## 5 Application of eNO\({}_{3}\)R to Brackish Groundwater: A Case Study
### pH  
During the reduction process, electrocatalytic nitrate reduction consumes protons in both the rate-limiting step and the subsequent reduction steps, which means that the pH increases with the electrolysis process. A thorough investigation was carried out to identify the optimal circumstances in which the pH could exert an influence. eNO\({}_{3}\)R process indicated that the best removal rates were achieved under acidic conditions, in contrast  
\begin{table}
\begin{tabular}{l l l} \hline
**Influence Factors** & **Descriptions** & **Refs.** \\ \hline \multirow{3}{*}{Initial chloride concentration} & high initial concentration of chlorides & \\  & increases the production of active & \\  & chlorine and Cl\({}^{\bullet}\), thereby enhancing & [185; 190; 181] \\  & Ammonia oxidation & \\ \hline \multirow{3}{*}{pH} & pH decreases during the process. & \\  & Acidic and neutral media are preferable & \\  & for ammonia oxidation. & [185; 192; 193] \\  & Alkaline media decrease ammonia & \\  & oxidation efficiency & \\ \hline \multirow{3}{*}{Current density} & Increasing current density improves & \\  & ammonia oxidation efficiency (rule & \\  & applied for lower current density than & \\  & 37.5 mA\(\cdot\)cm\({}^{-2}\)) & \\ \hline \multirow{3}{*}{Ammonia concentration} & High ammonia in solution inhibits the & \\  & formation of active chlorine and Cl\({}^{\bullet}\) & [194; 195] \\ \hline \multirow{3}{*}{Temperature} & Increasing temperature promotes the & \\  & oxidation of ammonia. However, at & \\ \cline{1-1}  & temperatures higher than 55 \({}^{\circ}\)C the & [193] \\ \cline{1-1}  & efficiency decreases due to the lose of & \\ \cline{1-1}  & active chlorine. & \\ \hline \end{tabular}
\end{table}
Table 5: Parameters Affecting the Oxidation Process of Ammonium generated.  
to neutral conditions that led to poor results [105]. Lowering the pH of the electrolyte solution, used in electrochemical nitrate reduction processes, can also provide additional benefits as reported by [PERSON] et al. [200] where an electrolyte with a lower pH facilitates the formation of ions (e.g., \(\text{T}^{2+}\text{or}/\text{and }\text{T}^{3+}\)), which can reduce nitrates and nitrites. On the other hand, the Pourbaix diagram of the N2-H2O system (Figure 15) indicated that the hydrogen evolution reaction (HER) cannot be ignored as it competes with the eNO3R due to electron consumption. The FE of HER is higher in acidic media than in alkaline media, which results in a significantly lower FE of nitrate reduction in acidic media compared to alkaline media. Due to competition with H2, the direct charge transfer process has low efficiency. However, the indirect electrochemical autocatalytic process (Vetter and/or Schmid) can be improved under acidic conditions due to the compensation of this effect. [PERSON] et al. [181] found that a neutral pH of 7, was the most favorable for electrocatalytic nitrate reduction, as acidic conditions caused hydrogen bubbles to form and prevent contact between the active site and the electrolyte, while higher pH led to the active surface being occupied by adsorbed hydroxide. On the other hand, [PERSON] et al. [201] indicated that solutions with pH values between 6.0 and 6.5 induced a lower conversion of nitrate than in alkaline media. Notably, nitrite formation was significantly minimized, and gaseous compound generation was higher in solutions at given pH values due to the availability of \(\text{H}^{+}_{\text{(aq)}}\), which in turn promotes gaseous compound generation.