## 4 Application of Fe-MOF in Advanced Oxidation Processes
### Electrochemical AOP
#### 4.3.2 Photoelectro-Fenton  
This process is a combination of electro-Fenton with radiation (UV or sunlight) and allows to achieve higher production of the hydroxyl radical (Figure 11) [195]. Based on the literature is was confirmed that the UV photons can reduce the \(\equiv\)Fe\({}^{3+}\), either complexed with a carboxylated organic (R-COO\(-\)) via reaction (Equation (28)) or in its hydrated form via photo-Fenton reaction (Equation (29)), thus promoting the continuous \(\equiv\)Fe\({}^{2+}\) regeneration. \[\equiv Fe(OCR)^{2+}+h\
u\rightarrow\equiv Fe^{2+}+CO_{2}+R^{\bullet} \tag{28}\]  
\[\equiv[Fe(OH)]^{2+}+h\
u\rightarrow\equiv Fe^{2+}+OH^{\bullet} \tag{29}\]  
To improve this hybrid process, it is important the selection of a good photoelectric catalysis performance since it determines the additional generation of these radicals [205]. A Fe-O cluster, which is photosensitive, is also an advantage of Fe-MOFs in the photo-electro-Fenton process, as discussed in Section 4.2. The studies of [PERSON] et al. [205] and [PERSON] et al. [206] should be emphasized due to their incorporation of new bimetallic Fe-MOFs within the photo-electro-Fenton processes. Regarding the study of [PERSON] et al. [205], MOF-525-Fe/Zr is synthesized and with this MOF the CF is coated to fabricate the modified cathode, which is called MOF-525-Fe/Zr@CF. When carrying out the characterization study of this material, the main difference between MOF-525 and MOF-525-Fe/Zr is the large increase in pore area and pore volume. In fact, in this study, it was found that most of the pores of both MOFs are mesoporous and over a diameter of 2-10 nm. In this study, these authors evaluated the catalytic capacity of this new material in comparison to four other materials such as CF, MOF-525@CF (which is the pure MOF-525), Fe-MOF-525@CF and Fe\({}_{3}\)O\({}_{4}\)@MOF-525@CF, to check the improvement that the MOF-525-Fe/Zr provides. From this study, it could be concluded that there is an influence on the catalyst preparation as the post-modified Fe-MOF-525@CF showed no significant difference in degradation performance from pure MOF-525 and common CF, both having degradation rates below 50%. As for the Fe\({}_{3}\)O\({}_{4}\)@MOF-525@CF, it showed a slightly worse SMX removal rate (70%) than the MOF-525-Fe/Zr (97.3%). This is due to the Fe\({}_{3}\)O\({}_{4}\)@MOF-525@CF being a post-assembly synthesis, as it can only control MOF growth and failed to achieve further charge transfer beyond the interfacial energy barrier. A drawback of this cathode is that its performance depends on the environmental conditions to which it is subjected as it can only work at pH below 4, with an optimum pH of 3. The main reason for this considerable reduction is that the carboxylic acid groups, which are the connectors between Zr-oxo SBUs and Fe-TCPP, can be partially polarized. Despite this drawback, this modified cathode showed promising results in SMX removal in different wastewaters (tap water and river water) and improves the mineralization rate (85% total organic carbon (TOC) mineralization rate) compared to other works using MOFs such as [PERSON] et al. [207], which using a photo-Fenton process manages to completely remove SMX in 40 min but its mineralization rate is 41%. Similarly, [PERSON] et al. [206] synthesized a bimetallic catalyst, called FeCu@porous carbon (PC). This catalyst was derived from Fe/Cu-MOFs through the pyrolysis process. In order to check the efficiency of the catalyst in the photo-electro-Fenton process, a comparison with the electro-Fenton process and photocatalysis was made to remove SMT. From this study, it was concluded that the FeCu@PC/photo-electro-Fenton system presented the highest removal efficiency after 30 min (96.4%) compared to the FeCu@PC/electro-Fenton (79.3%) and FeCu@PC/UV (30.8%) systems.