Long-term effect of set potential on biocathodes in microbial fuel cells: electrochemical and phylogenetic characterization

Influence of carbon-based cathodes on biofilm composition and electrochemical performance in soil microbial fuel cells

Increasing energy demands and environmental pollution concerns press for sustainable and environmentally friendly technologies. Soil microbial fuel cell (SMFC) technology has great potential for carbon-neutral bioenergy generation and self-powered electrochemical bioremediation. In this study, an in-depth assessment on the effect of several carbon-based cathode materials on the electrochemical performance of SMFCs is provided for the first time. An innovative carbon nanofibers electrode doped with Fe (CNFFe) is used as cathode material in membrane-less SMFCs, and the performance of the resulting device is compared with SMFCs implementing either Pt-doped carbon cloth (PtC), carbon cloth, or graphite felt (GF) as the cathode. Electrochemical analyses are integrated with microbial analyses to assess the impact on both electrogenesis and microbial composition of the anodic and cathodic biofilm. The results show that CNFFe and PtC generate very stable performances, with a peak power density (with respect to the cathode geometric area) of 25.5 and 30.4 mW m^-2, respectively. The best electrochemical performance was obtained with GF, with a peak power density of 87.3 mW m^-2. Taxonomic profiling of the microbial communities revealed differences between anodic and cathodic communities. The anodes were predominantly enriched with Geobacter and Pseudomonas species, while cathodic communities were dominated by hydrogen-producing and hydrogenotrophic bacteria, indicating H₂ cycling as a possible electron transfer mechanism. The presence of nitrate-reducing bacteria, combined with the results of cyclic voltammograms, suggests microbial nitrate reduction occurred on GF cathodes. The results of this study can contribute to the development of effective SMFC design strategies for field implementation.

Effect of aerobic microbes' competition for oxygen on nitrogen removal in mainstream nitritation-anammox systems

The effects of C/N ratio in mainstream partial nitritation (PN)-anaerobic ammonia oxidation (ANAMMOX) considering competitive relationship of aerobic microbes competing for oxygen were investigated. Thy system was operated for 501 d with various C/N ratio. Competitive growth of aerobic heterotrophic bacteria (AHB) at ≥ 1 of C/N ratio acted effectively on the selective inhibition of nitrite-oxidizing bacteria (NOB) while contributing to stable PN-A. In-depth kinetic analysis indicated oxygen affinity of aerobic microbes was in the order of AHB > ammonia-oxidizing bacteria (AOB) > NOB. In addition, potential of denitritation by AHB could contributed to improving nitrogen removal up to 87.5 ± 4.3%. AHB was comparatively clustered into two groups with a C/N ratio of 1. Nitrosomonas sp. PY1 became predominant while Nitrospira spp. were the major NOB. The potential of AHB in establishing selective inhibition of NOB was identified, which could be a novel approach to stabilze the mainstream PN-A.

Recent advances in soil microbial fuel cells based self-powered biosensor

The soil microbial fuel cell (SMFC) is a new device that was originally designed to generate electricity from organic matter in soil using microorganisms. Currently, SMFC based biosensors are emerging as a new and promising research direction for real-time and rapid monitoring of soil quality or soil pollution. Compared to conventional biosensors, SMFC based biosensors exhibit advantages such as low-cost, simple design, in-situ, and long-term self-powering monitoring, which makes it become attractive devices for in-situ long-term soil quality or soil pollution monitoring. Thus, this review aims to provide a comprehensive overview of SMFC based biosensors. In this review, different prototypes of SMFC based biosensors developed in recent years are introduced, the biosensing mechanisms and the roles of SMFC are highlighted, and the emerging applications of these SMFC based biosensors are discussed. Since the SMFC based biosensors are applied in open-air conditions, the effects of different environmental factors on the biosensing response are also summarized. Finally, to further expand the understanding and boost the practical application of the SMFC based biosensors, future perspectives including fundamental mechanism exploration and investigation of the full-scale application are proposed.

Bioelectrochemical systems-based metal removal and recovery from wastewater and polluted soil: Key factors, development, and perspective

Bioelectrochemical systems (BES) are considered efficient and sustainable technologies for bioenergy generation and simultaneously removal/recovery metal (loid)s from soil and wastewater. However, several current challenges of BES-based metal removal and recovery, especially concentrating target metals from complex contaminated wastewater or soil and their economic feasibility of engineering applications. This review summarized the applications of BES-based metal removal and recovery systems from wastewater and contaminated soil and evaluated their performances on electricity generation and metal removal/recovery efficiency. In addition, an in depth review of several key parameters (BES configurations, electrodes, catalysts, metal concentration, pH value, substrate categories, etc.) of BES-based metal removal and recovery was carried out to facilitate a deep understanding of their development and to suggest strategies for scaling up their specific application fields. Finally, the future intervention on multifunctional BES to improve their performances of mental removal and recovery were revealed.

