Comparison of Dissipation Kinetics and Residual Behaviour for Fenitrothion Insecticide and Thiobencarb Herbicide in Clay Soil

Abstract

Comparsion study of the residual behaviour and kinetics of insecticide fenitrothion and herbicide thiobencarb in clay soil were carried out under laboratory conditions. The residues of the tested pesticides were determined by GC-MS at different interval times (0, 2, 5, 13, 20, 27, 48, 55 and 84 day) after treatment. The recoveries were performed at level of 75 μg/g soil and the recovery percentages were found to be 85.69 and 90.89 for fenitrothion and thiobencarb in the tested soil, respectively. The residue of fenitrothion was 64.27 µg/soil, after 48 days of incubation and the dropped to 5.07 % of the initial residue remaining. Thiobencarb residues level recorded after treated soil with 75 μg a.i / g soil wes 68.17 μg/g soil. This concentration gradually decreased with the time and reached to 0.68 μg/g soil at 55 days after treatment. Both compounds fenitrothion and thiobencarb disappeared very rapidly in the soil following a bi-phasic pattern. Graphical and integral methods were used to select the fit kinetic model. The determination coefficient R2 was the highest and the SD of K values were the lowest in 1st order model compared to other kinetic models. Therefor, the dissipation of fenitrothion and thiobencarb in soil fits with the 1st-order kinetic model. The estimated value of fenitrothion half-life was 11.21 days and that of thiobencarb was 10.61 days. In general, the dissipation of thiobencarb was slightly faster in the tested soil than fenitrothion.