Deconvoluting Hydrolysis and Photolysis of the Herbicide Isoxaflutole and Its Products at Circumneutral pH

The proherbicide isoxaflutole and its hydrolysis product diketonitrile are emerging contaminants of concern in surface water, where they both pose risks to human health and nontarget vegetation. Limited information is available regarding their transformation via abiotic hydrolysis and photolysis at circumneutral pH, which are both key processes impacting the fate of these contaminants. Herein, we report that isoxaflutole hydrolyzes to diketonitrile with a half-life of 19 d at pH 7, which is 23-fold slower than prior modeling suggested. After correcting for hydrolysis, isoxaflutole was found to photolyze under simulated sunlight with a quantum yield of (1.56 ± 0.12) × 10-3, corresponding to a predicted near-surface half-life of 12 d under clear sky irradiance at 30 °N in summer, which was 2-fold slower than previously suggested. Diketonitrile, which does not hydrolyze, underwent slow photolysis despite significant absorbance within the solar spectrum, resulting in a quantum yield of (9.65 ± 1.54) × 10-6 and a modeled half-life of 150 d under the same conditions. Isoxaflutole photolysis primarily generated products other than diketonitrile, some of which were also susceptible to hydrolysis. Our revised environmental half-lives indicate that isoxaflutole photolysis is potentially more important relative to hydrolysis than considered in prior modeling, although both processes are ultimately slower than previously reported.