Biodegradation of organic matter and anodic microbial communities analysis in sediment microbial fuel cells with/without Fe(III) oxide addition

Microbial Electrochemical Technologies: Sustainable Solutions for Addressing Environmental Challenges

Addressing global challenges of waste management demands innovative approaches to turn biowaste into valuable resources. This chapter explores the potential of microbial electrochemical technologies (METs) as an alternative opportunity for biowaste valorisation and resource recovery due to their potential to address limitations associated with traditional methods. METs leverage microbial-driven oxidation and reduction reactions, enabling the conversion of different feedstocks into energy or value-added products. Their versatility spans across gas, food, water and soil streams, offering multiple solutions at different technological readiness levels to advance several sustainable development goals (SDGs) set out in the 2030 Agenda. By critically examining recent studies, this chapter uncovers challenges, optimisation strategies, and future research directions for real-world MET implementations. The integration of economic perspectives with technological developments provides a comprehensive understanding of the opportunities and demands associated with METs in advancing the circular economy agenda, emphasising their pivotal role in waste minimisation, resource efficiency promotion, and closed-loop system renovation.

Organic carbon compounds removal and phosphate immobilization for internal pollution control: Sediment microbial fuel cells, a prospect technology

As a current technology that can effectively remove organic carbon compounds and immobilize phosphorus in sediment, sediment microbial fuel cells (SMFCs) can combine sediment remediation with power generation. This review discusses the removal efficiency of SMFCs on organic carbon compounds, including sediment organic matter, antibiotics, oil-contaminated sediments, methane, persistent organic pollutants, and other organic pollutants in sediment, with more comprehensive and targeted summaries, and it also emphasizes the mitigation of phosphorus pollution in water from the perspective of controlling endogenous phosphorus. In this review, the microbial community is used as a starting point to explore more about its roles on phosphorus and organic carbon compounds under SMFCs. Electrode modification, addition of exogenous substances and combinations with other technologies to improve the performance of SMFCs are also reviewed. It is further demonstrated that SMFCs have the prospect of long-term sustainability, but more attention needs to be paid to the study of the mechanism of SMFCs and the continuous improvement of devices for further application in practice.

Laccase-loaded CaCO3 sustained-release microspheres modified SBES anode for enhance performance in the remediation of soil contaminated with phenanthrene and pyrene

This study aimed to enhance the efficiency of SBES in remediating polycyclic aromatic hydrocarbon (PAH)-contaminated soils by modifying the anode with laccase. The experiment involved four SBES anodes: a carbon nanotube-modified anode (CNT), a free laccase-modified anode (Lac), a gelatin-encapsulated laccase-modified anode (Lac-Gel), and a CaCO3 sustained-release microsphere-loaded laccase-modified (CaCO3-SMs@Laccase) anode (Lac-SMs). The CaCO3-SMs@Laccase notably extended the active period of laccase, with laccase activity in the Lac-SMs measured at 1.646 U/g after 16 days, which was significantly higher than the 0.813 U/g observed in the Lac-Gel group and the 0.206 U/g in the Lac group. The superior electricity generation and degradation efficiency observed in the Lac-SMs group were due to the sustained enzymatic activity provided by the CaCO3-SMs@Laccase. The prevention of anode acidification through CaCO3 decomposition, and promote the forward progress of electrochemical reactions. The phenanthrene (Phe) and pyrene (Pyr) removal efficiency in the soil of the Lac-SMs reached 90.78 % and 84.72 %, surpassing those of the Lac-Gel (80.36 % and 79.14 %), Lac (79.38 % and 69.31 %), and CNT (63.22 % and 56.98 %). The degradation pathway from Pyr to Phe was possible started with hydroxylation. In addition, the laccase also transformed the predominant microbial communities and metabolism pathways.

Exogenous paths regulate electron transfer enhancing sediment phosphorus immobilization

The lack of electron acceptors in anaerobic sediments leads to endogenous phosphorus release and low removal efficiency of organic pollutants. This study introduced electrodes and iron oxides into sediments to construct electron network transport chains to supplement electron acceptors. The sediment total organic carbon (TOC) removal efficiencies of closed-circuit (CC) and closed-circuit with Fe addition (CC-Fe) were estimated to be 1.4 and 1.7 times of the control. Unlike the fluctuation of phosphorus in the overlying water of the controls, the CC-Fe was stabled at 0.04-0.08 mg/L during the 84-d operation. The phosphorus in interstitial water of CC-Fe was 30 % less than in control, whereas in sediment, the redox sensitive phosphorus was increased by 14 %, indicating phosphorus was preferred to fix into sediments rather than interstitial water. This is important to reduce the risk of endogenous phosphorus returning to the overlying water. Microbial community analysis showed that the multiplication of Fonticella in CC-Fe (20 %) was 1.8-fold of control (11 %) which improved the TOC removal efficiency. While electroactive microorganisms accumulated near the electrode reduced the abundance of Fe-reducing bacteria, such as Desulfitobacterium (2.4 %), leading to better phosphorus fixation. These findings suggest a strategy for the efficient bioremediation of endogenous pollution in water, with broader implications for regulating electron transport paths and element cycles in aquatic environments.

Synergistic reduction of pollution and carbon mitigation in constructed wetlands-microbial fuel cell using sludge-derived biochar

Integrating microbial fuel cells (MFC) into constructed wetland systems (CW) has been an efficient wastewater treatment to improve the pollutants removal and regenerate power energy. This study fabricated a sludge biochar material (SBM) to sequestrate the carbon of residual sludge. Thereafter used SBM and modified SBM as the substrate materials to construct three groups of CW-MFC for decreasing the greenhouse gas (GHG) emission. The water quality improvement in removal efficiency achieved (2.59 %, 3.10 %, 5.21 % for COD; 3.31 %, 3.60 %, 6.71 % for TN; 1.80 %, 7.38 %, 4.93 % for TP) by the application of MFC, SBM, and modified SBM in wastewater treatment, respectively. Additionally, the reduction in global warming potential (GWP) realized 17.2 %, 42.2 %, and 64.4 % resulting from these applications. The carbon flow and fate diagrams showed MFC shifted the gas phase‑carbon flow from CH4 to CO2, and SBM promoted this shift trends. Microbial diversity indicated enrichment of electrochemically active bacteria (EAB), denitrifying bacteria, and phosphate accumulating organisms (PAOs) by SBM. Metabolic pathways analysis showed that introduction of MFC and SBM exhibited significant increases of key functional genes in metabolic pathway of anaerobic oxidation of methane (AOM). This study highlights the benefit of CW-MFC in and provides a new strategy for removing pollutants and abating GHG emissions in wastewater treatment.

