# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Transformation-Based Technologies
#### 4.2.2 Chemical Nitrate Reduction
The influence of chloride ions on nitrate reduction by ZVI is controversial, although agreement is found for high concentrations of chloride ions, which negatively affect the reduction process of nitrate on microscopic ZVI [176]. The presence of phosphate ions causes competition with nitrate for active sites of iron, although better affinity of nitrate to iron is observed [176]. The reduction of the particle size of granular Fe\({}^{0}\) (mm) to 1-100 nm, increases its specific surface area and, thus also increases the reaction rates. It is known as nanoscopicFe\({}^{0}\) (nZVI) and can be easily synthesized by the reaction of a ferric salt solution with borohydride [177]:  
\[4\;\mathrm{Fe^{3+}}+3\;\mathrm{BH_{4}}^{-}+9\;\mathrm{H_{2}O}\to 4\;\mathrm{Fe^{0}}+3 \;\mathrm{H_{2}BO_{3}}^{-}+12\;\mathrm{H^{+}}+6\;\mathrm{H_{2}} \tag{9}\]  
A better performance of nZVI is observed at high pH values, compared to microscopic ZVI. A 100% nitrate removal is reported in experiments without pH regulation at a starting pH of 7, in aerobic or anaerobic conditions [178,179]. In either case, acidic conditions enhance the nitrate reduction process. In order to improve the performance of nZVI, and minimise its tendency to agglomerate, the incorporation of different encapsulating substances such as carbon-based materials, clays, or resins has been investigated. A nitrate elimination efficiency of 92% is reported with nZVI encapsulated with carbon nitride. In this laboratory experiment, the selectivity to N\({}_{2}\) reached 97%, with a constant performance at pH from 5 to 11 [180]. Also noteworthy is the 100% removal efficiency and N\({}_{2}\) selectivity close to 80%, without pH control, achieved with nZVI supported on NaY zeolite. [181]. The use of nZVI combined with certain support materials provides excellent nitrate removal efficiency and very good selectivity towards N\({}_{2}\). pH control can be a drawback of this treatment due to the need for additional chemicals to regulate the pH, and to restore neutral pH after treatment. The use of some support materials or the application of ultrasound can overcome this problem. A second major drawback is the release of oxidation products (Fe\({}^{2+}\)) into the treated solution. In addition of ZVI, other zero-valent metals have been studied over time. Among the alternative metals, particularly noteworthy are the studies with aluminium (ZVA) and magnesium (SVM). ZVA shows a higher reactivity than ZVI due to the more negative potential of aluminium (E\({}_{\mathrm{Al}}\)\({}^{0}=-1.662\) V and E\({}_{\mathrm{Fe}}\)\({}^{0}=-0.440\) V), although high pH values are needed to operate what may produce aluminium hydroxide precipitates in treated water according to reactions of Equations (10)-(12) [167]:  
\[3\;\mathrm{NO_{3}}^{-}+2\;\mathrm{Al}^{0}+3\;\mathrm{H_{2}O}\to 3\;\mathrm{NO_{2}}^{-}+2 \;\mathrm{Al(OH)_{3}}\;\mathrm{(s)} \tag{10}\]  
\[\mathrm{NO_{2}}^{-}+2\;\mathrm{Al}^{0}+5\;\mathrm{H_{2}O}\to\mathrm{NH_{3}}+2 \;\mathrm{Al(OH)_{3}}\;\mathrm{(s)}+\mathrm{OH}^{-} \tag{11}\]  
\[\mathrm{Fe}^{0}+\mathrm{NO}_{3}{}^{-}+2\;\mathrm{H}^{+}\rightarrow\mathrm{Fe }^{2+}+\mathrm{NO}_{2}{}^{-}+\mathrm{H}_{2}\mathrm{O} \tag{3}\]0  
The reduction potential (E\({}_{\mathrm{Mg}}\)\({}^{0}=-2.363\) V) and reactivity of zero-valent Mg (ZVM) are quite high and its use is most favourable, as Mg\({}^{2+}\) lacks the precipitation problems of its hydroxides and does not have any environmental risk [167]. * Catalytic reduction of nitrate over bimetallic catalysts with H\({}_{2}\)  
The use of a coating metal (mainly Cu\({}^{0}\)) on the particles of nZVI was tested to improve nitrate removal and N\({}_{2}\) selectivity. In this system, Cu acts as promoter metal, being the catalyst for the reaction with nitrate, while Fe\({}^{0}\) is the reductant source, restoring Cu\({}^{2+}\) to Cu\({}^{0}\). When adding H\({}_{2}\) to this reaction system, no improvement in nitrate reduction is observed, due to the poor adsorption of H\({}_{2}\) on the surface of Cu.