The importance of reactive oxygen species on the aqueous phototransformation of sulfonamide antibiotics: kinetics, pathways, and comparisons with direct photolysis

Sulfonamide antibiotics (SAs) are increasingly detected as aquatic contaminants and exist as different dissociated species depending on the pH of the water. Their removal in sunlit surface waters is governed by photochemical transformation. Here we report a detailed examination of the hydroxyl radical (center dot OH) and singlet oxygen (O-1(2)) mediated photooxidation of nine SAs: sulfamethoxazole, sulfisoxazole, sulfamethizole, sulfathiazole, sulfamethazine, sulfamerazine, sulfadiazine, sulfachloropyridazine and sulfadimethoxine. Both center dot OH and O-1(2) oxidation kinetics varied depending on the dominant protonated states of the SA in question (H2SAs+, HSAs0 and SAs-) as a function of pH. Based on competition kinetic experiments and matrix deconvolution calculations, HSAs0 or SAs- (pH similar to 5-8) were observed to be more highly reactive towards center dot OH, while SAs- (pH similar to 8) react the fastest with O-1(2) for most of the SAs tested. Using the empirically derived rates of reaction for the speciated forms at different pHs, the environmental half-lives were determined using typical O-1(2) and center dot OH concentrations observed in the environment. This approach suggests that photochemical O-1(2) oxidation contributes more than center dot OH oxidation and direct photolysis to the overall phototransformation of SAs in sunlit waters. Based on the identification of key photointermediates using tandem mass spectrometry, O-1(2) oxidation generally occurred at the amino moiety on the molecule, whereas center dot OH reaction experienced multi-site hydroxylation. Both these reactions preserve the basic parent structure of the compounds and raise concerns that the routes of phototransformation give rise to intermediates with similar antimicrobial potency as the parent SAs. We therefore recommend that these phototransformation pathways are included in risk assessments concerning the presence and fate of SAs in waste and surface waters. (C) 2018 The Authors. Published by Elsevier Ltd.