Catalytic persulfate activation for oxidation of organic pollutants: A critical review on mechanisms and controversies

Persulfate-based advanced oxidation processes (PS-AOPs) have received increasing attention as new approaches to oxidize refractory organic pollutants in contaminated water and soil. In particular, PS-AOPs using catalysts have been the most intensively studied because a broad spectrum of materials can catalyze the reaction without requiring external input of energy. Despite recent advances in catalytic PS-AOPs, their activation mechanisms are still challenged by uncertainties. Persulfate can be activated via a radical mechanism to generate sulfate radical and hydroxyl radical, as well as via a nonradical mechanism to mediate electron transfer or generate other nonradical reactive oxidants (e.g., single oxygen or high valent metal species). The mechanisms of catalytic persulfate activation and the identities of dominant reactive oxidants remain controversial for many reported catalysts. Herein, we endeavor to address radical and nonradical mechanisms and their identification methods in catalytic persulfate activation, focusing on controversies over widely investigated catalysts. This article will improve the mechanistic understanding of catalytic PS-AOPs and offer a theoretical foundation for exploring novel PS-AOPs with better specificity and performance.