## 5 Application of eNO\({}_{3}\)R to Brackish Groundwater: A Case Study
### Coexistence Ions  
The influence of the nature of the coexisting ions has been largely reported. For instance, [PERSON] et al. [122] noted that the reduction in nitrate increases slightly with an increase in NaCl concentration. Additionally, according to the Li\({}^{+}<\) Na\({}^{+}<\) K\({}^{+}<\) Cs\({}^{+}\) series of the supporting electrolyte, the reduction rate also increases significantly. Likewise, they demonstrated that the rate of reduction is notably higher in the presence of multivalent cations, namely Ca\({}^{2+}\) and La\({}^{3+}\) as well as NH\({}_{4}^{+}\)[122]. Moreover, [PERSON] et al. [209] showed that when the concentration of Na\({}_{2}\)SO\({}_{4}\) was high, the rate of removal of NO\({}_{3}^{-}\) was rapid and this can be explained by the fact that a higher concentration of electrolytes induced a higher rate of electron transfer and the effect of electrolytes on the selectivity to N\({}_{2}\) was remarkable. The effect of the supporting electrolyte cation on nitrate reduction has been explained by the theory of \"cationic catalysis\", which suggests that the cation of the supporting electrolyte acts as an attracting center for the reduced anion by forming an instantly neutral ion pair. The latter is not repelled by the negatively charged electrode, increasing the rate of nitrate reduction [122]. [PERSON] et al. [210] evaluated the effect of K\({}^{+}\) and Na\({}^{+}\) cations on nitrate reduction in an electrolyte solution using real-time on-line differential electrochemical mass spectrometry. The results indicated that the presence of K\({}^{+}\) led to the production of N\({}_{2}\) as the main product, while Na\({}^{+}\) favored the production of N\({}_{2}\)O. The study also reported a moderate HER in K\({}^{+}\)-containing electrolyte solutions, promoting the reaction between NO\({}_{3}^{-}\) and H\({}_{2}\) species, while the HER kinetics was more dominant in Na\({}^{+}\)-containing electrolyte solutions, leading to a suppression of molecular nitrogen generation. These results highlight the significant influence of different cations in the supporting electrolyte on the products and kinetics of nitrate reduction [210]. [PERSON] et al. [211] conducted also a study to investigate the influence of La\({}^{3+}\) and Al\({}^{3+}\) cations on nitrate reduction at a polycrystalline silver electrode. The outcomes showed that the presence of La\({}^{3+}\) caused a positive shift in the onset potential, indicating an enhancement of nitrate reduction. This may be attributed to acid-base catalysis, with La(III) species acting as proton donors and/or OH\({}^{-}\) ion acceptors at the electrode surface. The research studies also exhibited that films deposited onto the electrode surface consisted of La(OH)\({}_{3}\) and/or Al(OH)\({}_{3}\) in the solutions containing La\({}^{3+}\) and Al\({}^{3+}\) ions, respectively. The La(III) species in the solution then diffused to the electrode surface as nitrate complexes or ion pairs, thereby promoting the reduction reaction. In contrast, Al\({}^{3+}\) cations, which do not form ion pairs with NO\({}_{3}^{-}\) anions were found to be less effective in promoting the nitrate reduction reaction [211]. In contrast, the rate of reduction was reduced by the anion of the supporting electrolyte. Specifically, I\({}^{-}\) exhibited the greatest inhibitory effect, followed by Br\({}^{-}\), Cl\({}^{-}\), and F\({}^{-}\). [PERSON] et al. [201] noticed that pH control by sulfuric acid affected nitrate reduction, as the SO\({}_{4}^{2-}\) anion present in the solution can interfere with nitrate adsorption in the active sites of the catalyst, leading to a diminution of nitrate reduction.