Biochemical and Chemical Transformations of Phorate, Phorate Sulfoxide, and Phorate Sulfone in Natural and Sterile Mineral and Organic Soil

Abstract

A sandy loam and a muck soil were sterilized by autoclaving, and samples were treated at 10 ppm with phorate, phorate sulfoxide, or phorate sulfone. These were incubated in the dark at 28°C, and samples were removed after 1, 2, 4, 8, 12, and 16 weeks and examined for microbial populations, pH, and insecticide concentrations. These variables also were determined in correspondingly treated natural soils. The three applied insecticides disappeared much more rapidly from the natural soils than from the sterilized soils. Phorate sulfoxide and phorate sulfone were observed as intermediate transformation products. In the sterile soils, phorate oxidation to phorate sulfoxide was the major process observed, and further oxidation of the sulfoxide did not occur. The expected increases in rates and changes in the products formed were observed as the sterile soils became nonsterile in time. Reduction of phorate sulfoxide to phorate was detected as a minor process in these contaminated soils. The results show that biochemical processes are of major importance in the transformation of phorate and its oxidation products in soil. The usefulness of appropriate partial rate constants in describing the processes involved is discussed.