1. Introduction
Microbial Fuel Cells (MFCs) represent an innovative technology at the intersection of microbiology, environmental engineering, and renewable energy [1]. Although, there are many microbial electrochemical systems being used worldwide by the researchers for energy positive wastewater treatment, but MFCs poses to be the best fit for the treatment of domestic wastewater [2–7]. These devices utilize the metabolic activity of microorganisms to convert organic matter directly into electrical energy, offering a sustainable alternative to traditional fossil fuel-based power generation methods. Key to the efficiency and functionality of MFCs is the membrane component, which plays a critical role in separating the anodic and cathodic chambers while allowing the transport of protons necessary for electron transfer [8]. Electrogens are a family of bacteria that break down wastewater anaerobically when it is put into the MFC. Inside the anodic chamber, protons and electrons are produced as a consequence of the biological breakdown of the organic substrate [9]. Then, via a membrane that is selectively permeable and also referred to as a proton exchange membrane (PEM), protons are transferred to the aerobic cathode compartment [10–12]. Because of this, the separator is among the MFCs’ most crucial components.
Oxygen transport from cathode to anode, higher fabrication costs, and bio-fouling are the challenges that a membrane faces when operating in microbial fuel [13–14]. In research on microbial fuel cells, Nafion has been the most often utilized membrane [15]. However, Nafion’s biofouling and lack of biodegradability [1] have fueled the search for a novel substitute membrane for MFC application. The ubiquitous adoption of Nafion membranes is hindered by their extravagant cost, limited availability, and environmental concerns associated with their production and disposal [16]. Therefore, there is a growing interest in exploring alternative membrane materials that are cost-effective, environmentally friendly, and readily available. Many scholars have been working hard to find solutions to these problems in recent years [17,18]. Numerous polymeric substitutes and modified polymeric membranes have been studied [19–21]. Because of their biocompatibility, biopolymers such as naturally occurring rubber and polyhydroxyalkanoates have also been studied [22–23]. However, there is a drawback to these polymeric membranes, namely their high production costs. A number of alternatives to polymers have been suggested for lowering the cost of the membranes utilized in MFCs [24]. These choices include ceramic membranes [25–27] glass and nylon fiber filters [28], non-woven fabric filters [29] and coated cotton fabric [30].
Because of its affordability, durability, specialized chemical and thermal qualities, and stability, ceramic separators have shown to be superior to polymeric membranes [31,32]. To report a few instances are the usage of a pot made of earthenware [33,34,8] as well as the use of earthen membrane has been reported in few research. To improve the efficiency of the microbial fuel cells, modifications were also made to the earthen membranes [35]. The efficacy of the MFCs were improved by altering the earthen membranes by adding cation exchangers [8] known as montmorillonite (MMT) and vermiculite (VC) and also through conductive coating of Nafion solution. To create a clay composite membrane, starch has been used by [36] and vermiculite nano-milling was done to reduce the particle size and improve the performance of MFC [27].
One such material that has shown promise in various applications is wood ash, a byproduct of biomass combustion processes. Wood ash is rich in minerals and exhibits unique physicochemical properties that make it suitable for a number of applications, including soil amendment, treatment of wastewater, and construction materials [37]. Recent research has highlighted the potential of wood ash-derived biochar as an effective adsorbent for removing heavy metals from contaminated water [38], as well as its use in sustainable construction materials, such as geopolymer composites [39,40]. Wood ash when mixed with soil enhances the cation exchange capacity of the soil [41–44]. Despite its versatility, the utilization of wood ash in MFC membranes for enhancing their performance remains relatively unexplored. This presents an exciting opportunity for innovative research aimed at developing cost-effective and sustainable membrane materials for MFCs. In this study, the authors investigate the feasibility of using wood ash-modified red soil membranes to intensify the efficiency of microbial fuel cells.
