Enhancement of gaseous o-xylene degradation in a microbial fuel cell by adding Shewanella oneidensis MR-1

Integrated system of electro-catalytic oxidation and microbial fuel cells for the treatment of recalcitrant wastewater

The emission of recalcitrant wastewater poses serious threats to the environment. In this study, an integrated approach combining electrocatalytic oxidation (EC) for pretreatment and microbial fuel cells (MFC) for thorough pollutant degradation is proposed to ensure efficient degradation of target substances, with low energy input and enhanced bioavailability of refractory organics. When phenol was used as the pollutant, an initial concentration of 2000 mg/L phenol solution underwent EC treatment under constant current-exponential attenuation power supply mode, resulting in a COD removal rate of 54.53%, and a phenol degradation rate of 99.83%. Intermediate products such as hydroquinone and para-diphenol were detected in the solution. After subsequent MFC treatment, only minor amounts of para-diphenol were left, and the degradation rate of phenol and its intermediate products reached 100%, with an output power density of 110.4 mW m-2. When coal chemical wastewater was used as the pollutant, further examination of the EC-MFC system performance showed a COD removal rate of 49.23% in the EC section, and a 76.21% COD removal rate in the MFC section, with an output power density of 181.5 mW m-2. Microbiological analysis revealed typical electrogenic bacteria (such as Pseudomonas and Geobacter), and specific degrading functional bacteria (such as Stenotrophomonas, Delftia, and Brevundimonas). The dominant microbial communities and their proportions adapted to environmental changes in response to the variation of carbon sources.

Enhancement of gaseous chlorobenzene biodegradation and power generation in a microbial fuel cell by bifunctional Acinetobacter sp. HY-99C

The efficient biodegradation of volatile chlorinated hydrocarbons using microbial fuel cells (MFCs) offers a feasible approach for purifying waste gas and alleviating energy crises. However, power generation is limited by poor pollutant biodegradation and slow electron transfer. The bifunctional bacterium Acinetobacter sp. HY-99C was screened and used to improve the performance of a conventional MFC. The inoculation of strain HY-99C into the conventional MFC promoted the formation of a compact biofilm with high metabolic activity and an enriched bifunctional genus (Acinetobacter), which resulted in the accelerated decomposition of chlorinated aromatic compounds into biodegradable organic acids. This led to efficient chlorobenzene removal and power generation from the MFC, with a chlorobenzene elimination capacity of 70.8 g m-3 h-1 and power density of 89.6 mW m-2, which are improved over those of previously reported MFCs. This study provides novel insights into enhancing pollutant removal and power generation in MFCs.

Microbial co-culture mediated by intercellular nanotubes during DMAC degradation: Microbial interaction, communication mode, and degradation mechanism

Microbial co-culture has been proven as an effective technique for environmental remediation. In this study, co-culture mechanism of Rhodococcus ruber HJM-8 and Paracoccus communis YBH-X during N,N-dimethylacetamide (DMAC) degradation was studied. The comparison of degradation performance in monoculture and co-culture was presented; due to the efficient cooperation between the two strains via parallel and cascaded degradation, the removal efficiency of total nitrogen (TN) in co-culture could reach 90.1%, which was 1.35 and 1.21 times higher than that of HJM-8 and YBH-X, respectively. Then the communication mode of co-culture during DMAC degradation was determined as contact-independent and contact-dependent interactions between microorganisms. Meanwhile, intercellular nanotube between HJM-8 and YBH-X was found as a unique contact-dependent interaction. The cell staining experiments and RNA sequencing analyses revealed that the nanotube could be used as a bridge to exchange cytoplasmic molecules, and thus improved material transfer and enhanced cell connection in co-culture. The results of KEGG pathway showed that differentially expressed genes in co-culture have an association with cell metabolism, nanotube generation, and genetic material transfer. Furthermore, a mechanism diagram of DMAC biodegradation was proposed for co-culture, indicating that bidirectional cooperation was established between HJM-8 and YBH-X which was mediated by the conversions of acetate and nitrogen. Finally, the co-culture system was validated for treatment of an actual wastewater; results indicated that removal efficiencies of 100% and 68.2% were achieved for DMAC and TN, respectively, suggesting that co-culture had the potential for application.

