Are Free Radicals the Primary Reactive Species in Co(II)-Mediated Activation of Peroxymonosulfate? New Evidence for the Role of the Co(II)–Peroxymonosulfate Complex

The catalytic activation of peroxymonosulfate (PMS) is under intensive investigation with potentials as an alternative advanced oxidation process (AOP) in wastewater treatment. Among all catalysts examined, Co(II) exhibits the highest reactivity for the activation of PMS, following the conventional Fenton-like mechanism, in which free radicals (i.e., sulfate radicals and hydroxyl radicals) are reckoned as the reactive species. Herein, we report that the primary reactive species (PRS) is proposed to be a Co(II)–PMS complex (Co(II)–OOSO3–), while free radicals and Co(III) species act as the secondary reactive species (SRS) that play a minor role in the Co(II)/PMS process. This Co(II)–OOSO3– exhibits several intriguing properties including ability to conduct both one-electron-transfer and oxygen-atom-transfer reactions with selected molecules, both nucleophilic and electrophilic in nature, and strongly pH-dependent reactivity. This study provides novel insights into the chemical nature of the Co(II)-catalyzed PMS activation process.