Treatment of Oily Wastewater by the Optimization of Fe2O3 Calcination Temperatures in Innovative Bio-Electron-Fenton Microbial Fuel Cells

Due to the fact that Iron oxide (Fe2O3) is known to have a good effect on the photochemical reaction of catalysts, an investigation in this study into the enhancement of the degradation performance of bio-electro-Fenton microbial fuel cells (Bio-E-Fenton MFCs) was carried out using three photocatalytic cathodes. These cathodes were produced at different calcination temperatures of Fe2O3 ranging from 500 ◦C to 900 ◦C for realizing their performance as photo catalysts within the cathodic chamber of an MFC, and they were compared for their ability to degrade oily wastewater. Results show that a suitable temperature for the calcination of iron oxide would have a significantly positive effect on the performance of Bio-E-Fenton MFCs. An optimal calcination temperature of 500 ◦C for Fe2O3 in the electrode material of the cathode was observed to produce a maximum power density of 52.5 mW/m2 and a chemical oxygen demand (COD) degradation rate of oily wastewater (catholyte) of 99.3% within one hour of operation. These novel findings will be useful for the improvement of the performance and applications of Bio-E-Fenton MFCs and their future applications in the field of wastewater treatment.


Introduction
The Bio-Electro-Fenton Microbial Fuel Cells (Bio-E-Fenton MFCs) is a new framework and has been operated extensively because of its simultaneous wastewater treatment and power generation capability. Significantly, the electro-Fenton reaction consists of iron ions (Fe 2+ ) and hydrogen peroxide, and has become a useful technology for treating organic pollutants in wastewater [1][2][3][4][5][6][7][8][9][10] as hydroxyl radicals (•OH) are generated in abundance within the electro-Fenton reaction [11]. Considering the working principle of the cathode in Bio-E-Fenton MFCs because the photo catalysis of Fe2O3 can also produce hydroxyl radicals [11]. It can further combine with FeSO4 to act as an iron source with H2O2 for producing more hydroxyl radicals (•OH), resulting in a higher degradation rate within a short time. In addition, a pH of 3 was maintained at the cathode chamber to treat refractory organic wastewater and this condition will not affect the pH of the anode chamber.

Reactor Construction and Operation
An acrylic dual chambered tank with dimensions of 85 mm × 70 mm × 55 mm was selected for the construction of the Bio-E-Fenton MFC, as shown in Figure 2. Each chamber had a total volume of 200 mL with a proton exchange membrane Nafion-117 (80 mm × 70 mm) and carbon felt (CF, 60 mm × 60 mm × 5 mm) as the anode and cathode electrodes, respectively. The preparation method for the carbon felt is as follows: It was initially washed in hot H2O2 (10%, 90 °C) solution for 3 h to develop local Quinone sites on the carbon surface for improvement of the anode biocompatibility and increased quantity of anthraquinone [29]. In order to prevent the thermal heat transfer from the cathode to the anode, a Bakelite plate with dimensions of 100 mm × 50 mm × 20 mm ( Figure 2) was employed in this study. Therefore, the power density would be influenced by the effect of the calcination temperature of Fe2O3 in the cathode chamber, but not by the temperature of the anode. In addition, a control reactor without Fe2O3 was not constructed and it was considered from the similar work of [30]. This research study has been carried out for gaining an in-depth understanding of the effects of calcination temperatures (500 • C to 900 • C) on the performance of Bio-E-Fenton MFCs. Based on the calcination temperature of Fe 2 O 3 , the photo catalyst structure will change and can be applied in this study for investigating the performance of MFCs. The results showed that the intensity of Fe 2 O 3 will not vary greatly due to high calcination temperatures, but the crystalline particle size and surface morphology will be changed immensely [24]. This feature of Fe 2 O 3 will enhance the COD removal efficiency of the MFC significantly [25][26][27][28]. Fe 2 O 3 /carbon felt (CF) has been selected to act as part of the cathode in Bio-E-Fenton MFCs because the photo catalysis of Fe 2 O 3 can also produce hydroxyl radicals [11]. It can further combine with FeSO 4 to act as an iron source with H 2 O 2 for producing more hydroxyl radicals (•OH), resulting in a higher degradation rate within a short time. In addition, a pH of 3 was maintained at the cathode chamber to treat refractory organic wastewater and this condition will not affect the pH of the anode chamber.