By incorporating wood ash at different weight ratios into red soil membranes, this study aims to optimize membrane composition and evaluate its impact on MFC performance. Through systematic experimentation and characterization, we seek to elucidate the effects of wood ash modification on membrane properties, such as surface morphology, and proton mass transfer, oxygen mass transfer, water uptake, and ion exchange capacity. Furthermore, we aim to assess the performance of wood ash-modified membranes in MFCs in terms of power output, current density, and organic removal efficiency.
2. Materials and Methods
2.1. Materials
Red Soil used for fabricating membrane was acquired from Vadodara, Gujarat, India through a nearby ceramist. Wood ash was obtained from local sweet shop where the wood is used as a source for providing heat for cooking. Domestic wastewater was obtained from sewage treatment plant (121 MLD capacity) situated in Altadara, Vadodara, Gujarat, India.
2.2. Earthen Membrane Preparation
The same red soil that was employed in our earlier investigation [8] was utilized to fabricate earthen membranes. The prepared earthen membranes are composed of red soil and wood ash using different weight ratios as shown in Table S1. A bare soil membrane served as the control and the commercially available Nafion 117 membrane was used as standard, while the membrane made only from red soil (Soil) served as the control. Three distinct test membranes, designated X1, X2 and X3 were fabricated Fig. 1a with the composition mentioned in Table S1.
The red soil and wood ash were then thoroughly blended to form a dough. After that, a sheet with a thickness of 5 mm was created by pressing the dough between two glass plates. Following that, circular discs from the sheets were removed using hollow molds of 70 mm in diameter. After that, the membranes were allowed to dry at ambient temperature for seven to eight days in order to allow the membrane to dry gently and prevent voids. The clay membranes were allowed to air dry before being dried for five hours at 100°C in an oven. In the end, the earthen membranes were scorched for 35 minutes at 650°C in a muffle furnace [45]. The membrane was sanded using sandpaper to get its thickness down to 4 mm.
2.3. Fabrication and Operation of Microbial Fuel Cells
The technique used to fabricate single chambered air-cathode MFCs was taken from an earlier study [27] except for the size. The working volume was reduced to 50 mL in this study. The cylinders inside dimensions were 4 cm in diameter and 4.5 cm in height (Fig. 1c &1d). The anode chamber had a working capacity of 50 mL. The cathode electrode and the MFC lid were separated by an earthen membrane. Both the electrodes (anode and cathode) were prepared from carbon felt having an area of 14 cm2 each (Fig. 1b). The earthen separator and anode were spaced one centimeter apart. The electrodes in the MFC system were cleaned with 0.5M H2SO4 and then deionized water to get rid of any contaminants before employing the carbon felts [46]. Real wastewater was used to feed the anode chamber with influent COD of 876.41±153 mg/L. The MFCs’ performance was evaluated after they were set off in batch mode for three days per cycle. Ten cycles of closed-circuit operation were run and evaluated for MFCs. About 1% (v/v) anaerobic sludge from the sewage treatment plant’s anaerobic digester was used to inoculate all of the MFCs (STP).
2.4. Assessment of Membrane Properties and Performance of MFCs
Water absorption, proton diffusion, oxygen transport, conductivity and ion exchange capacity were measured for the constructed clay membranes. Every technique used to assess these earthen membrane qualities was taken from an earlier investigation [8]. A data collecting system (34972A, Keysight Technologies, USA) was used to monitor the voltage (V) and current (I) in order to assess the performance of the MFC reactors. In order to monitor the open-circuit potential (OCP), every MFC was maintained in the open circuit state. Polarization investigations were conducted by changing the external resistance from 100000 Ω to 1 Ω after the OCV reached stability. Evaluation of the MFCs’ current generation was conducted with a resistor of 100 Ω to close the circuit. Power was normalized to the MFC reactor’s volume to obtain the volumetric power density.
2.5. Treatment Efficiency and Normalized Energy Recovery
The MFCs’ organic removal efficiency was evaluated using the closed reflux colorimetric approach to calculate the COD contents of the feed and outflow at the end of each cycle (APHA, 1995). According to [47], the volumetric treatment rate (VTR) measures how well a system uses substrate to reduce COD concentrations to a suitable level while taking the reactor capacity, feed strength, volumetric flow rate, and COD removal into account. It makes it possible to compare MFCs under various operating situations with greater accuracy. The VTR may be computed using Eq. (1) [48].