Sediment microbial fuel cell coupled floating treatment wetland for enhancing non-reactive phosphorus removal

The presence of non-reactive phosphorus (NRP) in environmental waters presents a potential risk of eutrophication and poses challenges for the removal of all phosphorus (P) fractions. This study presents the first investigation on the removal performance and mechanism of three model NRP compounds, sodium tripolyphosphate (STPP), adenosine 5'-monophosphate (AMP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), in the sediment microbial fuel cell-floating treatment wetland (SMFC-FTW). Coupling SMFC with plants proved to be effective at removing NRP via electrochemical oxidation and plant uptake, particularly the challenging-to-degrade phosphonates that contain C-P bonds. Compared with the control group, the removal efficiencies of the model NRP in SMFC were observed to increase by 11.9%-20.8%. SMFC promoted the conversion of NRP to soluble reactive phosphorus (sRP) and the transfer of P to sediment. Furthermore, the electrochemical process enhanced both plant growth and P uptake, and increased P assimilation by 72.6%. The presence of plants in the bioelectrochemical system influenced the occurrence and fate of P by efficiently assimilating sRP and supporting microbial transformation of NRP. Consequently, plants enhanced the removal efficiencies of all P fractions in the overlying water. This study demonstrated that SMFC-FTW is a promising technology to remove various NRP species in environmental waters.

Magnetite-equipped algal-rich sediments for microbial fuel cells: Remediation of sediment organic matter pollution and mechanisms of remote electron transfer

Using the bio-electrochemical methods for the restoration of high algae sediments is full of potential and challenges. How to promote extracellular electron transfer (EET) process in microbial fuel cells (MFC) is the key bottleneck. The study had explored the potential application of magnetite on accelerating electron transfer for improving the output of MFC and sediment pollution remediation. The results indicated that the organic matter degradation rate showed a remarkable increase of 27.45 %, and the voltage output was approximately 1.68 times higher compared to the MFC configured with regular sediment. Abundant electroactive bacteria (EABs), such as Geobacter and Burkholderiaceae, and fermentative bacteria were responsible for these results, accompanied by the enhanced fluorescence of humic substances (HS), increased concentration and activity of cytochrome C (25.05 % and 21.12 %), as well as elevated extracellular polymeric substance content. Moreover, the intrinsic EET mechanisms among Fe-oxides, HS, and EABs were explored. According to the electrochemical analysis and substance transformation, the EET process involved four stages: magnetite-enhanced direct electron transfer via strong conductivity, iron respiration mediating electron transfer to the electrode, the model quinone substance acting as an electron shuttle facilitating EET and iron reduction, and iron cycling mediating electron transfer. This study provides an effective strategy for pollution remediation in algal-rich sediment, which was beneficial for the harmless treatment and resource utilization of both algae and sediment, simultaneously.

The application of ferrous and graphitic N modified graphene-based composite cathode material in the bio-electro-Fenton system driven by sediment microbial fuel cells to degrade methyl orange

In this work, the ferrous (Fe2+) and graphitic N modified graphene-based composite cathode materials (N-rGO/Fe3O4) were developed through an in-situ reduction method, aiming to facilitate the two-electron pathway in the oxidation-reduction process. This approach generated a specific concentration of H2O2, enabling the construction of a sediment bio-electro-Fenton system using Fe2+ released from the cathode materials. Notably, this system operates without the need for proton exchange membranes. During the cathode material preparation, the utilization of Fe2+ as a reduction agent for graphene oxide (GO), triggered ammonia water to form graphitic N in graphene sheets. This addition enhanced the two-electron pathway, resulting in increased H2O2 production. Specifically, when the Fe2+ concentration was maintained at 0.1 mol/L, precise preparation of N-rGO/Fe3O4 occurred, leading to a maximum output voltage of 0.528 V and a maximum power density of 178.17 mW/m2. The degradation of methyl orange (MO) reached 68.91% within a 25-h period, a phenomenon contributed to the presence of graphitic N in the graphene sheets. H2O2, a byproduct of the two-electron pathway in cathode oxidation reduction reaction, played a crucial role in constructing the bio-electro-Fenton system. This system, in conjunction with Fe2+ released from N-rGO/Fe3O4, facilitated the complete degradation process of MO.

Mineral transformation of poorly crystalline ferrihydrite to hematite and goethite facilitated by an acclimated microbial consortium in electrodes of soil microbial fuel cells

In this study, we investigated the biogenic mineral transformation of poorly crystalline ferrihydrite in the presence of an acclimated microbial consortium after confirming successful soil microbial fuel cell optimization. The acclimated microbial consortia in the electrodes distinctly transformed amorphous ferrihydrite into crystallized hematite (cathode) and goethite (anode) under ambient culture conditions (30 °C). Serial analysis, including transmission/scanning electron microscopy and X-ray/selected area electron diffraction, confirmed that the biogenically synthesized nanostructures were iron nanospheres (~100 nm) for hematite and nanostars (~300 nm) for goethite. Fe(II) ion production with acetate oxidation via anaerobic respiration was much higher in the anode electrode sample (3.2- to 17.8-fold) than for the cathode electrode or soil samples. Regarding the culturable bacteria from the acclimated microbial consortium, the microbial isolates were more abundant and diverse at the anode. These results provide new insights into the biogeochemistry of iron minerals and microbial fuel cells in a soil environment, along with physiological characters of microbes (i.e., iron-reducing bacteria), for in situ applications in sustainable energy research.

