High-valent metals in advanced oxidation processes: A critical review of their identification methods, formation mechanisms, and reactivity performance

The role of high-valent metals as dominant reactive oxidants in advanced oxidation processes (AOPs) has not been systematically reviewed. The aim of this study was to provide an in-depth and comprehensive overview of high-valent metals in AOPs with respect to the identification methods, formation mechanisms, and reactivity performance. First, we discuss current identification methods for high-valent metals including their drawbacks. Subsequently, theoretical formation pathways and key influencing factors (e.g., pH and ligands) of high-valent metals are summarized. Based on density functional theory calculations, high-valent metals are more favorable than radicals. We then compare high-valent metals with radicals with respect to their redox potential and reaction rate constant and observed that high-valent metals have a strong selectivity towards the electrophilic functional group due to their moderate oxidation performance. Finally, points for critical thinking and further prospects for high-valent metal-mediated oxidation processes are proposed.