Surface Photochemistry of Pesticides: An Approach Using Diffuse

Técnico, 1049-001 Lisboa, Portugal, and FCT, Universidade do. Algarve, Campus de Gambelas, 8005-139 Faro, Portugal. The photochemistry of pesticides ...
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Environ. Sci. Technol. 2004, 38, 2849-2856

Surface Photochemistry of Pesticides: An Approach Using Diffuse Reflectance and Chromatography Techniques J O S EÄ P . D A S I L V A * , † , ‡ A N D LUIS F. VIEIRA FERREIRA† Centro de Quı´mica-Fı´sica Molecular, Instituto Superior Te´cnico, 1049-001 Lisboa, Portugal, and FCT, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal

The photochemistry of pesticides triadimenol and triadimefon was studied on cellulose and β-cyclodextrin (β-CD) in controlled and natural conditions, using diffuse reflectance techniques and chromatographic analysis. The photochemistry of triadimenol occurs from the chlorophenoxyl moiety, while the photodegradation of triadimefon also involves the carbonyl group. The formation of 4-chlorophenoxyl radical is one of the major reaction pathways for both pesticides and leads to 4-chlorophenol. Triadimenol also undergoes photooxidation and dechlorination, leading to triadimefon and dechlorinated triadimenol, respectively. The other main reaction process of triadimefon involves R-cleavage from the carbonyl group, leading to decarbonylated compounds. Triadimenol undergoes photodegradation at 254 nm but was found to be stable at 313 nm, while triadimefon degradates in both conditions. Both pesticides undergo photochemical decomposition under solar radiation, being the initial degradation of rate per unit area of triadimefon 1 order of magnitude higher than the observed for triadimenol in both supports. The degradation rates of the pesticides were somewhat lower in β-CD than on cellulose. Photoproduct distribution of triadimenol and triadimefon is similar for the different irradiation conditions, indicating an intramolecular energy transfer from the chlorophenoxyl moiety to the carbonyl group in the latter pesticide.

1. Introduction It is now well-accepted that photochemical transformations of pesticides and other xenobiotics in environmental systems play a major role in determining their behavior and fate (1). Since pesticides are mainly localized at the solid/gas interface (2), any description of their behavior in the environment requires the description in these systems. Diffuse reflectance techniques are among the most important for surface photochemistry studies (3). Diffuse reflectance laser flash photolysis, developed by Wilkinson and co-workers in the 1980s (4-6), is a powerful tool for the identification and characterization of the main photophysical and photochemical processes, which are crucial in determining the main photochemical reaction pathways. Combined with * Corresponding author fax: 351 21 8464455; e-mail: jpsilva@ ualg.pt. † Instituto Superior Te ´ cnico. ‡ Universidade do Algarve. 10.1021/es0348501 CCC: $27.50 Published on Web 04/14/2004

 2004 American Chemical Society

FIGURE 1. Triadimenol (I) and triadimefon (II). chromatographic analysis and product identification, these techniques can be used to assess the role of photochemical processes of pesticides on model and natural surfaces. Diffuse reflectance techniques have been used to study the photochemical transformations of several xenobiotics such as dioxins (7, 8), polychlorinated biphenyls (9, 10), pyrene (11), and 4-chlorophenol (12, 13) adsorbed on different supports. The complexity of natural environmental surfaces usually does not allow for conventional photochemical experiments. Photochemistry on environmental systems such as leaf surfaces will not be successfully modeled until photochemical and sorptive processes on its pure components are better understood (1). Cellulose is one of the main structural components of vegetal cells. Microcrystalline cellulose has been widely used as a host to study the photophysics and photochemistry of several organic molecules, mainly dyes, at the solid/gas interface (14-16). This support can be used as the starting surface in assessing the photochemical behavior of pesticides on vegetal surfaces. Leaf surfaces also possess waxes (17); therefore, a model hydrophobic solid support is also needed. Among the several solid supports whose photophysics and photochemistry is well-studied, cyclodextrins are good candidates. They have an external hydrophilic surface, similar to that supplied by cellulose, and a relatively nonpolar cavity, being able to include a variety of compounds whose character may vary from hydrophobic to ionic (12, 18-22). Cyclodextrins offer therefore the starting point in going from cellulose to more hydrophobic solid supports. The inclusion in β-CD deeply affects the guest properties, being usually associated with a decrease of the nonradiative processes rate and rotational freedom and/or with the elimination of water molecules surrounding the probe up inclusion (18-23). This support can therefore be used as a first approximation to both nonpolar and rigidity characteristics typical of leaf waxes. Therefore, we will use cellulose and β-CD as the starting model supports to assess the photochemical behavior and fate of pesticides on leaf surfaces. Triadimenol, 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H1,2,4-triazol-1-yl) butan-2-ol (I), and triadimefon, 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl) butanone (II), (Figure 1) are two related systemic fungicides (24) that have been studied by us as model pesticides (25-29). Their photochemistry and photophysics are well-documented in solution (28-31), and the major photodegradation products of both pesticides were identified at the solid/gas interface (27, 31). In solution, among the several photoproducts identified, 4-chorophenol and 1,2,4-triazole are the major components. At the solid/gas interface, the photoproduct distribution is different. In the photolysis of triadimenol, the identified photoproducts were 1-phenoxy-3,3-dimethyl-1(1H-1,2,4-triazol-1-yl)butan-2-ol (dechlorinated triadimenol), 1-(4-chlorophenoxy)-3,3-dimethylbutan-2-one, and 4-chlorophenol (31). The main photoproducts of triadimefon at the solid/gas interface result from the R-cleavage of the carbonyl group (27). Transient absorption studies in solution showed the formation of the 4-chlorophenoxyl radical for both pesticides. VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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In this paper, we report a transient absorption study of triadimenol and triadimefon adsorbed on cellulose and β-CD. The main degradation products were identified, and the main degradation pathways were established in controlled and natural conditions.

