Anode-respiring bacteria (ARB) are able to transfer electrons from reduced substrates to a solid electrode. Previously, we developed a biofilm model based on the Nernst−Monod equation to describe the ...anode potential losses of ARB that transfer electrons through a solid conductive matrix. In this work, we develop an experimental setup to demonstrate how well the Nernst−Monod equation is able to represent anode potential losses in an ARB biofilm. We performed low-scan cyclic voltammetry (LSCV) throughout the growth phase of an ARB biofilm on a graphite electrode growing on acetate in continuous mode. The jV response of 9 LSCVs corresponded well to the Nernst−Monod equation, and the half-saturation potential (E KA) was −0.425 ± 0.002 V vs Ag/AgCl at 30 °C (−0.155 ± 0.002 V vs SHE). Anode-potential losses from the potential of acetate reached ∼0.225 V at current density saturation, and this loss was determined by our microbial community’s E KA value. The LSCVs at high current densities showed no significant deviation from the Nernst−Monod ideal shape, indicating that the conductivity of the biofilm matrix (κbio) was high enough (≥0.5 mS/cm) that potential loss did not affect the performance of the biofilm anode. Our results confirm the applicability of the Nernst−Monod equation for a conductive biofilm anode and give insights of the processes that dominate anode potential losses in microbial fuel cells.
Anode-respiring bacteria (ARB) are able to transfer electrons contained in organic substrates to a solid electrode. The selection of ARB should depend on the anode potential, which determines the ...amount of energy available for bacterial growth and maintenance. In our study, we investigated how anode potential affected the microbial diversity of the biofilm community. We used a microbial electrolysis cell (MEC) containing four graphite electrodes, each at a different anode potential (E anode = −0.15, −0.09, +0.02, and +0.37 V vs SHE). We used wastewater-activated sludge as inoculum, acetate as substrate, and continuous-flow operation. The two electrodes at the lowest potentials showed a faster biofilm growth and produced the highest current densities, reaching up to 10.3 A/m2 at the saturation of an amperometric curve; the electrode at the highest potential produced a maximum of 0.6 A/m2. At low anode potentials, clone libraries showed a strong selection (92−99% of total clones) of an ARB that is 97% similar to G. sulfurreducens. At the highest anode potential, the ARB community was diverse. Cyclic voltammograms performed on each electrode suggest that the ARB grown at the lowest potentials carried out extracellular electron transport exclusively by conducting electrons through the extracellular biofilm matrix. This is supported by scanning electron micrographs showing putative bacterial nanowires and copious EPS at the lowest potentials. Non-ARB and ARB using electron shuttles in the diverse community for the highest anode potential may have insulated the ARB using a solid conductive matrix from the anode. Continuous-flow operation and the selective pressure due to low anode potentials selected for G. sulfurreducens, which are known to consume acetate efficiently and use a solid conductive matrix for electron transport.
Anode-respiring bacteria (ARB) in a biofilm anode carry out an oxidation half-reaction of organic matter, producing an electrical current from renewable biomass, including wastes. At the same time, ...ARB produce protons, usually one proton for every electron. Our study shows how current density generated by an acclimated ARB biofilm was limited by proton transport out of the biofilm. We determined that, at high current densities, protons were mainly transported out of the biofilm by protonating the conjugate base of the buffer system; the maximum current generation was directly related to the transport of the buffer, mainly by diffusion, into and out of the biofilm. With non-limiting acetate concentrations, the current density increased with higher buffer concentrations, going from 2.21 ± 0.02 A m⁻² with 12.5-mM phosphate buffer medium to 9.3 ± 0.4 A m⁻² using a 100-mM phosphate buffer at a constant anode potential of Eanode = -0.35 V versus Ag/AgCl. Increasing the concentration of sodium chloride in the medium (0-100 mM) increased current density by only 15%, indicating that ion migration was not as important as diffusion of phosphate inside the biofilm. The current density also varied strongly with medium pH as a result of the buffer speciation: The current density was 10.0 ± 0.8 A m⁻² at pH 8, and the pH giving one-half the maximum rate was 6.5. A j-V curve analysis using 100 mM phosphate buffer showed a maximum current density of 11.5 ± 0.9 A m⁻² and half-saturation potential of -0.414 V versus Ag/AgCl, a value that deviated only slightly from the standard acetate potential, resulting in small anode-potential losses. We discuss the implications of the proton-transport limitation in the field of microbial fuel cells and microbial electrolytic cells. Biotechnol. Bioeng. 2008;100: 872-881.
