Microbial electrolysis cells turning to be versatile technology: recent advances and future challenges

Sustainable hydrogen production via microalgae: Technological advancements, economic indicators, environmental aspects, challenges, and policy implications

Hydrogen has emerged as an alternative energy source to meet the increasing global energy demand, depleting fossil fuels and environmental issues resulting from fossil fuel consumption. Microalgae-based biomass is gaining attention as a potential source of hydrogen production due to its green energy carrier properties, high energy content, and carbon-free combustion. This review examines the hydrogen production process from microalgae, including the microalgae cultivation technological process for biomass production, and the three main routes of biomass-to-hydrogen production: thermochemical conversion, photo biological conversion, and electrochemical conversion. The current progress of technological options in the three main routes is presented, with the various strains of microalgae and operating conditions of the processes. Furthermore, the economic and environmental perspectives of biomass-to-hydrogen from microalgae are evaluated, and critical operational parameters are used to assess the feasibility of scaling up biohydrogen production for commercial industrial-scale applications. The key finding is the thermochemical conversion process is the most feasible process for biohydrogen production, compared to the pyrolysis process. In the photobiological and electrochemical process, pure hydrogen can be achieved, but further process development is required to enhance the production yield. In addition, the high production cost is the main challenge in biohydrogen production. The cost of biohydrogen production for direct bio photolysis it cost around $7.24 kg-1; for indirect bio photolysis it costs around $7.54 kg-1 and for fermentation, it costs around $7.61 kg-1. Therefore, comprehensive studies and efforts are required to make biohydrogen production from microalgae applications more economical in the future.

Advances and challenges in biocathode microbial electrolysis cells for chlorinated organic compounds degradation from electroactive perspectives

Microbial electrolysis cell (MEC) is a promising in-situ strategy for chlorinated organic compound (COC) pollution remediation due to its high efficiency, low energy input, and long-term potential. Reductive dechlorination as the most critical step in COC degradation which takes place primarily in the cathode chamber of MECs is a complex biochemical process driven by the behavior of electrons. However, no information is currently available on the internal mechanism of MEC in dechlorination from the perspective of the whole electron transfer procedure and its dependent electrode materials. This review addresses the underlying mechanism of MEC on the fundamental of the generation (electron donor), transmission (transfer pathway), utilization (functional microbiota) and reception (electron acceptor) of electrons in dechlorination. In addition, the vital role of varied cathode materials involved in the entire electron transfer procedure during COC dechlorination is emphasized. Subsequently, suggestions for future research, including model construction, cathode material modification, and expanding the applicability of MECs to removal gaseous COCs have been proposed. This paper enriches the mechanism of COC degradation by MEC, and thus provides the theoretical support for the scale-up bioreactors for efficient COC removal.

Overcoming hydrogen loss in single-chamber microbial electrolysis cells by urine amendment

The effective hydrogen production in single-chamber microbial electrolysis cells (MECs) has been seriously challenged by various hydrogen consumers resulting in substantial hydrogen loss. In previous studies, the total ammonia nitrogen (TAN) has been used to inhibit certain hydrogen-consuming microorganisms to enhance hydrogen production in fermentation. In this study, we explored the feasibility of using source-separated urine to overcome hydrogen loss in the MEC, with the primary component responsible being TAN generated via urea hydrolysis. Experimental results revealed that the optimal TAN concentration ranged from 1.17 g N/L to 1.75 g N/L. Within this range, the hydrogen production rate substantially improved from less than 100 L/(m3·d) up to 520 L/(m3·d), and cathode recovery efficiency and energy recovery efficiency were greatly enhanced, with the hydrogen percentage achieved over 95 % of the total gas volume, while maintaining uninterrupted electroactivity in the anode. Compared to using chemically added TAN, using source separated urine as the source of ammonia also showed the effect of overcoming hydrogen loss but with lower Coulombic efficiency due to the complex organic components. Pre-adaptation of the reactor with urea enhanced hydrogen production by nearly 60 %. This study demonstrated the effectiveness of TAN and urine in suppressing hydrogen loss, and the results are highly relevant to MECs treating real wastewater with high TAN concentrations, particularly human fecal and urine wastewater.

Performance enhancement of integrating microbial electrolysis cell on two-stage anaerobic digestion of food waste: Electro-methanogenic stage versus electro-two stages

The effects of microbial electrolysis cell (MEC) integration stage on two-stage anaerobic digestion (TSAD) of food waste (FW) were studied via semi-continuous experiments. The results showed that both MEC (with 1.2 V) integrations enhanced the performances of the TSADs, with the enhancement of electro-two stages being higher. The methane production of TSAD increased from 1.36 ± 0.04 L/L/d to 1.53 ± 0.05 L/L/d (electro-methanogenic stage) and 1.54 ± 0.04 L/L/d (electro-two stages) during the steady period. Electro-acidogenesis decreased propionic acid production and enhanced hydrogen production, while electro-methanogenesis promoted the conversion of volatile fatty acids to methane. The MEC integration improved energy recovery from the organic matter in FW by 11.65-16.15% and reduced the mass loss, with those of the electro-two stages being higher and the electro-methanogenic stage being dominant in the electro-two stages. The integration of MEC enhanced anaerobic fermentation by enriching Olsenella, norank_f__ST-12K33 and Proteiniphilum and improved methanogenesis by enriching Methanobacterium and Candidatus_Methanofastidiosum.

Performance and microbial community analysis on nitrate removal in a bioelectrochemical reactor

In this experiment, we took reflux sludge, sludge from an aeration tank, and soil from roots as microbial inoculating sources for an electrochemical device for denitrification with high-throughput sequencing on cathodic biofilms. The efficiency of nitrate nitrogen removal using different microbial inoculates varied among voltages. The optimal voltages for denitrification of reflux sludge, aeration tank sludge, and root soil were 0.7V, 0.5V, and 0.5V, respectively. Further analysis revealed that the respective voltages had a significant effect upon microbial growth from the respective inoculates. Proteobacteria and Firmicutes were the main denitrifying microbes. With the addition of low current (produced by the applied voltage), the Chao1, Shannon and Simpson indexes of the diversity of microorganisms in soil inoculation sources increased, indicating that low current can increase the diversity and richness of the microorganisms, while the reflux sludge and aeration tank sludge showed different changes. Low-current stimulation decreased microbial diversity to a certain extent. Pseudomonas showed a trend of decline with increasing applied voltage, in which the MEC (microbial electrolysis cell) of rhizosphere soil as inoculates decreased most significantly from 77.05% to 12.58%, while the MEC of Fusibacter showed a significant increase, and the sludge of reflux sludge, aeration tank and rhizosphere soil increased by 31.12%, 18.7% and 34.6%, respectively. The applied voltage also significantly increased the abundance of Azoarcus in communities from the respective inoculates.

