Reaction of thiometon and disulfoton with reduced sulfur species in simulated natural environment.

The reactions of thiometon and its ethyl analogue, disulfoton, with reduced sulfur species [e.g., bisulfide (HS-), polysulfide (S(n)2-), thiophenolate (PhS-), and thiosulfate (S2O3(2-))] were examined in well-defined aqueous solutions under anoxic conditions. The role of reduced sulfur species was investigated in the abiotic degradation of thiometon and disulfoton. Experiments at 25 degrees C demonstrated that HS-, S(n)2-, PhS-, and S2O3(2-) promoted the degradation of thiometon to a great extent while only S(n)2- and PhS- showed a small accelerating effect in the degradation of disulfoton. Reactions were monitored at varying concentrations of reduced sulfur species to obtain the second-order rate constants. The reactivity of the reduced sulfur species decreased in the following order: S(n)2- > PhS- > HS- approximately S2O3(2-). Transformation products were confirmed by standards or characterized by gas chromatography mass spectrometry. The results illustrate that multiple pathways occur in the reactions with reduced sulfur species, among which the nucleophilic attack at the alpha-carbon of the alkoxy group was the predominant pathway. Activation parameters of the reaction of thiometon and disulfoton with HS- were also determined from the measured second-order rate constants over a temperature range. DeltaH( not equal) values indicated that the reactivity of thiometon toward HS- was much greater than for disulfoton. Nucleophilic attack at the alkoxy group was more important for thiometon than disulfoton. When the measured second-order rate constants at 25 degrees C are multiplied by [HS-] and Sigma[S(n)2-] reported in saltmarsh porewaters, predicted half-lives show that reduced sulfur species present at environmentally relevant concentrations may present an important sink for thiometon in coastal marine environments.