Biofilm metagenomic characteristics behind high coulombic efficiency for propanethiol deodorization in two-phase partitioning microbial fuel cell

Hydrophobic volatile organic sulfur compounds (VOSCs) are frequently found during sewage treatment, and their effective management is crucial for reducing malodorous complaints. Microbial fuel cells (MFC) are effective for both VOSCs abatement and energy recovery. However, the performance of MFC on VOSCs remains limited by the mass transfer efficiency of MFC in aqueous media. Inspired by two-phase partitioning biotechnology, silicone oil was introduced for the first time into MFC as a non-aqueous phase (NAP) medium to construct two-phase partitioning microbial fuel cell (TPPMFC) and augment the mass transfer of target VOSCs of propanethiol (PT) in the liquid phase. The PT removal efficiency within 32 h increased by 11-20% compared with that of single-phase MFC, and the coulombic efficiency of TPPMFC (11.01%) was 4.32-2.68 times that of single-phase MFC owing to the fact that highly active desulfurization and thiol-degrading bacteria (e.g., Pseudomonas, Achromobacter) were attached to the silicone oil surface, whereas sulfur-oxidizing bacteria (e.g., Thiobacillus, Commonas, Ottowia) were dominant on the anodic biofilm. The outer membrane cytochrome-c content and NADH dehydrogenase activity improved by 4.15 and 3.36 times in the TPPMFC, respectively. The results of metagenomics by KEGG and COG confirmed that the metabolism of PT in TPPMFC was comprehensive, and that the addition of a NAP upregulates the expression of genes related to sulfur metabolism, energy generation, and amino acid synthesis. This finding indicates that the NAP assisted bioelectrochemical systems would be promising to solve mass-transfer restrictions in low solubility contaminates removal.

Metal-organic framework derived bio-anode enhances chlorobenzene removal and electricity generation: Special Ru4+/Ru3+-bridged intracellular electron transfer

Efficient removal of chlorinated organic contaminants using the microbial fuel cell (MFC) provides a promising strategy to alleviate water pollution and energy crisis. However, bio-degradation is challenged by poor biofilm formation and sluggish intracellular electron transfer, causing unsatisfactory electricity generation. To address those problems, a metal-organic framework derivative, Ru-porous TiO2 (Ru-PT) bio-anode has been artfully designed herein for chlorobenzene removal. The Ru-PT bio-anode not only formed a compact anodic biofilm due to the large specific surface area of PT, but more importantly, it introduced special pseudocapacitance-enhanced intracellular electron transfer by slowly implanting Ru4+/Ru3+ redox pair into bacteria. Such a Ru4+/Ru3+ implantation was then found to directionally induce the enrichment of a dual-functional genus (degrader & exoelectrogen), Pseudomonas, thereby enhancing the conversion of bio-refractory chlorophenols towards biodegradable carboxylic acids. These features allowed our MFC to have a resilient chlorobenzene removal and accompanied satisfactory electricity generation, with power density, coulombic efficiency, and turnover frequency reaching 662 mW m-2, 8.7%, and 386,622 s-1, which outcompeted those of other MFCs reported. Further, benefiting from the reversible pseudocapacitance, the Ru-PT bio-anode intriguingly functioned as an internal capacitor for electricity storage. This work provided important insights into cost-effective bio-anode development and offered an avenue for engineering MFC.

Boosting o-xylene removal and power generation in an airlift microbial fuel cell system

Microbial fuel cells (MFCs) are widely acknowledged to be a promising eco-friendly abatement technology of pollutants, and are capable of generating electricity. However, the poor mass transfer and reaction rate in MFCs significantly decrease their treatment capacity for contaminants, especially hydrophobic substances. The present work developed a novel MFC integrated with an airlift (ALR) reactor using a polypyrrole modified anode to promote the bioaccessibility of gaseous o-xylene and attachment of microorganisms. The results indicated that the established ALR-MFC system showed excellent elimination capability, with removal efficiency exceeding 84% even at high o-xylene concentration (1600 mg m^-3). The maximum output voltage of 0.549 V and power density of 13.16 mW m^-2 obtained by the Monod-type model were approximately twice and sixfold higher than that of a conventional MFC, respectively. According to the microbial community analysis, the superior performances of the ALR-MFC in terms of o-xylene removal and power generation were mainly ascribed to the enrichment of degrader (i.e. Shinella) and electrochemical active bacteria (i.e. Proteiniphilum). Moreover, the electricity generation of the ALR-MFC did not decrease at a high O₂ concentration, as O₂ was conducive to o-xylene degradation and electron release. The supplication of an external carbon source such as sodium acetate (NaAc) was conducive to increasing output voltage and coulombic efficiency. The electrochemical analysis revealed that released electrons can be transmitted with the action of NADH dehydrogenase to OmcZ, OmcS, and OmcA outer membrane proteins via a direct or indirect pathway, and ended up transferring to the anode directly.

