Graphite anode surface modification with controlled reduction of specific aryl diazonium salts for improved microbial fuel cells power output

Graphite electrodes were modified with reduction of aryl diazonium salts and implemented as anodes in microbial fuel cells. First, reduction of 4-aminophenyl diazonium is considered using increased coulombic charge density from 16.5 to 200 mC/cm(2). This procedure introduced aryl amine functionalities at the surface which are neutral at neutral pH. These electrodes were implemented as anodes in "H" type microbial fuel cells inoculated with waste water, acetate as the substrate and using ferricyanide reduction at the cathode and a 1000 Ω external resistance. When the microbial anode had developed, the performances of the microbial fuel cells were measured under acetate saturation conditions and compared with those of control microbial fuel cells having an unmodified graphite anode. We found that the maximum power density of microbial fuel cell first increased as a function of the extent of modification, reaching an optimum after which it decreased for higher degree of surface modification, becoming even less performing than the control microbial fuel cell. Then, the effect of the introduction of charged groups at the surface was investigated at a low degree of surface modification. It was found that negatively charged groups at the surface (carboxylate) decreased microbial fuel cell power output while the introduction of positively charged groups doubled the power output. Scanning electron microscopy revealed that the microbial anode modified with positively charged groups was covered by a dense and homogeneous biofilm. Fluorescence in situ hybridization analyses showed that this biofilm consisted to a large extent of bacteria from the known electroactive Geobacter genus. In summary, the extent of modification of the anode was found to be critical for the microbial fuel cell performance. The nature of the chemical group introduced at the electrode surface was also found to significantly affect the performance of the microbial fuel cells. The method used for modification is easy to control and can be optimized and implemented for many carbon materials currently used in microbial fuel cells and other bioelectrochemical systems.     

生物电化学系统处理废气的研究进展

生物电化学系统(Bioelectrochemical systems,BESs)可将污染物的降解转化与电能紧密耦联,具有适用基质广泛、反应过程温和且效率高的特点,在环境污染治理中具有广阔的应用前景。近几年,BESs也逐渐被应用到废气处理。由于微生物和电化学过程的复合作用,BESs显示出较高的处理效率和良好的应用前景。本文在对废气类型、效果及反应器构型进行总结的基础上,还对BESs中的重要功能微生物和微生物电化学反应机理进行介绍和讨论,并对BESs在废气处理方面需要解决的问题和研究方向进行展望,以期为提高生物电化学系统的处理性能提供参考。     

Effect of electrode potentials on the microbial community of photo bioelectrochemical systems

Increasing attention is being paid to the adoption of photoautotrophic microbes in bioelectrochemical systems (BESs) because of the advantages of self-sustainability. Biased potential on the anode was capable of adjusting the performance of non-photo BESs, and the microbial community structure was also changed; however, few studies have been conducted to investigate the effects of potential on microbial community structure in photo-BESs. In this work, the response of microbial community structure to different potentials (i.e., 0, 0.2, 0.4 and 0.6 V vs. Ag/AgCl) was characterized with 454 pyrosequencing. Four samples were collected and they generated 42865 16S rDNA sequencing reads with an average length of 429 bp. The potential at 0.2 V resulted in the highest current density (378.8 mA m^?2) and showed a strong selection for γ-proteobacteria (30.8% of the sequences). α-Diversity analysis showed that microbial diversity increased with increased potential. Rhodopseudomonas palustris was dominant among known exoelectrogens in the biofilm biased at 0.4 V. The results provided an insight into the mechanism of potential regulation on the performance of photo-BESs and changes in microbial community structure.     

Effect of formation of biofilms and chemical scale on the cathode electrode on the performance of a continuous two-chamber microbial fuel cell

A two-chamber MFC system was operated continuously for more than 500 days to evaluate effects of biofilm and chemical scale formation on the cathode electrode on power generation. A stable power density of 0.57 W/m² was attained after 200 days operation. However, the power density decreased drastically to 0.2 W/m² after the cathodic biofilm and chemical scale were removed. As the cathodic biofilm and chemical scale partially accumulated on the cathode, the power density gradually recovered with time. Microbial community structure of the cathodic biofilm was analyzed based on 16S rRNA clone libraries. The clones closely related to Xanthomonadaceae bacterium and Xanthomonas sp. in the Gammaproteobacteria subdivision were most frequently retrieved from the cathodic biofilm. Results of the SEM-EDX analysis revealed that the cation species (Na⁺ and Ca²⁺) were main constituents of chemical scale, indicating that these cations diffused from the anode chamber through the Nafion membrane. However, an excess accumulation of the biofilm and chemical scale on the cathode exhibited adverse effects on the power generation due to a decrease in the active cathode surface area and an increase in diffusion resistance for oxygen. Thus, it is important to properly control the formation of chemical scale and biofilm on the cathode during long-term operation.     