Bioelectricity generation using long-term operated biocathode: RFLP based microbial diversity analysis

In the present work, power generation and substrate removal efficiencies of long-term operated microbial fuel cells, containing abiotic cathodes and biocathodes, were evaluated for 220 days. Among the two microbial fuel cell (MFC) types, the one containing biocathode showed higher power density (54 mW/m2), current density (122 mA/m2) coulombic efficiency (33%), and substrate removal efficiency (94%) than the abiotic cathode containing MFC. Voltammetric analysis also witnessed higher and sustainable electron discharge for the MFC with biocathode, when compared with the abiotic cathode MFC. Over the tested period, both MFC have shown a cell voltage drop, after 150 and 165, days, for the MFC with biocathode and abiotic cathodes, respectively. Polymerase chain reaction (PCR) based restriction fragment length polymorphism (RFLP) analysis identified 281 clones. Bacteria belonging to Acinetobacter, Acidovorax, Pseudomonas and Burkholderia were observed in the abiotic cathode MFC. Bacteria belonging to Geobacter, Cupriavidus and Acidobacteria were observed in the biocathode MFC. Almost similar types of archaea (Methanosarcinales, Methanolinea, Nitrososphaera and Methanomicrobiales) were observed in both MFCs.

An overview in the development of cathode materials for the improvement in power generation of microbial fuel cells

Since the high cost and low power generation hinder the overall practical application of microbial fuel cells (MFCs), numerous attempts have been made in the field of cathode materials to enhance the electrical performance of MFCs because they directly catalyze the oxygen reduction reactions (ORR). To choose a proper cathode material, following principles such as ORR activity, conductivity, cost-efficiency, durability, surface area, and accessibility should be taken into consideration. In preparation of cathode materials, versatile materials have been chosen, synthesized, or modified to achieve an improvement in power generation of MFCs. The most widely applied cathode materials could be categorized into three classes, namely carbon-base materials, metal-based materials, and biocatalysts. This review summarizes the utilization, development, and the cost of cathode materials applied in MFCs and tries to highlight the effective modification methods of cathode materials which have helped in achieving enhanced power generation of MFCs in recent years.

Catalysis of the electrochemical oxygen reduction reaction (ORR) by animal and human cells

Animal cells from the Vero lineage and MRC5 human cells were checked for their capacity to catalyse the electrochemical oxygen reduction reaction (ORR). The Vero cells needed 72 hours’ incubation to induce ORR catalysis. The cyclic voltammetry curves were clearly modified by the presence of the cells with a shift of ORR of 50 mV towards positive potentials and the appearance of a limiting current (59 μA.cm^-2). The MRC5 cells induced considerable ORR catalysis after only 4 h of incubation with a potential shift of 110 mV but with large experimental deviation. A longer incubation time, of 24 h, made the results more reproducible with a potential shift of 90 mV. The presence of carbon nanotubes on the electrode surface or pre-treatment with foetal bovine serum or poly-D-lysine did not change the results. These data are the first demonstrations of the capability of animal and human cells to catalyse electrochemical ORR. The discussion of the possible mechanisms suggests that these pioneering observations could pave the way for electrochemical biosensors able to characterize the protective system of cells against oxidative stress and its sensitivity to external agents.

Long-term operation of bio-catalyzed cathodes within continuous flow membrane-less microbial fuel cells

Microorganisms were observed to facilitate cathodic oxygen reduction and enhance cathode performance of microbial fuel cells (MFCs). However, the long-term activity and stability of bio-catalyzed cathode needs to be explored. This study evaluated the long-term performance of bio-catalyzed cathode and iron(II) phthalocyanine (FePc)-catalyzed cathode MFCs through effluent water quality, electricity production and electrochemical impedance spectroscopy (EIS) analysis under different scenarios, including conventional wastewater treatment and energy harvesting using a power management system (PMS). During the continuous operation, both systems demonstrated high chemical oxygen demand and ammonium removal, but bio-catalyzed cathode MFCs could achieve significantly better total nitrogen removal than FePc-catalyzed cathode MFCs. The FePc-coated cathode showed constant cathode potential during the entire operation period, but the biocathode showed varied but step-wise increased cathode potential to achieve more than 500 mV versus the standard hydrogen electrode, likely due to the gradual enrichment of biocathode biofilm. EIS analysis revealed that biocathode had higher ohmic resistance than bare carbon felt cathode but the microbial biofilm could largely decrease polarization resistance of cathode material. Microbial community analysis has shown the presence of nitrifying and denitrifying bacteria in the bio-catalyzed cathode biofilm. When connecting PMS, both bio-catalyzed cathode and FePc-catalyzed cathode MFCs successfully charged a capacitor, but the bio-catalyzed cathode MFC voltage significantly dropped to less than 100 mV after charging for 91 h, and gradually recovered when disconnecting PMS. This study has demonstrated the potential application of oxygen reduction bio-catalyzed cathode MFCs for continuous wastewater treatment and energy harvesting for long period of time.