High efficiency electrochemical separation of uranium(VI) from uranium-containing wastewater by microbial fuel cells with different cathodes

As an emerging versatile technology for separating uranium from uranium-containing wastewater (UCW), microbial fuel cell (MFC) offers a novel approach to UCW treatment. Its cathode is essential for the treatment of UCW. To thoroughly investigate the efficacy of MFC in treating UCW, investigations were conducted using MFCs with five materials (containing iron sheet (IP), stainless steel mesh (SSM), carbon cloth (CC), carbon brush (CB), and nickel foam (NF)) as cathodes. The results revealed that each MFC system performed differently in terms of carbon source degradation, uranium removal, and electricity production. In terms of carbon source degradation, CB-MFC showed the best performance. The best uranium removal method was NF-MFC, and the best electricity production method was carbon-based cathode MFC. Five MFC systems demonstrated stable performance and consistent difference over five cycles, with CC-MFC outperforming the others. Furthermore, SEM and XPS characterization of the cathode materials before and after the experiment revealed that a significant amount of U(IV) was generated during the uranium removal process, indicating that uranium ions were primarily removed by electrochemical reduction precipitation. This study confirmed that abiotic cathode MFC had a high UCW removal potential and served as a good guideline for obtaining the best cathode for MFC.

Peroxidase-like phosphate hydrate nanosheets bio-synthesized by a marine Shewanella algae strain for highly sensitive dopamine detection

The sensitive and rapid detection of dopamine (DA) is of great significance for early diagnosis of related diseases. Current detection strategies of DA are time-consuming, expensive and inaccurate, while biosynthetic nanomaterials were considered highly stable and environment friendly, which were promising on colorimetric sensing. Thus, in this study, novel zinc phosphate hydrate nanosheets (SA@ZnPNS) biosynthesized by Shewanella algae were designed for the detection of DA. SA@ZnPNS showed high peroxidase-like activity which catalyzed the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂. Results showed that the catalytic reaction of SA@ZnPNS followed Michaelis-Menton kinetics, and catalytic process conformed to ping-pong mechanism with chief active species of hydroxyl radicals. The colorimetric detection of DA in human serum samples was performed based on SA@ZnPNS peroxidase-like activity. The linear range of DA detection was 0.1-40 μM, and the detection limit was 0.083 μM. This study provided a simple and practical method for the detection of DA and expanded the application of biosynthesized nanoparticles to biosensing fields.

Interactions among microorganisms functionally active for electron transfer and pollutant degradation in natural environments

Compared to single microbial strains, complex interactions between microbial consortia composed of various microorganisms have been shown to be effective in expanding ecological functions and accomplishing biological processes. Electroactive microorganisms (EMs) and degradable microorganisms (DMs) play vital roles in bioenergy production and the degradation of organic pollutants hazardous to human health. These microorganisms can strongly interact with other microorganisms and promote metabolic cooperation, thus facilitating electricity production and pollutant degradation. In this review, we describe several specific types of EMs and DMs based on their ability to adapt to different environments, and summarize the mechanism of EMs in extracellular electron transfer. The effects of interactions between EMs and DMs are evaluated in terms of electricity production and degradation efficiency. The principle of the enhancement in microbial consortia is also introduced, such as improved biomass, changed degradation pathways, and biocatalytic potentials, which are directly or indirectly conducive to human health.