Towards practical implementation of bioelectrochemical wastewater treatment

Bioelectrochemical systems (BESs), such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), are generally regarded as a promising future technology for the production of energy from organic material present in wastewaters. The current densities that can be generated with laboratory BESs now approach levels that come close to the requirements for practical applications. However, full-scale implementation of bioelectrochemical wastewater treatment is not straightforward because certain microbiological, technological and economic challenges need to be resolved that have not previously been encountered in any other wastewater treatment system. Here, we identify these challenges, provide an overview of their implications for the feasibility of bioelectrochemical wastewater treatment and explore the opportunities for future BESs.     

New applications and performance of bioelectrochemical systems

Bioelectrochemical systems (BESs) are emerging technologies which use microorganisms to catalyze the reactions at the anode and/or cathode. BES research is advancing rapidly, and a whole range of applications using different electron donors and acceptors has already been developed. In this mini review, we focus on technological aspects of the expanding application of BESs. We will analyze the anode and cathode half-reactions in terms of their standard and actual potential and report the overpotentials of these half-reactions by comparing the reported potentials with their theoretical potentials. When combining anodes with cathodes in a BES, new bottlenecks and opportunities arise. For application of BESs, it is crucial to lower the internal energy losses and increase productivity at the same time. Membranes are a crucial element to obtain high efficiencies and pure products but increase the internal resistance of BESs. The comparison between production of fuels and chemicals in BESs and in present production processes should gain more attention in future BES research. By making this comparison, it will become clear if the scope of BESs can and should be further developed into the field of biorefineries.     

Biofilm stratification during simultaneous nitrification and denitrification (SND) at a biocathode

The aeration of the cathode compartment of bioelectrochemical systems (BESs) was recently shown to promote simultaneous nitrification and denitrification (SND). This study investigates the cathodic metabolism under different operating conditions as well as the structural organization of the cathodic biofilm during SND. Results show that a maximal nitrogen removal efficiency of 86.9 ± 0.5%, and a removal rate of 3.39 ± 0.08 mg N L⁻¹ h⁻¹ could be achieved at a dissolved oxygen (DO) level of 5.73 ± 0.03 mg L⁻¹ in the catholyte. The DO levels used in this study are higher than the thresholds previously reported as detrimental for denitrification. Analysis of the cathodic half-cell potential during batch tests suggested the existence of an oxygen gradient within the biofilm while performing SND. FISH analysis corroborated this finding revealing that the structure of the biofilm included an outer layer occupied by putative nitrifying organisms, and an inner layer where putative denitrifying organisms were most dominant. To our best knowledge this is the first time that nitrifying and denitrifying microorganisms are simultaneously observed in a cathodic biofilm.     

Effect of oxygen on the per‐cell extracellular electron transfer rate of Shewanella oneidensis MR‐1 explored in bioelectrochemical systems

Extracellular electron transfer (EET) is a mechanism that enables microbes to respire solid‐phase electron acceptors. These EET reactions most often occur in the absence of oxygen, since oxygen can act as a competitive electron acceptor for many facultative microbes. However, for Shewanella oneidensis MR‐1, oxygen may increase biomass development, which could result in an overall increase in EET activity. Here, we studied the effect of oxygen on S. oneidensis MR‐1 EET rates using bioelectrochemical systems (BESs). We utilized optically accessible BESs to monitor real‐time biomass growth, and studied the per‐cell EET rate as a function of oxygen and riboflavin concentrations in BESs of different design and operational conditions. Our results show that oxygen exposure promotes biomass development on the electrode, but significantly impairs per‐cell EET rates even though current production does not always decrease with oxygen exposure. Additionally, our results indicated that oxygen can affect the role of riboflavin in EET. Under anaerobic conditions, both current density and per‐cell EET rate increase with the riboflavin concentration. However, as the dissolved oxygen (DO) value increased to 0.42 mg/L, riboflavin showed very limited enhancement on per‐cell EET rate and current generation. Since it is known that oxygen can promote flavins secretion in S. oneidensis, the role of riboflavin may change under anaerobic and aerobic conditions. Biotechnol. Bioeng. 2017;114: 96–105. © 2016 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.     

