Mn/Fe bimetallic MOF-driven sulfite activation for synergistic non-radical oxidation and chemisorption of arsenite: Involvement of high-valent Fe(IV)/Mn(V) species.

The decontamination of carcinogenic arsenite (As(III)) in water remains a critical challenge. This difficulty stems from its high mobility and weak chemisorption at near-neutral pH. Herein, a Mn/Fe bimetallic metal-organic frameworks (MOFs, MFM) is designed to activate sulfite (S(IV)) for rapid As(III) oxidation and enhanced arsenate (As(V)) adsorption. The MFM/S(IV) system achieves a 3.9-fold higher efficiency than the monometallic Fe-MOF, removing 99.5 % of As(III) (1 mg L) within 20 min at pH = 7, reducing residual arsenic to below 10 μg L. Mechanism investigations reveal that Mn incorporation establishes dual Fe(II)↔Fe(III) and Mn(II)↔Mn(III) redox cycles. This facilitates efficient S(IV) activation to generate high-valent Fe(IV)/Mn(V) species as dominant oxidants. These species drive rapid As(III)-to-As(V) conversion via oxygen-atom transfer, diverging from radical-mediated pathways. Besides, S(IV) activation enhances the adsorption of As(V) to form inner-sphere monodentate complexes, amplified by S(IV)-induced surface hydroxylation and pore restructuring. The system demonstrates pH resilience (6 -9), thermal adaptability (288.15 -318.15 K), and efficacy in real waters (tap, underground, and lake water). Fixed-bed operation confirms its scalability (99.6 % removal over 12 h) and regenerability (88.3 % arsenic recovery). This work advances the design of bifunctional MOF for S(IV)-driven As(III) decontamination in complex aquatic environments, emphasizing dual-metal redox synergies and non-radical oxidation mechanisms.