Photochemical transformations of dichloroacetamide safeners

Safeners are widely used ingredients in commercial herbicide formulations, but their environmental fate has garnered relatively little scrutiny because of their classification as “inert” by the US EPA. Here, we investigated the photolysis of one popular class of safeners, dichloroacetamides, to better understand their persistence and formation potential for bioactive transformation products in surface waters. Of four commonly used dichloracetamide safeners only benoxacor underwent direct photolysis. Benoxacor had a half-life of 7.7 min when irradiated at pH 7, thus photolysis will likely be an important fate pathway in surface waters. Other dichloroacetamide safeners AD-67, dichlormid, and furilazole, while resistant to direct photolysis, were slowly degraded by indirect photolysis pathways in the presence of common photosensitizers including nitrate, nitrite, and humic acids. Half-lives of these compounds were greater than 8 hours. Reactive entities involved in these reactions are likely •OH and 1O2 as verified by selective quenchers, such as isopropanol (•OH) and sodium bromate (1O2). Where possible, we identified photoproducts using NMR and high-resolution mass spectrometry. Only benoxacor photolysis yielded detectable and identifiable transformation products. These products were generally more polar, and were entirely dechlorinated through photolysis, suggesting they are likely to have limited bioactivity relative to benoxacor.