CHANGES IN SORPTION OF IMIDACLOPRID WITH INCUBATION-TIME

Changes in sorption of the insecticide imidacloprid (1-[(6-chloro-3-py ridinyl)-methyl]-N-nitro-2 imidazolidinimine) with aging in three soil s have been determined. Soil moisture was adjusted to -33 kea and C-14 - and analytical-grade imidacloprid added to the soil at a rate of 1.0 mg kg(-1). Imidacloprid-spiked soils were incubated at 25 degrees C f or 16 wk, Replicate soil samples were periodically extracted successiv ely; with 0.01 M CaCl2, acetonitrile, and 1 M HCl, Sorption, as indica ted bg the sorption coefficient (K-d) values, was highest in the soil with highest organic C content, and increased by an average factor of 2.8 in the three soils during the incubation period. This increase was the result of a decrease in the imidacloprid extractable with CaCl2 ( solution phase) and the increase in the amount of imidacloprid extract able with acetonitrile and HCl (sorbed phase) with incubation time. it is not known whether the increase in sorption was the result of imida cloprid diffusion to less accessible or stronger binding sites with ti me, a rate of degradation in solution and on labile sites that is fast er than the rate of desorption, or a combination of the two processes, These results are, however, further evidence that increases in sorpti on during pesticide aging should be taken into account during characte rization of the sorption process for mathematical models of pesticide leaching.