Reducing Plastic Waste and Generating Bioelectricity Simultaneously through Fuel Cells Using the Fungus Pleurotus ostreatus

Abstract

Plastic waste, a persistent and escalating issue, and the high costs of installing electric power, particularly in remote areas, have become pressing concerns for governments. This research proposes a novel method for generating electric power from sugarcane bagasse waste and reducing plastic waste. The key to this method is the use of the fungus Pleurotus ostreatus in microbial fuel cells. Microbial fuel cells (MFCs) demonstrated their effectiveness by generating peaks of electric current (4.325 ± 0.261 mA) and voltage (0.427 ± 0.031 V) on day twenty-six, with a pH of 5.539 ± 0.278. The peak electrical conductivity of the substrate was 130.574 ± 4.981 mS/cm. The MFCs were able to reduce the chemical oxygen demand by 83%, showing a maximum power density of 86.316 ± 4.724 mW/cm² and an internal resistance of 37.384 ± 62.522 Ω. The infrared spectra of the plastic samples showed a decrease in the peaks 2850–2920, 1470, and 720 cm⁻¹, which are more characteristic of plastic, demonstrating the action of the Pleurotus ostreatus fungus on the plastic samples. Also, the micrographs taken by SEM showed the reduction in the thickness of the plastic film by 54.06 µm and the formation of microstructures on the surface, such as pores and raised layers of the sample used.

1. Introduction

Plastics, found in a myriad of applications such as toys, packaging, and construction materials, contribute to a substantial 16% of the total worldwide waste. This stark reality presents us with significant environmental challenges due to the accumulation of plastic waste [1,2]. The long biodegradation time of plastics and the adverse impacts on soil fertility and waste management systems in developing countries [3] are deeply troubling. The need for alternative methods of plastic waste disposal is not just a concern; it is a global urgency. This urgency is driven by the rapid growth in the generation of this waste and the limited space in landfills [4]. As landfills approach their maximum capacity, waste is often incinerated or deposited in illegal plastic dumps, exacerbating significant environmental problems [5].

The annual production of plastic waste in developing countries is increasing by 4%. A particularly worrying case is Delhi in India, where 690 tons of plastic waste is generated daily. Nationwide, India generates more than 5600 tons annually, but only 40% of this waste is recycled [6]. Moreover, it is crucial to note that excessive exposure to plastic waste, as well as its manufacturing, can lead to serious health issues [7]. These include eye disorders, congenital disabilities, malignant cancers, and other contagious diseases [8,9]. On the other hand, global energy consumption has experienced a considerable increase, with the industrial sector being the primary energy consumer. Traditional energy sources such as natural gas, oil, gasoline, coal, and liquefied petroleum gas are beginning to run out over the years [10]. Despite this, energy demand continues to increase; in the years 2022 and 2023, an increase of 2.4 and 2.2%, respectively, was observed, and an increase of 8.6% is forecast for the year 2026, reaching a projected consumption of 1400 TWh [11].

This growing energy demand has driven the search for new ways to generate electricity from renewable sources. For example, India’s Ministry of New and Renewable Energy (MNRE) has built large solar parks, increasing its generation capacity from 20 GW to 40 GW between 2020 and 2022 [12,13]. In addition, microbial fuel cell (MFC) technology has experienced significant advances in recent years. These cells use a wide variety of waste as fuel (substrate), reducing electricity costs [14,15]. MFCs generate electric current from the chemical reactions inside them, using the substrate as fuel. The anodic electrode captures the electrons released during these chemical processes. It is transported through an external circuit to the cathodic electrode, generating a flow of electrons and, therefore, the electric current [16,17,18]. Mango peel waste has been used as substrate in MFCs, generating 102 mV and a power density of 0.099 mW/m². These MFCs operated at a pH of 6.97; the microorganisms Bacillus, Klebsiella, Escherichia, and Actinobacillus were identified as the primary electron generators [19]. Likewise, the fungus Scedosporium dehoogii has also been used as a catalyst in MFCs, achieving maximum values of 550 mV with a power density of 50 mW/m², which demonstrates the importance of certain fungi as biodegrading agents [20].

