Vanadium trioxide mediated peroxymonosulfate for fast metronidazole oxidation: Stepwise oxidation of vanadium for donating electrons

In this study, vanadium trioxide (V 2 O 3 ) was adopted to activate PMS via a Fenton-like reaction to degrade metronidazole (MNZ). The V 2 O 3 -PMS system can almost completely degrade MNZ at 30 min with 42.4% TOC removal. Comparative tests reveal that V 2 O 3 stands out among a variety of heterogeneous catalysts, including metallic oxides and carbon materials. Sulfate radicals (SO 4 •− ) and hydroxyl radicals ( • OH) derived from PMS decomposition are major reactive oxygen species, based on quenching tests, electron spin resonance (ESR) analysis, the steady-state concentrations of radicals ([SO 4 •− ] ss  = 5.1 × 10 -13 M and [ • OH] ss  = 4.0 × 10 -14 M), and kinetics model. The process of stepwise electron transfer from vanadium species to PMS to produce reactive radicals was proved by small-molecule simulation experiments and pickling experiments of vanadium oxides. Possible pathways of MNZ degradation were proposed based on the results of LC-MS and Fukui function, including two stages of the hydroxylation and bond cleavage of nitro and the subsequent ring-opening. This study reveals the high reusability and practicability of the V 2 O 3 -PMS system over a relatively wide pH range, which puts forward a new vision on V 2 O 3 induced Fenton-like reactions and a new reference method for the removal of medical organic contaminants in water.