The influence of benzene on the composition, diversity and performance of the anodic bacterial community in glucose-fed microbial fuel cells

Bioelectrochemical systems offer unique opportunities to remove recalcitrant environmental pollutants in a net positive energy process, although it remains challenging because of the toxic character of such compounds. In this study, microbial fuel cell (MFC) technology was applied to investigate the benzene degradation process for more than 160 days, where glucose was used as a co-metabolite and a control. We have applied an inoculation strategy that led to the development of 10 individual microbial communities. The electrochemical dynamics of MFC efficiency was observed, along with their 1H NMR metabolic fingerprints and analysis of the microbial community. The highest power density of 120 mW/m2 was recorded in the final period of the experiment when benzene/glucose was used as fuel. This is the highest value reported in a benzene/co-substrate system. Metabolite analysis confirmed the full removal of benzene, while the dominance of fermentation products indicated the strong occurrence of non-electrogenic reactions. Based on 16S rRNA gene amplicon sequencing, bacterial community analysis revealed several petroleum-degrading microorganisms, electroactive species and biosurfactant producers. The dominant species were recognised as Citrobacter freundii and Arcobacter faecis. Strong, positive impact of the presence of benzene on the alpha diversity was recorded, underlining the high complexity of the bioelectrochemically supported degradation of petroleum compounds. This study reveals the importance of supporting the bioelectrochemical degradation process with auxiliary substrates and inoculation strategies that allow the communities to reach sufficient diversity to improve the power output and degradation efficiency in MFCs beyond the previously known limits. This study, for the first time, provides an outlook on the syntrophic activity of biosurfactant producers and petroleum degraders towards the efficient removal and conversion of recalcitrant hydrophobic compounds into electricity in MFCs.

Enriching electroactive microorganisms from ferruginous lake waters - Mind the sulfate reducers!

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.

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.

Enhanced Hydrocarbons Biodegradation at Deep-Sea Hydrostatic Pressure with Microbial Electrochemical Snorkels

In anaerobic sediments, microbial degradation of petroleum hydrocarbons is limited by the rapid depletion of electron acceptors (e.g., ferric oxide, sulfate) and accumulation of toxic metabolites (e.g., sulfide, following sulfate reduction). Deep-sea sediments are increasingly impacted by oil contamination, and the elevated hydrostatic pressure (HP) they are subjected to represents an additional limitation for microbial metabolism. While the use of electrodes to support electrobioremediation in oil-contaminated sediments has been described, there is no evidence on their applicability for deep-sea sediments. Here, we tested a passive bioelectrochemical system named ”oil-spill snorkel” with two crude oils carrying different alkane contents (4 vs. 15%), at increased or ambient HP (10 vs. 0.1 MPa). Snorkels enhanced alkanes biodegradation at both 10 and 0.1 MPa within only seven weeks, as compared to nonconductive glass controls. Microprofiles in anaerobic, contaminated sediments indicated that snorkels kept sulfide concentration to low titers. Bulk-sediment analysis confirmed that sulfide oxidation by snorkels largely regenerated sulfate. Hence, the sole application of snorkels could eliminate a toxicity factor and replenish a spent electron acceptor at increased HP. Both aspects are crucial for petroleum decontamination of the deep sea, a remote environment featured by low metabolic activity.

1,4-Dioxane-contaminated groundwater remediation in the anode chamber of a microbial fuel cell

A two-chambered microbial fuel cell (MFC) was used for the first time for the remediation of an emerging contaminant-1,4-dioxane in its anode chamber. Groundwater historically detected 1,4-dioxane contamination was sampled from a Superfund site. Comparative study was carried out between metabolic (i.e., 1,4-dioxane as sole carbon source) and cometabolic (i.e., 1,4-dioxane and methanol as carbon sources) anodic degradations. It was found that cometabolic degradation increased 1,4-dioxane removal by 10%-52% after 7 days and increased maximum power production of the MFC by 18% to 88.9 mW/m³ . Oxalic acid was detected as a main metabolic degradation product. Beside oxalic acid, acetic acid and isopropanol were also detected as main products for cometabolic degradation. The presence of a biofilm for 1,4-dioxane anodic degradation was observed by a scanning electron microscopy. Phyla of Bacteroidetes, Firmicutes, and Proteobacteria, as well as a variety of species, were identified for the first time-especially Rikenella sp. and Solitalea canadensis, whose relative abundances were the highest of 18.8% and 24.0% for metabolic and cometabolic degradation, respectively. This study provided an innovative and sustainable approach for 1,4-dioxane anodic biodegradation, which would be potentially utilized for remediation of groundwater contaminated by 1,4-dioxane. PRACTITIONER POINTS: Groundwater contaminated with 1,4-dioxane was remediated in the anode chamber of a two-chambered microbial fuel cell. Cometabolic pathway increased 1,4-dioxane removal and power production of the MFC compared to metabolic pathway. The presence of a biofilm for 1,4-dioxane anodic degradation was observed, and oxalic acid was a main degradation product. This study would be potentially utilized for 1,4-dioxane-contaminated groundwater remediation with simultaneous energy production. External voltage supply for bioelectrochemical remediation of groundwater would potentially be reduced when treating chlorinated hydrocarbons co-occurred with 1,4-dioxane.

