## 4 Application of Fe-MOF in Advanced Oxidation Processes
### Photo-Based AOPs
As heterogeneous porous catalysts, these MOFs should adsorb and degrade the target pollutants simultaneously or in series, allowing the operation in a continuous flow without catalyst regeneration. In this sense, [PERSON] et al. [169], designed a biochar-MOF hybrid to reduce the particle agglomeration of MIL-53-Fe MOF, increase the mechanical strength and permit the rapid water flow through fixed beds while dispersing tiny attached MOF particles. For that, the biochar was previously decorated with magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles to create a magnetically manipulable adsorbent. They confirmed that this material was able to adsorb and photodegrade a dye model pollutant such as Rhodamine B. The coexistence of three different adsorbing surface phases allowed a broader application of this unique sorbent. The separation and recovery of Fe-MOFs from aqueous media present a challenge in wastewater treatment. This limitation may be overcome by immobilizing a powder catalyst on a carrier. By this method, the catalyst could be effectively separated and recovered, and it is also prevented from aggregating, increasing its catalytic efficiency. [PERSON] et al. [187] immobilized MIL-88A(Fe) on cotton fibers (MC) via an in situ method to synthesize MIL-88A(Fe)/MC and evaluated its ability to degrade several antibiotics such as oxytetracycline, tetracycline and chlortetracycline via photoactivated sulfate radical operating in a fixed bed reactor in batch and continuous mode. In this proposed system, peroxdisulfate (PDS) is considered an appealing oxidant. Its activation by MIL-88A(Fe)/MC and UV light irradiation is necessary for sulphate radical generation due to the low reactivity of PDS (Figure 9). They evaluated the effect of the initial pH on antibiotic degradation efficiency. Thus, at pH values lower than 7 near complete degradation was achieved in 10 min by the sulfate radical generation following Equations (22) and (23). However, at pH values higher than 7, sulfate radicals could be transformed according to Equations (16) and (24), which reduced the concentration of sulphate radicals decreasing the degradation efficiency. In addition, it is important to consider the high stability of MIL-88A(Fe) in acidic conditions [18]. In this context, operation under acidic conditions is favored, so in most cases, it is not necessary to modify the pH of the water to be treated. \[S_{2}O_{8}^{-}+H^{+}\to HS_{2}O_{8}^{-} \tag{22}\]  
\[HS_{2}O_{8}^{-}+e^{-}\to SO_{4}^{-\bullet}+SO_{4}^{2-}+H^{+} \tag{23}\]  
\[SO_{4}^{-\bullet}+OH^{-}\to SO_{4}^{2-}+OH^{\bullet} \tag{24}\]  
Figure 9: Schematic of the proposed mechanism of PDS activation over Fe-MOFs under UV light towards persistent pollutants degradation. In this study, it was demonstrated that MIL-88A(Fe)/MC is extremely stable and reusable by running 30 cycles consecutively, with a well-maintained removal efficiency for each cycle (8 min). These good results were confirmed by the continuous operation of the photoactivated process in a fixed bed reactor with an annular channel and the central cavity was irradiated with a 10 W mercury UV light (UV-Hg) of wavelength 256 nm. Operating at residence times of 10 min, oxytetracycline, tetracycline and chlortetracycline were completely degraded during one day detecting a slight drop in the degradation efficiency at the end hour, showing that one kilogram of MIL-88A(Fe)/MC can treat 23.5 tonnes of polluted water (within a concentration of each antibiotic of 10 mg/L) in one day. MIL-88 AFe was also used to synthesize a photo composite by ball-milling strategy using perylene-34,910-tetracarboxylicimide (PDINH) [188]. The photocatalyst obtained PDINH/MIL-88A(Fe) was tested in the activation of PDS in the degradation of chloroquine phosphate. The comparison studies with individual compounds (MIL-88A and PDINH) showed a synergic effect with degradation levels around 6 times the level obtained using only photocatalyst pristine MIL-88A(Fe).