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BACKGROUND In this study, sulfur‐doped g‐C3N4 (SCN) nanosheets were synthesized using thiourea as a precursor, and then used to catalyze the degradation of oxytetracycline (OTC) under visible‐light irradiation. OTC was chosen as target contaminant because of it being an important precursor of N‐nitrosodimethylamine (NDMA). RESULTS The removal of OTC was influenced by its initial concentration, initial pH, SCN dosage and water matrices. OTC (≤10 mg L−1) was almost completely removed in 40 min under the conditions of an initial pH of 7.0 and a SCN dose of 1.0 g L−1, and its degradation was well described by the pseudo‐first‐order kinetics model. Water matrices would also influence the degradation of OTC in the system. Electron paramagnetic resonance analysis and trapping experiments confirmed that the function of the radicals for OTC removal followed the order of •O2− > •OH > h+. Eight transformation products were determined during the degradation of OTC, it being removed through four pathways: methyl oxidization, demethylation, decarbonylation and secondary alcohol oxidation. SCN was stable in the system, as the removal efficiency of OTC was not significantly reduced even after three recycles. In addition, 55.5% of NDMA formation potential of OTC was reduced after SCN catalytic oxidation. CONCLUSIONS The results imply that the visible‐light‐driven SCN catalytic oxidation is an effective technology not only for the removal of OTC but also for the reduction of NDMA formation potential. © 2021 Society of Chemical Industry

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