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.

Indirect electrocatalytic (EC) oxidation of ammonia into harmless N2 based on in situ generated reactive chlorine species from chlorinated aqueous streams has been considered as an effective technique for ammonia abatement from water/wastewater. However, the precise oxidation mechanism and contribution of reactive species to the ammonia oxidation are still under debate and deserved to be specifically investigated. In order to enhance mass transfer efficiency, this work systematically investigated and optimized a three-dimensional EC (3D-EC) system, which exhibits about two times higher ammonia oxidation rate compared to a two-dimensional EC system. The polarization characters of particle electrodes have been analyzed to correlate with the enhanced generation of ·OH and ClO· radicals under biased conditions, and the role of each oxidative species was specifically quantified. In the enlarged scale study, water flow-by mode was tested with varying experimental parameters including flow rate, hydraulic residence time, and secondary oxidation. Furthermore, the performance of the 3D-EC system for the treatment of six types of practical wastewater with different levels of ammonia and organics was evaluated. This study provides mechanistic insight on the indirect 3D-EC oxidation of ammonia and its potential application on ammonia wastewater treatment.