Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/1999
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 develop a method, an isotopic exchange technique, to characterize the irreversibility of pesticide sorption-desorption by soil. The isotopic exchange technique described in the paper is an easy and useful tool to characterize the irreversibility of the sorption-desorption of pesticides by soil. Using two pesticides as test compounds, we showed that the technique allows a direct characterization of the dynamics of the pesticide sorption-desorption equilibrium in soil, including the amount of pesticide irreversibly bound to soil, 10 percent in none soil and 30 percent in another soil. This is the amount of pesticide which is not available for offsite movement. This technique also eliminates inherent experimental artifacts of currently used methods. Scientists now 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: An isotopic exchange method is presented that characterizes the irreversibility of pesticide sorption-desorption by soil observed in batch equilibration experiments. The exchange between 12C- and 14C-labeled pesticide molecules was monitored in 24 h-preequilibrated soil suspensions, which allowed characterization of pesticide exchange kinetics at equilibrium and the estimation of amounts of sorbed pesticide that did not participate in the sorption equilibrium. Experiments were performed using two soils with different clay and organic C contents and triadimefon and imidacloprid-guanidine as test compounds. The isotopic exchange of riadimefon and imidacloprid-guanidine in a sandy loam soil indicated that these systems can be described by a two compartment model in which about 90 percent of sorption occurs on reversible, easily-desorbable sites, whereas 10 percent of the sorbed molecules are irreversibly sorbed on soil and do not participate in the sorption-desorption equilibrium. This model closely predicted the hysteresis observed in the desorption isotherms from batch experiments. The isotopic exchange of triadimefon and imidacloprid-guanidine in silty clay loam soil indicated that, even though all sorbed pesticide seemed to participate in the sorption equilibrium, there is a fraction that is resistant to desorption. This fraction increased as pesticide concentration decreased and was higher for triadimefon than for imidacloprid-guanidine. In contrast, the batch equilibration method resulted in ill-defined desorption isotherms for the Drummer soil, which made accurate desorption characterization problematic.