Protection of electroactive biofilms against hypersaline shock by quorum sensing

Quorum sensing (QS) is an ideal strategy for boosting the operating performance of electroactive biofilms (EABs), but its potential effects on the protection of electroactive biofilms against environmental shocks (e.g., hypersaline shock) have been rarely revealed. In this study, a QS signaling molecule, the N-(3-oxo-dodecanoyl)-L-homoserine lactone, was employed to promote the anti-shock property of the EABs against extreme saline shock. The maximum current density of the QS-regulated biofilm recovered to 0.17 mA/cm² after 10% salinity exposure, which was much higher than those of its counterparts. The laser scanning confocal microscope confirmed a thicker and more compact biofilm with the presence of the QS signaling molecule. The extracellular polymeric substances (EPS) might play a crucial role in the anti-shocking behaviors, as the polysaccharides in EPS of QS-biofilm had doubled compared to the groups with acylase (the QS quencher). The microbial community analysis indicated that the QS molecule enriched the relative abundance of key species including Pseudomonas sp. and Geobacter sp., which were both beneficial to the stability and electroactivity of the biofilms. The functional genes related to the bacterial community were also up-regulated with the presence of the QS molecule. These results highlight the importance of QS effects in protecting electroactive biofilm under extreme environmental shock, which provides effective and feasible strategies for the future development of microbial electrochemical technologies.

Efficient elimination of nonylphenol and 4-tert-octylphenol by weak electrical stimulated anaerobic microbial processes

The investigation into the degradation of alkylphenol pollutants (APs) has become a hotspot due to their harmful effects on the environment and human health. In this study, microbial electrolysis cells (MECs) were used to degrade nonylphenol (NP) and 4-tert-octylphenol (4-tert-OP). The study found that the degradation rates of NP and 4-tert-OP for a 6-day period were 83.6% and 96.3%, respectively, which were 30.53% and 26.7% higher than those of the group without applied voltage. The double layer area in the degradation of 4-tert-OP was larger than that of NP, and the resistance exhibited by 4-tert-OP (87.47 Ω) in MEC was lower than that of NP (99.42 Ω). Meanwhile, NP had a greater effect on the bioenzyme activity than 4-tert-OP. GC-MS analysis showed that the degradation pathways of both pollutants mainly included oxidation and hydroxylation reactions. Furthermore, the microbial community analysis indicated that the main functional bacteria in NP degradation were Citrobacter, Desulfovibrio and Advenella, and those in 4-tert-OP degradation were Stenotrophomonas, Chryseobacterium, Dokdonella, and the key microbiomes underlying the cooperative relationship. The biotoxicity test indicated that the toxicity of residual substances was significantly reduced. Therefore, the MEC system is efficient and environmentally friendly and has broad application prospects in phenol refractory organics.

A Review of Biohydrogen Production from Saccharina japonica

Saccharina japonica (known as Laminaria japonica or Phaeophyta japonica), one of the largest macroalgae, has been recognized as food and medicine for a long time in some Asian countries, such as China, South Korea, Japan, etc. In recent years, S. japonica has also been considered the most promising third-generation biofuel feedstock to replace fossil fuels, contributing to solving the challenges people face regarding energy and the environment. In particular, S. japonica-derived biohydrogen (H2) is expected to be a major fuel source in the future because of its clean, high-yield, and sustainable properties. Therefore, this review focuses on recent advances in bio-H2 production from S. japonica. The cutting-edge biological technologies with suitable operating parameters to enhance S. japonica’s bio-H2 production efficiency are reviewed based on the Scopus database. In addition, guidelines for future developments in this field are discussed.

Improving the performance of bioelectrochemical sulfate removal by applying flow mode

Treatment of wastewater contaminated with high sulfate concentrations is an environmental imperative lacking a sustainable and environmental friendly technological solution. Microbial electrochemical technology (MET) represents a promising approach for sulfate reduction. In MET, a cathode is introduced as inexhaustible electron source for promoting sulfate reduction via direct or mediated electron transfer. So far, this is mainly studied in batch mode representing straightforward and easy-to-use systems, but their practical implementation seems unlikely, as treatment capacities are limited. Here, we investigated bioelectrochemical sulfate reduction in flow mode and achieved removal efficiencies (E_sulfate , 89.2 ± 0.4%) being comparable to batch experiments, while sulfate removal rates (R_sulfate , 3.1 ± 0.2 mmol L^-1 ) and Coulombic efficiencies (CE, 85.2 ± 17.7%) were significantly increased. Different temperatures and hydraulic retention times (HRT) were applied and the best performance was achieved at HRT 3.5 days and 30°C. Microbial community analysis based on amplicon sequencing demonstrated that sulfate reduction was mainly performed by prokaryotes belonging to the genera Desulfomicrobium, Desulfovibrio, and Desulfococcus, indicating that hydrogenotrophic and heterotrophic sulfate reduction occurred by utilizing cathodically produced H₂ or acetate produced by homoacetogens (Acetobacterium). The advantage of flow operation for bioelectrochemical sulfate reduction is likely based on higher absolute biomass, stable pH, and selection of sulfate reducers with a higher sulfide tolerance, and improved ratio between sulfate-reducing prokaryotes and homoacetogens.

