Abhijeet P. Borole

Understanding long-term changes in microbial fuel cell performance using electrochemical impedance spectroscopy.

Changes in the anode, cathode, and solution/membrane impedances during enrichment of an anode microbial consortium were measured using electrochemical impedance spectroscopy. The consortium was enriched in a compact, flow-through porous electrode chamber coupled to an air-cathode. The anode impedance initially decreased from 296.1 to 36.3 Omega in the first 43 days indicating exoelectrogenic biofilm formation. The external load on the MFC was decreased in a stepwise manner to allow further enrichment. MFC operation at a final load of 50 Omega decreased the anode impedance to 1.4 Omega, with a corresponding cathode and membrane/solution impedance of 12.1 and 3.0 Omega, respectively. An analysis of the capacitive element suggested that most of the three-dimensional anode surface was participating in the bioelectrochemical reaction. The power density of the air-cathode MFC stabilized after 3 months of operation and stayed at 422 +/- 42 mW/m(2) (33 W/m(3)) for the next 3 months. The normalized anode impedance for the MFC was 0.017 kOmega cm(2), a 28-fold reduction over that reported previously. This study demonstrates a unique ability of biological systems to reduce the electron transfer resistance in MFCs, and their potential for stable energy production over extended periods of time.

Controlling accumulation of fermentation inhibitors in biorefinery recycle water using microbial fuel cells

BackgroundMicrobial fuel cells (MFC) and microbial electrolysis cells are electrical devices that treat water using microorganisms and convert soluble organic matter into electricity and hydrogen, respectively. Emerging cellulosic biorefineries are expected to use large amounts of water during production of ethanol. Pretreatment of cellulosic biomass results in production of fermentation inhibitors which accumulate in process water and make the water recycle process difficult. Use of MFCs to remove the inhibitory sugar and lignin degradation products from recycle water is investigated in this study.ResultsUse of an MFC to reduce the levels of furfural, 5-hydroxymethylfurfural, vanillic acid, 4-hydroxybenzaldehyde and 4-hydroxyacetophenone while simultaneously producing electricity is demonstrated here. An integrated MFC design approach was used which resulted in high power densities for the MFC, reaching up to 3700 mW/m2 (356 W/m3 net anode volume) and a coulombic efficiency of 69%. The exoelectrogenic microbial consortium enriched in the anode was characterized using a 16S rRNA clone library method. A unique exoelectrogenic microbial consortium dominated by δ-Proteobacteria (50%), along with β-Proteobacteria (28%), α-Proteobacteria (14%), γ-Proteobacteria (6%) and others was identified. The consortium demonstrated broad substrate specificity, ability to handle high inhibitor concentrations (5 to 20 mM) with near complete removal, while maintaining long-term stability with respect to power production.ConclusionUse of MFCs for removing fermentation inhibitors has implications for: 1) enabling higher ethanol yields at high biomass loading in cellulosic ethanol biorefineries, 2) improved water recycle and 3) electricity production up to 25% of total biorefinery power needs.

Enhancement in current density and energy conversion efficiency of 3-dimensional MFC anodes using pre-enriched consortium and continuous supply of electron donors

Using a pre-enriched microbial consortium as the inoculum and continuous supply of carbon source, improvement in performance of a three-dimensional, flow-through MFC anode utilizing ferricyanide cathode was investigated. The power density increased from 170 W/m(3) (1800 mW/m(2)) to 580 W/m(3) (6130 mW/m(2)), when the carbon loading increased from 2.5 g/l-day to 50 g/l-day. The coulombic efficiency (CE) decreased from 90% to 23% with increasing carbon loading. The CEs are among the highest reported for glucose and lactate as the substrate with the maximum current density reaching 15.1A/m(2). This suggests establishment of a very high performance exoelectrogenic microbial consortium at the anode. A maximum energy conversion efficiency of 54% was observed at a loading of 2.5 g/l-day. Biological characterization of the consortium showed presence of Burkholderiales and Rhodocyclales as the dominant members. Imaging of the biofilms revealed thinner biofilms compared to the inoculum MFC, but a 1.9-fold higher power density.

