Novel graphite rod electrode modified with iron- functionalized nanozeolite for efficient wastewater treatment by microbial fuel cells

Received 10 Nov 2020 Revised form 14 Jan 2021 Accepted 23 Feb 2021 Available online 29 Mar 2021 *Corresponding Author: Mohsen Zeeb Email: zeeb.mohsen@gmail.com https://doi.org/10.24200/amecj.v4.i01.133 ------------------------


Introduction
Microbial fuel cell (MFC) has different approach for wastewater treatment because the wastewater treatment process generates electricity or hydrogen gas instead of consuming electricity [1]. The MFC technology is depended on generating bioelectricity from bacterial biomass as the latest method for wastewater treatment. MFCs can divided into two main categories, mediated and unmediated groups. The MFCs separated the compartments of the anode (oxidation) and the cathode (reduction). Most of MFCs use an organic electron donor that is oxidized to produce CO 2 , protons, and electrons. The cathode acts by different electron acceptors such as oxygen (O 2 ). Other electron acceptors studied for metal treatment by reduction, nitrate reduction, and sulfate reduction in 25 °C and pH of 7 [2][3][4]. Microorganisms within an MFC, can be decomposed the organic matter by oxidizing, produce electrons that pass through a series of respiratory enzymes inside the cell and produce energy for the cell in the form of ATP. Then. the electrons are released towards a final electron acceptor. This receptor captures and reduces the electrons. For example, oxygen can be converted to water by the catalytic reaction of electrons with proton [5]. Previous research on electrodes used catalytic adhesives, carbon with non-platinum catalysts, flat carbon, carboncoated tube and bio electrodes in the fabrication of carbon-based cathodes. Therefore, this study employed a carbon rod electrode coated with ZSM-5/Fe nanocatalyst [6][7][8]. Zeolites are tetrahedral crystalline aluminosilicates bonded with oxygen bridges. Due to their SSA, the specific channel structure, high thermal and hydrothermal stability, they are widely used in industries such as chemistry and petrochemicals, and water and wastewater treatment [9][10][11]. As mentioned, extensive research has been performed throughout the world to make fuel cells exploiting new electrodes. The researchers developed a cathode made of nickel-doped reduced nanographene as well as acid-hydroionized reduced nanographene to determine and evaluate the efficiency of the power output density with each of these electrodes. According to the results, the acid-hydroionized reduced nanographene showed the higher power output density (37%) than the nickel-doped reduced nanographene [12]. In another study, the researchers were developed a triple nanocomposite cathode containing graphene oxide, polyethylene dioxythiophene and iron oxide nanorods to increase the current efficiency of MFCs. Due to the large specific surface area of the electrode, high electrical conductivity as well as large sites for oxygen uptake in this electrode, the oxidation-reduction reaction occurs very quickly; as far as the power output density of the cell could be maintained for more than 600 hours [13]. By previous studies, a cathode was made of carbon nanotubes doped with titanium oxide nanoparticles aimed at enhancing the current output density and increasing the elimination of residual chemical oxygen demand (COD). The results of this study revealed an increase in the specific surface area and the active sites for oxygen uptake, so that the maximum current output density produced was 15.16 mW m -2 and the COD removal efficiency was reported between 54-71% (after 10 days), which was related to the presence of active reaction sites on the electrode [14]. The results showed us, the specific surface area is a very effective factor in increasing the efficiency of MFCs. In this study, the graphite rod as a high stability and electrical conductivity was used for wastewater treatment. So, the surface modification of graphite rod by zeolite nanocatalyst will increase the specific surface area of the electrode. On the other hand, the modification of graphite rods with zeolite nanocatalyst were compared to simple graphite rod with the low price and poor efficiency [15,16]. Metal nanoparticles can greatly influence the oxygen reduction [17][18][19][20][21][22]. Hence, in this study the graphite rod electrodes were modified with iron particles (Fe) as a doping agent on ZSM-5 nanozeolite (G-Z/Fe/ ZSM-5) for increasing of MFC efficiency for wastewater treatment.

Preparation of ZSM-5/Fe Nanocatalyst
To Preparation the functionalized ZSM-5 nanocatalyst, first 2.5 g of ZSM-5 nanozeolite powder was placed in the furnace at a temperature of 500°C for 4 hours and calcined. Then, 0.5 g of ferric chloride (FeCl 3 ) powder was dissolved in distilled water twice for one hour, added to the calcined ZSM-5 nanozeolite powder and mixed for another 30 minutes, and filtered with a filter paper. The resulting powder was rinsed three times with distilled water and placed in an oven at a temperature of 80°C for 2 hours. Next, the powder was separated from the filter paper and re-calcined at a temperature of 500°C for 4 hours. The method of preparation above nanocatalyst is schematically illustrated in Figure1.

Characterization
X-ray diffraction (XRD, STADI-P, the USA) was used to investigate ferrous (Fe) metal in the nanocatalyst structure functionalized with these metal. Brunauer-Emmett-Teller (BET) surface area analysis (Belsorb apparatus, Japan) was used to determine the SSA of nanocatalyst particles, and energy-dispersive X-ray spectroscopy (EDX, MIRA III SAMX, Czech Republic) were applied to investigate the surface modification of the graphite electrode by each of the nanocatalysts.

Electrode Modification
To modify the graphite surface and to impregnate with the synthesized nanocatalyst powders, 0.5 g of each of the produced nanocatalysts (ZSM-5, ZSM-5/Fe) was poured into a test tube and 10 ml of ethanol was added and the graphite electrode was inserted into the test tube and placed in an  Then, the resulting electrode was rinsed twice with deionized water and placed in a furnace at a temperature of 300°C for 2 hours (Fig.2).

