Ammonia removal under free radical action in advanced oxidation processes: A review

Advanced oxidation processes(AOPs) are promising and potential strategies to handle the severe ammonia/nitrogen pollution in the global aquatic ecosystems. This review compares AOPs for ammonia removal from wastewater, emphasizing hydroxyl, sulfate, and chlorine radicals' mechanism and efficiencies. Due to the strong electrophilic nature of hydroxyl radicals, they show limited effectiveness in the direct oxidation of ammonia. Sulfate radicals are able to produce hydroxyl radicals and facilitate ammonia degradation pathways involving hydroxyl radicals, thus improving the effectiveness but with low selectivity to N 2 . With moderate redox potentials and hydrogen abstraction capabilities, chlorine radicals prevent excessive oxidation and ensure high selectivity, making CR-AOPs a highly promising candidate for the treatment of ammonium-rich wastewater. The review provides a comparative analysis of AOPs based on radical activity, evaluates their respective advantages and limitations, and outlines future research directions. These include innovative AOP designs, synergistic radical interactions, and enhanced process optimization, aiming to improve the sustainability and cost-efficiency of ammonia removal using AOPs.