Recent Implementations of Hydrogel-Based Microbial Electrochemical Technologies (METs) in Sensing Applications

Hydrogel materials have been used extensively in microbial electrochemical technology (MET) and sensor development due to their high biocompatibility and low toxicity. With an increasing demand for sensors across different sectors, it is crucial to understand the current state within the sectors of hydrogel METs and sensors. Surprisingly, a systematic review examining the application of hydrogel-based METs to sensor technologies has not yet been conducted. This review aimed to identify the current research progress surrounding the incorporation of hydrogels within METs and sensors development, with a specific focus on microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). The manufacturing process/cost, operational performance, analysis accuracy and stability of typical hydrogel materials in METs and sensors were summarised and analysed. The current challenges facing the technology as well as potential direction for future research were also discussed. This review will substantially promote the understanding of hydrogel materials used in METs and benefit the development of electrochemical biosensors using hydrogel-based METs.

Complexing agents-free bioelectrochemical trickling systems for highly-efficient mesothermal NO removal: The role of extracellular polymer substances

The biological treatments are promising for nitric oxide (NO) reduction, however, the biotechnology has long suffered from high demands of NO-complexing agents (i.e., Fe(II)EDTA), leading to extra operation costs. In this study, novel complexing agents-free bioelectrochemical systems have been developed for direct NO reduction. The electricity-driven bioelectrochemical trickling system (ED-BTS, a denitrifying biocathode driven by the external electricity and an acetate-consuming bioanode) achieved approximately 68% NO removal without any NO-complexing agents, superior to the bioanode-driven BTS and open-circuit BTS. The extracellular polymeric substances from the biofilms of ED-BTS contained more polysaccharides, humic substrates, and hydrophobic tryptophan that were beneficial for NO reduction. Additionally, the external electricity altered the microbial community toward more denitrifying bacteria and a higher abundance of NO reduction genes (nosZ and cnorB). This study provides a comprehensive understanding of microbial behaviors on the adsorption and reduction of NO and proposes a promising strategy for mesothermal NO biotreatment.

Effect of Different Acid and Base Potassium Ferrate Pretreatment on Organic Acid Recovery by Anaerobic Digestion of Sludge

Potassium ferrate has strong oxidation in both acid and alkali environments, which has attracted extensive attention. However, the impact of the pH environment on this coupling process with the goal of resource recovery has not received attention. Under the goal of the efficient recovery of organic acid, the changes of solid-liquid characteristics of sludge after acid and alkaline ferrate pretreatment and during anaerobic digestion were discussed. The results showed that compared with blank control groups, after alkaline ferrate pretreatment, the volatile suspended solids (VSSs) decreased the most, reaching 28.19%. After being pretreated with alkaline ferrate, the sludge showed the maximum VFA accumulation (408.21 COD/g VSS) on the third day of digestion, which was 1.34 times higher than that of the acid ferrate pretreatment. Especially in an alkaline environment, there is no need to add additional alkaline substances to adjust the pH value, and the effect of sludge reduction and acid production is the best.

Bioelectrochemical methanation by utilization of steel mill off-gas in a two-chamber microbial electrolysis cell

Carbon capture and utilization has been proposed as one strategy to combat global warming. Microbial electrolysis cells (MECs) combine the biological conversion of carbon dioxide (CO₂) with the formation of valuable products such as methane. This study was motivated by the surprising gap in current knowledge about the utilization of real exhaust gas as a CO₂ source for methane production in a fully biocatalyzed MEC. Therefore, two steel mill off-gases differing in composition were tested in a two-chamber MEC, consisting of an organic substrate-oxidizing bioanode and a methane-producing biocathode, by applying a constant anode potential. The methane production rate in the MEC decreased immediately when steel mill off-gas was tested, which likely inhibited anaerobic methanogens in the presence of oxygen. However, methanogenesis was still ongoing even though at lower methane production rates than with pure CO₂. Subsequently, pure CO₂ was studied for methanation, and the cathodic biofilm successfully recovered from inhibition reaching a methane production rate of 10.8 L m⁻²d⁻¹. Metagenomic analysis revealed Geobacter as the dominant genus forming the anodic organic substrate-oxidizing biofilms, whereas Methanobacterium was most abundant at the cathodic methane-producing biofilms.

Suppressing Methane Production to Boost High-Purity Hydrogen Production in Microbial Electrolysis Cells

Hydrogen gas (H₂) is an attractive fuel carrier due to its high specific enthalpy; moreover, it is a clean source of energy because in the combustion reaction with oxygen (O₂) it produces water as the only byproduct. The microbial electrolysis cell (MEC) is a promising technology for producing H₂ from simple or complex organics present in wastewater and solid wastes. Methanogens and non-archaeal methane (CH₄)-producing microorganisms (NAMPMs) often grow in the MECs and lead to rapid conversion of produced H₂ to CH₄. Moreover, non-archaeal methane production (NAMP) catalyzed by nitrogenase of photosynthetic bacteria was always overlooked. Thus, suppression of CH₄ production is required to enhance H₂ yield and production rate. This review comprehensively addresses the principles and current state-of-the-art technologies for suppressing methanogenesis and NAMP in MECs. Noteworthy, specific strategies aimed at the inhibition of methanogenic enzymes and nitrogenase could be a more direct approach than physical and chemical strategies for repressing the growth of methanogenic archaea. In-depth studies on the multiomics of CH₄ metabolism can possibly provide insights into sustainable and efficient approaches for suppressing metabolic pathways of methanogenesis and NAMP. The main objective of this review is to highlight key concepts, directions, and challenges related to boosting H₂ generation by suppressing CH₄ production in MECs. Finally, perspectives are briefly outlined to guide and advance the future direction of MECs for production of high-purity H₂ based on genetic and metabolic engineering and on the interspecific interactions.

Nanomaterial-enabled photothermal-based solar water disinfection processes: Fundamentals, recent advances, and mechanisms

The pathogenic microorganisms in water pose a great threat to human health. Photothermal and photothermocatalytic disinfection using nanomaterials (NPs) has offered a promising and effective strategy to address the challenges in solar water disinfection (SODIS), especially in the point-of-use operations. This review aims at providing comprehensive and state-of-the-art knowledge of photothermal-based disinfection by NPs. The fundamentals and principles of photothermal-based disinfection were first introduced. Then, recent advances in developing photothermal/photothermocatalytic catalysts were systematically summarized. The light-to-heat conversion and disinfection performance of a large variety of photothermal materials were presented. Given the complicated mechanisms of photothermal-based disinfection, the attacks from reactive oxygen species and heat, the destruction of bacterial cells, and the antibacterial effects of released metal ions were highlighted. Finally, future challenges and opportunities associated with the development of cost-effective photothermal/photothermocatalytic disinfection systems were outlined. This review will provide guidance in designing future NPs and inspire more research efforts from environmental nano-communities to move towards practical water disinfection operations.