An Overview of Microbial Fuel Cell Technology for Sustainable Electricity Production

The over-exploitation of fossil fuels and their negative environmental impacts have attracted the attention of researchers worldwide, and efforts have been made to propose alternatives for the production of sustainable and clean energy. One proposed alternative is the implementation of bioelectrochemical systems (BESs), such as microbial fuel cells (MFCs), which are sustainable and environmentally friendly. MFCs are devices that use bacterial activity to break down organic matter while generating sustainable electricity. Furthermore, MFCs can produce bioelectricity from various substrates, including domestic wastewater (DWW), municipal wastewater (MWW), and potato and fruit wastes, reducing environmental contamination and decreasing energy consumption and treatment costs. This review focuses on recent advancements regarding the design, configuration, and operation mode of MFCs, as well as their capacity to produce bioelectricity (e.g., 2203 mW/m2) and fuels (i.e., H2: 438.7 mg/L and CH4: 358.7 mg/L). Furthermore, this review highlights practical applications, challenges, and the life-cycle assessment (LCA) of MFCs. Despite the promising biotechnological development of MFCs, great efforts should be made to implement them in a real-time and commercially viable manner.

Natural hematite as low-cost auxiliary material for improving soil remediation by in-situ microbial community

Microbial-mineral interaction has a broad application prospect in the field of environmental remediation of organic pollutants. However, the disadvantages of long repair cycle and low repair rate limit its industrial application. In this study, natural hematite was used as an auxiliary material for soil remediation in a bio-electrochemical system. It was found that the power density of soil microbial fuel cell (SMFC) system composed of 2.0 mm hematite was 2.889 mW/m2, which is 2.7 times compared with the blank group (1.068 mW/m2) in the particle size optimization experiment. A similarly increased power density (1.068 to 2.467 mW/m2) was observed when the hematite content changed from 0 to 20% in the concentration optimization experiment. Under 20% and 2.0-mm hematite condition, the phenol removal rate was closed to 99% after 7 days, which is 1.9-folds compared with blank control (53%). These results suggest that addition of hematite enhances soil porosity and conductivity, and increases the number of electron acceptors in soil. These findings inspire that this economic and abundant natural mineral is expected to be a potential auxiliary material in the field of soil organic pollutant purification, and expand the understanding of interactions between hematite and microorganisms in nature.

Customized Multichannel Measurement System for Microbial Fuel Cell Characterization

This work presents the development of an automatic and customized measuring system employing sigma-delta analog-to-digital converters and transimpedance amplifiers for precise measurements of voltage and current signals generated by microbial fuel cells (MFCs). The system can perform multi-step discharge protocols to accurately measure the power output of MFCs, and has been calibrated to ensure high precision and low noise measurements. One of the key features of the proposed measuring system is its ability to conduct long-term measurements with variable time steps. Moreover, it is portable and cost-effective, making it ideal for use in laboratories without sophisticated bench instrumentation. The system is expandable, ranging from 2 to 12 channels by adding dual-channel boards, which allows for testing of multiple MFCs simultaneously. The functionality of the system was tested using a six-channel setup, and the results demonstrated its ability to detect and distinguish current signals from different MFCs with varying output characteristics. The power measurements obtained using the system also allow for the determination of the output resistance of the MFCs being tested. Overall, the developed measuring system is a useful tool for characterizing the performance of MFCs, and can be helpful in the optimization and development of sustainable energy production technologies.

Methane emission reduction oriented extracellular electron transfer and bioremediation of sediment microbial fuel cell: A review

Sediment is the internal and external source of water environment pollution, so sediment remediation is the premise of water body purification. Sediment microbial fuel cell (SMFC) can remove the organic pollutants in sediment by electroactive microorganisms, compete with methanogens for electrons, and realize resource recycling, methane emission inhibiting and energy recovering. Due to these characteristics, SMFC have attracted wide attention for sediment remediation. In this paper, we comprehensively summarized the recent advances of SMFC in the following areas: (1) The advantages and disadvantages of current applied sediment remediation technologies; (2) The basic principles and influencing factors of SMFC; (3) The application of SMFC for pollutant removal, phosphorus transformation and remote monitoring and power supply; (4) Enhancement strategies for SMFC in sediments remediation such as SMFC coupled with constructed wetland, aquatic plant and iron-based reaction. Finally, we have summarized the drawback of SMFC and discuss the future development directions of applying SMFC for sediment bioremediation.

Anode Modification with Fe2O3 Affects the Anode Microbiome and Improves Energy Generation in Microbial Fuel Cells Powered by Wastewater

This study investigated how anode electrode modification with iron affects the microbiome and electricity generation of microbial fuel cells (MFCs) fed with municipal wastewater. Doses of 0.0 (control), 0.05, 0.1, 0.2, and 0.4 g Fe2O3 per the total anode electrode area were tested. Fe2O3 doses from 0.05 to 0.2 g improved electricity generation; with a dose of 0.10 g Fe2O3, the cell power was highest (1.39 mW/m2), and the internal resistance was lowest (184.9 Ω). Although acetate was the main source of organics in the municipal wastewater, propionic and valeric acids predominated in the outflows from all MFCs. In addition, Fe-modification stimulated the growth of the extracellular polymer producers Zoogloea sp. and Acidovorax sp., which favored biofilm formation. Electrogenic Geobacter sp. had the highest percent abundance in the anode of the control MFC, which generated the least electricity. However, with 0.05 and 0.10 g Fe2O3 doses, Pseudomonas sp., Oscillochloris sp., and Rhizobium sp. predominated in the anode microbiomes, and with 0.2 and 0.4 g doses, the electrogens Dechloromonas sp. and Desulfobacter sp. predominated. This is the first study to holistically examine how different amounts of Fe on the anode affect electricity generation, the microbiome, and metabolic products in the outflow of MFCs fed with synthetic municipal wastewater.

Effects of carbon source on electricity generation and PAH removal in aquaculture sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) have been used for treating pollutants in sediment or overlying water. This study investigated the feasibility of constructing SMFCs under aquaculture conditions by employing indigenous carbohydrates as substrates to enhance the removal efficiency of polycyclic aromatic hydrocarbons (PAHs) in sediment, as well as the correlation between PAHs removal and electricity generation in SMFCs. The results showed that adding glucose could allow SMFCs to generate more electrical power and increase the removal efficiency of PAHs (by 57.2% for naphthalene, 41.3% for acenaphthene, and 36.5% for pyrene). In addition, starch enhanced PAHs removal by 49.9%, 35.8%, and 31.2%, respectively, whereas cellulose enhanced removal by 44.3%, 29.3%, and 26.9%, respectively. Pearson correlation coefficients between the level of electrical power generated and the removal masses of the three PAHs were 0.485, 0.830**, and 0.851**. Thus, the use of SMFCs could be an effective approach for PAH treatment in aquaculture, and the electrical power generated could be used as an in-situ indicator for the biodegradation rate of SMFCs.

