Recent advances in the separators for microbial fuel cells

Performance and mechanism of antibiotic resistance removal by biochar-enhanced sediment microbial fuel cell

This study is the first to explore the performance and mechanism of biochar-impacted sediment microbial fuel cell for removing antibiotic resistance genes (ARGs), and examines the effects of different biochar contents. The addition of 5% biochar produced the highest output voltage and power density, which increased by 100% and 219%, respectively, while simultaneously reducing the abundance and risk of ARGs. Comparatively, the addition of moderate amount of biochar (1-5%) promoted the removal of ARGs, while the opposite was true for excessive (10%) biochar. Biochar affected ARGs through prophages, insertion sequence, and transposons. Biological factors and voltage jointly influenced ARGs variation, with the former accounting for 56%. Further analysis of functional genes indicated that biochar controlled ARGs by regulating the synthesis of genetic material and amino acids to influence metabolism. Overall, findings of this study shed light on the potential removal of ARGs in microbial electrochemical systems.

A directional electrode separator improves anodic biofilm current density in a well-mixed single-chamber bioelectrochemical system

In this study, a directional electrode separator (DES) was designed and incorporated into a single-chamber bioelectrochemical system (BES) to reduce migration and reoxidation of hydrogen. This issue arises when H2, generated at the cathode, travels to the anode where anodic biofilms use H2. To test the feasibility of our design, a 3D-printed BES reactor equipped with a DES was inoculated with anaerobic digestor granules and operated under fed-batch conditions using fermented corn stover effluent. The DES equipped reactor achieved significantly higher current densities (∼53 A/m²) compared to a conventional single-chamber BES without a separator (∼16 A/m²), showing a 3.3 times improvement. Control abiotic electrochemical experiments revealed that the DES exhibited significantly higher proton conductivity (456±127 µS/mm) compared to a proton exchange membrane (67±21 µS/mm) with a statistical significance of P=0.03. The DES also effectively reduced H2 migration to the anode by 21-fold relative to the control. Overall, incorporating a DES in a single-chamber BES enhanced anodic current density by reducing H2 migration to the anode.

Sorting Out the Latest Advances in Separators and Pilot-Scale Microbial Electrochemical Systems for Wastewater Treatment: Concomitant Development, Practical Application, and Future Perspective

To date, dozens of pilot-scale microbial fuel cell (MFC) devices have been successfully developed worldwide for treating various types of wastewater. The availability and configurations of separators are determining factors for the economic feasibility, efficiency, sustainability, and operability of these devices. Thus, the concomitant advances between the separators and pilot-scale MFC configurations deserve further clarification. The analysis of separator configurations has shown that their evolution proceeds as follows: from ion-selective to ion-non-selective, from nonpermeable to permeable, and from abiotic to biotic. Meanwhile, their cost is decreasing and their availability is increasing. Notably, the novel MFCs configured with biotic separators are superior to those configured with abiotic separators in terms of wastewater treatment efficiency and capital cost. Herein, a highly comprehensive review of pilot-scale MFCs (>100 L) has been conducted, and we conclude that the intensive stack of the liquid cathode configuration is more advantageous when wastewater treatment is the highest priority. The use of permeable biotic separators ensures hydrodynamic continuity within the MFCs and simplifies reactor configuration and operation. In addition, a systemic comparison is conducted between pilot-scale MFC devices and conventional decentralized wastewater treatment processes. MFCs showed comparable cost, higher efficiency, long-term stability, and significant superiority in carbon emission reduction. The development of separators has greatly contributed to the availability and usability of MFCs, which will play an important role in various wastewater treatment scenarios in the future.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.

A comprehensive review of microbial fuel cells considering materials, methods, structures, and microorganisms

Microbial fuel cells (MFCs) are promising for generating renewable energy from organic matter and efficient wastewater treatment. Ensuring their practical viability requires meticulous optimization and precise design. Among the critical components of MFCs, the membrane separator plays a pivotal role in segregating the anode and cathode chambers. Recent investigations have shed light on the potential benefits of membrane-less MFCs in enhancing power generation. However, it is crucial to recognize that such configurations can adversely impact the electrocatalytic activity of anode microorganisms due to increased substrate and oxygen penetration, leading to decreased coulombic efficiency. Therefore, when selecting a membrane for MFCs, it is essential to consider key factors such as internal resistance, substrate loss, biofouling, and oxygen diffusion. Addressing these considerations carefully allows researchers to advance the performance and efficiency of MFCs, facilitating their practical application in sustainable energy production and wastewater treatment. Accelerated substrate penetration could also lead to cathode clogging and bacterial inactivation, reducing the MFC's efficiency. Overall, the design and optimization of MFCs, including the selection and use of membranes, are vital for their practical application in renewable energy generation and wastewater treatment. Further research is necessary to overcome the challenges of MFCs without a membrane and to develop improved membrane materials for MFCs. This review article aims to compile comprehensive information about all constituents of the microbial fuel cell, providing practical insights for researchers examining various variables in microbial fuel cell research.

A study of electron source preference and its impact on hydrogen production in microbial electrolysis cells fed with synthetic fermentation effluent

Fermentation effluents from organic wastes contain simple organic acids and ethanol, which are good electron sources for exoelectrogenic bacteria, and hence are considered a promising substrate for hydrogen production in microbial electrolysis cells (MECs). These fermentation products have different mechanisms and thermodynamics for their anaerobic oxidation, and therefore the composition of fermentation effluent significantly influences MEC performance. This study examined the microbial electrolysis of a synthetic fermentation effluent (containing acetate, propionate, butyrate, lactate, and ethanol) in two-chamber MECs fitted with either a proton exchange membrane (PEM) or an anion exchange membrane (AEM), with a focus on the utilization preference between the electron sources present in the effluent. Throughout the eight cycles of repeated batch operation with an applied voltage of 0.8 V, the AEM-MECs consistently outperformed the PEM-MECs in terms of organic removal, current generation, and hydrogen production. The highest hydrogen yield achieved for AEM-MECs was 1.26 L/g chemical oxygen demand (COD) fed (approximately 90% of the theoretical maximum), which was nearly double the yield for PEM-MECs (0.68 L/g COD fed). The superior performance of AEM-MECs was attributed to the greater pH imbalance and more acidic anodic pH in PEM-MECs (5.5-6.0), disrupting anodic respiration. Although butyrate is more thermodynamically favorable than propionate for anaerobic oxidation, butyrate was the least favored electron source, followed by propionate, in both AEM- and PEM-MECs, while ethanol and lactate were completely consumed. Further research is needed to better comprehend the preferences for different electron sources in fermentation effluents and enhance their microbial electrolysis.

