Thermal Decomposition of Captan and Formation Pathways of Toxic

Apr 30, 2010 - improved understanding of the formation pathways of toxic air pollutants in the accidental or deliberate combustion of captan. Introduc...
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Environ. Sci. Technol. 2010, 44, 4149–4154

Thermal Decomposition of Captan and Formation Pathways of Toxic Air Pollutants KAI CHEN, JOHN C. MACKIE,† ERIC M. KENNEDY, AND BOGDAN Z. DLUGOGORSKI* Process Safety and Environmental Protection Research Group, School of Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia

Received December 15, 2009. Revised manuscript received April 12, 2010. Accepted April 14, 2010.

This study investigates the thermal decomposition of a widely used fungicide, captan, under gas phase conditions, similar to those occurring in fires, cigarette burning, and combustion of biomass treated or contaminated with pesticides. The laboratory-scale apparatus consisted of a plug flow reactor equipped with sampling trains for gaseous, volatile organic compounds (VOC) and condensed products, with analysis performed by Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS), respectively.Underoxidativeconditions,thethermaldecomposition of captan generated gaseous pollutants including carbon disulfide, thiophosgene, phosgene, and hydrogen cyanide. The VOC analysis revealed the formation of tetrachloroethylene, hexachloroethane, and benzonitrile. Quantum chemical calculations indicated that captan decomposes unimolecularly, via fission of the C-S bond, with the ensuing radicals reacting with O2. The results of the present study provide an improved understanding of the formation pathways of toxic air pollutants in the accidental or deliberate combustion of captan.

the need for gaining better understanding of the combustion processes involving pesticides. In addition, the thermal decomposition of captan needs to be elucidated for another reason. During cigarette burning, oxygen levels down to 5% (v/v) and temperatures up to 600 °C can be encountered in the endothermic region (11) of the propagating combustion front of a cigarette. As a consequence, captan residue in tobacco products could engender the formation of toxic decomposition products (12). Surprisingly, there have been very few studies on the thermal decomposition of captan under conditions relevant to pesticide burning, especially its decomposition in fires. One of these studies established the formation of gaseous sulfides, acids, and organochlorines when captan was heated to 400-525 °C (13). However, no quantification of gaseous products and no analysis of condensed species were attempted, preventing one from postulating decomposition pathways or performing quantitative risk assessment. A subsequent investigation emphasized the need for absorbent tubes to trap products from the combustion of captafol, a pesticide possessing a tetrahydrophthalimide moiety similar to captan (14). Although gas chromatography-mass spectrometry (GC-MS) results revealed several condensed and volatile organic compounds (VOC), the lack of complete analysis and quantification of all gaseous products makes it impossible to understand the detailed decomposition mechanism. This paper reports the results from a combined experimental and quantum chemical study designed to obtain detailed knowledge of the mechanistic pathways of the formation of toxic species during the thermal decomposition of captan, under oxidative conditions. The experiments were performed at two oxygen levels, denoting underventilated and overventilated combustion, respectively. For brevity, we denote the former as low-oxygen pyrolysis (LOP) and the latter as oxidation (OXD). The results of this study will facilitate the evaluation of the potential hazards of captan to health and the environment of surrounding areas in case of accidental or deliberate combustion of this fungicide.

Experimental Section Introduction Captan, a nonspecific sulfenimide fungicide, is structurally similar to other sulfenimides, namely, folpet and captafol (1). First registered for fruit tree application in 1949, captan has become one of the most widely used fungicides, deployed to inhibit the growth of fungi on fruit trees, vegetable crops, and ornamental plants (2). In the United States alone, more than 160 registered products incorporate captan as their active ingredient (3). Captan is also broadly employed for applications in cosmetics, paints, and textiles (4, 5). The wide use of captan has stimulated research into its potential health effects, including possible carcinogenicity and degradation behavior in the environment (6, 7). The serious consequences of accidental fires occurring in production facilities and warehouses have motivated continuous efforts to determine the combustion products from burning pesticides (8-10). Mixtures of toxic gases and smoke from widespread burning of pesticide-treated plants can affect highly populated regions, as evidenced by recent Australian and U.S. bushfires. These catastrophes reinforce * Corresponding author e-mail: Bogdan.Dlugogorski@ newcastle.edu.au; phone: +61 2 4985 4433; fax +61 2 4921 6893. † Also at School of Chemistry, The University of Sydney. 10.1021/es9037935

