Electrochemical sulfite activation produces sulfur dioxide radical anions for bulk-phase arsenite reduction and passivation-free As(0) recovery

To address the global health crisis caused by the arsenic contamination of groundwater, the effective removal of the non-ionic form of arsenite (As(III)) is crucial but challenging. Electrochemical reduction offers a single-step route to convert As(III) into valuable elemental arsenic (As(0)), yet its efficiency is often compromised by electrode passivation due to the deposition of nonconductive As(0). Here, we introduce electrochemical advanced reduction processes to overcome this limitation by utilizing a cathodic H-to-sulfite radical relay. Specifically, electrons are transferred from the cathode via adsorbed hydrogen to sulfite, generating sulfur dioxide radical anions that drive As(III) reduction in the bulk-phase rather than on the electrode interface. The process was shown to achieve efficient As(III) removal and high-purity As(0) recovery at mildly acidic pH and ambient oxygen levels in a flow cell system equipped with only commercial nickel foam electrodes. This study provides a cost-effective route for remediation of arsenic-containing waters and can be extended to treat other reducible pollutants such as oxyanions and halogenated organics. • As(III) was removed by the sulfite-activated electrochemical reduction process. • High-purity elemental arsenic (>97.5 wt%) was recovered as a valuable product. • As(III) reduction was predominantly driven by ·SO 2 - radicals in bulk solution. • The in situ generation of ·SO 2 - was enabled by a cathodic *H-sulfite relay. • Flow-cell operation with commercial electrodes allows for scalable application.