Cathodic biofouling control by microbial separators in air-breathing microbial fuel cells

Microbial fuel cells (MFCs) incorporating air-breathing cathodes have emerged as a promising eco-friendly wastewater treatment technology capable of operating on an energy-free basis. However, the inevitable biofouling of these devices rapidly decreases cathodic catalytic activity and also reduces the stability of MFCs during long-term operation. The present work developed a novel microbial separator for use in air-breathing MFCs that protects cathodic catalytic activity. In these modified devices, microbes preferentially grow on the microbial separator rather than the cathodic surface such that biofouling is prevented. Trials showed that this concept provided low charge transfer and mass diffusion resistance values during the cathodic oxygen reduction reaction of 4.6 ± 1.3 and 17.3 ± 6.8 Ω, respectively, after prolonged operation. The maximum power density was found to be stable at 1.06 ± 0.07 W m^-2 throughout a long-term test and the chemical oxygen demand removal efficiency was increased to 92% compared with a value of 83% for MFCs exhibiting serious biofouling. In addition, a cathode combined with a microbial separator demonstrated less cross-cathode diffusion of oxygen to the anolyte. This effect indirectly induced the growth of electroactive bacteria and produced higher currents in air-breathing MFCs. Most importantly, the present microbial separator concept enhances both the lifespan and economics of air-breathing MFCs by removing the need to replace or regenerate the cathode during long-term operation. These results indicate that the installation of a microbial separator is an effective means of stabilizing power generation and ensuring the cost-effective performance of air-breathing MFCs intended for future industrial applications.

Comparative review on microbial electrochemical technologies for resource recovery from wastewater towards circular economy and carbon neutrality

Newly arising concepts such as the circular economy and carbon neutrality motivate resource recovery from wastewater. This paper reviews and discusses state-of-the-art microbial electrochemical technologies (METs), specifically microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial recycling cells (MRCs), which enable energy generation and nutrient recovery from wastewater. Mechanisms, key factors, applications, and limitations are compared and discussed. METs are effective in energy conversion, demonstrating advantages, drawbacks and future potential as specific scenarios. MECs and MRCs exhibited greater potential for simultaneous nutrient recovery, and MRCs offer the best scaling-up potential and efficient mineral recovery. Research on METs should be more concerned with lifespan of materials, secondary pollutants reduction and scaled-up benchmark systems. More up-scaled application cases are expected for cost structures comparison and life cycle assessment of METs. This review could direct the follow-up research, development and successful implementation of METs for resource recovery from wastewater.

Microbial Fuel Cell Construction Features and Application for Sustainable Wastewater Treatment

A microbial fuel cell (MFC) is a system that can generate electricity by harnessing microorganisms' metabolic activity. MFCs can be used in wastewater treatment plants since they can convert the organic matter in wastewater into electricity while also removing pollutants. The microorganisms in the anode electrode oxidize the organic matter, breaking down pollutants and generating electrons that flow through an electrical circuit to the cathode compartment. This process also generates clean water as a byproduct, which can be reused or released back into the environment. MFCs offer a more energy-efficient alternative to traditional wastewater treatment plants, as they can generate electricity from the organic matter in wastewater, offsetting the energy needs of the treatment plants. The energy requirements of conventional wastewater treatment plants can add to the overall cost of the treatment process and contribute to greenhouse gas emissions. MFCs in wastewater treatment plants can increase sustainability in wastewater treatment processes by increasing energy efficiency and reducing operational cost and greenhouse gas emissions. However, the build-up to the commercial-scale still needs a lot of study, as MFC research is still in its early stages. This study thoroughly describes the principles underlying MFCs, including their fundamental structure and types, construction materials and membrane, working mechanism, and significant process elements influencing their effectiveness in the workplace. The application of this technology in sustainable wastewater treatment, as well as the challenges involved in its widespread adoption, are discussed in this study.

Putting the electro-bugs to work: A systematic review of 22 years of advances in bio-electrochemical systems and the parameters governing their performance

Wastewater treatment using bioelectrochemical systems (BESs) can be considered as a technology finding application in versatile areas such as for renewable energy production and simultaneous reducing environmental problems, biosensors, and bioelectrosynthesis. This review paper reports and critically discusses the challenges, and advances in bio-electrochemical studies in the 21st century. To sum and critically analyze the strides of the last 20+ years on the topic, this study first provides a comprehensive analysis on the structure, performance, and application of BESs, which include Microbial Fuel Cells (MFCs), Microbial Electrolysis Cells (MECs) and Microbial Desalination Cells (MDCs). We focus on the effect of various parameters, such as electroactive microbial community structure, electrode material, configuration of bioreactors, anode unit volume, membrane type, initial COD, co-substrates and the nature of the input wastewater in treatment process and the amount of energy and fuel production, with the purpose of showcasing the modes of operation as a guide for future studies. The results of this review show that the BES have great potential in reducing environmental pollution, purifying saltwater, and producing energy and fuel. At a larger scale, it aspires to facilitate the path of achieving sustainable development and practical application of BES in real-world scenarios.

Investigating the Performance of a Zinc Oxide Impregnated Polyvinyl Alcohol-Based Low-Cost Cation Exchange Membrane in Microbial Fuel Cells

The current study investigated the development and application of lithium (Li)-doped zinc oxide (ZnO)-impregnated polyvinyl alcohol (PVA) proton exchange membrane separator in a single chambered microbial fuel cell (MFC). Physiochemical analysis was performed via FT-IR, XRD, TEM, and AC impedance analysis to characterize thus synthesized Li-doped ZnO. PVA-ZnO-Li with 2.0% Li incorporation showed higher power generation in MFC. Using coulombic efficiency and current density, the impact of oxygen crossing on the membrane cathode assembly (MCA) area was evaluated. Different amounts of Li were incorporated into the membrane to optimize its electrochemical behavior and to increase proton conductivity while reducing biofouling. When acetate wastewater was treated in MFC using a PVA-ZnO-Li-based MCA, the maximum power density of 6.3 W/m³ was achieved. These observations strongly support our hypothesis that PVA-ZnO-Li can be an efficient and affordable separator for MFC.

