Comprehensive Insight into the Common Organic Radicals in Advanced Oxidation Processes for Water Decontamination

Abstract

Radical-based advanced oxidation processes (AOPs) are among the most effective technologies employed to destroy organic pollutants. Compared to common inorganic radicals, such as ^•OH, O₂^•–, and SO₄^•–, organic radicals are widespread, and more selective, but are easily overlooked. Furthermore, a systematic understanding of the generation and contributions of organic radicals remains lacking. In this review, we systematically summarize the properties, possible generation pathways, detection methods, and contributions of organic radicals in AOPs. Notably, exploring organic radicals in AOPs is challenging due to (1) limited detection methods for generated organic radicals; (2) controversial organic radical-mediated reaction mechanisms; and (3) rapid transformation of organic radicals as reaction intermediates. In addition to their characteristics and reactivity, we examine potential scenarios of organic radical generation in AOPs, including during the peroxide activation process, in water matrices or with coexisting organic pollutants, and due to the addition of quenching agents. Subsequently, we summarize various methods for organic radical detection as reported previously, such as electron paramagnetic resonance spectroscopy (EPR), ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR), liquid/gas chromatography–mass spectroscopy (GC/LC–MS), and fluorescence probes. Finally, we review the contributions of organic radicals to decontamination processes and provide recommendations for future research.