## 4 Advanced Strategies for eNO\({}_{3}\)R Toward Ammonia and Nitrogen
### Advanced Strategies Toward eNO\({}_{3}\)R to Nitrogen
#### 4.2.2 Indirect eNO\({}_{3}\)R to Nitrogen via Cl-Electrochemical Advanced Oxidation  
The ammonia generated during eNO\({}_{3}\)R can be oxidized to nitrogen gas by free radicals (HO\({}^{\bullet}\), Cl\({}^{\bullet}\), ClO\({}^{\bullet}\)) or active chlorine, generated during electrolysis [182]. Studies confirm that HO\({}^{\bullet}\) alone does not affect ammonia specifically in the absence of chloride [183; 184]. In contrast, Cl\({}^{\bullet}\), ClO\({}^{\bullet}\), and active chlorine exhibits high ammonia oxidation capacity, achieving nitrogen selectivity exceeding 90% [185; 186; 183]. Ammonia oxidation in the Cl-EAO process primarily occurs via active chlorine and chlorine radicals. Initially, Cl\({}^{-}\) ions adsorb onto the anode surface and lose electrons, forming Cl\({}^{\bullet}\). These radicals may combine to form Cl\({}_{2}\) or react with Cl\({}^{-}\) to produce Cl\({}_{2}\), which then diffuses into the solution and reacts further to form HClO or ClO\({}^{-}\). Additionally, HO\({}^{\bullet}\) is generated on the anode surface, some of which reacts with Cl\({}^{-}\) in the solution to form Cl\({}^{\bullet}\). Both HO\({}^{\bullet}\) and Cl\({}^{\bullet}\) can further react with HClO or ClO\({}^{-}\) to form ClO\({}^{\bullet}\). These species react with ammonia through breakpoint chlorination and chlorine radical oxidation mechanisms, as illustrated in Figure 14[187]. In the Cl-EAO process, ammonia oxidation occurs at the electrode-solution interface, where the production rate of active chlorine and chlorine radicals is crucial and often limits the overall reaction rate [188]. The rate of chlorine radical production, particularly the production of Cl\({}^{\bullet}\), is identified as a slower step compared to other reaction rates [189]. During breakpoint chlorination, the mass transfer of active chlorine to water becomes another rate-limiting factor, influencing the rate of chlorine radical oxidation more than breakpoint chlorination itself. At the cathode, active chlorine in the solution is reduced back to Cl\({}^{-}\). Over time, the concentration of active chlorine in the solution stabilizes, maintaining a stable Cl\({}^{-}\) concentration due to this reduction process. Despite some Cl\({}_{2}\) escaping from the solution, the impact on active chlorine production remains minimal [187]. Furthermore, the ammonium generated during the process can be influenced by different factors, as demonstrated in Table 5.  
Figure 14: Mechanisms for ammonia oxidation by breakpoint chlorination and chlorine radical.  
The debatable aspect of this process is the formation of chloramine intermediates, which include NH\({}_{2}\)Cl, NHCl\({}_{2}\), and NCI\({}_{3}\), as well as chlorine-containing byproducts (CBPs) such as ClO\({}_{2}\), ClO\({}_{3}\), and ClO\({}_{4}\)[192; 196]. Researchers have found that chloramine concentrations are always below certain specific levels, such as 0.44 mg/L [197]. A study using a graphite/PbO\({}_{2}\) anode showed that the total chloramine concentration was always less than 2.2% of the initial ammonia concentration [192]. Therefore, it is important to focus on the concentration and behavior of chloramines and CBPs in the Cl-EAO process to enhance the understanding and application for the removal of ammonium generated during the electrochemical reduction in nitrates.