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Changes in Sorption of Imidacloprid with Incubation Time

Dep. of Soil, Water and Climate, Univ. of Minnesota, St. Paul, MN 55108

William C. Koskinen^*

USDA-ARS, Soil and Water Management Research Unit, 1991 Upper Buford Cir., Rm. 439, St. Paul, MN 55108

Pau Yong Yen

Bayer Corporation, 17745 South Metcalf, Stilwell, KS 660885

*Corresponding author (koskinen{at}soils.umn.edu).

ABSTRACT

Changes in sorption of the insecticide imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) with aging in three soils have been determined. Soil moisture was adjusted to –33 kPa and ¹⁴C- and analytical-grade imidacloprid added to the soil at a rate of 1.0 mg kg^–1. Imidacloprid-spiked soils were incubated at 25°C for 16 wk. Replicate soil samples were periodically extracted successively with 0.01 M CaCl₂, acetonitrile, and 1 M HCI. Sorption, as indicated by 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 extractable with acetonitrile and HCI (sorbed phase) with incubation time. It is not known whether the increase in sorption was the result of imidacloprid diffusion to less accessible or stronger binding sites with time, a rate of degradation in solution and on labile sites that is faster than the rate of desorption, or a combination of the two processes. These results are, however, further evidence that increases in sorption during pesticide aging should be taken into account during characterization of the sorption process for mathematical models of pesticide leaching.

Received for publication February 18, 1997.

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