Article pubs.acs.org/JAFC
Accelerated Dissipation of the Herbicide Cycloxydim on Wax Films in the Presence of the Fungicide Chlorothalonil and under the Action of Solar Light Shirin Monadjemi,† Alexandra ter Halle,†,‡ and Claire Richard*,†,‡ †
Institut de Chimie de Clermont-Ferrand, Clermont Université, B.P. 10448, 63000 Clermont-Ferrand, France Equipe Photochimie CNRS, UMR 6296, ICCF, F-63171 Aubière, France
‡
ABSTRACT: Photolysis is a known dissipation pathway of pesticides on leaves just after their spraying. This pathway may be affected by the residues of other pesticides. To illustrate this idea, this study investigated the mutual effect of two pesticides (chlorothalonil and cycloxydim) under simulated solar light. Cycloxydim was added at the agricultural rate (200 g ha−1) and chlorothalonil at 1.3−10% of the rate (20−150 g ha−1). These compounds were studied either pure or in their commercial formulation. Both analytical and kinetic data show that chlorothalonil significantly accelerates the decay of cycloxydim on wax films, promoting its oxidation, even at the lowest tested dose. Conversely, cycloxydim does not affect the fate of chlorothalonil. Moreover, the detection of oxidized forms of wax alkanes in the extracts demonstrates that chlorothalonil may have also a degrading effect on the leaves’ constituents under the action of solar light. KEYWORDS: photolysis, sensitization, oxidation, pesticides, leaves surface
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INTRODUCTION On foliage and under the action of sunlight, pesticides can undergo photodegradation.1 Nevertheless, this route of dissipation is scarcely documented, although understanding its mechanism and evaluating its rate is of utmost importance for the improvement of pesticide formulations. Besides, photochemical reactions taking place on vegetation can follow specific pathways and lead to degradation products different from those observed in water, the most commonly studied medium. For example, it has been reported that the herbicides mesotrione and sulcotrione deposited on leaf models mainly undergo an intramolecular photocyclization when irradiated in a solar light simulator, whereas this process is very minor in aqueous solution.2,3 Moreover, photochemical reaction rates of molecules in solution cannot be generalized to the solid state. For instance, the half-life of photolysis of the herbicide mesotrione is about 90 days in water against 8 h on leaf surface models.2 Another poorly considered issue is the mutual potential effect that pesticides may have on each other when they are applied in a mixture. To improve crop protection, it is common to apply pesticides in mixtures or to use successively different pesticides during annual crop treatments. Mixing pesticides might have consequences on their photostability and thus influence their effectiveness. For example, it was shown that mesotrione and nicosulfuron herbicides show higher rates of photodegradation in a mixture than separately.4 In this work, we studied the fate of two pesticides in a mixture: the herbicide cycloxydim (E isomer, CD) and the fungicide chlorothalonil (CT) (Scheme 1). CD and CT are foliar pesticides that are both used for the control of tomato, asparagus, and pea crops; their presence altogether on crops is thus very likely. CD, on the one hand, is a systemic herbicide used for the control of grass weeds and the control of agricultural crops. In solution, photoisomerization of the CN © 2014 American Chemical Society
Scheme 1
bond and cleavage of the oxime occur, leading to a rather fast loss of CD (E isomer).5 Only a few data are, however, available on the photostability of this molecule on plants. A recent patent proposes to stabilize the cyclohexane oxime formulations by the addition of aromatic solvents such as toluene or benzene, epoxidized oil fatty acids, or more adjuvants.6 This confirms that CD undergoes photodegradation on leaves. CD was shown also to react with singlet oxygen in acetonitrile.7 CT, on the other hand, is a persistent fungicide. It is also a powerful sensitizer, which has the ability to produce singlet oxygen with a quantum yield close to 1 in alkanes.8 Therefore, one might expect that CT is able to promote the photodegradation of CT. However, this enhancing effect of CT remains to be evidenced when both compounds are in the solid state. Moreover, its actual contribution to the CD photodissipation in simulated solar light and using commercial formulations has to be evaluated. Finally, the potential effect of CD on CT phototransformation is unknown so far. Comparative experiReceived: Revised: Accepted: Published: 4846
