Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/1999
Publication Date: N/A
Interpretive Summary: Sorption-desorption is one of the most important processes influencing the fate of agrochemicals in soil. Laboratory equilibrium sorption-desorption data are used to predict pesticide efficacy and potential of groundwater and surface water contamination. While pesticide sorption by soil has been extensively documented, desorption remains much less understood. The objective of the present study was to characterize the irreversibility of sorption-desorption of a widely used pesticide, imidacloprid, and two of its metabolites (breakdown products) in soil using a method newly developed in our lab, an isotopic exchange technique. Using the technique, we were able to directly characterize the dynamics of the pesticide sorption- desorption equilibrium in soil, including the amount of pesticide irreversibly bound to soil. We found greater irreversibility of binding of the metabolites than imidacloprid. Depending on the soil, 15-51% of applied dchemical was irreversibly bound for the metabolites compared to 6-32% for imidacloprid. This is the amount of pesticide which is not available for offsite movement. Scientists how have an easy technique to obtain data that can be used in pesticide transport models to better predict potential offsite movement of pesticides to surface water and groundwater.
Technical Abstract: The reversibility of pesticide sorption-desorption in soil is of fundamental importance in the understanding of their fate in the environment. We used an isotopic exchange method to characterize the irreversibility of the sorption-desorption process of the insecticide imidacloprid and its degradation product imidacloprid-urea on a silty clay loam (SiCL) soil, and that of the metabolite imidacloprid-guanidine on a loamy sand (LS) soil. The exchange between 12C-pesticide molecules and 14C-labeled pesticide molecules in 24 h-preequilibrated soil suspensions 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 non-desorbable, irreversible component. The quantitative estimation of the irreversible and reversible components of sorption indicated increased irreversibility (% irreversibly bound) in the orderimidacloprid-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 also 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 isotherms.