 2010 American Chemical Society

Published on Web 04/30/2010

The apparatus employed to study the thermal decomposition of captan comprised a pesticide vaporizer, an isothermal tubular-flow reactor, and a product collection system. Details of the apparatus have been provided elsewhere (15) with a schematic illustration of the experimental facility included in Figure S1 of the Supporting Information. Captan (>98%, TCI) powder was loaded into a polytetrafluoroethylene (PTFE) tube installed vertically inside the pesticide vaporizer, which itself was housed in a GC oven maintained at 150 °C. This temperature afforded slow evaporation of captan at a rate of approximately 0.045 mg min-1. Diluted with nitrogen (99.999%), the vapor/nitrogen mixture entrained a controlled amount of oxygen, with the total flow entering the reactor of 19 cm3 min-1. The O2 concentration in carrier gas was monitored by a Varian CP 2003 micro gas chromatograph (µGC). The reactor included an alumina tube maintained at a preset temperature by a three-zone furnace, coupled by a heated transfer line (175 °C) to the vaporizer. Manipulating the length of the reaction zone enables one to retain the same residence time of 1 s for all experiments performed between 200 and 500 °C. We conducted the OXD experiments under an atmosphere of 6.0 ( 0.5% O2 in N2 (v/v) and the LOP under 280 ppmv O2 in nitrogen, corresponding to fuel equivalence ratios (φ) of 0.03 and 7.1, respectively. The OXD VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Comparison of Yields of Condensed Products from the LOP and OXD Experiments (Mole Percent) 250 °C

FIGURE 1. Yield-temperature profiles of undecomposed captan and condensed products under the OXD conditions. and LOP conditions span the entire range of fuel equivalence ratios expected to exist in combustion systems, such as fires of pesticides. Sampling from the reactor extended for 1 h in each run. Connection tubes, made solely of PTFE, were cleaned by a solution of methanol and acetone between runs to recover adsorbed species and to avoid contamination of products in subsequent runs (16). The gaseous species passed through a desiccant tube into a tedlar sampling bag for Fourier transform infrared (FTIR) spectroscopic analysis while, in a separate run, VOC were trapped in a glass tube loaded with around 100 mg of activated charcoal. To conclude the sampling train, a dichloromethane-methanol solvent trap was chilled in a cold glycol bath (0 °C) to collect the condensed products. The analysis of condensed products was performed on a Varian CP 3800 GC and 1200 quadrupole mass spectrometer equipped with a 30 m Varian VF-5 ms FactorFour column (0.25 mm i.d., 0.25 µm film thickness). For the analysis of VOC, we modified the general procedure of NIOSH method 1003, as was necessary for our instrument. Extracts obtained by washing the activated charcoal with 1 mL of CS2 were filtered prior to injection into the same GC-MS. Infrared spectra were recorded on a Varian 660-IR spectrometer equipped with a gas cell characterized by a 10 m long IR path. The results from repeated experiments suggested reasonable reproducibility. Further details of the analytical procedures, purity of reactants, estimates of errors (in terms of relative standard deviations, RSD), and the limits of detection (LOD) are discussed in section 2 of the Supporting Information, with Table S1 listing numerical values for RSD and LOD.

