Contribution of Weak Bases to Isoxaflutole Hydrolysis: Implications for Hydrolysis in Laboratory and Natural Waters

The hydrolysis rates of many organic chemicals are accelerated under alkaline conditions by the presence of hydroxide (HO^-), which is typically assumed to be the predominant species contributing to base-catalyzed hydrolysis in both natural waters and laboratory buffers used in standard protocols. In this study, we demonstrated that weak bases (e.g., carbonates and phosphates) are able to catalyze the hydrolysis of the proherbicide isoxaflutole, an emerging contaminant of concern in water resources, under conditions relevant to both natural waters and laboratory protocols. Second-order rate constants for isoxaflutole hydrolysis catalyzed by individual bases were correlated with the p<i>K</i>_a of the conjugate acid of the base using the Brønsted relationship. Using these values, we determined that buffer effects accounted for 43-99% of previously reported rate constants for base-catalyzed isoxaflutole hydrolysis misattributed to HO^-, resulting in hydrolysis rates predicted in natural waters overestimated by 5- to 30-fold from pH 7.0 to 8.5. In comparison, our corrected rate constants accurately predicted isoxaflutole hydrolysis measured in natural waters within a factor of 5. While isoxaflutole hydrolysis in surface water remains predominantly catalyzed by HO^- following standard assumptions, carbonate species (particularly bicarbonate) are predicted to drive the reaction in as many as 83% of groundwater systems.