Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/1997
Publication Date: N/A
Interpretive Summary: Pesticide sorption-desorption processes (i.e. binding to and release of chemicals from soil) are important in determining the fate and distribution of agrochemicals in the soil/water environment, since they determine the amount of pesticide that can reach the target organism and the amounts that tcan be volatilized, degraded and leached. The objective of the present research was to characterize the sorption-desorption of the insecticide imidacloprid over a wide range of initial solution concentrations on seven soils with different physical and chemical properties. Although the normal field application rate is less that 1 lb/Ac, imidacloprid is also used for termite control at rates 100 times greater. It was found that soil organic carbon was the single most important soil property affecting sorption of imidacloprid in the soils studied. It was also found that at the low initial concentration, which is the normal agronomic application rate, sorption was medium to high. In contrast, at the high initial concentration, which would be in the range of rates used for termite control, sorption was low to medium. The change in sorption at the higher initial solution concentration would significantly change the potential mobility in soil. Users of imidacloprid, such as farmers and commercial termite control applicators, can use this information to reduce the risk of potential for contamination of surface and ground water. For instance, by using low rates of application in soils with typical organic matter and clay contents, farmers and other applicators would decrease the potential risk for imidacloprid leaching through soil.
Technical Abstract: Imidacloprid [1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2- imidazolidinimine] is a new systemic insecticide used as soil treatment, seed dressing, and foliar treatment. Sorption- desorption studies were conducted using the batch equilibration method with seven U.S. soils varying in their physicochemical properties (OC = 0.3 - 4 percent, clay = 3 - 43 percent, pH = 5.7 - 7.8, CEC = 3.6 - 41). Initial imidacloprid concentrations ranged from 3 to 300 mg L**-1. Sorption coefficients Kf and the soil properties organic carbon and CEC gave the higher single correlation coefficients; the latter due to the positive correlation between OC and CEC. Sorption was irreversible, and desorption hysteresis coefficients in general, were higher at low soil solution concentrations. Kf values were lower at 1:5 soil:solution ratio than at 1:1, suggesting a possible effect of a soluble soil component, such as salt concentration, on sorption; no significant differences in pH between solutions were detected.