Insights into the Electron Transfer Behaviors of a Biocathode Regulated by Cathode Potentials in Microbial Electrosynthesis Cells for Biogas Upgrading

Electrically Driven Biocatalysis for Sustainable CO2-to-Chemicals Transformation

The catalytic transformation of CO2 into value-added chemicals has become a critical strategy for mitigating environmental issues and generating economic benefits. Although substantial progress has been made in renewable electricity-driven CO2 conversion into C1/C2 products, the efficient synthesis of high-value, and long-chain compounds remains a significant challenge. Biosynthesis offers a feasible route for producing long-chain value-added products at mild conditions. Consequently, the integration of electrocatalysis with bioconversion has emerged as a promising approach for sustainable CO2 conversion. This short review outlines recent advances in the sustainable synthesis of long-chain compounds from CO2 via electrically driven biocatalysis, highlighting innovative coupling strategies that combine electrochemical and biochemical processes. Furthermore, the remaining challenges and prospects are tentatively discussed for further advancing CO2-based sustainable synthesis.

Microbial electrosynthesis technology for CO2 mitigation, biomethane production, and ex-situ biogas upgrading

Currently, global annual CO2 emissions from fossil fuel consumption are extremely high, surpassing tens of billions of tons, yet our capacity to capture and utilize CO2 remains below a small fraction of the amount generated. Microbial electrosynthesis (MES) systems, an integration of microbial metabolism with electrochemistry, have emerged as a highly efficient and promising bio-based carbon-capture-and-utilization technology over other conventional techniques. MES is a unique technology for lowering the atmospheric CO2 as well as CO2 in the biogas, and also simultaneously convert them to renewable bioenergy, such as biomethane. As such, MES techniques could be applied for biogas upgrading to generate high purity biomethane, which has the potential to meet natural gas standards. This article offers a detailed overview and assessment of the latest advancements in MES for biomethane production and biogas upgrading, in terms of selecting optimal methane production pathways and associated electron transfer processes, different electrode materials and types, inoculum sources and microbial communities, ion-exchange membrane, externally applied energy level, operating temperature and pH, mode of operation, CO2 delivery method, selection of inorganic carbon source and its concentration, start-up time, and system pressure. It also highlights the current MES challenges associated with upscaling, design and configuration, long-term stability, energy demand, techno-economics, achieving net negative carbon emission, and other operational issues. Moreover, we provide a summary of current and future opportunities to integrate MES with other unique biosystems, such as methanotrophic bioreactors, and incorporate quorum sensing, 3D printing, and machine learning to further develop MES as a better biomethane-producer and biogas upgrading technique.

Carbon chain elongation characterizations of electrode-biofilm microbes in electro-fermentation

The higher efficiency of electro-fermentation in synthesizing medium-chain fatty acids (MCFAs) compared to traditional fermentation has been acknowledged. However, the functional mechanisms of electrode-biofilm enhancing MCFAs synthesis remain research gaps. To address this, this study proposed a continuous flow electrode-biofilm reactor for chain elongation (CE). After 225 days of operation, stable electrode-biofilms formed and notably improved caproate yield by more than 38 %. The electrode-biofilm was enriched with more CE microorganisms and electroactive bacteria compared to the suspended sludge microorganisms, including Caproicibacterium, Oscillibacter and Pseudoramibacter. Besides, the upregulated CE pathways were evaluated by metagenomic analysis, and the results indicated that the pathways such as acetyl-CoA and malonyl-[acp] formation, reverse beta-oxidation, and fatty acid biosynthesis pathway were all markedly enhanced in cathodic biofilm, more than anodic biofilm and suspended microorganisms. Moreover, microbial community regulated processes like bacterial chemotaxis, flagellar assembly and quorum sensing, crucial for electrode-biofilm formation. Electron transfer, energy metabolism, and microbial interactions were found to be prominently upregulated in the cathodic biofilm, surpassing levels observed in anodic biofilm and suspended sludge microorganisms, which further enhanced CE efficiency. In addition, the statistical analyses further highlighted key microbial functions and interactions within the cathodic biofilm. Oscillospiraceae_bacterium was identified to be the most active microbe, alongside pivotal roles played by Caproiciproducens_sp._NJN-50, Clostridiales_bacterium, Prevotella_sp. and Pseudoclavibacter_caeni. Eventually, the proposed microbial collaboration mechanisms of cathodic biofilm were ascertained. Overall, this study uncovered the biological effects of the electrode-biofilm on MCFAs electrosynthesis, thereby advancing biochemicals production and filling the knowledge gaps in CE electroactive biofilm reactors.