Enhanced performance and microbial interactions of shallow wetland bed coupling with functional biocathode microbial electrochemical system (MES)

It is essential to remediate the polluted urban river, which endangers the aquatic creatures and affected human body's senses. The treatment wetland combined with microbial electrochemical system (MES) used for the remediation is becoming a new research focus due to its ideal pollutants removal efficiency and small footprint. Here this paper provided a kind of novel shallow wetland bed coupling with close-circuit microbial electrochemical system (WB-CMES) to remove pollutants in surface water. In contrast to the shallow wetland bed coupling with open-circuit MES (WB-OMES) and the shallow wetland bed without MES (WB), the enhancing effects and pollutants removal pathway were evaluated. After 62-day operation, average TN removal efficiency in WB-CMES was 87.7%, which was 19.7% and 13.8% higher than that of WB-OMES and WB respectively. The rate coefficient k of NO₃^--N degradation in WB-CMES was 1.6 and 1.8 times higher than that in WB-OMES and WB. The results of chlorophyll, protein and superoxide dismutase (SOD) in WB-CMES were 27.3%, 44.3% and 12.9% higher than those in WB. The microbial community structure analysis indicated that electroactive bacteria on anode like Desulfobulbus could oxidize organics and generate electrons to compensate cathode, meanwhile, cathode could enrich more species of functional bacteria like Rhodobacter, Pirellula, Hyphomicrobium, Thauera, which had a synergistic effect on oxygen reduction, nitrogen removal and plant growth. The results indicated that oxygen produced by submerged plants could be utilized by the oxygen-reducing functional biocathode of MES and the proper aerobic and anoxic environment might enhance nitrate removal mainly through simultaneous nitrification and denitrification (SND), aerobic denitrification and anammox. This research provided a novel technology with advantages of simple operation, flexible configuration, easy scale-up and low cost for application in remediation of highly polluted surface water.

Studying microbial fuel cells equipped with heterogeneous ion exchange membranes: Electrochemical performance and microbial community assessment of anodic and membrane-surface biofilms

In this study, microbial fuel cells deploying heterogeneous ion exchange membranes were assessed. The behavior of the cells as a function of the membrane applied was evaluated in terms of maximal current density, electron recovery efficiency and energy production rate (up to 427.5 mA, 47.7 % and 660 J m^-2h^-1, respectively) at different substrate (acetate) feedings (2.15 - 8.6 mM). System performance was characterized in the light of oxygen and acetate crossovers. The effect of membranes (in relation to the oxygen mass transfer coefficient, k_O) on the microbial diversity of anodic and membrane-surface biofilms was investigated. Based on the relative abundance of bacterial orders, the two populations could be distinguished and membranes with larger k_O tended to promote more the air-tolerant microbes in the biofouling layer. This indicates that membrane k_O has a direct effect on membrane foulant microbial composition, and thus, on the expected time-stability of the membrane.

Microbial fuel cell treatment energy-offset for fertilizer production from human urine

Microbial fuel cells (MFCs) are a promising technology for simultaneous wastewater treatment and the biological conversion of organics to electrical energy. Yet effective MFC utilization of complex waste streams like human urine is limited by interference from high-strength organics (>5000 mg L^-1 total organic carbon) and concentrated macronutrients (>500 mg L^-1 nitrogen and phosphorus). This research assesses potential gains in MFC energy performance and organics treatment achieved by incorporating MFCs as a tertiary step in a human urine nutrient recovery system. The bioelectrochemical performance of benchtop-scale, low-cost MFCs was assessed using pre-treated human urine that was depleted in ammonium-nitrogen and phosphate (the "waste bottoms" of the urine nutrient recovery system). Performance of MFCs with waste bottoms as feedstock was compared to MFC performance with hydrolyzed real urine and synthetic urine as feedstocks. MFCs with waste bottoms produced 16.2 ± 14.8 mW m_Cat^-2 (2.14 ± 1.95 W m_Cat^-3), equivalent to 93% of the mean power density achieved by hydrolyzed urine after 32 days of operation. Coulombic efficiency over the full experimental runtime was 32.3 ± 4.1% higher for waste bottoms than urine. Waste bottoms helped avoid fouling of the ceramic membrane separator that occurs with urea hydrolysis and phosphate precipitation from urine. Enhanced ion separation was also observed, producing neutral pH in the anolyte and high pH (11.5) and electrical conductivity (25 dS m^-1) in the catholyte. While several gains in performance were observed when using waste bottoms as feedstock, anolyte organics removal decreased 36.5% in MFCs with waste bottoms. This research indicates that pretreatment of source-separated urine via nutrient removal improves MFC electrical power generation and ion separation.

Enhanced performance of novel carbon nanotubes - sulfonated poly ether ether ketone (speek) composite proton exchange membrane in mfc application

Sulfonated poly ether ether ketone (SPEEK) nanocomposite proton exchange membrane (PEM) was prepared by incorporating multi-walled carbon nanotubes (CNT) at different weight percentages for microbial fuel cell (MFC) applications. Physico-chemical, thermal, mechanical and morphological characteristics of the prepared CNT-SPEEK composite membranes were analyzed using various techniques. Further, the water uptake capacity, Ion exchange capacity (IEC) and MFC performance of the CNT-SPEEK composite membranes were evaluated and compared with the pristine SPEEK membrane. Results show that incorporation of CNTs in SPEEK membranes exhibited a better water uptake capacity (34.18%-36.02%) and IEC (1.94-2.15 meq/g) compared to the SPEEK membrane. Improvement in membrane properties resulted in 2-fold higher power density compared to SPEEK membrane. Composite membrane with 0.75% CNT-SPEEK produced the higher power density (1.77 W/m²) in comparison with all the membranes evaluated. Chemical oxygen demand (COD) removal efficiency values of the MFC with SPEEK composite membranes were also found to be around 90%. Overall, the results reveal that CNT-SPEEK composite membrane as a potential PEM for MFC applications.

Advanced bioelectrochemical system for nitrogen removal in wastewater

Nitrogen (N) pollution in water has become a serious issue that cannot be ignored due to the harm posed by excessive nitrogen to environmental safety and human health; as such, N concentrations in water are strictly limited. The bioelectrochemical system (BES) is a new method to remove excessive N from water, and has attracted considerable attention. Compared with other methods, it is highly efficient and has low energy consumption. However, the BES has not been applied for N removal in practice due to lack of in-depth research on the mechanism and construction of high-performance electrodes, separators, and reactor configurations; this highlights a need to review and examine the efforts in this field. This paper provides a comprehensive review on the current BES research for N removal focusing on the reaction principles, reactor configurations, electrodes and separators, and treatment of actual wastewater; the corresponding performances in these realms are also discussed. Finally, the prospects for N removal in water using the BES are presented.