There are fungi that can break down plastic by producing enzymes that break the polymeric bonds of plastics [21]. This process turns plastic into simpler compounds that microorganisms can use [22]. For example, polyethylene terephthalate (PET) plastic is broken down into smaller molecular components such as BHET and MHET [23]. These components are linked by ester bonds, and fungal hydrolytic enzymes such as lipases, esterases, and cutinases can break them down [24]. In this sense, the fungus Pleurotus ostreatus is a basidiomycete that can degrade chemical substances [25] and produce secondary metabolites for pharmaceutical purposes. Despite the applications having generated interest in several research fields, there are existing limitations in the knowledge about its capabilities [26,27]. However, the potential of Pleurotus ostreatus to degrade plastic has not been fully explored. Polyethylene terephthalate was used as a substrate for the fungus by means of the immersion technique to reduce the initial concentration by 10% in 60 days [28]. Similarly, the Pleurotus ostreatus fungus has been used as a catalyst in MFCs, generating maximum voltage values of 940.00 ± 10.00 mV with a power density of 13.22 ± 0.78 mW/m² [29]. However, Pleurotus ostreatus-based MFCs have not been used for plastic waste reduction and bioelectricity production so far.

The primary objective of this research is to delve into the potential of the Pleurotus ostreatus fungus in a single-chamber MFC for plastic waste reduction and power generation, simultaneously, over a period of 45 days, using sugarcane bagasse waste. This study will meticulously monitor power density, electric current, current density, electrical conductivity, pH, chemical oxygen demand, and voltage values. Furthermore, the internal resistance of the MFCs will be calculated, the FTIR (Fourier transform infrared) spectra (initial and final) of the plastic waste will be obtained, and so will their respective micrographs of their initial and final states. The results of this study will lead the way in using this fungus to reduce plastic waste and generate electric current, thereby making a significant contribution to addressing two major social, environmental, and political issues commonly found in cities.

2. Materials and Methods

a. Assembly of SC-MFCs

The single-chamber microbial fuel cells (SC-MFCs) were built by Xin Tester (Shanghai, China) using silica (SiO₂) with a 100 mL substrate volume capacity. A zinc electrode was the cathode, and a carbon electrode was the anode, with precise areas of 24.7 cm² and 22.6 cm², respectively. An external circuit connected to a 100 Ω external resistor (R_ext.) facilitated electron transfer from the anode to the cathode. Nafion was used as the proton exchange membrane (PEM) to separate the anode and cathode chambers, as illustrated in Figure 1.

b. Electrochemical and morphological tests.

The monitoring of electrical values (current and voltage) was carried out using a digital multimeter (Truper MUT–830 Digital Multimeter, MI, USA) along with an external resistor (Rext). The measurements were taken at regular intervals over a specific period of time under controlled temperature and humidity conditions. The chemical oxygen demand (COD) was monitored using the closed reflux colorimetric method [26]. The internal resistance of the MFCs was determined using Ohm’s law and the energy sensor (Vernier- ± 30 V and ±1000 mA). The voltage and electric current values were plotted on the “Y” and “X” axes, respectively, to calculate the internal resistance of the MFC [30]. The power density (PD) and current density (CD) values were determined using the formula PD = V²/(R_EXT.A) y CD = V/(R_EXT.A) with external resistances of 0.2 (±0.05), 5 (±0.50), 20 (±2.4), 50 (±6.52), 120 (±10.55), 240 (±15.62), 480 (±20.64), 520 (±30.88), 780 (±50.75), and 1000 (±60.55) Ω [26]. The analysis of the anodic electrode was performed through scanning electron microscopy (SEM) using a TECSAN VEGA 3 LM equipped with an SPI 11430-AB gold coating system (TESCAN, Kohoutovice, Czech Republic). Additionally, the transmittance spectra of the plastic films were measured using a Fourier Transform Infrared Spectrometer (FTIR, Thermo Scientific IS50, Waltham, MA, USA).

c. Obtaining Pleurotus Ostreatus mycelium

The Pleurotus ostreatus mycelium was carefully obtained from wheat grains and was identified and verified for purity at the Cesar Vallejo University laboratory. The fungal species was confirmed using the microculture plate method by comparing its morphology and structure with the taxonomic keys for filamentous fungi according to the identification guide by Barnett S. A. [31]. The inoculum was prepared with great care and was then incubated at 28 ± 1 °C for 15 days until the formation of new mycelium. The mycelium attached to the sugarcane bagasse was used as the inoculum in the microbial fuel cells (MFCs).

d. Microbial Fuel Cell (MFC) Start-up

The microbial fuel cell was meticulously assembled, with each component carefully balanced and placed. The substrate, a perfectly balanced mixture of sugarcane bagasse and Bushner broth (ratio 2:8), was sterilized before use. The microbial inoculum, a carefully measured 10 g of mycelium attached to the sugarcane bagasse, was added. A total of 400 g of substrate, 10 g of mycelium, and a 1.5 × 1.5 cm low-density polyethylene sheet were placed inside the MFC.