Modular Engineering Strategy to Redirect Electron Flux into the Electron-Transfer Chain for Enhancing Extracellular Electron Transfer in Shewanella oneidensis

Efficient extracellular electron transfer (EET) of exoelectrogens is critical for practical applications of various bioelectrochemical systems. However, the low efficiency of electron transfer remains a major bottleneck. In this study, a modular engineering strategy, including broadening the sources of the intracellular electron pool, enhancing intracellular nicotinamide adenine dinucleotide (NADH) regeneration, and promoting electron release from electron pools, was developed to redirect electron flux into the electron transfer chain in Shewanella oneidensis MR-1. Among them, four genes include gene SO1522 encoding a lactate transporter for broadening the sources of the intracellular electron pool, gene gapA encoding a glyceraldehyde-3-phosphate dehydrogenase and gene mdh encoding a malate dehydrogenase in the central carbon metabolism for enhancing intracellular NADH regeneration, and gene ndh encoding NADH dehydrogenase on the inner membrane for releasing electrons from intracellular electron pools into the electron-transport chain. Upon assembly of the four genes, electron flux was directly redirected from the electron donor to the electron-transfer chain, achieving 62% increase in intracellular NADH levels, which resulted in a 3.5-fold enhancement in the power density from 59.5 ± 3.2 mW/m² (wild type) to 270.0 ± 12.7 mW/m² (recombinant strain). This study confirmed that redirecting electron flux from the electron donor to the electron-transfer chain is a viable approach to enhance the EET rate of S. oneidensis.

Coproduction and enhancement of electricity and biobutanol using adsorption carrier solid-state fermentation

Background

Electric energy is not collected and utilized in biobutanol fermentation. The reason is that the yields of electron shuttles and nanowires are not enough to gather and transfer all electrons to the electrode in liquid fermentation. However, the solid matrix of the adsorption carrier may be conducive to the collection and transfer of electrons because of its good adsorption and conductivity. Therefore, this first-attempt study coupled microbial fuel cell (MFC) with adsorption carrier solid-state fermentation (ACSF). In addition, the effect and mechanism of adsorption carrier solid-state fermentation on power generation were explored.

Results

The power generation performance and fermentation performance were improved by ACSF. The power density by polyurethane and carbon felt carrier solid-state fermentation (PC) was 12 times that by no carrier fermentation (NC). The biobutanol yield of absorbent cotton and carbon felt carrier solid-state fermentation (ACC) was increased by 36.86%. Moreover, the mechanism was explored via metabolic flux analysis, cyclic voltammetry and scanning electron microscopy. The results of metabolic flux analysis showed that more electrons were produced and more carbon flowed to biobutanol production. The cyclic voltammetry results revealed that more riboflavin was produced to enhance extracellular electron transport (EET) by ACSF. The scanning electron microscopy image showed that the adsorption capacity and aggregation degree of bacteria were increased on the electrode and nanowires were observed by ACSF.

Conclusions

A new fermentation mode was established by coupling MFC with ACSF to improve substrate utilization, which will provide crucial insights into the fermentation industry. In addition, the ACSF is an effective method to enhance power generation performance and fermentation performance.

Graphical Abstract

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

Performance of a packed-bed anode bio-electrochemical reactor for power generation and for removal of gaseous acetone

The packed anode bioelectrochemical system (Pa-BES) developed in this study is a type of BES that introduces waste gas into a cathode and then into an anode, thereby providing the cathode with sufficient oxygen and reducing the amount of oxygen to the anode to promote the output of electricity. When the empty-bed residence time was 45 s and the liquid flowrate was 35 mL/s, the system achieved optimal performance. Under these conditions, removal efficiency, mineralization efficiency, voltage output, and power density were 93.86%, 93.37%, 296.3 mV, and 321.12 mW/m³, respectively. The acetone in the waste gas was almost completely converted into carbon dioxide, indicating that Pa-BES can effectively remove acetone and has the potential to be used in practical situations. A cyclic voltammetry analysis revealed that the packings exhibited clear redox peaks, indicating that the Pa-BES has outstanding biodegradation and power generation abilities. Through microbial community dynamics, numerous organics degraders, electrochemically active bacteria, nitrifying and denitrifying bacteria were found, and the spatial distribution of these microbes were identified. Among them, Xanthobacter, Bryobacter, Mycobacteriums and Terrimonawas were able to decompose acetone or other organic substances, with Xanthobacter dominating. Bacterium_OLB10 and Ferruginibacter are the electrochemically active bacteria in Pa-BES, while Ferruginibacter is the most abundant in the main anode, which is responsible for electron collection and transfer.

