Differences in the Oxidizing Species on the Surface of Metal-Doped Ferrihydrite and Hematite in the Alkaline S(IV)-O₂ Solution

Although inorganic Fe(IV)=O oxo and Fe(III)-OOH peroxo have attracted more attention in aqueous environments, direct evidence for the characterization and identification of those oxidizing species is still lacking. In this study, the doped Cu/Co increases the reactivity of ferrihydrite/hematite in oxidizing As(III) by S(IV)-O-2 under alkaline and dark conditions. Quenching experiments exclude the contributions of center dot OH, O-2(center dot-), SO3 center dot-, SO4 center dot-, SO5 center dot-, HSO5-, and H2O2 and confirm that peroxo and oxo are primary oxidizing species in As(III) oxidation. Electrochemical analyses indicate that Fe(IV)=O oxo and Fe(III)-OOH peroxo are primarily located on the surface of ferrihydrite and hematite. Selective oxidation of methyl phenyl sulfoxide and methyl phenyl sulfide to the corresponding oxidation products further reveals the presence of Fe(IV)=O oxo on both ferrihydrite and hematite, and the doped Cu/Co significantly decreases the produced Fe(IV)=O oxo on ferrihydrite, while increases the produced Fe(IV)=O oxo on hematite. Meanwhile, selective oxidation of benzyl alcohol to benzaldehyde indicates the presence of Fe(III)-OOH peroxo on both ferrihydrite and hematite, and the doped Cu/Co apparently increases the produced Fe(III)-OOH peroxo on ferrihydrite and hematite. Raman analysis further reveals the presence of Fe(IV)=O oxo and Fe(III)-OOH peroxo on pure and metal-doped ferrihydrite/hematite, which also confirms that the doped Cu/Co decreases the produced Fe(IV)=O oxo on ferrihydrite and increases Fe(IV)=O oxo on hematite, while apparently increasing Fe(III)-OOH peroxo on both ferrihydrite and hematite. This study enriches our understanding on the oxidizing species in the S(IV)-mediated oxidation processes under alkaline conditions, providing new insight on the oxidation reactivity of surface peroxo and oxo species mediated by the doped metal ions.