Role of nitrite ligands in enhancing sulfate radical production via catalytic peroxymonosulfate activation by cobalt complexes

In this study, hexanitrocobaltate (HN-Co(III)) was compared against Co²⁺ with respect to the capability for peroxymonosulfate (PMS) activation. The role of NO₂⁻ ligands in enhancing Co(II)/Co(III) cycling was confirmed based on the two-fold increase in the benzoic acid (BA) degradation efficiency of HN-Co(III)/PMS vs. Co²⁺/PMS and the negligible reduction in the activation performance of HN-Co(III) over repeated use. This was in line with accelerated BA degradation and catalytic activity recovery upon NO₂⁻ addition to Co²⁺/PMS. A negative shift in the anodic peak potential of the Co(II)/Co(III) couple and an increase in the anodic current over time imply that NO₂⁻ coordination promoted the capacity of Co(II) for PMS activation and facilitated the Co(III)-to-Co(II) conversion through hydrolysis. Along with sulfate radical as the primary oxidant, high-valent cobalt acted as the non-radical secondary oxidant based on the transformation of phenyl methyl sulfoxide to the corresponding sulfone in the HN-Co(III)/PMS mixtures.