Simultaneously enhancing degradation of refractory organics and achieving nitrogen removal by coupling denitrifying biocathode with MnOx/Ti anode

To simultaneously remove carbon and nitrogen from refractory organic wastewater, this study couples the denitrifying biocathode and MnO_x/Ti anode to oxidize refractory organic pollutants in the anode chamber and remove NO₃^--N in the cathode chamber (denitrifying biocathode-electrocatalytic reactor, DBECR). After inoculation, DBECR started up at 1.3 and 1.5 V with NO₃^--N reduction peak appearing on the cyclic voltammetry curve and increased NO₃^--N removal by approximately 90 %. Compared to the electrocatalytic reactor without inoculation (ECR), NO₃^--N removal of DBECR significantly increased from 0.09 to 0.45 kg NO₃^--N/m³ NCC/d (NCC: net cathodic compartment). NO₃^--N removal correlated well with charges/current flowing through the circuit of DBECR, further validating the presence of electrotrophic denitrifiers. Moreover, coupling of denitrifying biocathode significantly enhanced methylene blue (MB) removal in the anode chamber (0.18 ± 0.002 and 2.92 ± 0.02 g COD/m²/d for ECR and DBECR, respectively). This was because the growth of eletrotrophic denitrifiers increased the cathodic potential and thus the potential of MnO_x/Ti anode. The higher potential of MnO_x/Ti anode promoted the generation of hydroxyl radicals and consequently promoted MB removal. This study demonstrated that DBECR not only realized nitrogen removal in the cathode chamber, but also enhanced refractory organic carbon degradation in the anode chamber.

Electrochemical and phylogenetic comparisons of oxygen-reducing electroautotrophic communities

The mechanisms of extracellular electron transfer and the microbial taxa associated with the observed electroactivity are fundamental to oxygen-reducing microbial cathodes. Here we confirmed the apparent 'electroautotrophic' behavior of electroactive biofilms (EABs) grown on carbon electrodes at + 0.20V vs. Ag/AgCl under air. The EABs catalyzed O₂ electroreduction into water ─ as demonstrated by a rotating ring disc experiment ─ and performed quasi-reversible heterogeneous electron transfer (HET). By using electrodes of low surface capacitance, we report for the first time nonturnover redox peaks that are very likely intrinsic to the redox protein(s) performing the HET. Because the formal potential of redox proteins is pH-dependent, we investigated the evolution of characteristic potentials of the EABs with the solution pH: (i) open circuit potential, (ii) half-wave potential, and (iii) averaged peak potential of nonturnover cyclic voltammograms, which is presumably the formal potential of the primary electron acceptor(s) for the community. In addition to describing the redox thermodynamics behind HET, we suggest that the corresponding data provides an electrochemical fingerprint that could help in comparing the electroactivity of diverse microbial communities. The taxon with the highest relative abundance in our EABs was an unclassified member of the Gammaproteobacteria that was phylogenetically closely related to most other abundant unclassified Gammaproteobacteria commonly reported in EABs reducing O₂ at high potentials, further suggesting that those taxa are responsible for the bioelectroactivity. Phylogenetic and electrochemical similarities between reported EABs jointly support the hypothesis that similar biomolecular mechanisms may be responsible for this highly probable electroautotrophic metabolism.

Development of an electroactive aerobic biocathode for microbial fuel cell applications

Microbial biocathodes are gaining interest due to their low cost, environmental friendliness and sustainable nature. In this study, a microbial consortium was enriched from activated sludge obtained from a common textile effluent treatment plant in the absence of organic carbon source to produce an electroactive biofilm. Chronoamperometry method of enrichment was carried out for over 70 days to select for electroactive bacteria that could be used as a cathode catalyst in microbial fuel cells (MFC). The resultant biofilm produced an average peak current of -0.7 mA during the enrichment and produced a maximum power density of 64.6 ± 3.5 mW m^-2 compared to platinum (72.7 ± 1.2 mW m^-2 ) in a Shewanella-based MFC. Microbial community analysis of the initial sludge sample and enriched samples, based on 16S rRNA gene sequencing, revealed the selection of chemolithotrophs with the most dominant phylum being Bacteroidetes, Proteobacteria, Firmicutes, Actinobacteria and Acidobacteria in the enriched samples. A variety of CO₂ fixing and nitrate-reducing bacteria was present in the resultant biofilm on the cathode. This study suggests that microbial consortia are capable of replacing expensive platinum as a cathode catalyst in MFCs.