Use of activated carbon and camphor carbon as cathode and clay cup as proton exchange membrane in a microbial fuel cell for the bioenergy production from crude glycerol biodegradation

This work characterizes two alternative materials to substitute the most expensive microbial fuel cells (MFCs) components: proton exchange membrane (PEM) and cathode. Crude glycerol biodegradation was studied in MFCs using a clay cup as a PEM and activated carbon and camphor carbon mixture (CAC) as a cathode. The cathode performance was compared with Platinum on carbon cloth. Two clay cup single-chamber MFCs were operated with each cathode and fed with 2000 mg/L of crude glycerol. Electrochemical properties were characterized by linear sweep voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. Biodegradation efficiencies were estimated with the chemical oxygen demand (COD) removal percentage. MFCs with CAC showed a maximum power density of 100 mW/m². This result was a 43.47% power response regarding MFCs with Platinum. COD removal efficiencies of 94% were achieved in 37 days for both cells. The Columbic efficiencies were 24.04% and 22.78% for the MFCs with Platinum and CAC. The economic analysis showed a cost of USD 9.97 for MFCs with CAC. This cost is five times lower than when using Platinum. MFCs utilizing clay cups and CAC showed an acceptable performance for the bioenergy production from crude glycerol biodegradation above all economic advantage in the cell cost.

Improvement of zero waste sustainable recovery using microbial energy generation systems: A comprehensive review

Microbial energy generation systems, i.e., bioelectrochemical systems (BESs) are promising sustainable technologies that have been used in different fields of application such as biofuel production, biosensor, nutrient recovery, wastewater treatment, and heavy metals removal. However, BESs face great challenges such as large-scale application in real time, low power performance, and suitable materials for their configuration. This review paper aimed to discuss the use of BES systems such as conventional microbial fuel cells (MFCs), as well as plant microbial fuel cell (P-MFC), sediment microbial fuel cell (S-MFC), constructed wetland microbial fuel cell (CW-MFC), osmotic microbial fuel cell (OsMFC), photo-bioelectrochemical fuel cell (PBFC), and MFC-Fenton systems in the zero waste sustainable recovery process. Firstly, the configuration and electrode materials used in BESs as the main sources to improve the performance of these technologies are discussed. Additionally, zero waste recovery process from solid and wastewater feedstock, i.e., energy recovery: electricity generation (from 12 to 26,680 mW m^-2) and fuel generation, i.e., H₂ (170 ± 2.7 L^-1 L^-1 d^-1) and CH₄ (107.6 ± 3.2 mL^-1 g^-1), nutrient recovery of 100% (PO₄³-P), and 13-99% (NH₄⁺-N), heavy metal removal/recovery: water recovery, nitrate (100%), sulfate (53-99%), and sulfide recovery/removal (99%), antibiotic, dye removal, and other product recovery are critically analyzed in this review paper. Finally, the perspective and challenges, and future outlook are highlighted. There is no doubt that BES technologies are an economical option for the simultaneous zero waste elimination and energy recovery. However, more research is required to carry out the large-scale application of BES, as well as their commercialization.

Novel study on microbial fuel cells via a comprehensive bibliometric and dynamic approach

Microbial fuel cells (MFCs) are eco-friendly and useful bioelectrical devices that harness the natural metabolisms of microbes to produce electrical power directly from organic materials. In this study, a bibliometric analysis is conducted to evaluate MFC research from 2001 to 2018 on the basis of the Science Citation Index Expanded database. Overall, MFC research has experienced a dramatic increase over last 18 years, with an exponential growth in the accumulated number of publications. Most publications are closely related to the industrialization and commoditization of MFCs, along with environmental issues, which are currently the biggest global challenges in MFC studies. A small proportion (4.34%) of the scientific journals published more than half (54.34%) of the total articles in the MFC field. Articles from the top 10 countries/regions accounted for the majority (83.16%) of the total articles, clearly indicating that advanced MFC technologies are currently dominated by these countries/regions. Moreover, an increasing number of MFC researchers are considering two-chamber and three-chamber MFC reactions. In particular, they are focusing on environmental technology instead of merely improving the efficiency of electricity generation. Materials research in the MFC field is still a popular area worldwide, and many researchers have focused on novel and eco-friendly cathode and anode developments. Meanwhile, only a few MFC studies are concerned with biological research.

Advances and prospects on the aquatic plant coupled with sediment microbial fuel cell system

Energy resource scarcity and sediment pollution perniciousness have become enormous challenges, to which research has been focused on energy recovery and recycle technologies to solve both above problems. The organic matter stored in anoxic sediments of freshwater ecosystem represents a tremendous potential energy source. The system of aquatic plant coupled with sediment microbial fuel cell (AP-SMFC) has attracted much attention as a more feasible, economical and eco-friendly way to remediate sediment and surface water and generate electricity. However, the research on AP-SMFC has only been carried out in the last decade, and relevant studies have not been well summarized. In this review, the advances and prospects on AP-SMFC were systematically introduced. Firstly, the annual publication counts and keywords co-occurrence cluster of AP-SMFC were identified and visualized by resorting to the CiteSpace software, and the result showed that the research on AP-SMFC increased significantly in the last decade on the whole and will continue to increase. The bibliometric results provided valuable references and information on potential research directions for future studies. And then, the research progress and reaction mechanism of AP-SMFC were systematically described. Thirdly, the performance of AP-SMFC, including nutrients removal, organic contaminants removal, and electricity generation, was systematically summarized. AP-SMFC can enhance the removal of pollutants and electricity generation compared with SMFC without AP, and is considered to be an ideal technology for pollutants removal and resource recovery. Finally, the current challenges and future perspectives were summarized and prospected. Therefore, the review could serve as a guide for the new entrants to the field and further development of AP-SMFC application.