Soil Amoebae Affect Iron and Chromium Reduction through Preferential Predation between Two Metal-Reducing Bacteria

Soil protists are essential but often overlooked in soil and could impact microbially driven element cycling in natural ecosystems. However, how protists influence heavy metal cycling in soil remains poorly understood. In this study, we used a model protist, Dictyostelium discoideum, to explore the effect of interactions between soil amoeba and metal-reducing bacteria on the reduction of soil Fe(III) and Cr(VI). We found that D. discoideum could preferentially prey on the Fe(III)-reducing bacterium Shewanella decolorationis S12 and significantly decrease its biomass. Surprisingly, this predation pressure also stimulated the activity of a single S. decolorationis S12 bacterium to reduce Fe(III) by enhancing the content of electron-transfer protein cyt c, intracellular ATP synthesis, and reactive oxygen species (e.g., H₂O₂). We also found that D. discoideum could not prey on the Cr(VI)-reducing bacterium Brevibacillus laterosporus. In contrast, B. laterosporus became edible to amoebae in the presence of S. decolorationis S12, and their Cr(VI) reduction ability decreased under amoeba predation pressure. This study provides direct evidence that protists can affect the Cr and Fe cycling via the elective predation pressure on the metal-reducing bacteria, broadening our horizons of predation of protists on soil metal cycling.

Economic affordable carbonized phenolic foam anode with controlled structure for microbial fuel cells

In microbial fuel cells (MFCs), the anode electrode is a core structure as the catalytic area of exoelectrogens. The anode material for large-scale MFCs needs excellent bioelectrochemical performance and low fabrication costs. Herein, carbonized phenolic foam with controllable porous structures was developed as the bio-capacitor of MFCs. The proportion of sodium dodecylbenzene sulfonate (SDBS), which improved mixing and dissolution between the resin liquid and the foaming agent, was adjusted to form open pores on the foam film and skeletons, which promoted both the capacitance and biocompatibility of the anode. Within SDBS proportion from 0 to 1.2 wt%, the anode SPF-9 (0.9 wt%) obtained the best capacitance (37 ± 0.13 F g^-1), electrochemical active surface area (87 ± 0.38 cm²) and hydrophilia (contact angle 79 ± 0.2°). The MFCs with SPF-9 obtained the highest power density of 3980 ± 178 mW m^-2, while those of carbon-cloth anodes were 1600 ± 28 mW m^-2. The biofilm of SPF-9 also demonstrated higher activity and obtained larger abundance of exoelectrogens (68 ± 0.38%). The increased capacitance and biocompatibility mainly resulted in the good performance of SPF-9. The carbonized phenolic foam anode material was worth considering for the future application of MFCs due to its superior electrochemical performance and large-quantity fabrication capability.

Gaseous isopropanol removal in a microbial fuel cell with deoxidizing anode: Performance, anode characteristics and microbial community

A deoxidizing packing material (DPM) with an encapsulated deoxidizing agent (DA) was developed to construct the packed anodes of a trickle-bed microbial fuel cell (TB-MFC) for treating waste gas. The encapsulated DA can consume O₂ in waste gas and increase the voltage output and power density (PD) of the constructed TB-MFC. The DPM effectively enables the circulating water in TB-MFC for maintaining a low level of dissolved oxygen for 80 h. The results revealed that when the concentration of isopropanol (IPA) in waste gas was 0.74 g/m³, the TB-MFC (DPM with DA) exhibited an IPA removal efficiency (RE) of up to 99.7%. When DPM with DA was used as the packing material of the TB-MFC (486.6 mW/m³), the PD was 2.54 times that obtained when using coke as the packing material (191.6 mW/m³). The next-generation sequencing results demonstrated that because the oxygen content of the MFC anode chamber decreased over time in the TB-MFC, the richness of anaerobic electrogens (Pseudoxanthomonas, Flavobacterium, and Ferruginibacter) in the packing materials was increased. These electrogens mainly attached to the DPM, and IPA-degraders appeared in the circulating water of the TB-MFC. This enabled the TB-MFC to simultaneously achieve a high voltage output and IPA RE.