2. Experimental Procedures 2.1 Materials. Triadimenol (isomer A), triadimefon (Riedelde-Hae¨n, 98%), 4-chlorophenol, phenol, β-cyclodextrin (Aldrich), cellulose (Fluka-DSO), 1,2,4-triazole (Fluka, 99%), methanol, and acetonitrile (Merck Lichrosolv) were used without further treatment. Water was deionized and distilled. 2.2 Sample Preparation. Samples containing 20, 50, 150, and 500 µmol g-1 of both pesticides in undried microcrystalline cellulose were prepared by adding a solution of the probe in methanol (10 mL) to the correspondent quantity of the adsorbent. Samples were then stirred during solvent evaporation. Two more aliquots of 10 mL were added, and the procedure was repeated. The samples were then dried under reduced pressure (∼10-3 mbar). Solid pesticide/β-CD complexes of molar ratios 1:1, 1:2.5, and 1:5 were prepared by slowly mixing the correspondent solution of the probe solved in 0.5 mL of methanol with a saturated solution of β-CD (∼10-2 M, 50 mL volume). The resulting mixture was then magnetic stirred during 48 h and then lyophilized (Heto, Model FD 1.0-110). The final concentration of pesticide was determined by extracting the samples with methanol (a known weight of sample in a known volume of solvent) followed by centrifugation and analysis by HPLC. 2.3 Methods. Diffuse Reflectance Ground-State Absorption Spectra. Ground-state absorption spectra of the solid powdered samples were recorded using a Cintra 40 GCB Scientific Equipment spectrophotometer, with a diffuse reflectance attachment. The measured reflectance, R, was used to calculate the remission function F(R) using the Kubelka-Munk equation defined by

F(R) )

(1 - R)2 K ) 2R S

where K and S are the absorption and scattering coefficients, respectively. The Kubelka-Munk equation applies to optically thick samples (i.e., those where any further increase in the thickness does not affect the experimentally determined reflectance). For an ideal diffuser, where the radiation has the same intensity in all directions, K ) 2C, where  is the Naperian absorption coefficient and C is the concentration. Since the support usually absorbs at the excitation wavelength, F(R)probe ) F(R) - F(R)support ) Σi2iCi/S, where F(R)support is the blank obtained with a cell containing only the powdered solid support. Whenever the probe is only in the form of monomer, this equation predicts a linear relationship for the remission function of the probe as function of the concentration (for a constant scattering coefficient). Diffuse Reflectance Laser Flash Photolysis System. Laser flash photolysis experiments were carried out with the fourth harmonic of a Nd:YAG laser (266 nm, 6 ns fwhm, 10-30 mJ/pulse) from B. M. Industries (Thomson-CSF), model Saga 12-10. Transmission geometry was used for solutions study, and diffuse reflectance mode was used for solid samples. A schematic diagram of the system is presented in ref 3. The light arising from the irradiation of the samples by the laser pulse is collected by a collimating beam probe coupled to optical fibers (fused silica) and detected by a gated intensified charge coupled device (ICCD, Oriel model Instaspec V) after passing via a compact fixed imaging spectrograph (Oriel, model FICS 77441). The system can be used either by capturing all light emitted by the sample or in time-resolved mode, by the use of a delay box (Stanford Research Systems, 2850

