New microbial electrosynthesis system for methane production from carbon dioxide coupled with oxidation of sulfide to sulfate.

Microbial electrosynthesis system (MES) is a promising method that can use carbon dioxide, which is a greenhouse gas, to produce methane which acts as an energy source, without using organic substances. However, this bioelectrical reduction reaction can proceed at a certain high applied voltage when coupled with water oxidation in the anode coated with metallic catalyst. When coupled with the oxidation of HS- to SO42-, methane production is thermodynamically more feasible, thus implying its production at a considerably lower applied voltage. In this study, we demonstrated the possibility of electrotrophic methane production coupled with HS- oxidation in a cost-effective bioanode chamber in the MES without organic substrates at a low applied voltage of 0.2 V. In addition, microbial community analyses of biomass enriched in the bioanode and biocathode were used to reveal the most probable pathway for methane production from HS- oxidation. In the bioanode, electroautotrophic SO42- production accompanied with electron donation to the electrode is performed mainly by the following two steps: first, incomplete sulfide oxidation to sulfur cycle intermediates (SCI) is performed; then the produced SCI are disproportionated to HS- and SO42-. In the biocathode, methane is produced mainly via H2 and acetate by electron-accepting syntrophic bacteria, homoacetogens, and acetoclastic archaea. Here, a new eco-friendly MES with biological H2S removal is established.

Biological methane production coupled with sulfur oxidation in a microbial electrosynthesis system without organic substrates.

Methane is produced in a microbial electrosynthesis system (MES) without organic substrates. However, a relatively high applied voltage is required for the bioelectrical reactions. In this study, we demonstrated that electrotrophic methane production at the biocathode was achieved even at a very low voltage of 0.1 V in an MES, in which abiotic HS- oxidized to SO42- at the anodic carbon-cloth surface coated with platinum powder. In addition, microbial community analysis revealed the most probable pathway for methane production from electrons. First, electrotrophic H2 was produced by syntrophic bacteria, such as Syntrophorhabdus, Syntrophobacter, Syntrophus, Leptolinea, and Aminicenantales, with the direct acceptance of electrons at the biocathode. Subsequently, most of the produced H2 was converted to acetate by homoacetogens, such as Clostridium and Spirochaeta 2. In conclusion, the majority of the methane was indirectly produced by a large population of acetoclastic methanogens, namely Methanosaeta, via acetate. Further, hydrogenotrophic methanogens, including Methanobacterium and Methanolinea, produced methane via H2.

An external magnetic field moderating Cr(VI) stress for simultaneous enhanced acetate production and Cr(VI) removal in microbial electrosynthesis system.

A stressful heavy metal circumstance disfavors production of acetate from bicarbonate reduction in the biocathode of microbial electrosynthesis system (MES) with simultaneous function of heavy metal removal/recovery. It is of great interest to explore effective approaches to moderate the heavy metal stress with achievement of simultaneous enhanced acetate production and heavy metal removal in MES. Herein, a magnetic field strength of 100 mT was successfully employed to moderate Cr(VI) stress, achieving simultaneous production of acetate at a rate of 1.48 ± 0.01 mg/L/h and Cr(VI) removal at a rate of 1.67-2.42 mg/L/h in the Serratia marcescens Q1 catalyzed cathode of MES under periodical addition of bicarbonate and Cr(VI), 1.35-fold (acetate production) and 1.34-1.46 times (Cr(VI) removal) of those in the controls in the absence of magnetic field. This simultaneous efficient acetate production and Cr(VI) removal was regulated by the magnetic field and the stressful Cr(VI), which induced the S. marcescens to physiologically release additive amounts of extracellular polymeric substances with a compositional diversity and containing the electrochemically active c-type cytochromes to facilitate extracellular electron transfer. This study confirmed the importance of magnetic field in developing the S. marcescens catalytic activity for moderating Cr(VI) stress, and thus provided a feasible approach for simultaneous efficient acetate production and Cr(VI) removal/recovery in MES, from waters contaminated with Cr(VI).

Semiquantitative Detection of Hydrogen-Associated or Hydrogen-Free Electron Transfer within Methanogenic Biofilm of Microbial Electrosynthesis.