February 24, 2014 May 9, 2014 May 12, 2014 May 12, 2014 dx.doi.org/10.1021/jf500771s | J. Agric. Food Chem. 2014, 62, 4846−4851
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was 6 μL. The flow rate was set at 0.5 mL min−1, and elution was carried out in the gradient mode. The mobile phase composition started with 58% water (acidified with formic acid, 0.5‰ v/v) and 42% acetonitrile. The percentage of acetonitrile was increased linearly to 75% over 3 min and then kept constant during 0.5 min. The mobile phase was then returned to 42% acetonitrile in 0.5 min and kept for 0.5 min. Detection was set at 280 nm. CD and CT were monitored at 280 nm (retention times of 3.1 and 2.3 min, respectively). The MS system consisted of an LC/QTOF equipped with an orthogonal geometry Z-spray ion source (Waters/Micromass, Manchester, UK). A photodiode array detector, Waters Alliance 2695, system was used for UV detection. A volume of 25.0 μL was injected on a reversed-phase column (Xterra, Waters, C18, 3.5 μm, 100 mm × 2.1 mm). A flow rate of 0.2 mL min−1 was set. The binary solvent system used was composed of solvent A (acetonitrile) and solvent B (water acidified with 0.5‰ v/v formic acid). The gradient elution started with 5% A and reached 95% A in 15 min linearly. The gradient was held for 10 min and then decreased to 5% A in 10 min. The desolvation chamber and ion source block temperatures were set at 250 and 100 °C, respectively. N2 was used as the nebulizer gas (35 L h−1) and the desolvation gas (350 L h−1). The electrospray interface was operating in positive ion mode. The capillary voltage was 3000 V; the sample cone and the extraction cone were set at 35 and 1 V, respectively. Data were acquired over the m/z 90−1000 range at a scan rate of 1 s per spectrum. The data recorded were processed with MassLynx (version 4.0).
ments were conducted using phenalenone, a well-known singlet oxygen producer.9
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EXPERIMENTAL PROCEDURES
Chemicals. All solvents and chemicals were used as received. CD (98%) and CT (99%) were obtained from Fluka (Saint-Quentin Fallavier, France). Commercial formulations of CD and CT, respectively Stratos Ultra (100 g L−1 of CD) and Fongil FL (500 g L−1 CT), were obtained from a regular agricultural shop. Phenalenone (97%) and paraffin wax (mp 70−80 °C) were both purchased from Aldrich (Saint-Quentin Fallavier, France). Water was purified using a Millipore Milli-Q system (Millipore αQ, resistivity = 18 MΩ cm−1, DOC < 0.1 mg L−1). Acetonitrile (99%, HPLC grade) was provided by Riedel de Haën (Saint-Quentin Fallavier, France). Sample Preparation and Irradiation. Stock solutions of pure CD and CT (10−4 M) were prepared in acetonitrile and stored at 4 °C. Solutions of formulated CD (Stratos Ultra) and formulated CT (Fongil FL) were prepared in Milli-Q water just before use. The exact pesticide concentration was determined using a calibration curve established for the corresponding active ingredient in acetonitrile. Paraffin wax films were made by weighing 0.8 g of wax in each dish (3.2 cm diameter) and by heating them at 90 °C for wax melting and achieving film formation. Films were allowed to cool and ready for use. In all of the experiments, the surface concentration of CD was equal to 200 g ha−1, this concentration corresponding to the recommended dose applications. In the set of experiments in which CD and CT were applied as nonformulated active ingredients, the surface concentration of CT was also chosen as 150 g ha−1. When Stratos Ultra and Fongil FL formulations were used, the chosen CT concentrations were 20 and 150 g ha−1. This choice aimed to mimic a situation in which CT traces are remaining on leaves when CD is applied. Actually, in the case of tomatoes, for instance, the recommended field application for CT is 1500 g ha−1. We chose a situation in which 10 and 1.3% of CT are still present on the leaves when CD is applied. Acetonitrile solutions of pure CD and the CD + CT mixture were deposited on wax films as 1 mL volume. Stratos (diluted 125× in water) and a mixture of Stratos (diluted 125× in water) with Fongil (diluted 833× and 4550× in water) were applied as 10 droplets of 2 μL using a micropipet (Eppendorf). The solvent was allowed to evaporate overnight. After drying, 24 dishes (3 replicates for each sampling) were irradiated inside a Suntest CPS photosimulator (Atlas) equipped with a xenon lamp filtered below 290 nm (irradiance within the range of 290−800 nm). The intensity of the lamp was set at 400 W m−2 to simulate