# 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  
Chemical nitrate reduction proceeds through sequential reactions in which NO3\({}^{-}\) is reduced to NO2\({}^{-}\), which is transformed to NH4\({}^{+}\), as an undesirable co-product, and to harmful N2 as the target product. Different metals, in particular Fe and Al, but also Cu, Pd, and Rh, have been applied as catalysts for this chemical nitrate reduction [142]. Compared to biological denitrification, catalytic denitrification exhibits several advantages, such as being more flexible in operation, less space requirements, easy automatic control,no start-up time, and no odour problems [165]. Compared to electrochemical reduction, the main advantage is its low energy requirement, and it is a technology particularly suitable for the purification of waterbodies with low organic matter content [166]. Chemical denitrification of nitrates by means of zero-valent metals has attracted a great deal of interest, with a special focus on zero-valent iron (ZVI) [167, 168, 142, 166]. Relevant investigations on the use of bimetallic catalysts, using H\({}_{2}\) as reductant agent, are also significant [169, 170, 171, 172]. * Removal of nitrate by zero-valent iron (ZVI)  
The application of zero-valent metals (mainly iron) for the reduction of nitrates in water presents many practical benefits, such as simple handling, high efficiency, and low cost. Different reactions occur during the reduction of nitrate by Fe\({}^{0}\). In fact, nitrate can be transformed directly to ammonium:  
\[4\;\mathrm{Fe}^{0}+\mathrm{NO}_{3}{}^{-}+10\;\mathrm{H}^{+}\to 4\;\mathrm{Fe}^{2+}+ \mathrm{NH}_{4}{}^{+}+3\;\mathrm{H}_{2}\mathrm{O} \tag{2}\]  
or to nitrite in a first step, with subsequent reduction to N\({}_{2}\) or NH\({}_{4}{}^{+}\):  
\[\mathrm{Fe}^{0}+\mathrm{NO}_{3}{}^{-}+2\;\mathrm{H}^{+}\rightarrow\mathrm{Fe }^{2+}+\mathrm{NO}_{2}{}^{-}+\mathrm{H}_{2}\mathrm{O} \tag{3}\]  
\[3\;\mathrm{Fe}^{0}+\mathrm{NO}_{2}{}^{-}+8\;\mathrm{H}^{+}\to 3\;\mathrm{Fe }^{2+}+\mathrm{NH}_{4}{}^{+}+2\;\mathrm{H}_{2}\mathrm{O} \tag{4}\]  
The reduction agent is Fe\({}^{0}\), although the production of H\({}_{2}\) by corrosion of Fe\({}^{0}\) in acidic media can also contribute to the reduction of nitrate:  
\[2\;\mathrm{H}^{+}+\mathrm{Fe}^{0}\rightarrow\mathrm{H}_{2}+\mathrm{Fe}^{2+} \tag{5}\]  
\[\mathrm{NO}_{3}{}^{-}+4\;\mathrm{H}_{2}+2\;\mathrm{H}^{+}\rightarrow\mathrm{NH }_{4}{}^{+}+3\;\mathrm{H}_{2}\mathrm{O} \tag{6}\]  
The reduction directly to N\({}_{2}\) is also possible, with the formation of OH\({}^{-}\), the accumulation of which can be prevented by pH regulation. \[5\;\mathrm{Fe}^{0}+2\;\mathrm{NO}_{3}{}^{-}+6\;\mathrm{H}_{2}\mathrm{O} \to 5\;\mathrm{Fe}^{2+}+\mathrm{N}_{2}+12\;\mathrm{OH}^{-} \tag{7}\]  
\[10\;\mathrm{Fe}^{0}+6\;\mathrm{NO}_{3}{}^{-}+3\;\mathrm{H}_{2}\mathrm{O} \to 5\;\mathrm{Fe}_{2}\mathrm{O}_{3}+3\;\mathrm{N}_{2}+6\;\mathrm{OH}^{-} \tag{8}\]  
The reduction of nitrate by Fe\({}^{0}\) leads to pH increase. The neutralisation of the OH\({}^{-}\) produced in the reaction will lead to higher yields in the process; hence, pH control is one of the key factors. [173]. For experiments using microscopic iron, it is observed that nitrate removal efficiencies of 100% are only obtained at low pH values [166]. The usual method of pH regulation is the addition of HCl or acetic acid or the bubbling of CO\({}_{2}\). An oxidation product can appear on the surface of Fe particles in aerobic conditions. It can be black (magnetic, Fe\({}_{3}\)O\({}_{4}\)) or brown (lepidocrocite, FeO(OH)). The presence of lepidocrocite can slow down the nitrate reduction because the transition of electrons is easier in magnetite. [168]. The reduction of nitrates with ZVI can be enhanced through the pre-treatment of Fe particles and with increasing temperatures [174]. Several mechanisms are involved in the nitrate reduction on FeO, such as diffusion, adsorption, and chemical reduction, the latter being the controlling step of the process [175].