## 4 Application of Fe-MOF in Advanced Oxidation Processes
### Catalytic AOP
The molar ratio of Fe/Cu in the catalyst was adjusted in the function of the ratio of the two metallic precursors, obtaining several bimetallic MOFs with a different initial molar ratio of Fe/Cu. The best SMX degradation was obtained using a ratio Fe/Cu of 0.75/0.25 at pH around 5.6. This catalyst overcame one of the limitations of the Fenton reaction that requests acid conditions. The results clearly show that this bimetallic MOF was effective at a wide range of pH from 4 to 8.6. As is shown in Figure 8 the presence of both metallic species on the catalyst surface served as active sites for H\({}_{2}\)O\({}_{2}\) activation and also to the generation of hydroxyl radicals increasing the efficiency of SMX degradation. By the use of this catalyst, the anchored Fe\({}^{2+}\) (\(\equiv\)Fe\({}^{2+}\)) to the Fe\({}_{x}\)Cu\({}_{1-x}\)(BDC) surface followed the reaction shown in Figure 8 and the lost electron to activate H\({}_{2}\)O\({}_{2}\) was captured by \(\equiv\)Fe\({}^{3+}\) to form HO\({}_{2}\bullet\) and regenerate Fe\({}^{2+}\) (Figure 8). Similarly, the Cu active sites also catalyze the decomposition of H\({}_{2}\)O\({}_{2}\) and the same radicals were generated with the regeneration of \(\equiv\)Cu\({}^{2+}\) to \(\equiv\)Cu\({}^{+}\). Thus, considering the standard reduction potentials of Cu (0.17 V) and Fe (0.77 V), part Cu\({}^{+}\) generated on the Fe\({}_{x}\)Cu\({}_{1-x}\)(BDC) surface could promote the regeneration of Fe\({}^{2+}\) by a thermodynamically favorable electron transfer process (Figure 8), that increased the \(\equiv\)Fe\({}^{2+}\) on the catalyst surface. Recently, [PERSON] et al. [162] explored the introduction of -NH\({}_{2}\) groups as electron-rich donors to promote the cycling of \(\equiv\)Fe\({}^{3+}\) to \(\equiv\)Fe\({}^{2+}\). They determined the reduction in the activation energy barrier in the dissociation of H\({}_{2}\)O\({}_{2}\) adsorbed on the surface of Fe-BDC-NH\({}_{2}\) in relation to Fe-BDC due to the amino groups providing more electrons for hydrogen peroxide activation. The generation of hydroxyl radicals was also increased due to the direct injection of internal electrons (Equations (4)-(6)) that favor the reduction of \(\equiv\)Fe  
Figure 8: Graphical scheme of the reactions occurring on the Fe\({}_{x}\)Cu\({}_{1-x}\)(BDC) surface for SMX degradation. generating more radicals (OH**\(\bullet\)** and O**\(\bullet^{-}\)**) that attacked the target pollutants until the total mineralization. \[H_{2}O_{2}\ \stackrel{{-NH_{2}}}{{\rightarrow}}\ e^{-}+HO_{2}^{ \bullet}+H^{+} \tag{4}\]  
\[HO_{2}^{\bullet}\rightarrow\mathcal{O}_{2}^{-\bullet}+H^{+} \tag{5}\]  
\[\equiv\ Fe^{3+}+e^{-}\ \stackrel{{-NH_{2}}}{{\rightarrow}}\equiv\ Fe^{2+} \tag{6}\]  
The electron-donating properties of Fe-MOFs are likely to make them suitable for PS activation through Fenton-like reaction pathways. In Table 6, several studies in which Fe-MOFs were used as persulfate activators for the degradation of several pollutants are summarized. [PERSON] et al. [163] evaluated the SMX degradation by different Fe-MOFs with cyclopentadienyl iron(II) dicarbonyl dimer [Fe(Cp)(CO)\({}_{2}\)]\({}_{2}\) as an iron precursor. In this study, different carboxyl group-containing pyridine compounds were used as organic ligands: Nicotinic acid (HNic), chinchomeronic acid (Py-3,4-H\({}_{2}\)BDC), and 5-(pyridine-4-yl) isophthal acid (H\({}_{2}\)PIP) obtaining three MOFs named as Fe(Nic), Fe(PyBDC) and Fe(PIP), respectively, by hydrothermal self-assembled coordination synthesis. They mentioned that the activation process was due to the reaction on the Fe-MOF surface (\(\equiv\)Fe\({}^{2+}/\equiv\)Fe\({}^{3+}\)) and iron soluble form in water, with the heterogeneous process the main responsible for the activation reactions. Thus, it was detected that the Fe\({}^{2+}/\)Fe\({}^{3+}\) ratio of Fe(Nic) and Fe(PyBDC) decreased after activation, which indicates that \(\equiv\)Fe\({}^{2+}\) acted as the electron donor for PS decomposition with \(\equiv\)Fe\({}^{3+}\) generation reducing the surface activate sites (Equation (7)).