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Model D6335). The ICCD has high-speed gating electronics (2.2 ns) and an intensifier and works in the 200-900 nm wavelength range with appropriate time gates and amplification. Time-resolved absorption and emission spectra are available in the nanosecond to second time range. Experiments conducted in solution were made using O.D. ∼1.5 at the excitation wavelength in a 1 cm cell, on air-equilibrated and argon-purged samples. For solid samples, transient absorption data are reported as a percentage of absorption (% Abs) defined as 100∆Jt/ J0 ) (1 - Jt/J0) × 100, where J0 and Jt are the diffuse reflected light before exposure to the laser pulse and at time t after excitation, respectively. In all samples, the initial transient absorption (e20%) increased proportionally with laser intensity, giving evidence for the validity of this treatment, rather than the Kubelka-Munk analysis (14, 20). For solution samples, transient absorption data are reported as ∆O.D. Irradiation, Degradation Kinetics, and Product Analysis. Photolysis studies were conducted in a system previously used to study pesticides in solution (28, 29); pesticides at the solid/gas interface (27); and 4-chlorophenol on silicalite, β-CD, cellulose, and silica (12, 13). The system uses a merrygo-round and an immersion-well photochemical reactor (Applied Photophysics). The 313 nm radiation was obtained using a medium-pressure 400 W mercury lamp (Applied Photophysics) and a filter solution of potassium chromate and sodium carbonate (32) circulated inside the doublewalled well. The merry-go-round and the immersion-well photochemical reactor were both immersed in water for cooling. The water temperature was kept constant (22 °C) using external circulation through a cooling bath. The 254 nm radiation was obtained using a 16 W low-pressure mercury lamp (Applied Photophysics) without filters. No refrigeration was required in this case. The solid samples were irradiated in 0.5 cm quartz cells at a distance of 10 cm from the lamp housing. The degradation and product formation kinetics under sunlight irradiation were made using samples prepared by spreading the solid powder on glass microscope slides (30 mg spread on ∼6 cm2) covered with quartz slides. The study was performed in Algarve (South Portugal latitude, 37° N; longitude, 8° W) in April. The initial degradation rates were measured in June using the same sample preparation method, but a known opaque irradiated surface was taken for analysis. After irradiation, the pesticide residue and its photoproducts were extracted as described for the nonirradiated samples. The solar radiation was monitored using a International Light IL 700 A Research Radiometer, equipped with a SEE240 3358 detector, a W 6237 diffuser, and a UVB 12813 filter. The radiation transmission through the filter has a maximum close to 295 nm and obeys a near Gaussian profile between 270 and 320 nm. The measured irradiation was 4.98 × 10-5 W cm-2 at 11:00, rose up to 8.13 × 10-5 W cm-2 at 12:30, reached a maximum of 8.80 × 10-5 W cm-2 between 13:00 and 14:00, and then decreased to 2.70 × 10-5 W cm-2 (17:00). Photolysis was followed by HPLC using a Merck-Hitachi 655A-11 chromatograph equipped with detectors 655A-22 UV and Shimadzu SPD-M6A Photodiode Array. A column LiChroCART 125 (RP-18, 5 µm) Merck was used, and the runs were performed using several fixed water/acetonitrile mixtures (50%-50%; 60%-40%; 75%-25%) as the eluent. Analyses were conducted on irradiated and control samples kept in the dark during irradiation. Controls showed no sign of pesticide degradation. Photoproducts were studied by irradiating I at 254 nm and II at 254 and 313 nm. GC-MS analyses were performed using a Hewlett-Packard 5890 series II gas chromatograph with a 5971 series mass selective detector (EI 70 eV). A OPTIMA-5-MS capillary column with and 0.25 mm i.d.

FIGURE 2. Remission function, normalized to the absorption maximum, of triadimenol (A) and triadimefon (B) on cellulose. The inset shows the ground-state diffuse reflectance absorption spectra of triadimefon on cellulose (250 µmol g-1), obtained after 0 min (1), 20 min (2), and 40 min (3) irradiation time (313 nm). (Machery-Nagel GmbH & Co) was used. The initial temperature of 50 °C was maintained during 5 min, and then a rate of 5°C/min was used until a final temperature of 250 °C was reached. Analyses were conducted at conversions lower than 10%.

