Role of Soil Organic Matter on the Sorption and Cosorption of Endosulfan and Chlorpyrifos on Agricultural Soils

Abstract

Sorption of endosulfan and chlorpyrifos was studied through batch experiments with three agricultural soils (organic carbon 0.8, 1.37, and 2%) and different initial concentrations of pesticides below the limits of their solubility. The effect of the presence of one pesticide on sorption of the other was also examined. The sorption of both pesticides followed the Lagergren kinetic model, and more than 95% of sorption was completed within 4–6 hours for endosulfan and 6–12 hours for chlorpyrifos. The equilibrium sorption of both pesticides was primarily on the soil organic matter, but the isotherms were nonlinear, especially at low-solute concentrations. The nonlinearity was the result of the sorption onto the hard carbon portion of soil organic carbon, which was also indicated by the simulations with a dual reactive domain model (DRDM). The sorption of either pesticide was suppressed in the presence of the other, and the isotherm nonlinearity decreased with increasing concentration of the cosolute. The effect of a cosolute was more prominent for the sorption of endosulfan compared with that of chlorpyrifos. The simulations with DRDM indicated a competitive sorption in the portion of organic carbon showing site-limited sorption. No competition was observed in the portion showing linear partition behavior. Among the two isomers of endosulfan, the