Adsorption-Desorption Kinetics of Imidacloprid in Soils: Influence of Phosphate

In this study, the influence of P in the form of phosphate on the adsorption and desorption of the insecticide imidacloprid by different soils was investigated. Kinetic batch experiments were carried out over time and followed by desorptions using successive dilutions. Both linear and Freundlich equations provided good description of imidacloprid adsorption isotherms and for different levels of initial P concentrations. Moreover, the distribution coefficient K(d) values decreased with addition of P in solution, illustrating that P decreased the amount of imidacloprid adsorption for all soils. Distinct discrepancies between adsorption and desorption isotherms indicated extensive hysteresis for imidacloprid release for all soils. Addition of P increased imidacloprid desorption indicative of decreased affinity for imidacloprid. Soil organic matter was a major component not only in increased imidacloprid adsorption but also in minimizing the extent of desorption regardless of the P concentration in the soil solution. A multireaction model, which accounts for nonlinear equilibrium and kinetic retention reactions, was successfully used to describe time-dependent imidacloprid adsorption and for different P concentrations. The addition of different amounts of P in the soil solution did not alter adsorption-desorption trends versus time. Based on model simulations, the use of a fully kinetic model for describing imidacloprid retention results is recommended.