Comparative Study of Different Production Methods of Activated Carbon Cathodic Electrodes in Single Chamber MFC Treating Municipal Landfill Leachate

The treatment of real waste extracts with simultaneous energy production is currently under research. One method of addressing this dual task is using biochemical reactors named microbial fuel cells (MFCs). MFCs consist of a bioanode and a cathode where the oxygen reduction reaction (ORR) occurs. Cathodes are currently under optimization regarding the nature of their support, their catalytic efficiency and their configurations. In this work, we present facile preparation methods for the production of activated carbon ceramic-supported cathodic electrodes produced with three different techniques (wash-coat, brush-coat, and ultrasound-assisted deposition/infiltration). The produced cathodic electrodes were tested in a single-chamber MFC, filled with the concentrated liquid residue, after the reverse osmosis (RO-CLR) treatment of leachate from a municipal waste landfill, in order to exploit their electrochemical potential for simultaneous waste treatment and energy production. The electrode produced utilizing 20 kHz ultrasounds proved to be more effective in terms of energy harvesting (10.7 mW/g·L of leachate) and wastewater treatment (COD removal 85%). Internal resistances of the ultrasound-produced electrodes are lower, as compared to the other two methods, opening new exploitation pathways in the use of ultrasound as a means in producing electrodes for microbial fuel cells.

Effect of Food Waste Condensate Concentration on the Performance of Microbial Fuel Cells with Different Cathode Assemblies

The aim of this study is to examine the effect of food waste condensate concentration (400–4000 mg COD/L) on the performance of two microbial fuel cells (MFCs). Food waste condensate is produced after condensing the vapors that result from drying and shredding of household food waste (HFW). Two identical single-chamber MFCs were constructed with different cathodic assemblies based on GoreTex cloth (Cell 1) and mullite (Cell 2) materials. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements were carried out to measure the maximum power output and the internal resistances of the cells. High COD removal efficiencies (>86%) were observed in all cases. Both cells performed better at low initial condensate concentrations (400–600 mg COD/L). Cell 1 achieved maximum electricity yield (1.51 mJ/g COD/L) at 500 mg COD/L and maximum coulombic efficiency (6.9%) at 400 mg COD/L. Cell 2 achieved maximum coulombic efficiency (51%) as well as maximum electricity yield (25.9 mJ/g COD/L) at 400 mg COD/L. Maximum power was observed at 600 mg COD/L for Cell 1 (14.2 mW/m2) and Cell 2 (14.4 mW/m2). Impedance measurements revealed that the charge transfer resistance and the solution resistance increased significantly with increasing condensate concentration in both cells.

A review on bio-electro-Fenton systems as environmentally friendly methods for degradation of environmental organic pollutants in wastewater

Bio-electro-Fenton (BEF) systems have been potentially studied as a promising technology to achieve environmental organic pollutants degradation and bioelectricity generation. The BEF systems are interesting and constantly expanding fields of science and technology. These emerging technologies, coupled with anodic microbial metabolisms and electrochemical Fenton's reactions, are considered suitable alternatives. Recently, great attention has been paid to BEFs due to special features such as hydrogen peroxide generation, energy saving, high efficiency and energy production, that these features make BEFs outstanding compared with the existing technologies. Despite the advantages of this technology, there are still problems to consider including low production of current density, chemical requirement for pH adjustment, iron sludge formation due to the addition of iron catalysts and costly materials used. This review has described the general features of BEF system, and introduced some operational parameters affecting the performance of BEF system. In addition, the results of published researches about the degradation of persistent organic pollutants and real wastewaters treatment in BEF system are presented. Some challenges and possible future prospects such as suitable methods for improving current generation, selection of electrode materials, and methods for reducing iron residues and application over a wide pH range are also given. Thus, the present review mainly revealed that BEF system is an environmental friendly technology for integrated wastewater treatment and clean energy production.

Bioelectrochemical technology for recovery of silver from contaminated aqueous solution: a review

A large amount of silver-rich wastewater is generated from different industrial processes. This wastewater is not considered a waste, but a valuable source for recovery due to the precious silver (Ag). Previous studies have used traditional methods such as membrane filtration, electrolysis, chemical precipitation, electrochemical, and cementation for Ag recovery. However, many drawbacks have been reported for these techniques such as high cost, hazardous waste generation, and the needed refinement of recovered products. In this study, a bioelectrochemical system (BES) for Ag recovery from aqueous solution is introduced as an effective and innovative method, as compared with other techniques. Different types of Ag(I)-containing solutions that have been investigated in recent BES studies (e.g., Ag⁺ solution, [Ag(NH₃)₂]⁺, [Ag(S₂O₃)]^-, [Ag(S₂O₃)₂]^3- complexes) are reported. A BES is an anaerobic system consisting of anode and cathode chambers, which are normally separated by an ion-exchange membrane. The electron flow obtained from the anodic biological oxidation of organic matter is used directly for the cathodic electrochemical reduction of Ag(I) ions. The recovered product is Ag electrodeposits, formed at the cathode surface. Several studies have reported high Ag recovery efficiency by using a BES (i.e., > 90%), with high purity of metallic silver, and simultaneous electricity production. Furthermore, a BES can be employed for a wide range of initial Ag(I) concentrations (e.g., 50-3000 mg/L). The advantages of BES technology for Ag recovery are highlighted in this study for further practical applications.

Management of Cattle Dung and Novel Bioelectricity Generation Using Microbial Fuel Cells: An Ingenious Experimental Approach

Microbial fuel cells (MFCs) are the rising modern equipment for the generation of bioelectricity from organic matters. In this study, MFCs in two formats are assembled and concurrently operated for a 30-day period in a batch mode manner. Natural biowaste cattle dung slurry with mediators is used as a substrate persistently for the enhancement of electron transfer rate and additionally for the augmentation of required electrical parameters. Under similar conditions, the MFC setups are experimented with a variety of anode-cathode material combinations, namely carbon-carbon, copper-carbon, and zinc-carbon. The performance of these MFCs during the testing period is evaluated independently and compared by plotting polarization data generated by them. It is revealed that maximum current and power densities are achieved from all these MFCs and the best attained values are 1858 mA/m2 and 1465 mW/m2, respectively, for the novel single-chamber zinc-carbon electrode MFC. The corresponding findings present that the MFC with zinc-carbon electrodes has the better power density than other MFCs. Being conductive and higher standard potential metal electrodes have improved the capability to act in place of carbon family electrodes for MFC-based power applications. Although the MFC power generation is low, but modifications in configurations, electrodes, microbe-rich biowaste, mediators, and power management may enhance the power output to a significant level for commercialization of this technology. The unique feature of this research is to explore the pertinent use of conductive metal electrodes to enhance the power generation capability of MFCs through biowaste as an alternative power source for small applications. The novelty of this research is presented through usage of conductive metal electrodes for the performance analysis of MFCs.

