Soil Degradation of Malathion, a Phosphorodithioate Insecticide†

Abstract

Rates of malathion degradation in soils were related directly to extent of malathion adsorption, suggesting that degradation occurred by a chemical mechanism which was catalyzed by adsorption. Malathion degradation was rapid (50 to 90% in 24 hours, depending on the type of soil) in both sterile and nonsterile soil systems, and no lag phase occurred prior to degradation. In aqueous soil-free systems inoclulated with a soil extract, a lag phase (7 days) occurred, followed by rapid malathion loss, likely due to microbial degradation. Thus in soils, complete chemical degradation of malathion occurred prior to microbial adaptation to malathion. To obtain information on the pathway and products of malathion degradation, chemical hydrolysis in soil-free systems was investigated using liquid:liquid partition techniques with ¹⁴C-labeled malathion. Hydrolysis did not occur in acid systems (> pH 2), was slow at pH 9 (< 50% in 20 days) and rapid at pH 11 (> 99% in 1 day). At pH 9 the hydrolysis resulted in formation of thiomalic acid and dimethyl thiophosphoric acid as final products with accumulation of diethyl thiomalate as an intermediate due to unequal rates of hydrolysis of the ester linkages. In soils, both ester linkages are hydrolyzed, although not at the same rate, resulting in the accumulation of diethyl thiomalate in some soils.