Enhanced anodic oxidation and energy saving for dye removal by integrating O2-reducing biocathode into electrocatalytic reactor

Simultaneous enhancement of dye removal and reduction of energy consumption is critical for electrochemical oxidation in treating dyeing wastewater. To address this issue, this work presented a novel process termed biocathode-electrocatalytic reactor (BECR). The dual-chamber BECR employed O₂-reducing biocathode instead of normal stainless steel (SS) cathode and MnO_x/Ti anode to reduce O₂ in the cathode chamber and treat methylene blue (MB) in the anode chamber, respectively. BECR successfully started up at 0.7 and 1 V and substantially improved MB and total organic carbon (TOC) removal compared with the electrocatalytic reactor with SS cathode (ECR-SS), e.g., removal of MB (150 mg L^-1) increased from 27.0 ± 0.2% to 78.1 ± 0.4% at 1 V. To achieve the same TOC removal, BECR reduced the energy consumption by approximately 45.7% compared with ECR-SS (19.5 and 35.9 kWh (kg TOC) ^-1 for BECR and ECR, respectively). To explain the above merits of BECR, M(·OH) (·OH adsorbed on the anode surface) generation, potential of MnO_x/Ti anode (E_a), and their correlation were investigated. When coupled with O₂-reducing biocathode, MnO_x/Ti anode considerably accelerated M(·OH) generation because E_a increased. The increased E_a in BECR was due to the fact that its cathodic reaction was converted to the four-electron O₂ reduction, which exhibited a higher cathodic potential than hydrogen evolution reaction on SS cathode in ECR-SS. Thereby, BECR simultaneously promoted dye removal and reduced energy consumption, showing promise in treating dyeing wastewater.

Optimal set of electrode potential enhances the toxicity response of biocathode to formaldehyde

The autotrophic biocathode was promising as a broad spectrum, rapid-responding and sensitive sensing element for the early warning of toxicants in water. However, we found that the baseline current and the responsivity strongly relied on the cathode potential. Here we poised cathode potentials at 0, -0.2 and -0.4 V to investigate the effect of electrode potential on the sensor responsivity. With formaldehyde as the tested toxicant, the biocathode poised at -0.2 V had the highest baseline current (118.2 ± 10.7 A m^-2) and the lowest toxicity response concentration (0.00148%), which exhibited a 6-64 times higher response ratio (1.4 × 10⁴ A%^-1 m^-3) than those controlled at 0 V (2.3 × 10³ A%^-1 m^-3) and -0.4 V (2.2 × 10² A%^-1 m^-3). First derivative of cyclic voltammetries revealed that the biocathode acclimated at -0.2 V had a highest main peak centered at 0.301 ± 0.006 V and several minor peaks between -0.2 to 0.2 V. Bacterial community analysis showed that Proteobacteria and Bacteroidetes families closely related to the sensing performance. Interestingly, Nitrospirae was obviously acclimated at -0.2 V, indicating that bacteria belonging to this phylum possibly contributed to the highest responsivity as well. Our findings revealed that the optimal set of electrode potential was critical to promote the toxicity responses of biocathode to the formaldehyde, and the differences were mainly from the microbial communities selected by different cathode potentials.

Azo dye degradation pathway and bacterial community structure in biofilm electrode reactors

In this study, the degradation pathway of the azo dye X-3B was explored in biofilm electrode reactors (BERs). The X-3B and chemical oxygen demand (COD) removal efficiencies were evaluated under different voltages, salinities, and temperatures. The removal efficiencies increased with increasing voltage. Additionally, the BER achieved maximum X-3B removal efficiencies of 66.26% and 75.27% at a NaCl concentration of 0.33 g L^-1 and temperature of 32 °C, respectively; it achieved a COD removal efficiency of 75.64% at a NaCl concentration of 0.330 g L^-1. Fourier transform infrared spectrometry and gas chromatography-mass spectrometry analysis indicated that the X-3B biodegradation process first involved the interruption of the conjugated double-bond, resulting in aniline, benzodiazepine substance, triazine, and naphthalene ring formation. These compounds were further degraded into lower-molecular-weight products. From this, the degradation pathway of the azo dye X-3B was proposed in BERs. The relative abundances of the microbial community at the phylum and genus levels were affected by temperature, the presence of electrons, and an anaerobic environment in the BERs. To achieve better removal efficiencies, further studies on the functions of the microorganisms are needed.