Emulsified polycolloid substrate biobarrier for benzene and petroleum-hydrocarbon plume containment and migration control - A field-scale study

The objective of this field-scale study was to assess the effectiveness of applying an emulsified polycolloid substrate (EPS; containing cane molasses, soybean oil, and surfactants) biobarrier in the control and remediation of a petroleum-hydrocarbon plume in natural waters. An abandoned petrochemical manufacturing facility site was contaminated by benzene and other petroleum products due to a leakage from a storage tank. Because benzene is a petroleum hydrocarbon with a high migration ability, it was used as the target compound in the field-scale study. Batch partition and sorption experiment results indicated that the EPS to water partition coefficient for benzene was 232 mg/mg at 25 °C. This suggests that benzene had a higher sorption affinity to EPS, which decreased the benzene concentrations in groundwater. The EPS solution was pressure-injected into three remediation wells (RWs; 150 L EPS in 800 L groundwater). Groundwater samples were collected from an upgradient background well, two downgradient monitor wells (MWs), and the three RWs for analyses. EPS injection increased total organic carbon (TOC) concentrations (up to 786 mg/L) in groundwater, which also resulted in the formation of anaerobic conditions. An abrupt drop in benzene concentration (from 6.9 to below 0.04 mg/L) was observed after EPS supplementation in the RWs due to both sorption and biodegradation mechanisms. Results show that the EPS supplement increased total viable bacteria and enhanced bioremediation efficiency, which accounted for the observed decrease in benzene concentration. The first-order decay rate in RW1 increased from 0.003 to 0.023 d^-1 after EPS application. Injection of EPS resulted in significant growth of indigenous bacteria, and 23 petroleum-hydrocarbon-degrading bacterial species were detected, which enhanced the in situ benzene biodegradation efficiency. Results demonstrate that the EPS biobarrier can effectively contain a petroleum-hydrocarbon plume and prevent its migration to downgradient areas, which reduces the immediate risk presented to downgradient receptors.

Restructured fungal community diversity and biological interactions promote metolachlor biodegradation in soil microbial fuel cells

Soil microbial fuel cells (MFCs) provide an inexhaustible electron acceptor for the removal of metolachlor and in situ biocurrent stimulation for fungal activity was investigated. The metolachlor degradation rates enhanced by 33%-36% upon the introduction of electrodes after 23 d. In closed MFCs, the abundance of Mortierella as the most dominant genus increased to 43%-54% from 17% in the original soil, whereas those of Aphanoascus and Penicillium decreased to 0.24%-0.39% and 0.38-0.72% from 14% to 11%, respectively. Additionally, a 10-fold amplification of unique OTUs was observed, mainly from increase on the electrode surface. The different treatments were clustered, especially samples near the cathode. The linear discriminant analysis showed that Aphanoascus fulvescens acted as a biomarker between the original and treated soils. The co-occurrence networks demonstrated that Mortierella universally competed for growth with coexisting species while Cladosporium exhibited the most affiliations with species from the 36 other genera present. The correlation analysis indicated that the species associated with degradation belonged to Mortierella, Kernia, Chaetomium and Trichosporon, while the species associated with electrogenesis were Debaryomyces hansenii and Mortierella polycephala. Importantly, this study is the first to reveal fungal community structure in soil MFCs with degrading pollutants and producing electricity.

