Simulated Solar Light Phototransformation of Organophosphorus

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Simulated Solar Light Phototransformation of Organophosphorus Azinphos Methyl at the Surface of Clays and Goethite Matthieu Menager† and Mohamed Sarakha*,‡ †

Clermont Université, Université Blaise Pascal, ICCF UMR CNRS 6296, équipe de Photochimie, BP 80026, F-63171 Aubière Cedex, France ‡ CNRS, UMR 6296, ICCF, BP 80026, F-63171 Aubière, France S Supporting Information *

ABSTRACT: The photochemical behavior of the pesticide azinphos methyl at the surface of clays (kaolinite, bentonite) and goethite was studied using Suntest setup (λ > 300 nm). The quantum yield on the clays was found to be roughly three times lower than that in aqueous solution. However, the photochemical efficiency was much higher at the surface of goethite owing to its photocatalytic activity through the hydroxyl radical production. The added humic substances on kaolonite show an inhibition of azinphos methyl degradation while the incorporation of iron(III) aquacomplexes leads to an important increase of the disappearance together with the formation of iron(II). Hydroxyl radical species were found to be formed either by excitation of goethite or clays. The goethite support acts as a more efficient catalyst for the formation of these reactive oxygen species. The photodecomposition reactions observed were (i) hydrolysis process leading to the formation of benzotriazone and the oxidation of the P = S bond giving rise to the formation of the oxon derivative, and (ii) homolytic cleavage of the N−C and C−S bonds of the organophosphorus bridge leading to the formation of dimers that appear to be specific to the irradiation at the surface of solid supports since they were not observed when the irradiation was performed in aqueous media: a statement that is related to the presence of aggregates at the surface of solid supports.

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INTRODUCTION Nowadays, the extension of intensive agriculture worldwide together with the desire to improve the yield of crops has led to the use of varieties and great amounts of pesticides.1 However, these compounds induce great pollution problems of the environment. This concerns not only a specific environmental compartment owing to the unavoidable transport to other compartments such as surface or ground waters,2−4 atmosphere,5,6 and soil.7,8 On this last medium, such organic molecules may reside at the upper surface layer for a certain period of time and undergo various transformation processes such as photochemical reactions. It is now averred that this abiotic degradation connected with the irradiation by solar light may represent an important pathway for the transformation of these pollutants9 and can then provide an alternative way to biodegradation. Investigating pesticide degradation occurring in the environment is of high interest, as both parent and generated byproducts can be hazardous because of their toxicity. In the course of our investigations on the photochemistry of pesticides, we have recently cast our attention on photolysis studies of polluted clays and soils. Due the complexity of the soil texture, several studies reported on the transformation of pesticides at the surface of soils or clays from the analytical point of view.10,11 Ciani et al.12 report a suitable method for the evaluation of the molar extinction coefficient at the surface of mineral porous media. Such evaluation permitted © XXXX American Chemical Society

an interesting experimental approach and model that leads to the kinetic description of the photolysis process of organic pollutants on clays and soils by the evaluation of the disappearance quantum yield .13,14 Thus, understanding photochemical reaction at the surface of homogeneous and well characterized clays may represent a first step to complete mechanistic studies in natural soil. Such an approach is of great importance since several papers have reported that nonpolar compounds in dry soils can adsorb in significant proportion on mineral part,15,16 mainly by clays. At the surface of soils, it is well-known that natural organic matter can act, under sunlight excitation, as a photosensitizer through the generation of various oxygen reactive species.17−19 On the contrary, the photoreactivity of clays is still not understood and is poorly reported in the literature. Katagi suggested the production of hydroxyl radical during the irradiation of montmorillonite and kaolinite involving electron transfer20,21 processes in the presence of oxygen. Moreover, clays may contain different amount of various iron species such as iron oxide (such as goethite) and titanium dioxide impurities22−25 that are able to absorb sunlight and lead to Received: June 28, 2012 Revised: December 6, 2012 Accepted: December 7, 2012