The Implications of Membranes Used as Separators in Microbial Fuel Cells

Microbial fuel cells (MFCs) are electrochemical devices focused on bioenergy generation and organic matter removal carried out by microorganisms under anoxic environments. In these types of systems, the anodic oxidation reaction is catalyzed by anaerobic microorganisms, while the cathodic reduction reaction can be carried out biotically or abiotically. Membranes as separators in MFCs are the primary requirements for optimal electrochemical and microbiological performance. MFC configuration and operation are similar to those of proton-exchange membrane fuel cells (PEMFCs)-both having at least one anode and one cathode split by a membrane or separator. The Nafion® 117 (NF-117) membrane, made from perfluorosulfonic acid, is a membrane used as a separator in PEMFCs. By analogy of the operation between electrochemical systems and MFCs, NF-117 membranes have been widely used as separators in MFCs. The main disadvantage of this type of membrane is its high cost; membranes in MFCs can represent up to 60% of the MFC's total cost. This is one of the challenges in scaling up MFCs: finding alternative membranes or separators with low cost and good electrochemical characteristics. The aim of this work is to critically review state-of-the-art membranes and separators used in MFCs. The scope of this review includes: (i) membrane functions in MFCs, (ii) most-used membranes, (iii) membrane cost and efficiency, and (iv) membrane-less MFCs. Currently, there are at least 20 different membranes or separators proposed and evaluated for MFCs, from basic salt bridges to advanced synthetic polymer-based membranes, including ceramic and unconventional separator materials. Studies focusing on either low cost or the use of natural polymers for proton-exchange membranes (PEM) are still scarce. Alternatively, in some works, MFCs have been operated without membranes; however, significant decrements in Coulombic efficiency were found. As the type of membrane affects the performance and total cost of MFCs, it is recommended that research efforts are increased in order to develop new, more economic membranes that exhibit favorable properties and allow for satisfactory cell performance at the same time. The current state of the art of membranes for MFCs addressed in this review will undoubtedly serve as a key insight for future research related to this topic.

Effect of simple interventions on the performance of a miniature MFC fed with fresh urine

The aim of the present study is to enhance the performance of a microbial fuel cell (MFC) design by making simple interventions. Specifically, terracotta "t" and mullite "m" ceramics are tested as membranes while carbon veil and carbon cloth are used as electrodes. In the case of "m" cylinders different dimensions are examined (m: ID 30 mm x height 11.5 mm; sm: ID 18 mm x height 18 mm). The units operated continuously with urine as the feedstock. The best performing is the sm type (60-100 μW), followed by the t type (40-80 μW) and the m type (20-40 μW). Polarisation experiments indicated that activated carbon on the anode enhances the power output (t: 423 μW, sm: 288 μW). Similarly, the increase of the surface area and the addition of stainless steel mesh on the cathode improves the power performance for the "sm" and the "t" units. Furthermore, it is shown that the design with the smaller internal diameter, performs better and is more stable through time.

Phenolphthalein Anilide Based Poly(Ether Sulfone) Block Copolymers Containing Quaternary Ammonium and Imidazolium Cations: Anion Exchange Membrane Materials for Microbial Fuel Cell

A class of phenolphthalein anilide (PA)-based poly(ether sulfone) multiblock copolymers containing pendant quaternary ammonium (QA) and imidazolium (IM) groups were synthesized and evaluated as anion exchange membrane (AEM) materials. The AEMs were flexible and mechanically strong with good thermal stability. The ionomeric multiblock copolymer AEMs exhibited well-defined hydrophobic/hydrophilic phase-separated morphology in small-angle X-ray scattering and atomic force microscopy. The distinct nanophase separated membrane morphology in the AEMs resulted in higher conductivity (IECw = 1.3-1.5 mequiv./g, σ(OH^-) = 30-38 mS/cm at 20 °C), lower water uptake and swelling. Finally, the membranes were compared in terms of microbial fuel cell performances with the commercial cation and anion exchange membranes. The membranes showed a maximum power density of ~310 mW/m² (at 0.82 A/m²); 1.7 and 2.8 times higher than the Nafion 117 and FAB-PK-130 membranes, respectively. These results demonstrated that the synthesized AEMs were superior to Nafion 117 and FAB-PK-130 membranes.

Effect of iron oxide content and microstructural porosity on the performance of ceramic membranes as microbial fuel cell separators

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Suitable bioelectrochemical application of ceramic clays with varying Fe2O3 content.

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5.75% of Fe2O3 and 1100 °C of sintering temperature allow MFCs to reach 1.045 mW.

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The Fe2O3 content mitigates the effect of porosity features on the MFC power output.

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Good stability of the MFCs working during 65 days in continuous mode.

Ceramic materials based on naturally occurring clays are a low cost and environmentally friendly alternative to commercial polymer-based membranes in bioelectrochemical systems. In this work, ceramic membranes containing different amounts of iron oxide (1.06, 2.76 and 5.75 vol.%) and sintered at different temperatures (1100, 1200 and 1300 °C) have been elaborated and tested as separators in urine-fed microbial fuel cells (MFCs). The results reveal that the presence of iron oxide in the ceramic membrane composition increases the structural porosity and reduces the pore size for the three temperatures investigated. On the other hand, it was also observed that the iron content mitigates the negative effect of the high sintering temperature on the power performance of the MFCs. In the case of the ceramic membranes sintered at 1300 °C, power output improved ca. 10-fold when the iron oxide content in the membrane increased from 1.06 up to 5.75 vol.% (30.9 and 286.6 µW, respectively). Amongst the different combinations of iron phase content and sintering temperatures, the maximum power output was obtained by MFCs working with separators containing 5.75 vol. % of iron oxide and sintered at 1100 °C (1.045 mW). Finally, the system was stable for 65 days, which supports the long-term functionality of the different materials assessed.

