Direct generation of high-valent iron-oxo species to eliminate oxytetracycline at circumneutral pH via paper mill sludge ash activating peroxymonosulfate

The excellent performance of high-valent iron-oxo species (Fe IV O) in the degradation of pollutants by activating peroxymonosulfate (PMS) had attracted much attention. However, the generation and reaction mechanisms were still unclear. In this study, iron-rich paper mill sludge was used as a raw material for the first time to prepare iron-based catalysts with abundant octahedral Fe III sites (Fe oct III ). The results showed that the highest content of Fe oct III was found in the paper mill sludge ash obtained by aerobic calcination at 800 °C ( pm SA800), and the pm SA800 could exhibit excellent catalytic activity (0.4785 min −1 ) and stability during the oxytetracycline (OTC) degradation process. The characterization and density functional theory (DFT) calculations further indicated that the Fe oct III was the dominant active site for Fe IV O production. PMS tended to be adsorbed on two adjacent Fe oct III sites to form a [Fe oct III -OSO 2 OO(H)-Fe oct III ] complex with a two-site adsorption configuration. Subsequently, the O O and O H bonds in [Fe oct III -OSO 2 OO(H)-Fe oct III ] broken by absorbing energy, generating Fe IV O. It's worth noting that the Ca species of pm SA800 effectively stabilized the pH of the reaction solution, which ensured the stable and continuous production of Fe IV O. This work provides a new idea and insight for preparing catalysts that could produce Fe IV O active species.