Reactor Construction and Operation
An acrylic dual chambered tank with dimensions of 85 mm × 70 mm × 55 mm was selected for the construction of the Bio-E-Fenton MFC, as shown in Figure 2. Each chamber had a total volume of 200 mL with a proton exchange membrane Nafion-117 (80 mm × 70 mm) and carbon felt (CF, 60 mm × 60 mm × 5 mm) as the anode and cathode electrodes, respectively. The preparation method for the carbon felt is as follows: It was initially washed in hot H 2 O 2 (10%, 90 • C) solution for 3 h to develop local Quinone sites on the carbon surface for improvement of the anode biocompatibility and increased quantity of anthraquinone [29]. In order to prevent the thermal heat transfer from the cathode to the anode, a Bakelite plate with dimensions of 100 mm × 50 mm × 20 mm (Figure 2) was employed in this study. Therefore, the power density would be influenced by the effect of the calcination temperature of Fe 2 O 3 in the cathode chamber, but not by the temperature of the anode. In addition, a control reactor without Fe 2 O 3 was not constructed and it was considered from the similar work of [30].

Anolyte/Catholyte and Fe2O3 Catalyst Preparation
Kim et al. [31] proved that lactic acid wastewater can be employed to sustain the production of electrical energy in MFCs. Therefore, dairy wastewater was used in this study as the anolyte. During the early stage of fermentation of the dairy wastewater in the chamber, precipitate particulates were formed. Three layers were naturally formed when the wastewater was kept stagnant and were classified as top-level supernatant (clear fluid), a mid-level interface, and a bottom-level precipitate. Prior to the study, a conductivity experiment confirmed that the top-level supernatant in the chamber had the best conductivity of all, with an open-circuit voltage of 0.70 V, limiting current of 0.547 mA/m 2 , and an achievable maximum power density of 101.4 mW/m 2 . In addition, considering the fact that viscosity would influence microbial activity and also affect the power generation of MFCs [32], samples from the top-level supernatant were selected for further studies.
The water bodies [33] have long been contaminated by oil pollution [34][35][36], so artificially prepared oily wastewater was used as the catholyte in this study. A total of 1 mL of diesel was added to 1 L of water and heated in a magnetic heater and blended with a blender for 1 day at 50 °C. As diesel was immiscible in water, 10 g of emulsifying (Tween 80) agent was added to aid the dissolution of the diesel in water. Molognoni et al. [37] assessed the bio-electrochemical treatability of industrial (dairy) wastewater by MFCs, and an MFC was built and continuously operated for 72 days, during which time the anodic chamber was fed with dairy wastewater and the cathodic chamber with an aerated mineral solution. The study demonstrated that industrial effluents from agrifood facilities can be treated by bio-electrochemical systems (BESs) with >85% (average) organic matter removal. The study by Marashi and Kariminia [38] proved that the wastewater concentration had an influence on the MFC performance and using the raw wastewater with the concentration of 8000 mg COD L −1 resulted in the highest power density (65.6 mW m −2 ) production. Kim et al. [39] reported that the MFC-Anaerobic fluidized membrane bioreactors (AFMBRs) achieved 89 ± 3% removal of the COD with an effluent of 36 ± 6 mg-COD/L over 112 days of operation.
A total of 270 mg of FeCl3 (98%, Acros, Taipei, Taiwan) was dissolved in 30 mL of deionized water, and then 1 M of NaOH (98%, Fisher, Hampton, VA, USA) 1 mL and 0.5 M oxalic acid (98%, Acros) 750 μL were added to the Teflon tank, and the microwave was set up at 160 °C for 30 min. After that, the Fe2O3 sample solution was filtered through a membrane filter with a pore size of 0.2 μm (Advantec, Toyo, Japan) and mixed with the 80 °C deionized water and Fe2O3 powder (by freezedrying at −50 °C) under a vacuum for 5 h. Calcination of Fe2O3 was achieved by loading it into a fused aluminum oxide boat and the temperatures were set to 500 °C, 700 °C, and 900 °C for 30 min per/sample in a high temperature furnace. Following the calcination, the furnace was cooled to room temperature by natural convection.

