The effect of physico-chemically immobilized methylene blue and neutral red on the anode of microbial fuel cell

Microbial Biofilms: Features of Formation and Potential for Use in Bioelectrochemical Devices

Microbial biofilms present one of the most widespread forms of life on Earth. The formation of microbial communities on various surfaces presents a major challenge in a variety of fields, including medicine, the food industry, shipping, etc. At the same time, this process can also be used for the benefit of humans-in bioremediation, wastewater treatment, and various biotechnological processes. The main direction of using electroactive microbial biofilms is their incorporation into the composition of biosensor and biofuel cells This review examines the fundamental knowledge acquired about the structure and formation of biofilms, the properties they have when used in bioelectrochemical devices, and the characteristics of the formation of these structures on different surfaces. Special attention is given to the potential of applying the latest advances in genetic engineering in order to improve the performance of microbial biofilm-based devices and to regulate the processes that take place within them. Finally, we highlight possible ways of dealing with the drawbacks of using biofilms in the creation of highly efficient biosensors and biofuel cells.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Redox mediators stimulated chain elongation process in fluidized cathode electro-fermentation systems for caproate production

Medium chain fatty acids (MCFAs), the secondary products of traditional anaerobic fermentation, can be produced via chain elongation (CE), a process often retarded due to the difficulty during interspecies electron transfer (IET). This study employed redox mediators, neutral red (NR), methyl viologen (MV), and methylene blue (MB) as electron shuttles to expedite the electro-fermentation for caproate production by improving IET. Results showed that MV increased the MCFAs production by promoting acetate to ethanol conversion, leading to the highest MCFAs selectivity of 68.73%. While NR was indicated to improve CE by encouraging H2 production, and the biocathode had the highest electrical activity due to the smallest internal resistance and largest capacitance increase of 96% than the control. A higher proportion of Sutterella, Prevotella, and Hydrogenophaga, linked with the H2 mediated interspecies electron transfer (MIET) during CE process, was observed across redox mediators supplied groups compared to the control. The presence of mediators led to an elevated abundance of key enzymes for enhanced CE process and electron transfer. This study provided the perspective of the stimulated electron transfer for improved MCFAs production in electro-fermentation systems.

Isolation of a heterotrophic nitrification-aerobic denitrification strain and identification of its potential electricity generation ability in microbial fuel cells

To construct a simultaneous nitrification and denitrification-microbial fuel cell (SND-MFC) reactor for stable electricity generation, a heterotrophic nitrification-aerobic denitrification strain was isolated and purified from aerobic activated sludge from a sewage treatment plant. The strain with an optimal nitrogen removal performance, which was identified as Pseudomonas, was inoculated into the SND-MFC as a single strain. Different electrode materials and electrode distances (EDs) were investigated. The results showed that a maximum ammonium removal rate of 92.31% and a maximum power density of 134.28 mW/m³ were obtained by the SND-MFC using graphene film as the electrodes material. Decreasing the ED did not significantly improved the power generation performance of the pure strain SND-MFC at the initial stage. When the electrode distance of the SND-MFC was 4 cm, the best generation efficiency was achieved with a maximum power density of 151.84 mW/m³.

Solid neutral red/Nafion conductive layer on carbon felt electrode enhances acetate production from CO2 and energy efficiency in microbial electrosynthesis system

Renewable electricity-based microbial electrosynthesis can upgrade CO₂ into value-added chemicals and simultaneously increase the number of biocatalysts by cell growth, helping to achieve sustainable carbon-negative processes. In most studies, the main strategy for improving the MES performance was to enhance H₂-based electron uptake by decreasing the overpotential and electrical conductivity of the electrode. Less is known about the electrode-based direct electron uptake for CO₂ conversion in MES. In this study, a solid neutral red/Nafion conductive layer was developed on the carbon electrode surface using a feasible dip and dry method. The modified electrode showed higher HER overpotential and lower capacitance but enhanced redox capability and hydrophobicity, which increased direct electron transport to the bacteria rather than hydrogen-based indirect electron delivery. The Neutral red/Nafion-implemented MES showed faster start-up, higher acetate production, and energy efficiency than the non-modified electrode.

