# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Transformation-Based Technologies
#### 4.2.3 Electrochemical Denitrification  
Electrochemically driven technologies for water decontamination are well positioned as efficient solutions because of their great suitability, modular reactor designs, and possibilities to transform pollutants into harmful products [189]. The electrochemical removal of NO3- has the advantage of reducing the oxidation number of nitrogen, transforming nitrates into N2 or NH3/NH4+, as the main products, without the requirement for the addition of chemical compounds. It was shown that reaction products may also contain hydrazine (N2H4) and other oxygen-containing nitrogen species (NO2, NO2-, NO, N2O, and NH2 OH) [190]. The electroreduction of nitrates occurs over the cathode, which is negatively charged (Figure 8). Due to electrostatic forces, nitrates do not touch the cathode surface unless agitation is supplied. It can be accomplished with a static cathode in an externally agitated solution or with a rotating electrode, which achieves turbulent flow conditions at low rotation rates [191]. Depending on the composition of the treated solution, the reduction reaction of nitrates may compete on the cathode with other reduction reactions. According to the potential supplied, hydrogen evolution may occur due to the reduction of the solvent:  
\[2\;\mathrm{H_{2}O}+2\;\mathrm{e^{-}}\rightarrow\mathrm{H_{2}}+2\;\mathrm{OH^{-}} \tag{13}\]  
At the same time, there are oxidation reactions taking place over the anode, with oxygen evolution being the main one:  
\[\mathrm{H_{2}O}\rightarrow\frac{1}{2}\;\mathrm{O_{2}}+2\;\mathrm{H^{+}}+2\; \mathrm{e^{-}} \tag{14}\]  
Due to the presence of other ions, such as chloride, another oxidation reaction may occur:  
\[2\;\mathrm{Cl^{-}}\rightarrow\mathrm{Cl_{2}}+2\;\mathrm{e^{-}} \tag{15}\]  
The generation of chlorine gas is of crucial importance in this process. In fact, the chlorine generated can disproportionate, giving hypochlorite and chloride:  
\[\mathrm{Cl_{2}}+\mathrm{H_{2}O}\rightarrow\mathrm{HClO}+\mathrm{H^{+}}+ \mathrm{Cl^{-}} \tag{16}\]  
\[\mathrm{HClO}\rightarrow\mathrm{ClO^{-}}+\mathrm{H^{+}} \tag{17}\]  
Figure 8: Electrochemical reduction of nitrate ions in dual- and single-chamber cells. The presence of hypochlorite in the solution promotes N\({}_{2}\) as the primary end-product of nitrate reduction, due to the breakpoint chlorination reaction:  
\[2\;\mathrm{NH_{4}}^{+}+3\;\mathrm{ClO^{-}}\rightarrow\mathrm{N_{2}}+3\;\mathrm{H_{2 }O}+2\;\mathrm{H^{+}}+3\;\mathrm{Cl^{-}} \tag{18}\]  
Although this is not the only product of the reaction between ammonium and hypochlorite, chloramines (mono, di and tri-chloramine) can be produced [192]. These molecules are disinfectants more stable than hypochlorite. The effect of chloride concentration on the electrochemical nitrate reduction rate is controversial. [PERSON] and co-workers, with Pd-Cu/SS, Pd-Co/Ti, and Pd-Co-Cu/Ti cathodes [193], and [PERSON] and co-workers, with Fe, Cu and Ni cathodes [194], reported no improvement or decrease in the nitrates reduction rate with chloride concentration. However, a positive effect of chloride concentration was observed for cathodes such as Zn [195], BDD (boron-doped diamond) [196], and graphite [197]. A positive effect of chloride concentration was also reported using SS (stainless steel) cathode and BDD or DSA anodes in a single chamber cell [192]. In this work, the evolution of chlorine atoms is analysed. It is observed that the BDD anode oxidizes chlorine to its higher oxidation number, giving perchlorate as the final product, specially at low chloride concentrations. With DSA anodes, chlorine is partially oxidized to chlorate. The best performance of these anodes occurs for high chloride concentration (3000 mg/L of NaCl), giving a maximum nitrate reduction rate, excellent selectivity to N\({}_{2}\) gas, and no formation of chlorate or perchlorate [192].