3. Results and Discussion 3.1 Ground-State Diffuse Reflectance Absorption Spectra. Figure 2 presents the ground-state absorption spectra (normalized to the maximum absorption) of triadimenol and triadimefon adsorbed on cellulose. The spectra of triadimefon and triadimenol are similar to the ones obtained in solution (28). The band centered at 275 nm can be attributed to absorption by the chlorophenoxyl group. The low intensity band observed for triadimefon between 290 and 340 nm can be attributed to the carbonyl group. The absence of this band in triadimenol confirms the assignment. The spectra obtained also indicate a small blue shift (∼2 nm) when going from cellulose to β-CD (data not shown). The solvatochromic behavior of ππ* bands is well-known: polar and polar protic solvents promote bathochromic shifts because the dipole moment in the excited state is larger than in the ground state (33). The energy level of the excited state is therefore more stabilized by solvent interaction than the ground state. The comparison with solid supports suggests an increase of polarity in the environment of the guest molecule when going from β-CD to cellulose, which is in agreement with the relatively nonpolar environment supplied by β-CD upon inclusion (18). This explains the deviation of the π f π* absorption band to the red and indicates the formation of pesticide/β-CD inclusions complexes. Therefore, β-CD can be used as a starting model support in accessing the photochemistry of triadimefon and triadimenol adsorbed on hydrophobic solid supports. The solution behavior of the ππ* absorption band of triadimefon was reported to show a bathochromic shift in nonpolar solvents (28). This result suggests that the rigidity increase upon adsorption on the used solid supports has a crucial role in the solvathochromic behavior of the used pesticides. In natural conditions, only the solar radiation above 290 nm arrives to the Earth’s surface (34). The absorption spectra indicate that triadimefon is expected to undergo higher direct photodegradation rate in natural conditions, as compared to triadimenol, due to a larger overlap with the solar radiation at ground level. It is also expected that in environmental conditions the photochemistry of triadimefon will be mainly that of the carbonyl group, and for triadimenol, it will be centered on the chlorophenoxyl group.