the sunlight average intensity in summer in France. Cold water (16 °C) was flowed through the bottom of the photosimulator to maintain the internal temperature as low as possible. Four dishes were taken after each selected irradiation times. For each, wax was rinsed using 1 mL of acetonitrile to recover remaining pesticides and photoproducts. These mixtures were further analyzed by UPLC and LC-MS. By this method, the recovery yields of CD and CT prior to irradiation were good (∼95%). Mixtures of CD and phenalenone were also applied on wax films in acetonitrile. The same procedure as for CD + CT was used for photolytic tests. Analyses. Absorption spectra were recorded using a Cary 3 UV− visible spectrometer (Varian). Solid state spectra were recorded using a DRA-CA-30I integrating sphere accessory (Varian) and a BaSO4 reflectance standard (Spectralon). Acetonitrile solutions of CD (1 g L−1) and CT (1 g L−1) were deposited on quartz plates as 3 drops of 10 μL, and spectra were recorded after acetonitrile was completely evaporated. For the laser flash photolysis experiments, the apparatus consisted of a laser flash photolysis spectrometer from Applied Photophysics (LKS.60) equipped with a Quanta Ray GCR 130-01 Nd:YAG laser.10 Solutions were deoxygenated by bubbling argon directly in the quartz cell. UPLC-UV analyses were performed using an Acquity Waters UPLC system equipped with a thermostated autosampler, a degasser, a pump, and a photodiode array detector. Chromatographic separation was performed on an Acquity UPLC BEH C18 column (2.1 mm × 50 mm, 1.7 μm). The operating temperature was maintained at 25 °C, and the sample volume injected
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RESULTS AND DISCUSSION Phototransformation of Pure CD on Wax Films. The percentage of transmission of solid CD (12 kg ha−1) between 500 and 250 nm is shown in Figure 1. Absorption of CD starts
Figure 1. Transmission spectra of CD (12 kg ha−1) (solid line) and CT (7 kg ha−1) (dashed line) on quartz plates.
below 380 nm, meaning that, upon solar irradiation, CD can undergo direct photolysis. By extrapolating absorbances of Figure 1 to a dose of CD of 200 g ha−1, one gets 0.0019 at 300 nm. Dark control experiments showed no CD decay on the time scale of the experiments, indicating that other phenomena such as volatilization, thermal degradation, or penetration into the wax are negligible. Pure CD undergoes fast photolysis on wax films at the recommended agricultural rate of 200 g ha−1. The apparent first-order rate constant of phototransformation is 0.11 h−1 (see Figure 2). The typical chromatogram of irradiated CD is given in Figure 3a. CD in E form is eluted at 17.8 min and CD in Z form at 13.2 min. The Z form is very minor before and after irradiation. Thus, only the E-form peak variation was used for photodegradation rate determination. The small variation of the Zform peak during irradiation indicates that photoisomerization 4847
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is a minor process. Five other chromatographic peaks are detected, showing m/z at 298, 342, 282, and 358 u. Two peaks have the same m/z of 342 u. Using the exact mass measurement and MS/MS data (in the positive mode), we propose the elemental compositions given in Table 1 and the structures given in Scheme 2. P2, detected at 12.1 min, has gained O compared to CD. The oxidation can occur either on the sulfur atom or on the cyclohexane ring. Thanks to the MS/MS data, differentiation is possible. First, a fragment at m/z 101 u corresponding to the ion C5H9S+ is detected in P2 fragmentation pattern as in that of CD. This shows that the S atom is not oxidized. Second, the fragment at 324 u corresponding to the loss of water is in good accordance with the presence of an OH group on the cyclohexane ring, because the neighborhood of H atoms makes possible water elimination. The other fragments of P2 show loss of C2H4O by cleavage of the N−O bond (m/z 298 u) and also subsequent loss of water (m/z 280 u). P3, detected at 13 min, gives a positive ion at m/z 282 u, resulting from the loss of C2H4O. The main fragments of the ion are at m/z 265 u (loss of NH3), 254 u (loss of CO), and 182 u (loss of neutral C5H8S). Again, the fragment at 101 u
Figure 2. Decay of CD (200 g ha−1) irradiated on paraffin wax in a solar simulator: CD alone (○, k = 0.11 ± 0.01 h−1) and in the presence of CT at 150 g ha−1 (●, k = 0.30 ± 0.03 h−1); Stratos alone (△, k = 0.41 ± 0.04 h−1), in the presence of Fongil at 20 g ha−1 (☆, k = 0.58 ± 0.06 h−1), and in the presence of Fongil at 150g ha−1(▲, k = 1.3 ± 0.1h−1).