Direct electricity is one way that energy may be recovered in MFCs. Either in terms of the amount of wastewater treated per unit time (kWh/m3) Eq. (2) or the COD eliminated in a unit of time (kWh/kgCOD) Eq. (3) are used to assess the normalized energy recovery (NER) [27]. These formulas can be used to determine the NER:
where ΔCOD is difference in COD, HRT is hydraulic retention time, where, P is power, T is time and Va is wastewater volume.
2.6. Evaluation of Electrochemical Performance of MFCs
Cyclic Voltammetry has been carried out at a scan rate of 0.01V/s within the range of −0.8 to 0.8 V. Electrochemical impedance spectroscopy (EIS) was performed on all of the MFCs with an amplitude of 0.01 V and the frequency ranging from 105 to 1 Hz [46]. With a three-electrode arrangement consisting of a working anode, a counter cathode (both made of carbon felt) and a KCl saturated, Ag/AgCl reference electrode, all electrochemical analysis was carried out using a CHI 760 electrochemical analyzer.
3. Results and Discussion
3.1. Assessment of Membrane Properties
Water uptake, or the membrane’s capacity to retain water inside its matrix, is one of its most crucial characteristics. Table 1 shows that when the weight percentage of wood ash increases, so does the membrane’s ability to absorb water. The maximal water absorption of the X3 membrane, which has the maximum wood ash, was 39.67±0.32%, which is ~2 folds that of the control and comparable to Nafion 117 membrane (Table 1). The more wood ash provides greater surface area for water retention, which accounts for the increased water intake [49]. Table 1 shows that when pore size decreases, the membranes’ ability to absorb water increases. When the wood ash percentage is increased, it provides water a lot of surface area to attach to [50].
To assess the earthen membrane’s capacity to move protons from the anode to the cathode, measurements were made of its proton mass transfer coefficient (Pmt). From Table 1, it is evident that among all the membranes under test, the X3 membrane achieved the highest proton mass transfer coefficient (11.72±0.11 cm/sec).10−3). The Pmt of X5 value obtained is around 4 times better than the control soil membrane, almost similar to that of standard. It is evident from the study that a membrane’s ability to store water determines how well it transfers protons [11]. The proton mass transfer coefficient increased in the order of Soil < X1 < X2 < X3 < Nafion, corresponding to the membrane’s increasing ability to retain water. It’s a crucial feature of the membrane since achieving electro-neutrality requires the transport of protons to the cathode.
In a microbial fuel cell, the oxygen transfer towards the anode reduces the ability of extracellular elution of electrons by anaerobes. Therefore, the membrane’s oxygen permeability should ideally be zero in order to sustain the optimum conditions for the synthesis of anaerobic electrogens. As a result, the oxygen mass transfer coefficient (Omt) was calculated to assess the oxygen penetration of each membrane. Table 1 lists the oxygen mass transfer coefficient (Omt) for each membrane. The Omt are arranged in decreasing order as follows: Soil > X1 > X2 > Nafion > X3. 4.37±0.24 x 10−5 (cm/sec) is the lowest oxygen mass transfer coefficient value of X3 membrane. The value obtained was almost half of that of the control membrane and even less than what was observed in Nafion membrane. With the increase in weight percent of wood ash in membrane, there is a noticeable decrease in oxygen diffusion.
The ability of a membrane to transfer ions is measured by its conductivity. Conductivity (Ce) study was performed on all the manufactured earthen membranes, and Table 1 shows that the Ce values increase with increase in wood ash weight percentage. The X3 membrane had the greatest conductivity value because of its highest wood ash content. The X3 membrane has ~2 times the conductivity of the control membrane and is nearly identical to that of standard. The results suggest that wood ash has a favorable impact on conductivity. According to [44], the addition of wood ash offers more cation exchange sites, which in turn causes the conductivity to rise.
