Kinetics and mechanism of degradation of dichlorvos in aqueous solutions containing reduced sulfur species.

Reactions of dichlorvos with five reduced sulfur species (hydrogen sulfide, bisulfide, thiosulfate, thiophenol, and thiophenolate) were examined in well-defined anoxic aqueous solutions to investigate their role in its degradation. Reactions were monitored at varying concentrations of reduced sulfur species over pH range to obtain the second-order reaction rate constants. Experiments at 25 degrees C demonstrated that degradation of dichlorvos promoted by bisufide, thiosulfate, and thiophenolate were of much greater importance than hydrolysis under the experimental conditions in our study. In contrast, hydrogen sulfide and thiophenol were not effective in the degradation of dichlorvos. The activation parameters of the reaction of dichlorvos with bisulfide, thiosulfate, and thiophenolate were also determined from the measured second-order rate constants over a temperature range of 12-50 degrees C. The relative reactivity of the reduced sulfur species decreases in the following order: PhS- > HS- approximately equal to S2O3(2-). When the second-order rate constants at 25 degrees C are multiplied by the environmentally relevant concentration of the reduced sulfur species, predicted half-lives of dichlorvos ranged from hours to days. The results indicated that reduced sulfur species could play a very important role in the chemical fate of dichlorvos in coastal marine environments.

Degradation of naled and dichlorvos promoted by reduced sulfur species in well-defined anoxic aqueous solutions.

This work examines the reaction of reduced sulfur species (e.g., bisulfide, thiosulfate, thiophenolate) with naled, a registered insecticide, in well-defined anoxic aqueous solutions at 5 degrees C. High concentrations of reduced sulfur species can occur in the porewater of sediments and in anoxic subregions of estuaries. The dominanttransformation product from the reaction of naled with reduced sulfur species is dichlorvos, which indicates that debromination is the major reaction pathway. Dichlorvos is also a registered insecticide which is more toxic than naled. The second-order rate constants for reaction of naled with bisulfide and thiophenolate at 5 degrees C are 10.2 +/- 0.4 M(-1) s(-1) and 27.3 +/- 0.9 M(-1) s(-1), respectively, while the second-order rate constant for the reaction of naled with hydrogen sulfide and thiophenol are not significantly different from zero. The second-order rate constant of the reaction of naled with thiosulfate at 5 degrees C is 5.0 +/- 0.3 M(-1) s(-1). In contrast, the second-order rate constant of the reaction of dichlorvos with bisulfide at 25 degrees C is (3.3 +/- 0.1) x 10(-3) M(-1) s(-1). The activation parameters of the reaction of naled with bisulfide were also determined from the measured second-order rate constants over a temperature range. The results indicate that reduced sulfur species can play a very important role in the transformation of naled and dichlorvos in the coastal marine environment. It can be expected that in the presence of reduced sulfur species, naled is almost immediately transformed into the more toxic dichlorvos, which has an expected half-life of 4 days to weeks.

Half-life of naled under three test scenarios.

Decline of naled residue on filter paper was studied after exposure to ultra-low volume droplets in a settling chamber. Naled-treated filter papers were stored under 3 treatment scenarios: 1) in a dark environmental chamber at an average relative humidity (RH) of 46.9% and temperature of 24 degrees C, 2) in a dark environmental chamber at an average RH of 87.7% and temperature of 24 degrees C, and, 3) in direct sunlight in the field. Decline of naled followed first order kinetics in all cases; consequently, half-life of naled under each treatment was determined from the slope of each line. Half-life (+/- 1 SD) of naled was 8.17 +/- 1.24, 4.81 +/- 1.18, and 1.37 +/- 0.24 h, for treatment scenarios 1, 2, and 3, respectively. In each test, a significant (P < 0.05) decline in naled residue occurred between initial assessment and 4 h postapplication. The half-life of each treatment scenario was significantly (P < 0.05) different from that of the other 2 scenarios, indicating that both humidity and sunlight affect naled degradation rates.