Degradation of sulfamethoxazole in microbubble ozonation process: Performance, reaction mechanism and toxicity assessment

[Display omitted] •Better performance of SMX removal and mineralization in microbubble ozonation.•Most SMX removal in microbubble ozonation attributed to OH and 1O2.•Predicted preferential ozonation reaction sites of SMX by DFT calculation.•Proposed SMX degradation pathways by intermediates detection and DFT calculation.•Test, prediction and assessment of toxicity variation during SMX degradation. Sulfamethoxazole (SMX) as a widely used antibiotic has caused ecological problems in aquatic environment due to high toxicity and poor biodegradability. In this study, the microbubble ozonation (MB/O3) process was applied for SMX removal, compared with common bubble ozonation (CB/O3) process under the same conditions. The results showed that the SMX degradation and mineralization were much more efficient in MB/O3 process than those in CB/O3 process. The enhancement generation of reactive oxygen species (ROS) in MB/O3 process was proven by electron paramagnetic resonance (EPR) detection, including hydroxyl free radicals (OH), superoxide free radicals (O2−), and singlet oxygen (1O2). In addition, it was found that OH and 1O2 contributed 84.82% and 67.74% of SMX removal in MB/O3 and CB/O3 processes, respectively. The preferential reaction sites of SMX were predicted by density functional theory (DFT) calculation in ozonation process. The sulfonamide group, 17(N), 13(C), 15(C), and 9(C) regions were determined as the more active electrophilic reaction sites. More various and complex degradation pathways of SMX in MB/O3 process were proposed according to intermediates detection and DFT calculation, because both active and inactive sites were found to be attacked due to enhanced ROS oxidation. The more difficult destruction of isoxazole ring than benzene ring in SMX molecule was also observed, especially in CB/O3 process. The increased toxicity was observed during SMX degradation in both ozonation processes. The toxicity prediction and assessment of SMX and its degradation intermediates showed that many intermediates were more toxic than SMX, due to hydroxyl group addition on benzene ring or isoxazole ring by hydroxylation reaction probably.