Enhanced microbial electrosynthesis performance with 3-D algal electrodes under high CO2 sparging: Superior biofilm stability and biocathode-plankton interactions

Microbial electrosynthesis (MES) shows great promise for converting CO2 into high-value chemicals. However, cathode biofilm erosion by high CO2 sparging and the unclear role of plankton in MES hinders the continuous improvement of its performance. This study aims to enhance biofilm resistance and improve interactions between bio-cathode and plankton by upgrading waste algal biomass into 3-D porous algal electrode (PAE) with rough surface. Results showed that the acetate synthesis of PAE under 20 mL/min CO2 sparging (PAE-20) was up to 3330.61 mol/m3, 4.63 times that of carbon felt under the same conditions (CF-20). The microbial loading of PAE-20 biofilm was twice that of CF-20. Furthermore, higher cumulative abundance of functional microorganisms was observed in plankton of PAE-20 (55 %), compared to plankton of CF-20 (14 %), and enhanced biocathode-plankton interactions significantly suppressed acetate consumption. Thus, this efficient and sustainable 3-D electrode advances MES technology and offers new perspectives for waste biomass recycling.

A novel Anaerobic Cathodic Dynamic Membrane Bioreactor (AnCDMBR) for efficient mitigating fouling and recovering bioenergy from municipal wastewater

Concerns regarding membrane fouling and suboptimal bioenergy recovery have constrained the implementation of anaerobic membrane bioreactor (AnMBR) for treating low-strength municipal wastewater. This study presents a novel anaerobic cathodic dynamic membrane bioreactor (AnCDMBR) designed to address these challenges. A self-formed cathodic dynamic membrane (CDM) on inexpensive carbon cloth was developed to function as both a membrane and biocathode to achieve dual-function effects of mitigating membrane fouling and accelerating organics conversion. Compared with common dynamic membrane (1.52 kPa/d) and commercial membranes (7.52 kPa/d), the developed CDM presented a significantly reduced fouling rate (1.02 kPa/d), exhibiting the potential as a substitute for high-cost conductive membranes. Furthermore, efficient and stable biomethanation occurred in AnCDMBR with a superior methane yield rate of 0.26 L-CH4/g-COD (CH4 content > 95 %), which was 1.42 times higher than the control, linked to the higher activities of microbial metabolism and methanogenic-related key enzymes. Further analysis revealed that electrostimulation-induced niche differentiation of microbiota regulated interspecies interactions between electroactive microorganisms and complex anaerobic digestion microbiomes, facilitating organic matter conversion to methane and leading to superior bioenergy recovery. This study offered a new strategy for effectively mitigating fouling and recovering bioenergy from low-strength wastewater, potentially expanding the application of AnMBRs.

Potential and challenge in accelerating high-value conversion of CO2 in microbial electrosynthesis system via data-driven approach

Microbial electrosynthesis for CO2 utilization (MESCU) producing valuable chemicals with high energy density has garnered attention due to its long-term stability and high coulombic efficiency. The data-driven approaches offer a promising avenue by leveraging existing data to uncover the underlying patterns. This comprehensive review firstly uncovered the potentials of utilizing data-driven approaches to enhance high-value conversion of CO2 via MESCU. Firstly, critical challenges of MESCU advancing have been identified, including reactor configuration, cathode design, and microbial analysis. Subsequently, the potential of data-driven approaches to tackle the corresponding challenges, encompassing the identification of pivotal parameters governing reactor setup and cathode design, alongside the decipheration of omics data derived from microbial communities, have been discussed. Correspondingly, the future direction of data-driven approaches in assisting the application of MESCU has been addressed. This review offers guidance and theoretical support for future data-driven applications to accelerate MESCU research and potential industrialization.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Electro-mediated cathodic oxygen drives respiration chain electron transfer of electroactive bacteria to enhance refractory organic biological oxidation

In electro-mediated biological system (EMBS), biological anode and cathode components were incorporated into an anaerobic bioreactor, providing a small amount of oxygen to the cathode as an electron acceptor. Oxygen diffusion also impacts the anode's anaerobic ecological environment. This study unraveled how oxygen influences the metabolism and electron transport chain during the biological oxidation of refractory organics. Under the influence of electromotive force, the straight-chain model pollutant N,N-dimethylformamide (DMF) showed rapid degradation and better ammonification, with maximum rates reaching 0.53 h-1 and 26.6 %, respectively. Elevated electromotive force promoted the enrichment of functional electroactive bacteria on the anode and enhanced the availability of electron storage sites, thereby facilitating electron transfer at the anode-biofilm interface. Conversely, the anodic micro-aerobic environment disrupted the anaerobic microbial community structure, and the competitive interactions among fermentative bacteria and electroactive bacteria inhibited DMF degradation. Metagenomic analysis confirmed that cathodic oxygen up-regulated the pyruvate metabolism and the tricarboxylic acid (TCA) cycle to generate NADH and synthesize ATP. The electromotive force induced by cathodic oxygen accelerated the electron transfer in respiratory chains of electroactive bacteria, driving the oxidation of NADH and enhancing the degradation of organics. This study improves our understanding of the regulatory mechanisms governing metabolic pathways under the influence of cathodic oxygen. It offers potential for developing more efficient EMBS in industrial wastewater pretreatment, ensuring that oxygen is prevented from diffusing to the anode during micro-aeration at the cathode.