A higher ion exchange capacity is the result of the membrane’s conductivity and water-holding capacity working together. Because the X3 highest weight percentage of wood ash of all the manufactured earthen membranes, it has the greatest IEC due to ions moving over the aqueous medium and exchanging at the sites of cation exchange. The X3 membrane has the largest ion exchange capacity due to its maximal water absorption and conductivity. Better proton transport through the membrane is indicated by a higher ion exchange capacity [51].
3.2. Performance of MFCs in terms of Electric Output
Batch mode operation was used to construct and equip five MFC installations using earthen membranes made using wood ash. Initially, the MFCs operated in open circuit arrangement till a stable potential is attained. The reactors operated for 12 days (4 cycles) in open circuit mode (Fig. 2a). Following the achievement of its maximum open-circuit voltage on the eighth day of operation, MX3 continued to function steadily until the twelfth day. After reaching their highest OCP on the ninth day, the other MFCs stabilized. At 0.732 V, the maximum OCP was reached in MX3. The remaining MFCs’ OCVs ranged from 0.510 to 0.670 V.
The MFC’s capacity to maintain a voltage at a specific current may be determined using polarization information. Maximum voltage potential is used to capture measurement data for different resistances during MFC operation (Fig. 2b). A popular way to display polarization curves is to chart MFC’s potential and volumetric power density as a function of volumetric current density. Once the reactors were stable, the external resistance was changed from 100000 Ω to 1Ω to analyze the polarization of the reactors. All of the polarization curves (Fig. 2b) show the activation losses and had an abrupt steep. These activation losses are caused by the anaerobic electrogens’ need for activation energy in order to oxidize the substrate [52]. Every MFC showed comparable activation losses. The polarization curve’s subsequent portion (Fig. 2b) provides information on the ohmic losses. Higher ohmic losses are evident in reactor with soil membrane due to the sharp decline in voltage that occurs as the current density increases. The MX3 reactor with the X3 membrane had the lowest ohmic losses. The graph of current density vs voltage density (Fig. 2c) reveals that MX3 has the maximum power density of 364.32 mW/m3. Since the MX3 has the highest proton transfer, additional electrons must also be supplied to the cathode via the external circuit in order to electro-neutralize the protons there.
The systems were first kept in open circuit condition. The systems were connected through an external resistor only after the open circuit voltage became constant. The constant open circuit voltage indicated the growth of biofilm. After the system was closed through a resistor, the output became constant after almost 3 weeks which showed that the biofilm has matured [53]. Following the completion of the polarization investigations, an external resistance of 100 Ω was used to shut the external circuit of each MFC [53,54]. Every observation of the current generation throughout the study was done at this resistance (unless stated otherwise). All of the MFCs had their closed-circuit voltage measured for 30 days (10 cycles). Next, the power density and current generation were computed to assess each reactor’s performance. The data shown in Fig. 2d makes it evident that of all the installed reactors, the MX3 reactor achieved the most current. The current in MX3 was 1.04±0.29 mA, about 2.5 times that of control, and analogous to that of the reactor fitted with Nafion membrane. MX3 had the highest power density (1450.09±151.3 mW/m3), 6.2 times higher than soil evident from (Fig. 5b). The order in which the MFCs produced the highest power densities is Nafion > MX3 > MX2 > MX1 > Soil. Table 2 showcases the performance comparison of this study with other prior studied in terms of power density.
Proton mass transfer is the primary factor influencing the power density of MFCs [62]. Higher power densities are produced by more proton diffusion across the membrane. However, other factors also play a role in enhancing power density, such as the electrons flow towards the cathode in order to maximize electro-neutralization. In order to prevent protons from building up in the anode that eventually create an acidic environment and would negatively impact exoelectrogen metabolism, proton transfer should be similar to the rate of electron transfer [8].
After being operated in closed circuit mode for 30 days, the MFC reactors were disassembled. On the membranes, no appreciable fouling was seen. In the long run, however, biofouling may be a crucial factor to consider when working with membranes. If so, the membranes could be autoclaved and cleaned with DI water [8].