Graphene Modified Electro-Fenton Catalytic Membrane for in Situ Degradation of Antibiotic Florfenicol

The removal of low-concentration antibiotics from water to alleviate the potential threat of antibiotic-resistant bacteria and genes calls for the development of advanced treatment technologies with high efficiency. In this study, a novel graphene modified electro-Fenton (e-Fenton) catalytic membrane (EFCM) was fabricated for in situ degradation of low-concentration antibiotic florfenicol. The removal efficiency was 90%, much higher than that of electrochemical filtration (50%) and single filtration process (27%). This demonstrated that EFCM acted not only as a cathode for e-Fenton oxidation process in a continuous mode but also as a membrane barrier to concentrate and enhance the mass transfer of florfenicol, which increased its oxidation chances. The removal rate of florfenicol by EFCM was much higher (10.2 ± 0.1 mg m^-2 h^-1) than single filtration (2.5 ± 0.1 mg m^-2 h^-1) or batch e-Fenton processes (4.3 ± 0.05 mg m^-2 h^-1). Long-term operation and fouling experiment further demonstrated the durability and antifouling property of EFCM. Four main degradation pathways of florfenicol were proposed by tracking the degradation byproducts. The above results highlighted the feasibility of this integrated membrane catalysis process for advanced water purification.

Electron transfer mechanism of biocathode in a bioelectrochemical system coupled with chemical absorption for NO removal

A biocathode with the function of Fe(III)EDTA and Fe(II)EDTA-NO reduction was applied in a microbial electrolysis cell coupled with chemical absorption for NO removal from flue gas. As the mediated electron transfer was excluded by the same electrochemical characterizations of the biocathodes before and after a 48 h continuous operation, the profiles of reduction experiments indicated that direct electron transfer was the main mechanism of Fe(III)EDTA reduction, while Fe(III)EDTA-NO was mainly reduced via Fe(II)-assisted autotrophic denitrification. The microscopy of the biocathode confirmed the existence of pili, which was supposed to be bacterial nanowires for electron transfer. The analysis of microbial community revealed that iron-reducing bacteria, including Escherichia coli, had the possibility of electron uptake from electrode via physical contact. These results first time gave us in-depth understanding of the electron transfer in the multifunctional biocathode and mechanism for further enhancement of the bioreduction processes.

Effects of residual organics in municipal wastewater on hydrogenotrophic denitrifying microbial communities

Hydrogenotrophic denitrification is promising for tertiary nitrogen removal from municipal wastewater. To reveal the influence of residual organics in municipal wastewater on hydrogenotrophic denitrifiers, we adopted high-throughput 16S rRNA gene amplicon sequencing to examine microbial communities in hydrogenotrophic denitrification enrichments. Using effluent from a municipal wastewater treatment plant as water source, COD, nitrate and pH were controlled the same except for a gradient of biodegradable carbon (i.e., primary effluent (PE), secondary effluent (SE), or combined primary and secondary effluent (CE)). Inorganic synthetic water (IW) was used as a control. Hydrogenophaga, a major facultative autotroph, accounted for 17.1%, 5.3%, 32.7% and 12.9% of the sequences in PE, CE, SE and IW, respectively, implicating that Hydrogenophaga grew well with or without organics. Thauera, which contains likely obligate autotrophic denitrifiers, appeared to be the most dominant genera (23.6%) in IW and accounted for 2.5%, 4.6% and 8.9% in PE, CE and SE, respectively. Thermomonas, which is related to heterotrophic denitrification, accounted for 4.2% and 7.9% in PE and CE fed with a higher content of labile organics, respectively. In contrast, Thermomonas was not detected in IW and accounted for only 0.6% in SE. Our results suggest that Thermomonas are more competitive than Thauera in hydrogenotrophic denitrification with biodegradable organics. Moreover, facultative autotrophic denitrifiers, Hydrogenophaga, are accommodating to residual organic in effluent wastewater, thus we propose that hydrogenotrophic denitrification is amenable for tertiary nitrogen removal.

Effect of engineered environment on microbial community structure in biofilter and biofilm on reverse osmosis membrane

Four dual media filters (DMFs) were operated in a biofiltration mode with different engineered environments (DMF I and II: coagulation with/without acidification and DMF III and IV: without/with chlorination). Designed biofilm enrichment reactors (BERs) containing the removable reverse osmosis (RO) coupons, were connected at the end of the DMFs in parallel to analyze the biofilm on the RO membrane by DMF effluents. Filtration performances were evaluated in terms of dissolved organic carbon (DOC) and assimilable organic carbon (AOC). Organic foulants on the RO membrane were also quantified and fractionized. The bacterial community structures in liquid (seawater and effluent) and biofilm (DMF and RO) samples were analyzed using 454-pyrosequencing. The DMF IV fed with the chlorinated seawater demonstrated the highest reductions of DOC including LMW-N as well as AOC among the other DMFs. The DMF IV was also effective in reducing organic foulants on the RO membrane surface. The bacterial community structure was grouped according to the sample phase (i.e., liquid and biofilm samples), sampling location (i.e., DMF and RO samples), and chlorination (chlorinated and non-chlorinated samples). In particular, the biofilm community in the DMF IV differed from the other DMF treatments, suggesting that chlorination exerted as stronger selective pressure than pH adjustment or coagulation on the biofilm community. In the DMF IV, several chemoorganotrophic chlorine-resistant biofilm-forming bacteria such as Hyphomonas, Erythrobacter, and Sphingomonas were predominant, and they may enhance organic carbon degradation efficiency. Diverse halophilic or halotolerant organic degraders were also found in other DMFs (i.e., DMF I, II, and III). Various kinds of dominant biofilm-forming bacteria were also investigated in RO membrane samples; the results provided possible candidates that cause biofouling when DMF process is applied as the pretreatment option for the RO process.

