(Xe) lamp, an Oriel CornerStone 130 1/8 m mono-chromator with grating 200–1600 nm and a 1-cm quartz cell containing the samplesolution (degassed by purging with argon for 15 min before irradiation). The lightintensity was monitored by a Si photodiode detector (UV enhanced) with an Oriel

Table 1Major products detected after methyl-parathion (1) photolysis in both water and ice(both aerated and degassed start solutions).a

Product Characteristic ions (m/z)

paraoxon (3) [M]þ ¼ 247, 230, 109, 964-nitrophenol (5) [M]þ ¼ 139, 109, 65

a 20 or �15 �C; c w10�5 mol L�1.

J. Weber et al. / Environmental Pollution 157 (2009) 3308–33133310

OPM multifunction optical power meter controlled by TRACQ32 software. Valer-ophenone (c¼ 5�10�3 mol L�1) was used as an actinometer (Wagner et al., 1972) incyclohexane with addition of hexadecane (c ¼ 1 � 10�4 mol L�1) as an internalstandard. Samples of fenitrothion in water (c¼ 3�10�5 mol L�1) and valerophenonein cyclohexane were purged for 30 min with argon to remove oxygen, a knowntriplet quencher, and were irradiated for w40 min at 290 � 5 nm at 20 �C. Afterirradiation, samples were immediately measured on HPLC-DAD and GC-FID. Theconversions were kept below 5% in order to avoid interferences from photoproducts.

3. Results

The UV spectra of methyl-parathion (1) and fenitrothion (2) inaqueous solutions are presented in Fig. 2. Their main absorptionbands have a similar shape (lmax

1 w276 nm; lmax2 w272 nm);

a small hypsochromic shift of the absorption maximum in 2 and thecorresponding lower molar absorption coefficient are apparentlycaused by the steric interactions of an ortho methyl group to thenitro group, which is, however, opposed by its electron-donatingeffect. Although both derivatives absorb significantly below310 nm; they also have considerable tail absorption within the solarregion of 310–400 nm. The concentration of both parentcompounds (in methanolic aqueous solutions) was found to haveno effect on the character of their absorption spectra.

3.1. Photoproduct identification

The water (20 �C) and ice samples (�15 �C) of methyl-parathion(1) and fenitrothion (2), in the absence of any organic co-solventswere UV-irradiated through a Pyrex-filter in an ethanol bath ofa cryostat box. The use of Pyrex resulted in >300 nm multi-wave-length radiation that approximately simulated natural solar radia-tion. The initial sample pesticide concentrations were as low asw10�5 mol L�1. For photoproduct identification, the irradiationconversions were kept below 10%, thus allowing the formation ofthe major photoproducts, and, at the same time reducing secondaryphotoreactions as much as possible. Identification and quantifica-tion of the photoproducts was carried out by comparison of theirretention times and MS spectra with those of the authenticcompounds and the mass spectral library. The major photoproductsidentified in both aqueous and ice samples (both aerated anddegassed start solutions) are shown in Tables 1 and 2. In all cases,oxons (3 and 4) were identified as the major products formed withat least a 20% chemical yield, which, surprisingly, was also the casefor the degassed samples. The concentration of the nitrophenol

Fig. 2. Absorption spectra of methyl-parathion (1;d) and fenitrothion (2;– – –) inaqueous (liquid) solutions (c w3 � 10�5 mol L�1), compared with the calculated solarspectral actinic flux (F) at the Earth’s surface for cloud-free skies and solar 45� zenithangle (Hutzinger, 1982) (/).

derivatives (5 and 6), corresponded to w5% chemical yield, whichwas observed in the initial stages of the photolysis experiment andremained constant during the whole experiment. In addition,a small amount of methyl-parathion (1) (i.e. demethylated photo-product of 2) was detected in all photolyzed samples of 2. Bothqualitative and quantitative analyses of the photoproducts were,however, very difficult because of the low initial reagent concen-trations (w10�5 mol L�1). Furthermore, photoproduct conversionwas very low, and as some of these products were also photolabile,this resulted in photoproduct concentrations to be several orders ofmagnitude lower than the concentrations of the parentcompounds. Numerous trace photoproducts were also observedbut at concentrations below the limit of unambiguous identifica-tion. Since the complete mass balance was never achieved in any ofthe analyses (usually below 30%), and it always declined withincreasing conversions of the starting material, it was not possibleto complete an exhaustive list of photoproduct formation andassociated yields.

