Dichloroacetamide safeners are co-applied with chloroacetamide herbicides to selectively protect crops from herbicide toxicity. Despite annual global application exceeding 8x10<sup>6</sup> kg, minimal data exist describing dichloroacetamide occurrence, fate, and transformation in the environment. Some studies suggest that dichloroacetamide transformation products may be more bioactive than parent compounds and could pose human and environmental health risks. To evaluate safeners’ transformation in aquatic systems, we measured the rate and extent of dichloroacetamide hydrolysis, identified products, and elucidated pathways for multiple dichloroacetamide species. For benoxacor, the most widely used of the dichloroacetamides, we found that hydrolysis proceeds rapidly at elevated pH but that co-solute species can strongly influence the rate of hydrolysis. Faster rates were observed in borate buffer at pH 10.65, corresponding to the ambient pH in the University of Iowa Drinking Water Plant chemical softening basin. However, in softening basin samples spiked with benoxacor, half-lives were seven-fold longer than in the borate buffer, suggesting that an as yet unidentified matrix effect limits hydrolytic transformation. High-resolution mass spectrometry indicates that benoxacor hydrolysis proceeds via base-catalyzed acyl cleavage, where hydroxide ion attacks the amide group to yield a benzoxazine derivative and dichloroacetate, which is a regulated disinfection by-product. Our findings suggest that hydrolysis can contribute to the environmental fate of benoxacor in certain environments such as alkaline surface waters and in treatment facilities that use chemical (i.e., lime-soda) softening.

Hydrolysis of dichloroacetamide herbicide safeners: Rates and transformation products

2019 Fall National Meeting

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