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.