Hydrogen-entangled electron transfer has been verified as an important extracellular pathway of sharing reducing equivalents to regulate biofilm activities within a diversely anaerobic environment, especially in microbial electrosynthesis systems. However, with a lack of useful methods for in situ hydrogen detection in cathodic biofilms, the role of hydrogen involvement in electron transfer is still debatable. Here, a cathodic biofilm was constructed in CH4-produced microbial electrosynthesis reactors, in which the hydrogen evolution dynamic was analyzed to confirm the presence of hydrogen-associated electron transfer near the cathode within a micrometer scale. Fluorescent in situ hybridization images indicated that a colocalized community of archaea and bacteria developed within a 58.10-µm-thick biofilm at the cathode, suggesting that the hydrogen gradient detected by the microsensor was consumed by the collaboration of bacteria and archaea. Coupling of a microsensor and cyclic voltammetry test further provided semiquantitative results of the hydrogen-associated contribution to methane generation (around 21.20% ± 1.57% at a potential of -0.5 V to -0.69 V). This finding provides deep insight into the mechanism of electron transfer in biofilm on conductive materials.IMPORTANCE Electron transfer from an electrode to biofilm is of great interest to the fields of microbial electrochemical technology, bioremediation, and methanogenesis. It has a promising potential application to boost more value-added products or pollutant degradation. Importantly, the ability of microbes to obtain electrons from electrodes and utilize them brings new insight into direct interspecies electron transfer during methanogenesis. Previous studies verified the direct pathway of electron transfer from the electrode to a pure-culture bacterium, but it was rarely reported how the methanogenic biofilm of mixed cultures shares electrons by a hydrogen-associated or hydrogen-free pathway. In the current study, a combination method of microsensor and cyclic voltammetry successfully semiquantified the role of hydrogen in electron transfer from an electrode to methanogenic biofilm.

Enhanced biodegradation of perfluorooctanoic acid in a dual biocatalyzed microbial electrosynthesis system.

The toxic perfluorooctanoic acid (PFOA) is widely spread in terrestrial and aquatic habitats owing to its resistance to conventional degradation processes. Advanced techniques to degrade PFOA requires drastic conditions with high energy cost. In this study, we investigated PFOA biodegradation in a simple dual biocatalyzed microbial electrosynthesis system (MES). Different PFOA loadings (1, 5, and 10 ppm) were tested and a biodegradation of 91% was observed within 120 h. Propionate production improved and short-carbon-chain PFOA intermediates were detected, which confirmed PFOA biodegradation. However, the current density decreased, indicating an inhibitory effect of PFOA. High-throughput biofilm analysis revealed that PFOA regulated the microbial flora. Microbial community analysis showed enrichment of the more resilient and PFOA adaptive microbes, including Methanosarcina and Petrimonas. Our study promotes the potential use of dual biocatalyzed MES system as an environment-friendly and inexpensive method to remediate PFOA and provides a new direction for bioremediation research.

Enhanced bio-electrochemical performance of microbially catalysed anode and cathode in a microbial electrosynthesis system.

Most bio-electrochemical systems (BESs) use biotic/abiotic electrode combinations, with platinum-based abiotic electrodes being the most common. However, the non-renewability, cost, and poisonous nature of such electrode systems based on noble metals are major bottlenecks in BES commercialisation. Microbial electrosynthesis (MES), which is a sustainable energy platform that simultaneously treats wastewater and produces chemical commodities, also faces the same problem. In this study, a dual bio-catalysed MES system with a biotic anode and cathode (MES-D) was tested and compared with a biotic cathode/abiotic anode system (MES-S). Different bio-electrochemical tests revealed improved BES performance in MES-D, with a 3.9-fold improvement in current density compared to that of MES-S. Volatile fatty acid (VFA) generation also increased 3.2-, 4.1-, and 1.8-fold in MES-D compared with that in MES-S for acetate, propionate, and butyrate, respectively. The improved performance of MES-D could be attributed to the microbial metabolism at the bioanode, which generated additional electrons, as well as accumulative VFA production by both the bioanode and biocathode chambers. Microbial community analysis revealed the enrichment of electroactive bacteria such as Proteobacteria (60%), Bacteroidetes (67%), and Firmicutes + Proteobacteria + Bacteroidetes (75%) on the MES-S cathode and MES-D cathode and anode, respectively. These results signify the potential of combined bioanode/biocathode BESs such as MES for application in improving energy and chemical commodity production.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies.

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Enhancing the selective synthesis of butyrate in microbial electrosynthesis system by gas diffusion membrane composite biocathode.

