Chlorine oxide radical: An emerging free radical for denitrification and pollutant degradation

Excessive emissions of ammonia and organic pollutants have been a challenge for conventional wastewater treatment. Chlorine oxide radical-mediated advanced oxidation processes (ClO•-AOPs) have recently garnered significant interest for their benefits, such as ease of operation and high selectivity. This review presents the latest insights into the removal mechanism of ammonia and organic pollutants by ClO•, especially focusing on the similarities and distinctions between representative radicals. ClO• has a redox potential of 1.5 V and is formed via photo/photoelectrochemical and electrochemical reactions, whose concentration is significantly higher than that of Cl• and OH•. ClO• shows selective reactivity toward compounds with electron-rich moieties at higher second-order rate constants, and the major degradation mechanism is electron transfer. The reaction kinetics, free radical quenching, electron paramagnetic resonance, and radical probes are reviewed. ClO• is not easily consumed by water matrix components (carbonate and dissolved organic matter (DOM)), and its proper potential can abate the chlorinated disinfection byproducts after treatment. Lastly, we offer recommendations for further investigation of ClO• in the fields of photo/photoelectrocatalysis and electrocatalysis, as well as emphasize essential topics for future research.