400 °C

450 °C

500 °C

product

LOP

OXD

LOP

OXD

LOP

OXD

LOP

OXD

captan folpet THPI PI

94.2 0.9 1.3 0.0

89.7 1.2 4.2 0.0

15.6 4.1 36.7 5.4

0.0 0.0 39.8 14.9

0.0 0.0 38.8 11.1

0.0 0.0 2.9 32.6

0.0 0.0 24.5 19.7

0.0 0.0 0.0 28.3

dominant condensed product up to 425 °C, with a yield of 39.8% at 400 °C. At temperatures >400 °C, the yield of THPI rapidly declines while phthalimide (PI) gradually becomes the major product. The yield of PI attains its highest value of 32.6% at 450 °C, decreasing above this temperature. Folpet reaches its maximum yield of 9.7% at 350 °C but decomposes completely at higher temperatures. We detected no presence of folpet in product gases of experiments performed at temperatures >400 °C. The GC-MS analyses for the experiments conducted under the LOP and OXD conditions identified the formation of the same condensed species, but at different yields (Table 1). Under the LOP conditions, the decomposition of captan also initiates at 250 °C, but proceeds at lower conversion with 15.6% of captan remaining undecomposed even at 400 °C. The peak value of THPI shifts from 400 to 450 °C, with less PI formed in the reaction. Three major VOC products were unequivocally identified in GC-MS analysis of the CS2 extract from the activated charcoal trap by comparing our measurements to the NIST library and injecting reference standards. Two of these products, namely, tetrachloroethylene (TCE) and hexachloroethane (HCE), are listed as suspected carcinogens. The third, benzonitrile (BZN), has been documented to cause the chromosomal end point of genotoxicity (17). Figure 2 compares the yields of TCE, HCE, and BZN from the LOP and OXD experiments. The formation of HCE is significantly enhanced under the LOP conditions with its peak yield amounting to 39.9% compared with only 1.9% in the OXD experiments, both detected at 450 °C. The processes leading to TCE are also promoted under the LOP conditions with monotonic increase in its yields at both O2 levels. Conversely, our results indicate that the OXD conditions facilitate the formation of BZN with the yield gradually increasing to 24.5% at 500 °C, much higher than the measurements under LOP conditions. The interpretation of FTIR spectra is described in the Supporting Information. Under the OXD conditions, we identified 11 gaseous species including phosgene (COCl2), thiophosgene (SCCl2), chloroform (CHCl3), hydrogen chloride (HCl), carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2), benzene (C6H6), hydrogen cyanide (HCN), and

Computational Details Quantum chemical computations have been employed to obtain information on reaction pathways and energetics, with the detailed methods outlined in the Supporting Information.

Results and Discussion Under oxidative conditions, the chromatograms of condensed species indicated four compound peaks comprising undecomposed captan and three major products. In Figure 1, we report the yield-temperature profiles for each compound as mole percent of the initial captan loading, together with the corresponding molecular structures. The amount of undecomposed captan initially follows a slow decrease and then, above 300 °C, a steep conversion increasing to 100% at 400 °C. Although present initially as a trace impurity, cis-1,2,5,6-tetrahydrophthalimide (THPI) constitutes the 4150

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FIGURE 2. Yields of VOC compounds under the LOP and OXD conditions.

FIGURE 3. Yield-temperature profiles of gas products measured in the OXD experiments: (a) chlorine-containing gases; (b) sulfur-containing gases.

TABLE 2. Comparison of Yields of Major Toxic Gases in the LOP and OXD Experiments (Mole Percent) 250 °C

400 °C

450 °C

500 °C

product

LOP

OXD

LOP

OXD

LOP

OXD

LOP

OXD

COCl2 SCCl2 CS2a HCN

0.3 0.5 0.6 0.0

1.0 1.0 0.6 0.0

10.3 4.7 2.4 0.8

22.3 11.9 5.0 5.1

7.6 10.4 13.4 2.1

3.4 11.2 4.3 8.1

1.9 5.7 18.7 2.7

0.3 2.0 3.0 10.1

a We multiplied the concentration of CS2 by 2 to calculate the mole percentage because its molecule contains two S atoms.

