New insights into transformation mechanisms for sulfate and chlorine radical-mediated degradation of sulfonamide and fluoroquinolone antibiotics

Zheng, Jinshuai and Niu, Junfeng and Halsall, Crispin and Guo, Yadi and Zhang, Peng and Ge, Linke (2024) New insights into transformation mechanisms for sulfate and chlorine radical-mediated degradation of sulfonamide and fluoroquinolone antibiotics. Chinese Chemical Letters: 110202. ISSN 1001-8417

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Abstract

As antibiotic pollutants cannot be incompletely removed by conventional wastewater treatment plants, ultraviolet (UV) based advanced oxidation processes (AOPs) such as UV/persulfate (UV/PS) and UV/chlorine are increasingly concerned for the effective removal of antibiotics from wastewaters. However, the specific mechanisms involving degradation kinetics and transformation mechanisms are not well elucidated. Here we report a detailed examination of SO4 •−/Cl•-mediated degradation kinetics, products, and toxicities of sulfathiazole (ST), sarafloxacin (SAR), and lomefloxacin (LOM) in the two processes. Both SO4 •−/Cl•-mediated transformation kinetics were found to be dependent on pH (P < 0.05), which was attributed to the disparate reactivities of their individual dissociated forms. Based on competition kinetic experiments and matrix calculations, the cationic forms (H2ST+, H2SAR+, and H2LOM+) were more highly reactive towards SO4 •− in most cases, while the neutral forms (e.g., HSAR0 and HLOM0) reacted the fastest with Cl• for the most of the antibiotics tested. Based on the identification of 31 key intermediates using tandem mass spectrometry, these reactions generated different products, of which the majority still retained the core chemical structure of the parent compounds. The corresponding diverse transformation pathways were proposed, involving S−N breaking, hydroxylation, defluorination, and chlorination reactions. Furthermore, the toxicity changes of their reaction solutions as well as the toxicity of each intermediate were evaluated by the vibrio fischeri and ECOSAR model, respectively. Many primary by-products were proven to be more toxic than the parent chemicals, raising the wider issue of extended potency for these compounds with regards to their ecotoxicity. These results have implications for assessing the degradative fate and risk of these chemicals during the AOPs.