The bacterial communities of bioelectrochemical systems associated with the sulfate removal under different pHs

Sulfate contamination in ecosystems has been a serious problem. Among various technologies, bioelectrochemical systems (BESs) show the advantage of no-pollution and low-cost for removing sulfate. In order to further expound the biological process of sulfate removal in BESs, 454 pyrosequencing was applied to analyze the bacterial communities under different pH conditions. The bacterial community profiles were analyzed from three aspects: (a) the α-diversity and β-diversity of bacterial communities, (b) the distribution of bacterial phylotypes, and (c) the characterizations of dominant operational taxonomic units (OTUs). We demonstrated that the indexes of phylotype richness and phylogenetic diversity were positively correlated across the pH gradient in the BESs. Among the dominant OTUs, the OTUs which were highly similar to Desulfatirhabdium butyrativorans, Desulfovibrio marrakechensis and Desulfomicrobium sp. might participate in removing sulfate. Standing on genus level, Desulfomicrobium and Sulfuricurvum play conducing and adverse roles for sulfate removal in alkaline condition, respectively. Desulfovibrio contributed to removing sulfate in the neutral and acidic conditions, while Thiomonas mainly weakened the performance of sulfate removal in neutral pH condition. These results further clarified how pH condition directly affected the bacterial communities, which consequently affected the performance of sulfate pollutant treatment using BESs.     

Minimizing losses in bio-electrochemical systems: the road to applications

Bio-electrochemical systems (BESs) enable microbial catalysis of electrochemical reactions. Plain electrical power production combined with wastewater treatment by microbial fuel cells (MFCs) has been the primary application purpose for BESs. However, large-scale power production and a high chemical oxygen demand conversion rates must be achieved at a benchmark cost to make MFCs economical competitive in this context. Recently, a number of valuable oxidation or reduction reactions demonstrating the versatility of BESs have been described. Indeed, BESs can produce hydrogen, bring about denitrification, or reductive dehalogenation. Moreover, BESs also appear to be promising in the field of online biosensors. To effectively apply BESs in practice, both biological and electrochemical losses need to be further minimized. At present, the costs of reactor materials have to be decreased, and the volumetric biocatalyst activity in the systems has to be increased substantially. Furthermore, both the ohmic cell resistance and the pH gradients need to be minimized. In this review, these losses and constraints are discussed from an electrochemical viewpoint. Finally, an overview of potential applications and innovative research lines is given for BESs.     

Resource recovery from low strength wastewater in a bioelectrochemical desalination process

In this research, low strength synthetic wastewaters with chemical oxygen demand less than 300 mg L^?1 were treated at different concentrations in a bioelectrochemical desalination process. A process optimization model was utilized to study the performance of the photosynthetic bioelectrochemical desalination process. The variables include substrate (chemical oxygen demand) concentration, total dissolved solids, and microalgae biomass concentration in the cathode chamber. Relationships between the chemical oxygen demand concentration, microalgae, and salt concentrations were evaluated. Power densities and potential energy benefits from microalgal biomass growth were discussed. The results from this study demonstrated the reliability and reproducibility of the photosynthetic microbial desalination process performance followed by a response surface methodology optimization. This study also confirms the suitability of bioelectrochemical desalination process for treating low substrate wastewaters such as agricultural wastewaters, anaerobic digester effluents, and septic tank effluents for net energy production and water desalination.     

Power production enhancement with a polyaniline modified anode in microbial fuel cells

In this paper, an approach of improving power generation of microbial fuel cells (MFCs) by using a HSO(4)(-) doped polyaniline modified carbon cloth anode was reported. The modification of carbon cloth anode was accomplished by electrochemical polymerization of aniline in 5% H(2)SO(4) solution. A dual-chamber MFC reactor with the modified anode achieved a maximum power density of 5.16 Wm(-3), an internal resistance of 90 Ω, and a start-up time of 4 days, which was respectively 2.66 times higher, 65.5% lower, and 33.3% shorter than the corresponding values of the MFC with unmodified anode. Evidence from X-ray photoelectron spectroscopy and scanning electron microscopy results proved that the formation of biofilm on the anode surface could prevent the HSO(4)(-) doped polyaniline to be de-doped, and the results from electrochemical tests confirmed that the electrochemical activity of the modified anode was enhanced significantly after inoculation. Charge transfer was facilitated by polyaniline modification. All the results indicated that the polyaniline modification on the anode was an efficient approach of improving the performance of MFCs.     