Anolyte/Catholyte and Fe 2 O 3 Catalyst Preparation
Kim et al. [31] proved that lactic acid wastewater can be employed to sustain the production of electrical energy in MFCs. Therefore, dairy wastewater was used in this study as the anolyte. During the early stage of fermentation of the dairy wastewater in the chamber, precipitate particulates were formed. Three layers were naturally formed when the wastewater was kept stagnant and were classified as top-level supernatant (clear fluid), a mid-level interface, and a bottom-level precipitate. Prior to the study, a conductivity experiment confirmed that the top-level supernatant in the chamber had the best conductivity of all, with an open-circuit voltage of 0.70 V, limiting current of 0.547 mA/m 2 , and an achievable maximum power density of 101.4 mW/m 2 . In addition, considering the fact that viscosity would influence microbial activity and also affect the power generation of MFCs [32], samples from the top-level supernatant were selected for further studies.
The water bodies [33] have long been contaminated by oil pollution [34][35][36], so artificially prepared oily wastewater was used as the catholyte in this study. A total of 1 mL of diesel was added to 1 L of water and heated in a magnetic heater and blended with a blender for 1 day at 50 • C. As diesel was immiscible in water, 10 g of emulsifying (Tween 80) agent was added to aid the dissolution of the diesel in water. Molognoni et al. [37] assessed the bio-electrochemical treatability of industrial (dairy) wastewater by MFCs, and an MFC was built and continuously operated for 72 days, during which time the anodic chamber was fed with dairy wastewater and the cathodic chamber with an aerated mineral solution. The study demonstrated that industrial effluents from agrifood facilities can be treated by bio-electrochemical systems (BESs) with >85% (average) organic matter removal. The study by Marashi and Kariminia [38] proved that the wastewater concentration had an influence on the MFC performance and using the raw wastewater with the concentration of 8000 mg COD L −1 resulted in the highest power density (65.6 mW m −2 ) production. Kim et al. [39] reported that the MFC-Anaerobic fluidized membrane bioreactors (AFMBRs) achieved 89 ± 3% removal of the COD with an effluent of 36 ± 6 mg-COD/L over 112 days of operation.
A total of 270 mg of FeCl 3 (98%, Acros, Taipei, Taiwan) was dissolved in 30 mL of deionized water, and then 1 M of NaOH (98%, Fisher, Hampton, VA, USA) 1 mL and 0.5 M oxalic acid (98%, Acros) 750 µL were added to the Teflon tank, and the microwave was set up at 160 • C for 30 min. After that, the Fe 2 O 3 sample solution was filtered through a membrane filter with a pore size of 0.2 µm (Advantec, Toyo, Japan) and mixed with the 80 • C deionized water and Fe 2 O 3 powder (by freeze-drying at −50 • C) under a vacuum for 5 h. Calcination of Fe 2 O 3 was achieved by loading it into a fused aluminum oxide boat and the temperatures were set to 500 • C, 700 • C, and 900 • C for 30 min per/sample in a high temperature furnace. Following the calcination, the furnace was cooled to room temperature by natural convection.

