# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Transformation-Based Technologies
#### 4.2.3 Electrochemical Denitrification
There are many factors that govern the rate of nitrate removal and the products resulting from this reduction in an electrochemical reactor, the main ones being the configuration of the cell, the electrode material, the applied potential, the pH of the solution, and the nature of the supporting electrolyte [198]. Regarding the cell configuration, the electrolytic reduction of nitrates has been studied in a single-chamber cell (SCC) and in a dual-chamber cell (DCC). The distinguishing feature of a DCC is the presence of a cation exchange membrane (CEM) separating the cathodic chamber from the anodic chamber. In DCC configuration, ammonium is the only reduction product able to reach the anodic chamber. In the case of SCC configurations, nitrites can reach the anode and re-oxidate to nitrates, decreasing the cell performance (Figure 8). When chloride ions are present in solution, its oxidation takes place on the anode (Figure 9). In DCC configuration, the breakpoint chlorination reaction only occurs when ammonium passes across the cation exchange membrane into the anodic compartment [193]. The improvement of DCC configurations has been demonstrated with different cathodic materials (graphite, Cu-Ni, Ti, Pd-Cu/SS, Pd-Co/Ti, and Pd-Co-Cu/Ti) [193; 197]. In SCC configurations, chloride ions (Cl\({}^{-}\)) are oxidized on the anode to hypochlorite (ClO\({}^{-}\)), which reach ammonium ions directly in the solution (Figure 9). The presence of  
Figure 9: Electrochemical reduction of nitrate ions, in presence de Cl\({}^{-}\), in dual- and single-chamber cells. chloride ions in SCC configuration increases the selectivity towards N\({}_{2}\) in the reduction of nitrates [192; 199; 200]. An interesting DCC configuration is the one designed with a Pt anode and Pt-Cu cathode, deposited on both sides of a nafion membrane [201]. In this reactor, O\({}_{2}\) is produced on the anode surface and H\({}^{+}\) flow across the membrane to the cathodic chamber, where they are further converted to H\({}_{2}\) (g), which contributes to the reduction of nitrates. Regarding electrode materials, different cathode materials have been studied, such as graphite, SS, Ni, Cu, Pt, Pd, Pb, Bi, Sn, Ti, Rh, Fe, BDD, etc. Pd cathodes showed almost no activity in nitrate reduction [202; 203], while graphite and Cu electrodes showed the higher activity, yielding nitrite and ammonium as the main products [197; 204]. Selectivity towards N\({}_{2}\) was raised up to 100% in the presence of chloride [194]. Significant improvement was obtained using bimetallic cathodes, mainly combinations of Cu with Ni, Rh, Zn, Pd, and Sn. When Cu-Pd electrodes are used, the functional role of Cu is to reduce nitrate into nitrite, while Pd reduces nitrite into N\({}_{2}\) with high selectivity. The cathode combination Cu-Zn performed excellent conversion of nitrates, reaching values of 90% [205] and 94.4% [206], with the suppression of ammonium products when NaCl was added as an electrolyte. With a cathode obtained via the electrodeposition of Cu on a Ni foam matrix, 100% elimination of nitrate was reported, which slightly decreased when NaCl was added to promote the almost complete removal of ammonium [199]. A novel Co\({}_{3}\)O\({}_{4}\)/Ti cathode was recently tested for nitrate removal with different concentrations of Cl\({}^{-}\). In the absence of chloride, 98% of nitrate removal was reached, with a selectivity of 32% to NH\({}_{4}\)\({}^{+}\) in 3 h. On increasing the Cl\({}^{-}\) content to 1500 mg/L, the nitrate removal slightly decreased to 96%, but no NH\({}_{4}\)\({}^{+}\) was detected as the final product, with almost complete transformation of nitrogen to gaseous species [200]. The pH is also a key factor in the electroreduction rate of NO\({}_{3}\)\({}^{-}\), increasing at acidic pH values. At pH \(>\) 7.0, hydroxyl species passivate the electrode surface, decreasing the reduction rate [190]. It has been reported an increase of pH from 1 to 7 during the electroreduction process due to the production of OH\({}^{-}\) ions in non-buffered solutions.