Recent advances on the role of high-valent metals formed during persulfate activation

Over the past years, persulfate-based advanced oxidation processes have been widely employed in scientific research and practical application because of their outstanding oxidizing capability for decontamination and sterilization. In contrast to SO4•− and •OH radicals that are commonly considered to be the primary reactive oxidants, high-valent metals, such as Fe(IV), Co(IV), Cu(III), and Mn(V), formed during persulfate activation have only been observed in recent years. The advantages of high-valent metals over radicals lie in the better tolerance of background ions and the selective reaction towards compounds containing electron-rich groups. In this review, current research progress on high-valent metals generated during persulfate activation is outlined, and the formation mechanism, oxidation performance, and practical application are discussed. Specifically, the formation of high-valent metals has been observed via 18O-labeled water combined with chemical probes and in situ spectroscopy; compared to radicals, the formation of these metals was more thermodynamically favorable. Unlike •OH and SO4•−, which exhibit strong and nonselective reactivity, high-valent metals possess moderate oxidation power and selectively react with compounds containing electron-rich groups; these reactions are affected by several parameters, such as pH, temperature, activator/persulfate dosage, and ligand. The applications of high-valent metal-mediated oxidation processes and challenges encountered are discussed. Overall, potential applications of high-valent metal-mediated oxidation based on persulfate could be extended for practical wastewater treatment.