The herbicide isoxaflutole is a drinking water contaminant due to its mobility in water and toxicity. However, isoxaflutole undergoes base-catalyzed hydrolysis to form diketonitrile, which can also act as a drinking water contaminant or as a precursor for the toxic disinfection-by-product, dichloroacetonitrile (DCAN). Because only diketonitrile rapidly reacts with drinking water disinfectants (e.g., free chlorine), the impact of isoxaflutole in drinking water is dependent on the hydrolysis reaction. However, general base-catalyzed reactions (i.e., reactions where all base species are catalysts) in the environment often are assumed to be pH dependent (i.e., driven by hydroxide) due to both the low environmental concentrations of common base species like carbonate and their respective smaller bimolecular rate constants. In this study we demonstrated the rate of isoxaflutole hydrolysis is dominated by carbonate rather than hydroxide. First, we investigated the general-base catalysis mechanism of isoxaflutole hydrolysis, determining the carbonate and hydroxide bimolecular rate constants. We evaluated their respective contributions to the overall rate in environmentally relevant conditions (e.g., pH: 8-9, total carbonate: 1mM), identifying carbonate as the driver of the hydrolysis reaction relative to hydroxide. Next, we investigated the hydrolysis of isoxaflutole in natural water from 3 sampling sites with varying pH and alkalinity. The observed first order rate constants across the natural water samples increased with increasing alkalinity but did not correlate well with pH. This was consistent with carbonate rather than hydroxide driving the reaction rate. Finally, using the alkalinity correlation to determine a bimolecular rate constant (k = 0.0258 ± 0.0004 (mg/L as CaCO3)-1d-1) and alkalinity data from the United States Geological Survey, we modelled the rate of isoxaflutole hydrolysis across surface waters in states where isoxaflutole is approved for use. This model predicts that in 88% of the 555 stream sites isoxaflutole half-lives would be less than or equal to 2 days. As an emerging contaminant in agricultural systems, isoxaflutole will predominantly be converted to diketonitrile. Therefore, isoxaflutole in agricultural runoff will impact drinking water quality in the form of diketonitrile as a high-yield DCAN precursor.

Isoxaflutole hydrolysis is driven by carbonate in natural water

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