A review on the role of high-valent metals in peracetic acid-based advanced oxidation processes

In contrast to •OH and CH3C(O)O•/CH3C(O)OO• radicals, which are commonly considered to be the primary reactive oxidants, high-valent metals generated during peracetic acid (PAA) activation have only recently been proposed. Compared with radicals, high-valent metals possess longer lifetimes (e.g., 0.02 μs for •OH versus 7 s for Fe(IV)) and are more tolerant to background ions despite their relatively moderate oxidation power. Although several review papers on high-valent metals have been published, there is a lack of information on the generation of high-valent metals in peracetic acid systems. Therefore, the aim of this work was to review high-valence metals generated during peracetic acid activation, primarily focusing on i) formation pathways based on homo- and heterogeneous Fe(II)/PAA and Co(II)/PAA systems, ii) reaction mechanisms involving hydride/H-atom transfer, oxygen atom transfer, electrophilic addition, and nucleophilic attack, iii) oxidation performance including substrate-specific reactivity, influencing factors, and environmental water matrices. Furthermore, critical thinking and further prospects for high-valent metal-mediated oxidation processes are proposed.