A novel electrochemical sensor based on autotropic and heterotrophic nitrifying biofilm for trichloroacetic acid toxicity monitoring

Trichloroacetic acid (TCA), a toxic substance produced in the disinfection process of wastewater treatment plants, will accumulate in the receiving water. The detection of TCA in the water can achieve the purpose of early warning. However, currently there are few reports on microbial sensors used for TCA detection, and the characteristics of their microbial communities are still unclear. In this work, a toxicity monitoring microbial system (TMMS) with nitrifying biofilm as a sensing element and cathode oxygen reduction as a current signal was successfully constructed for TCA detection. The current and nitrification rate showed a linear relationship with low TCA concentration from 0 to 50 μg/L (R²_current = 0.9892, R²_{nitrification} = 0.9860), and high concentration range from 50 to 5000 μg/L (R²_current = 0.9883, R²_{nitrification} = 0.9721). High-throughput sequencing revealed that the TMMS was composed of autotrophic/heterotrophic nitrifying and denitrifying microorganisms. Further analysis via symbiotic relationship network demonstrated that Arenimonas and Hyphomicrobium were the core nodes for maintaining interaction between autotropic and heterotrophic nitrifying bacteria. Kyoto Encyclopedia of Genes and Genomes analysis showed that after adding TCA to TMMS, the carbon metabolism and the abundance of the tricarboxylic acid cycle pathway were reduced, and the activity of microorganisms was inhibited. TCA stress caused a low abundance of nitrifying and denitrifying functional enzymes, resulting in low oxygen consumption in the nitrification process, but more oxygen supply for cathode oxygen reduction. This work explored a novel sensor combined with electrochemistry and autotrophic/heterotrophic nitrification, which provided a new insight into the development of microbial monitoring of toxic substances.

Perspectives on Microbial Electron Transfer Networks for Environmental Biotechnology

The overlap of microbiology and electrochemistry provides plenty of opportunities for a deeper understanding of the redox biogeochemical cycle of natural-abundant elements (like iron, nitrogen, and sulfur) on Earth. The electroactive microorganisms (EAMs) mediate electron flows outward the cytomembrane via diverse pathways like multiheme cytochromes, bridging an electronic connection between abiotic and biotic reactions. On an environmental level, decades of research on EAMs and the derived subject termed “electromicrobiology” provide a rich collection of multidisciplinary knowledge and establish various bioelectrochemical designs for the development of environmental biotechnology. Recent advances suggest that EAMs actually make greater differences on a larger scale, and the metabolism of microbial community and ecological interactions between microbes play a great role in bioremediation processes. In this perspective, we propose the concept of microbial electron transfer network (METN) that demonstrates the “species-to-species” interactions further and discuss several key questions ranging from cellular modification to microbiome construction. Future research directions including metabolic flux regulation and microbes–materials interactions are also highlighted to advance understanding of METN for the development of next-generation environmental biotechnology.

Increase of antibiotic resistance genes via horizontal transfer in single- and two-chamber microbial electrolysis cells

Microbial electrolysis cells (MECs) have been applied for antibiotic degradation but simultaneously induced antibiotic resistance genes (ARGs), thus representing a risk to disseminate antibiotic resistance. However, few studies were on the potential and risk of ARGs transmission in the MECs. This work assessed conjugative transfer of ARGs under three tested conditions (voltages, cell concentration, and donor/recipient ratio) in both single- and two-chamber MECs. The results indicated that voltages (> 0.9 V) facilitated the horizontal frequency of ARGs in the single-chamber MECs and anode chamber of two-chamber MECs. The donor cell number (donor/recipient ratio was 2:1) increased the transfer frequency of ARGs. Furthermore, voltages ranged from 0.9 to 2.5 V increased reactive oxygen species (ROS) production and cell membrane permeability in MECs. These findings offer new insights into the roles of ARG transfer under different applied voltages in the MECs, which should not be ignored for horizontal transfer of antibiotic resistance.

Enhanced Cr(VI) reduction in biocathode microbial electrolysis cell using Fenton-derived ferric sludge

Hexavalent chromium (Cr(VI)) is one of the major concerns for water environment and human health due to its high toxicicity, while ferric sludge produced from Fenton processes is also a tough nut to crack. In this study, the synergetic impact of ferric sludge derived from the Fenton process on the bioreduction of Cr(VI) in biocathode microbial electrolysis cell was investigated for the first time. As a result, Cr(VI) reduction efficiency at biocathode increased by 1.1-2.6 times with 50 mg/L ferric sludge under different operation conditions. Besides, the Cr(VI) reduction enhancement decreased with the increase of pH and initial Cr(VI) concentration or increased with the increase of ferric sludge dosage. Correspondingly, relatively higher power density (1.027 W/m³ with 100 mg/L ferric sludge while 0.827 W/m³ for control) and lower activation energy and resistance were also observed. Besides, the presence of ferric sludge increased biomass protein (1.7 times higher with 100 mg/L ferric sludge) and cytochrome c (1.4 times higher with 100 mg/L ferric sludge). The evolution of microbial community structure for a higher abundance of Cr(VI) and Fe(III)-reducing microorganisms were exhibited, implying the enhancement of Cr(VI) reduction was due to the formation of Fe(II) from the reduction of ferric sludge. These findings provide insights and theoretical support for developing a viable biotechnology platform to realize waste treatment using waste.

Review on microbial fuel cells applications, developments and costs

The microbial fuel cell (MFC) technology has attracted significant attention in the last years due to its potential to recover energy in a wastewater treatment. The idea of using an MFC in industry is very attractive as the organic wastes can be converted into energy, reducing the waste disposal costs and the energy needs while increasing the company profit. However, taking aside these promising prospects, the attempts to apply MFCs in large-scale have not been succeeded so far since their lower performance and high costs remains challenging. This review intends to present the main applications of the MFC systems and its developments, particularly the advances on configuration and operating conditions. The diagnostic techniques used to evaluate the MFC performance as well as the different modeling approaches are described. Towards the introduction of the MFC in the market, a cost analysis is also included. The development of low-cost materials and more efficient systems, with high higher power outputs and durability, are crucial towards the application of MFCs in industrial/large scale. This work is a helpful tool for discovering new operation and design regimes.