Current advances in microbial fuel cell technology toward removal of organic contaminants - A review

At present, water pollution and demand for clean energy are most pressing global issues. On a daily basis, huge quantity of organic wastes gets released into the water ecosystems, causing health related problems. The need-of-the-hour is to utilize proficient and cheaper techniques for complete removal of harmful organic contaminants from water. In this regard, microbial fuel cell (MFC) has emerged as a promising technique, which can produce useful electrical energy from organic wastes and decontaminate polluted water. Herein, we have systematically reviewed recently published results, observations and progress made on the applications of MFCs in degradation of organic contaminants, including organic synthetic dyes, agro pollutants, health care contaminants and other organics (such as phenols and their derivatives, polyhydrocarbons and caffeine). MFC-based hybrid technologies, including MFC-constructed wetland, MFC-photocatalysis, MFC-catalysis, MFC-Fenton process, etc., developed to obtain high removal efficiency and bioelectricity production simultaneously have been discussed. Further, this review assessed the influence of factors, such as nature of electrode catalysts, organic pollutants, electrolyte, microbes and operational conditions, on the performance of pristine and hybrid MFC reactors in terms of pollutant removal efficiency and power generation simultaneously. Moreover, the limitations and future research directions of MFCs for wastewater treatment have been discussed. Finally, a conclusive summary of the findings has been outlined.

Flavin mononucleotide-stimulated microbial fuel cell for efficient gaseous toluene abatement

Chemical park is regarded as a major contributor of VOCs emissions in China. Currently, a green and safe technology, microbial fuel cells (MFCs), is being developed for the VOCs abatement. Noting that effective electron transfer is critical to the MFC performance. In this work, flavin mononucleotide (FMN) was dosed as an electron shuttle to improve the removal of the typical toxic VOCs, toluene. The experimental results revealed that the performance of toluene removal and power generation were accelerated with the dosage of 0.2-2 μM FMN. With the addition of 1 μM FMN, the removal efficiency, the maximum output voltage and the coulombic efficiency of MFC were increased by 18.4%, 64.4% and 56.3%, respectively. However, a further increase in FMN concentration to 2 μM caused a reduction in the removal efficiency and coulombic efficiency. The images of scanning electron microscopy and confocal laser scanning microscopy showed that the presence of FMN greatly promoted the microbial growth and its activity. Furthermore, microbial community analysis also implied that the moderate dosage of FMN (0.2-1 μM) was beneficial for the growth of the typical exoelectrogens, Geobacter sp., and thus the coulombic efficiency was increased. In addition, an electron transfer pathway involving in cytochrome b, OMCs, cytochrome c, and MtrA was proposed based on the cyclic voltammetry analysis. This work will provide a fundamental theoretical support for its application of toxic VOCs abatement from the chemical park.

A vertically configured photocatalytic-microbial fuel cell for electricity generation and gaseous toluene degradation

A vertically configured photocatalytic-microbial fuel cell (photo-MFC) is developed by combining a nanodiamond-decorated ZnO (ZnO/ND) photocathode with a bioanode. The system can effectively couple the light energy with bioenergy to enhance the degradation of volatile organic compounds (VOCs) and boost electricity output. Results show that the composite system exhibits increased performance for toluene removal (60.65%), higher than those of individual parts (ZnO/ND-photocatalysis: 37.16%, MFC: 17.81%). Furthermore, its electrochemical performance is dramatically increased. The peak power density of 120 mW/m² and the current density of 1.07 A/m² are generated under light illumination, which are about 1.57-fold and 1.37-fold higher than that under dark (76 mW/m², 0.78 A/m²), respectively. Microbial community analysis demonstrates Proteobacteria and Firmicute are dominant phyla, implying they play important roles on accelerating the extracellular-electron transfer and toluene degradation. In addition, the underlying mechanism for toluene degradation in the photo-MFC system is preliminary explored. Our results suggest that the photo-MFC has great potential for simultaneous treatment of VOCs with energy recovery.

Exogenous electron transfer mediator enhancing gaseous toluene degradation in a microbial fuel cell: Performance and electron transfer mechanism

Effective electron transfer (ET) between microorganisms and electrodes is essential for the toluene degradation and power generation in a microbial fuel cell (MFC). In this work, the neutral red, with excellent electrochemical reversibility and compatible redox potential as NADH/NAD⁺, was selected as electron mediator to boost the performance of the MFC. Experimental results revealed that, with the 0.5 μM neutral red, the removal efficiency and coulombic efficiency of the gaseous toluene powered MFC was increased by ~19% and ~400%, respectively. However, further increase in neutral red concentration resulted in a decreased in removal efficiency and coulombic efficiency, which was attributed by the toxicity of neutral red to the microbes. The microbial community analysis indicated that, with the dosage of the neutral red, the dominated bacteria shifted from Geobacter to Ignavibacteriales, resulting in a high coulombic efficiency. With the further increase in the neutral red, the amount of Ignavibacteriales gradually decreased and thus the coulombic efficiency declined at a high neutral red concentration. Based on the cyclic voltammetry analysis, an electron transport pathway involving neutral red, cytochromes, and OMCs in neutral red mediated MFC was proposed. Overall, the dosage of neutral not only enhanced the electron transfer but also induced the growth of the exoelectrogens, and thus significantly improve the MFC performance.

