Enhanced degradation of metronidazole by cobalt doped TiO₂/sulfite process under visible light

The degradation of metronidazole (MNZ) by cobalt doped TiO₂ activation of sulfite has been studied in this work. The Co-TiO₂ was synthesized using a sol–gel method and characterized by TEM, EDS, DRS, XPS and EPR. The catalytic stability was tested by consecutive reuses. Up to 94% of MNZ was removed after 18 min reaction with Co-TiO₂ and sulfite dosage being 0.1 g/L and 5 mM, respectively. The influencing parameters, such as the initial concentration of MNZ (0.03–0.2 mM), sulfite dosage (1–10 mM), catalyst dosage (0.1–0.8 g/L) and pH (2–12) were examined. The MNZ removal was pH-dependent with an optimal pH of 7.0. The presence of halides and nitrite could inhibit the process effectively. The reaction mechanisms were explored by radical quenching experiments and EPR measurements, and investigated by the XPS spectra of the catalysts before and after reactions. The formation of Co-SO₃ complex and the Co²⁺–Co³⁺–Co²⁺ redox processes on the surface of the catalyst were crucial for sulfite activation. Compared to other Co species/persulfate processes, Co-TiO₂/sulfite/LED combined the advantages of TiO₂ photocatalysis and activation of sulfite with transition metal. The study suggests a novel and promising process for the treatment of antibiotics in water.