Enhancing microbial fuel cell performance using anode modified with Fe3O4 nanoparticles

Low electricity generation efficiency is one of the key issues that must be addressed for the practical application of microbial fuel cells (MFCs). Modification of microbial electrode materials is an effective method to enhance electron transfer. In this study, magnetite (Fe₃O₄) nanoparticles synthesized by co-precipitation were added to anode chambers in different doses to explore its effect on the performance of MFCs. The maximum power density of the MFCs doped with 4.5 g/L Fe₃O₄ (391.11 ± 9.4 mW/m²) was significantly increased compared to that of the undoped MFCs (255.15 ± 24.8 mW/m²). The COD removal efficiency of the MFCs increased from 85.8 ± 2.8% to 95.0 ± 2.1%. Electrochemical impedance spectroscopy and cyclic voltammetry tests revealed that the addition of Fe₃O₄ nanoparticles enhanced the biocatalytic activity of the anode. High-throughput sequencing results indicated that 4.5 g/L Fe₃O₄ modified anodes enriched the exoelectrogen Geobacter (31.5%), while control MFCs had less Geobacter (17.4%). Magnetite is widely distributed worldwide, which provides an inexpensive means to improve the electrochemical performance of MFCs.

Effects of water flow on performance of soil microbial fuel cells: Electricity generation, benzo[a]pyrene removal, microbial community and molecular ecological networks

Soil microbial fuel cells with water flow (W-SMFCs) as a driven force of substrate transport were constructed. Electricity generation, benzo[a]pyrene (BaP) removal, microbial communities and microbial molecular ecological networks were compared between W-SMFCs and their control reactors (without water flow, C-SMFCs) in 240 days of operation. The W-SMFCs started up faster than C-SMFCs (37 days vs. 50 days) and output higher startup voltage (148.45 mV vs. 111.90 mV). The water flow caused higher removal efficiency of BaP at sites >1 cm from the anode (S _{> 1 cm}) than at sites <1 cm from the anode (S _{< 1 cm}) in W-SMFCs, whereas in C-SMFCs, the removal efficiency of BaP at S_{< 1 cm} was higher than that at S_{> 1 cm}. The removal efficiency of BaP at S_{> 1 cm} in W-SMFCs was up to 1.7 times higher than that at S_{> 1 cm} in C-SMFCs on the 91^st day. After 240 days of operation, the biodegradation efficiency of absolute BaP amount was 45.95% in W-SMFCs, being 20% higher than that in C-SMFCs (38.17%). Moreover, the water flow caused highly tight interaction among the microbial species, which could be beneficial to BaP biodegradation. Conclusively, the water flow in soil was very beneficial for startup and biodegradation of BaP in SMFCs.

Accelerated bioremediation of a complexly contaminated river sediment through ZVI-electrode combined stimulation

Complexly contaminated river sediment caused by reducible and oxidizable organic pollutants is a growing global concern due to the adverse influence on ecosystem safety and planetary health. How to strengthen indigenous microbial metabolic activity to enhance biodegradation and mineralization efficiency of refractory composite pollutants is critical but poorly understood in environmental biotechnology. Here, a synergetic biostimulation coupling electrode with zero-valent iron (ZVI) was investigated for the bioremediation of river sediments contaminated by 2,4,6-tribromophenol (TBP, reducible pollutant) and hydrocarbons (oxidizable pollutants). The bioremediation efficiency of ZVI based biostimulation coupling electrode against TBP debromination and hydrocarbons degradation were 1.1-3 times higher than the electrode used solely, which was attributed to the shape of distinctive microbial communities and the enrichment of potential dehalogenators (like Desulfovibrio, Desulfomicrobium etc.). The sediment microbial communities were significantly positively correlated with the enhanced degradation efficiencies of TBP and hydrocarbons (P < 0.05). Moreover, the coupled system predominately increased positive microbial interactions in the ecological networks. The possible mutual relationship between microbes i.e., Thiobacillus (iron-oxidizing bacteria) and Desulfovibrio (dehalogenator) as well as Pseudomonas (electroactive bacteria) and Clostridium (hydrocarbons degraders) were revealed. This study proposed a promising approach for efficient bioremediation of complexly contaminated river sediments.

Recent advances in soil microbial fuel cells for soil contaminants remediation

The cost-effective and eco-friendly approaches are needed for decontamination of polluted soils. The bio-electrochemical system, especially microbial fuel cells (MFCs) offer great promise as a technology for remediation of soil, sediment, sludge and wastewater. Recently, soil MFCs (SMFCs) have been attracting increasing amounts of interest in environmental remediation, since they are capable of providing a clean and inexhaustible source of electron donors or acceptors and can be easily controlled by adjusting the electrochemical parameters. In this review, we comprehensively covered the principle of SMFCs including the mechanisms of electron releasing and electron transportation, summarized the applications for soil contaminants remediation by SMFCs with highlights on organic contaminants degradation and heavy metal ions removal. In addition, the main factors that affected the performance of SMFCs were discussed in details which would be helpful for performance optimization of SMFCs as well as the efficiency improvement for soil remediation. Moreover, the key issues need to be addressed and future perspectives are presented.

Generation of electrical energy in a microbial fuel cell coupling acetate oxidation to Fe3+ reduction and isolation of the involved bacteria

An iron reducing enrichment was obtained from sulfate reducing sludge and was evaluated on the capability of reducing Fe³⁺ coupled to acetate oxidation in a microbial fuel cell (MFC). Three molar ratios for acetate/Fe³⁺ were evaluated (2/16, 3.4/27 and 6.9/55 mM). The percentages of Fe³⁺ reduction were in a range of 80-90, 60-70 and 40-50% for the MFCs at closed circuit for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Acetate consumption was in a range of 80-90% in all cases. The results obtained at closed circuit for current density were: 11.37 mA/m², 4.5 mA/m² and 7.37 mA/m² for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Some microorganisms that were isolated and identified in the MFCs were Azospira oryzae, Cupriavidus metallidurans CH34, Enterobacter bugandensis 247BMC, Citrobacter freundii ATCC8090 and Citrobacter murliniae CDC2970-59, these bacteria have been reported as exoelectrogens in MFC and in MFC involving metals removal but not all of them have been reported to utilize acetate as preferred substrate. The results demonstrate that the isolates can utilize acetate as the sole source of carbon and suggest that Fe³⁺ reduction was carried out by a combination of different mechanisms (direct contact and redox mediators) utilized by the bacteria identified in the MFC. Storage of the energy generated from the 2/16 mM MFC system arranged in a series of three demonstrated that it is possible to utilize the energy to charge a battery.