FIGURE 3. Typical chromatogram (GC-MS) of the extract of an irradiated sample of triadimenol on cellulose (A) and mass spectrum of dechlorinated triadimenol (B). The inset of Figure 2 shows the evolution on time of the ground-state diffuse reflectance absorption spectra of a sample of triadimefon on cellulose, upon irradiation at 313 nm. Photodegradation leads to an increase of the groundstate absorption at all wavelengths between 250 and 400 nm, showing the formation of new products. The increase of the absorbance in the spectral region where triadimefon absorbs indicates the formation of products with higher extinction coefficients than the pesticide itself. These results also show that to follow the triadimefon degradation and/or its products formation, chromatographic analyses are required. The photodegradation of triadimenol followed by ground-state absorption in the same conditions showed no change within the experimental error. This result also indicates that triadimenol is expected to undergo a lower direct photodegradation rate in natural conditions than triadimefon. 3.2 Degradation Products and Transient Absorption of Triadimenol. The analyses of samples of triadimenol irradiated at 313 nm confirmed its stability at this excitation wavelength and is in agreement with the low direct photodegradation rates predicted in natural conditions. The analyses (GC-MS) of the extracts of samples of triadimenol on cellulose irradiated at 254 nm (conversions lower than 10%) showed four major degradation products: 4-chlorophenol (retention time (rt) ) 19.64 min, m/z ) 128), triadimefon (rt ) 40.93 min, m/z ) 293), a compound with rt ) 37.30 min (m/z ) 261), and a compound with rt ) 21.56 min (see Figure 3A). The formation of 1,2,4-triazole was also detected. Dechlorinated triadimenol and 4-chlorophenol VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Transient absorption of triadimenol on cellulose 1 µs (1), 5 µs (2), and 1 ms (3) after the laser pulse (excitation at 266 nm, ∼20 mJ/pulse). The inset shows the transient absorption of triadimenol/β-CD complexes 1 µs (a) and 20 ms (b) after the laser pulse. were already reported as photodegradation products of solid triadimenol (31). The mass spectrum of the compound with rt ) 37.30 min shows a low abundance molecular ion peak (m/z ) 261), and the fragmentation pattern is similar to that obtained for triadimenol, except the absence of the chlorine atom in all ions (see Figure 3B). This product was assigned to the dechlorinated triadimenol (1-phenoxy-3,3-dimethyl1-(1H-1,2,4-triazol-1-yl)butan-2-ol). Compound with rt ) 21.56 min is also an important photoproduct. However, due to the lack of other spectroscopic data and of literature results, no structure was assigned to the obtained mass spectrum. The photoproduct analyses suggest that the cleavage of the chlorine bond to the ring, the cleavage of the C-O bond to the C1 carbon, and the photooxidation to triadimefon are three of the major degradation pathways. In β-CD, the degradation products are higher in number. Besides 4-chlorophenol, 1,2,4-triazole, and dechlorinated triadimenol, compounds with molecular ion m/z ) 94, 192, and 226 (attributable to 1,2,4-triazole, phenol, 1-phenoxy3,3-dimethylbutan-2-one, and 1-(4-chlorophenoxy)-3,3dimethylbutan-2-one, respectively) were detected. The formation of triadimefon is a minor path in this support. The identifications of 4-chlorophenol, phenol, 1,2,4-triazole, and triadimefon were made by comparison with authentic samples and the other degradation products by comparison with mass spectra library data (Hewlett-Packard library) and with the published mass spectra of these products (30). The results indicate, as expected from the ground-state absorption spectra, that the main primary photochemical processes involve the chlorophenoxyl group. The differences on the photoproduct distribution observed between cellulose and β-CD indicate that the overall photodegradation processes are support dependent. The photodegradation of triadimenol in cyclohexane and methanol leads to 4-chlorophenol through the 4-chlorophenoxyl radical formation (28). In methanol/ water mixtures, a pathway involving the chlorophenolate formation was also detected (29). Both transients were observed by flash photolysis. Since 4-chlorophenol was observed in both supports, one should be able to observe transient absorption in the diffuse reflectance mode. The observation of any transient absorption from the triazolyl group is not expected since triazole itself does not absorb to any appreciable extent above 200 nm (28, 35). Figure 4 shows the transient absorption of triadimenol on cellulose. A main absorption band, centered at 400 nm, was detected at 1 µs after the laser pulse. This band was assigned to the 4-chlorophenoxyl radical by comparison with the absorption spectrum obtained in silicalite (12). This absorption band almost vanishes after 1 ms, and a broad 2852

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FIGURE 5. Typical chromatogram (GC-MS) of the extract of an irradiated sample of triadimefon on cellulose. band appears at shorter wavelengths, indicating the formation of other transients. Therefore, as observed in solution, one of the primary photochemical processes of triadimenol is the homolytic cleavage of the C-O bond to the benzenic ring, which leads to 4-chlorophenol through the chlorophenoxyl radical formation. Transient absorption of triadimenol/ β-CD complexes showed also the formation of the 4-chlorophenoxyl radical, but several differences are observed (see inset of Figure 4): at 1 µs after the laser pulse, transients absorbing at lower wavelength relative to the absorption of the chlorophenoxyl radical are already present and show higher absorption than this radical; the decay of the chlorophenoxyl radical absorption is slower. These differences suggest that other reaction pathways are available in β-CD, which effectively compete with the formation of 4-chlorophenol. Transient absorption is therefore in accordance with the photoproduct distribution, which showed a higher number of photoproducts on this support. 3.3 Degradation Products and Transient Absorption of Triadimefon. The main photodegradation products of triadimefon on cellulose, upon irradiation at 313 nm (detected by GC-MS at conversions lower than 10%) were 4-chlorophenol (rt ) 19.62 min, m/z ) 128), 1-[(4-chlorophenoxy) methyl]-1,2,4-triazole (rt ) 33.38 min, m/z ) 209), and 1-(4chlorophenoxy)-2,2-dimethyl-1-(1,2,4-triazol-1-yl)propane (rt ) 35.87 min, m/ z ) 265) (see Figure 5). 1,2,4-Triazole and a compound with m/z ) 167, attributable to 1-(1,2,4-triazol-1-yl)-3,3-dimethylbutan-2-one, were also detected. The identifications of 4-chlorophenol and 1,2,4triazole were made by comparison with authentic samples and the other degradation products by comparison with mass spectra library data (Hewlett-Packard library) and with the published results (27, 30). The UV-vis spectra of compounds with rt ) 33.38 min (m/z ) 209) and rt ) 35.87 min (m/z ) 265), obtained by HPLC with the diode array detector, are in agreement with these assignments, showing the absence of the n f π* absorption band of the carbonyl group. These compounds were already detected on the commercial formulation Bayleton 5 (27). These results suggest that the main photodegradation paths are weakly dependent on the solid support. In fact, the same degradation products were also found for this pesticide adsorbed on β-CD, which supplies a relatively hydrophobic environment. The degradation paths that involve the cleavage of the C-O bond in the β-position to the carbonyl group and the cleavage of the R C-C bond (see Figure 1), already proposed for triadimefon on its commercial formulation Bayleton 5 (27), appear to be quite general at the solid/gas interface. The latter degradation path was not detected in organic solvents or in aqueous solution (28-30). This result indicates that the photochemical

