## 4 Application of Fe-MOF in Advanced Oxidation Processes
### Photo-Based AOPs  
The hydroxyl radicals could be generated by means of radiation, such as ultraviolet light (UV) or sunlight. Catalysts, also known as photocatalysts, are normally used to encourage this photochemical reaction to take place [165]. As a semiconductor material, it is defined by two separate energy bands: the highest energy band with electrons and the lowest without electrons, known as the valence and conduction bands, respectively [166]. The more common photocatalyst listed in the databases is TiO\({}_{2}\) (alone and modified), due to its great advantages such as low cost, low level of toxicity and thermal stability [165]. However, in recent years, other photocatalysts such as MOFs have been synthesized with similar results. In 2007, the [PERSON] group [167] evaluated the photocatalytic activity of MOF-5 (Zn\({}_{4}\)O(BDC)\({}_{3}\)) for phenol degradation under UV light irradiation, confirming the low stability of this photocatalyst which decomposed gradually in air or water. From this study, several MOFs with good photocatalytic activities were synthesized and tested in several degradation processes [168]. Due to their magnificent photocatalytic properties, some MOFs were compared with the most widely used semiconducting materials to date (TiO\({}_{2}\)) [169]. Among these MOFs, Fe-MOFs stand out for their high degradation power as photocatalysts, partly because of their suitable band gap and stability. Compared to MOFs with wide bandgaps, Fe-MOFs are highly attractive. As visible light can directly excite large Fe-O clusters, solar energy can be utilized more efficiently. In addition, they are low-cost and non-toxic catalysts, which make them suitable catalysts for the purification of polluted water [170]. Table 6, shows several examples of modified Fe-MOFs in which the photo-Fenton process has been carried out to degrade a great variety of organic pollutants. The photochemical reactions that occur when iron is used as a transition metal for catalysis are shown below (Equations (17)-(19)) [154]. In the photo-Fenton process, the hydrogen peroxide functions in two ways: by capturing the electrons generated from Fe-MOF (Equation (17)) to produce the hydroxyl radicals (Equation (18)) or by Fenton reaction and to promote the cycling of \(\equiv\)Fe\({}^{3+}\) to \(\equiv\)Fe\({}^{2+}\) in the Fe-MOF (Equations (20) and (21)). \[Fe-MOF+h\
u\to h^{+}+e^{-} \tag{17}\]  
\[H_{2}O_{2}+e^{-}\to OH^{\bullet}+OH^{-} \tag{18}\]  
\[H_{2}O_{2}+OH^{\bullet}\to H_{2}O+HO_{2}^{\bullet} \tag{19}\]  
\[\equiv Fe^{3+}+H_{2}O_{2}\ +h\
u\rightarrow\equiv Fe^{2+}+HO_{2}^{\bullet}+H^{+} \tag{20}\]  
\[\equiv Fe^{2+}+H_{2}O_{2}+h\
u\rightarrow\equiv Fe^{3+}+OH^{\bullet}+OH^{-} \tag{21}\]  
Over these years, several limitations were detected such as the same MOFs exhibiting only photocatalytic activity under UV light radiation, poor conductivity or stability. To overcome these limitations, several strategies have been developed by the inclusion of amino groups and the synthesis of Fe-MOFs and heterostructures to utilize the visible light or sunlight. In general, when compared to the parent MOF structures, Fe-MOF composites provide a great improvement in catalytic performance. Thus, by the inclusion of metal oxides such as ZnO nanoparticles to MIL-100-Fe, [PERSON] et al. [171] determined that the load of ZnO reduced the electron-hole recombination and enhanced the photodegradation of phenol, bisphenol A and atrazine. Thus, by the inclusion of metal oxides such as ZnO nanoparticles to MIL-100-Fe, [PERSON] et al. [171] determined that the load of ZnO reduced the electron-hole recombination and enhanced the photodegradation of phenol, bisphenol A and atrazine. An effective photocatalyst was also obtained through the combination of a semiconductor such as WO\({}_{3}\) with the photosensitive MOF (MIL-53 Fe). It was demonstrated that this material acted as a catalyst for the photo-reduction of Cr(VI) and the photo-oxidation of a known organochlorinated herbicide, 2,4-dichlorophenoxyayacetic acid (2,4-D).