Activation of MnFe₂O₄ by sulfite for fast and efficient removal of arsenic(III) at circumneutral pH: Involvement of Mn(III)

As(III) oxidation to As(V) is deemed necessary for better arsenic removal, and separation is still the optimal approach for water remediation from As(III). Herein, sulfite (SIV) was adopted to activate MnFe₂O₄ for simultaneous oxidation and adsorption of As(III) in neutral water. The As(III) removal was more efficient than a peroxidation of As(III) followed by adsorption. The adsorption capacity of MnFe₂O₄/S(IV) for As(III) (26.257 mg g⁻¹) was much higher than those of MnFe₂O₄ alone for As(III) (9.491 mg g⁻¹) and As(V) (9.142 mg g⁻¹). The mechanistic study corroborated that intermediate Mn(III) was the dominant oxidant responsible for rapid oxidation of As(III), and the dual roles of S(IV) as a complexing ligand and a precursor of oxysulfur radicals accelerated the redox cycle of Mn(II)/Mn(III). Moreover, S(IV) enhanced arsenic adsorption by driving more production of monodentate complexes. As(III) can be effectively removed over a wide range of temperatures (283.15–313.15 K) and pH (3–10) with the optimal pH of 7. The effect of coexisting ions and reusability of MnFe₂O₄ were also investigated. Especially, the superior performance of MnFe₂O₄/S(IV) for As(III) removal in various water matrixes may help develop new removal technologies based on active Mn(III) for the water decontamination from As(III).