Figure 3. (a) TIC chromatogram in positive mode of the CD solution obtained by rinsing the 1-h-irradiated wax films by acetonitrile. (b) TIC chromatogram in positive mode of the solution used to rinse wax films after 30 min of irradiation of CD and CT mixture on pure wax films. 4848
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Table 1. Fragmentation of CD and Main Photoproducts protonated compound
measured m/z in u (ES+)
elemental composition
error in ppm
[CD + H]+
326.1792
280 [M − C2H6O + H]+ 180 [M − C2H6O − C5H8S + H]+ 101 [C5H9S]+
fragments in u (ES+)
C17H28NO3S
0.4
[P1 + H]+
298.1472
280 [M − H2O + H]+ 263 [M − H2O − NH3 + H]+ 101 [C5H9S]+
C15H24NO3S
0.3
[P2 + H]+ and [P′2 + H]+
342.1741
324 298 280 178 101
[M − H2O + H]+ [M − C2H4O + H]+ [M − H2O − C2H4O + H]+ [M − C2H4O − H2O − C5H10S + H]+ [C5H9S]+
C17H28NO4S
2.1
[P3 + H]+
282.1530
265 254 182 101
[M − NH3 + H]+ [M − CO + H]+ [M − C5H8S + H]+ [C5H9S]+
C15H24NO2S
2.8
[P4 + H]+
358.1691
340 312 296 215 197 117
[M − H2O + H]+ [M − C2H6O + H]+ [M − H2O − C2H4O + H]+ [M − C2H6O − C4H7N − CO + H]+ [M − C2H6O − C4H7N − CO − H2O + H]+ [C5H9SO]+
C17H28NO5S
2.3
[P5 + H]+ [P6 + H]+
341.3486 369.3805
296 [M − COOH + H]+ 324 [M − COOH + H]+
C22H45O2 C24H49O2
21.2 21.2
P4 with m/z 358 u is detected at 16.1 min. The [P4 + H]+ ion has two extra oxygen atoms compared to the [CD + H]+ ion. Among fragments, the ion at m/z 117 u is detected. This ion corresponds to [C5H9SO]+, and thus in P4 the sulfur atom is oxidized. The second oxygen atom is on the cychohexane ring as indicated by the fragments at m/z 340 corresponding to the loss of water as observed for the other oxidized compounds. Phototransformation of CD in the Presence of CT. As shown in Figure 1, CT absorbs solar light between 300 and 350 nm. On the basis of the transmission spectrum recorded at 7 kg ha−1, one calculates that absorption of CT (200 g ha−1) should be around 0.0012 at the maximum (340 nm) and 0.0006 at 300 nm. This means that CT absorbs solar light more weakly than CD when they are applied at the same dose. In the presence of CT (150 g ha−1), there is a marked increase of the rate of CD photolysis (Figure 2). The apparent first-order rate constant of phototransformation is equal to 0.3 ± 0.03 h−1, thus 3-fold higher than for pure CD. On the contrary, the concentration of CT remains unchanged after 6 h of irradiation by considering a 5% error margin. The photolysis of pure CT was previously found to be quite slow (first-order rate constant of 0.065 days−1 under continuous irradiation at 500 W m−2);8 therefore, CD does not significantly affect its photoreactivity. The phototransformations of CD and CT on wax film were also studied using commercial formulations, Stratos and Fongil, respectively. In this case, pesticide solutions were deposited as 2 μL droplets to better represent field conditions, and the surface concentration of CD was again fixed at 200 g ha−1. Stratos alone disappears with a first-order rate constant of 0.4 ± 0.03 h−1 (Figure 2). In the presence of Fongil, the photodissipation
Scheme 2
corresponding to C5H9S+ is detected. P3 is assigned to the imine. P2′ (retention time at 15.7 min) is a minor photoproduct giving the same mass and fragmentation pattern as P2. This is thus an isomer of P2. P1, eluted at 1.6 min, has lost C2H4 compared to CD. Using data on the other photoproducts, this can be understood by the replacement of the ethoxyl group by H with the formation of the imine and oxidation with O addition. By MS/MS the main fragments are m/z 280, 263, and 101 u. The presence of a fragment at 101 u is again in accordance with oxidation of the cyclohexane ring. 4849