Enhanced oxygen reducing biocathode electroactivity by using sediment extract as inoculum

Autotrophic bacteria are able to catalyze cathodic oxygen reduction as a renewable and sustainable inexpensive catalyst. However, the performance of biocathode varied over reactors, and we still not know how inoculums affect this system. Using three different inoculum of wastewater (WW), sediment extract (SE) and soil extract (SO) in parallel reactors, we found that SE achieved the shortest setup time (17-25% shorter) as well as the highest power density compared to those of SO and WW. Cyclic voltammetry (CV) further revealed that the current densities of SE biocathodes (100±1A/m³) was 150% and 67% higher than those of WW biocathodes (40±1A/m³) and SO biocathodes (65±1A/m³). Community analysis showed the selective pressure on biocathode facilitated the growth of Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria families. Different from WW and SO biocathodes, Nitrospirae was selectively enriched in SE biocathodes, corresponding to an obvious increase in Unidentified Nitrospiraceae population at genus level, which may play an important role on the cathodic electroactivity. These results confirmed that sediment extract is a better bacteria source than soil and wastewater for the acclimation of autotrophic electroactive bacteria, and the community comparison provided broader knowledge on biocathode microbiology.

Chemolithotrophic processes in the bacterial communities on the surface of mineral-enriched biochars

Biochar and mineral-enriched biochar (MEB) have been used as soil amendments to improve soil fertility, sequester carbon and mitigate greenhouse gas emissions. Such beneficial outcomes could be partially mediated by soil bacteria, however little is known about how they directly interact with biochar or MEB. We therefore analyzed the diversity and functions of bacterial communities on the surfaces of one biochar and two different MEBs after a 140-day incubation in soil. The results show that the biochar and the MEBs harbor distinct bacterial communities to the bulk soil. Communities on biochar and MEBs were dominated by a novel Gammaproteobacterium. Genome reconstruction combined with electron microscopy and high-resolution elemental analysis revealed that the bacterium generates energy from the oxidation of iron that is present on the surface. Two other bacteria belonging to the genus Thiobacillus and a novel group within the Oxalbacteraceae were enriched only on the MEBs and they had the genetic capacity for thiosulfate oxidation. All three surface-enriched bacteria also had the capacity to fix carbon dioxide, either in a potentially strictly autotrophic or mixotrophic manner. Our results show the dominance of chemolithotrophic processes on the surface of biochar and MEB that can contribute to carbon sequestration in soil.

Biocathodes reducing oxygen at high potential select biofilms dominated by Ectothiorhodospiraceae populations harboring a specific association of genes

Biocathodes polarized at high potential are promising for enhancing Microbial Fuel Cell performances but the microbes and genes involved remain poorly documented. Here, two sets of five oxygen-reducing biocathodes were formed at two potentials (-0.4V and +0.1V vs. saturated calomel electrode) and analyzed combining electrochemical and metagenomic approaches. Slower start-up but higher current densities were observed at high potential and a distinctive peak increasing over time was recorded on cyclic voltamogramms, suggesting the growth of oxygen reducing microbes. 16S pyrotag sequencing showed the enrichment of two operational taxonomic units (OTUs) affiliated to Ectothiorodospiraceae on high potential electrodes with the best performances. Shotgun metagenome sequencing and a newly developed method for the identification of Taxon Specific Gene Annotations (TSGA) revealed Ectothiorhodospiraceae specific genes possibly involved in electron transfer and in autotrophic growth. These results give interesting insights into the genetic features underlying the selection of efficient oxygen reducing microbes on biocathodes.

