Cobalt(II) mediated calcium sulfite activation for efficient oxidative decontamination in waters: Performance, kinetics and mechanism

To overcome the drawback that excess SO₃^2- from soluble Na₂SO₃ captures the generated reactive intermediates in sulfite (S(IV))-based advanced oxidation processes (AOP), CaSO₃ of the ability to slowly release SO₃^2- is selected as an alternative S(IV) source to establish an enduring S(IV)-based AOP with Co(II). Herein, the Co(II)/CaSO₃ process triggers a much better ofloxacin (OFL) degradation than the Co(II)/Na₂SO₃ process (degradation rate constant: 12.1 > 3.18 mM^-1 min^-1). The mechanism investigation corroborates that the Co(II) mediated CaSO₃ activation follows a Fenton-like process (complexation followed by intramolecular electron transfer). Apart from the conventional sulfate radical (SO₄^•-), Co(IV⁾ species and singlet oxygen (¹O₂) are also certifiably involved in Co(II)/CaSO₃ process, and their role and formation mechanisms are elucidated comprehensively. Further, the proposed Co(II)/CaSO₃ process exhibits an excellent tolerance to complex water matrices (e.g., background ions and humic acid), suggesting its practical application potential for various contaminants abatement in actual wastewater.