# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Transformation-Based Technologies
#### 4.2.4 Photocatalytic Denitrification  
Heterogeneous photocatalysis has received increasing attention in recent years as an interesting alternative to conventionally applied technologies for the treatment of polluted water. In this context, photocatalytic reduction is identified as a suitable technology for the denitrification of waterbodies [209; 210]. It was first reported in 1998 by [PERSON] and [PERSON] [211]. The process starts with the absorption of radiation of a certain wavelength, which excites a semiconductor that functions as a photocatalyst. Irradiation induces the formation of electrons (ec\({}_{\rm CB}\)\({}^{-}\)) in the conduction band and holes (h\({}_{\rm VB}\)\({}^{+}\)) in the valence band (electron-hole pairs) in the semiconductor, which can migrate to its surface, generating highly reactive radicals capable of degrading the pollutants in the medium. The minimum energy to excite an electron from the valence band to the conduction band is known as band gap energy. The incident photon must reach or exceed this energy to generate the (ec\({}_{\rm CB}\)\({}^{-}\))/(h\({}_{\rm VB}\)\({}^{+}\)) pair [210]. The light-driven excited states of the photocatalyst enable the transformation of NO\({}_{3}\)\({}^{-}\) to N\({}_{2}\) gas. The photoreaction of nitrate ions occurs by reaction with electrons ec\({}_{\rm CB}\)\({}^{-}\) or by reaction with CO\({}_{2}\)\({}^{\bullet-}\) radicals generated by the interaction of hole scavengers (formic acid, methanol) with the holes (h\({}_{\rm VB}\)\({}^{+}\)) in the valence band (Figure 10) [212; 213]. The photocatalytic activity and selectivity are affected by the nature of the photocatalyst and hole scavengers used in the process [214]. Titanium dioxide (TiO\({}_{2}\)) is the classic photocatalyst on account of its reduced cost, excellent stability to chemical corrosion, catalytic activity in the whole pH range, harmless behaviour towards humans, and efficiency to avoid the production of NO\({}_{2}\)\({}^{-}\) and NH\({}_{4}\)\({}^{+}\) as co-products [215]. However, its broad band gap energy (3.2 eV) limits its absorption of solar radiation to the UV light range (\(\lambda<\) 390 nm),which only represents about 5% of the solar spectrum. This material also shows a high regeneration rate of electron-hole pairs that restricts its photocatalytic activity. Alternative materials are investigated to overcome these drawbacks; essentially, materials that can be activated in the region of the visible range make the best use of the solar spectrum. In this respect, it has been proven that the addition of Ag nanoparticles improved the catalytic activity of TiO\({}_{2}\) to reduce NO\({}_{3}\)\({}^{-}\), due to the existence of Ag-TiO\({}_{2}\) junctions on the catalyst surface that increased its photoactivity [214]. Both monometallic (Pd, Ag, Cu) and bimetallic (Pd/Ag, Pd/Cu) photocatalysts were investigated for the removal of nitrate under solar irradiation, assessing the effect of different scavengers as reducing agents (acetic acid, formic acid, ammonium oxalate, and oxalic acid) on the efficiency of the denitrification process [216]. To improve the activity of bimetallic catalysts, different support materials such as hydrotalcite, acrylic resins, ZrO\({}_{2}\) or NaTaO\({}_{3}\) nanoparticles are incorporated [217]. In recent years, bismuth-based photocatalysts have also exhibited outstanding performance in efficient nitrate reduction via visible-light photochemical reactions [218]. Photocatalytic denitrification frequently leads to the formation of non-desired NO\({}_{2}\)\({}^{-}\) or NH\({}_{4}\)\({}^{+}\) instead of up to N\({}_{2}\), which is linked to intrinsic characteristics of the catalysts, mainly a high valence band potential. Finally, the global yield is affected by electron-hole recombination, as is common in other photocatalytic reactions [213].