Biosynthetic FeS/BC hybrid particles enhanced the electroactive bacteria enrichment in microbial electrochemical systems

Modifying the surface of an anode can improve electroactive bacteria (EAB) enrichment, thereby enhancing the performance of the associated microbial electrochemical systems (MESs). In this study, biosynthetic FeS nanoparticles were used to modify the anode in MESs. The experimental results demonstrated that the stable maximum voltage of the FeS composited biochar (FeS/BC)-modified anode reached 0.72 V, which is 20% higher than that of the control. The maximum power density with the FeS/BC anode was 793 mW/m², which is 46.31% higher than that obtained with the control (542 mW/m²). According to cyclic voltammetry (CV) analysis, FeS/BC facilitates the direct electron transfer between bacteria and the electrode. The biomass protein concentration of the FeS/BC anode was 841.75 μg/cm², which is almost 1.5 times higher than that of the carbon cloth anode (344.25 μg/cm²); hence, FeS/BC modification can promote biofilm formation. The composition of Geobacter species on the FeS/BC anode (75.16%) was much higher than that on the carbon cloth anode (4.81%). All the results demonstrated that the use of the biosynthetic FeS/BC anode is an environmentally friendly and efficient strategy for enhancing the electroactive biofilm formation and EAB enrichment in MESs.

Microorganisms in sediment microbial fuel cells: Ecological niche, microbial response, and environmental function

A sediment microbial fuel cell (SMFC) is a device that harvests electrical energy from sediments rich in organic matter. SMFCs have been attracting increasing amounts of interest in environmental remediation, since they are capable of providing a clean and inexhaustible source of electron donors or acceptors and can be easily controlled by adjusting the electrochemical parameters. The microorganisms inhabiting sediments and the overlying water play a pivotal role in SMFCs. Since the SMFC is applied in an open environment rather than in an enclosed chamber, the effects of the environment on the microbes should be intense and the microbial community succession should be extremely complex. Thus, this review aims to provide an overview of the microorganisms in SMFCs, which few previous review papers have reported. In this study, the anodic and cathodic niches for the microorganisms in SMFCs are summarized, how the microbial population and community interact with the SMFC environment is discussed, a new microbial succession strategy called the electrode stimulation succession is proposed, and recent developments in the environmental functions of SMFCs are discussed from the perspective of microorganisms. Future studies are needed to investigate the electrode stimulation succession, the environmental function and the electron transfer mechanism in order to boost the application of SMFCs for power generation and environmental remediation.

Effects of anode materials on the performance and anode microbial community of soil microbial fuel cell

Five soil microbial fuel cells (SMFCs) with graphite felt, aluminium sheet, activated carbon fibre felt, graphite paper and carbon cloth as anodes were constructed using the petroleum hydrocarbon polluted soils as substrates. After 115 days of operation, the SMFC with graphite felt anode performed the best in both bioelectricity output and removal of target pollutants, with the bioelectricity output parameters of 345 mV for stable voltage, 24.0 mW/m² for power density and 774 Ω for internal resistance, and the removal rates of 59.14 % for total petroleum hydrocarbon, 61.65 % for anthracene, and 55.92 % for pyrene, respectively. The conductivity of the material was the key factor affecting the electron transfer rate of the anode, which determined the electric acclimation and screening intensity of SMFC to soil microbes, leading to the growth and succession of the electricigens-dominanted anode microbial community with various abundances of phyla and genera. The surface structure of the anode material played a critical role in the internal resistance of SMFC through affecting the mass transfer of substrate and metabolites, and it might also change the abundance of microbes especially those non-electricigens on the community through different adhesion.

Insights into Advancements and Electrons Transfer Mechanisms of Electrogens in Benthic Microbial Fuel Cells

Benthic microbial fuel cells (BMFCs) are a kind of microbial fuel cell (MFC), distinguished by the absence of a membrane. BMFCs are an ecofriendly technology with a prominent role in renewable energy harvesting and the bioremediation of organic pollutants through electrogens. Electrogens act as catalysts to increase the rate of reaction in the anodic chamber, acting in electrons transfer to the cathode. This electron transfer towards the anode can either be direct or indirect using exoelectrogens by oxidizing organic matter. The performance of a BMFC also varies with the types of substrates used, which may be sugar molasses, sucrose, rice paddy, etc. This review presents insights into the use of BMFCs for the bioremediation of pollutants and for renewable energy production via different electron pathways.

Chlorella vulgaris on the cathode promoted the performance of sediment microbial fuel cells for electrogenesis and pollutant removal

The lack of electron acceptors in cathode has limited the widespread application of sediment microbial fuel cells (SMFCs). In this study, Chlorella vulgaris (C. vulgaris) was added to the cathode to produce oxygen as an electron acceptor. The synergistic effects between C. vulgaris and electrogenic microorganisms in SMFCs were investigated, and were shown to enhance biodegradation of organic matter in sediments and convert chemical energy into electrical energy. Results showed that the addition of C. vulgaris on the cathode of SMFCs significantly reduced their internal resistance. The low algae concentration SMFC group reduced the initial internal resistance by 67.4% under illumination and produced a maximum power density of 5.17 W/m³, which was 6 times higher than that of SMFCs without addition of C. vulgaris. We also obtained organic matter removal efficiencies 37.2% higher after 16 days, which accelerated the startup time for three times. It was demonstrated that IEF-N and OP, respectively, were forms of nitrogen and phosphorus removed by SMFCs. Additionally, high-throughput sequencing of microbial communities indicated that C. vulgaris increased the abundance of electrogenic bacteria (Geobacter and Desulfobulbaceae) in the anode and types of photosynthetic bacteria that support oxygen production in the cathode. The combined application of microalgae- and SMFC-based technologies offer a promising remediation approach for organically-contaminated sediments.