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dx.doi.org/10.1021/es301866f | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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determined using the procedure given in the SI. The evolution of iron(III) and iron(II) concentration as a function of irradiation time was determined using the reported method.34 Hydroxyl Radical Detection. ESR experiments were conducted on a Bruker ER 200D, X band (γ = 9.30 GHz), modulation frequency 100 kHz, from CRMP center at the University Blaise Pascal. In situ irradiations were carried out with a Xe−Hg lamp equipped with a filter λ > 340 nm. All the experiments were undertaken at room temperature in a quartz cell of 0.1 mm thickness by using 5,5-dimethylpyrroline as OH• radical scavenger. The evolution of hydroxyl radical formation was also performed by using spectroscopic techniques such as fluorescence according to the method adapted from Louit et al.35 and Newton et al.36 In the presence of mineral supports, slurry solutions were used at the concentration of 1.0 g L−1 of mineral support in the presence of coumarin, used as hydroxyl radical scavenger, at a concentration of 5.0 × 10−4 M. To maintain the homogeneity of the mixture (usually 50 mL), the irradiation was performed under vigorous agitation (400 rpm). It was undertaken with a MAZDA MAW 125 W lamp (up to 6) emitting at 365 nm and a 100-mL cylindrical quartz reactor. The deaeration of the solution was accomplished by continuous nitrogen or argon bubbling. Experiments with dry samples were conducted with mixtures prepared with the procedure described above using a layer thickness of 23 μm. The concentration of coumarin was 1.3 g per g of mineral support. The irradiations were performed in the Suntest device filtered at λ > 345 nm to avoid any direct photodegradation of coumarin. The solid−liquid extraction was performed with 4 mL of water and immediately filtered at 22 μm. Supernate was analyzed by spectrofluorometry. The chromatographic details and the spectroscopic analysis are described in the SI.

the formation of reactive species. In most of the cases, hydroxyl radical appears to be the main specie after clay irradiation. Organophosphorus compounds attained growing importance in pest control as insecticides, where their acute lethality is owing to the inhibition of acethylcholinesterase.26 They have been used as an alternative to organochlorine compounds which are suspected to be bioaccumulated up the food chain.27Azinphos methyl is tolerated in several countries.28 It is one of the most toxic organophosphorus pesticides for nontarget organisms. The photodecomposition of azinphos methyl was shown to occur efficiently on soils and leaf surfaces.29 Upon these surfaces, the rate of disappearance appeared to increase with increasing the moisture content. In our recent work, the photochemistry of azinphos methyl was deeply studied in aqueous solution.30 The two major byproducts were identified as azinphos methyl oxon which represents up to 50% of azinphos methyl conversion, and anthranillic acid. Both of them were previously detected in other studies.31 It is of great importance to note that the oxon derivative was not detected under these studies. This paper covers different aspects of the photochemical disappearance of azinphos methyl deposited at the surface of clays (kaolinite, montmorillonite, bentonite) with the evaluation of quantum yield. Because of the complexity of the studied media, it was therefore a challenge to study the complete photodegradation process of azinphos methyl not only from the kinetic, but also from the analytical, point of view.