carbon oxides (CO, CO2). There have been a number of studies indicating that most of these chemicals exhibit negative effects on health (18-22). Both COCl2 and SCCl2 were released in World War I as chemical warfare agents (23, 24). Two more species were identified under the LOP conditions, namely, dichloromethane (CH2Cl2) and carbon tetrachloride (CCl4). Standard samples of several product species were unavailable; thus, reference spectra in the QAsoft package were applied to quantify the products’ spectra with the integration wavenumber ranges for each gas listed in Table S2 of the Supporting Information. We calculated the yields of all gas products as mole percent of captan initially present in the experiments. The yields of toxicants are mainly compared hereafter, whereas the results of detailed measurements of the evolution of remaining species under the LOP and OXD conditions are listed in Table S3 of the Supporting Information. The yield-temperature profiles of the chlorine-containing gases, under the OXD conditions, are indicated in Figure 3a. The yield of COCl2 increases with temperature, attaining a maximum of 22.3% at 400 °C, and then descends sharply to zero. Similarly, the yield of SCCl2 increases up to 425 °C with the peak value determined as 12.3% and shows a sudden decline to 2.0% at 500 °C. Unlike these two compounds, CHCl3 and HCl exhibit a monotonic increase in yield over the entire temperature range. A rapid increase in the formation of HCl occurs at temperatures of 300-400 °C. Table 2 compares the yields of COCl2 and SCCl2 from the LOP and OXD experiments. Under the LOP conditions, the conversion of COCl2 and SCCl2 to HCl increases monotonically with temperature, whereas that of COCl2 and SCCl2 peaks at 400 and 450 °C, respectively. With less O2 present, CCl4 forms in trace amounts of around 0.2%. With no undecomposed captan and no chlorine-containing condensed products detected at higher temperatures,

