Radical and non-radical mechanisms for removal of micropollutants in peroxymonosulfate activation systems: Their generation and identification

Generation and identification of reactive oxygen species (ROS) in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) hold great significance for understanding reaction mechanism and controllable synthesis of catalytic materials. Chemical quenching and probe experiments, electron paramagnetic resonance (EPR), and other advanced detection techniques have been widely used to identify the oxidation mechanisms in PMS-AOPs, although some recent reports have indicated the inherent limitations of these strategies. In this review, we systematically summarize the typical characteristics, generation pathways and corresponding oxidation mechanisms of various radical and non-radical processes. A comprehensive inventory of commonly used scavengers, chemical probes, and relevant experimental design methodologies is compiled to facilitate thorough discussions on the identification methods of distinct reaction mechanisms. By dissecting the quenching effect of scavengers on target ROS in reaction systems, we emphasize the validity of these methods and corresponding quenchers in assessing the contributions of ROS to pollutant degradation. Subsequently, a more rational and comprehensive protocol for ROS verification is proposed. In addition, recent advancements in mechanism switching from radical to non-radical oxidation in PMS-AOPs are discussed. Accordingly, we expound the intrinsic conversion mechanism by exploring relationships among catalyst properties, active species generation, and pollutant types. Finally, major challenges and future perspectives of catalyst design and mechanism modulation are suggested. This review offers valuable references for ROS identification in future research, providing new impetus to enhance catalyst performance and regulate reaction mechanisms in PMS-AOP systems.