FIGURE 6. Transient absorption of triadimefon on cellulose at pulse end (1) and 0.3 µs (2), 0.5 µs (3), and 1 ms (4) after the laser pulse (A) and transient absorption of triadimenol in air equilibrated (a) and argon purged (b) cyclohexane solutions 30 ns after the laser pulse (B) (excitation at 266 nm, ∼20 mJ/pulse). studies in organic solvents cannot be used to access the photochemical behavior at the solid/gas interface. For comparative purposes, we have studied also the photodegradation of crystals of triadimefon upon irradiation at 313 nm. The major degradation products in this case were 4-chlorophenol and 1-(1,2,4-triazol-1-yl)-3,3-dimethylbutan2-one (m/z ) 167). Compounds with rt ) 33.38 and 35.87 min were not detected. However, the irradiation of the solid residue of pesticide resulting from the solubilization of the crystal with a few drops of dichloromethane followed by volatilization of the solvent showed the formation of these degradation products. Therefore, the cleavage of the R C-C bond to the carbonyl group, observed at the solid/gas interface in all supports, is not allowed in crystalline triadimefon. This behavior was attributed to spatial constraints imposed by the crystalline structure. This result indicates that the photochemistry results obtained in the crystalline state cannot also be used to access the photochemical behavior in the adsorbed state. Upon irradiation at 313 nm, only the carbonyl group absorbs (see Figure 2), and the major degradation products indicate the cleavage of the R- and β-bonds to the carbonyl group. Since this is a wavelength of the solar spectrum at ground level, it is clear that direct photodegradation will play an important role on the dissipation of triadimefon from solid/gas interfaces in natural conditions. At 254 nm, the π f π* transition of the chlorophenoxyl group dominates (see Figure 2) (28). However, the same main photodegradation products were observed, indicating also the photochemistry of the carbonyl group. In cyclohexane, a fast intramolecular energy transfer process from the localized ππ* state of the chlorophenoxyl group to the nπ* state of the carbonyl group was detected (28). A similar process can by invoked to explain the photochemical behavior on solids. Therefore, upon irradiation at 254 nm, an intramolecular electronic energy transfer process from the localized ππ* of the chlorophenoxyl group to the nπ* state of the carbonyl leads to photochemistry from the latter. As for triadimenol, transient absorption should show the formation of chlorophenoxyl radical.

