Degradation performance and conversion mechanisms of MNZ by advanced oxidation systems with O3: Comparison of O3, O3/UV, O3/H2O2 and UV-H2O2/O3 systems

This study evaluated the degradation performance of ozone (O 3 ), ozone/ultraviolet (O 3 /UV), ozone/hydrogen peroxide (O 3 /H 2 O 2 ), and ultraviolet/hydrogen peroxide/ozone (UV-H 2 O 2 /O 3 ) systems of removing antibiotic metronidazole (MNZ) in aqueous environments. The UV-H 2 O 2 /O 3 system achieved complete degradation of 100 μg·L −1 MNZ within 2 min. Ozone utilization in the UV-H 2 O 2 /O 3 system was enhanced by 30.05 % compared with O 3 alone. Hydroxyl radicals (·OH) and superoxide radicals (·O 2 − ) played crucial roles in process, with ·OH exposure and yields being 4.7 and 28.5 times greater, respectively, than those in the O 3 system. The initial stage rate constant of O 3 conversion increased tenfold in the presence of ·O 2 − . LC–MS analysis and density functional theory (DFT) calculations identified fifteen major intermediates and proposed three primary transformation pathways: alcohol oxidation, hydroxyethyl dissociation, methyl elimination, nitro-group cleavage, and imidazole ring-opening reactions. Cytotoxicity assessments indicated the UV-H 2 O 2 /O 3 system exhibited low biotoxicity during the degradation process. Pilot-scale experiments verified the effectiveness of the UV-H 2 O 2 /O 3 process in removing 500 ng·L −1 MNZ in real water bodies, achieving a 93.8 % removal rate for MNZ, with corresponding removal rates of 66.53 % for UV 254 and 39.52 % for total organic carbon (TOC). This study provides significant insights for removing antibiotics in practical water treatment applications, underscoring their potential engineering implications.