Experimental Analysis
Electrochemical analysis was performed by the workstation (Jiehan ECW-5600, Taipei, Taiwan) to measure the polarization performance of the Bio-E-Fenton MFCs. For COD analysis, an instrument (SUNTEX-V2000 photometer, Taipei, Taiwan) was utilized with solutions diluted till 100× with deionized water. The temperature for the calcination of Fe 2 O 3 (500 • C-900 • C) was applied using high temperature furnaces (Riki JH-2, Taipei, Taiwan). The pH was measured in the anode/cathode chambers by using a pH meter (SUNTEX SP-2300, Taipei, Taiwan) and dissolved oxygen (DO) by using the DO analyzer (CLEAN DO-200, Taipei, Taiwan). For material analysis, instruments like an X-ray diffractometer (Bruker D2 Phaser), field emission scanning electron microscope (FE-SEM) (JSM-6500F), and Field Emission Gun Transmission Electron Microscopy, FEG-TEM (FEI Tecnai™ G2 F-20 S-TWIN), were used.

Performance of the Fe 2 O 3 -C500 • C/CF in Bio-E-Fenton MFCs Power Generation
Polarization curves for the Bio-E-Fenton MFCs using Fe 2 O 3 calcination from 500 • C-900 • C are portrayed in Figure 3. The conditions adopted for the polarization test were listed as follows; two-electrode measurement, anode electrode connected (by copper wire) to the working electrode (WE); reference electrode 2 (RE2) was in the dairy wastewater solution; the cathode electrode was connected (by copper wire) to the counter electrode (CE) and reference electrode 1 (RE1) in oily wastewater solution. Here, the carbon felt (60 mm × 60 mm × 5 mm) was selected as the electrode and can be used to calculate the power density (power/working area of electrode) and current density (current/working area of electrode). It was clear that Fe 2 O 3 -500 • C /CF produced an open circuit voltage of 0.55 V, a maximum current density of 349 mA/m 2 , and a maximum power density of 52.5 mW/m 2 , which was 2.6 fold higher compared to Fe 2 O 3 -900 • C. A comparison between previous studies of Bio-E-Fenton MFCs and this study has been made and the results are shown in Table 1. In addition, an original report related to TiO 2 has been addressed for showing its optoelectronic ability to generate hydrogen in water [40]. Table 2 shows the list of related cases of photo catalysts.

Experimental Analysis
Electrochemical analysis was performed by the workstation (Jiehan ECW-5600, Taipei, Taiwan) to measure the polarization performance of the Bio-E-Fenton MFCs. For COD analysis, an instrument (SUNTEX-V2000 photometer, Taipei, Taiwan) was utilized with solutions diluted till 100× with deionized water. The temperature for the calcination of Fe2O3 (500 °C-900 °C) was applied using high temperature furnaces (Riki JH-2, Taipei, Taiwan). The pH was measured in the anode/cathode chambers by using a pH meter (SUNTEX SP-2300, Taipei, Taiwan) and dissolved oxygen (DO) by using the DO analyzer (CLEAN DO-200, Taipei, Taiwan). For material analysis, instruments like an X-ray diffractometer (Bruker D2 Phaser), field emission scanning electron microscope (FE-SEM) (JSM-6500F), and Field Emission Gun Transmission Electron Microscopy, FEG-TEM (FEI Tecnai™ G2 F-20 S-TWIN), were used.