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of Stratos increases. When Fongil is added at 20 and 150 g ha−1, the photodegradation rate constants of formulated CD are 0.6 ± 0.03 and 1.3 ± 0.03 h −1, respectively. Thus, the photoreactivity of formulated CD is 1.5- and 3.2-fold higher, respectively. The same enhancement was measured when using the pure active ingredients; therefore, one can conclude that the accelerating effect is mainly due to the active ingredient CT and not the additives present in Fongil. This experiment shows that the lowest level of Fongil, corresponding to a 70th of the recommended application dose on tomato crops, still has an impact on the photodissipation rate of CD. This result is amazing when one takes into account that the absorption of CT at this dose is very low compared to that of CD. CT is thus a very efficient sensitizer. Typical HPLC-MS analysis of reactional mixtures is shown Figure 3b. In this case, six peaks are observed. On the basis of retention times and MS data, P1, P2, P3, and P4 correspond to the photoproducts identified in the case of pure CD. However, TIC data show that P1 and P2 are formed in much greater amount in the presence of CT than for pure CD. Two new photoproducts are also detected: P5 and P6. The chemical formulas of the respective ions [P5 + H]+ and [P6 + H]+ are [C22H44O2 + H]+ and [C24H48O2 + H]+, and the unique fragments are [C21H43 + H]+ and [C23H47O2 + H]+, respectively, demonstrating the loss of CO2H radical in both cases. Clearly, these oxidation products do not arise either from CT or from CD, but from the wax that contains long-chain alkanes C22H44 and C24H48 as main constituents.11 It must be pointed out that CT cannot be protonated and, consequently, cannot be detected by electrospray MS in the positive mode. Phototransformation of CD in the Presence of Phenalenone. Experiments were repeated by replacing CT by phenalenone, a strong singlet oxygen sensitizer. To conduct the tests at the same molar concentration for phenalenone and CT, the surface concentration of P on wax films was set at 100 g ha−1. Considering a 10% error margin, the apparent firstorder rate constant of phototransformation of CD in the presence of P measured was 0.25 ± 0.03 h−1, which is close to the rate constant measured for CD in the presence of CT at 150 g ha−1. However, unlike CT, P disappeared rapidly during the irradiation. The same photoproducts as in the case of CD + CT mixtures were detected. Photoproducts P 5 and P 6 corresponding to the oxidation of the wax were also formed. Laser-Flash Photolysis. When CD is irradiated in the presence of CT or phenalenone, it may disappear by reaction with singlet oxygen produced by the CT and phenalenone triplets8,9 or be oxidized by the triplets themselves. The reactivity of CD with the triplet of phenalenone was reported elsewhere.7 The reaction rate constant is (9.6 ± 1) × 106 M−1 s−1 in acetonitrile. Here, we measured the rate constant of reaction between the triplet of chlorothalonil and CD. To this end, acetonitrile solutions of CT were irradiated at 266 nm with the fourth harmonic of a Nd:YAG laser, and the triplet decay was monitored at 320 nm. In deoxygenated medium, the firstorder rate constant for the triplet decay was equal to (1 ± 0.2) × 105 s−1. In the presence of CD (5 × 10−5 M), the triplet decayed more quickly with an apparent first-order rate constant of ∼1.5 × 105 s−1. In the reaction, the triplet of chlorothalonil is reduced into the radical anion that absorbs in the same wavelength range as the triplet.12 As a consequence, the rate constant of reaction of CD with the triplet of chlorothalonil cannot be obtained accurately, but the experiment demon-
strates that CD is oxidized by the triplet of chlorothalonil with a rate constant ∼109 M−1 s−1.
