# 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 incorporation of a noble metal (Pt, Pd, Au, or Rh) to this catalyst, in the presence of H\({}_{2}\) as a reducing agent, shows an enhanced effect on the nitrate reduction performance and the selectivity to N\({}_{2}\), as a result of the better fixation of H\({}_{2}\) on the noble metal sites [169,182]. The general structure of bimetallic catalysts (Figure 7) involves two different metals (a transition metal and a noble metal) on a support material that can be either active, with catalytic reducing properties (ZVI), or passive (zeolite, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), activated carbon (AC), carbon nanotubes (CNT), etc.). Considering the noble metal, Pd showed the highest activation capacity for H\({}_{2}\)[183]. The transition metals acting as promoters, with the most efficient performance were In, Cu, and Sn. Investigations analysing three bimetallic catalysts with nano-crystalline beta-zeolite (NBeta) as support material (Sn-Pd/NBeta, Cu-Pd/NBeta, and In-Pd/NBeta) concluded that all of them showed high reduction efficiency (>95%) and high N\({}_{2}\) selectivity (>80%). The highest reduction rate was observed for Sn-Pd/NBeta, while Cu-Pd/NBeta exhibited the greatest selectivity towards N\({}_{2}\) (92.7%). When Cu and Sn metals were analysed after the reaction, it was observed that Sn showed a higher rejuvenation ability than Cu, with the Sn surface showing a higher affinity to nitrate [169]. In any case, these results are not extensive for all catalysts, due to synergistic effects in the reduction process between promoter metal and support material. Regarding the support material, many researchers have compared the removal performance and N\({}_{2}\) selectivity of different materials [170]. Alumina is the most widely studied support. Other metal oxides studied were TiO\({}_{2}\), ZrO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\). Carbon-based material, such as AC and CNT, zeolites (NBeta, NZSM-5, NaY), SiO\({}_{2}\), and ZVI-based materials were also investigated [184,185]. The physical properties of these support materials, such as specific surface area, morphology, and porosity, are crucial in the denitrification process. Support materials with higher reactive surface area and porosity, such as \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)-dyatomita and zeolites, showed high catalytic effect and high selectivity towards N\({}_{2}\) due to a good dispersion of the metal particles on their surfaces. Supports based on ZVI showed high reduction rates, but a high selectivity towards NH\({}_{4}\)\({}^{+}\)[183,186,187]. To maximize the process efficiency, several factors need to be taken into account, including: (a) the effect of the loading of the noble and promoter metals; (b) the effect of the loading of catalyst and NO\({}_{3}\)\({}^{-}\) concentration; (c) the effect of pH and buffer system; and (d) the effect of the H\({}_{2}\) flow rate in the system [185]. The deactivation of the catalyst is one of the main drawbacks of this technology. Factors such as catalyst passivation, leaching of the metal from its support, and aggregation of metal particles after several cycles of treatment affect the life of the catalyst. Results of the leaching test for bimetallic catalysts showed that zeolites and ZVI-based supports are less prone to leaching than metal oxides and carbon-based supports [172]. Moreover, metal leaching was found to have pH dependence. Most of the research about the catalytic reduction of nitrates was carried out in batch reactors. Experiments in packed beds or trickle-bed reactors present the drawbacks of partial loss of the catalyst, as it is swept away by the flow rate. To overcome this problem, the use of catalytic membrane reactors (CMRs) was implemented. Commercial ceramic membranes, usually alumina nanotubes of a varied porosity, are used to support the bimetallic catalyst. The determinant influence of the membrane pore size on the catalyst performance was verified. Membranes with a smaller pore size (5 nm) achieved up to 92% NO\({}_{3}\)\({}^{-}\) conversion and 80% N\({}_{2}\) selectivity [188].