Cathodic microbial community adaptation to the removal of chlorinated herbicide in soil microbial fuel cells

The microbial fuel cell (MFC) that uses a solid electrode as the inexhaustible electron acceptor is an innovative remediation technology that simultaneously generates bioelectricity. Chlorinated pollutants are better metabolized by reductive dechlorination in proximity to the cathode. Here, the removal efficiency of the herbicide metolachlor (ML) increased by 262 and 176% in soil MFCs that were spiked with 10 (C10) and 20 mg/kg (C20) of ML, respectively, relative to the non-electrode controls. The bioelectricity output of the C10 and C20 increased by over two- and eightfold, respectively, compared to that of the non-ML control, with maximum current densities of 49.6 ± 2.5 (C10) and 78.9 ± 0.6 mA/m² (C20). Based on correlations between ML concentrations and species abundances in the MFCs, it was inferred that Azohydromonas sp., Sphingomonas sp., and Pontibacter sp. play a major role in ML removal around the cathode, with peak removal efficiencies of 56 ± 1% (C10) and 58 ± 1% (C20). Moreover, Clostridium sp., Geobacter sp., Bacillus sp., Romboutsia sp., and Terrisporobacter sp. may be electricigens or closely related microbes due to the significant positive correlation between the bioelectricity generation levels and their abundances around the anode. This study suggests that a directional adaptation of the microbial community has taken place to increase both the removal of chlorinated herbicides around the cathode and the generation of bioelectricity around the anode in bioelectrochemical remediation systems.

Composition and role of the attached and planktonic microbial communities in mesophilic and thermophilic xylose-fed microbial fuel cells

A mesophilic (37 °C) and a thermophilic (55 °C) two-chamber microbial fuel cell (MFC) were studied and compared for their power production from xylose and the anode-attached, membrane-attached and planktonic microbial communities involved.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.

Electrobioremediation of oil spills

Annually, thousands of oil spills occur across the globe. As a result, petroleum substances and petrochemical compounds are widespread contaminants causing concern due to their toxicity and recalcitrance. Many remediation strategies have been developed using both physicochemical and biological approaches. Biological strategies are most benign, aiming to enhance microbial metabolic activities by supplying limiting inorganic nutrients, electron acceptors or donors, thus stimulating oxidation or reduction of contaminants. A key issue is controlling the supply of electron donors/acceptors. Bioelectrochemical systems (BES) have emerged, in which an electrical current serves as either electron donor or acceptor for oil spill bioremediation. BES are highly controllable and can possibly also serve as biosensors for real time monitoring of the degradation process. Despite being promising, multiple aspects need to be considered to make BES suitable for field applications including system design, electrode materials, operational parameters, mode of action and radius of influence. The microbiological processes, involved in bioelectrochemical contaminant degradation, are currently not fully understood, particularly in relation to electron transfer mechanisms. Especially in sulfate rich environments, the sulfur cycle appears pivotal during hydrocarbon oxidation. This review provides a comprehensive analysis of the research on bioelectrochemical remediation of oil spills and of the key parameters involved in the process.

Microbial fuel cells as pollutant treatment units: Research updates

Microbial fuel cells (MFC) are a device that can convert chemical energy in influent substances to electricity via biological pathways. Based on the consent that MFC technology should be applied as a waste/wastewater treatment unit rather than a renewable energy source, this mini-review discussed recent R&D efforts on MFC technologies for pollutant treatments and highlighted the challenges and research and development needs. Owing to the low power density levels achievable by larger-scale MFC, the MFC should be used as a device other than energy source such as being a pollutant treatment unit.

Enhancement and monitoring of pollutant removal in a constructed wetland by microbial electrochemical technology

A bench-scale constructed wetland combined with microbial electrochemical technology (MET-CW) was run for 400days with groundwater contaminated with benzene, methyl-tert-butyl ether (MTBE), and ammonium (NH4(+)). Four vertically stacked anode modules were embedded into a sand bed and connected with a stainless steel cathode placed in an open water pond. In the zone of presence of anode modules, significantly more benzene and MTBE were removed in the MET-CW compared to the control CW without MET in the first 150 operation days. Benzene was identified as primary electron donor at the anode. Benzene removal and current densities were linearly correlated, implying the potential of the system for electrochemically monitoring benzene biodegradation. Compound-specific isotope analysis (CSIA) indicated that benzene was initially activated by monohydroxylation forming intermediates which were subsequently oxidized accompanied by extracellular electron transfer, leading to current production. NH4(+) removal was not stimulated by MET.

Improved biodegradation of total organic carbon and polychlorinated biphenyls for electricity generation by sediment microbial fuel cell and surfactant addition

The biodegradation of total organic carbon (TOC) and polychlorinated biphenyls (PCBs) in sediment was studied in different treatments.

Effect of zeolite on SPEEK /zeolite hybrid membrane as electrolyte for microbial fuel cell applications

A zeolite (H-faujasite) incorporated SPEEK membrane was demonstrated as an effective proton exchange membrane for Microbial Fuel Cell (MFC) application.