Making glucose oxidase fit for biofuel cell applications by directed protein evolution

Progress in miniature chip-design raises demands for implantable power sources in health care applications such as continuous glucose monitoring of diabetic patients. Pioneered by Adam Heller, miniaturized enzymatic biofuel cells (mBCs) convert blood sugars into electrical energy by employing for example glucose oxidase (GOx) on the anode and bilirubin oxidase on the cathode. To match application demands it is crucial to increase lifetime and power output of mBCs. The power output has been limited by the performance of GOx on the anode. We developed a glucose oxidase detection assay (GODA) as medium-throughput screening system for improving GOx properties by directed protein evolution. GODA is a reaction product detection assay based on coupled enzymatic reactions leading to NADPH formation which is recorded at 340 nm. The main advantage of the assay is that it detects the production of d-gluconolactone instead of the side-product hydrogen peroxide and enables to improve bioelectrochemical properties of GOx. For validating the screening system, a mutagenic library of GOx from Aspergillus niger (EC 1.1.3.4) was generated and screened for improved activity using Saccharomyces cerevisiae as host. Directed evolution resulted in a GOx mutant I115V with 1.4-1.5-fold improved activity for beta-d-glucose (Vmax from 7.94 to 10.81 micromol min(-1) mg(-1); Km approximately 19-21 mM) and oxygen consumption kinetics correlate well [Vmax (O2) from 5.94 to 8.34 micromol min(-1) mg(-1); Km (O2) from 700 to 474 microM]. The developed mutagenic protocol and GODA represent a proof-of-principle that GOx can be evolved by directed evolution in S. cerevisiae for putative use in biofuel cells.     

The electric picnic: synergistic requirements for exoelectrogenic microbial communities

Characterization of the various microbial populations present in exoelectrogenic biofilms provides insight into the processes required to convert complex organic matter in wastewater streams into electrical current in bioelectrochemical systems (BESs). Analysis of the community profiles of exoelectrogenic microbial consortia in BESs fed different substrates gives a clearer picture of the different microbial populations present in these exoelectrogenic biofilms. Rapid utilization of fermentation end products by exoelectrogens (typically Geobacter species) relieves feedback inhibition for the fermentative consortia, allowing for rapid metabolism of organics. Identification of specific syntrophic processes and the communities characteristic of these anodic biofilms will be a valuable aid in improving the performance of BESs.     

Stratified microbial structure and activity within anode biofilm during electrochemically assisted brewery wastewater treatment

In a bioelectrochemical system (BES), microbial community of anode biofilm is crucial to BES performance. In this study, the stratified pattern of community structure and activity of an anode-respiring biofilm in a BES fueled with brewery wastewater was investigated over time. The anode biofilm exhibited a superior performance in the removal of ethanol to that of an open-circuit system. The electrical current density reached a high level of 0.55mA/cm² with a Coulombic efficiency of 71.4%, but decreased to 0.18mA/cm² in the late stage of operation. A mature biofilm developed a more active outer layer covering a less active inner core, although the activities of the outer and inner layers of biofilm were similar in the early stage. More Geobacter spp., typical exoelectrogens, were enriched in the outer layer than in the inner layer of biofilm in the early stage, while more Geobacter spp. were distributed in the inner layer than in the outer layer in the late stage. The inactive and Geobacter-occupied inner layer of biofilm might be responsible for the decreased electricity generation from wastewater in the late stage of operation. This study provides better understanding of the effect of anode biofilm structure on BES performance.     

Insights in the anode chamber influences on cathodic bioelectromethanogenesis – systematic comparison of anode materials and anolytes

Cathode and catholyte are usually optimized to improve microbial electrosynthesis process, whereas the anodic counter reaction was not systematically investigated and optimized for these applications yet. Nevertheless, the anolyte and especially the anode material can limit the cathodic bioelectrochemical process. This paper compares for the first time the performance of different anode materials as counter electrodes for a cathodic bioelectrochemical process, the bioelectromethanogenesis. It was observed that depending on the anode material the cathodic methane production varies from 0.96 µmol/d with a carbon fabric anode to 25.44 µmol/d with a carbon felt anode of the same geometrical surface area. The used anolyte also affected the methane production rate at the cathode. Especially, the pH of the anolyte showed an impact on the system; an anolyte with pH 5 produced up to 2.0 times more methane compared to one with pH 8.5. The proton availability is discussed as one reason for this effect. Although some of the measured effects cannot be explained completely so far this study advises researchers to strongly consider the anode impact during process development and optimization of a cathodic bioelectrochemical synthesis process.     