3.2. Initial quantum yields

The initial quantum yields of methyl-parathion (1) and feni-trothion (2) degradation (Fdeg), are shown in Table 3 and wereobtained by the actinometric measurements using the Norrish typeII reaction of valerophenone (Wagner et al., 1972). The quantumyields of fenitrothion degradation were approximately 1 order ofmagnitude higher than those of methyl-parathion in liquid aqueoussolutions, which is in good agreement with the data reported byWan et al. (1994), who reported Fdeg

1 ¼ 0.17 � 10�3 andFdeg

2 ¼ 3.1 � 10�3 at 313 nm in aqueous solutions containinga small percentage of methanol. The quantum yields measured inour study were found to be the same for both the degassed andaerated liquid solutions. Unexpectedly, the apparent initialquantum yields (F0deg) obtained for the ice samples of 1 and 2 werehigher than those of Fdeg measured in the liquid solutions byfactors of 6 and 3, respectively. F0deg were calculated assuming thesame quantity of light was absorbed by the reactant in both liquidand ice matrices. However, these numbers are considered to be atthe lower limits, because light scattering and reflection of ice allowsonly a fraction of the incident radiation to reach the chromophoricimpurities within the ice (Heger et al., 2005).

3.3. Photodegradation kinetics

Figs. 3 and 4 show the time-dependent consumption of 1 and 2,respectively, by photolysis in both liquid water and ice. No

Table 2Major products detected after fenitrothion (2) photolysis in both water and ice (bothaerated and degassed start solutions).a

Product Characteristic ions (m/z)

fenitrooxon (4) [M]þ ¼ 261, 244, 1093-methyl-4-nitrophenol (6) [M]þ ¼ 153, 136, 77methyl-parathion (1) [M]þ ¼ 263,125,109

a 20 or �15 �C; c w10�5 mol L�1.

Table 3Initial quantum yields of the degradation of 1, 2 and 3 (Fdeg) at 290 nm.a

Compound Fdeg � 103; water; 20 �C(degassed samples)b

F0deg � 103; ice; �20 �Cc

(degassed samples)b

methyl-parathion (1) 0.3 (0.3) 1.7 (1.5)fenitrothion (2) 2.7 (2.7) 8.4 (7.1)paraoxon (3) (0.3) (3.8)

a Non-degassed aqueous samples; the conversion was kept below 5% in order toavoid the photoproduct interference. Fdegs were measured by reference to valer-ophenone irradiated in cyclohexane (F ¼ 0.30) (Wagner et al., 1972).

b The samples were purged with argon for 30 min.c F0deg obtained for ice were calculated assuming the same quantity of light was

absorbed by the reactant in both liquid and ice matrices (see the text for discussion).

Fig. 4. Photodegradation of 2 in aerated aqueous liquid (20 �C) and ice (�15 �C)samples by multi-wavelength radiation >290 nm. The curve represents the expo-nential fit of the first order. The bars represent relative standard deviations derivedfrom triplicate measurements for each time step.

J. Weber et al. / Environmental Pollution 157 (2009) 3308–3313 3311

correlation for the absorption properties of the reactants wasmade; therefore, the data represent the relative values and servefor comparing the degradation efficiencies of the particular chro-mophore only. Methyl-parathion (1) degradation was more effi-cient in ice than in water during the course of the irradiationexperiments, which is in agreement with the measured (initial)quantum yields. The degradation profiles obtained for the liquidsolutions were fitted with monoexponential curves, which for theice samples corresponded to an exponential dependence of thethird order. In contrast, the degradation efficiencies of 2 arecomparable for both phases. This is somewhat contradictory to thequantum yield values shown in Table 3; however, it is worth notingthat both Fdeg and F0deg were calculated at very low (initial) reac-tion conversions, when the primary processes still dominate.

