Characterization of pesticide desorption from soil by the isotopic exchange technique

The reversibility of pesticide sorption-desorption in soil is of fundamental importance in the understanding of the fate of these agrochemicals in the environment. We used an isotopic exchange method to characterize the irreversibility of the sorption-desorption process of the insecticide imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) and its degradation product imidaclopridurea (1-[(6 chloro-3-pyridinyl)-methyl]-2-imidazolidinone) on a silty clay loam (SiCL) soil, and that of the metabolite imidacloprid-guanidine (1-[6-chloro-3-(pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine) on a loamy sand (LS) soil. The exchange between C-12-pesticide molecules and C-14-labeled pesticide molecules in soil suspensions preequilibrated for 24 h was monitored and indicated that a fraction of the sorbed chemicals was resistant to desorption. A two-compartment model was applied to describe the experimental sorption data points of the sorption isotherms as the sum of a reversible component and a nondesorbable, irreversible component. The quantitative estimation of the irreversible and reversible components of sorption, experimentally derived from isotopic exchange experiments, indicated degree of irreversibility (percentage irreversibly bound) in the order: imidacloprid-SiCL soil (6-32%) < imidacloprid urea-SiCL soil (15-23%) < imidacloprid guanidine-LS soil (32-51%), with greater irreversibility at lower pesticide concentration. Increasing the preequilibration time and decreasing pH in the imidacloprid-SiCL soil system resulted in increased sorption irreversibility. The irreversible component of sorption determined by the isotopic exchange technique also allowed accurate prediction of the sorption-desorption hysteretic behavior during successive desorption cycles for all three soil-pesticide systems studied. The isotopic exchange technique appears to be a suitable method to quantitatively characterize pesticide desorption from soil, allowing prediction of hysteresis during sorption-desorption experiments.