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EXPERIMENTAL SECTION Chemicals and Materials. Azinphos methyl (O,O-diethyl S-[(4-oxo-1,2,3-benzotriazin-3(4H)-yl) methyl] ester) and azinphos methyl oxon were purchased as the highest purity from Riedel-de Haën (France) and Supelco (France), respectively. 1,2,3 Benzotriazin-4(3H)-one, coumarin, iron(III) as (Fe(ClO4)3), bentonite, montmorrilonite, and DMPO were provided by Aldrich (France). Kaolinite and humic acid were purchased from Fluka (France). All these products were used as received. The other reactants were of the highest grade available. Goethite was synthesized according to the procedure of Atkinson and co-workers.32 Aqueous solutions were prepared with deionized ultrapure water which was purified with a Milli Q device (Millipore) with purity controlled by its resistivity. The solid samples were used as films (see Supporting Information (SI) for details). The equilibrium adsorption was measured using batch experiments within the range 0.1 to 1.3 mg g−1 of azinphos methyl. Equilibrium was achieved by vigorously shaking the mixture azinphos methyl/clays in methanol for 8 h at 20 °C. Irradiation Apparatus. The prepared samples were irradiated horizontally in a Suntest CPS photoreactor (Atlas) equipped with a Xenon lamp and a filter that prevents the transmission of wavelength below 290 nm. The lamp was set at the intensity of 750 W m−2. The temperature of the sample was roughly maintained at 20 °C by a continuous flow of cold water through the bottom of the photoreactor. The actinometry was performed by the method reported by Dulin et al.33 After a given irradiation time, the irradiated sample was used for extraction with methanol in order to determine the Azinphos methyl conversion (see SI for details). and also for the elucidation of the byproducts. The dry sample was extracted with 2 mL of methanol and agitated for 15 min and then the solution was centrifugated for 10 min at 13 500 tr·min−1 and analyzed by HPLC experiment. The quantum yield was

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RESULTS AND DISCUSSION Photodegradation of Azinphos Methyl in Kaolinite. Prior to photodegradation experiments at the surface of clays, we examined the possibility of adsorption of the pesticide azinphos methyl (AZM). Under our experimental conditions, the adsorption process was studied at five concentrations ranging from 0.1 to 1.3 mg per g of kaolinite using the batch procedure and found that the adsorption varied with the concentration, namely 2% adsorption was obtained for the lowest concentration and 7% was obtained for the highest. Thus, a high proportion of AZM will be considered deposited at the surface of the clay either in a monolayer or multilayer structure. Except for the experiments where the effect of humidity was studied, all the experiments were conducted under dry conditions. The recovery of the pesticide was realized by a solid−liquid extraction which was estimated to be higher than 85−90%. It has to be pointed out that AZM was found to be stable when deposited on clays and kept in dark at room temperature for 24 h. To have a better insight into the spectroscopic features of AZM on kaolinite, we recorded the reflection spectrum using an integration sphere at various concentrations (within the range 0.32−7.9 mg g−1). The reflection diffuse spectra of AZM at the surface of kaolinite layer of 200 μm at various concentrations show that the percentage of reflected light decreases when the concentration of AZM increases. The analysis of the solid sample within the entire surface confirmed the homogeneous dispersion of the pesticide into the clay sample. By subtracting B



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the reflection part due to kaolinite, the reflection spectrum of AZM is obtained (Figure 1) where the evaluation of the molar

Figure 2. Disappearance of AZM at the surface of kaolinite as a function of irradiation time (irradiation on Suntest, 1.3 mg g−1 of kaolinite, 23 μm thickness). The insert represents the pseudo-firstorder kinetics. The results were an average of three individual experiments and the error bars correspond to the deviation from the average value.

Figure 1. Reflection spectrum of azinphos methyl at the surface of kaolinite (6.3 mg g−1, 200 μm thickness) compared to the absorption spectrum in aqueous solution.

as reported in the literature13,19 where the disappearance rate was assumed to be dependent on the photolysis process and on the pesticide diffusion phenomenon in the clay. The latter process may be considered minor for relatively thin films of clays. Figure 3 indicates that kobs × Z is constant within the range 12−70 μm (evaluated to 0.040 min−1 μm) and decreased

absorption coefficient was carried out using Kubelka−Munk function, f(R∞,i), since the film is thick enough:11,38 f (R ∞ , i ) =