4. Discussion

4.1. Photochemistry

In this work, the full identification of all the photoproducts wasnot possible due to the low initial pesticide concentrations,although these were still considerably higher than those observedin remote polar or mountainous environments. Surprisingly, oxons(3 and 4) were formed as the major photoproducts in both theaerated and deoxygenated solutions. There are many possibleoxidation species that can convert thiophosphates to phosphates.However, under the conditions of these experiments, only oxygen

Fig. 3. Photodegradation of 1 in aerated liquid solution (20 �C) and ice (�15 �C) bymulti-wavelength radiation >290 nm. The curves represent the exponential fit of thefirst (– – –) and third (d) order. The bars represent the relative standard deviationsfrom triplicate measurements conducted at each time step.

is likely to be a readily available reagent. For example, the groundstate triplet oxygen, trapped by a radical, forms peroxy radicalswhich have been shown to oxidize the P]S bonds (Buckland andDavidson, 1987) (Scheme 2). Singlet oxygen formed by sensitizationof 3O2 by triplet excited species, may also be a reactant.

However, since the oxons were formed in the absence of oxygen(degassed solutions) we have considered other oxidation pathways.While we have no evidence of HO� production in our samples, it isknown that the hydroxyl radicals can be formed directly fromaromatic nitro compounds (i.e. 1 and 2) both in liquid solutions (Alifet al., 1987) and ice (Dubowski and Hoffmann, 2000), or that thenitro compounds can release NO, NO2

�, and NO3� in degassed

aqueous solutions (Alif et al., 1987), which can be further photo-lyzed to create HO� (Mack and Bolton, 1999) (Scheme 3). The effectof O2 on the degradation quantum yields in the ice samples isapparent from Table 3, although it is very small . It suggests thatoxygen may partially quench the excited species in a layer coveringthe ice-crystal surfaces, where both hydrophobic organic andinorganic compounds are known to accumulate prior to and duringirradiation (Matykiewiczova et al., 2007b; Petrenko and Whit-worth, 1999).

The nitrophenol (3 and 4, see Tables 1 and 2) concentrations werelow (<5%) and approximately constant during the whole course ofirradiation. These compounds are known to be readily photode-gradable (Dubowski and Hoffmann, 2000; Wan et al., 1994), andboth their production and degradation appeared to be at steady-state during the irradiation experiments. The nucleophilic attack ofwater molecules via the O–P bond cleavage is possible in liquidsolutions but is highly unlikely in frozen samples (Klanova et al.,2003a,b). However, since we do not know the intrinsic quantumyield of this process, we cannot evaluate its importance. Photo-heterolysis of the Carom-OP bond (Torrisi and Sortino, 2004) shouldnot take place here because no stabilization of the aryl cation isavailable (the nitro group is significantly destabilizing).

PH3COOCH3

SR

+ R-OO PH3COOCH3

SR OO-R

PH3COOCH3

OR

Scheme 2. The role of the peroxy radical in oxon formation.

PH3COOCH3

SR

+ HO PH3COOCH3

SR OH

PH3COOCH3

SHR O

PH3COOCH3

OR

Scheme 3. The role of the aqueous hydroxyl radical (HO) in oxon formation.

J. Weber et al. / Environmental Pollution 157 (2009) 3308–33133312

4.2. Degradation differences between water and ice

The reaction quantum yields (Table 3) demonstrate that 2degrades initially more efficiently than 1 by factors of 9 and 5 inwater and ice, respectively. The only structural difference betweenthe two compounds is the presence of the methyl group ortho to thenitro group in 2, and this must be responsible for the difference inbehaviour. Intramolecular photoreduction of the nitro group in o-nitrobenzyl derivatives is one of the most intensely studied reac-tions in photochemistry; however, the reaction is not very efficient(e.g., F w0.01 for 2-nitrotoluene) and is largely reversible(Schworer and Wirz, 2001). Therefore, it is possible that the methylgroup, which has been shown to be photochemically oxidized inaqueous solutions (Greenhalgh and Marshall, 1976; Durand et al.,1992, 1994), is responsible for more than one specific photo-degradation pathway.

