Hydrolysis and Soil Adsorption of the Labile Herbicide Isoxaflutole

Abstract

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4-trifluoromethylphenyl ketone) is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the soil behavior and environmental fate of the compound. The herbicide exhibits an unusual behavior in which it is functionally reactivated by rainfall events, providing control of small weeds that have emerged. Isoxaflutole is extremely labile in aqueous solution, thus measuring equilibrium sorption is challenging. A qualitative kinetic evaluation was performed to characterize the sorption of isoxaflutole, during rapid hydrolysis to its bioactive product, a diketonitrile derivative (2-cyclopropyl-3-(2-mesyl-4-trifluoromethylphenyl)-3-oxopropanenitrile). The transformation was measured over time in a herbicide-treated aqueous solution with or without soil. At 25 °C, 83% of the parent compound remained in solution at 24 h in the soil free system, but only 15% remained in the solution in the presence of soil. The sorbed phase consisted mainly of isoxaflutole, although a small percentage of diketonitrile was also detected in increasing concentrations as the study progressed. Hydrolysis prevented the attainment of sorption equilibrium, thus the apparent Kd of isoxaflutole increased over time, while that of diketonitrile remained close to zero at both 5 and 25 °C. Batch sorption isotherms were conducted with both isoxaflutole and diketonitrile using four Illinois soils of the Drummer, Flanagan, Catlin, and Cisne series ranging in organic carbon (OC) from 1.0 to 2.5%. Freundlich Kd values were 6−12-fold greater for isoxaflutole than diketonitrile, with the greatest difference in the lower organic carbon soils. After removing the hydrolysis effect, sorption of the isoxaflutole and diketonitrile was independent of temperature, suggesting that it was an entropy-driven process. Based on soil OC content, Koc values of 134 and 17 mL g-1 were calculated for isoxaflutole and diketonitrile, respectively. Results suggest that desorption coupled to hydrolysis promotes reactivation of the herbicide's function after rainfall and contributes to the efficacy of the compound by resupplying the soil solution with a bioactive product.