3.3. Morphology of Prepared Membranes
To examine the morphological characteristics of the developed membranes, scanning electron microscopy was used. The average pore size and pore area of the separators were measured using Image J software (Fig. 3). The X3 separator has the least average pore size of 0.119 μm and the highest pore area of 2.06 μm2 among all the test membranes that were employed. The membranes’ average pore sizes can be arranged as follows: Soil > X1 > X2 > X3 (Table. 1). It is evident that the membrane’s pore size reduces in tandem with the increase in wood ash content into the membrane.
3.4. Organic Removal and Coulombic Efficiency
The MFCs’ COD removal efficiency was assessed when the systems were switched to closed circuit mode. After a modest rise, the MFCs’ COD elimination efficiency stabilized after the third cycle. With an average value of 93.89±0.73%, MX3 had the greatest efficiency of COD removal (Fig. 4). The MFC using the X3 membrane performed better than any other MFC, according to the data. [63] have demonstrated a clear correlation between COD elimination and the current generation. Furthermore, the microbial community in the anode consumes COD for their development and metabolism rather than directly using it to produce energy [64]. The greatest proton diffusion via the X3 membrane is thought to be responsible for the higher COD removal in the MX3 [65]. In order to achieve electro-neutrality, the increased proton transfer to the cathode pushes an additional quantity of electrons to the cathode. This, in turn, accelerates the rate at which the electrogens oxidize the substrate to produce more electrons [34].
The electric current generated from the breakdown of organic substrates is measured by coulombic efficiency [66]. Coulombic efficiency changed in a linear fashion with the study’s generation. The MFC MX3 has the highest CE (54.10 ± 2.08%) and current generation (1.04±0.29 mA). The maximum CE in MX3 is sustained by elements such as low oxygen diffusion rate, high ion exchange capacity, and high proton mass transfer coefficient. The coulombic efficiency of the MFCs is in the order of Nafion (66.10±2.53%) > MX3 (54.10±2.08%) > MX2 (49.27±0.96%) > MX1 (37.06±1.34%) > Soil (23.91±1.21%). The findings suggested that the MFCs’ improved coulombic efficiency is due to the inclusion of wood ash in increment. This is because the wood ash increases the number of cation exchange sites available, which increases the separators’ capacity for cation exchange and, in turn, raises the MFC’s CE. Because of its bigger pores, which permits more oxygen penetration and impedes the anaerobic electrogens’ ability to metabolize in the anode chamber, the control MFC with control membrane has the lowest CE. By examining the current (Fig. 2d) and COD removal (Fig. 4), it is evident that the efficacy of COD removal and improvement in the current generation are subsequent. As a result, CE provides us with a clear understanding of how to better utilize the substrate to generate more current.
The volumetric treatment efficiency provides information on the removal of COD over the hydraulic retention time (HRT), as the formula suggests. Out of all the installed MFCs, the MX3 has the greatest VTR (0.275 kgCOD/m3.day) and the best COD removal efficiency among the fabricated membranes. As seen in Fig. 5a, the VTR declined in agreement with the COD removal. According to [46], the reactor capacity, feed concentration, and rate of COD removal are all taken into account when calculating volumetric treatment efficiency. However, in the current investigation, every parameter was fixed, with the exception of the COD removal rate. As a result, it can be said that VTR is a direct function of COD removal.
The term “normalized energy recovery” (NER) to describe energy recovery in MFCs. Of the installed reactors [67], MFC MX3 has the greatest NER (0.052 kWh/m3) and (0.063 kWh/kgCOD). To analyze the correlation of power output and NER, the peak volumetric power density (PDmax) was plotted against the NER acquired during the investigation in Fig. 5a. Power generation is contingent upon the microbial population’s consumption of substrate, and the study indicates that the elimination of COD in each MFC leads to an increase in power density. Additionally, Fig. 5b shows that when NER increases, power density increases. Normalized energy recovery may be used to examine energy recovery regardless of the reactor’s size or scale since it takes into account both the feed rate and the organic removal efficiency [68].
