Effect of pH on bacterial distributions within cathodic biofilm of the microbial fuel cell with maltodextrin as the substrate

PDA-Fe3O4 decorated carbon felt anode enhancing electrochemical performance of microbial fuel cells: Effect of electrode materials on electroactive biofilm

Anode modification is an effective strategy for enhancing the electrochemical performance of microbial fuel cell (MFC). However, the impacts of the modified materials on anode biofilm development during MFC operation have been less studied. We prepared a novel PDA-Fe3O4-CF composite anode by coating original carbon felt anode (CF) with polydopamine (PDA) and Fe3O4 nanoparticles. The composite anode material was characterized by excellent hydrophilicity and electrical conductivity, and the anodic biofilm exhibited fast start-up, higher biomass, and more uniform biofilm layer after MFC operation. The MFC reactor assembled with the composite anode achieved a maximum power density of 608 mW m-2 and an output voltage of 586 mV, which were 316.4% and 72.4% higher than the MFC with the original CF anode, respectively. Microbial community analysis indicated that the modified anode biofilm had a higher relative abundance of exoelectrogen species in comparison to the unmodified anode. The PICRUSt data revealed that the anodic materials may affect the bioelectrochemical performance of the biofilm by influencing the expression levels of key enzyme genes involved in biofilm extracellular polymer (EPS) secretion and extracellular electron transfer (EET). The growth of the anodic biofilm would exert positive or negative influences on the efficiency of electricity production and electron transfer of the MFCs at different operating stages. This work expands the knowledge of the role that anodic materials play in the development and electrochemical performance of anodic biofilm in MFCs.

Response of current distribution in a liter-scale microbial fuel cell to variable operating conditions

Microbial fuel cells (MFCs) are an emerging technology in renewable energy and waste treatment and the scale-up is crucial for practical applications. Undoubtedly, the analysis and comprehension of MFC operation necessitate essential information regarding the response of the current distribution to variable operating conditions, which stands as one of its significant dynamic characteristics. In this study, the dynamic responses of current distribution to external stimuli (external load, temperature, pH, and electrolyte concentration) were investigated by employing a segmented anode current collector in a liter-scale MFC. The results demonstrated that, with respect to the anodic segment close to the cathode, a major response of the segment current to changes in load, temperature and pH was observed while minor response to changes in ion concentration. It was also found that external stimuli-induced high current usually led to a worse current distribution while increasing electrolyte ion concentration could simultaneously improve the maximal power generation and current distribution. In addition, the response time of segment current to input stimulus followed the pattern of temperature ˃ pH ˃ ion concentration ˃ external load. The results and implication of this study would be helpful in enhancing the operational stability of scale-up MFCs in future practical application.

Generation of green electricity from sludge using photo-stimulated bacterial consortium as a sustainable technology

Microbial fuel cell (MFC) is a bio-electrical energy generator that uses respiring microbes to transform organic matter present in sludge into electrical energy. The primary goal of this work was to introduce a new approach to the green electricity generation technology. In this context a total of 6 bacterial isolates were recovered from sludge samples collected from El-Sheikh Zayed water purification plant, Egypt, and screened for their electrogenic potential. The most promising isolates were identified according to 16S rRNA sequencing as Escherichia coli and Enterobacter cloacae, promising results were achieved on using them in consortium at optimized values of pH (7.5), temperature (30°C) and substrate (glucose/pyruvate 1%). Low level red laser (λ = 632.8nm, 8mW) was utilized to promote the electrogenic efficiency of the bacterial consortium, maximum growth was attained at 210 sec exposure interval. In an application of adding standard inoculum (107 cfu/mL) of the photo-stimulated bacterial consortium to sludge based MFC a significant increase in the output potential difference values were recorded, the electricity generation was maintained by regular supply of external substrate. These results demonstrate the future development of the dual role of MFCs in renewable energy production and sludge recycling.

