Microbial Fuel Cells

STATEMENT OF GOVERNMENT INTEREST

[001] This work was supported by the United States Department of Energy Shewanella Federation program (Award No. 58486720) and the United States Department of Defense (Air Force) AFOSR MURI program (Award No. FA9550-06- 1-0292), thus the U.S. government may have certain rights in the invention.

BACKGROUND

[002] This application relates to microbial fuel cells, and more particularly to microbial fuel cells with biocatalysts on both the anode and the cathode of the fuel cell.

[003] Various microbial fuel cell (MFC) designs focus primarily on power generation utilizing oxygenic reduction. In these designs, the anode compartment is operated anaerobically while the cathode compartment is operated aerobically. MFCs rely on bacteria to catalyze oxidation reactions in the anodic chamber of the MFC system. In this manner, electrons can be passed from this oxidation reaction to the cathode electrode where an appropriate acceptor can be reduced. Reported electron donors include substances such as lactate, glucose, and organic substances found in wastewater. An inorganic catalyst such as platinum is used to drive the reduction of oxygen at the cathode. The use of such catalysts can be costly and limits the variety of electron acceptors that can be reduced in the fuel cell.

SUMMARY

[004] In one aspect, a microbial fuel cell includes an anode compartment with an anode and an anode biocatalyst, a cathode compartment with a cathode and a cathode biocatalyst, a membrane positioned between the anode compartment and the cathode compartment, and an electrical pathway between the anode and the cathode. The anode biocatalyst is capable of catalyzing

oxidation of an organic substance, and the cathode biocatalyst is capable of catalyzing reduction of an inorganic substance.

[005] In some implementations, the anode compartment is substantially impermeable to the anode biocatalyst and the cathode compartment is substantially impermeable to the cathode biocatalyst. The cathode compartment can be maintained under substantially anaerobic conditions.

[006] In some cases, the anode biocatalyst and the cathode biocatalyst are substantially the same. The cathode biocatalyst can include metal-reducing bacteria or metal-oxidizing bacteria. The cathode biocatalyst can be oxygen-tolerant. In some embodiments, the cathode biocatalyst is a species of Shewanella. The inorganic substance can be a radionuclide or a metal, including a semi-conductor. In some cases, the inorganic substance is chromium(VI), and the cathode biocatalyst is capable of reducing chromium(VI) to chromium(III). In certain embodiments, the cathode biocatalyst is capable of reducing a concentration of chromium(VI) in the cathode compartment to less than about 1 ppb.

[007] In some implementations, the fuel cell is self-contained and/or self-powered. The fuel cell is operable to generate electricity. The fuel cell is operable for in situ soil or water remediation in marine or freshwater environments.

[008] In another aspect, a microbial fuel cell includes an anode compartment with an anode and an anode biocatalyst, a cathode compartment with a cathode and a cathode biocatalyst, a membrane positioned between the anode compartment and the cathode compartment, and an electrical pathway between the anode and the cathode. The anode biocatalyst is capable of catalyzing oxidation of an inorganic substance, and the cathode biocatalyst is capable of catalyzing reduction of an organic substance.

[009] In another aspect, a method of generating electricity includes positioning a microbial fuel cell in a location proximate a medium with an inorganic contaminant and providing the contaminated medium to the fuel cell. The method further includes catalyzing oxidation of an

organic substance with bacteria in the fuel cell, catalyzing reduction of the contaminant with bacteria in the fuel cell to form a reduced contaminant, forming a precipitate comprising the reduced contaminant, and substantially containing the precipitate in the fuel cell.

[010] In some implementations, positioning the fuel cell in a location proximate the medium includes placing the fuel cell in the medium. In other implementations, positioning the fuel cell in a location proximate the medium includes placing the fuel cell outside of the medium. The medium can include soil or water. Providing the contaminant to the fuel cell can include allowing the medium to diffuse toward the fuel cell, transporting the medium toward the fuel cell, and/or electrokinetically assisting the contaminants toward the fuel cell.

[Oi l] In some embodiments, the method further includes reducing a concentration of the contaminant in the fuel cell or in the medium to less than about 1 ppb. In certain cases, substantially containing the precipitate in the fuel cell includes inhibiting oxidation of the reduced contaminant. The bacteria can be substantially contained in the fuel cell. The fuel cell, including the precipitate and/or the bacteria can be removed from the medium. In some embodiments, oxidation of the reduced contaminant is inhibited.

[012] In another aspect, a method of generating electricity includes positioning a microbial fuel cell in a location proximate a medium with an inorganic contaminant and providing the contaminated medium to the fuel cell. The method further includes catalyzing reduction of an organic or inorganic substance with bacteria in the fuel cell, catalyzing oxidation of the contaminant with bacteria in the fuel cell to form an oxidized contaminant, forming a precipitate including the oxidized contaminant at the anode, and substantially containing the precipitate in the fuel cell.

[013] Using bacteria as biocatalysts on both the anode and cathode of the microbial fuel cell eliminates the need for expensive inorganic catalysts, such as platinum. Pure culture biocatalysis of both anode and cathode reactions in an MFC, as seen in this example, have the advantage that self-sustenance and repair can prevent poisoning/fouling seen in inorganic catalysts. Using bacteria as biocatalysts on both the anode and cathode of the microbial fuel cell increases the

activity level of reducing microorganisms exposed, but not released, to local environments. This high level of activity will generate faster kinetics and ultimately shorten treatment times for in situ operation, while reducing biomass in both compartments of the fuel cell. Containment of precipitated substances allows substantially complete removal of the contaminants, essentially eliminating subsequent re-oxidation and/or mobilization of the contaminants. By confining the biological reactions to compartments within the fuel cell, no non-indigenous microorganisms will be introduced into environmental systems. Solid precipitates formed from reduced contaminants can be trapped in the fuel cell for recovery and disposal/reuse when the fuel cell operations have been completed. Additionally, the microbial fuel cells described herein can produce electrical energy for process energy demands or other uses.

