RPA 201772 is a preemergence herbicide that undergoes rapid conversion to a diketonitrile metabolite (DKN) in soil. The half-life of RPA 201772 is very short, but the half-life of DKN is much longer; hence, DKN remains for an extended time in soil. Sorption studies were conducted with five soils varying in physical and chemical properties using the batch equilibration technique. Analysis of 14C-ring–labeled DKN was performed using liquid scintillation counting, and sorption data were fitted to the Freundlich model. Isotherms of DKN were nonlinear in all the soils as depicted by the Freundlich exponent (n < 1.0), indicating differential distribution of site energies for sorption. Multiple regression of the sorption constants against selected soil properties indicated that soil organic matter content was the best single determining factor of DKN sorption (r2 = 0.961) followed by soil pH (r2 = 0.947). The Freundlich sorption coefficient (KF) decreased in the following order Chelsea, MI > Amherst, MA > Moorhead, MN > East Monroe, CO > South Deerfield, MA. The organic matter content of the soils decreased in the same order. Clay content had a minimal effect on the sorption of DKN, whereas the sorption of DKN increased with an increase in organic matter content and a decrease in soil pH. There was an increase in the sorption of DKN with an increase in Ca2 concentration of the soil solution, whereas the net sorption constant (Kd) was correlated to the organic matter content of the soils. The site energy distribution of DKN sorption was governed mainly by the organic matter content of the soils.
Nomenclature:RPA 201772, 4-(2-methanesulphonyl-4-trifluoromethylbenzoyl)-5-cyclopropyl isoxazole; diketonitrile metabolite (DKN), 2-cyano-3-cyclopropyl-1-(2-methylsulfonyl-4-trifluoromethylphenyl)propan-1,3-dione.