Hydrolysis and soil adsorption of the labile herbicide isoxaflutole

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4-trifluoromethylphenyl k etone) is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the soil behavior and en vironmental fate of the compound. The herbicide exhibits an unusual behavio r in which it is functionally reactivated by rainfall events, providing con trol of small weeds that have emerged. Isoxaflutole is extremely labile in aqueous solution,thus measuring equilibrium sorption is challenging. A qual itative kinetic evaluation was performed to characterize the sorption of is oxaflutole, during rapid hydrolysis to its bioactive product, a diketonitri le derivative (2-cyclopropyl-3-(2- mesyl-4 trifluoromethylphenyl)-3-oxoprop anenitrile). The transformation was measured over time in a herbicide-treat ed aqueous solution with or without soil. At 25 degrees C, 83% of the paren t compound remained in solution at 24 h in the soil free system, but only 1 5% remained in the solution in the presence of soil. The sorbed phase consi sted mainly of isoxaflutole, although a small percentage of diketonitrile w as also detected in increasing concentrations as the study progressed. Hydr olysis prevented the attainment of sorption equilibrium, thus the apparent K-d Of isoxaflutole increased over time, while that of diketonitrile remain ed close to zero at both 5 and 25 degrees C. Batch sorption isotherms were conducted with both isoxaflutole and diketonitrile using four Illinois soil s of the Drummer, Flanagan, Catlin, and Cisne series ranging in organic car bon (OC) from 1.0 to 2.5%. Freundlich K-d 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 th e isoxaflutole and diketonitrile was independent of temperature, suggesting that it was an entropy-driven process. Based on soil OC content, K-oc valu es of 134 and 17 mL g(-1) were calculated for isoxaflutole and diketonitril e, respectively. Results suggest that desorption coupled to hydrolysis prom otes reactivation of the herbicide's function after rainfall and contribute s to the efficacy of the compound by resupplying the soil solution with a b ioactive product.