Revisiting the Iron(II)/Cobalt(II)-Based Homogenous Fenton-like Processes from the Standpoint of Diverse Metal-Oxygen Complexes.

The aqueous Fe^IV-oxo complex and Fe^III-peroxy complex (e.g., ligand-assisted or interfacial Fe^III-hydroperoxo intermediates) have been recognized as crucial reactive intermediates for decontamination in iron-based Fenton-like processes. Intermediates with terminal oxo ligands can undergo the oxygen atom exchange process with water molecules, whereas peroxides are unable to induce such exchanges. Therefore, these distinct metal-oxygen complexes can be distinguished based on the above feature. In this study, we identified previously unknown intermediates with a peroxy moiety and cobalt center that were generated during peroxymonosulfate (PMS) activation via aqueous Co^II ions under acidic conditions. Results of theoretical calculations and tip-enhanced Raman spectroscopy revealed that the Co^II ion tended to coordinate with the PMS anion to form a bidentate complex with a tetrahedral structure. These reactive cobalt intermediates were collectively named the Co^II-PMS* complex. Depending on the inherent characteristics of the target contaminants, the Co^II-PMS* complex can directly oxidize organic compounds or trigger PMS disproportionation to release hydroxyl radicals and sulfate radicals for collaborative decontamination. This work provides a comparative study between iron- and cobalt-based Fenton-like processes and proposes novel insights from the standpoint of diverse metal-oxygen complexes.