Comparison of batchstirred and electrospray reactors for biodesulfurization of dibenzothiophene in crude oil and hydrocarbon feedstocks

Biological removal of organic sulfur from petroleum feedstocks offers an attractive alternative to conventional thermochemical treatment, because of the mild operating conditions afforded by the biocatalyst. In order for biodesulfurization to realize commercial success, reactors must be designed that allow for sufficient liquid-liquid and gas-liquid mass transfer, while simultaneously reducing operating costs. Electro-spray bioreactors were investigated for use as desulfurization reactors because of their reported operational cost savings relative to mechanically agitated reactors. Unlike batch-stirred reactors, which mix the biocatalystcontaining aqueous phase with the organic feedstock by imparting momentum to the entire bulk solution, electro-spray reactors have the potential for tremendous cost savings, creating an emulsion <5 (μm in diameter, at a cost of only 3 W/L. Power law relationships indicate that mechanically stirred reactors would require 100-1000-fold more energy to create such a fine emulsion, but these relationships generally do not account for the effect of endogenously produced surfactant in the system. Here, the rates dibenzothiophene (DBT) oxidation to 2-hydroxybiphenyl (2-HBP) in hexadecane, byRhodococcus sp IGTS8 are compared in the two reactor systems. Desulfurization rates ranged from 1.0 to 5.0 mg 2-HBP/(dry g cells · h), independent of the reactor employed. The batch-stirred reactor was capable of forming a very fine emulsion in the presence of the biocatalyst IGTS8, similar to that formed in the emulsion phase contactor (EPTM), presumably because the biocatalyst produces its own surfactant. Although EPC did not prove to be advantageous for the IGTS8 desulfurization system, it may prove advantageous for systems that do not produce surface-active bioagents, in addition to being mass-transport limited.

Conversion of residual organics in corn stover-derived biorefinery stream to bioenergy via a microbial fuel cell.

A biorefinery process typically uses about 4-10 times more water than the amount of biofuel generated. The wastewater produced in a biorefinery process contains residual sugars, 5-furfural, phenolics, and other pretreatment and fermentation byproducts. Treatment of the wastewater can reduce the need for fresh water and potentially add to the environmental benefits of the process. Use of microbial fuel cells (MFCs) for conversion of the complete range of phenolic compounds and furan aldehyde derivatives present in a postfermentation biorefinery stream is reported here. The consortium was capable of removing the molecules simultaneously with sugars, which were present at 2 orders of magnitude higher concentrations. Organic loading in a fed-batch MFC affected Coulombic efficiency, which decreased from 40% at 0.66 g/L loading to 1.8% at 66.4 g/L loading. Power density increased with loading reaching 1180 mW/m(2) at 5.3 g/L (8% dilution), but decreased thereafter. Excessive loading leads to poor electrogenic performance; therefore, operation of an MFC at an intermediate loading using dilution and recirculation of the process stream can enable effective treatment with bioenergy recovery.

Performance of Chloroperoxidase Stabilization in Mesoporous Sol-Gel Glass Using In Situ Glucose Oxidase Peroxide Generation

A unique mesoporous sol-gel glass possessing a highly ordered porous structure (with three pore sizes of about 50, 150, and 200 A diameter) was used as a support material for immobilization of the enzyme chloroperoxidase (CPO). CPO was bound onto the glass via a bifunctional ligand, trimethoxysilylpropanal. In situ production of the cosubstrate, H2O2, was achieved using glucose oxidase. Solvent stability in acetonitrile mixtures was enhanced when a pore size larger than the size of CPO was used (i.e., 200 A). From these results, it appears that the glass-enzyme complex developed through the present work can be used as high-performance biocatalysts for various chemical-processing applications, particularly in harsh conditions.

Biotransformation of Furanic and Phenolic Compounds with Hydrogen Gas Production in a Microbial Electrolysis Cell

Furanic and phenolic compounds are problematic byproducts resulting from the breakdown of lignocellulosic biomass during biofuel production. The capacity of a microbial electrolysis cell (MEC) to produce hydrogen gas (H2) using a mixture of two furanic (furfural, FF; 5-hydroxymethyl furfural, HMF) and three phenolic (syringic acid, SA; vanillic acid, VA; and 4-hydroxybenzoic acid, HBA) compounds as the substrate in the bioanode was assessed. The rate and extent of biotransformation of the five compounds and efficiency of H2 production, as well as the structure of the anode microbial community, were investigated. The five compounds were completely transformed within 7-day batch runs and their biotransformation rate increased with increasing initial concentration. At an initial concentration of 1200 mg/L (8.7 mM) of the mixture of the five compounds, their biotransformation rate ranged from 0.85 to 2.34 mM/d. The anode Coulombic efficiency was 44-69%, which is comparable to that of wastewater-fed MECs. The H2 yield varied from 0.26 to 0.42 g H2-COD/g COD removed in the anode, and the bioanode volume-normalized H2 production rate was 0.07-0.1 L/L-d. The biotransformation of the five compounds took place via fermentation followed by exoelectrogenesis. The major identified fermentation products that did not transform further were catechol and phenol. Acetate was the direct substrate for exoelectrogenesis. Current and H2 production were inhibited at an initial substrate concentration of 1200 mg/L, resulting in acetate accumulation at a much higher level than that measured in other batch runs conducted with a lower initial concentration of the five compounds. The anode microbial community consisted of exoelectrogens, putative degraders of the five compounds, and syntrophic partners of exoelectrogens. The MEC H2 production demonstrated in this study is an alternative to the currently used process of reforming natural gas to supply H2 needed to upgrade bio-oils to stable hydrocarbon fuels.

