MP5 Agrochemicals and Pesticides
Monday, 14 November 2005: 8:00 AM - 5:30 PM in Exhibit Hall

(JAN-1117-830306) Kinetics and Mechanism of the Reaction of Organophosphorus Insecticides with Reduced Sulfur Species.

Wu, Tong¹, Guo, Xiaofen¹, Jans, Urs¹, ¹ City College of CUNY, New York, NY, USA

ABSTRACT- High concentrations of reduced sulfur species can occur in porewater of sediments and in anoxic subregions of estuaries. The kinetics of the reactions of organophosphorus insecticides (e.g., chlorpyrifos-methyl, chlorpyrifos, parathion-methyl) with reduced sulfur nucleophiles (e.g., bisulfide, thiosulfate, thiophenolate, polysulfide) is determined using well-defined aqueous solutions over a pH range from 5 to 9. The resulting pseudo-first-order rate constants yielded a second-order rate constant for the different pesticides and sulfur nucleophiles. The second-order rate constant for bisulfide is more than 50 times larger than for hydrogen sulfide, while polysulfide reacts 20 times faster than bisulfide. The reactivity for thiophenolate is similar to bisulfide, while thiophenolate is a little less reactive than polysulfides. Product studies clearly indicate that the nucleophilic attack of the sulfur nucleophile occurs primarily at the carbon of the methoxy/ethoxy groups of the organophosphorus insecticide and not at the phosphorus atom. The effect of natural organic matter (NOM) and reduced sulfur species on the fate of organophosphorus insecticides has also been investigated. The results show that NOM is increasing the rate of reaction of some of the investigated pesticides in the presence of bisulfide. It was found that in the presence of NOM different degradation products can be formed. The results indicate that reduced sulfur species (bisulfide, polysulfides) present at environmentally relevant concentrations may represent an important sink for organophosphorus insecticides in a coastal marine environment.

Key words: insecticides, bisulfide, polysulfide, chlorpyrifos-methyl