Performance of the Fe2O3-C500 °C/CF in Bio-E-Fenton MFCs Power Generation
Polarization curves for the Bio-E-Fenton MFCs using Fe2O3 calcination from 500 °C-900 °C are portrayed in Figure 3. The conditions adopted for the polarization test were listed as follows; twoelectrode measurement, anode electrode connected (by copper wire) to the working electrode (WE); reference electrode 2 (RE2) was in the dairy wastewater solution; the cathode electrode was connected (by copper wire) to the counter electrode (CE) and reference electrode 1 (RE1) in oily wastewater solution. Here, the carbon felt (60 mm × 60 mm × 5 mm) was selected as the electrode and can be used to calculate the power density (power/working area of electrode) and current density (current/working area of electrode). It was clear that Fe2O3-500 °C /CF produced an open circuit voltage of 0.55 V, a maximum current density of 349 mA/m 2 , and a maximum power density of 52.5 mW/m 2 , which was 2.6 fold higher compared to Fe2O3-900 °C. A comparison between previous studies of Bio-E-Fenton MFCs and this study has been made and the results are shown in Table 1. In addition, an original report related to TiO2 has been addressed for showing its optoelectronic ability to generate hydrogen in water [40]. Table 2 shows the list of related cases of photo catalysts.   In consideration of the COD degradation rate, the Fe 2 O 3 -500 • C/CF performed better with 99.3% removal in 1 h and with an effluent concentration of 152 ± 5 mg-COD/mL/1 day. Generally speaking, a higher COD degradation rate was observed in this study compared to the other studies involving general bio-electro-Fenton systems [41] for the treatment of oily wastewater, with an efficiency that was 1.4 times higher ( Table 3). The pH and DO at 700 • C were lower than those at 500 • C and 900 • C because under the constant resistance (1 KΩ) discharge at 700 • C, the DO and pH values affected the voltage output recorded from the long-term measurement. Nevertheless, the results of Table 3 indicated that a better power performance of the system and COD removal rate in the cathode chamber were observed at a calcination temperature of Fe 2 O 3 -500 • C, as it had a suitable pH and DO for befitting the biocompatibility [42][43][44] and electrical conductivity [45,46] in the system. The results showed that the maximum power density was 52.5 mW/m 2 and it was influenced by the calcination temperature of Fe 2 O 3 , which obviously showed that the power density decreased with increasing calcination temperature. In addition, the average temperature of the anode chamber was controlled at about 35~45 • C (by Bakelite plate) and the cathode temperature was higher than 70 • C (Shown in Table 3), which would be the effect of evaporation. Therefore, the measurement time was one hour and the time interval was based on the experimental reliability and neglecting the effect of evaporation in the cathode chamber. To further understand the impact of Fe 2 O 3 -C500-/CF on the degradation rate, the following three reasons were formulated: (1) the in situ reaction between Fe 2+ leached from Fe 2 O 3 /FeSO 4 and H 2 O 2 generated from the CF was kinetically more favorable than the reaction between H 2 O 2 from the CF and Fe 2+ in the bulk solution; (2) the calcination of Fe 2 O 3 resulted in the formation of different crystal structures that affected the COD removal efficiency [47]; and (3) the photo catalyst accelerated chemical reactions [48] so that the strong oxidation of the hydroxyl radicals could quickly break the covalent bonds of diesel and enhance the degradation of wastewater [49].  [11]. Fe 2 O 3 crystallographic structures can be controlled by varying their calcination temperatures. At a high calcination temperature, the intensity did not vary greatly, but the crystalline particle size and surface morphology changed immensely [24]. were ascribed to the (110) reflection of Fe2O3 [11]. Fe2O3 crystallographic structures can be controlled by varying their calcination temperatures. At a high calcination temperature, the intensity did not vary greatly, but the crystalline particle size and surface morphology changed immensely [24].

The Morphologies of the Fe2O3 at Different Calcination Temperatures
For analyzing the surface morphology of Fe2O3 at different temperatures, field emission scanning electron microscopy was used (Figure 5a-c). The Fe2O3 had an average particle size of 581 nm at 500 °C, 984 nm at 700 °C, and 1255 nm at 900 °C. Fe2O3 at 500 °C showed a layered stacked morphology and the particles were uniformly distributed with a large surface area. This showed that Fe2O3-C at 500 °C was better in terms of surface modification [50][51][52], which resulted in a maximum power density of 52.5 mW/m 2 . In addition, the surface modification of Fe2O3 was indeed the main reason behind its best performance compared to the other systems.

