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 (FeIVO) 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 FeIII sites (FeoctIII). The results showed that the highest content of FeoctIII was found in the paper mill sludge ash obtained by aerobic calcination at 800 °C (pmSA800), and the pmSA800 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 FeoctIII was the dominant active site for FeIVO production. PMS tended to be adsorbed on two adjacent FeoctIII sites to form a [FeoctIII-OSO2OO(H)-FeoctIII] complex with a two-site adsorption configuration. Subsequently, the OO and OH bonds in [FeoctIII-OSO2OO(H)-FeoctIII] broken by absorbing energy, generating FeIVO. It's worth noting that the Ca species of pmSA800 effectively stabilized the pH of the reaction solution, which ensured the stable and continuous production of FeIVO. This work provides a new idea and insight for preparing catalysts that could produce FeIVO active species.