Simultaneous wastewater treatment and energy harvesting in microbial fuel cells: an update on the biocatalysts

The development of microbial fuel cell (MFC) makes it possible to generate clean electricity as well as remove pollutants from wastewater. Extensive studies on MFC have focused on structural design and performance optimization, and tremendous advances have been made in these fields. However, there is still a lack of systematic analysis on biocatalysts used in MFCs, especially when it comes to pollutant removal and simultaneous energy recovery. In this review, we aim to provide an update on MFC-based wastewater treatment and energy harvesting research, and analyze various biocatalysts used in MFCs and their underlying mechanisms in pollutant removal as well as energy recovery from wastewater. Lastly, we highlight key future research areas that will further our understanding in improving MFC performance for simultaneous wastewater treatment and sustainable energy harvesting.

Carbon nanomaterial-modified graphite felt as an anode enhanced the power production and polycyclic aromatic hydrocarbon removal in sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) can be used to generate electricity and remove organic contaminants. For electricity generation and contaminant removal, the anode material is one of important factors influencing the performance of SMFCs. In this study, graphene (GR), graphene oxide (GO) and carbon nanotubes (CNTs) were applied to modify the graphite felt (GF) anode in SMFCs during 110 d operation. An economical and easy modification method with the carbon nanomaterials was applied. The carbon nanomaterials increased the electrochemically active surface areas and biomass content of the anodes and correspondingly effectively enhanced the generation of electricity and the removal rates of loss on ignition (LOI) and polycyclic aromatic hydrocarbons (phenanthrene and pyrene). During the steady period from 50 d to 110 d, the GO-SMFCs favored the enrichment of EAB and thus output the highest voltages of 30.60-48.61 mV. The GR-SMFCs and GO-SMFCs generated high electric power of approximate 0.98 ± 0.14 kJ and 0.87 ± 0.04 kJ, followed by CNT-SMFCs (0.57 ± 0.06 kJ) and GF-SMFCs (0.49 ± 0.07 kJ) during the 110 d operation. The PAH degradation was not directly related to the electric current in the SMFCs. Near the anodes, the order of the phenanthrene removal rates was CNT-SMFCs (78.1%) > GR-SMFCs (73.0%) ≈ GO-SMFCs (71.2%) > GF-SMFCs (45.6%), and the order of the pyrene removal rates was GO-SMFCs (69.6%) ≈ GR-SMFCs (68.2%) ≈ CNT-SMFCs (66.7%) > GF-SMFCs (42.3%). The three carbon nanomaterials increased the microbial community diversity and slightly changed the microbial community distribution of biofilms on the anodes. Correlation analysis indicated that the degradation of phenanthrene was positively correlated with the abundances of Pseudomonas, Thauera, Diaphorobacter, Tumebacillus and Lysobacter. Pyrene degradation was strongly correlated with LOI degradation.

Bioelectrochemically-assisted vermibiofilter process enhancing stabilization of sewage sludge with synchronous electricity generation

Bioelectrochemically-assisted vermifilter (VBF_BE) with sewage sludge as the anode fuel was constructed to accelerate composting of sewage sludge, which could increase the quality of the compost and harvest electric energy in comparison with vermicomposting and electrochemical only. Results revealed that the sludge stabilization with a higher soluble chemical oxygen demand (SCOD) and lower NH₄⁺-H during 40 days of composting. At the composting, pH, C/N, electrical conductivity (EC) and germination index (GI) results demonstrated that the maturity degree of VBF_BE4 was higher than that of other VBF_BE. The VBF_BE4 yielded a voltage of 1.024 V and maximum power density of 105.28 mW/m² on 3th day. The bacteria in VBF_BE4 were richer and higher in terms of diversity than those in other VBF_BE, that was demonstrated that combination vermicomposting and electrochemistry could improve the sludge stabilization degree, accelerate sludge composting process and enhance composting maturity.

Hydrogen and electrical energy co-generation by a cooperative fermentation system comprising Clostridium and microbial fuel cell inoculated with port drainage sediment

This research work has succeeded in recovering energy from glucose by generating H₂ with the aid of a Clostridium beijerinckii strain and obtaining electrical energy from compounds present in the H₂ fermentation effluent in a microbial fuel cell (MFC) seeded with native port drainage sediment. In the fermentation step, 49.5% of the initial glucose concentration (56 mmol/L) was used to produce 104 mmol/L H₂; 5, 33, 3, and 1 mmol/L acetate, butyrate, lactate, and ethanol also emerged, respectively. MFC tests by feeding the anodic compartment with acetate, butyrate, lactate (individually or as a mixture), or the H₂ fermentation effluent provided power density values ranging between 0.6 and 1.2 W/m². Acetate furnished the highest power density with a nanowire-rich biofilm despite the lowest anode bacterial concentration (10¹² 16S gene copies/g of sediment). Non-conventional exoelectrogenic microbial communities were observed in the acetate-fed MFC; e.g., Pseudomonadaceae (Pseudomonas) and Clostridia (Acidaminobacter, Fusibacter).

Effects of the presence of nanoscale zero-valent iron on the degradation of polychlorinated biphenyls and total organic carbon by sediment microbial fuel cell

The degradation of polychlorinated biphenyls (PCBs) and total organic carbon (TOC) by sediment microbial fuel cell (SMFC) with/without nanoscale zero-valent iron (NZVI) addition was investigated. It was found that the combined application led to the highest removal efficiencies of PCBs (37.55 ± 1.11%) and TOC (49.72 ± 1.54%) in all circumstances and produced a higher power density (108.89 mW/m²) and a corresponding lower internal resistance (264 Ω) than operation employing SMFC only. The TOC removal efficiency and the total production of electricity were linear. High-throughput sequencing of anodic microbial communities indicated that the electrode participation can increase the abundance of electrogenic bacteria (Geobacter and Pseudomonas) and the NZVI addition can reduce the oxidation reduction potential of the system and therefore enrich some bacteria (Longilinea and Desulfofustis) beneficial to the degradation of organic matter.