The Morphologies of the Fe 2 O 3 at Different Calcination Temperatures
For analyzing the surface morphology of Fe 2 O 3 at different temperatures, field emission scanning electron microscopy was used (Figure 5a-c). The Fe 2 O 3 had an average particle size of 581 nm at 500 • C, 984 nm at 700 • C, and 1255 nm at 900 • C. Fe 2 O 3 at 500 • C showed a layered stacked morphology and the particles were uniformly distributed with a large surface area. This showed that Fe 2 O 3 -C at 500 • C was better in terms of surface modification [50][51][52], which resulted in a maximum power density of 52.5 mW/m 2 . In addition, the surface modification of Fe 2 O 3 was indeed the main reason behind its best performance compared to the other systems. were ascribed to the (110) reflection of Fe2O3 [11]. Fe2O3 crystallographic structures can be controlled by varying their calcination temperatures. At a high calcination temperature, the intensity did not vary greatly, but the crystalline particle size and surface morphology changed immensely [24].

The Morphologies of the Fe2O3 at Different Calcination Temperatures
For analyzing the surface morphology of Fe2O3 at different temperatures, field emission scanning electron microscopy was used (Figure 5a-c). The Fe2O3 had an average particle size of 581 nm at 500 °C, 984 nm at 700 °C, and 1255 nm at 900 °C. Fe2O3 at 500 °C showed a layered stacked morphology and the particles were uniformly distributed with a large surface area. This showed that Fe2O3-C at 500 °C was better in terms of surface modification [50][51][52], which resulted in a maximum power density of 52.5 mW/m 2 . In addition, the surface modification of Fe2O3 was indeed the main reason behind its best performance compared to the other systems. The TEM images depicted that highly-agglomerated Fe2O3 with a uniform size of ~28 nm in diameter affected the material structure of Fe2O3 after the calcination processes, as shown in Figure  6a, b. It should be noted that the structures after calcination at 500 °C to 700 °C led to a greater particle size and the lattice plane spacing of 0.21 to 0.26 nm. As shown in Figure 6c, calcination at 900 °C resulted in sheet-like structures and lattice plane spacing of 0.27 nm. The selected area diffraction pattern (SADP) showed excellent crystalline structure formations at 500 °C, which improved the COD degradation efficiency [25][26][27][28]. In addition, Fe2O3-C500 °C/CF had a better degradation rate of 99.3% in 1 h. This was a novel attempt where the Fe2O3-C500 °C/CF was combined with the bio-electro-Fenton reagent in MFCs.

Conclusions
In this study, the photo catalytic ability of iron oxide (Fe2O3) was investigated at different calcination temperatures ranging from 500 °C to 900 °C in a Bio-E-Fenton MFC for evaluating its effect on the degradation of dairy and oily wastewaters and power generation. A series of studies were executed and useful findings were addressed as follows: For the study of Fe2O3 in Bio-E-Fenton MFCs, firstly, the calcination at 500 °C brought about a better output voltage than in the case of the calcination at 900 °C, and its power density was also 2.6 times higher than at 900 °C. In addition, the COD degradation efficiency showed that Fe2O3-C at 500 °C/CF had a better performance of 99.3% removal in 1 h and with an effluent of (152 ± 5 mg-COD/mL/1 day) operation, which was 1.3 times higher than the Fe2O3-C at 900 °C/CF. Further, these evidences proved that calcination temperatures of 500 °C with 52.5 mW/m 2 would be optimal for Fe2O3 in Bio-E-Fenton MFCs because of its morphology with a uniform particle size and a larger surface area. In contrast, the degradation performance of 80.1% in 1 h was obtained by combining Fe2O3-C500 °C/CF with a bio-electro-Fenton reagent under non-illuminated conditions. Last but not the least, this innovative process was very promising for the treatment of organic wastewater treatment and the improved performance of related bio-electrochemical systems in the future. The TEM images depicted that highly-agglomerated Fe 2 O 3 with a uniform size of~28 nm in diameter affected the material structure of Fe 2 O 3 after the calcination processes, as shown in Figure 6a, b. It should be noted that the structures after calcination at 500 • C to 700 • C led to a greater particle size and the lattice plane spacing of 0.21 to 0.26 nm. As shown in Figure 6c, calcination at 900 • C resulted in sheet-like structures and lattice plane spacing of 0.27 nm. The selected area diffraction pattern (SADP) showed excellent crystalline structure formations at 500 • C, which improved the COD degradation efficiency [25][26][27][28]. In addition, Fe 2 O 3 -C500 • C/CF had a better degradation rate of 99.3% in 1 h. This was a novel attempt where the Fe 2 O 3 -C500 • C/CF was combined with the bio-electro-Fenton reagent in MFCs. The TEM images depicted that highly-agglomerated Fe2O3 with a uniform size of ~28 nm in diameter affected the material structure of Fe2O3 after the calcination processes, as shown in Figure  6a, b. It should be noted that the structures after calcination at 500 °C to 700 °C led to a greater particle size and the lattice plane spacing of 0.21 to 0.26 nm. As shown in Figure 6c, calcination at 900 °C resulted in sheet-like structures and lattice plane spacing of 0.27 nm. The selected area diffraction pattern (SADP) showed excellent crystalline structure formations at 500 °C, which improved the COD degradation efficiency [25][26][27][28]. In addition, Fe2O3-C500 °C/CF had a better degradation rate of 99.3% in 1 h. This was a novel attempt where the Fe2O3-C500 °C/CF was combined with the bio-electro-Fenton reagent in MFCs.