Improved Microbial Fuel Cell Performance by Engineering E. coli for Enhanced Affinity to Gold

Microorganism affinity for surfaces can be controlled by introducing material binding motifs into proteins such as fimbrial tip and outer membrane proteins. Here, controlled surface affinity is used to manipulate and enhance electrical power production in a typical bioelectrochemical system, a microbial fuel cell (MFC). Specifically, gold-binding motifs of various affinity were introduced into two scaffolds in Escherichia coli: eCPX, a modified version of outer membrane protein X (OmpX), and FimH, the tip protein of the fimbriae. The behavior of these strains on gold electrodes was examined in small-scale (240 µL) MFCs and 40 mL U-tube MFCs. A clear correlation between the affinity of a strain for a gold surface and the peak voltage produced during MFC operation is shown in the small-scale MFCs; strains displaying peptides with high affinity for gold generate potentials greater than 80 mV while strains displaying peptides with minimal affinity to gold produce potentials around 30 mV. In the larger MFCs, E. coli strains with high affinity to gold exhibit power densities up to 0.27 mW/m2, approximately a 10-fold increase over unengineered strains lacking displayed peptides. Moreover, in the case of the modified FimH strains, this increased power production is sustained for five days.

Sorting the main bottlenecks to use paper-based microbial fuel cells as convenient and practical analytical devices for environmental toxicity testing

Three of the primary bottlenecks, which should be consider for practical, point-of-need use of microbial fuel cell (MFC) analytical devices were surpassed in this work: i) the use of a diffusive barrier, hence, an electrogenic biofilm; ii) longer enrichment/stabilization times to produce a biofilm, made in a laboratory environment, over the electrode; and iii) difficulty comparing results obtained from MFCs based on electrogenic biofilms with standardized bioassays, a setback to be adopted as a new method. Here we show an easy way to determine water toxicity employing planktonic bacteria as biorecognition agents. The paper-based MFC contain an electron carrier (or mediator) to facilitate charge transfer from bacteria to the anode. In this way, there is no need to use biofilms. As far as we know this is the first paper-based MFC containing P. putida KT2440, a well characterized non-pathogenic bacteria previously used in standardized water toxicity bioassays. Results were obtained in 80 min and an effective concentration 50 of 9.02 mg L^-1, calculated for Zn²⁺ (a reference toxic agent), was successfully compared with previously published and ISO standardized bioassays, showing a promising future for this technology. The practical design and cost (less than one U.S. dollar) of the paper-based MFC toxicity test presented will open new market possibilities for rapid and easy-to-use MFC analytical devices.

Enhanced microbial fuel cell (MFC) power outputs through Membrane Permeabilization using a branched polyethyleneimine

This study demonstrates the impact outer membrane permeability has on the power densities generated by E. coli-based microbial fuel cells with neutral red as the mediator, and how increasing the permeability improves the current generation. Experiments performed with several lipopolysaccharide (LPS) mutants (ΔwaaC, ΔwaaF and ΔwaaG) of E. coli BW25113 that increase the outer membrane permeability found the power generated by two of the truncated LPS mutants, i.e., ΔwaaC and ΔwaaF, to be significantly higher (5.6 and 6.9 mW/m², respectively) than that of the wild-type E. coli BW25113 (2.6 mW/m²). Branched polyethyleneimine (BPEI, 400 mg/L), a strong chemical permeabilizer, was more effective, however, increasing the power output from E. coli BW25113 cultures to as much as 29.7 mW/m², or approximately 11-fold higher than the control MFC. BPEI also increased the activities of the mutant strains (to between 10.6 and 16.3 mW/m²), as well as when benzyl viologen was the mediator. Additional tests found BPEI not only enhanced membrane permeability but also increased the zeta potential of the bacterial cells from a value of -43.4 mV to -21.0 mV. This led to a significant increase in auto-aggregation of the bacterial cells and, consequently, better adherence of the cells to the anode electrode, as was demonstrated using scanning electron microscopy. In conclusion, our study demonstrates the importance of outer membrane permeabilities on MFC performances and defines two benefits that BPEI offers when used within MFCs as an outer membrane permeabilizer.