Adaptively Evolving Bacterial Communities for Complete and Selective Reduction of Cr(VI), Cu(II), and Cd(II) in Biocathode Bioelectrochemical Systems

Bioelectrochemical systems (BESs) have been shown to be useful in removing individual metals from solutions, but effective treatment of electroplating and mining wastewaters requires simultaneous removal of several metals in a single system. To develop multiple-reactor BESs for metals removal, biocathodes were first individually acclimated to three different metals using microbial fuel cells with Cr(VI) or Cu(II) as these metals have relatively high redox potentials, and microbial electrolysis cells for reducing Cd(II) as this metal has a more negative redox potential. The BESs were then acclimated to low concentrations of a mixture of metals, followed by more elevated concentrations. This procedure resulted in complete and selective metal reduction at rates of 1.24 ± 0.01 mg/L-h for Cr(VI), 1.07 ± 0.01 mg/L-h for Cu(II), and 0.98 ± 0.01 mg/L-h for Cd(II). These reduction rates were larger than the no adaptive controls by factors of 2.5 for Cr(VI), 2.9 for Cu(II), and 3.6 for Cd(II). This adaptive procedure produced less diverse microbial communities and changes in the microbial communities at the phylum and genus levels. These results demonstrated that bacterial communities can adaptively evolve to utilize solutions containing mixtures of metals, providing a strategy for remediating wastewaters containing Cr(VI), Cu(II), and Cd(II).

The biocathode of microbial electrochemical systems and microbially-influenced corrosion

The cathode reaction is one of the most important limiting factors in bioelectrochemical systems even with precious metal catalysts. Since aerobic bacteria have a much higher affinity for oxygen than any known abiotic cathode catalysts, the performance of a microbial fuel cell can be improved through the use of electrochemically-active oxygen-reducing bacteria acting as the cathode catalyst. These consume electrons available from the electrode to reduce the electron acceptors present, probably conserving energy for growth. Anaerobic bacteria reduce protons to hydrogen in microbial electrolysis cells (MECs). These aerobic and anaerobic bacterial activities resemble those catalyzing microbially-influenced corrosion (MIC). Sulfate-reducing bacteria and homoacetogens have been identified in MEC biocathodes. For sustainable operation, microbes in a biocathode should conserve energy during such electron-consuming reactions probably by similar mechanisms as those occurring in MIC. A novel hypothesis is proposed here which explains how energy can be conserved by microbes in MEC biocathodes.

Removal of organic matters and nitrogenous pollutants simultaneously from two different wastewaters using biocathode microbial fuel cell

In this study, a dual chamber MFC was constructed for simultaneous removal of organic matter and nitrogenous pollutants and bioelectricity generation from synthetic and complex industrial wastewaters and it was operated in batch and continuous mode. When the cell potential was stable after 16 days of batch mode operation, the MFC was converted to continuous mode (from batch mode) and operated for 125 days with different organic loading rates (OLR) and ammonia loading rates (ALR) and fixed hydraulic retention time (HRT) of 40 h. The OLR of 1.49 kg COD m(-3) d(-1) and ALR of 0.58 kg NH3(-) m(-3) d(-1), for anodic and cathodic chambers, respectively, gave the best results. The highest value of cell potential on these OLRs was 310 mV with current density of 85.11 mA m(-2), power density of 26.38 mW m(-2) and volumetric power density of 192.20 mW m(-3). During this period, COD reduction was 78-83% in the anodic chamber and the ammonia reduction was 36-38%. After stable operation with synthetic wastewater one case study was performed with complex industrial wastewater. Continuous mode operation was performed at two different OLR and HRT with a constant ALR. A stable power density and volumetric power density of 23.56 mW m(-2) and 112.50 mW m(-3), respectively were achieved after 24 days of continuous operation at an OLR of 0.35 kg COD/m(3) day with an ALR of 0.43 kg NH3(-) m(-3) day(-1) and corresponding HRT of 68 h. A maximum of 89% COD removal and 40% removal of ammonia was obtained after 50 days. A stable voltage of 300 mV was obtained across 1000 Ω resistance. These findings suggest that BMFC can be used for the treatment of industrial wastewater, with carbon removal in anodic chamber and electricity generation.

Autotrophic biocathode for high efficient sulfate reduction in microbial electrolysis cells

The aim of this study was to utilize the biocathode microbial electrolysis cell (MEC) for sulfate removal from wastewater. Experiments were conducted using the two-chambered MEC under fed-batch and continuous flow modes, respectively, with different cathode potentials. With the fed-batch operation, the average reductive rate of sulfate was 0.49 mg d(-1) and the sulfide concentration increased to 3.1 ± 0.7 mg L(-1) in the catholyte. Sulfate removal rate and electron production rate in the continuous flow mode were 49% and 11 times higher than in the fed-batch mode. With cathode potentials from -0.6 to -1.0 V, electron recovery efficiencies ranged from 5.3% to 50% with the maximum obtained at -0.8 V. The maximum sulfate removal efficiency of (39 ± 9.2)% was achieved at -0.9 V. This study suggests the MEC can be a valuable alternative to remove sulfate in wastewater treatment.

Coupling interaction of cathodic reduction and microbial metabolism in aerobic biocathode of microbial fuel cell

Certain mixed consortia colonized on aerobic biocathodes can improve the 4-electron oxygen reduction of cathodes; however, the coupling interaction of the cathodic reaction and microbial metabolism remains unclear.

