| Organophosphate (OP) and carbamate compounds are wildly used in agriculture. The exposure to these chemicals may be a high risk for adverse health effects. Recent studies have shown that organophosphate chemicals are consistently present in the air, rain and surface waters, suggesting a long environmental half-life. Although transformation is the only ultimate sink for these compounds in the environment, some transformation reactions produce compounds that are even more toxic than the original compounds. One possible type of transformation of OPs and carbamates is nucleophilic substitution, the focus of this dissertation.; It is reported that high concentrations of reduced sulfur species can occur under anaerobic conditions. Reduced sulfur species are good nucleophiles and reductants that can possibly undergo reactions with OPs and carbamates. In this work, model systems consisting of homogeneous aqueous solutions with reduced sulfur species (hydrogensulfide/bisulfide (H2S/HS-), polysulfides (Sn2-), thiophenolate (PhS-) or thiosulfate (S2O32-)) were used to study the degradation of OPs and carbamates. The model systems, being simpler than environmental milieus, help to clarify the mechanisms. For carbamates, the reaction rates are independent of the concentration of reduced sulfur species. For the OPs, the reaction rates are first-order in the concentration of the different reduced sulfur species. The activation parameters of the reaction of OPs with bisulfide were determined from the measured second-order rate constants over a temperature range of 5-60°C. The resulting data indicates that OPs undergo a SN2 reaction with the reduced sulfur species. The transformation products indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of an alkoxy group to form the desalkyl OP. The formation of corresponding phenol, a minor degradation product, could be attributed entirely to hydrolysis. The determined second-order rate constants show that the reaction of an OP with polysulfides is about 15 to 50 times faster than the reaction of the same OP with bisulfide. The reaction rate constants for chlorpyrifos and parathion with the bisulfide and polysulfide are about factor of 10 smaller than for chlorpyrifos-methyl and parathion-methyl, while the reaction rate constants for fenchlorphos with the bisulfide and polysulfide are similar as chlorpyrifos-methyl and parathion-methyl. |