Efficient hydrogen production in single-chamber microbial electrolysis cell with a fermentable substrate under hyperalkaline conditions

Hydrogen production from food waste is of great significance for energy conversion and pollution control. The aim of this study was to investigate the glucose fermentation from food waste and hydrogen (H2) production in the single-chamber microbial electrolysis cell (MEC) under hyperalkaline conditions. Single-chamber MECs were tested with 1 g/L glucose as substrate under different pH values (i.e., 7.0, 9.5, and 11.2) and applied voltages (i.e., 0.8, 1.2, and 1.6 V). With pH increase from 7.0 to 11.2, H2 production with methanogenesis inhibition was significantly improved in the MEC. At pH of 11.2, the maximum current density reached 180 ± 9 A/m3 with the H2 purity of 93.3 ± 1.2% and average H2 yield of 7.72 ± 0.23 mol H2/ mol glucose under 1.6 V. Acetate from glucose fermentation was the largest electron sink within 12 h. Methanobacterium alcaliphilum dominated the archaeal communities with the relative abundance of > 99.0% in the cathodic biofilms. The microbial communities and mcr A gene copy numbers analyses showed that high pH enhanced the acetate production from glucose fermentation, inhibited syntrophic acetate-oxidizing with hydrogenotrophic methanogenesis in the anodic biofilms, and inhibited hydrogenotrophic methanogenesis in the cathodic biofilms. Our results of hyperalkaline conditions provide a feasible way to harvest H2 efficiently from fermentable substrates in the single-chamber MEC.

Synergistic interactions of microbial fuel cell and microbially induced carbonate precipitation technology with molasses as the substrate

The application of microbially induced carbonate precipitation (MICP) technology is critical, but many challenges remain. In this paper, a microbial fuel cell (MFC) is used to treat molasses wastewater, and the effluent is used as the substrate to promote the growth of urease-producing bacteria. The results showed that the maximum voltage of MFC was 500 mV, and the maximum power density was 169.86 mW/m². The mineralization rate reached 100% on the 15^th day, and the mineralized product was calcite CaCO₃. According to the microbial community analysis, the unclassified_Comamondaceae, Arcobacter, and Aeromonas, which could improve the OH^-, signal molecular transmission and small molecular nutrients to promote the urease activity of urease-producing bacteria. The above conclusions provide a new way to reuse molasses wastewater efficiently and to apply MICP technology in dust suppression.

An electricity-generating bacterium separated from oil sludge microbial fuel cells and its environmental adaptability

Electricity-generating bacteria as biocatalysts for microbial fuel cells (MFCs), their species, and power generation performance determine the pollution control and power generation performance of MFCs. And there are few studies on the types and performance of electricity-generating bacteria isolated from oily sludge microbial fuel cells. For improving the power generation performance of oily sludge MFCs, an electricity-generating bacterium was isolated from the oily sludge. More importantly, the adaptability of nitrogen to phosphorus ratio, temperature, and pH of the electricity-generating bacteria were adjusted by a controlled variable method. The results of this study showed that the electricity-generating bacterium was identified as Bacillus cereus, with a rod-shaped cell, about 0.5-1.0 μm in length. The optimal nitrogen-phosphorus ratio, temperature, and pH of MFCs were 4.67:1, 25 ℃, and pH = 7, respectively. Its maximum power density, COD, and oil removal rate was up to 65 mW·m^-3, 90.51%, and 87.76%, respectively. The study of this functional bacterium will provide beneficial assistance for the improvement of oil removal and power generation performance of oily sludge MFCs.

High current density with spatial distribution of Geobacter in anodic biofilm of the microbial electrolysis desalination and chemical-production cell with enlarged volumetric anode

The aim of this study was to establish the relationship between spatial distribution of Geobacter and electric intensity in the microbial electrolysis desalination and chemical-production cell (MEDCC) and to investigate the effect of enlarged volumetric anode on the performance of MEDCC. The MEDCC was constructed with nine carbon brush anodes (length × diameter = 11 cm × 3 cm) as enlarged volumetric anode, and operated by feeding with 1 g/L acetate as substrate and 35 g/L NaCl as artificial seawater under the applied voltages of 1.2-4.5 V. Spatial distribution of Geobacter in the anodic biofilm was determined according to the bacterial community analysis on 27 biofilm samples from the top, middle and bottom layers of anodes (i.e., with distance of 4.5, 10, and 15.5 cm to the cathode, respectively). Results showed that the enlarged volumetric anode significantly improved the performance of MEDCC. The maximum desalination rate and current density reached 338.5 ± 21.8 mg/L∙h and 55.7 ± 3.7 A/m² in the MEDCC, respectively. The electric intensity values decreased with the distance from the anode to the cathode and formed an uneven distribution in the anode chamber. The samples in the top layer of anodes had the highest average 16S rRNA gene copy number of Geobacter of 1.55 × 10⁷ copies/μL, which was 18 times higher than that in the bottom layer of anodes. A linear relation was established between the spatial distribution of Geobacter and electric intensity (R² = 0.994-0.999). The electric intensity gradient created the uneven spatial distribution of Geobacter in the biofilms of volumetric anode. Results from this study could be useful to enrich Geobacter in the anodic biofilm thus to improve the performance of MEDCC.

