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

To overcome the drawback that excess SO32− from soluble Na2SO3 captures the generated reactive intermediates in sulfite (S(IV))-based advanced oxidation processes (AOP), CaSO3 of the ability to slowly release SO32− 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. CaSO3 of the slow-released SO32− ability replaced Na2SO3 to establish a novel advanced oxidation process (AOP) with Co(II) for efficient removal of various organic contaminants. To our delight, apart from the conventional sulfate radical (SO4•−), Co(IV) species and singlet oxygen (1O2) were also probably involved in the Co(II)/CaSO3 process (barely known in other similar researches), and their formation mechanism and role had been systematically elucidated in this work. Overall, this work provided new mechanistic insight into the nonradical degradation pathways during the metal mediated sulfite activation process, and significantly extended the sulfite-based water treatment technologies.