Characterization of pesticide desorption from soil by the isotopic exchange technique

The reversibility of pesticide sorption-desorption in soil is of fundamenta l importance in the understanding of the fate of these agrochemicals in the environment. We used an isotopic exchange method to characterize the irrev ersibility of the sorption-desorption process of the insecticide imidaclopr id (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) and its degradation product imidaclopridurea (1-[(6 chloro-3-pyridinyl)-methyl]-2-i midazolidinone) on a silty clay loam (SiCL) soil, and that of the metabolit e imidacloprid-guanidine (1-[6-chloro-3-(pyridinyl)methyl]-4,5-dihydro-1H-i midazol-2-amine) on a loamy sand (LS) soil. The exchange between C-12-pesti cide molecules and C-14-labeled pesticide molecules in soil suspensions pre equilibrated for 24 h was monitored and indicated that a fraction of the so rbed chemicals was resistant to desorption. A two-compartment model was app lied to describe the experimental sorption data points of the sorption isot herms as the sum of a reversible component and a nondesorbable, irreversibl e component. The quantitative estimation of the irreversible and reversible components of sorption, experimentally derived from isotopic exchange expe riments, indicated degree of irreversibility (percentage irreversibly bound ) in the order: imidacloprid-SiCL soil (6-32%) < imidacloprid urea-SiCL soi l (15-23%) < imidacloprid guanidine-LS soil (32-51%), with greater irrevers ibility at lower pesticide concentration. Increasing the preequilibration t ime and decreasing pH in the imidacloprid-SiCL soil system resulted in incr eased sorption irreversibility. The irreversible component of sorption dete rmined 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 p esticide desorption from soil, allowing prediction of hysteresis during sor ption-desorption experiments.