Bioelectrosynthetic Conversion of CO2 Using Different Redox Mediators: Electron and Carbon Balances in a Bioelectrochemical System

Microbial electrosynthesis (MES) systems can convert CO2 to acetate and other value-added chemicals using electricity as the reducing power. Several electrochemically active redox mediators can enhance interfacial electron transport between bacteria and the electrode in MES systems. In this study, different redox mediators, such as neutral red (NR), 2-hydroxy-1,4-naphthoquinone (HNQ), and hydroquinone (HQ), were compared to facilitate an MES-based CO2 reduction reaction on the cathode. The mediators, NR and HNQ, improved acetate production from CO2 (165 mM and 161 mM, respectively) compared to the control (without a mediator = 149 mM), whereas HQ showed lower acetate production (115 mM). On the other hand, when mediators were used, the electron and carbon recovery efficiency decreased because of the presence of bioelectrochemical reduction pathways other than acetate production. Cyclic voltammetry of an MES with such mediators revealed CO2 reduction to acetate on the cathode surface. These results suggest that the addition of mediators to MES can improve CO2 conversion to acetate with further optimization in an operating strategy of electrosynthesis processes.

Redox mediators as charge agents for changing electrochemical reactions

This review provides a comprehensive discussion toward understanding the effects of RMs in electrochemical systems, underlying redox mechanisms, and reaction kinetics both experimentally and theoretically.

Microbial electrocatalysis: Redox mediators responsible for extracellular electron transfer

Redox mediator plays an important role in extracellular electron transfer (EET) in many environments wherein microbial electrocatalysis occurs actively. Because of the block of cell envelope and the low difference of redox potential between the intracellular and extracellular surroundings, the proceeding of EET depends mainly on the help of a variety of mediators that function as an electron carrier or bridge. In this Review, we will summarize a wide range of redox mediators and further discuss their functional mechanisms in EET that drives a series of microbial electrocatalytic reactions. Studying these mediators adds to our knowledge of how charge transport and electrochemical reactions occur at the microorganism-electrode interface. This understanding would promote the widespread applications of microbial electrocatalysis in microbial fuel cells, bioremediation, bioelectrosynthesis, biomining, nanomaterial productions, etc. These improved applications will greatly benefit the sustainable development of the environmental-friendly biochemical industries.

Effect of biochar on bio-electrochemical dye degradation and energy production

The effect of coconut shell biochar on dye degradation in a microbial fuel cell (MFC) was investigated in the present study. Two different doses of biochar (0.5 g and 1 g) and one control without bio-char were studied. The highest COD removal efficiency was about 77.7% (0.5 g biochar), maximum current (1.07 mA) and voltage (722 mV) were obtained with 1 g biochar. Biofilm optical microscopy characterization revealed the micro colonies intricate plate-like structures. High adsorbent dosage might provide a high surface area for biofilm to generate electricity. BET results of coconut shell biochar showed the maximum surface area of 0.9669 m²/g and macroporosity (0.0032 cm³/g). The overall results highlighted the possibility of using biochar as an additive in MFC for efficient dye degradation.

Carbon quantum dots shuttle electrons to the anode of a microbial fuel cell

Electrodes based on graphite, graphene, and carbon nanomaterials have been used in the anode chamber of microbial fuel cells (MFCs). Carbon quantum dots (C-dots) are a class of versatile nanomaterials hitherto not reported in MFCs. C-dots previously synthesized from coconut husk were reported to possess hydroxyl and carboxyl functional groups on their surface. The presence of these functional groups on a carbon matrix conferred on the C-dots the ability to conduct and transfer electrons. Formation of silver nanoparticles from silver nitrate upon addition of C-dots confirmed their reducing ability. DREAM assay using a mixed microbial culture containing C-dots showed a 172% increase in electron transfer activity and thus confirmed the involvement of C-dots in supplementing redox activity of a microbial culture. Addition of C-dots as a suspension in the anode chamber of an MFC resulted in a 22.5% enhancement in maximum power density. C-dots showed better performance as electron shuttles than methylene blue, a conventional electron shuttle used in MFCs.

Performance of a single chamber microbial fuel cell at different organic loads and pH values using purified terephthalic acid wastewater

BACKGROUND

Purified terephthalic acid (PTA) wastewater from a petrochemical complex was utilized as a fuel in the anode of a microbial fuel cell (MFC). Effects of two important parameters including different dilutions of the PTA wastewater and pH on the performance of the MFC were investigated.