A loop of catholyte effluent feeding to bioanodes for complete recovery of Sn, Fe, and Cu with simultaneous treatment of the co-present organics in microbial fuel cells

A loop of catholyte effluent feeding to the bioanodes of air-cathode microbial fuel cells (MFCs) achieved complete recovery of mixed Sn(II), Fe(II) and Cu(II), with simultaneous treatment of the co-present organics in synthetic wastewater of printed circuit boards (PrCBs). This in-situ utilization of caustic in the cathodes and the neutralization of acid in the anodes achieved superior metal recovery performance at an optimal hydraulic retention time (HRT) of 24 h. Cathode chambers primarily removed Sn of 91 ± 4% (bottom: 74 ± 3%; electrode: 17 ± 1%), Fe of 89 ± 8% (bottom: 64 ± 4%; electrode: 25 ± 2%), and Cu of 92 ± 7% (electrode: 63 ± 5%; bottom: 29 ± 1%), compared to Sn of 9 ± 3% (electrode: 7 ± 1%; bottom: 2 ± 1%), Fe of 9 ± 3% (electrode: 8 ± 3%; bottom: 1 ± 0%), and Cu of 7 ± 3% (electrode: 4 ± 1%; bottom: 3 ± 1%) in the bioanodes. Bacterial communities on the anodes were well evolutionarily developed after the feeding of catholyte effluent, with the increase in abundance of Rhodopseudomonas and Geobacter, and the shift from Thiobacillus and Acinetobacter to Pseudomonas, Comamonas, Aeromonas and Azospira. This loop of cathodic effluent feeding to the bioanodes of MFCs may represent a unique method for complete metal recovery with simultaneous extraction of renewable electrical energy from the co-present organics. This study also offers new insights into the development of compact microbial electro-metallurgical processes for simultaneous recovery of value-added products from PrCBs processing wastewaters and accomplishing the national wastewater discharge standard for both metals and organics.

Construction of a Self-Powered System for Simultaneous In Situ Remediation of Nitrate and Cr(VI) Contaminated Synthetic Groundwater and River Sediment

A novel self-powered system was constructed to in situ remove nitrate and Cr(VI) from synthetic groundwater and achieve river sediment remediation simultaneously. The sediment organic matter in an anodic chamber was used as a carbon source to provide self-powered energy to reduce the cathode’s contaminants. With the acceptance of protons and electrons, nitrate and Cr(VI) were transformed into nitrite and Cr(III), respectively. In a 72 h test with both nitrate and Cr(VI) present, nitrate was removed at a rate of 70.96 mg/m3·h and Cr(VI) at a rate of 8.95 mg/m3·h. When a phosphate buffer was used in the test, their removal rates were changed to 140.83 mg/m3·h and 8.33 mg/m3·h, respectively. The results showed that the self-powered system could achieve the simultaneous reduction of nitrate and Cr(VI), although the presence of Cr(VI) hindered nitrate reduction. This system could realize simultaneous in situ groundwater and sediment remediation, with no need for additional energy or materials.

Effects of nitrate and sulfate on the performance and bacterial community structure of membrane-less single-chamber air-cathode microbial fuel cells

Membrane-less, single-chamber, air-cathode, microbial fuel cells (ML-SC MFCs) have attracted attention as being suitable for wastewater treatment. In this study, the effects of nitrate and sulfate on the performance of ML-SC MFCs and their bacterial structures were evaluated. The maximum power density increased after nitrate addition from 8.6 mW·m^-2 to 14.0 mW·m^-2, while it decreased after sulfate addition from 11.5 mW·m^-2 to 7.7 mW·m^-2. The chemical oxygen demand removal efficiencies remained at more than 90% regardless of the nitrate or sulfate additions. The nitrate was removed completely (93.0%) in the ML-SC MFC, while the sulfate removal efficiency was relatively low (17.6%). Clostridium (23.1%), Petrimonas (20.0%), and unclassified Rhodocyclaceae (6.2%) were dominant on the anode before the addition of nitrate or sulfate. After the addition of nitrate, Clostridium was still the most dominant on the anode (23.6%), but Petrimonas significantly decreased (6.0%) and unclassified Rhodocyclaceae increased (17.1%). After the addition of sulfate, the amount of Clostridium almost doubled in the composition on the anode (43.2%), while Petrimonas decreased (5.5%). The bacterial community on the cathode was similar to that on the anode after the addition of nitrate. However, Desulfovibrio was remarkably dominant on the cathode (32.9%) after the addition of sulfate. These results promote a deeper understanding of the effects of nitrate or sulfate on the ML-SC MFCs' performance and their bacterial community.

Electrochemistry and microbiology of microbial fuel cells treating marine sediments polluted with heavy metals

Novel laboratory-designed aerated and non-aerated sediment microbial fuel cell (SMFC) models were constructed for power generation and heavy metal bioremediation.

Bacterial community shift and improved performance induced by in situ preparing dual graphene modified bioelectrode in microbial fuel cell

Dual graphene modified bioelectrode (D-GM-BE) was prepared by in situ microbial-induced reduction of graphene oxide (GO) and polarity reversion in microbial fuel cell (MFC). Next Generation Sequencing technology was used to elucidate bacterial community shift in response to improved performance in D-GM-BE MFC. The results indicated an increase in the relative ratio of Proteobacteria, but a decrease of Firmicutes was observed in graphene modified bioanode (GM-BA); increase of Proteobacteria and Firmicutes were observed in graphene modified biocathode (GM-BC). Genus analysis demonstrated that GM-BE was beneficial to enrich electrogens. Typical exoelectrogens were accounted for 13.02% and 8.83% in GM-BA and GM-BC. Morphology showed that both GM-BA and GM-BC formed 3D-like graphene/biofilm architectures and revealed that the biofilm viability and thickness would decrease to some extent when GM-BE was formed. D-GM-BE MFC obtained the maximum power density by 124.58±6.32mWm^-2, which was 2.34 times over C-BE MFC.

Simultaneous removal of heavy metals and biodegradation of organic matter with sediment microbial fuel cells

To in situ remediate rivers polluted by organic matter and heavy metals, lab-scale sediment microbial fuel cells (SMFCs) were operated under different conditions.