we observed chlorine deficiency especially in oxidation experiments performed at higher temperatures. Typical chlorine balance amounted only to 40.2% at 450 °C and decreased with increasing temperature. The yields of COCl2 and SCCl2 followed similar trends with large increases in HCl yields at elevated temperature. We propose that HCl is underestimated in the FTIR measurements, even though the yields reported here reflect the trend of HCl formation as a function of temperature. It appears that part of the HCl is removed by the water condensing in the moisture trap and by adsorption on surfaces in colder regions of the experimental apparatus. To test this explanation of the observed results, we bubbled the reaction gases, obtained from two additional oxidation experiments, into a solution of sodium hydroxide. Ion chromatographic analysis of Cl- demonstrated improved chlorine recoveries of 85.3% at 450 °C and 90.1% at 500 °C, respectively. The sulfur-containing gases identified in FTIR spectra include COS, CS2, and SO2. As displayed in their yieldtemperature profiles for oxidation experiments (Figure 3b), SO2 dominates the product distribution of the sulfurcontaining species, especially at temperatures exceeding 350 °C. The yield of COS increases over the whole temperature range, leveling off to a plateau at the highest temperature investigated. The yield of CS2 increases up to 425 °C, to attain 5.3%, and then declines at higher temperatures. Under the LOP conditions, we identified no new sulfur-containing species in the FTIR spectra. Yields of COS and SO2 are lower than those measured in the OXD experiments. In contrast, CS2 exhibits higher yields at higher temperatures, compared to the results of the OXD experiments. Generally, satisfactory sulfur and carbon elemental balances were achieved (recovery > 80%). However, the recovery of sulfur showed a decline in some experiments at high temperatures. Under the OXD conditions, the recovered sulfur was only 72.8% at 400 °C. Upon investigation of the inner surfaces of the reactor tubes at the conclusion of the experiment, we discovered a residue with yellowish brown color, thought to be elemental sulfur. The tubes were washed with dichloromethane to remove any possible organic residues, then purged with O2, and heated to 700 °C. Subsequently, the identification of SO2 in the FTIR spectrum of exhaust gases confirmed that elemental sulfur is a product species. The formation of sulfur may be the reason for the observed deficiency in sulfur balance at 400 °C. Under the LOP conditions, the sulfur elemental balances show even lower recoveries, for example, 43.8% at 500 °C. We observed soot residue left on the tube inner surfaces, which contributes to poor carbon recovery. The formation of soot may lead to release of H radicals, resulting in the formation of hydrogen VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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sulfide (H2S) previously identified as a gas product (13). However, no signal from H2S appeared in our spectra; H2S possesses extremely weak FTIR response with a limit of detection (LOD) of 100 ppm in our instrument. Benzene, HCN, CO, and CO2 all exhibit monotonic increase in yields over the studied range of temperatures in the OXD experiments (for profiles refer to Figure S4 of the Supporting Information). The yield of HCN exceeds that of benzene, with the highest yields of HCN and benzene of 10.1 and 7.5%, respectively. However, under the LOP conditions, the yields of HCN and C6H6 decrease, compared to the measurements of the OXD experiments. The yield of CO increases with temperature up to 425 °C, at which it plateaus. Finally, the yield of CO2 exhibits a rapid ascent in the region of 300-450 °C and a slow increase in the remaining temperature ranges. With less O2 present under LOP conditions, we observed a significant decrease in the yields of both CO and CO2. Under the oxidative pyrolysis conditions of this study, initial reaction would be expected to take place via unimolecular decomposition of captan, bimolecular reaction with O2, or a combination of both. A previous study on oxidative photolysis (25) suggested that unimolecular fission of captan might compete with bimolecular reaction between O2 and captan in oxidative photolysis. Here we employ quantum chemical calculation to assist in unraveling probable initiation pathways. Unimolecular reaction would take place by an initial rearrangement, expulsion of an appropriate leaving group, or bond fission. Bimolecular reaction with O2 would involve abstraction of a relatively weakly bonded H atom (most likely from a methylene carbon) or addition of O2 or O (with O-O fission), most probably to the S atom. We have been unable to discover any facile rearrangements that might take place in captan, nor does the molecule appear to have any facile leaving groups. Hence, unimolecular initiation is likely to occur by bond fission. The quantum chemical calculation of precise bond energies requires a high level of computational theory. Ideally, such calculations would be performed at the G3 or similar level. However, captan is far too large a molecule to study at this level. The framework of N-S-CCl3 is crucial to understand the initial decomposition of captan, owing to the lack of potentially weak bonds present in the remaining structure. To estimate the bond energies in this framework, we adopted the model compound H2N-S-CCl3 and tested its appropriateness. For more details, refer to the Supporting Information. Initially, we considered the bimolecular abstraction reaction between captan and O2, which would form HO2 and a captan-derived radical. The rate of the bimolecular reaction is proportional to [captan][O2], whereas the rate of a unimolecular initiation depends only on [captan]. Given comparable rate constants, the bimolecular reaction at modest concentrations of O2 will not be able to compete with bond fission. The S-C bond energy corresponds to 55 ( 2 kcal mol-1 (0 K), which is 17.0 kcal mol-1 less than the required energy for the rupture of the N-S bond. The C-Cl bond energy amounts to 72.4 kcal mol-1, very similar to that of N-S. Therefore, the decomposition of captan in the vapor phase (at least in the absence of O2) most probably initiates through the fission at the weakest bond (C-S bond). The experimental rate constants for formation of HO2 from hydrocarbons are around 2 × 1013 exp(-52 kcal mol-1/RT) cm3 mol-1 s-1 (26). In the structure of captan, several abstractable H atoms are available in the cyclohexene ring. Although the activation energies for H abstraction by O2 and for S-C bond fission are comparable, the latter would be expected to have an A factor of 1016-1017 s-1. When the A factor for abstraction is multiplied by [O2] e 1.5 × 10-6 mol 4152