The reduction of carbon dioxide (CO2) to high value-added multi-carbon compounds at the cathode is an emerging application of microbial electrosynthesis system (MES). In this study, a composite cathode consisting of hollow fiber membrane (HFM) and the carbon felt is designed to enhance the CO2 mass transfer of the cathode. The result shows that the main products are acetate and butyrate without other substances. The electrochemical performance of the electrode is significantly improved after biofilm becomes matures. The composite cathode significantly reduces the "threshold" for the synthesis of butyrate. Moreover, CO2 is dissolved and protons are consumed by synthesizing volatile fatty acids (VFAs) to maintain a stable pH inside the composite electrode. The synthesis mechanism of butyrate is that CO2 is converted sequentially into acetate and butyrate. The microenvironment of the composite electrode enriches Firmicute. This composite electrode provides a novel strategy for regulating the microenvironment.

Enhanced electron transfer on microbial electrosynthesis biocathode by polypyrrole-coated acetogens.

Extracellular electron transfer (EET) is a significant pathway to transport electrons between bacteria and electrode in microbial electrosynthesis systems (MESs). To enhance EET in the MES, a high-conductivity polymer, polypyrrole (PPy), was coated on the surface of mixed culture acetogens in situ and the PPy-coated bacteria were inoculated on the cathode of MES. The charge transfer resistance of PPy-coated biocathode was 33%-70% of that with PPy-uncoated. Acetate production rate and Faradic efficiency in PPy-coated biocathodes increased by 3 to 6 times. After 960 h operation, Acetobacterium, Desulfovibrio, and Acinetobacter dominate the community on the coated and uncoated biocathode. Quinone loop and NADH dehydrogenase to ubiquinone were involved in electron transfer pathway of biocathode and stimulated by PPy coating. Low-level expression of C-type cytochromes on biocathode indicated its less important role in inward EET. The study provided useful information for applications of high-conductivity chemicals in microbial electrosynthesis.

Understanding the interdependence of strain of electrotroph, cathode potential and initial Cu(II) concentration for simultaneous Cu(II) removal and acetate production in microbial electrosynthesis systems.

Metallurgical microbial electrosynthesis systems (MES) are holding great promise for simultaneous heavy metal removal and acetate production from heavy metal-contaminated and organics-barren waters. How critical parameters of strain of electrotroph, cathode potential and initial heavy metal concentration affect MES performance, however, is not yet fully understood. Heavy metal of Cu(II) and four Cu(II)-tolerant electrotrophs (Stenotrophomonas maltophilia JY1, Citrobacter sp. JY3, Pseudomonas aeruginosa JY5 and Stenotrophomonas sp. JY6) were employed to evaluate MES performance at various cathode potentials (-900 or -600 mV vs. standard hydrogen electrode) and initial Cu(II) concentrations (60-120 mg L-1). Each electrotrophs exhibited incremental Cu(II) removals with increased Cu(II) at -900 mV, higher than at -600 mV or in the abiotic controls. Acetate production by JY1 and JY6 decreased with the increase in initial Cu(II), compared to an initial increase and a decrease thereafter for JY3 and JY5. For each electrotrophs, the biofilms than the planktonic cells released more amounts of extracellular polymeric substances (EPS) with a compositional diversity and stronger Cu(II) complexation at -900 mV. These were higher than at -600 mV, or in the controls either under open circuit conditions or in the absence of Cu(II). This work demonstrates the interdependence of strain of electrotroph, cathode potential and initial Cu(II) on simultaneous Cu(II) removal and acetate production through the release of different amounts of EPS with diverse composites, contributing to enhancing the controlled MES for efficient recovery of value-added products from Cu(II)-contaminated and organics-barren waters.

Electrosynthesis of acetate from inorganic carbon (HCO3-) with simultaneous hydrogen production and Cd(II) removal in multifunctional microbial electrosynthesis systems (MES).