Recent progress of graphene based nanomaterials in bioelectrochemical systems

The application of graphene (Gr) to microbial fuel cells (MFCs) and microbial electrolysis cell (MECs) is considered a very promising approach in terms of enhancing their performance. The superior Gr properties of high electrical and thermal conductivities, along with: superior specific surface area, high electron mobility, and mechanical strength, are the key features that endorse this. Factors impeding the advancement of a microbial fuel cell into commercialization involve primarily the cost of their components, and their production on a small scale. Gr with such outstanding characteristics can help mitigate these challenges, when used as electrode material. The application of Gr as an anode material improves the efficiency of electron transfer and bacterial attachment. When used as a cathode material, it supports the oxygen reduction reaction. This investigation, presents a thorough analysis of the feasibility of Gr as an electrode material in both MFC and MEC applications - based on experimental results from the investigation. Current technological advancements in the implementation of Gr in MFC and MEC are also highlighted in this review. To summarise, the investigation exposes critical issues impeding the advancement of microbial fuel cells, and proposes possible solutions to mitigate these challenges.

Impact of Inoculum Type on the Microbial Community and Power Performance of Urine-Fed Microbial Fuel Cells

Bacteria are the driving force of the microbial fuel cell (MFC) technology, which benefits from their natural ability to degrade organic matter and generate electricity. The development of an efficient anodic biofilm has a significant impact on the power performance of this technology so it is essential to understand the effects of the inoculum nature on the anodic bacterial diversity and establish its relationship with the power performance of the system. Thus, this work aims at analysing the impact of 3 different types of inoculum: (i) stored urine, (ii) sludge and (iii) effluent from a working MFC, on the microbial community of the anodic biofilm and therefore on the power performance of urine-fed ceramic MFCs. The results showed that MFCs inoculated with sludge outperformed the rest and reached a maximum power output of 40.38 mW·m^-2_anode (1.21 mW). The power performance of these systems increased over time whereas the power output by MFCs inoculated either with stored urine or effluent decreased after day 30. These results are directly related to the establishment and adaptation of the microbial community on the anode during the assay. Results showed the direct relationship between the bacterial community composition, originating from the different inocula, and power generation within the MFCs.

The reaction of wastewater treatment and power generation of single chamber microbial fuel cell against substrate concentration and anode distributions

This study demonstrated the effectiveness of single chamber up-flow membrane-less microbial fuel cell (UFML-MFC) in wastewater treatment concurrently with bioelectricity generation. The objectives of this study were to examine the effect of influent substrate concentration (0.405 g/L, 0.810 g/L, 1.215 g/L, 1.620 g/L), anode distributions (11 cm, 17 cm, 23 cm ) and surface morphologies for biofilm formation on the performance of wastewater treatment and power generation. The optimum performance was obtained with substrate concentration of 0.810 g/L. The COD removal efficiency, output voltage, internal resistance, power density and current density obtained were 84.64%, 610 mV, 200 Ω, 162.59 mW/m2 and 468.74 mA/m2, respectively. The Coulombic Efficiency (CE), Normalized Energy Recovery (NERS and NERv) were 1.03%, 789.38 kWh/kg COD and 22.56 kWh/m3, respectively. The results also indicate that the output voltage and power generation obtained in a continuous up-flow MFC were higher with A3 (23 cm), which is of larger electrodes spacing followed by A2 (17 cm) and A1 (11 cm) caused by the enrichment of anaerobic microbial population at A1.

Spontaneous reduction of low-potential silver(I) dithiosulfate complex in bioelectrochemical systems for recovery of silver and simultaneous electricity production

Waste fixer solutions generated from photographic processing are silver-rich effluents, in which silver exists in the form of a dithiosulfate complex ([Ag(S₂O₃)₂]^3-), a stable and water-soluble chemical compound. This study investigated the electrochemical reduction of [Ag(S₂O₃)₂]^3- at different initial concentrations for silver recovery, combined with electricity production in a two-chamber bio-electrochemical system using a cation exchange membrane as the separator. During the biological oxidation of acetate to produce electrons in the anode chamber, [Ag(S₂O₃)₂]^3- was reduced spontaneously by acting as an electron acceptor in the cathode chamber, despite its low standard redox potential ([Ag(S₂O₃)₂]^3-/Ag⁰, E ⁰ = 0.016 V). After 48 h in each batch of operation, a Ag recovery efficiency of 81.7-95.2%, with a columbic efficiency of 12.9-21.4% and a maximum power density of 1500-2647 mW/m³, were obtained with an initial [Ag(S₂O₃)₂]^3- concentration of 10-20 mM, respectively. When the initial [Ag(S₂O₃)₂]^3- concentration increased to 30 mM, cell voltage production did not improve significantly, and a small decrease in Ag recovery efficiency to 93.2% was found. After 61 days of operation, the cathode surface was covered by different-sized silver clusters under SEM observation. The results were confirmed by EDX and XRD characterization, in which metallic silver with high purity was detected. SEM-EDX-XRD characterization of the membrane and the measurements in the control reactor confirmed that there was no diffusion of negatively charged [Ag(S₂O₃)₂]^3- complex through the membrane. Thus, this study showed a successful recovery of Ag from the low-potential [Ag(S₂O₃)₂]^3- complex without energy consumption, secondary waste generation, and loss of Ag.

Study of the Effect of Activated Carbon Cathode Configuration on the Performance of a Membrane-Less Microbial Fuel Cell

In this paper, the effect of cathode configuration on the performance of a membrane-less microbial fuel cell (MFC) was evaluated using three different arrangements: an activated carbon bed exposed to air (MFCE), a wetland immersed in an activated carbon bed (MFCW) and a cathode connected to an aeration tower featuring a water recirculation device (MFCT). To evaluate the MFC performance, the efficiency of the organic matter removal, the generated voltage, the power density and the internal resistance of the systems were properly assessed. The experimental results showed that while the COD removal efficiency was in all cases over 60% (after 40 days), the MFCT arrangement showed the best performance since the average removal value was 82%, compared to close to 70% for MFCE and MFCW. Statistical analysis of the COD removal efficiency confirmed that the performance of MCFT is substantially better than that of MFCE and MFCW. In regard to the other parameters surveyed, no significant influence of the different cathode arrangements explored could be found.

