Biochemical and Chemical Transformations of Terbufos, Terbufos Sulfoxide, and Terbufos 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 terbufos, terbufos sulfoxide, or terbufos sulfone. These were incubated in the dark at 28°C; samples were removed after 1, 2, 4, 8, 12, 16, and 20 weeks and were examined for microbial populations, pH, and insecticide concentrations. These variables were determined also in correspondingly treated natural soils. Residual insecticide determinations were simplified by acid treatment of the solid support used to prepare gas-liquid chromatography (GLC) columns, which permitted direct analysis of at least 0.05 ng of terbufos sulfoxide and separation from 100-fold greater quantities of terbufos sulfone by GLC. In the natural soils, all three applied insecticides disappeared much more rapidly than from the sterilized soils, and terbufos sulfoxide and terbufos sulfone were observed as intermediate transformation products. In the sterilized soils, terbufos oxidation to terbufos sulfoxide was the major observable process, but the rate of oxidation was slower than in the natural soils and further oxidation to the sulfone was not observed. Terbufos sulfone also was stable in the sterilized soils. These results show that biochemical processes are of major importance in the transformation of terbufos and its oxidation products in soil. Partial-rate constants for the processes described showed that the greater persistence of the “total residue” from terbufos treatment in the organic soil was due to more rapid conversion of the terbufos to terbufos sulfoxide combined with a slower rate of degradation of terbufos sulfoxide and terbufos sulfone.