Microbial electrolysis: a promising approach for treatment and resource recovery from industrial wastewater

Wastewater is one of the most common by-products of almost every industrial process. Treatment of wastewater alone, before disposal, necessitates an excess of energy. Environmental concerns over the use of fossil fuels as a source of energy have prompted a surge in demand for alternative energy sources and the development of sophisticated procedures to extract energy from unconventional sources. Treatment of municipal and industrial wastewater alone accounts for about 3% of global electricity use while the amount of energy embedded in the waste is at least 2–4 times greater than the energy required to treat the same effluent. The microbial electrolysis cell (MEC) is one of the most efficient technologies for waste-to-product conversion that uses electrochemically active bacteria to convert organic matter into hydrogen or a variety of by-products without polluting the environment. This paper highlights existing obstacles and future potential in the integration of Microbial Electrolysis Cell with other processes like anaerobic digestion coupled system, anaerobic membrane bioreactor and thermoelectric micro converter.

Enhancement of methane production from waste activated sludge using hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) process - A review

Hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) was proved to increase methane productivity and methane yield of waste activated sludge (WAS) by establishing direct interspecies electron transfer method and enriching functional microorganisms. This review first summarized the pretreatment methods of WAS for MEC-AD and then reviewed the reactor configurations, operation parameters, and the economic benefit of MEC-AD. Furthermore, the enhancement mechanisms of MEC-AD were reviewed based on the analysis of thermodynamics and microbial community. It was found that the decrease of hydrogen partial pressure due to the hydrogenotrophic methanogens enriched in cathodic biofilm and direct interspecies electron transfer between exoelectrogens and anode were the core mechanisms for improving acidogenesis, acetogenesis, and methanogenesis. Finally, the potentially technological issues that need to be addressed to increase energy efficiency in large-scale MEC-AD processes were discussed.

Three-dimensional biofilm electrode reactors (3D-BERs) for wastewater treatment

Three-dimensional biofilm electrode reactors (3D-BERs) are highly efficient in refractory wastewater treatment. In comparison to conventional bio-electrochemical systems, the filled particle electrodes act as both electrodes and microbial carriers in 3D-BERs. This article reviews the conception and basic mechanisms of 3D-BERs, as well as their current development. The advantages of 3D-BERs are illustrated with an emphasis on the synergy of electricity and microorganisms. Electrode materials utilized in 3D-BERs are systematically summarized, especially the critical particle electrodes. The configurations of 3D-BERs and their integration with wastewater treatment reactors are introduced. Operational parameters and the adaptation of 3D-BERs to varieties of wastewater are discussed. The prospects and challenges of 3D-BERs for wastewater treatment are then presented, and the future research directions are proposed. We believe that this timely review will help to attract more attentions on 3D-BERs investigation, thus promoting the potential application of 3D-BERs in wastewater treatment.

Construction of an Acetate Metabolic Pathway to Enhance Electron Generation of Engineered Shewanella oneidensis

Background: Microbial fuel cells (MFCs) are a novel bioelectrochemical devices that can use exoelectrogens as biocatalyst to convert various organic wastes into electricity. Among them, acetate, a major component of industrial biological wastewater and by-product of lignocellulose degradation, could release eight electrons per mole when completely degraded into CO₂ and H₂O, which has been identified as a promising carbon source and electron donor. However, Shewanella oneidensis MR-1, a famous facultative anaerobic exoelectrogens, only preferentially uses lactate as carbon source and electron donor and could hardly metabolize acetate in MFCs, which greatly limited Coulombic efficiency of MFCs and the capacity of bio-catalysis. Results: Here, to enable acetate as the sole carbon source and electron donor for electricity production in S. oneidensis, we successfully constructed three engineered S. oneidensis (named AceU1, AceU2, and AceU3) by assembling the succinyl-CoA:acetate CoA-transferase (SCACT) metabolism pathways, including acetate coenzyme A transferase encoded by ato1 and ato2 gene from G. sulfurreducens and citrate synthase encoded by the gltA gene from S. oneidensis, which could successfully utilize acetate as carbon source under anaerobic and aerobic conditions. Then, biochemical characterizations showed the engineered strain AceU3 generated a maximum power density of 8.3 ± 1.2 mW/m² with acetate as the sole electron donor in MFCs. In addition, when further using lactate as the electron donor, the maximum power density obtained by AceU3 was 51.1 ± 3.1 mW/m², which approximately 2.4-fold higher than that of wild type (WT). Besides, the Coulombic efficiency of AceU3 strain could reach 12.4% increased by 2.0-fold compared that of WT, which demonstrated that the engineered strain AceU3 can further utilize acetate as an electron donor to continuously generate electricity. Conclusion: In the present study, we first rationally designed S. oneidensis for enhancing the electron generation by using acetate as sole carbon source and electron donor. Based on synthetic biology strategies, modular assembly of acetate metabolic pathways could be further extended to other exoelectrogens to improve the Coulombic efficiency and broaden the spectrum of available carbon sources in MFCs for bioelectricity production.

Microbial Electrochemical Fluidized Bed Reactor: A Promising Solution for Removing Pollutants From Pharmaceutical Industrial Wastewater

The capacity of electroactive bacteria to exchange electrons with electroconductive materials has been explored during the last two decades as part of a new field called electromicrobiology. Such microbial metabolism has been validated to enhance the bioremediation of wastewater pollutants. In contrast with standard materials like rods, plates, or felts made of graphite, we have explored the use of an alternative strategy using a fluid-like electrode as part of a microbial electrochemical fluidized bed reactor (ME-FBR). After verifying the low adsorption capacity of the pharmaceutical pollutants on the fluid-bed electrode [7.92 ± 0.05% carbamazepine (CBZ) and 9.42 ± 0.09% sulfamethoxazole (SMX)], our system showed a remarkable capacity to outperform classical solutions for removing pollutants (more than 80%) from the pharmaceutical industry like CBZ and SMX. Moreover, the ME-FBR performance revealed the impact of selecting an anode potential by efficiently removing both pollutants at + 200 mV. The high TOC removal efficiency also demonstrated that electrostimulation of electroactive bacteria in ME-FBR could overcome the expected microbial inhibition due to the presence of CBZ and SMX. Cyclic voltammograms revealed the successful electron transfer between microbial biofilm and the fluid-like electrode bed throughout the polarization tests. Finally, Vibrio fischeri-based ecotoxicity showed a 70% reduction after treating wastewater with a fluid-like anode (+ 400 mV), revealing the promising performance of this bioelectrochemical approach.