3. Results and Analysis

Microbial fuel cells showed a significant increase in voltage values from day 3 (0.012 ± 0.001 V) to day 26 (0.427 ± 0.031 V) and then showed a continuous decrease until day 45 (0.223 ± 0.098 V); see Figure 2a. The first days did not show significant voltage values due to the acclimatization of the microorganisms that generate the redox process, which is responsible for the potential differential between the electrodes. The initial days showed low voltage values due to the microorganisms needing time to adjust and generate the redox process, which creates the potential difference between the electrodes. The decrease in voltage from day 27 to day 45 occurred because the carbon sources started to deplete, which hindered the growth of the fungus Pleurotus ostreatus. Additionally, the fungus Psathyrella candolleana acted as a catalyst in the MFC, producing voltage peaks of 400 mV. It was noted that the low porosity of the electrodes used in this research negatively impacted the MFC’s performance [32]. Votat et al. (2024) used the fungus Trichoderma harzianum as a catalyst for biodegrading dyes used in the textile industry, managing to generate voltage peaks of 300.1 mV. In their study, they observed the same phenomenon shown in our research (decrease in values after a maximum value), mentioning that this is because the proliferation of the fungus depletes the sources used as food for its metabolism [33]. Likewise, Umar et al. (2023) used the fungus Ganoderma gibbosum in their MFCs, managing to generate a maximum voltage value of 810 mV on the tenth day of operation and then stopping generating voltage on the 18th day. The authors mentioned that voltage loss is due to the rapid depletion of oxygen in the MFC due to excessive fungal growth [34]. The MFCs showed a considerable value of the electric current from the third day (0.198 ± 0.004 mA) to the 26th day (4.325 ± 0.261 mA) and then showed a continuous decrease until the 45th day (2.272 ± 0.294 mA). Consistent with that shown in Figure 2a, the values of the electric current increased from the third day due to the acclimatization period of the fungi in the MFCs; one of the most important factors was the formation of the anodic biofilm in the first days of operation, while the increase in the values of the electric current in the following days was due to the electrons generated by the metabolism of the fungi during their proliferation. The decrease in the electric current values is because by not having food sources such as carbon, the fungi begin to reach the end of their life and generate sediments in the lower part of the MFC, which increases the resistance of the substrate and therefore of the MFC, harming the performance of electron generation [35]. The yeast Rhodotorula glutinis has been used as a biocatalyst to generate electric current and, achieving maximum values of 1.76 ± 0.16 mA on day 14, holds promising implications for the field of microbial fuel cells. The increase in electric current values was attributed to the intermediate molecules (simple and complex) existing in the substrate, which facilitated the fungus’s metabolism through various processes involved in microbial growth and electricity production [36]. Similarly, in the literature, the potential of glucose in MFCs has been demonstrated, as it enriches the substrate with carbon sources used by various microbes to increase their metabolism. This process leads to an increase in the release of electrons that are captured by the anodic electrode [37]. These findings not only pave the way for more efficient and sustainable energy production but also inspire further research and practical applications in the field.

The pH levels increased from moderately acidic to neutral, with the pH on day 26 being 5.539 ± 0.278, Figure 3a. The increase in pH values in the substrate is due to the movement of protons from the anodic to the cathodic chamber during the operation of the microbial fuel cell (MFC). It is important to note that the microbes in the substrate, which tend to produce weak acids, have a significant impact on the high pH levels. Previous studies have suggested that the optimal operating pH levels are close to neutral. However, standardization has not been achieved yet, as different microbes, such as fungi, thrive at different pH levels [38]. The bacteria Anaerolineaceae, Rhodocy-claceae, and Geobacteraceae have been used in effluents as substrates in MFCs, generating voltage peaks of 394.12 mV when operating at a pH of 6 [39]. The fungus Pleurotus eryngii has been used as a catalyst in waste, generating maximum voltage values of 510 mV, mentioning that the modification of the electrodes with fluctuating electrodes increased the electric current values by 20% [40].

