Revisiting the Iron(II)/Cobalt(II)-Based Homogenous Fenton-like Processes from the Standpoint of Diverse Metal–Oxygen Complexes
Yingxu Gong, Zhonglin Chen, Yining Wu, Aijie Wang, Shengxin Zhao
Abstract
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.