Electrochemical surface modification of carbon mesh anode to improve the performance of air-cathode microbial fuel cells

A convenient and promising alternative to surface modification of carbon mesh anode was fulfilled by electrochemical oxidation in the electrolyte of nitric acid or ammonium nitrate at ambient temperature. It was confirmed that such an anode modification method was low cost and effective not only in improving the efficiency of power generation in microbial fuel cells (MFCs) for synthetic wastewater treatment, but also helping to reduce the period for MFCs start-up. The MFCs with anode modification in electrolyte of nitric acid performed the best, achieving a Coulombic efficiency enhancement of 71 %. As characterized, the electrochemical modification resulted in the decrease of the anode potential and internal resistance but the increase of current response and nitrogen-containing and oxygen-containing functional groups on the carbon surface, which might contribute to the enhancement on the performances of MFCs.     

Effects of the Pt loading side and cathode-biofilm on the performance of a membrane-less and single-chamber microbial fuel cell

In order to analyze the effect of cathode's Pt loading side on the performance of single-chamber microbial fuel cells (MFCs), power generation of a bamboo charcoal membrane-less air-cathode MFC was examined. The maximum power outputs obtained were 0.144 and 1.16 mW, while the maximum voltage outputs were 0.400 and 0.500 V (external resistance was 500 Omega), respectively, when the Pt loading side facing to the air and to the anode chamber solution; after a long time of operation with the side of cathode loaded Pt facing to anode chamber solution, a biofilm was developed on the inner side of cathode. With the formation of this biofilm, the power outputs of MFC increased first, and then decreased to 0.8 mW; oxidation-reduction potentials (ORP) dropped first, and then achieved the level of stability. Coulombic efficiency (CE) increased at a certain extent. In addition, the impact of cathode-biofilm on the loss of water in anode chamber solution was determined.     

Production of bioelectricity,bio-hydrogen,high value chemicals and bioinspired nanomaterials by electrochemically active biofilms

Microorganisms naturally form biofilms on solid surfaces for their mutual benefits including protection from environmental stresses caused by contaminants, nutritional depletion or imbalances. The biofilms are normally dangerous to human health due to their inherited robustness. On the other hand, a recent study suggested that electrochemically active biofilms (EABs) generated by electrically active microorganisms have properties that can be used to catalyze or control the electrochemical reactions in a range of fields, such as bioenergy production, bioremediation, chemical/biological synthesis, bio-corrosion mitigation and biosensor development. EABs have attracted considerable attraction in bioelectrochemical systems (BESs), such as microbial fuel cells and microbial electrolysis cells, where they act as living bioanode or biocathode catalysts. Recently, it was reported that EABs can be used to synthesize metal nanoparticles and metal nanocomposites. The EAB-mediated synthesis of metal and metal–semiconductor nanocomposites is expected to provide a new avenue for the greener synthesis of nanomaterials with high efficiency and speed than other synthetic methods. This review covers the general introduction of EABs, as well as the applications of EABs in BESs, and the production of bio-hydrogen, high value chemicals and bio-inspired nanomaterials.     

Influence of anodic biofilm growth on bioelectricity production in single chambered mediatorless microbial fuel cell using mixed anaerobic consortia

The effect of anodic biofilm growth and extent of its coverage on the anodic surface of a single chambered mediatorless microbial fuel cell (MFC) was evaluated for bioelectricity generation using designed synthetic wastewater (DSW) and chemical wastewater (CW) as substrates and anaerobic mixed consortia as biocatalyst. Three MFCs (plain graphite electrodes, air cathode, Nafion membrane) were operated separately with variable biofilm coverage [control; anode surface coverage (ASC), 0%], partially developed biofilm [PDB; ASC approximately 44%; 90 days] and fully developed biofilm [FDB; ASC approximately 96%; 180 days] under acidophilic conditions (pH 6) at room temperature. The study depicted the effectiveness of anodic biofilm formation in enhancing the extracellular electron transfer in the absence of mediators. Higher specific power production [29mW/kg COD(R) (CW and DSW)], specific energy yield [100.46J/kg VSS (CW)], specific power yield [0.245W/kg VSS (DSW); 0.282W/kg VSS (CW)] and substrate removal efficiency of 66.07% (substrate degradation rate, 0.903kgCOD/m(3)-day) along with effective functioning fuel cell at relatively higher resistance [4.5kOmega (DSW); 14.9kOmega (CW)] correspond to sustainable power [0.008mW (DSW); 0.021mW (CW)] and effective electron discharge (at higher resistance) and recovery (Coulomb efficiency; 27.03%) were observed especially with FDB operation. Cyclic voltammetry analysis documented six-fold increment in energy output from control (1.812mJ) to PDB (10.666mJ) operations and about eight-fold increment in energy from PDB to FDB (86.856mJ). Biofilm configured MFC was shown to have the potential to selectively support the growth of electrogenic bacteria with robust characteristics, capable of generating higher power yields along with substrate degradation especially operated with characteristically complex wastewaters as substrates.