2ln(10)εi(λ) k + Ci s s

where f(R∞,i) is the Kubelka−Munk function, k and s are the absorption and the scattering coefficient of kaolinite respectively; εi(λ) is the molar absorption coefficient of AZM at a specific wavelength; and Ci is the deposited concentration of AZM (mol cm−3). Under our experimental conditions, f(R∞,i) was found to be linear within the studied concentration range. The absorption maximum was then evaluated to 300 nm with ε300 nm = 15 × 103 mol−1 L cm−1 showing a bathochromic shift of about 16 nm as well as an increase of the molar absorption coefficient by roughly a factor of 2 when compared to the spectrum in aqueous solution. Such effect is well reported in the literature21,39 and can be attributed to different chemical and physical interactions between the clay and the pesticide. Both spectroscopic effects are in favor of an efficient and a better absorption of solar light (λ > 300 nm) by AZM when deposited at the surface of clays since the overlap with the sun emission spectrum will be higher. When AZM at the surface of kaolinite (1.3 mg g−1 of kaolinite with a thickness of 23 μm) is irradiated using a Suntest setup (λ > 295 nm), an efficient disappearance was observed. As shown in Figure 2, 50% conversion of the pesticide was observed after 6 h irradiation. The results were reproducible as far as temperature and light intensity were perfectly controlled during the irradiation. The decrease of the concentration of AZM follows a first order kinetics (insert of Figure 2) with an observed rate constant, kobs, evaluated to 2.0 × 10−3 min−1. The effect of the layer thickness, Ztot, which controls the light penetration, was studied within the range 12−118 μm with a constant concentration of AZM (1.3 mg g−1 of kaolinite) (SI Figure S1). The observed rate constant rapidly decreased when the layer thickness increased in agreement with the limitation of the light penetration through the clay particles for thick samples

Figure 3. Plot of kobs × Ztot as a function of the thickness layer Ztot.

rapidly for higher thickness leading us to the conclusion that the pesticide diffusion may be neglected for thin samples. Under these conditions the following expression may be used:13 kobsZtot = 1.433kpoZ0.5

where kobs represents the observed rate constant determined for a given layer thickness Ztot; kop is the rate constant for pesticide disappearance at the surface of the clay and Z0.5 is the thickness for which the light intensity is decreased by a factor 2. The latter parameter depends on the mineral clay used (Z0.5 = 6.2 μm for kaolinite) and was considered independent of the nature of the pesticide. The rate constant at the surface of the C



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Table 1. Photodegradation Rate Constant kobs of AZM on Kaolinite Film (Ztot = 23μm) at Various Conditions of Moisture, Temperature, Humic Acid, and Iron(III) Contentsb AZM mg g‑1

moisture

temperature °C

humic acid content %

iron(III) content %

0.30 0.66 1.3 6.6 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3

dry dry dry dry moista dry dry dry dry dry dry dry

15 15 15 15 15 5 35 15 15 15 15 15

0 0 0 0 0 0 0 0.5 2.0 0 0 0

0 0 0 0 0 0 0 0 0 0.5 1.0 3.0

kobs (Z = 23 μm) 1.4 2.4 2.0 1.2 5.0 1.3 2.7 2.0 1.2 2.6 4.1 9.3

(±0.2) (±0.2) (±0.3) (±0.2) (±0.5) (±0.6) (±0.4) (±0.2) (±0.3) (±0.4) (±0.9) (±1.3)

× × × × × × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

The moisture at the top surface of the sample was increased by the addition of 100 μL of ultrapure water. bIrradiation with Suntest, 1.3 mg g−1 of kaolinite. a