Development and life cycle assessment of an auto circulating bio-electrochemical reactor for energy positive continuous wastewater treatment

Bioelectrochemical systems like microbial fuel cells (MFCs) are quaint systems known to metamorphose the chemical energy of organic matter into electrical energy using catalytic activity of microorganisms. A novel continuous Auto Circulating Bio-Electrochemical Reactor (AutoCirBER) was developed to fulfil the gap of 'simple, inexpensive and compact design' that can continuously treat larger amount of organic wastewater at shorter residence time and without consuming external energy for liquid mixing. AutoCirBER eliminated the need for external agitation for liquid-mixing and therefore, energy requirements. AutoCirBER was operated in continuous-mode and hydraulic retention time was optimized. The reactor underwent performance check-up viz. COD removal, net power output, columbic efficiency, sludge generation and an attributional life cycle assessment (LCA) was also conducted. AutoCirBER was sustainable to run in continuous-mode and showed more than 90.4% of COD removal, and 59.55 W.h net annual energy recovery. Experimental LCA of AutoCirBER also displays its environmental feasibility in longer run.

Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes

An ion exchange membrane (IEM) is an important component in dual-chamber microbial fuel cells (MFCs) to separate cathodic chromium from anode bacteria to avoid toxicity. Common used IEMs (e.g., BPM, CEM, PEM, AEM) have different ionic transfer abilities which could influence MFC performance and chromium removal. Additionally, to distinguish chromium "removal" or "reduction" by MFCs, the chromium removal in this study was further analyzed into cathodic reduction, adsorption on the membrane and permeation through membrane to the anode chamber. It was found that BPM achieved the best performance in removing hexavalent chromium (99.4 ± 0.2 %) and balancing pH and conductivity in both chambers, followed by AEM (97.9 ± 0.8 %) and CEM (95.6 ± 0.8 %), while PEM can not well maintain pH and conductivity leading to the worst anode performance and lowest chromium removal efficiency. However, the adsorption of chromium on the AEM accounts for 91.1 ± 0.7 %, which was much higher than the other three membranes. The permeation of chromium through the membrane were all lower than 0.2% which can be ignored. SEM and EDS results showed that chromium deposits and bacteria were detected on the membrane facing cahtode and anode, respectively, indicating that membrane scaling and fouling were inevitable and happened within 24 h operation.

Development and Application of Supported Ionic Liquid Membranes in Microbial Fuel Cell Technology: A Concise Overview

Membrane separators are key elements of microbial fuel cells (MFCs), especially of those constructed in a dual-chamber configuration. Until now, membranes made of Nafion have been applied the most widely to set-up MFCs. However, there is a broader agreement in the literature that Nafion is expensive and in many cases, does not meet the actual (mainly mass transfer-specific) requirements demanded by the process and users. Driven by these issues, there has been notable progress in the development of alternative materials for membrane fabrication, among which those relying on the deployment of ionic liquids are emerging. In this review, the background of and recent advances in ionic liquid-containing separators, particularly supported ionic liquid membranes (SILMs), designed for MFC applications are addressed and evaluated. After an assessment of the basic criteria to be fulfilled by membranes in MFCs, experiences with SILMs will be outlined, along with important aspects of transport processes. Finally, a comparison with the literature is presented to elaborate on how MFCs installed with SILM perform relative to similar systems assembled with other, e.g., Nafion, membranes.

The Potential of Microbial Fuel Cells for Remediation of Heavy Metals from Soil and Water-Review of Application

The global energy crisis and heavy metal pollution are the common problems of the world. It is noted that the microbial fuel cell (MFC) has been developed as a promising technique for sustainable energy production and simultaneously coupled with the remediation of heavy metals from water and soil. This paper reviewed the performances of MFCs for heavy metal removal from soil and water. Electrochemical and microbial biocatalytic reactions synergistically resulted in power generation and the high removal efficiencies of several heavy metals in wastewater, such as copper, hexavalent chromium, mercury, silver, thallium. The coupling system of MFCs and microbial electrolysis cells (MECs) successfully reduced cadmium and lead without external energy input. Moreover, the effects of pH and electrode materials on the MFCs in water were discussed. In addition, the remediation of heavy metal-contaminated soil by MFCs were summarized, noting that plant-MFC performed very well in the heavy metal removal.

Dominance of electroactive microbiomes in bioelectrochemical remediation of hydrocarbon-contaminated soils with different textures

Bioelectrochemical systems (BESs) are known to enhance the remediation of hydrocarbon-contaminated soil and sediments compared with natural attenuation, and the primary mechanism has been assumed as anaerobic degradation facilitated by electroactive bacteria (EAB) using the electrode as electron acceptor. However, known EAB were rarely found on the anodes of reported BESs, which challenged the fundamental mechanism of BESs although significant current generation was always observed during degradation of these recalcitrant substrates. This study however found the abundant EAB Geobacter (∼27.3%) in the anodic biofilms, which confirmed the role of electroactive bio-anode on the conversion of hydrocarbons into the current for the enhancement of remediation. Widespread occurrence of aerobic hydrocarbon-degrading bacteria (HDB) (e.g. ∼24.0% Parvibaculum and ∼30.6% Pseudomonas) was observed in soils with limited dissolved oxygen (∼0.4 mg/L). The higher abundance of dehydrogenase genes was found in the anode biofilms than that in soils, indicating anodic microorganisms may be mainly responsible for the removal of intermediates of aerobic hydrocarbons degradation in soils. High water saturation level and sandy soil texture showed positive impacts on bioelectrochemical remediation, while clay soil and unsaturation condition pose challenges in mass transfers in the matrix. The reactor performance was consistent with the phylogenetic molecular ecological network (pMENs) analysis, which showed that sandy soil BESs had tighter microbial network interactions than clay soil reactors.

