Unraveling the Contribution of Co(IV) and Free Radicals in Co2+-activated Peroxymonosulfate Process: Reaction Mechanism, Degradation Pathway, and DFT Calculation

The Co2+ and Peroxymonosulfate (PMS) reaction is a highly regarded advanced oxidation process (AOP) that has been extensively researched. The consensus regarding the relative contribution of reactive species in the Co2+/PMS system under different pH conditions has not been established. In this study, quenching experiments and electron spin resonance (ESR) tests demonstrated the coexistence of Co(IV), sulfate radical (SO4•-), hydroxyl radical (•OH), and singlet oxygen (1O2), and there fell to CBZ degradation were Co(IV) > SO4•->•OH ≈ 1O2 in the optimal conditions, where almost 100% of CBZ was degraded in the Co2+/PMS system within 30 min. Furthermore, Cl− would suppress the formation of Co(IV), SO4•-, and •OH but enhance the yield of 1O2, thereby showing a trend of inhibiting CBZ degradation. The probe compound used in this study was Methyl Phenyl Sulfoxide (PMSO), while methyl phenyl sulfone (PMSO2) as the indicator of Co(IV), the generation efficiency of PMSO2 (η'[PMSO2]) increased with reaction time and reached 70%–98% at 40 min at all set pH values (3, 4, 6, and 8). These results suggest that Co(IV) played a significant role in the Co2+/PMS system and its importance became more pronounced as the reaction progressed. PMSO probe and SO4•- quantification experiments jointly revealed that Co(IV) had the greatest performance at pH 4, and compared to other pH values, SO4•- had the largest contribution in the initial stage of the reaction at pH 6. The Ecological Structure Activity Relationships (ECOSAR) software was utilized to predict the toxicity of the degradation by-products. Additionally, luminescent bacteria experiments were conducted to assess the acute toxicity of the reaction solution. The results of these experiments demonstrated that the Co2+/PMS process exhibited a remarkable ability to detoxify the solution.