MODELING ADSORPTION-DESORPTION KINETICS OF ALACHLOR IN A TYPIC FRAGIUDALF

Adsorption and desorption kinetics are essential components for modeling the fate of pesticides in soils. This study was conducted to characterize the kinetics of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide] adsorption and desorption in Gigger (fine-silty, mixed, thermic Typic Fragiudalf) surface soil samples collected from long term no-till and conventional tillage plots. A batch method was used to determine alachlor adsorption with time up to 528 h for a range of input concentrations from 0.5 to 50 mg/L. Successive dilution was used to quantify desorption kinetics following 16, 32, 64, 128, 256, and 512 h of adsorption. For both samples, alachlor time-dependent results indicated initially fast adsorption followed by slow adsorption where kinetic reactions appeared dominant. Adsorption-desorption showed extensive hysteretic behavior resulting from discrepancies between adsorption and desorption isotherms. Hysteresis was more pronounced with increased reaction time of adsorption. Four variations of a nonlinear multireaction kinetic model successfully described retention results. Adsorption and desorption, however, were best described by a model version incorporating nonlinear equilibrium, a kinetic reversible mechanism, and a consecutive irreversible mechanism. The model was successfully used in describing alachlor adsorption and desorption results. The model predicted the overall alachlor hysteresis satisfactorily except that, at large times, desorption isotherms were not considered adequate. The model also was capable of predicting alachlor desorption kinetics based solely on parameters obtained from adsorption experiments.