clay, kop, may then be estimated for pesticide disappearance and can be used for comparison purposes. It was evaluated to kop = 4.6 × 10−4 min−1. Such value may be used for the determination of the quantum yield of AZM disappearance on kaolinite (see Experimental Section) that was found equal to 1.4 × 10−3 which is roughly three times lower than that obtained in air saturated water−water upon irradiation at λ >290 nm.30 Influence of Physicochemical Parameters and Clays. Various experimental conditions such as pesticide concentration, moisture, temperature, humic substances, and iron contents were studied from the kinetic point of view. Table 1 gathers the observed rate constants, kobs, that were obtained by irradiation of AZM on kaolinite clay at a concentration of 1.3 mg g−1 and a thickness of 23 μm. When the concentration of AZM increased from 0.30 to 0.66 mg g−1, the observed rate constant increased owing to the increase of fraction of light absorbed by the pesticide. However, for higher concentrations, we clearly observe a decrease in the rate constant more likely due to a screen effect phenomenon. In the presence of high content of moisture, the degradation rate was found to increase rapidly which can be explained by both (i) a deeper penetration of light since the Z0.5 value increased from 6.2 μm in dry to 14 μm in humid kaolinite37 and (ii) the participation of photochemical processes that involve water molecules. The increase of temperature from 5 to 35 °C led to an increase of the observed rate constant from 1.3 × 10−3 min−1 to 2.7 × 10−3 min−1 more likely due a better diffusion of the pesticide. The presence of humic substances (HS) with a content that varies from 0.1 to 0.5% did not show any improvement of AZM disappearance but an increase of HS to a maximum of 2.0% led to the decrease of the degradation more probably due to a screen effect of HS and also as either selfquenching of reactive excited states or as a trap of reactive radical species. The addition of iron(III) species as potential photoinducer within the content range 0.5−3.0% showed an important increase of the observed rate constant. Together with the degradation of AZM, iron(III) was efficiently converted to iron(II) (SI Figure S2). The disappearance of AZM is correlated with iron(II) formation indicating that the pesticide degradation is mainly owing to the excitation of iron(III) species more likely through the formation of hydroxyl radicals as reported in the literature.40,41 Under these conditions the

nature of the generated byproducts will be different when compared to the experiments in the absence of iron(III). The degradation of AZM was also analyzed as a function of the nature of the mineral support. Two clays, kaolinite and bentonite, and also goethite (as iron oxide) were used (SI Table S1). The quantum yields were evaluated to 1.4 × 10−3 and 1.2 × 10−3 for kaolinite and bentonite, respectively, while it was found higher by more than 1 order of magnitude when the irradiation was undertaken at the surface of goethite. It was found equal to 4.1 × 10−2. This effect is mainly due to the role of goethite as photoinducer as largely reported in the literature.42−44 The irradiation of goethite at λ > 290 nm leads to the formation of hydroxyl radicals that efficiently react with the majority of organic compounds. This result is well correlated with the increase of AZM disappearance when the content of iron(III) species increased. It should be pointed out that the increase of humidity in the studied mineral supports led to an important increase of the rate constant indicating the influence of water molecule in the photochemical process as well as in the chemical structure of the mineral support.45,46 The possible hydroxyl radical formation was studied under heterogeneous and dry conditions with goethite and clays. The detection of such radical was performed using both ESR technique by employing DMPO as a radical trap leading to a stable nitroxide radical47 and by fluorescence spectroscopy using Coumarin as a radical quencher leading to the formation of 7-hydroxycoumarin, a highly fluorescent compound (λemission = 455 nm).35 When an aerated aqueous suspension of goethite (1.0 g L−1) was irradiated at λ > 345 nm with a Xenon lamp equipped with a filter in the presence of DMPO, a signal corresponding to a quartet with intensities 1,2,2,1 was observed (Figure 4). This is attributed to the adduct •OH-DMPO48,49 which is a clear evidence for the photochemical generation of hydroxyl radical by excitation of goethite. It should be pointed out that the same ESR spectrum was also observed when the experiment was undertaken by excitation of kaolinite. To compare the different rate constants of hydroxyl radical formation with the various mineral supports at λ = 365 nm, fluorescence experiments were performed using coumarin as hydroxyl radical scavenger. When the irradiation was performed in oxygenated suspensions the fluorescence intensity increased with the irradiation time (SI Figure S3). In oxygen free suspension of kaolinite, an initial increase was observed more D