METHODS

The MFC used was a membrane-less single chamber consisted of a stainless steel mesh as anode electrode and a carbon cloth as cathode electrode. Both power density and current density were calculated based on the projected surface area of the cathode electrode. Power density curve method was used to specify maximum power density and internal resistance of the MFC.

RESULTS

Using 10-times, 4-times and 2-times diluted wastewater as well as the raw wastewater resulted in the maximum power density of 10.5, 43.3, 55.5 and 65.6 mW m(-2), respectively. The difference between the power densities at two successive concentrations of the wastewater was considerable in the ohmic resistance zone. It was also observed that voltage vs. initial wastewater concentration follows a Monod-type equation at a specific external resistance in the ohmic zone. MFC performance at three different pH values (5.5, 7.0 and 8.5) was evaluated. The power generated at pH 8.5 was higher for 40% and 66% than that for pH 7.0 and pH 5.4, respectively.

CONCLUSIONS

The best performance of the examined MFC for industrial applications is achievable using the raw wastewater and under alkaline or neutralized condition.

Combination of a novel electrode material and artificial mediators to enhance power generation in an MFC

This study focuses on two main aspects: developing a novel cost-effective electrode material and power production from domestic wastewater using three different mediators. Methylene blue (MB), neutral red (NR) and 2-hydroxy-1,4-naphthoquinone (HNQ) were selected as electrode mediators with different concentrations. A tin-coated copper mesh electrode was tested as anode electrode. Maximum power density of the microbial fuel cell (MFC) with 300 μM MB was 636 mW/m². Optimal mediator concentrations with respect to the achieved maximum power output for MB, NR and HNQ were 300 μM, 200 μM and 50 μM, respectively. The results demonstrate that tin-coated copper mesh showed a higher biocompatibility and electrical conductivity.

Removal of sulfide and production of methane from carbon dioxide in microbial fuel cells-microbial electrolysis cell (MFCs-MEC) coupled system

Removal of sulfide and production of methane from carbon dioxide in microbial electrolysis cells (MECs) at the applied voltage of 0.7 V was achieved using sulfide and organic compound as electron donors. The removal rate of sulfide was 72% and the Faraday efficiency of methane formation was 57% within 70 h of operation. Microbial fuel cell (MFCs) can be connected in series to supply power and drive the reaction in MECs. Removal of sulfide and production of methane from carbon dioxide in MFCs-MEC coupled system was achieved. The sulfide removal rates were 62.5, 60.4, and 57.7%, respectively, in the three anode compartments. Methane accumulated at a rate of 0.354 mL h(-1) L(-1) and the Faraday efficiency was 51%. Microbial characterization revealed that the biocathode of MEC was dominated by relatives of Methanobacterium palustre, Methanobrevibacter arboriphilus, and Methanocorpusculum parvum. This technology has a potential for wastewater treatments and biofuel production from carbon dioxide.

Simultaneous co-metabolic decolourisation of azo dye mixtures and bio-electricity generation under thermophillic (50 °C) and saline conditions by an adapted anaerobic mixed culture in microbial fuel cells

In this study, azo dye adapted mixed microbial consortium was used to effectively remove colour from azo dye mixtures and to simultaneously generate bio-electricity using microbial fuel cells (MFCs). Operating temperature (20-50 °C) and salinity (0.5-2.5%w/v) were varied during experiments. Reactor operation at 50 °C improved dye decolourisation and COD removal kinetic constants by approximately 2-fold compared to the kinetic constants at 30 °C. Decolourisation and COD removal kinetic constants remained high (0.28 h(-1) and 0.064 h(-1) respectively) at moderate salinity (1%w/v) but deteriorated approximately 4-fold when the salinity was raised to 2.5% (w/v). Molecular phylogenetic analysis of microbial cultures used in the study indicated that both un-acclimated and dye acclimated cultures from MFCs were predominantly comprised of Firmicutes bacteria. This study demonstrates the possibility of using adapted microbial consortia in MFCs for achieving efficient bio-decolourisation of complex azo dye mixtures and concomitant bio-electricity generation under industrially relevant conditions.