Figure 3b demonstrates the progressive increases in electrical conductivity from day one (34.211 ± 1.248 mS/cm) to day 26 (130.574 ± 4.981 mS/cm), followed by a decline until the last day (79.817 ± 5.568 mS/cm). This change in electrical conductivity is directly linked to the ionic release in the oxidation–reduction process in the substrate. As the compounds present were depleted, the electrical conductivity values also decreased, leading to an increase in the substrate’s resistance. In previous studies, they showed an electrical conductivity of 20 mS/cm and emphasized the direct relationship between the electrical conductivity of the substrate and the internal resistance of the MFC. This is because the electrons released in the metabolic process will have a higher resistance to be captured by the anodic electrode [41]. An electrical conductivity of 3500 mS/cm in their wastewater used as effluents has also been shown, which produced a voltage peak of 610 mV. They observed that following the peak voltage and electric current values, there was a decrease due to environmental factors from the substrate itself [42]. The chemical oxygen demand (COD) values showed a reduction of 72.15% in the 45 days of operation, while on day 25, a reduction of 59.87% (405.84 ± 16.58 mg/L) was observed, as shown in Figure 3c. The decrease in COD values was due to the different processes within the MFC during electrical energy generation. The consumption of organic matter by the Pleurotus Ostreatus fungus helped to reduce these concentrations. The concentration of COD was also reduced by 83% while generating voltage peaks of 0.07 V in the fruit waste used as a substrate, mentioning that the reduction was due to the oxidation processes that occurred in the anodic chamber and the hydrolysis rate that occurred in the fermentation of the substrate [43]. The tendency towards a decrease in COD values is because the complex carbohydrates in the various types of substrates release soluble sugars used by the microorganisms present for their metabolism [44].

The maximum observed power density was 86.316 ± 4.724 mW/cm² at a current density of 5.163 mA/cm² with a peak voltage of 372.349 ± 25.155 mV; see Figure 4a. The linear relationship observed between voltage and current density corresponds to the presence of the dominant ohmic loss by the proton exchange membrane. Liu et al. used the fungus Ganoderma lucidum in their MFCs, achieving a peak power density (PD) of 223 mW/m², reporting that the addition of metallic particles such as copper to the electrode improves the PD values because they serve as catalysts in electron transport [45]. Likewise, the fungus Sporotrichum pruinosum has been used in MFCs as a biodegrader in industrial water waste, achieving a maximum PD of 20.13 ± 0.052 mW/m², varying a large number of fungi in different types of electrodes. They reported that the compatibility of the fungus with the electrode is vital since there are toxic materials that can harm the growth of this microorganism [46]. MFC usage has become a cost-effective process that uses different types of waste as fuel for energy generation. However, power density efficiency remains one of the most significant challenges to overcome. According to the literature, the study of variations in electrode size and separation between them seems to be the right way, offering potential solutions to these challenges [47]. Figure 4b shows the value of the internal resistance found, which was 37.384 ± 62.522 Ω of the MFCs on day 26; the low value shown could be due to the high electrical conductivity shown by the MFC on that day, which reinforces the values shown for voltage and electric current found in the research since a lower resistance generates higher values of electric current. Another study has also reported an internal resistance of 243.3 Ω and a power density of 145.11 mW/m² in MFCs using carbon electrodes and agricultural waste as substrate [48]. The activated sludge and carbon felts were used as substrates, generating an internal resistance of 339.25 Ω and a maximum voltage of 121.40 mV [49]. Previous studies have also shown that when metallic nanoparticles are incorporated into anodic electrodes, they enhance the electrical performance of the MFC. These nanoparticles serve as biocatalysts, effectively capturing electrons generated in the substrate and aiding in their swift transportation from the anodic to the cathodic chamber [50].

