Iron (hydr)oxides-induced activation of sulfite for contaminants degradation: The critical role of structural Fe(III)

Iron-based sulfite (S(IV)) activation has emerged as a novel strategy to generate sulfate radicals (SO 4 •- ) for contaminants degradation. However, numerous studies focused on dissolved iron-induced homogeneous activation processes while the potential of structural Fe(III) remains unclear. In this study, five iron (hydr)oxide soil minerals (FeO x ) including ferrihydrite , schwertmannite , lepidocrocite , goethite and hematite , were successfully employed as sources of structural Fe(III) for S(IV) activation. Results showed that the catalytical ability of structural Fe(III) primarily depended on the crystallinity of FeO x instead of their specific surface area and particle size, with ferrihydrite and schwertmannite being the most active. Furthermore, in-situ ATR-FTIR spectroscopy and 2D-COS analysis revealed that HSO 3 - was initially adsorbed on FeO 6 octahedrons of FeO x via monodentate inner-sphere complexation, ultimately oxidized into SO 4 2- which was then re-adsorbed via outer-sphere complexation. During this process, strong oxidizing SO 4 •- and •OH were formed for pollutants degradation , confirmed by radical quenching experiments and electron spin resonance . Moreover, FeO x /S(IV) system exhibited superior applicability with respect to recycling test, real waters and twenty-six pollutants degradation. Eventually, plausible degradation pathways of three typical pollutants were proposed. This study highlights the feasibility of structural Fe(III)-containing soil minerals for S(IV) activation in wastewater treatment .