In microbial fuel cells and electrolysis cells (MXCs), anode-respiring bacteria (ARB) oxidize organic substrates to produce electrical current. In order to develop an electrical current, ARB must ...transfer electrons to a solid anode through extracellular electron transfer (EET). ARB use various EET mechanisms to transfer electrons to the anode, including direct contact through outer-membrane proteins, diffusion of soluble electron shuttles, and electron transport through solid components of the extracellular biofilm matrix. In this review, we perform a novel kinetic analysis of each EET mechanism by analyzing the results available in the literature. Our goal is to evaluate how well each EET mechanism can produce a high current density (>10 A m⁻²) without a large anode potential loss (less than a few hundred millivolts), which are feasibility goals of MXCs. Direct contact of ARB to the anode cannot achieve high current densities due to the limited number of cells that can come in direct contact with the anode. Slow diffusive flux of electron shuttles at commonly observed concentrations limits current generation and results in high potential losses, as has been observed experimentally. Only electron transport through a solid conductive matrix can explain observations of high current densities and low anode potential losses. Thus, a study of the biological components that create a solid conductive matrix is of critical importance for understanding the function of ARB.
•Critical transport processes in microbial electrochemistry technologies (METs) are reviewed.•Current production is intimately linked to fluxes of several chemical species in MET.•MET design has ...evolved over the years as knowledge of transport limitations has been gained.•Future MET designs are discussed.
Microbial electrochemistry technologies (METs) take advantage of the connection of microorganisms with electrodes. In the classic case of a microbial anode, the maximization of current density produced is often the goal. But, current production is dependent on many transport processes occurring, which can be rate-limiting. These include the fluxes of electron donor and acceptor, the ionic flux, the acidity and alkalinity fluxes at anode and cathode respectively, the electron transport flux at the biofilm, and the reactant/product crossover flux. Associated with these fluxes are inherent concentration gradients that can affect performance. This critical review provides an analysis on how these transport processes have hindered the development of METs, and how MET designs have evolved as more knowledge of these transport limitations is gained. Finally, suggestions are provided on how to design MET systems taking into consideration critical transport processes that are intimately linked to the current produced.
When a mixed-culture microbial electrolysis cell (MEC) is fed with a fermentable substrate, such as glucose, a significant fraction of the substrate’s electrons ends up as methane (CH4) through ...hydrogenotrophic methanogenesis, an outcome that is undesired. Here, we show that free ammonia-nitrogen (FAN, which is NH3) altered the glucose fermentation pathways in batch MECs, minimizing the production of H2, the “fuel” for hydrogenotrophic methanogens. Consequently, the Coulombic efficiency (CE) increased: 57% for 0.02 g of FAN/L of fed-MEC, compared to 76% for 0.18 g of FAN/L of fed-MECs and 62% for 0.37 g of FAN/L of fed-MECs. Increasing the FAN concentration was associated with the accumulation of higher organic acids (e.g., lactate, iso-butyrate, and propionate), which was accompanied by increasing relative abundances of phylotypes that are most closely related to anode respiration (Geobacteraceae), lactic-acid production (Lactobacillales), and syntrophic acetate oxidation (Clostridiaceae). Thus, the microbial community established syntrophic relationships among glucose fermenters, acetogens, and anode-respiring bacteria (ARB). The archaeal population of the MEC fed 0.02 g FAN/L was dominated by Methanobacterium, but 0.18 and 0.37 g FAN/L led to Methanobrevibacter becoming the most abundant species. Our results provide insight into a way to decrease CH4 production and increase CE using FAN to control the fermentation step, instead of inhibiting methanogens using expensive or toxic chemical inhibitors, such as 2-bromoethanesulfonic acid.
The human and financial costs of treating surgical site infections (SSIs) are increasing. The number of surgical procedures performed in the United States continues to rise, and surgical patients are ...initially seen with increasingly complex comorbidities. It is estimated that approximately half of SSIs are deemed preventable using evidence-based strategies.
To provide new and updated evidence-based recommendations for the prevention of SSI.
A targeted systematic review of the literature was conducted in MEDLINE, EMBASE, CINAHL, and the Cochrane Library from 1998 through April 2014. A modified Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) approach was used to assess the quality of evidence and the strength of the resulting recommendation and to provide explicit links between them. Of 5759 titles and abstracts screened, 896 underwent full-text review by 2 independent reviewers. After exclusions, 170 studies were extracted into evidence tables, appraised, and synthesized.