Aerobic biocathodes with potential regulation for ammonia oxidation with concomitant cathodic oxygen reduction and their microbial communities

Aerobic biocathodes are effective construct for the simultaneous nitrification and denitrification, but the disturbance of cathodic oxygen reduction on ammonia oxidation and denitrification remains unclear. In this study, we revealed the oxygen reduction peak at -0.4 V (versus silver/silver chloride) by cyclic voltammetry analysis at a cathode without a biofilm. The reduction peak, however, showed a right shift from -0.4 to -0.3 V for the biocathode, indicating that the aerobic biocathode could simultaneously perform traditional nitrification and cathode oxygen reduction. Therefore, different electrode potentials ranging from -0.5 to -0.1 V were designed for regulating the ammonia oxidation rate, and the results showed that the highest oxidation rate reached 3.08 mg/h/L at a potential of -0.2 V under a low-aeration rate of 5 mL/min. High-throughput sequencing showed that Nitrosomonas and Rhodococcus were the dominant nitrogen removal genera in the biocathode, and the abundance of Devosia was related to the interactions between the aeration rate and the electrode potential. Furthermore, the amoC and hao genes responded to aeration and electrode potential regulation, and -0.2 V was more suitable for promoting the denitrification process under low-aeration conditions. Therefore, these findings provided new insights on cathodic potential control for ammonia oxidation and nitrogen removal as well as for the regulation of microbial communities.

A review of recent advances in electrode materials for emerging bioelectrochemical systems: From biofilm-bearing anodes to specialized cathodes

Bioelectrochemical systems (BES), mainly microbial fuel cells (MEC) and microbial electrolysis cells (MFC), are unique biosystems that use electroactive bacteria (EAB) to produce electrons in the form of electric energy for different applications. BES have attracted increasing attention as a sustainable, low-cost, and neutral-carbon option for energy production, wastewater treatment, and biosynthesis. Complex interactions between EAB and the electrode materials play a crucial role in system performance and scalability. The electron transfer processes from the EAB to the anode surface or from the cathode surface to the EAB have been the object of numerous investigations in BES, and the development of new materials to maximize energy production and overall performance has been a hot topic in the last years. The present review paper discusses the advances on innovative electrode materials for emerging BES, which include MEC coupled to anaerobic digestion (MEC-AD), Microbial Desalination Cells (MDC), plant-MFC (P-MFC), constructed wetlands-MFC (CW-MFC), and microbial electro-Fenton (BEF). Detailed insights on innovative electrode modification strategies to improve the electrode transfer kinetics on each emerging BES are provided. The effect of materials on microbial population is also discussed in this review. Furthermore, the challenges and opportunities for materials scientists and engineers working in BES are presented at the end of this work aiming at scaling up and industrialization of such versatile systems.

Macrophytes as wastewater treatment agents: Nutrient uptake and potential of produced biomass utilization toward circular economy initiatives

Macrophytes have been widely used as agents in wastewater treatment. The involvement of plants in wastewater treatment cannot be separated from wetland utilization. As one of the green technologies in wastewater treatment plants, wetland exhibits a great performance, especially in removing nutrients from wastewater before the final discharge. It involves the use of plants and consequently produces plant biomasses as treatment byproducts. The produced plant biomasses can be utilized or converted into several valuable compounds, but related information is still limited and scattered. This review summarizes wastewater's nutrient content (macro and micronutrient) that can support plant growth and the performance of constructed wetland (CW) in performing nutrient uptake by using macrophytes as treatment agents. This paper further discusses the potential of the utilization of the produced plant biomasses as bioenergy production materials, including bioethanol, biohydrogen, biogas, and biodiesel. This paper also highlights the conversion of plant biomasses into animal feed, biochar, adsorbent, and fertilizer, which may support clean production and circular economy efforts. The presented review aims to emphasize and explore the utilization of plant biomasses and their conversion into valuable products, which may solve problems related to plant biomass handling during the adoption of CW in wastewater treatment plants.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

Recent Developments in Microbial Electrolysis Cell-Based Biohydrogen Production Utilizing Wastewater as a Feedstock

Carbon constraints, as well as the growing hazard of greenhouse gas emissions, have accelerated research into all possible renewable energy and fuel sources. Microbial electrolysis cells (MECs), a novel technology able to convert soluble organic matter into energy such as hydrogen gas, represent the most recent breakthrough. While research into energy recovery from wastewater using microbial electrolysis cells is fascinating and a carbon-neutral technology that is still mostly limited to lab-scale applications, much more work on improving the function of microbial electrolysis cells would be required to expand their use in many of these applications. The present limiting issues for effective scaling up of the manufacturing process include the high manufacturing costs of microbial electrolysis cells, their high internal resistance and methanogenesis, and membrane/cathode biofouling. This paper examines the evolution of microbial electrolysis cell technology in terms of hydrogen yield, operational aspects that impact total hydrogen output in optimization studies, and important information on the efficiency of the processes. Moreover, life-cycle assessment of MEC technology in comparison to other technologies has been discussed. According to the results, MEC is at technology readiness level (TRL) 5, which means that it is ready for industrial development, and, according to the techno-economics, it may be commercialized soon due to its carbon-neutral qualities.