MFC construction and operation
This study applied with a separate two-part cell consisting of anaerobic anode and aerobic cathode. The chambers were made based on 500 mL pyrex glass with 75% of the volume as a working volume (375 mL). The two chambers were separated by a pyrex tube with an inner diameter of 0.8 cm and a length of 13.4 cm embedded in the middle portion with the proton exchange Nafion 117 membrane. The electrodes were made with rod graphite and heated at 3000°C with an area of 22.62 cm 2 . In order to remove any impurities and improve membrane performance, the membrane was first boiled for an hour in 3% H 2 O 2 and then washed in 1 M sulfuric acid for 1 hour. Oxygen gas was injected into the cathode with a sparger at a flow rate of 20 ml min -1 , and nitrogen gas was injected into the anode chamber to provide anaerobic conditions. A magnetic stirrer was used to stir the solutions inside the anode and cathode chambers, and a copper wire was used to bond the anode and the cathode electrodes. Acidification of the medium inhibits the optimal growth of the bacteria in the anode chamber, so it is necessary to use a buffer with appropriate pH in the bacterial growth medium. For this purpose, in order to maintain the acid strength in the cell, 50 mM of phosphate buffered solution (PBS) (0.13 g L -1 of potassium chloride, 3.32 g L -1 of sodium di-hydrogen phosphate dihydrate, 5.13 g L -1 of di-sodium hydrogen phosphate dihydrate, and 0.31 g L -1 of ammonium chloride) was prepared in the cathode chamber and 375 mL was poured into the cathode chamber [23].

Microorganisms
In the anodic chamber of the fuel cell, the anaerobic wastewater prepared from the industrial town treatment plant was used as inoculum. The samples from the treatment plant were stored in stainless steel containers at 4°C, and transferred to the laboratory. The combined inoculum was inoculated into the pre-prepared culture medium containing 1 g L -1 of glucose, 3 g L -1 of yeast extract, 11 g L -1 of peptone, 0.5 g L -1 of ammonium chloride [24]. During the experiments, the cells were kept at room temperature and stirred at 50 rpm for 72 hours (Table 1).

Analytical method
A multimeter (MASTECH MS8360G, China) was used to measure the output voltage of the cell. The residual COD of the samples was measured with COD meter (Model 76133, Aqua Litik, Germany). Three-electrode systems including, anode electrode Spectrophotometric method was used to examine the treated wastewater. Initially, standard solutions with concentrations of 100-800 with 3 ml of digestion solution (containing potassium dichromate, sulfuric acid and silver sulfate) and 7 ml of stock solution (potassium hydrogen phethalate) are prepared and placed in an oven at 150 ° C for 1.5 hours was placed. After cooling, it was placed in a spectrophotometer (600nm) and the calibration curve was drawn. It was measured by placing the absorbance of the unknown sample in the residual COD calibration equation.

BET characterization
By comparing the as, BET parameter as in Figure  3 and the results in Table 2, in each of the four BET analysis curves of the nanocatalysts, the highest SSA was related to the catalyst functionalized with Fe metal (ZSM-5/Fe, which was determined to be 408.41 m 2 g -1 ).

X-Ray Diffraction (XRD) analysis
The XRD spectrum for the ZSM-5 and the ZSM-5/ Fe nanocatalyst was shown in Figure 4. The ZSM-5/Fe nanocatalyst confirms the presence of iron particles doped with silicate particles (Fig. 4).

Energy dispersive X-ray spectroscopy (EDX) analysis
The curves of EDX analyzes for the surface of G-Z and G-Z/Fe electrodes compared as Figure  5a

Electrochemical characterization
Comparing the cyclic voltammetry (CV) cerves for G, G-Z and G-Z/Fe was shown in Figure 6a, 6b and 6c. The peak of the graphite electrode modified with iron-doped nanocatalyst (ZSM-5/Fe), which has a higher specific surface area, has the maximum current compared to other electrodes. Due to Figure  7, the square wave (Sqw) voltammetric peaks of the G, GZ-5 and GZ-5/Fe electrodes, with the scan rate of 5mV•s −1 in 50 mM phosphate-buffered saline (PBS) at an ambient temperature and in the potential range of 1 to 90 volts were compared. Comparing the electrode peaks, the G-Z/Fe electrode peak has the highest current (3500 μA cm -2 ) relative to other electrodes. The graphite electrode peak has the lowest current (2000 μA cm -2 ), which indicates that the ZSM-5 nanocatalyst doped with iron caused to increase the current of analysis.

Conclusions
By procedure, a new microbial fuel cell was made by graphite rod electrodes. The surface of the cathode was modified by ZSM-5 and ZSM-5 functionalized with iron nanocatalyst. All three electrodes (G, G-ZSM and G-ZSM/Fe) were analyzed by square wave and cyclic voltammetry. Both analyses were introduced that the G-Z/Fe electrode had the higher efficiency as compared to others (Fig. 6-7). Experimental results of fuel cell steering was also studied as the Figure 8 and 9 by the G, G-Z and G-Z/ Fe electrode, the results showed that the efficiency of fuel cell output current (I) and residual chemical oxygen demand (%COD) based on this electrode increased up to 21.8% and 36.9%, respectively as compared to other graphite electrode. The high efficiency of G-ZSM/Fe nanocatalyst electrode is due to high specific surface area and the presence of iron particles with high electrical conductivity.