Recent Advances on Photocatalytic and Electrochemical Oxidation for Ammonia Treatment from Water/Wastewater

Heterogeneous photocatalytic (PC) and electrochemical (EC) oxidation of ammonia/ammonium pollutants in water/wastewater have been thoroughly investigated for ammonia abatement from aqueous streams, as there are continuing needs for developing alternative on-site easily controllable treatment approaches in contrast to conventional methods. Depending on the contamination level, water matrix characters, and regulatory consideration, the PC and EC oxidation of wastewater pollutants or integration with other treatment processes exhibit their own advantages and disadvantages at specific conditions. The PC oxidation of ammonia primarily relies on in situ generated strong oxidants such as hydroxyl radicals and holes, but their reactivities with ammonia are relatively slower at environmentally relevant pH conditions. In contrast, indirect EC oxidation of ammonia based on active chlorine species is more efficient and exhibits some advantages compared to the chemical chlorination approach. Although we have gained much scientific knowledge on this research topic in recent years, the research so far has not yet lead to broad industry adoption due to the many concerns about technical drawbacks and economic feasibilities; hence, it is necessary to reexamine the efforts made in this field. Upon survey of the recent literature, the aim of this review is to summarize and discuss the recent achievements and key issues on the PC and EC driven oxidation processes for ammonia abatement in order to avoid redundant studies that concentrate on well-established issues and point out the research directions to move these techniques forward in future.