Direct electron transfer from electrode to electrochemically active bacteria in a bioelectrochemical dechlorination system

Pentachlorophenol (PCP) was dechlorinated by electrochemically active bacteria using an electrode as the direct electron donor. Dechlorination efficiency and coulombic efficiency (CE) were investigated. When hydrogen evolution reaction was eliminated by controlling the potential, both dechlorination efficiency and CE increase as the potential decreases, which implied the dechlorination was stimulated by electric current rather than hydrogen gas. Further investigation of the cyclic voltammetry characterization of the medium revealed nearly no redox mediator secreted by the bacteria. Moreover, the comparison of dechlorination experiments carried out with filtered and unfiltered medium provided convincible evidence that the dominating electron transfer mechanism for the dechlorination is direct electron transfer. Additionally, 454 pyrosequencing technique was employed to gain a comprehensive understanding of the biocathodic microbial community. The results showed Proteobacteria, Bacteroidetes and Firmicutes were the three predominant groups. This paper demonstrated the direct electron transfer mechanism could be involved in PCP dechlorination with a biocathode.

Analysis of oxygen reduction and microbial community of air-diffusion biocathode in microbial fuel cells

Microbes play irreplaceable role in oxygen reduction reaction of biocathode in microbial fuel cells (MFCs). In this study, air-diffusion biocathode MFCs were set up for accelerating oxygen reduction and microbial community analysis. Linear sweep voltammetry and Tafel curve confirmed the function of cathode biofilm to catalyze oxygen reduction. Microbial community analysis revealed higher diversity and richness of community in plankton than in biofilm. Proteobacteria was the shared predominant phylum in both biofilm and plankton (39.9% and 49.8%) followed by Planctomycetes (29.9%) and Bacteroidetes (13.3%) in biofilm, while Bacteroidetes (28.2%) in plankton. Minor fraction (534, 16.4%) of the total operational taxonomic units (3252) was overlapped demonstrating the disproportionation of bacterial distribution in biofilm and plankton. Pseudomonadales, Rhizobiales and Sphingobacteriales were exoelectrogenic orders in the present study. The research obtained deep insight of microbial community and provided more comprehensive information on uncultured rare bacteria.

Prolonged applied potential to anode facilitate selective enrichment of bio-electrochemically active Proteobacteria for mediating electron transfer: microbial dynamics and bio-catalytic analysis

Prolonged application of poised potential to anode was evaluated to understand the influence of applied potentials [500 mV (E500); 1000 mV (E1000); 2000 mV (E2000)] on bio-electrogenic activity of microbial fuel cell (MFC) and the resulting dynamics in microbial community in comparison to control operation. E1000 system documented higher electrogenic activity (309 mW/m(2)) followed by E500 (143 mW/m(2)), E2000 (112 mW/m(2)) and control (65 mW/m(2)) operations. The improved power output at optimum applied potential (1000mV) might be attributed to the enrichment of electrochemically active bacteria majorly belonging to the phylum Proteobacteria with less extent of Firmicutes which helped in effective electron (mediated) transfer through release of exogenous shuttlers. Improved bio-electrogenic activity due to enrichment at 1000mV applied potential also correlated well with the observed cyctochrome-c peaks on the voltamatogram, lower ion ohmic losses and bio-electro kinetic analysis. Electric-shock at higher applied potential (E2000) resulted in the survival of less number of microbial species leading to lower electrogenesis.

Oxygen-reducing biocathodes operating with passive oxygen transfer in microbial fuel cells

Oxygen-reducing biocathodes previously developed for microbial fuel cells (MFCs) have required energy-intensive aeration of the catholyte. To avoid the need for aeration, the ability of biocathodes to function with passive oxygen transfer was examined here using air cathode MFCs. Two-chamber, air cathode MFCs with biocathodes produced a maximum power density of 554 ± 0 mW/m(2), which was comparable to that obtained with a Pt cathode (576 ± 16 mW/m(2)), and 38 times higher than that produced without a catalyst (14 ± 3 mW/m(2)). The maximum current density with biocathodes in this air-cathode MFC was 1.0 A/m(2), compared to 0.49 A/m(2) originally produced in a two-chamber MFC with an aqueous cathode (with cathode chamber aeration). Single-chamber, air-cathode MFCs with the same biocathodes initially produced higher voltages than those with Pt cathodes, but after several cycles the catalytic activity of the biocathodes was lost. This change in cathode performance resulted from direct exposure of the cathodes to solutions containing high concentrations of organic matter in the single-chamber configuration. Biocathode performance was not impaired in two-chamber designs where the cathode was kept separated from the anode solution. These results demonstrate that direct-air biocathodes can work very well, but only under conditions that minimize heterotrophic growth of microorganisms on the cathodes.