W8 AM Agrochemicals and Pesticides
Wednesday, 16 November 2005: 8:00 AM - 11:40 AM in 337-338

(JAN-1117-835951) Role of reduced sulfur species in the degradation of naled and dichlorovos in aqueous solution.

Gan, Qiu¹, Singh, Raphael¹, Jans, Urs¹, ¹ City College of CUNY, New York, NY, USA

ABSTRACT- 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. High concentrations of reduced sulfur species can occur in porewater of sediments and in anoxic subregions of estuaries. Dichlorovos is the dominant transformation product when naled is reacting with reduced sulfur species. This indicates that debromination is the major reaction mechanism. Dichlorovos is also a registered insecticide which is significantly more toxic than naled. The second-order rate constants for the reaction of naled with bisulfide and thiophenolate at 5 ^oC are 10.2 ± 0.4 M^-1s^-1 and 27.3 ± 0.9 M^-1s^-1, respectively, while the second-order rate constant for hydrogen sulfide and thiophenol are not significantly different from zero. The second-order rate constant of the reaction of naled with thiosulfate at 5 ^oC is 5.0 ± 0.3 M^-1s^-1. In contrast, the second-order rate constant of the reaction of dichlorovos with bisulfide at 25 ^oC is (3.3 ± 0.1) × 10^-3 M^-1s^-1, which is orders of magnitude slower than for naled. The activation parameters of the reaction of naled and dichlorovos with bisulfide were also determined from the pseudo-first-order rate constants measured in the buffered solutions performed over a temperature range. The results indicate that reduced sulfur species present at environmentally relevant concentrations can play a very important role in the transformation of naled and dichlorovos in a coastal marine environment. It can be expected that in the presence of reduced sulfur species naled is almost immediately transformed into the more toxic dichlorovos which has an expected half-life of a couple of days to weeks.

Key words: naled, dichlorovos, bisulifide, debromination