FIGURE 7. Photodegradation of triadimefon under 254 nm (A) and sunlight radiation (B) as function of the irradiation time, on cellulose. Triadimefon (1); 1-(4-chlorophenoxy)-2,2-dimethyl-1-(1,2,4-triazol1-yl)propane (m/z ) 165) (2); 4-chlorophenol (m/z ) 128) (3); and 1-[(4-chlorophenoxy) methyl]-1,2,4-triazole (m/ z ) 209) (4). The inset shows the normalized solution ground-state absorption spectra of the first three compounds. Figure 6A shows the transient absorption spectra of triadimefon on cellulose. At pulse end, the only observed transient absorption is localized below 350 nm. A few hundreds of nanoseconds later, the growing of a band at 400 nm can be observed, which almost vanishes after 1 ms. Rigorous transient absorption spectra at shorter times could not be obtained for the other samples (triadimefon in β-CD and triadimenol in both supports) due to the corrections associated to the strong luminescence detected in this time scale. In fact, the fluorescence quantum yield of triadimenol is 2 orders of magnitude higher than triadimefon (28), and β-CD itself also showed luminescence. The spectra obtained for triadimefon/β-CD complexes are similar to those obtained for triadimenol in the same support (data not shown): there is formation of a lower and long-lived absorption of the chlorophenoxyl radical as compared to the results obtained on cellulose. However, unlike triadimenol, the same main photoproducts were obtained for both supports. For this pesticide, the differences in the transient absorption spectra did not result in differences on the main photoproducts. Since the chlorophenoxyl radical is formed in both supports, differences in the transient absorption can be due to transients formed by minor reaction pathways or even transients that did not lead to the final degradation products. The main reaction pathways (other than the chlorophenoxyl radical formation) involve the R-C-C bond of the carbonyl group, which leads to •C(CH3)3 and •CO(CH3)3 radicals. These radicals only absorb significantly below 250 nm (36), being therefore much difficult to follow using our flash photolysis system since the monitoring light source used in this work (450 W xenon arc lamp) shows a low output below this wavelength. Both pesticides exhibit strong ground-state absorption below 300 nm (see Figure 2). Since transient absorption spectra are difference spectra, the results obtained below this wavelength reflect the absorbance of the transients VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Proposed mechanism for the photochemical reaction of triadimenol and triadimefon on the solid/gas interface. formed after the laser pulse and the depletion of the pesticide, the assignment being therefore quite difficult. The degradation pathways that lead to these radicals are therefore difficult to follow by flash photolysis. A shoulder centered at 330 nm can also be observed. The triplet-triplet absorption of chlorophenol was observed in hexane and leads to a broad band centered at 330 nm (37). Since both pesticides show a nonconjugated chlorophenoxyl group, one can expect similar localization for their triplettriplet absorption. To evaluate this possibility, we measured the transient absorption of triadimenol in air-equilibrated and argon-purged cyclohexane solutions using the described laser flash photolysis system in the transmission mode. The results are shown in Figure 6B. The small band near 410 nm, as reported before in this solvent (28), is due to absorption of the 4-chlorophenoxyl radical. A broad band near 330 nm appears in the argon-purged solutions. By its spectral localization and behavior with oxygen, this band was assigned to the triplet-triplet absorption. Although less pronounced, an increase in the transient absorption in argon-purged samples was also observed for triadimefon in cyclohexane (data not shown). Therefore, the transient absorption observed for both pesticides is this region can be in part due to the triplet-triplet absorption. As observed for triadimenol, the formation of the 4-chlorophenoxyl radical is in agreement with the photoproduct distribution since the chlorophenoxyl 2854

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radical can lead to chlorophenol by hydrogen abstraction (38). 3.3 Photolysis of Triadimenol and Triadimefon on β-CD and Cellulose. Because of the change of the concentration profile of the excited pesticide on the powdered samples in space and time upon irradiation in stationary conditions, and also due to formation of degradation products that absorb at the excitation wavelength, no simple models can be used to compare the photodegradation kinetics in different conditions. However, the initial photodegradation rates in mol time-1 area-1 can be compared, provided the same initial concentration is used. This is reasonable since at lower conversions the pesticide concentration keeps essentially constant at the surface and the photoproduct concentrations are low. The initial photodegradation rates of both pesticides under solar radiation were determined for samples containing 150 µmol g-1. For triadimenol, the results were (2.8 ( 0.2) × 10-2 µmol cm-2 h-1 and (3.8 ( 0.5) × 10-2 µmol cm-2 h-1 for β-CD and cellulose, respectively. These values are the average for 6 h irradiation (between 11:00 a.m. and 5:00 p.m., clear sky) in June. Therefore, unlike the result obtained at 313 nm, triadimenol undergoes some photodegradation upon solar radiation, although, as we will see latter, at a much lower rate than the observed for triadimefon in the same conditions. For triadimenol to undergo direct photodegradation, some