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DISCUSSION The photolysis of CD in solid form on wax films is very fast in simulated solar light. This reaction mainly leads to the imine by cleavage of the oxime function and to oxidation. Analytical data demonstrate that the oxidation takes place preferentially on the cyclohexane ring and not on the S atom. It must be stressed that the oxidation product is firmly identified for the first time. Traces of oxidized imine and doubly oxidized CD with one oxygen on the cyclohexane ring and another on the S atom are detected, too. The photoreactivity of CD is similar to that observed in acetonitrile.13 In particular, photoisomerization of the E isomer into the Z isomer, which is observed in water but not in acetonitrile, does not occur in solid form. This means that the intramolecular hydrogen bond between the OH and the N detected in acetonitrile exists also in the solid.13 When CD is formulated, the photoreactivity is faster, probably because of the better spreading of the molecules thanks to the formulation adjuvants. Photoproducts are the same as for pure CD; therefore, adjuvants do not affect photoreactivity. CD is also phototransformed by sensitized reactions. At the chosen level of added CT that remains well below the rate applied in the field, the accelerating effect is very significant. CD photoproducts are the same as for pure CD, but their distribution is different. In particular, the oxidized imine and the photoproduct resulting from oxidation of the cyclohexane ring are formed in much greater amounts. This confirms that sensitized reactions induce oxidation. The oxidation can imply two pathways: oxidation by singlet oxygen and oxidation by the sensitizer triplet itself. As the two routes probably give similar products, it is difficult at this stage to delineate the role of each of them in the CD photooxidation. The only indication is given by comparing the effects of CT and phenalenone. Both sensitizers photogenerate singlet oxygen with a yield close to 1.8,9 As phenalenone is much more absorbing than CT within the wavelength range of 290−500 nm,7 one might expect a much faster photodegradation of CD in the presence of phenalenone than of CT if singlet oxygen were the involved oxidant species. Actually, this is not the case as close rates of phototransformation are measured for both sensitizers. As far as the reactivity of CD with triplets is concerned, one found that the reaction rate constant is about 100-fold higher for triplet CT than for triplet phenalenone. This higher oxidant property of triplet CT may counterbalance a lower absorptivity and finally rationalize the close rates of phototransformation measured in the presence of both sensitizers. To sum up, data are consistent with an oxidation of CD by the sensitizer triplets as the main pathway of oxidation. This oxidation occurs on CD and on P3, the main CD photoproduct (Scheme 3). In the course of the reaction, the concentration of CT remains very stable, indicating that it is not measurably consumed, whereas phenalenone disappears quite rapidly. This difference may originate from the reactivity of the sensitizers’ reduced radicals. Oxidation of CD by the triplets yields the reduced forms of sensitizers: CT* + CD → CTH• + CD−H•
3
phenalenone* + CD → phenalenone H• + CD−H•
3
