An Isotopic Exchange Method for the Characterization of the Irreversibility of Pesticide Sorption−Desorption in Soil

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Abstract
An isotopic exchange method is presented that characterizes the irreversibility of pesticide sorption−desorption by soil observed in batch equilibration experiments. The isotopic exchange of 12C- and 14C-labeled triadimefon [(1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl)-2-butanone] and imidacloprid-guanidine [1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine] in Hanford sandy loam soil indicated that these systems can be described by a two-compartment model in which about 90% of sorption occurs on reversible, easily desorbable sites, whereas 10% 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 equilibration experiments. The isotopic exchange of triadimefon and imidacloprid-guanidine in Drummer silty clay loam soil indicated that there was a fraction of the sorbed 14C-labeled pesticide that was resistant to desorption, which 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. Keywords: Desorption; hysteresis; isotopic exchange; pesticides; sorption