Recent advances in the improvement of bi-directional electron transfer between abiotic/biotic interfaces in electron-assisted biosynthesis system

As an important biosynthesis technology, electron-assisted biosynthesis (EABS) system can utilize exogenous electrons to regulate the metabolic network of microorganisms, realizing the biosynthesis of high value-added chemicals and CO₂ fixation. Electrons play crucial roles as the energy carriers in the EABS process. In fact, efficient interfacial electron transfer (ET) is the decisive factor to realize the rapid energy exchange, thus stimulating the biosynthesis of target metabolic products. However, due to the interfacial resistance of ET between the abiotic solid electrode and biotic microbial cells, the low efficiency of interfacial ET has become a major bottleneck, further limiting the practical application of EABS system. As the cell membrane is insulated, even the cell membrane embedded electron conduit (no matter cytochromes or channel protein for shuttle transferring) to increase the cell membrane conductivity, the ET between membrane electron conduit and electrode surface is kinetically restricted. In this review, the pathway of bi-directional interfacial ET in EABS system was summarized. Furthermore, we reviewed representative milestones and advances in both the anode outward interfacial ET (from organism to electrode) and cathode inward interfacial ET (from electrode to organism). Here, new insights from the perspectives of material science and synthetic biology were also proposed, which were expected to provide some innovative opinions and ideas for the following in-depth studies.

Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Nitrogen removal from wastewater is an indispensable but highly energy-demanding process, and thus more energy-saving treatment processes are required. Here, we investigated the performance of bioelectrochemical ammonium nitrogen (NH₄⁺-N) removal from real domestic wastewater without energy-intensive aeration by a single chamber microbial electrolysis cell (MEC) that was electrically powered by a double chamber microbial fuel cell (MFC). Anoxic NH₄⁺-N oxidation and total nitrogen (TN) removal rates were determined at various applied voltages (0-1.2 V), provided by the MFC. The MEC achieved a NH₄⁺-N oxidation rate of 151 ± 42 g NH₄⁺-N m^-3 d^-1 and TN removal rate of 95 ± 42 g-TN m^-3 d^-1 without aeration at the applied voltage of 0.8 V (the anode potential E_anode = +0.633 ± 0.218 V vs. SHE). These removal rates were much higher than the previously reported values and conventional biological nitrogen removal processes. Open and closed-circuit MEC batch experiments confirmed that anoxic NH₄⁺-N oxidation was an electrochemically mediated biological process (that is, an anode acted as an electron acceptor) and denitrification occurred simultaneously without NO₂^- and NO₃^- accumulation. Moreover, ex-situ¹⁵N tracer experiment and microbial community analysis revealed that anammox and heterotrophic denitrification mainly contributed to the TN removal. Thus, the bioelectrochemical anodic NH₄⁺-N oxidation was coupled with anammox and denitrification in this MFC-assisted MEC system. Taken together, our MFC-driven single chamber MEC could be a high rate energy-saving nitrogen removal process without external carbon and energy input and high energy-demanding aeration.

Operationalization of a microbial electrolysis cell: The interaction of the primary factors for energy storage efficiency

Microbial electrolysis cells have attracted attention as a method to enhance anaerobic digesters' performance. However, optimization of individual factors is not directly transferrable among systems as many are intimately linked and influenced by the system design, influent, and inoculum. To avoid this, here the effects and interactions between the relative electrode size, hydraulic retention time (HRT), and voltage imposed have been explored within a pair of otherwise identical reactors. Methane production has a positive correlation with the applied voltage, reaching 12.9 mL_CH4 L^-1h^-1 with 10 days HRT and 1000 mV, also achieving 35% energy storage efficiency, despite the higher electrical input. Shorter HRTs led to bacterial washouts, reducing the methane production below 10 mL_CH4 L^-1h^-1. Contour plots were constructed to relate the energy storage efficiency with operational conditions changes. These highlighted the benefits of using a relatively larger cathode than anode for improving energy storage efficiency.

Pushing microbial desalination cells towards field application: Prevailing challenges, potential mitigation strategies, and future prospects

Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.

Transport of organic solutes in ion-exchange membranes: Mechanisms and influence of solvent ionic composition

Ion-exchange membrane (IEM)-based processes are used in the industry or in the drinking water production to achieve selective separation. The transport mechanisms of organic solutes/micropollutants (i.e., paracetamol, clofibric acid, and atenolol) at a single-membrane level in diffusion cells were similar to that of salts (i.e., diffusion, convection, and electromigration). The presence of an equal concentration of salts at both sides of the membrane slightly decreased the transport of organics due to lower diffusion coefficients of organics in salts and the increase of hindrance and/or decrease of partitioning in the membrane phase. In the presence of a salt gradient, diffusion was the main transport mechanism for non-charged organics, while the counter-transport of salts promoted the transport of charged organics through electromigration (electroneutrality). Conversely, the co-transport of salts hindered the transport of charged organics, where diffusion was the main transport mechanism of the latter. Although convection played a role in the transport of non-charged organics, its influence on the charged solutes was minimal due to the dominant electromigration. Positron annihilation lifetime spectroscopy showed a bimodal size distribution of free-volume elements of IEMs, with both classes of free-volume elements contributing to salt transport, while larger organics can only transport through the larger class.

A review on self-sustainable microbial electrolysis cells for electro-biohydrogen production via coupling with carbon-neutral renewable energy technologies

Microbial electrolysis cell (MEC) technology is a promising bioelectrochemical hydrogen production technology that utilizes anodic bio-catalytic oxidation and cathodic reduction processes. MECs require a lower external energy input than water electrolysis; however, as they also require the application of external power sources, this inevitably renders MEC systems a less sustainable option. This issue is the main obstacle hindering the practical application of MECs. Therefore, this review aims to introduce a self-sustainable MEC technology by combining conventional MECs with advanced carbon-neutral technologies, such as solar-, microbial-, osmotic-, and thermoelectric-powers (and their combinations). Moreover, new approaches to overcome the thermodynamic barriers and attain self-sustaining MECs are discussed in detail, thereby providing a working principle, current challenges, and future perspective in the field. This review provides comprehensive insights into reliable hydrogen production as well as the latest trends towards self-sustainable MECs for practical application.