Enhanced sulfur recovery and sulfate reduction using single-chamber bioelectrochemical system

The aim of this study was to investigate the feasibility of sulfate removal and elemental sulfur (S⁰) recovery in the single-chamber bioelectrochemical system (S-BES). The performance of S-BES was compared with that of dual-chamber bioelectrochemical system (D-BES). The S-BES was constructed with graphite felt as the anode and graphite brush as the cathode. The D-BES was constructed with proton exchange membrane as the separator between anode and cathode chambers. With an applied voltage of 1.0 V and 1 g/L acetate as the substrate, the S-BES and D-BES were tested by feeding with 480 mg/L SO₄^2- in the phosphate buffer. Results showed that the maximum current density of 37.6 ± 4.5 mA/m³ was reached in the S-BES, which was higher than that in the D-BES (i.e., 22.2 ± 2.6 mA/m³). The SO₄^2- removal was much higher in the S-BES than in the D-BES (99.5% vs. 57.2%). In the effluent and the electrodes of S-BES, S⁰ was identified with Raman and X- Ray diffraction analyses. The S⁰ recovery on the anode was 13.7 times of that on the cathode of S-BES, indicating that S⁰ was mainly produced on the anode. The measured total S⁰ recovery reached 67.5% in the S-BES. High relative abundance of Desulfurella (47.1%) and Geobacter (26.1%) dominated the community in the anode biofilm of S-BES. The excellent performance of S-BES may be attributed to the neutral pH in the solution and the synergistic reaction between the anode and cathode. Results from this study should be useful to enhance the S-BES applications in treating wastewater containing sulfate.

Heterogeneous Structure Regulated by Selection Pressure on Bacterial Adhesion Optimized the Viability Stratification Structure of Electroactive Biofilms

As the core of microbial fuel cells (MFCs), the components and structure of electroactive biofilms (EABs) are essential for MFC performance. Bacterial adhesion plays a vital role in shaping the structure of EABs, but the effect of bacterial adhesion under selection pressure on EABs has not been systematically studied. Here, the response of the composition, structure, and electrochemical performance of EABs to the selective adhesion pressure due to the selective coordination of Fe(III) and Co(II) with thiol and the different affinities for bacteria on hybrid electrodes (Fe₁Co, Fe₄Co, and Fe₁₀Co) were comprehensively investigated. Compared with carbon cloth (CC), the appropriate selective adhesion pressure of Fe₄Co activated the dead inner core of EABs and optimized their viability stratification structure. Both the total viability and the viability of the inner core layer in the Fe₄Co EAB (0.67, 0.70 ± 0.01) were higher than those of the CC (0.46, 0.54 ± 0.01), Fe₁Co (0.50, 0.48 ± 0.03), and Fe₁₀Co (0.51, 0.51 ± 0.03). Moreover, a higher proportion of proteins was detected in the Fe₄Co EAB, enhancing the redox activity of extracellular polymeric substances. Fe₄Co enriched Geobacter and stimulated microbial metabolism. Electrochemical analysis revealed that the Fe₄Co EAB was the most electroactive EAB, with a maximum power density of 2032.4 mW m^-2, which was 1.7, 1.3, and 1.1 times that of the CC (1202.6 mW m^-2), Fe₁Co (1610.3 mW m^-2), and Fe₁₀Co (1824.4 mW m^-2) EABs, respectively. Our findings confirmed that highly active EABs could be formed by imposing selection pressure on bacterial adhesion.