Role of the reduced sulfur species in the degradation of selected organophosphorus pesticides in simulated natural environment

Esters and thioesters of phosphoric acid and thiophosphoric acid are widely applied as pesticides in agricultural and urban environments, taking advantage of their inhibitory action on cholinesterase. Organophosphorus pesticides (OPs) are among EPA's highest priority for review under the Food Quality Protection Act due to their influence on the function of the nervous system and their reported neurodevelopmentally toxic effects at very low dose. They enter the environment primarily during their uses in crops, and in homes and gardens, which result in the release into sensitive costal environments such as estuaries, salt marshes and sediments. Anoxic conditions are common in such environments and relative high concentrations of reduced sulfur species are reported. This dissertation presents the kinetic, thermodynamic and mechanistical studies of the reactions of reduced sulfur species, including hydrogen sulfide (H 2S), bisulfide (HS-), polysulfide (Sn 2-), thiophenol (PhSH), thiophenolate (PhS-) and thiosulfate (S2O32-) in the degradation of selected organophosphorus pesticides (OPs) under anoxic condition. Selected OPs investigated in this study include thiometon, disulfoton, phorate, terbufos, naled and dichlorvos. Reactions were monitored at varying concentrations of reduced sulfur species over a pH range to obtain the second-order reaction rate constants. The activation parameters were investigated via the temperature dependence of the reaction rate constants of selected OPs with reduced sulfur species. This study also represents an experimental investigation of structural effects on reactivity and reaction mechanism. The subtle difference in structure may result in drastic differences in reactivity and even mechanism. Studies of the structural analogs can provide invaluable, though indirect and unclear, information regarding the mechanisms through which OPs react with the reduced sulfur species. When the second-order rate constants at 25°C are multiplied by the environmentally relevant concentration of the reduced sulfur species, half-lives of selected OPs can be predicted. The results indicated that reduced sulfur species could play a very important role in the chemical fate of selected OPs in coastal marine environments.