Synergistic mechanism and kinetic modeling of Fe ions coupled with electrochemical activation of sulfite for efficient organic contaminants degradation: A unique two-stage oxidation process

An integrated sulfite activation process combing electrochemistry and Fe ions (Electro-Fe/S(IV)) was developed for efficient emerging contaminants (ECs) degradation. Leveraging the strong synergy between Fe ions and electrochemical activation (synergy factor = 2.39), over 95 % of carbamazepine, sulfamethoxazole, bisphenol-A, and atrazine were removed in 10 min. It was found that electro-generated O₂ sustained high conversion rates of Fe ions and accelerated oxysulfur species conversion. Additionally, protons released during S(IV) electro-activation mitigated Fe ions precipitation and improved the complexation between Fe ions and S(IV). Importantly, Fe ions conversion, reactive species generation and ECs degradation in the unique two-stage oxidation process were elucidated through kinetic modeling and density functional theory calculations. Sufficient S(IV) and Fe(II) in Stage I drove rapid cycle of FeHSO₃⁺ /FeSO₃⁺ for SO₄•^- generation, which dominated 95–99 % of ECs degradation. However, with S(IV) consumption and Fe(II) oxidation in Stage II, Fe(II)/Fe(III) acted as electron shuttle in S(IV) and HSO₅^- activation, favoring Fe(IV)=O formation, with its contribution ranging from 3 % to 42 %. Process optimization by multiple S(IV) additions maintained the high SO₄•^- concentration, significantly increasing carbamazepine degradation rate and decreasing electrical cost compared to single addition. Furthermore, Electro-Fe/S(IV) process demonstrated cost-effectiveness in real water matrices. This study advances mechanistic understanding and kinetic optimization of coupling electrochemistry and Fe-based catalysts for S(IV) activation. © 2025 The Institution of Chemical Engineers.