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(5) Metabolic Pathways of Agrochemicals; Roberts, T. R., Ed.; Royal Society of Chemistry: Cambridge, UK, 1998. (6) Stabilized herbicidal formulations and methods of use. U.S. Patent 2011/0015074 A1, Jan 20, 2011. (7) Monadjemi, S.; Ter Halle, A.; Richard, C. Reactivity of cycloxydim toward singlet oxygen in solution and on wax film. Chemosphere 2012, 89, 269−273. (8) Monadjemi, S.; El Roz, M.; Richard, C.; Ter Halle, A. Photoreduction of chlorothalonil fungicide on plant leaf models. Environ. Sci. Technol. 2011, 45, 9582−9589. (9) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. Phenalenone, a universal reference compound for the determination of quantum yields of singlet oxygen O2(lΔg) sensitization. J. Photochem. Photobiol. A 1994, 19, 11−17. (10) Bonnichon, F.; Richard, C. Phototransformation of 3hydroxybenzonitrile in water. J. Photochem. Photobiol., A 1998, 119, 25−32. (11) Chastain, J. Unpublished results. (12) Bouchama, S.; de Sainte-Claire, P.; Arzoumanian, E.; Oliveros, E.; Boulkamh, A.; Richard, C. Photoreactivity of the fungicide chlorothalonil in aqueous medium. Environ. Sci.: Processes Impacts 2014, DOI: 10.1039/C3EM00537B. (13) Monadjemi, S.; de Sainte-Claire, P.; Abrunhosa-Thomas, I.; Richard, C. Photolysis of cycloxydim, a cyclohexanedione oxime herbicide. Detection, characterization and reactivity of the iminyl radical. Photochem. Photobiol. Sci. 2013, 12, 2067−2075. (14) Trivella, A.; Monadjemi, S.; Worrall, D. R.; Kirkpatrick, I.; Arzoumanian, E.; Richard, C. Perinaphthenone phototransformation in a model of leaf epicuticular waxes. J. Photochem. Photobiol., B 2014, 130, 93−101.
Scheme 3
The nonconsumption of CT indicates that the radical CTH• gives the H atom back to oxygen, regenerating CT and producing HO2•. From phenalenone, the hydroxylperinaphthenyl radical is expected to be formed. In solid medium, this radical is poorly reactive toward oxygen. It preferentially reacts by addition with other radicals; consequently, phenalenone is not regenerated.14 This explains the disappearance of phenalenone and shows that, in the solid state, CT is a much better sensitizer than phenalenone. The last but not the least interesting point here is the detection of oxidized long-chain alkanes. To our knowledge this is the first time it is shown that alkane wax constituents can be oxidized by a mild oxidation process involving light, oxygen, and an organic sensitizer. This implies that leaf constituents should be also subject to oxidation. In conclusion, on leaf models, direct photolysis of cycloxydim leads to the loss of ethoxyl group and to the oxidation of the cyclohexane moiety. Applied at a dose 75-fold less than the recommended one, and although showing a weak absorptivity, chlorothalonil increases the phototransformation rate of cycloxydim through oxidation. Besides, chlorothalonil is also able to oxidize the wax components of the leaves. This exemplifies the necessity to test the photochemical reactivity of pesticides in mixtures. It is also worth noting that the sensitizers chlorothalonil and phenalenone do not have the same properties in the solid state. Chlorothalonil is quite stable during the irradiation experiment in the presence of cycloxydim, whereas phenalenone undergoes photodegradation.
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AUTHOR INFORMATION
Corresponding Author
*(C.R.) E-mail:
[email protected]. Phone: +33 (0)4 73 40 71 42. Fax: +33 (0)4 73 40 77 00. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Katagi, T. Photodegradation of pesticides on plant and soil surfaces. Rev. Environ. Contam. Toxicol. 2004, 182, 1−195. (2) Lavieille, D.; Ter Halle, A.; Richard, C. Understanding mesotrione photochemistry when applied on leaves. Environ. Chem. 2006, 5, 420−425. (3) Ter Halle, A.; Drncova, D.; Richard, C. Phototransformation of the herbicide sulcotrione on maize cuticular wax. Environ. Sci. Technol. 2006, 40, 2989−2995. (4) Ter Halle, A.; Lavieille, D.; Richard, C. The effect of mixing two herbicides mesotrione and nicosulfuron on their photochemical reactivity on cuticular wax film. Chemosphere 2010, 79, 482−487. 4851
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