The herbicide isoxaflutole, which is increasingly used in recent years, has the potential to contaminate drinking waters directly, as well as after hydrolyzing to its active form diketonitrile. Diketonitrile present in surface water is a threat to drinking water quality itself as a toxic contaminant and as a precursor for dichloroacetonitrile (DCAN), which is an unregulated but highly toxic nitrogenous disinfection-by-product (DBP). In this study, we investigated the reaction kinetics and product formation of diketonitrile with water treatment oxidants (i.e., free chlorine, chloramine, permanganate) to understand diketonitrile’s impact on water quality. We found that free chlorine rapidly degraded diketonitrile. In contrast, monochloramine reacted with diketonitrile least 10<sup>5</sup>-fold slower compared to chlorine, while permanganate was unreactive. We measured the formation of DCAN at circumneutral pH in the presence of excess free chlorine and chloramine. DCAN was formed at ~100% molar yield from reacted diketonitrile in the presence of both oxidants. Our results suggest that diketonitrile will be degraded and DCAN will be formed at high yields from diketonitrile at water treatment plants that use either free chlorine or chloramine during disinfection. We also measured DCAN and additional product formation (i.e., cyclopropane carboxylic acid, CPCA; benzoic acid derivative, BD) under limited chlorine conditions (e.g., chlorine:diketonitrile molar ratio &lt; 2), to determine the possible reaction pathways of diketonitrile degradation. We found a relative molar yield of CPCA (80±8%) &gt; BD (60.6±0.2%) &gt; DCAN (38±1%) upon reaction completion. Consistent with this product formation, we propose a new reaction pathway for diketonitrile degradation in which diketonitrile degradation occurs via base-catalyzed halogenation followed by nucleophilic attack at diketonitrile’s ketones. Our results indicate increasing isoxaflutole use will result in greater occurrences of a significant DCAN precursor during disinfection, thereby increasing the toxicity of drinking water.

Product formation from isoxaflutole and diketonitrile in water treatment

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