Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/24/1997
Publication Date: N/A
Citation: Interpretive Summary: Sorption-desorption processes are important in determining the fate and distribution of agrochemicals in the soil/water environment, since they determine the amount of pesticide that can be volatilized, degraded and leached. Imidacloprid is a new systemic insecticide. There is no information available on sorption-desorption of imidacloprid or any of its metabolites. The objective of this research was to characterize the sorption-desorption of imidacloprid and three metabolites in soils. Sorption of imidacloprid and its metabolites is affected by soil properties, increasing with soil organic carbon content. The data show the importance of the range of concentration in experimentally determining sorption coefficients. Sorption of imidacloprid determined at high concentrations greatly overpredicts potential leaching compared to sorption determined at field application rates. Two metabolites were sorbed to a greater extent than imidacloprid indicating a lower leaching potential in soil, which can be attributed to irreversible binding of the molecules to soil surfaces. At normal field application rates, it appears the leaching potential of imidacloprid and its metabolites in soil is minimal. This information is important for farmers who want to use pesticides with minimal potential environmental impact and for state regulatory agencies involved in local registration decisions.
Technical Abstract: Sorption-desorption of imidacloprid and three metabolites 1-[(6-chloro-3- pyridinyl)methyl]-2-imidazolidinone (imidacloprid-urea), 1-[(6-chloro- 3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine (guanidine) and 1- [(6-chloro-3-pyridinyl)methyl]-1H-imidazol-2-amine (guanidine-olefin) in three different soils was determined using the batch equilibration technique and data fit to the Freundlich equation. Initial concentration range for the 4 chemicals was 0.05-1.5 ug/mL, which corresponds to a field application rate of 0.2-1.0 kg/ha **-1. Sorption isotherms showed marked curvature with calculated slopes of the Freundlich isotherm significantly less than 1. The order of sorption (Kf) was guanidine greater than guanidine-olefin greater than imidacloprid greater than imidacloprid-urea in the 3 soils. For all chemicals, sorption Kf values were correlated to organic carbon content. Average Kf-oc values were 203, 412, 2740, and 3200 0imidacloprid-urea, imidacloprid, guanidine-olefin, and guanidine, respectively. Desorption was hysteretic for all chemicals in all soils. Greatest hysteresis was observed with guanidine and guanidine-olefin metabolites, which were sorbed to the greatest extent. Sorption-desorption of imidacloprid was also determined at concentrations up to half its solubility (250 ug/mL). In the case of imidacloprid, sorption determined at half the solubility (Koc equals 77) greatly overpredicts potential leaching compared to Koc determined at field application rates (Kf-oc equals 411).