Fleroxacin (FLE) is a widely used fluoroquinolones to cure urinary tract infections and respiratory disease, which has been frequently detected in the aquatic environment. The reactivity kinetics of FLE by chlorine and chlorine dioxide (ClO) and transformation mechanism were investigated in this study. The results showed that FLE was degraded efficiently by chlorine and ClO, and both reactions followed second-order kinetics overall. The increase of disinfectant dosage and temperature would enhance the degradation of FLE. The highest removal of FLE by chlorine was achieved at a neutral condition (pH 7.4), whereas ClO reaction rates increased dramatically with the increasing pH in this study condition. The number of intermediates identified in FLE chlorination and ClO oxidation was seven and ten, respectively. The piperazine ring cleavage was the principal and initial reaction in both above reactions. Then, the removal of the piperazine group was predominantly in FLE removal by chlorine, while the decarboxylation mainly occurred in FLE removal by ClO. The intermediates increased first and then decreased with time, while three kinds of halogenated DBPs increased with time, indicating the above-identified intermediates were further transformed to the halogenated DBPs. Additionally, compared to chlorine reaction, the reaction of ClO with FLE reduced the formation of halogenated DBPs, but it also induced the formation of chlorite. The analysis of toxicity showed that compared with chlorination, the oxidation of ClO was more suitable for FLE removal.
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