Retention of Imidacloprid in Different Louisiana Soils: Adsorption-Desorption Kinetics.

Recently developed imidacloprid is a widely used insecticide because of its relative safety and of mode of action. In this study, our goal was to quantify the reactivity of imidacloprid by soils having different physical and chemical properties; namely Sharkey clay, Mhoon silty clay loam, Mahan sandy clay loam, and Vacherie loam. Kinetic batch type experiments were carried out for a wide range of initial concentrations and reaction times, followed by imidacloprid desorption using successive dilution with 0.005M CaCl₂ solution. Although Freundlich and linear equations provided good predictions of imidacloprid adsorption isotherm at different times (p <0.01), the best descriptions (higher r²) were realized using the Freundlich model. Adsorption isotherms for Mahan and the Vacherie soils exhibited stronger kinetics behavior for imidacloprid adsorption compared to the Mhoon and Sharkey soils. Adsorbed isotherm for the Mhoon soil was highest amounts sorbed among all soils but it also showed least kinetic behavior. The behavior of kinetic retention was not coordinated with the binding capacity of the different soil. Desorption of imidacloprid illustrated the hysteresis behavior, and the extents of hysteresis were varied in among the soils. Recoveries of imidacloprid were 60.0, 36.2, 36.8, and 32.4 % for Mahan, Mhoon, Sharkey, and Vacherie soil, respectively. The sequence of recovery rates demonstrated that the controlling factor for imidacloprid adsorption-desorption reaction is soil organic matter rather than soil clay contents. A multireaction model (MRM) with nonlinear equilibrium and kinetic sorption was capable of predicting imidacloprid adsorption and desorption reactions, and the model exhibited robust predictions when irreversible reaction was incorporated.