The simultaneous production of acetate from bicarbonate (from CO2 sequestration) and hydrogen gas, with concomitant removal of Cd(II) heavy metal in water is demonstrated in multifunctional metallurgical microbial electrosynthesis systems (MES) incorporating Cd(II) tolerant electrochemically active bacteria (EAB) (Ochrobactrum sp. X1, Pseudomonas sp. X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7). Strain X5 favored the production of acetate, while X7 preferred the production of hydrogen. The rate of Cd(II) removal by all EAB (1.20-1.32 mg/L/h), and the rates of acetate production by X5 (29.4 mg/L/d) and hydrogen evolution by X7 (0.0187 m3/m3/d) increased in the presence of a circuital current. The production of acetate and hydrogen was regulated by the release of extracellular polymeric substances (EPS), which also exhibited invariable catalytic activity toward the reduction of Cd(II) to Cd(0). The intracellular activities of glutathione (GSH), catalase (CAT), superoxide dismutase (SOD) and dehydrogenase were altered by the circuital current and Cd(II) concentration, and these regulated the products distribution. Such understanding enables the targeted manipulation of the MES operational conditions that favor the production of acetate from CO2 sequestration with simultaneous hydrogen production and removal/recovery of Cd(II) from metal-contaminated and organics-barren waters.

Reduction of Cu(II) and simultaneous production of acetate from inorganic carbon by Serratia Marcescens biofilms and plankton cells in microbial electrosynthesis systems.

Simultaneous Cu(II) reduction (6.42 ±â€¯0.02 mg/L/h), acetate production (1.13 ±â€¯0.02 mg/L/h) from inorganic carbon (i.e., CO2 sequestration), and hydrogen evolution (0.0315 ±â€¯0.0005 m3/m3/d) were achieved in a Serratia marcescens Q1 catalyzed microbial electrosynthesis system (MES). The biofilms released increasing amounts of extracellular polymeric substances (EPS) with a higher compositional diversity and stronger Cu(II) complexation, compared to the plankton cells, at higher Cu(II) concentrations (up to 80 mg/L) and circuital currents (cathodic potential of -900 mV vs. standard hydrogen electrode (SHE)). Moreover, the biofilms reduced Cu(II) to Cu(0) more effectively than the plankton cells. At Cu(II) concentrations below 80 mg/L, the dehydrogenase activity in the biofilms was higher than in the plankton cells, and increased with circuital current, which was converse to the lower activities of catalase (CAT), superoxide dismutase (SOD) and antioxidative glutathione (GSH) in the biofilms than the plankton cells, although all these physiological activities were positively correlated with the concentration of Cu(II). This is the first study that evaluates the EPS constituents and the physiological activities of the biofilms and the plankton cells in the MESs, that favors the production of acetate from CO2 sequestration and the simultaneous reduction of Cu(II) from organics-barren waters contaminated with heavy metals.

Acetate production and electron utilization facilitated by sulfate-reducing bacteria in a microbial electrosynthesis system.

The aim of this study is to investigate the effect of sulfate-reducing bacteria on performance of a mixed culture microbial electrosynthesis system (MES). The two-chamber MESs were operated under different cathode potentials (-0.5, -0.6, -0.7, and -0.8V) with or without addition of 6mM sulfate. At -0.7V, acetate production and electrons harvesting in the MES with the sulfate addition were 31.81mM and 5152C, respectively, which improved by 2.7 and 2.4times compared to that without sulfate. With sulfate, the biomass, proportion of live cells, and electrochemical activity of cathode biofilm were enhanced at all the potentials. At -0.7V, the relative abundance of Acetobacterium and Desulfovibrionaceae was 14.2% and 36.7% with sulfate, respectively, compared to 17.4% and 7.3% without sulfate. At -0.7 and -0.8V, the sulfate-reducing bacteria can promote the electron transfer of cathode biofilm and enhance the acetate production.

High-efficient acetate production from carbon dioxide using a bioanode microbial electrosynthesis system with bipolar membrane.

The aim of this study was to develop an efficient bioanode microbial electrosynthesis system (MES) to convert carbon dioxide into acetate using bioenergy from the wastewater. The bioanode MESs were constructed using proton exchange membrane (PEM) and bipolar membrane (BPM) as separator, respectively, and operated under different voltages (i.e., 0.8, 1.0, 1.2, and 1.4V). Since BPM could dissociate H2O into H+ and OH- in situ to buffer the pH change in the chambers, the BPM-MES achieved 238% improvement in cathodic acetate production rate, 45% increase in anodic substrate removal efficiency, and more than five times enhancement in current output, as compared to the PEM-MES. The biomass on the surface of anode and cathode, and the relative abundance of Acetobacterium in the cathode of BPM-MES was higher than that in PEM-MES. Bioanode MES with BPM should be a useful microbial electrosynthesis strategy for acetate production using bioenergy from wastewater treatment.