The maximum observed power density was 86.316 ± 4.724 mW/cm² at a current density of 5.163 mA/cm² with a peak voltage of 372.349 ± 25.155 mV. The linear relationship observed between voltage and current density corresponds to the presence of the dominant ohmic loss by the proton exchange membrane. Liu et al. used the fungus Ganoderma lucidum in their MFCs, achieving peak power density (PD) of 223 mW/m², reporting that the addition of metallic particles such as copper to the electrode improves the PD values because they serve as catalysts in electron transport [45]. The initial and final reading of the spectra of the plastic samples incubated in the MFCs for 45 days can be observed in Figure 5. The initial and final spectra show the vibrations called “stretching” between the regions of 2850 cm⁻¹ and 2920 cm⁻¹. The peak at 1470 cm⁻¹ corresponds to the molecular vibrations called “bending” and the peak at 720 cm⁻¹ to the vibration mode called “rocking”. These peaks coincide in the initial and final states of the plastic sample. However, other peaks can be observed in the final spectrum, such as the peak at 3280 cm⁻¹ belonging to the methylene group, 1650 cm⁻¹ belonging to the carbonyl group, and 1070 cm⁻¹ to the ether group [28,51]. The intensity variations in the characteristic peaks of the plastics in the initial and final states are possibly due to the activity of the Pleurotus ostreatus fungus, which generates molecular changes due to its activity. It is known that Pleurotus ostreatus produces the enzyme laccase at high levels, which is considered a plastic biodegrading agent [52]. In addition, the presence of methylene, carbonyl, and ether groups and the reduction in the initial peaks of the plastic confirm the alterations generated by the plastic and the fungus. This phenomenon has also been observed in other investigations with similar results [53,54]. The use of fungi to break down plastic and the decrease in peaks identified by FTIR are a clear indication of plastic biodegradation [55]. Other studies have also shown that a reduction in C–H bonds (peaks at 720, 1470, and 2920 cm⁻¹) suggests that polyethylene has undergone biodegradation in the presence of fungi, which aligns with the degradation observed in our research [56].

Figure 6 shows the initial and final micrographs of the plastic samples used. The initial surface of the plastic (Figure 6a) is smooth, without any deformation, with a thickness of 740.65 µm. Meanwhile, on the final surface (Figure 6b), the formation of irregular protruding structures is observed due to the activity of the fungus Pleurotus ostreatus, whose plastic sample in its final state showed a thickness of 686.59 µm, demonstrating a reduction of 54.06 µm on day 45. These results confirm that the fungus Pleurotus ostreatus can grow on polymeric surfaces. According to previous studies, the white structures observed in the final state of the plastic sample are due to the formation of calcium oxalate, known to donate electrons in the degradation process of lignocellulose and also to act as bioremediation in solid waste treatment processes [53]. In our groundbreaking research, we have discovered that fungal hyphae’s ability to secrete hydrophobic exoenzymes is closely connected to their capacity to penetrate surfaces [21]. For example, fungal hydrolases (such as lipases, carboxylesterases, cutinases, and proteases) have been found to change the surface of plastic, making it more hydrophilic [57]. Likewise, the involvement of oxidoreductases (such as laccases and peroxidases) in breaking down plastic into smaller molecules like oligomers, dimers, and monomers is a new and fascinating discovery. This is particularly true for plastic polymers like PE, PS, PP, and PVC, which require oxidation before the depolymerization process can occur [58].

4. Conclusions

This research has been conducted with meticulous precision, successfully generating electrical energy and reducing plastic waste using microbial fuel cells, sugarcane bagasse as a substrate, and the Pleurotus ostreatus fungus as a biodegrader for 45 days. The highest electrical values recorded by the MFCs were observed on day 26, reaching a value of 0.427 ± 0.031 V and 4.325 ± 0.261 mA for voltage and electric current, respectively. The pH values increased from the beginning, reaching an optimal operating value of 5.539 ± 0.278, while the electrical conductivity was 130.574 ± 4.981 mS/cm on day 26. In addition, the COD was reduced by 83% during the 45 days of operation of the MFCs. A maximum power density (PD) value of 86.316 ± 4.724 mW/cm² was obtained for a current density (CD) of 5.163 mA/cm², with an internal resistance of 37.384 ± 62.522 Ω. The initial and final FTIR spectra showed characteristic peaks of the plastic sample in the regions 2850–2920 cm⁻¹, 1470 cm⁻¹, and 720 cm⁻¹, corresponding to the molecular vibrations known as stretching, bending, and rocking. These peaks showed a decrease in intensity in the final spectrum. In addition, the micrographs revealed a reduction of 54.06 µm in the thickness of the plastic film and the formation of irregular structures on the surface due to the activity of Pleurotus ostreatus on the plastic sample used.