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-1H-imidazol-2-amine (imidacloprid−guanidine), and 1-[(6-chloro-3-pyridinyl)methyl]-1H-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). Keywords: Imidacloprid; metabolites; sorption; desorption; hysteresis

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 (Kd) 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.

Soil clay mineral and organic components have been shown to be responsible for sorption of many pesticides. This study assessed the influence of the different components of the clay fraction of five soils and model sorbents (montmorillonite, Fe oxide, and humic acid) and their binary and ternary associations on sorption of the insecticide imidacloprid (1‐[(6‐chloro‐3‐pyridinyl)‐methyl]‐N‐nitro‐2‐imidazoli‐dinimine). Sorption was very high on a clay fraction containing a high content of low‐laminar‐charge montmorillonite and relatively high organic C content. When organic matter of the soil clays was reduced by H2O2, sorption decreased; however, the sorption coefficient based on organic C (Koc) increased. The increase in Koc was greater on clays whose surface areas substantially increased after H2O2 treatment. This indicates a contribution of the surfaces of mineral components of the soil clay to sorption of imidacloprid. Imidacloprid sorbed on both montmorillonite and humic acid, whereas sorption on Fe oxide was very low. The sorption distribution coefficient, Kd, on montmorillonite increased from 15 to 180 when pH was decreased from 3.5 to 1.0, suggesting protonation of the ‐NH group of the imidazol ring. Sorption on binary and ternary associations of these model sorbents indicated that organic matter is the single most important component affecting sorption of imidacloprid, although there is some contribution of the mineral surfaces.

stimulate soil microbial activity, which could potentially lead to accelerated degradation, reducing the total Organic amendment (OA) addition is an agricultural practice that amount of chemical available for leaching and to injure can greatly affect pesticide behavior in soil. The influence of three OAs, consisting of a liquid amendment (LOA), solid (SOA) humic crops planted in rotation (Felsot and Dzantor, 1995; amendment, and a solid urban waste (SUW), on sorption, persistence,

The effect of aging in the soil of three organic amendments (OAs), one liquid (LF) and two solid ones (SF and AL), has been investigated and related to changes in soil adsorption of metalaxyl and tricyclazole. LF and AL have very high dissolved organic carbon (DOC) contents with low humification index values, whereas SF has a low DOC content but the highest amounts of highly humified material. All OAs increased the adsorption of tricyclazole, whereas adsorption of metalaxyl decreased in soils amended with LF and AL, due to competition with DOC for mineral adsorption sites. With aging, DOC from SF amended soils is not significantly affected and neither is adsorption behavior. On the contrary, the great reduction of DOC from LF and AL with aging has been shown to affect adsorption of metalaxyl and tricyclazole, and this effect is dependent on the pesticide, the nature of the DOC, and the type of soil, in particular its clay mineralogy.