Sorption−Desorption of Imidacloprid and Its Metabolites in Soils

Sorption−desorption of imidacloprid [1-[(6-chloro-3-pyridinyl)methyl]- N -nitro-2-imidazolidinimine] and metabolites 1-[(6-chloro-3-pyridinyl)methyl]-2-imidazolidinone (imidacloprid−urea), 1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1 H -imidazol-2-amine (imidacloprid−guanidine), and 1-[(6-chloro-3-pyridinyl)methyl]-1 H -imidazol-2-amine (imidacloprid−guanidine−olefin) in three soils was determined using the batch equilibration technique with initial concentrations for the four chemicals ranging from 0.05 to 1.5 μg mL - 1, which corresponds to a field application rate of 0.2−1.0 kg ha -1 . Calculated slopes of the Freundlich sorption isotherms were significantly less than 1. The order of sorption ( K f ) was imidacloprid−guanidine > imidacloprid−guanidine−olefin > imidacloprid > imidacloprid−urea in the three soils. Average K f-oc values were 203, 412, 2740, and 3200 for imidacloprid−urea, imidacloprid, imidacloprid−guanidine−olefin, and imidacloprid−guanidine, respectively. Desorption was hysteretic for all chemicals in all soils. Greatest hysteresis was observed with imidacloprid−guanidine and imidacloprid−guanidine−olefin. Sorption−desorption of imidacloprid determined at half the solubility (250 μg mL - 1 ) ( K oc = 77) greatly overpredicts potential leaching compared to K oc determined at field application rates ( K f-oc = 411).