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.

Biogas upgrading and membrane anti-fouling mechanisms in electrochemical anaerobic membrane bioreactor (EC-AnMBR): Focusing on spatio-temporal distribution of metabolic functionality of microorganisms

Electrochemical anaerobic membrane bioreactor (EC-AnMBR) by integrating a composite anodic membrane (CAM), represents an effective method for promoting methanogenic performance and mitigating membrane fouling. However, the development and formation of electroactive biofilm on CAM, and the spatio-temporal distribution of key functional microorganisms, especially the degradation mechanism of organic pollutants in metabolic pathways were not well documented. In this work, two AnMBR systems (EC-AnMBR and traditional AnMBR) were constructed and operated to identify the role of CAM in metabolic pathway on biogas upgrading and mitigation of membrane fouling. The methane yield of EC-AnMBR at HRT of 20 days was 217.1 ± 25.6 mL-CH4/g COD, about 32.1 % higher compared to the traditional AnMBR. The 16S rRNA analysis revealed that the EC-AnMBR significantly promoted the growth of hydrolysis bacteria (Lactobacillus and SJA-15) and methanogenic archaea (Methanosaeta and Methanobacterium). Metagenomic analysis revealed that the EC-AnMBR promotes the upregulation of functional genes involved in carbohydrate metabolism (gap and kor) and methane metabolism (mtr, mcr, and hdr), improving the degradation of soluble microbial products (SMPs)/extracellular polymeric substances (EPS) on the CAM and enhancing the methanogens activity on the cathode. Moreover, CAM biofilm exhibits heterogeneity in the degradation of organic pollutants along its vertical depth. The bacteria with high hydrolyzing ability accumulated in the upper part, driving the feedstock degradation for higher starch, sucrose and galactose metabolism. A three-dimensional mesh-like cake structure with larger pores was formed as a biofilter in the middle and lower part of CAM, where the electroactive Geobacter sulfurreducens had high capabilities to directly store and transfer electrons for the degradation of organic pollutants. This outcome will further contribute to the comprehension of the metabolic mechanisms of CAM module on membrane fouling control and organic solid waste treatment and disposal.

Delineating cathodic extracellular electron transfer pathways in microbial electrosynthesis: Modulation of polarized potential and Pt@C addition

Microbial electrosynthesis (MES) is an innovative technology that employs microbes to synthesize chemicals by reducing CO2. A comprehensive understanding of cathodic extracellular electron transfer (CEET) is essential for the advancement of this technology. This study explores the impact of different cathodic potentials on CEET and its response to introduction of hydrogen evolution materials (Pt@C). Without the addition of Pt@C, H2-mediated CEET contributed up to 94.4 % at -1.05 V. With the addition of Pt@C, H2-mediated CEET contributions were 76.6 % (-1.05 V) and 19.9 % (-0.85 V), respectively. BRH-c20a was enriched as the dominated microbe (>80 %), and its relative abundance was largely affected by the addition of Pt@C NPs. This study highlights the tunability of MES performance through cathodic potential control and the addition of metal nanoparticles.

Tandem Acidic CO2 Electrolysis Coupled with Syngas Fermentation: A Two-Stage Process for Producing Medium-Chain Fatty Acids

The tandem application of CO2 electrolysis with syngas fermentation holds promise for achieving heightened production rates and improved product quality. However, the significant impact of syngas composition on mixed culture-based microbial chain elongation remains unclear. Additionally, effective methods for generating syngas with an adjustable composition from acidic CO2 electrolysis are currently lacking. This study successfully demonstrated the production of medium-chain fatty acids from CO2 through tandem acidic electrolysis with syngas fermentation. CO could serve as the sole energy source or as the electron donor (when cofed with acetate) for caproate generation. Furthermore, the results of gas diffusion electrode structure engineering highlighted that the use of carbon black, either alone or in combination with graphite, enabled consistent syngas generation with an adjustable composition from acidic CO2 electrolysis (pH 1). The carbon black layer significantly improved the CO selectivity, increasing from 0% to 43.5% (0.05 M K+) and further to 92.4% (0.5 M K+). This enhancement in performance was attributed to the promotion of K+ accumulation, stabilizing catalytically active sites, rather than creating a localized alkaline environment for CO2-to-CO conversion. This research contributes to the advancement of hybrid technology for sustainable CO2 reduction and chemical production.