Before surgery, patients should shower or bathe (full body) with soap (antimicrobial or nonantimicrobial) or an antiseptic agent on at least the night before the operative day. Antimicrobial prophylaxis should be administered only when indicated based on published clinical practice guidelines and timed such that a bactericidal concentration of the agents is established in the serum and tissues when the incision is made. In cesarean section procedures, antimicrobial prophylaxis should be administered before skin incision. Skin preparation in the operating room should be performed using an alcohol-based agent unless contraindicated. For clean and clean-contaminated procedures, additional prophylactic antimicrobial agent doses should not be administered after the surgical incision is closed in the operating room, even in the presence of a drain. Topical antimicrobial agents should not be applied to the surgical incision. During surgery, glycemic control should be implemented using blood glucose target levels less than 200 mg/dL, and normothermia should be maintained in all patients. Increased fraction of inspired oxygen should be administered during surgery and after extubation in the immediate postoperative period for patients with normal pulmonary function undergoing general anesthesia with endotracheal intubation. Transfusion of blood products should not be withheld from surgical patients as a means to prevent SSI.
This guideline is intended to provide new and updated evidence-based recommendations for the prevention of SSI and should be incorporated into comprehensive surgical quality improvement programs to improve patient safety.
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•FA-MFC system for domestic wastewater treatment was operated during 8months.•FA-MFCs could successfully satisfy COD and TN discharge limits at an HRT of∼2.5h.•FA-MFC system showed a ...significantly high TN removal rate up to 0.62kg-N/m3/d.•Nitrosomonas, Nitratireductor and Acidovorax spp. contributed to nitrogen removal.•FA-MFC system has a promise for energy-sustainable domestic wastewater treatment.
Microbial fuel cells (MFCs) can treat organic compounds from domestic wastewater without aeration, but an additional procedure is required to remove nitrogen. This study developed a flat-panel air-cathode MFC (FA-MFC) that was comprised of five MFC units connected in series and operated to remove organic and nitrogen compounds from domestic wastewater with a short hydraulic retention time (HRT) of 2.5h. During eight months of operation, the removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN) increased, reaching 85% and 94%, respectively, and the effluent COD and TN concentrations were 20.7±2.5mg/L and 1.7±0.1mg/L, respectively. The greatest removals of COD and TN were in the first and second unit (0.62kg-N/m3/d of TN removal rate). The FA-MFC system allowed simultaneous removals of COD and TN from domestic wastewater, although it led to minimal power output (6.3W/m3 in the first unit). Because any abiotic ammonia loss was not found under the supplied potential of∼1.1V at a short HRT of 30min, the biological nitrogen removal was thought as a dominant mechanism for TN removal in the FA-MFCs. Microbial community analysis revealed that, near the cathode, Nitrosomonas-like strains contributed to nitrification and Nitratireductor-like strains led to denitrification. Acidovorax-like strains, known for their metabolic diversity, were ubiquitous and appeared to contribute to organics and nitrogen removal in anode and cathode biofilms. This study provides proof of concept that the FA-MFC system has a promise for energy sustainable wastewater treatment.
The substrate-utilization rate of anode-respiring bacteria (ARB) directly correlates to the current density, one of the main factors in a microbial electrolysis/fuel cell. This study first evaluates ...the effects of donor-substrate diffusion and anode potential on the estimation of the half-maximum-rate concentration (K s) and the maximum specific substrate-utilization rate (q max) of a mixed culture biofilm in a microbial electrolysis cell oxidizing acetate. The intrinsic K s value is 119 g COD/m3, substrate diffusion has a significant impact on K s estimation, and the effect of the anode potential on K s is small. The intrinsic q max value is 22.3 g COD/g VS-d for an assumed biomass density of 50,000 g VS/m3 (q maxXf = 1120 kg COD/m3-d). The maximum specific growth rate (μmax) is 3.2/d which is substantially faster than for acetate-utilizing methanogens and homoacetogens. Although the anode potential affects q max, substrate diffusion has a negligible effect. The measured half-saturation anode potential (E KA) is very negative, −0.448 V (vs Ag/AgCl), and this low value minimizes anode-potential limitation on the current density and the substrate-utilization rate. Thus, the ARB selected in our biofilm anode were relatively fast growers able to take advantage of their low E KA value (−0.448 V).
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