Bioelectricity production and xylene biodegradation through double chamber benthic microbial fuel cells fed with sugarcane waste as a substrate

Xylene, a recalcitrant compound present in wastewater from activities of petrochemical and chemical industries causes chronic problems for living organisms and the environment. Xylene contaminated wastewater may be biodegraded through a benthic microbial fuel cell (BMFC) as seen in this study. Xylene was oxidized into intermediate 3-methyl benzoic acid and entirely converted into non-toxic carbon dioxide. The highest voltage of the BMFC reactor was generated at 410 mV between 23 and 90 days when cell potential was 1 kΩ. The reactor achieved a maximum power density of about 63 mW/m², and a current of 0.4 mA which was optimized from variable resistance (20 Ω - 1 kΩ). However, the maximum biodegradation efficiency of the BMFC was at 87.8%. The cyclic voltammetry curve helped to determine that the specific capacitance was 0.124 F/g after 30 days of the BMFC operation. Furthermore, the fitting equivalent circuit was observed with the help of Nyquist plot for calculating overall internal resistance of 65.82 Ω on 30th day and 124.5 Ω on 80th day. Staphylococcus edaphicus and Staphylococcus sparophiticus were identified by 16S rRNA sequencing as the dominant species in the control and BMFC electrode, presumably associated with xylene biodegradation.

Hydrogen sulfide affects the performance of a methanogenic bioelectrochemical system used for biogas upgrading

Methanogenic bioelectrochemical systems (BESs) can convert carbon dioxide (CO₂) to methane (CH₄) and may be used for anaerobic digester biogas upgrading. However, the effect of hydrogen sulfide (H₂S), a common biogas component, on BES performance is unknown. Thus, the objective of this study was to assess the effect of H₂S addition to the cathode headspace on BES performance at a range of initial gas-phase H₂S concentrations (0-6% v/v), as well as its effect on the anode and cathode microbial communities. As the initial cathode headspace H₂S increased from 0 to 2% (v/v), biocathodic CH₄ production increased by two-fold to 3.56 ± 0.36 mmol/L-d, due to dissolved H₂S transport from the cathode to the anode where H₂S was oxidized. Elemental sulfur and sulfate were H₂S oxidation products detected in the anode. Above 3% initial cathode headspace H₂S, biocathodic CH₄ production declined due to inhibition. A phylotype most closely related to Methanobrevibacter arboriphilus dominated the cathode archaeal communities. In the sulfide-amended BES, a phylotype similar to the exoelectrogen Ochrobactrum anthropi was enriched in both the anode and cathode, whereas phylotypes related to sulfate-reducing and sulfur oxidizing Bacteria were detected in the bioanode. Thus, sulfide transport and oxidation in the anode play an important role in methanogenic BESs treating sulfide-bearing biogas.

Advancement in Benthic Microbial Fuel Cells toward Sustainable Bioremediation and Renewable Energy Production

Anthropogenic activities are largely responsible for the vast amounts of pollutants such as polycyclic aromatic hydrocarbons, cyanides, phenols, metal derivatives, sulphides, and other chemicals in wastewater. The excess benzene, toluene and xylene (BTX) can cause severe toxicity to living organisms in wastewater. A novel approach to mitigate this problem is the benthic microbial fuel cell (BMFC) setup to produce renewable energy and bio-remediate wastewater aromatic hydrocarbons. Several mechanisms of electrogens have been utilized for the bioremediation of BTX through BMFCs. In the future, BMFCs may be significant for chemical and petrochemical industry wastewater treatment. The distinct factors are considered to evaluate the performance of BMFCs, such as pollutant removal efficiency, power density, and current density, which are discussed by using operating parameters such as, pH, temperature and internal resistance. To further upgrade the BMFC technology, this review summarizes prototype electrode materials, the bioremediation of BTX, and their applications.