3.5. Electrochemical Analyses
To get a general understanding of the electrochemical behavior of the reactors under turnover conditions, cyclic voltammetry (CV) was performed [52,69]. Through the CV, the oxidation current was assessed. The MFC MX3 exhibited the maximum oxidation current, with the OC peak being measured at 1.25 mA Fig. 6a. Better electron transport and substrate oxidation are indicated by a larger oxidation current. X2 and X1 both had an oxidation peak that was measured at 1.14 mA and 1.09 mA Fig. 6a. Due to the lack of any wood ash, the control MFC (soil) exhibited the lowest oxidation current (0.85 mA). The findings unequivocally demonstrated that the oxidation current of membrane is increased when its wood ash content is increased because this affects the substrate oxidation rate. Better proton transport via the membrane was suggested by enhanced current values [70]. According to [8], the biofilm’s increased electrogen activity close to the anode surface is ultimately responsible for the rise in current. Increase in wood ash content contributes to the membrane’s smaller pore size as seen in Table S1, which lessens oxygen diffusion on the anode chamber and improves the anode’s ability to generate biofilm.
The MFCs internal resistances may be computed using the electrochemical impedance spectroscopy (EIS) method [71]. The solution resistance (RS) is displayed in the initial part of the Nyquist plot, which is the part between the zero and the beginning of the semi-circular part. The plot’s semicircular section provides information about the charge transfer resistance (RCT). By fitting the Nyquist spectra with an analogous circuit that assesses the system’s internal resistance components, it was noted that the MFC MX3 had the least internal resistance (RS: 6.9Ω and RCT: 13.42Ω) from the analogous circuit displayed in Fig. 6b. The control MFC (soil) had the greatest internal resistances, with solution resistances of 2.02 Ω and 69.01 Ω RCT. The drop in internal resistances seen in MFCs upon increase in wood ash content implies that the membrane containing more wood ash facilitates superior proton transfer. Because there are more protons being transported across the earthen membrane as a result of greater electron transport, even the enhanced current in system MX3 led to the same conclusion.
3.6. Proposed Mechanism of Wood Ash Improving the Performance of Ceramic Membrane
The inclusion of wood ash into red soil for fabrication of ceramic membrane improves its performance via the given proposed mechanism. To begin, alkaline wood ash (pH 9–13) raises the pH of the soil, resulting in deprotonation of surface hydroxyl groups on clay particles. This results in a greater negative surface charge, which increases the membrane’s cation attraction capability. Second, wood ash particles modify the soil’s microstructure by intercalating between soil particles, forming new micropores and channels. These structural alterations improve ion mobility and accessibility to exchange sites in the membrane. The combination of these mechanisms, surface charge adjustment and microstructure alteration, results in better membrane performance. The increased negative charge improves cation adsorption, whereas the increased porosity promotes more efficient ion transport through the membrane. This synergistic effect produces a ceramic membrane with enhanced ion exchange capabilities, making it more suitable for a variety of applications including water treatment and selective ion removal.
3.7. Costing
In comparison to the commercial Nafion 117 membrane, the X3 membrane exhibited a cost reduction of around 99.3%. The Nafion 117 (Vinpro Technologies, India) costs $15,000 per square meter, while the X3 membrane created for this investigation came out to cost $100 per square meter when all production costs were taken into account. In addition to the reduction in costing, the overall performance of the fabricated X3 membrane is comparable to Nafion in all the performance parameters.
4. Conclusions
After conducting extensive testing and research, it was determined that addition of wood ash in 20% weight contributes to improved earthen membrane performance, which in turn raises the MFC’s total output. The X3 separator satisfies every requirement needed for a membrane to be used in an MFC. It improves proton diffusion and electricity generation by lowering the MFC’s total internal resistance. When compared to the commercially available membrane that is being utilized worldwide, the cost of the manufactured earthen membrane was also rather inexpensive. This membrane can significantly lower construction costs because membranes account for the largest portion of the cost of building a microbial fuel cell. All things considered, it is possible to conclude that this membrane will be useful when microbial fuel cells are scaled up to treat wastewater and simultaneously produce energy.
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