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support. The elucidation was performed using either standards samples or LC/MS techniques. Two products with retention times of 12.58 and 21.16 min were easily identified by comparison with commercial standards. They correspond respectively to benzotriazone (BZT) at tret= 12.6 min and azinphos methyl oxon (AZM oxon) at tret = 21.2 min. For a given AZM conversion, BZT was found to represent, in relatively dry conditions, roughly 15% whatever the mineral support. Its amount increased rapidly when the moisture content increased. Azinphos oxon represented 8% when the irradiation was performed at the surface of kaolinite, 25% at the surface of bentonite, and found in trace concentrations on goethite. The products with retention times at 16.1 and 18.6 min were identified as methyl benzotriazone (methyl BZT) at tret= 18.6 min and demethylated azinphos methyl (demeythyl AZM) at tret = 16.1 min, respectively, by means of the results obtained in our previous work on direct excitation of AZM in aqueous solution.30,31 Two products with retention times of 26.5 and 27.4 min were not observed when the experiments were performed in aqueous solutions in aerated or in oxygen free conditions. They appear then to be specific to the irradiation at the surface of clays. At the surface of goethite, the amount of these products was even more important, highlighting the role of iron oxide in their formation. The product at the retention time of 26.5 min (P26.5) presents, in mass spectrometry with ES+ mode, an ion molecular mass of m/z [M + H+] = 353 and that at the retention time of 27.4 min (P27.4) an ion molecular mass of m/z [M + H+] = 385. They both present the same fragmentation and important ion masses of sodium and potassium adducts, [M + Na+] and [M + K+], as usually observed in the used mass spectrometry mode. The same fragmentation together with the similarity of the absorption spectrum with that of AZM is in favor of the presence of the benzotriazine part in the chemical structure (SI Figure S4). The high molecular masses and the similarity in the spectroscopic features led us to the conclusion that P26.5 and P27.4 may correspond to dimer structures with a sulfur bridge and SO2 bridge, respectively (Scheme 1). The former may be the precursor of the second compound through an oxidation process as reported in the literature.51 The formation of disulfur dimer is also a possible hypothesis for P27.4. The deaeration of the clays was not sufficient to differentiate between these two processes. These compounds were not observed when the experiments were performed in aqueous solutions or in suspension systems. Under the latter conditions and because of its solubility, the photochemical reactions were mainly owing to the absorption of AZM in aqueous medium.

Figure 4. ESR spectra of aqueous suspension of goethite and kaolinite (1.0 g L−1) in the presence of DMPO under irradiation at λ > 345 nm, t = 30 s. The coupling constants are aN = aH = 14.2 G.

likely owing to the consumption of residual oxygen and then the fluorescence intensity rapidly leveled off. Similar results were obtained when the irradiation was undertaken at 365 nm in dry conditions using clays and coumarin with a layer thickness of 23 μm. The hydroxyl radical formation may be attributed to the presence of impurities in the clay samples, such as titanium dioxide or iron(III) species (mainly as αFe2O3), and also to the possible formation of hydrogen peroxide via superoxide anion disproportionation. The latter species is suggested to be formed through an induced electron transfer process that involves the excited clay and molecular oxygen.20,21,50 The surface of clays can act as potential catalysts to activate oxygen molecules through chemisorption leading to the formation of superoxide anion radical, O2·−.50 These results indicate that the formation of hydroxyl radical species cannot be neglected in the presence of oxygen and that they can be involved in the degradation of organic molecules deposited or adsorbed on clays and should be taken into account for the elucidation of the byproducts. Byproducts Elucidation. Figure 5 represents the HPLC chromatogram obtained for the irradiation of AZM deposited on kaolinite (1.3 mg g−1 of kaolinite, 23 μm thickness) at the conversion percentage of about 25%. It shows the formation of several products with retention times lower than that of the parent compound. The same byproducts were observed when the irradiation was performed at the surface of bentonite and goethite but their amount varied with the nature of the mineral