Treatment of Low C/N Ratio Wastewater by a Carbon Cloth Bipolar Plate Multicompartment Electroenhanced Bioreactor (CBM-EEB)

The traditional biological denitrification process has the problems of low removal rates and lack of a carbon source when treating wastewater with high ammonia nitrogen concentration and a low carbon-nitrogen ratio. Based on a bio-electrochemical system (BES), a novel carbon cloth bipolar plate multicompartment electroenhanced bioreactor (CBM-EEB) system was constructed. In this study, nitrogen removal efficiency and enrichment of functional bacteria using CBM-EEB under different voltage conditions were investigated. The results from next-generation sequencing indicated that the CBM-EEB included heterotrophic nitrification and aerobic denitrification (HNAD) and was dominated by heterotrophic nitrification aerobic denitrifying bacteria (HNADB). The applied voltage was confirmed as having the ability to regulate the microbial community structure and abundance of functional genes, thereby further enhancing the nitrogen removal efficiency of the system. The total nitrogen removal efficiency was 77.70 ± 1.14, 87.10 ± 0.56, 86.40 ± 0.59, and 89.30 ± 0.53% under applied voltages of 0.4, 0.7, 1.0, and 1.3 V, respectively. All values were significantly higher than the control group (62.86 ± 2.06%). HNADB had the highest abundance among the 17 detected genera related to nitrogen metabolism. Facultative denitrifying bacteria, Pseudoxanthomonas, along with key bacteria of HNADB, such as Flavobacterium, constructed a shortcut simultaneous nitrification-denitrification (SND) process. Poisson analysis and redundancy analysis (RDA) showed that the applied voltage improved the denitrification efficiency by changing the microbial community structure, reducing the abundance of heterotrophic bacteria, and increasing the unit abundance of key functional genes so that less organics were required for the denitrification process. The increased nitrogen removal efficiency in the experimental group was mainly related to simultaneous nitrification-denitrification process and cooperation of microbial communities in the anode and the cathode. This study highlighted the feasibility of CBM-EEB to enhance the HNAD reaction and the response of wastewater with a low C/N ratio to enhance the abundance of microbial bacteria and their functional gene abundance.

Bioelectrochemical chlorate reduction by Dechloromonas agitata CKB

Chlorate has been described as an emerging pollutant that compromises water sources. In this study, bioelectrochemical reactors (BERs) using Dechloromonas agitata CKB, were evaluated as a sustainable alternative for chlorate removal. BERs were operated under flow-recirculation and batch modes with an applied cell-voltage of 0.44 V over a resistance of 1 kΩ. Results show chlorate removal up to 607.288 mg/L. After 115 days, scanning electron microscopy showed biofilm development over the electrodes, and electrochemical impedance spectroscopy confirmed the biocatalytic effect of CKB. The theoretical chlorate bioreduction potential (ε° = 0.792 V) was proven, and a kinetic study indicated that 6 electrons were involved in the reduction mechanism. Finally, a hypothetical bioelectrochemical mechanism for chlorate reduction in a BER was proposed. This research expands upon current knowledge of novel electrochemically active microorganisms and widens the scope of BER applications for chlorate removal.

Turning date palm waste into carbon nanodots and nano zerovalent iron composites for excellent removal of methylthioninium chloride from water

Novel carbon nanodots (nCD-DBC) and nano zero-valent iron composites (nZVI-DBC) were synthesized using date palm waste-derived biochar (DBC). The synthesized materials were analyzed for chemical and structural composition by using FTIR, SEM, XRD, and TGA, and evaluated for their methylthioninium chloride dye (MB) removal efficiency from contaminated aqueous solutions. pH 7.0 was found optimum for the highest MB removal in sorption batch studies. Kinetics sorption of MB onto the sorbents was best described by pseudo-second-order (R2 = 0.93-0.99) and Elovich models (R2 = 0.86-0.97) implying that sorption was being controlled by chemisorption. Langmuir model predicted maximum sorption capacities for nCD-DBC, nZVI-DBC, and DBC were 1558.66, 1182.90, and 851.67 mg g-1, respectively, which correlated with the results of kinetics sorption. Likewise, nCD-DBC yielded the highest partition coefficient (7067 mL g-1), followed by nZVI-DBC (1460 mL g-1), and DBC (930 mL g-1). Post-sorption XRD, FTIR, and SEM analyses depicted the binding of MB onto the sorbents. It was suggested that electrostatic interactions, π-π electron donor-accepter interactions, degradation, and diffusion were responsible for MB removal by the synthesized materials. Therefore, the nCD-DBC, nZVI-DBC, and DBC can potentially be used for scavenging MB dye from contaminated aqueous solutions.

Microbial electro-fermentation for synthesis of chemicals and biofuels driven by bi-directional extracellular electron transfer

Electroactive bacteria could perform bi-directional extracellular electron transfer (EET) to exchange electrons and energy with extracellular environments, thus playing a central role in microbial electro-fermentation (EF) process. Unbalanced fermentation and microbial electrosynthesis are the main pathways to produce value-added chemicals and biofuels. However, the low efficiency of the bi-directional EET is a dominating bottleneck in these processes. In this review, we firstly demonstrate the main bi-directional EET mechanisms during EF, including the direct EET and the shuttle-mediated EET. Then, we review representative milestones and progresses in unbalanced fermentation via anode outward EET and microbial electrosynthesis via inward EET based on these two EET mechanisms in detail. Furthermore, we summarize the main synthetic biology strategies in improving the bi-directional EET and target products synthesis, thus to enhance the efficiencies in unbalanced fermentation and microbial electrosynthesis. Lastly, a perspective on the applications of microbial electro-fermentation is provided.

In situ Biogas Upgrading by CO2-to-CH4 Bioconversion

Biogas produced by anaerobic digestion is an important renewable energy carrier. Nevertheless, the high CO₂ content in biogas limits its utilization to mainly heat and electricity generation. Upgrading biogas into biomethane broadens its potential as a vehicle fuel or substitute for natural gas. CO₂-to-CH₄ bioconversion represents one cutting-edge solution for biogas upgrading. In situ bioconversion can capture endogenous CO₂ directly from the biogas reactor, is easy to operate, and provides an infrastructure for renewable electricity storage. Despite these advantages, several challenges need to be addressed to move in situ upgrading technologies closer to applications at scale. This opinion article reviews the state of the art of this technology and identifies some obstacles and opportunities of biological in-situ upgrading technologies for future development.