Sulfur specificity in the bench-scale biological desulfurization of crude oil by Rhodococcus IGTS8†

Biological removal of organic sulfur from petroleum feedstocks may offer an attractive alternative to conventional thermochemical treatment due to the mild operating conditions and greater reaction specificity afforded by the nature of biocatalysis. Previous investigations have either reported the desulfurization of model sulfur compounds in organic solvents or gross desulfurization of crude oil without data on which sulfur species were being removed. This study reports initial sulfur speciation data for thiophenic sulfur compounds present in crude oil which may be used as a guide both as to which species are treated by the biocatalyst investigated as well as to where biocatalyst development is needed to improve the extent of biological desulfurization when applied to whole crudes. Biodesulfurization of two different crude oils in the 22-31° API specific gravity range with total sulfur contents between 1 and 2% is demonstrated in 1-dm 3 batch stirred reactors using wild type Rhodococcus sp IGTS8. While analysis of the crudes before and after biodesulfurization did not reveal a decrease in total sulfur, GC-MS did reveal significant (43-99%) desulfurization of dibenzothiophenes (DBT) and substituted DBTs. Fractionation of the whole crude, followed by analysis using gas chromatography-sulfur chemiluminescence detection (GC-SCD) of the aromatic fraction of the Van Texas crude oil, demonstrated a reduction of sulfur in this fraction from 3.8% to 3.2%. This research indicates that IGTS8 may be capable of biodesulfurization of refined products such as gasoline and diesel whose predominant sulfur species are dibenzothiophenes. Further biocatalyst development would be needed for effective treatment of the spectrum of sulfur-bearing compounds present in whole crudes.

Investigating microbial fuel cell bioanode performance under different cathode conditions.

A compact, three‐in‐one, flow‐through, porous, electrode design with minimal electrode spacing and minimal dead volume was implemented to develop a microbial fuel cell (MFC) with improved anode performance. A biofilm‐dominated anode consortium enriched under a multimode, continuous‐flow regime was used. The increase in the power density of the MFC was investigated by changing the cathode (type, as well as catholyte strength) to determine whether anode was limiting. The power density obtained with an air‐breathing cathode was 56 W/m3 of net anode volume (590 mW/m2) and 203 W/m3 (2160 mW/m2) with a 50‐mM ferricyanide‐based cathode. Increasing the ferricyanide concentration and ionic strength further increased the power density, reaching 304 W/m3 (3220 mW/m2, with 200 mM ferricyanide and 200 mM buffer concentration). The increasing trend in the power density indicated that the anode was not limiting and that higher power densities could be obtained using cathodes capable of higher rates of oxidation. The internal solution resistance for the MFC was 5–6 Ω, which supported the improved performance of the anode design. A new parameter defined as the ratio of projected surface area to total anode volume is suggested as a design parameter to relate volumetric and area‐based power densities and to enable comparison of various MFC configurations. Published 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009

Methane production in a 100-L upflow bioreactor by anaerobic digestion of farm waste.

Manure waste from dairy farms has been used for methane production for decades, however, problems such as digester failure are routine. The problem has been investigated in small scale (1–2 L) digesters in the laboratory; however, very little scale-up to intermediate scales are available. We report production of methane in a 100-L digester and the results of an investigation into the effect of partial mixing induced by gas upflow/recirculation in the digester. The digester was operated for a period of about 70 d (with 16-d hydraulic retention time) with and without the mixing induced by gas recirculation through an internal draft tube. The results show a clear effect of mixing on digester operation. Without any mixing, the digester performance deteriorated within 30–50 d, whereas with mixing continuous production of methane was observed. This study demonstrates the importance of mixing and its critical role in design of large scale anaerobic digesters.