Enhancement of nitrate reduction in microbial fuel cells by acclimating biocathode potential: Performance, microbial community, and mechanism

The enhancement of nitrate reduction in microbial fuel cells (MFCs) by acclimating biocathode potential was studied. An MFC system was started up, and measured by cyclic voltammetry to determine a suitable potential region for acclimating biocathode. The experimental results revealed that potential acclimation could efficiently improve denitrification performance by relieving the phenomenon of nitrite accumulation, and optimum performance was obtained at -0.4 V with a total nitrogen removal efficiency of 87.4 %. Subsequently, the characteristics of electron transfer behaviors were measured, suggesting that a positive correlation between nitrate reduction and the contribution of direct electron transfer emerged. Furthermore, a denitrification mechanism was proposed. The results indicated that potential acclimation was conducive to enhancing denitrifying enzyme activity and that the electron transport system activity could be increased by 5.8 times. This study provides insight into the electron transfer characteristics and denitrification mechanisms in MFCs for nitrate reduction at specific acclimatization potentials.

Enhancing electricity-driven methanogenesis by assembling biotic-abiotic hybrid system in anaerobic membrane bioreactor

Biotic-abiotic hybrid systems are promising technologies to enhance methane production in anaerobic wastewater treatment. However, the dense structure of the extracellular polymeric substances (EPS) present in anaerobic granular sludge (AGS) poses challenges with respect to the implementation of hybrid systems and efficient interspecies electron transfer. In this study, the use of AGS with a Ni/Fe layered double hydroxide@activated carbon (Ni/Fe LDH@C-AGS) was investigated in an anaerobic membrane bioreactor (AnMBR). The hybrid system showed a significant increase of 82% in methane production. Further research revealed that Ni/Fe LDH@C regulated the dense structure of EPS, stimulated the production of cytochromes, and facilitated the decomposition of nonconductive substances. Surprisingly, the hybrid system also promoted resistance to membrane fouling and extended membrane life by 81%. This study provides insights into the operation of a biotic-abiotic hybrid system by regulating the dense structure of EPS ultimately resulting in an enhanced methane production.

Insight into the Pseudocapacitive Behavior of Electroactive Biofilms in Response to Dynamic-Controlled Electron Transfer and Metabolism Kinetics for Current Generation in Water Treatment

Electroactive biofilms (EBs) engage in complex electron transfer and storage processes involving intracellular and extracellular mediators with temporary electron storage capabilities. Consequently, electroactive biofilms exhibit pseudocapacitive behaviors during substrate degradation processes. However, comprehensive systematic research in this area has been lacking. This study demonstrated that the pseudocapacitive property was an intrinsic characteristic of EBs. This property represents dynamic-controlled electron transfer and is critical in current generation, unlike noncapacitive responses. Nontransient charge and discharge experiments revealed a correlation between capacitive charge accumulation and current generation in EBs. Additionally, analysis of substrate degradation suggested that the maximum power density (Pmax) changed with the kinetic constants of COD degradation, with pseudocapacitances of EBs directly proportional to Pmax. The interaction networks of key latent variables were evaluated through partial least-squares path modeling analysis. The results indicated that cytochrome c was closely associated with the formation of pseudocapacitance in EBs. In conclusion, pseudocapacitance can be considered a valuable indicator for assessing the complex electron transfer behavior of EBs. Pseudocapacitive biofilms have the potential to efficiently regulate biological reactions and serve as a promising carbon-neutral and renewable strategy for energy generation and storage. An in-depth understanding of the intrinsic property of pseudocapacitive behavior in EBs can undoubtedly advance the development of this concept in the future.

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

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

Enhanced electron recovery by optimizing sandwich structure agricultural waste corncob filled anode in microbial electrochemical system to facilitate wastewater denitrification

Microbial electrochemical system autotrophic denitrification has attracted much attention due to its cost-efficiency and clean advantages. The autotrophic denitrification rate highly depends on the input electrons to the cathode. In this study, agricultural waste corncob was filled into sandwich structure anode as low-cost carbon source for electron production. The COMSOL software was used to guide the construction of sandwich structure anode to control carbon source release and enhance electron collection, including suitable pore size (4 mm) and current collector arrangement (five branches). Optimized sandwich structure anode system with the help of 3D printing obtained a higher denitrification efficiency (21.79 ± 0.22 gNO₃^--N/m³d) than anodic systems without pore and current collector. Statistical analysis showed that enhanced autotrophic denitrification efficiency was the responsible for enhanced denitrification performance of the optimized anode system. This study provides a strategy to improve the autotrophic denitrification performance of the microbial electrochemical system by optimizing the anode structure.