Nonradical reactions in environmental remediation processes: Uncertainty and challenges

Recent discoveries of nonradical oxidation in aqueous-phase advanced oxidation processes (AOPs) have induced tremendous interest in environmental remediation of wastewater, whereas different findings from a variety of investigations have also raised severe controversies in the occurrence and mechanism of the nonradical reaction. Hence, critical understandings of the nonradical reaction will significantly advance the knowledge and its application for catalytic oxidation and wastewater treatment. In this review, we would like to present state-of-the-art research on nonradical pathways in persulfate-based AOPs, with emphases on the controversial methodologies for identifying the oxygen reactive species (ROS), ambiguous reaction mechanisms, intrinsic impacts of metal/carbon catalysts and organic substrates in the nonradical-based catalytic oxidation reactions. Moreover, further research directions on mechanistic investigation of the nonradical pathway with rational experimental design and advanced strategies, as well as the potential applications of the nonradical system are proposed.