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. [Display omitted] • Structural Fe(III) of five iron (hydr)oxides can activate S(IV) to generate ROS. • Ferrihydrite and schwertmannite presented the most active performance. • Catalytical ability of structural Fe(III) depended on the crystallinity of FeO x. • In-situ ATR-FTIR spectroscopy and 2D-COS analysis unveiled possible mechanisms. • The FeO x /S(IV) system exhibited superior applicability. [ABSTRACT FROM AUTHOR]