Figure 5. HPLC chromatogram of irradiated AZM deposited at the surface of kaolinite (1.3 mg g−1 of kaolinite, 23 μm thickness; λdetection = 280 nm). E



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Scheme 1. Chemical Structures for the Products P26.5 and P27.4

Scheme 2. Formation Process of the Dimers P26.5 and P27.4

It should be pointed out that several products with m/z + [M + H ] = 334 owing to the hydroxylated compounds of AZM (OH-AZM) were also detected by LC/MS method at the retention time around 10.8 min. Mechanistic Scheme. According to the obtained results, the sunlight irradiation of AZM at the surface of the various mineral supports clays and goethite leads to the involvement of two degradation photochemical processes: (i) direct photodegradation of the pesticide due to the its absorbance at λ > 295 nm as clearly shown from spectroscopic features at the surface of solid supports and (ii) induced degradation through the photochemical generation of hydroxyl radicals from clays20,21,50 and goethite.42 With the latter support and because its important light absorption, the photoinduced process appeared to be the main process that leads to the disappearance of AZM. The increase of the degradation rate as a function of moisture may be attributed to the occurrence of photohydrolysis process but also to the increase of the light penetration in the clay layer as reported in the literature.37 Titanium dioxide and iron(III) content, as trace impurities, may also play a role in AZM degradation via induced processes. The formation of BZT as the major product and demethyl AZM may be the result of a photohydrolysis process upon excitation of AZM which is in perfect agreement with the increase of its amount when the moisture content increased31 (see SI for the mechanistic scheme, Scheme S1). In dry conditions, a homolytic scission of N−C and C−S bonds may occur as generally proposed on solid supports52,53 leading to the formation of BZT and methyl-benzotriaozne. The formation of BZT on the goethite support may also be owing to an efficient hydroxyl radical oxidation since the production of such reactive species is the main process under excitation of goethite (see SI for the mechanistic scheme, Scheme S2).

The oxon derivative is the result of the oxidation of AZM in its excited state by molecular oxygen. This highly toxic compound may represent an important problem of toxicity toward nontargeted organisms.54−56 The photochemical scission of the C−S and the S−P bond may be considered as the precursor process for dimer formation via various radical recombination reactions (Scheme 2). These reactions are obviously favored upon irradiation on solid supports leading to the conclusion that AZM aggregates are formed prior to irradiation. Such situation is in favor of a better radical recombination process. Similar conclusions were obtained when chlorophenol was irradiated on ice.57 Their formation may also be suggested through the action of hydroxyl radicals on the C−S and S−P bonds leading to the formation of similar radicals (Scheme 2). The involvement of hydroxyl radicals in the latter reactions is highly favored in the presence of iron oxides in the clays or at the surface of goethite as reported in the literature.42,58 These dimers were not formed when the irradiation was undertaken in aqueous solution or in suspension system, where AZM is mainly present in the aqueous phase, in complete agreement with this hypothesis. Hydroxylated AZM derivatives are also in perfect agreement with the generation of hydroxyl radical species upon irradiation under our experimental conditions. Their amount appeared to be important with goethite as mineral support. It is clear that the sunlight irradiation of azinphos methyl at solid surfaces such as clays and more probably at the surface of soils permits the formation of several byproducts among them azinphos oxon known as a toxic compound and more specifically to different dimers that can be harmful to the environment and for which toxicity tests should be performed. F



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ASSOCIATED CONTENT

S Supporting Information *

Details for the preparation and extraction of the sample, characterization of the clays, quantum yield calculation, and analysis. The evolution of the observed rate constant as a function of thickness, the evolution of the concentration of iron(II), and the evolution of the fluorescence corresponding to the formation of 7-hydroxycoumarin. Absorption spectra of the products P27.4, P26.5, and AZM as obtained by HPLC diode array and the scheme corresponding to the formation of BZT and methyl benzotriazone. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 00-33-473407170; fax: 00-33-473407700; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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