Conclusions
In this study, the photo catalytic ability of iron oxide (Fe2O3) was investigated at different calcination temperatures ranging from 500 °C to 900 °C in a Bio-E-Fenton MFC for evaluating its effect on the degradation of dairy and oily wastewaters and power generation. A series of studies were executed and useful findings were addressed as follows: For the study of Fe2O3 in Bio-E-Fenton MFCs, firstly, the calcination at 500 °C brought about a better output voltage than in the case of the calcination at 900 °C, and its power density was also 2.6 times higher than at 900 °C. In addition, the COD degradation efficiency showed that Fe2O3-C at 500 °C/CF had a better performance of 99.3% removal in 1 h and with an effluent of (152 ± 5 mg-COD/mL/1 day) operation, which was 1.3 times higher than the Fe2O3-C at 900 °C/CF. Further, these evidences proved that calcination temperatures of 500 °C with 52.5 mW/m 2 would be optimal for Fe2O3 in Bio-E-Fenton MFCs because of its morphology with a uniform particle size and a larger surface area. In contrast, the degradation performance of 80.1% in 1 h was obtained by combining Fe2O3-C500 °C/CF with a bio-electro-Fenton reagent under non-illuminated conditions. Last but not the least, this innovative process was very promising for the treatment of organic wastewater treatment and the improved performance of related bio-electrochemical systems in the future.

Conclusions
In this study, the photo catalytic ability of iron oxide (Fe 2 O 3 ) was investigated at different calcination temperatures ranging from 500 • C to 900 • C in a Bio-E-Fenton MFC for evaluating its effect on the degradation of dairy and oily wastewaters and power generation. A series of studies were executed and useful findings were addressed as follows: For the study of Fe 2 O 3 in Bio-E-Fenton MFCs, firstly, the calcination at 500 • C brought about a better output voltage than in the case of the calcination at 900 • C, and its power density was also 2.6 times higher than at 900 • C. In addition, the COD degradation efficiency showed that Fe 2 O 3 -C at 500 • C/CF had a better performance of 99.3% removal in 1 h and with an effluent of (152 ± 5 mg-COD/mL/1 day) operation, which was 1.3 times higher than the Fe 2 O 3 -C at 900 • C/CF. Further, these evidences proved that calcination temperatures of 500 • C with 52.5 mW/m 2 would be optimal for Fe 2 O 3 in Bio-E-Fenton MFCs because of its morphology with a uniform particle size and a larger surface area. In contrast, the degradation performance of 80.1% in 1 h was obtained by combining Fe 2 O 3 -C500 • C/CF with a bio-electro-Fenton reagent under non-illuminated conditions. Last but not the least, this innovative process was very promising for the treatment of organic wastewater treatment and the improved performance of related bio-electrochemical systems in the future.