Removal of Hepatitis B virus surface HBsAg and core HBcAg antigens using microbial fuel cells producing electricity from human urine

Microbial electrochemical technology is emerging as an alternative way of treating waste and converting this directly to electricity. Intensive research on these systems is ongoing but it currently lacks the evaluation of possible environmental transmission of enteric viruses originating from the waste stream. In this study, for the first time we investigated this aspect by assessing the removal efficiency of hepatitis B core and surface antigens in cascades of continuous flow microbial fuel cells. The log-reduction (LR) of surface antigen (HBsAg) reached a maximum value of 1.86 ± 0.20 (98.6% reduction), which was similar to the open circuit control and degraded regardless of the recorded current. Core antigen (HBcAg) was much more resistant to treatment and the maximal LR was equal to 0.229 ± 0.028 (41.0% reduction). The highest LR rate observed for HBsAg was 4.66 ± 0.19 h⁻¹ and for HBcAg 0.10 ± 0.01 h⁻¹. Regression analysis revealed correlation between hydraulic retention time, power and redox potential on inactivation efficiency, also indicating electroactive behaviour of biofilm in open circuit control through the snorkel-effect. The results indicate that microbial electrochemical technologies may be successfully applied to reduce the risk of environmental transmission of hepatitis B virus but also open up the possibility of testing other viruses for wider implementation.

Assessment of expanded polystyrene as a separator in microbial fuel cell

Separators are considered as an important component in microbial fuel cells (MFCs) to facilitate ion transport and to prevent electrode short circuiting. In the present study, expanded polystyrene (EPS) was evaluated for the first time as a separator in a single-chamber air cathode and dual chamber aqueous cathode MFCs. The characteristics and performance of EPS were analyzed and compared with other conventionally used separators used in MFCs and was found to be competitive. Initially, the EPS was less impermeable to protons, resulting in delayed process startup (17 days) and stabilization (57 days), but gradually exhibited improved and stable performance. In the air cathode MFC with the EPS as the separator and domestic wastewater as the substrate, power production was 391 mW/m², while power output of the aqueous cathode MFC was 328 mW/m². The characteristics and cost analysis of EPS indicate that it can be a potential candidate as a separator in scaled-up MFC applications.

New ecological dam for sediment and overlying water pollution treatment based on microbial fuel cell principle

In this study, a new ecological dam based on the microbial fuel cell principle (MFCED) was designed to remove pollutants from river sediments and water bodies. Sediment organics were better removed in the MFCED mode in comparison with the other two modes (ecological dam with open circuit (OCED) and ecological dam filled with gravel in cathode chamber (GMFCED)). The difference of nitrogen source in water had little effect on the removal of chemical oxygen demand (COD) (70-80%), while nitrate was more readily removed in the MFCED. The voltage curve and power curve were measured to understand the bioelectricity generation of MFCED. During the stable operation phase of MFCED, the voltage was stabilized between 0.09-0.15 V. The results of high-throughput sequencing indicated that the anode and cathode diversities of MFCED were more than the other systems, and the species diversity of the anode was more than that of the cathode in the MFCED. Graphical abstract.

Biofouling of membranes in microbial electrochemical technologies: Causes, characterization methods and mitigation strategies

The scope of the review is to discuss the current state of knowledge and lessons learned on biofouling of membrane separators being used for microbial electrochemical technologies (MET). It is illustrated what crucial membrane features have to be considered and how these affect the MET performance, paying particular attention to membrane biofouling. The complexity of the phenomena was demonstrated and thereby, it is shown that membrane qualities related to its surface and inherent material features significantly influence (and can be influenced by) the biofouling process. Applicable methods for assessment of membrane biofouling are highlighted, followed by the detailed literature evaluation. Finally, an outlook on e.g. possible mitigation strategies for membrane biofouling in MET is provided.

Towards the optimisation of ceramic-based microbial fuel cells: A three-factor three-level response surface analysis design

Microbial fuel cells (MFCs) are an environment-friendly technology, which addresses two of the most important environmental issues worldwide: fossil fuel depletion and water scarcity. Modelling is a useful tool that allows us to understand the behaviour of MFCs and predict their performance, yet the number of MFC models that could accurately inform a scale-up process, is low. In this work, a three-factor three-level Box-Behnken design is used to evaluate the influence of different operating parameters on the performance of air-breathing ceramic-based MFCs fed with human urine. The statistical analysis of the 45 tests run shows that both anode area and external resistance have more influence on the power output than membrane thickness, in the range studied. The theoretical optimal conditions were found at a membrane thickness of 1.55 mm, an external resistance of 895.59 Ω and an anode area of 165.72 cm2, corresponding to a maximum absolute power generation of 467.63 μW. The accuracy of the second order model obtained is 88.6%. Thus, the three-factor three-level Box-Behnken-based model designed is an effective tool which provides key information for the optimisation of the energy harvesting from MFC technology and saves time in terms of experimental work.

Microbial recycling cells (MRCs): A new platform of microbial electrochemical technologies based on biocompatible materials, aimed at cycling carbon and nutrients in agro-food systems

This article reviews the mechanisms that drive nutrients and carbon sequestration from wastewaters by microbial electrochemical technologies (METs). In this framework, a new generation of METs is also presented (to be called microbial recycling cells, MRCs), based on 100%-recyclable materials (biomass-derived char coal, clay, terracotta, paper, ligno-cellulosic plant materials, etc.), which can act as bio-electrodes, separators and structural frames. In traditional METs architectures (based on technological materials such as carbon cloths, plastic panels, membranes, binders), inorganic salts precipitation and adsorption, as well as biofouling due to organic-matter deposition, are considered as main drawbacks that clog and hinder the systems over relatively short periods. In MRCs, these mechanisms should be maximized, instead of being avoided. In this perspective, both inorganic and organic forms of the main nutrients are sequestered from wastewater and deposited on METs modules. Once the systems become saturated, they can entirely be recycled as agricultural soil conditioners or as base for organic-mineral fertilizers.

Regenerated silk fibroin membranes as separators for transparent microbial fuel cells

In recent years novel applications of bioelectrochemical systems are exemplified by phototrophic biocathodes, biocompatible enzymatic fuel cells and biodegradable microbial fuel cells (MFCs). Herein, transparent silk fibroin membranes (SFM) with various fibroin content (2%, 4% and 8%) were synthesised and employed as separators in MFCs and compared with standard cation exchange membranes (CEM) as a control. The highest real-time power performance of thin-film SFM was reached by 2%-SFM separators: 25.7 ± 7.4 μW, which corresponds to 68% of the performance of the CEM separators (37.7 ± 3.1 μW). Similarly, 2%-SFM revealed the highest coulombic efficiency of 6.65 ± 1.90%, 74% of the CEM efficiency. Current for 2%-SFM reached 0.25 ± 0.03 mA (86% of CEM control). Decrease of power output was observed after 23 days for 8% and 4% and was a consequence of deterioration of SFMs, determined by physical, chemical and biological studies. This is the first time that economical and transparent silk fibroin polymers were successfully employed in MFCs.