Perhaps the most interesting finding was the enhanced initialphotodegradation efficiency of both 1 and 2 in ice compared to theliquid solutions. Light scattering and reflection (Heger et al., 2005)lower the inherent quantum yields of the photoprocesses takingplace in ice or snow, and this actual efficiency increase is indeednon-trivial. The photodegradation profile for methyl-parathion (1)shows that the compound is still consumed efficiently even afterextensive irradiation when secondary photoprocesses mustalready occur. In fact, the data obtained for ice correspond to anexponential fit of the third order, suggesting that different andcomplex photodegradation pathways must be responsible forphotoinduced degradation. This is not, however, the case for liquidwater, where the fit is monoexponential. Enhanced local concen-trations of the pesticide molecules at the grain boundaries of theice crystals are likely to account for this difference, whereby thespecific concentration effect (Heger et al., 2005; Petrenko andWhitworth, 1999) increases the probability of any intermolecularreactions. Interestingly, there was no apparent difference in thephotodegradation profiles of 2 in both liquid and ice samples(Fig. 3), with degradation in both media corresponding to a mon-oexponential curve. Nevertheless, the initial degradation quantumyield was higher in ice than liquid water by a factor of 3 (see Table3) although the difference was smaller than that observed formethyl-parathion (1). Since the quantum yields were obtained inthe initial stage of the reaction, they may reflect the intrinsicdifferences in reactivity, better than the degradation dependenciesto higher conversions. The additional methyl group in fenitrothioncan certainly undergo intramolecular photoprocesses and sincethe absolute quantum yield of fenitrothion degradation was higherthan that of methyl-parathion in both phases, then this explana-tion seems to be the most conceivable.

4.3. Environmental relevance

There are many factors controlling the presence of thesecompounds in remote polar or mountainous snowpacks. Differ-ences in physical–chemical properties for example, may account forthe wider atmospheric dispersal of one compound over the other.The lower vapour pressure of fenitrothion (1.3 � 10�4 Pa at 25 �C)compared to methyl-parathion (2.69 � 10�3 Pa) (Mackay et al.,2000) may make this compound more susceptible to ice-crystalsorption and hence efficient deposition with snowfall.

Furthermore, the absence of sunlight during the winter at higherlatitudes will prevent photolytic degradation. Most importantly,the major photoproducts in the ice samples were the respectiveoxons and nitrophenols. This raises the issue that these toxicolog-ically relevant compounds are likely to be formed in sunlit snowand ice; possibly complimenting other formation routes in theenvironment, such as photooxidation in the atmosphere andsurface waters. For example, chlorpyrifos-oxon (another widelydistributed phosphorothioate pesticide) has been detected in air,snow and surface water samples in remote mountainous environ-ments (LeNoir et al., 1999; Hageman et al., 2006) and formationthrough photochemical processes in seasonal snow and ice maynow be a plausible pathway. In this study, further photo-degradation of methyl-paraoxon (3) was also observed in ice witha degradation quantum yield comparable to its parent (see Table 3).As both methyl-paraoxon and fenitroxon have similar absorptionspectra to their parent compounds, then they would be expected toundergo degradation in the presence of sunlight. It is thereforeunlikely in the environment, that toxicologically relevant quantitiesof these transformation products would be released followingspring melt in either upland or arctic catchments.

Acknowledgements

The project was supported by the Czech Ministry of Education,Youth and Sport (MSM 0021622412). The authors would like tothank Laxminarayana Kammari and Ceslav Ulrich. JW and CH aregrateful to the UK branch of SETAC and the NERC ‘POPs Network’ forfunding travel to Brno, Czech Republic and purchasing pesticidestandards, respectively.

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