Unlocking sustainable water decontamination: Controlled sulfite release from solid calcium sulfite for enhanced...

Electrochemical sulfite activation represents a novel member of advanced oxidation processes effective for pollutant degradation, yet systems based on soluble sulfite sources (e.g., Na₂SO₃) remain plagued by radical self-quenching, excessive sulfate accumulation, and limited long-term sustainability. To overcome these limitations, we developed a controlled-release sulfite-electrochemical oxidation system (EC/CaSO₃) employing CaSO₃ as a solid-phase sulfite reservoir. Arsenite (As(III)) was chosen as the model contaminant due to its pronounced toxicity and environmental prevalence. Under mild operating conditions (2.0 V, pH 8.5, 25°C), the EC/CaSO₃ system achieved complete As(III) oxidation within 12 min, exhibiting a kinetic rate constant 18-fold higher than that of the EC-only process. The controlled dissolution of CaSO₃ facilitated the sustained and stable release of SO₃^2 -, concurrently promoting the generation of reactive oxygen species (ROS) and reactive oxysulfur species (RSS). Mechanistic interrogation identified SO₄^•- as the predominant oxidant, with HO^• and SO₅^•- acting as secondary contributors. The process ultimately converted sulfite into stable CaSO₄ precipitates, effectively mitigating secondary salt contamination. Based on this mechanism, the oxidation performance of this system significantly outperforms that of conventional EC/Na₂SO₃ systems. By supplanting soluble Na₂SO₃ with solid-phase CaSO₃, this study pioneered a self-regulating and environmentally benign electrochemical oxidation strategy that integrated controlled sulfite supply with synergistic RSS/ROS generation, presenting an energy-efficient and scalable platform for water remediation.