The fate of dichlorvos in soil

Abstract

Experiments were conducted to determine the factors responsible for the loss (adsorption, chemical hydrolysis, microbial degradation, etc.) of dichlorvos (2,2-dichlorovingl 0 , 0 -dimethyl phosphate) in soil perfusion systems of Houston Black clay. The rate of disappearance from the perfusate (hence the rate of dichlorvos degradation in soil) was related directly to the presence of Bacillus cereus in the perfusion system, the pH of the system, and the extent of dichlorvos adsorption. Gas liquid chromatographic analyses of the perfusates showed that dichlorvos disappearance was rapid when B . cereus was added to a previously sterilized soil perfusion system (50% in 3.9 days). Under sterile conditions, 50% of the added dichlorvos was recovered after 10 days. When B . cereus was added to a mineral salts medium containing dichlorvos as sole ccrbon source, 49% of the initial dichlorvos concentration was degraded in 4 days. The organism was not capable of utilizing dichlorvos as a sole phosphorus source. Chemical hydrolysis of dichlorvos in aqueous, buffered, soil-free systems showed that hydrolysis did not occur in very acid systems (< pH 3.3), but increased with increasing pH values (26% in 4 days at pH 6.9), and was rapid at pH 9.3 (> 99% in 2 days). The extent of dichlorvos adsorption was determined by comparing the initial loss of dichlorvos in a sterile, soil-free extract solution with the initial loss in a sterile soil perfusion system. The rapid initial disappearance of dichlorvos in the presence of sterilized soil was attributed to soil adsorption of the pesticide. After 10 days both systems contained equal concentrations (50%) of dichlorvos. Non-biological mechanisms accounted for 70% of the total degradation of dichlorvos, while bacterial degradation accounted for 30% in the soil perfusion systems.