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cm-3, it becomes clear that the abstraction rate is negligible in comparison with S-C fission to initiate decomposition. Possible transition states for the formation of a peroxy radical or of O addition have also been investigated. Captan is again too large for transition state calculations even at the B3LYP/6-31G(d) level, and hence the H2N-S-CCl3 model was again employed. We found no transition state for the formation of a peroxy radical via addition at the S atom (or at any other atom). A discrete transition state for O addition to the S atom and formation of O(g) from O2(g) was located. Although the reaction energy for H2N-S-CCl3 + O2 f H2N-SO-CCl3 + O is only 49.2 kcal mol-1, the reaction has an energy barrier of 72.0 kcal mol-1. A discrete transition state for O(g) addition to the S atom was also located. Although this is a very exothermic reaction (-73.2 kcal mol-1), the barrier for addition is 50.3 kcal mol-1. Thus, the addition of O to S from O2, or from a subsequently formed O atom, would not be expected to compete favorably with S-C bond fission. Therefore, we would expect that both pyrolysis and oxidation are initiated similarly, with the decomposition pathways displayed in Figure 4. The fission of the S-C bond dominates the beginning stage of captan oxidation. Then O2 can react very rapidly with any radicals produced in the initiation, that is, CCl3 and radical R1. With the assistance of the NIST Chemical Kinetics Database (26), we postulate how phosgene might be formed during the oxidation of captan. Addition of O2 to CCl3 radical to form trichloromethylperoxy (reaction A) has been extensively studied because of its importance in atmospheric pollution (27). CCl3 + O2 (+M) f CCl3O2 (+M)

(A)

CCl3O2 + CCl3O2 f O2 + CCl3O + CCl3O

(B)

CCl3 + CCl3O2 f CCl3O + CCl3O

(C)

CCl3O f COCl2 + Cl

(D)

(+M) denotes that the reaction is pressure-dependent. The peroxy radical can then react with either itself or CCl3, in reaction B or C (28, 29). Both B and C are fast reactions with rate constants in the order of 108 cm3 mol-1 s-1. CCl3O will decompose fairly readily to phosgene and chlorine atom (reaction D). On the basis of this mechanism, phosgene should rapidly arise from CCl3 oxidation. The chlorine atom provides a source of further chlorination, as evidenced by CCl4 detected in the LOP experiments. The decline in yield of phosgene (>400 °C) coincides with the fact that phosgene can react with O(g) atom to produce two reactive intermediates COCl and ClO (30), which can convert to carbon oxides, HCl, and trace of Cl2. A slower decrease in phosgene yields revealed in the LOP results suggests that its destruction involves the participation of O2. Meanwhile, CCl3O2 radical will cease to have an appreciable existence owing to the weak Cl3C-OO bond strength at high temperatures, which may also be responsible for the decreased COCl2 yield (31). CCl3 radicals can also undergo self-combination, resulting in the formation of observed HCE. Comparison between the OXD and LOP results suggest that the self-combination process may compete with the oxidation of CCl3 to phosgene. With much less O2 present under the LOP conditions, the HCE yields exceed those in the OXD process, whereas the peak yield of COCl2 shows an opposite trend. At high temperatures, TCE can be expected because the chlorine in HCE can be abstracted by H or free Cl radicals. Among condensed phase products, THPI was the first product detected, indicating that the N-S bond can also be broken. Because of its relatively high bond energy (72 kcal