overlap between the triadimenol and the solar irradiation spectra must exist (see Figure 2). The solar radiation was measured during the irradiations with the UVB filter described in the Experimental Procedures. An average value of 6.5 × 10-5 W cm-2 was obtained during the 6 h exposure time. This result, although not comparable with the literature values (34) (the result was not corrected with the solar spectrum at ground level nor with the instrument response function), clearly indicates that spectral overlap and intensity are available for triadimenol to undergo direct photodegradation in natural conditions. The direct photodegradation of compounds showing low overlap with solar spectrum, like 4-chlorophenol, was already observed at the solid/gas interface (13). For triadimefon, the initial photodegradation rates under solar radiation were (2.4 ( 0.4) × 10-1 µmol cm-2 h-1 and (3.7 ( 0.4) × 10-1 µmol cm-2 h-1, for β-CD and cellulose, respectively. These values are 1 order of magnitude higher than the obtained for triadimenol. This difference is mainly due to a larger overlap between the absorption spectrum of this pesticide with the solar radiation at ground level (see Figure 2). The initial photodegradation rates are somewhat higher in cellulose for both pesticides, which is in agreement with the faster decay of the formed transients in this support. The direct photodegradation quantum yield of triadimenol and triadimefon are lower in low polarity solvents, indicating that this behavior at the solid/gas interface should be related with the polarity of the supports under study (28, 29). The photodegradation kinetics of triadimenol and triadimefon on both supports were followed under 254 nm and solar irradiation, in air-equilibrated conditions. For triadimefon, the kinetics of its product formation was also followed (see Figure 7). 4-Chlorophenol was quantified by comparison with standard solutions. For the other products, it was assumed that both show an extinction coefficient similar to that of triadimefon in the analysis wavelength (275 nm). This is reasonable since the chlorophenoxyl group is present in both products and is responsible for the absorption at this wavelength. The inset of Figure 7A shows the normalized absorption spectra of triadimefon, 4-chlorophenol, and of the product with rt ) 35.86 min (m/z ) 265). It can be verified that the absorption spectra of triadimefon down 290 nm and of compound with rt ) 35.86 min (m/z ) 265) almost coincide in the region of the analysis wavelength (275 nm). 4-Chlorophenol absorbs at higher wavelengths, indicating that this compound is one that increases the ground-state absorption spectra upon irradiation (see inset of Figure 2). The quantification of the main degradation products allowed the estimate of the molar balance for triadimefon. At lower conversions (less than 10%), the detected photoproducts account for more than 90% of the decomposed pesticide. At higher conversions, the effect of photoreaction of the photoproducts is more pronounced; therefore, a correct molar balance cannot be made. Products that are not amenable to HPLC and GC-MS analysis, volatile, insoluble in methanol, and resulting from reaction of the formed radicals with the solid support were not detected using the described procedure. Therefore, the main degradation products of triadimefon were detected. The time evolution of triadimefon and of its main degradation products is similar under lamp (254 nm) and natural irradiation (Figure 7B). This was expected by considering the intramolecular energy transfer process from the chlorophenoxyl moiety to the carbonyl group described in the literature (28). This process ensures that the main degradation pathways occur from the same electronic state (nπ*), independently of the excitation wavelength. The main differences are the relative higher concentration of the formed products under solar irradiation.

This can be attributed to lower photodegradation of the formed products owing to their lower absorbance of natural radiation (see inset of Figure 7A). 3.4 Photodegradation Mechanism. The main photoreaction pathways of triadimenol and triadimefon at the solid/ gas interface are significantly different from those observed in solution. In solution, the main degradation pathways involve the cleavages of the C-O and C-N bonds to the C1 carbon for both pesticides (see Figure 1). In the studied solid supports, besides these pathways, triadimenol undergoes dechlorination and photoreduction to triadimefon (see Figure 8). Compound with rt ) 21.56 min should result from the reaction of the radical containing the triazolyl group that is produced upon C-O cleavage to the C1 carbon (see Figure 8, path A). The main photodegradation pathways of triadimefon on β-CD and cellulose are similar to those reported on its commercial formulation Bayleton 5, being the main difference from solution photoreaction the R-cleavage of the C-C bond to the carbonyl group. The elimination of the triazolyl group, unlike the solution behavior, is not the main photodegradation pathway for triadimefon at the solid/gas interface. In fact, the molar balance at lower conversions (less than 10%) indicated that the detected photoproducts account for more than 90% of the decomposed pesticide. Photodegradation on natural surfaces is not therefore expected to deactivate triadimefon since its biological activity is related with this group (24).

Acknowledgments We thank Dr. Anabela Oliveira for his kind and helpful assistance with the transient absorption measurements. Postdoctoral Grant SFRH/BPD/5589/2001, support by Fundac¸ a˜o para a Cieˆncia e a Tecnologia, is gratefully acknowledged. This paper is dedicated to Professor Abı´lio Marques da Silva in honor of his 70th birthday.

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Received for review July 31, 2003. Revised manuscript received January 20, 2004. Accepted March 3, 2004. ES0348501