Selective oxidation of ammonium to nitrogen gas by advanced oxidation processes: Reactive species and oxidation mechanisms

Ammonia nitrogen pollution is still a global concern, leading to eutrophication of aquatic ecosystems. Various technologies have been studied and applied for ammonium removal from water and wastewater, including physical, chemical and biological methods. For wastewater purification purposes, advanced oxidation processes (AOPs) are considered as promising approaches and potential alternatives for advanced treatment to further improving water quality. Although there are many reviews on AOPs for degradation of refractory pollutants, few reviews on the ammonia oxidation. In recent years, the selective oxidation of ammonia by AOPs has received increasing attention. In this review, the selective oxidation of ammonium to nitrogen gas (N2) by various AOPs, such as photocatalytic process, electrochemical process, catalytic ozonation, sulfate radical-based process, and integrated processes were systematically summarized. Firstly, the formation, reactivity toward NH4+/NH3, and reactive species generated in AOPs were analyzed and compared; then, the radical mediated and non-radical mediated reaction mechanisms of NH4+/NH3 oxidation were discussed in detail; thirdly, the selective oxidation of ammonia to nitrogen gas under specific operational conditions were summarized, and finally, various AOPs for selective oxidation of ammonia to nitrogen gas were discussed from the perspective of merits and demerits, influencing factors, key material development, integrated processes, current status, main challenges, and future research directions. This review will deepen to understand the selective oxidation of ammonia by various AOPs.