FIGURE 4. Summary of pathways of captan decomposition; red arrows indicate additional reactions that operate under the OXD conditions. The species detected in this study are set in bold font. mol-1), direct bond fission is unlikely. CCl3 radicals produced by initiation can readily abstract hydrogen atoms from the cyclohexene ring in captan. Abstraction of a ring H leads to a cyclohexenyl radical. Quantum chemical calculation shows that this radical can lose a further H by fission with a low energy barrier to form a cyclohexadiene ring. Further abstraction by CCl3, Cl, or H radicals, which build up as decomposition proceeds, forms a cyclohexadienyl radical, which again can rapidly lose a H atom, eventually leading to the aromatic ring-containing phthalimide, also an observed product. The growing availability of H atoms now facilitates the rupture of the N-S bond by H addition to the nitrogen of captan, forming THPI and SCCl3. We have established the feasibility of the above mechanism by locating a transition state for H + H2N-S-CCl3 f NH3 + SCCl3. This reaction has an exothermicity of -35 kcal mol-1 and a barrier of only 7 kcal mol-1 (at the B3LYP/6-31G(d) level). This transition state clearly indicates that as the H atom approaches the N atom, the N-S bond lengthens and ultimately breaks when the H gets close enough to bond to the N. Because C-Cl bond energy in SCCl3 amounts only to 16.1 kcal mol-1, SCCl3 should easily lose a Cl to give SCCl2. The decreasing yield of SCCl2 at elevated temperatures suggests that further reaction relating to SCCl2 can take place in our experiment. The oxidation of SCCl2 by O was reported to form SO and CCl2 radical (32), with SO2 and HCl expected as the final products. On the other hand, SCCl2 may lead to CS intermediate, owing to the Cl abstraction by H or other radical processes. At the G3B3 level, the reaction H + SCCl2 f HCl + SCCl is exothermic at -26.4 kcal mol-1. We have also been able to locate a transition state for this reaction, and it is only 9.0 kcal mol-1. With a weak C-Cl bond of only 35.7 kcal mol-1, SCCl may readily decompose to CS either by direct fission or by reaction with H. CS2 has been detected in previous studies of captan decomposition (13, 33), and it can readily form by recombination with S atoms: CS + S + M f CS2 + M. As mentioned above, free sulfur was observed in the products. Its origin is somewhat uncertain, but one possibility, explored here by

quantum chemistry, is that it might arise via H addition and concomitant S fission. We have located a transition state for the model reaction, H + H2NS f NH3 + S. This reaction has an exothermicity of 28 kcal mol-1 with a barrier of 21.6 kcal mol-1. As decomposition proceeds and the radical pool builds up, HCl yields increase from sources including Cl abstraction of the methylene H atoms, abstraction of Cl from captan by H, and radical-radical processes. The presence of SO2 is evidence that the direct oxidation of sulfur to gaseous product occurs. The oxidation of sulfur may also lead to SO as a reactive intermediate, which might react with carbon source and contribute to the formation of COS. Hydrocarbon moieties present in radical R1 can react with O2, producing CO and radicals such as H and OH, which will promote the oxidation process. As the concentration of CO builds, it is also possible that COS arises from the reaction of CO with the R1 radical directly. We have an analogy for this proposition in CH3S radical reacting with CO to form COS and methyl radical (34). Because the structure of folpet might be considered an “aromatized version” of captan, its identification in the condensed products strongly supports the conclusion that the cyclohexene ring in captan can serve as a hydrogen donor to the radical process. Analogously, THPI also possesses the cyclohexene ring with abstractable H available, leading to phthalimide. Furthermore, phthalimide can arise from folpet ˙ urakowskaby the fission of either its S-C or N-S bond. Z Orsza`gh et al. (35), during their pyrolysis research on phthalimide, observed the formation of benzonitrile through the loss of CO2, with results comparable to those of our experiments. We suggest that the H abstraction by radicals can also facilitate the fission of the CsC bond between CdO and the ring. As evidence, higher benzonitrile yields were observed in the OXD experiments, as a consequence of more abundant radicals forming in the presence of elevated levels of O2. We also observed that the yield-temperature profiles of benzene and HCN appear to correlate with that of benzonitrile. Benzene and HCN have been reported in the major products of benzonitrile decomposition (36). Thus, VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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their formation in our experiments can probably be attributed to the further reactions involving benzonitrile.

Acknowledgments This study has been funded by the Australian Research Council. We thank Professor Adam Grochowalski of the Cracow University of Technology for advice on capturing VOC.

(16)

(17)

Supporting Information Available (1) Schematics of experimental setup; (2) materials and analytical procedure; (3) identification of gaseous species in FTIR; (4) integration wavenumber range and stoichiometric modification of some compounds for yield calculations; (5) comparison of gaseous products between LOP and OXD conditions; (6) yield-temperature profiles of benzene, HCN, carbon oxides; (7) computational details including a discussion on the validity of selecting H2NSCCl3 as a model of captan. This material is available free of charge via the Internet at http://pubs.acs.org.

(18) (19) (20) (21) (22)

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