[014] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the Claims.

DESCRIPTION OF DRAWINGS

[015] FIG. 1 is a schematic diagram of an exemplary microbial fuel cell.

[016] FIG. 2 is a schematic diagram showing reaction processes in a microbial fuel cell.

[017] FIG. 3 depicts a microbial fuel cell in a wellbore.

[018] FIG. 4A shows MRC current density for a lactate/oxygen system.

[019] FIG. 4B shows MRC current density for a lactate/fumarate system.

[020] FIG. 5A shows anode organic acid concentration for a lactate/oxygen system.

[021] FIG. 5B shows anode organic acid concentration for a lactate/fumarate system.

[022] FIG. 6 shows % of initial Cr(VI) remaining in solution during a remediation process in microbial fuel cells with Shewanella species as biocatalysts on both the anode and cathode.

[023] FIG. 7 shows voltage vs. time data for chromium reduction in a microbial fuel cell with Shewanella species as biocatalysts on both the anode and cathode.

[024] FIG. 8 shows current vs. time for chromium reduction in microbial fuel cells with Shewanella species as biocatalysts on one or both electrodes.

[025] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[026] Microbial fuel cells (MFCs) described herein can be used for remediation of soil or water contaminated with pollutants that form precipitates with the completion of a reduction or oxidation reaction. Such MFCs can allow soluble substances to be reduced on the cathode or oxidized on the anode in an anerobic redox reaction to form insoluble substances with different valence states. For example, soluble hexavalent chromium, Cr(VI), can be reduced at the cathode to form trivalent chromium, Cr(III). The trivalent form of chromium is known to readily form insoluble precipitates with commonly found environmental anions (OH ^" , PO4 ^" , etc.). After treatment, the MFC containing the insoluble species can be removed from the treatment site for disposal or reuse.

[027] MFCs described herein can be used in batch or continuous flow operation. FIG. 1 depicts a schematic diagram of an exemplary MFC 100 designed for batch operation. MFC 100 includes anode compartment 102 with anode 104 and cathode compartment 106 with cathode 108. Anode 104 and cathode 108 can be, for example, graphite felt electrodes. Membrane 110 is positioned between anode compartment 102 and cathode compartment 106. Membrane 110 can be any membrane that is substantially impermeable to the substance targeted for removal from the medium undergoing treatment. In some cases, membrane 110 is an ion exchange membrane or a nanoporous filter or membrane including nylon, cellulose, or polycarbonate. Anode

compartment 102 and cathode compartment 108 include gas ports 112 and conductive leads 114. In some embodiments, one or more leads 114 include platinum. MFC 100 can be operated with the addition of feed and effluent lines to form a continuous flow MFC system.

[028] FIG. 2 depicts a schematic diagram of oxidation and reduction processes in MFC 200. Biocatalysts 202 and 204, shown on anode 104 and cathode 108, respectively, are used as the catalytic agents for anodic and cathodic electron transfer. The catalytic agents can be pure culture biofilms or enriched microbial cultures or consortia. Biocatalysts 202 and 204 may be the same or different.

[029] Metal reducing bacteria capable of attaching and forming a biofilm at the anode and/or cathode can be used in MFCs to catalyze oxidation and/or reduction reactions. The metal- reducing bacteria are able to chemically reduce, and render insoluble, a variety of inorganic substance, including ions of metals, semiconductors, etc. Some metal-reducing bacteria that can be used without mediators in fuel cells include Geobacter metallireducens, Geobacter sulfurreducens, Rhodoferax ferrireducens, Pseudomonas aeruginosa, and members of the genus Shewanella. Of these, only Shewanella and P. aeruginosa are facultative anaerobes; the rest are obligate anaerobes.

[030] Shewanella species are soil- and sediment-dwelling bacteria common in both fresh and marine water environments. These facultative anaerobic bacteria respire a wide variety of inorganic and organic electron acceptors including soluble toxic metals or ions thereof, such as uranium and chromium, under anaerobic conditions. Suitable Shewanella species include, but are not limited to, S. putrefaciens SP200, S. oneidensis MR-I , S. species PV4, and S amazonensis SB2B.

[031] In one embodiment, both the anodic and the cathodic reactions of an MFC are driven with S. oneidensis MR-I . MR-I is able to efficiently pass electrons to the anode electrode without the introduction of an external electron shuttle/mediator such as anthraquinone disulfuonate or neutral red. MR-I has versatile metabolism at both the anode and cathode. As a

biocatalyst on the cathode, MR-I is able to utilize most substances reduced in anaerobic batch mode, such as NO ₃ ⁺ , SO4 ^2" , and Fe ³⁺ .

[032] Increased efficiency of MFCs can be achieved through optimization of gene expression in Shewanella or other metal-reducing bacteria. Over-expression of certain genes may lead to increased electron transfer from the electrode to the metal, resulting in improved kinetics and reductive capabilities.

[033] Referring to FIG. 2, biocatalyst 202 catalyzes oxidation reactions in anode compartment 102. Resulting electrons are passed from the oxidation reactions at the anode 104 to the cathode 108, where reduction of an electron acceptor is catalyzed by biocatalyst 204. Electron donors in the anode compartment 102 can include any respirable organic substances found in the medium undergoing treatment such as, for example, lactate, glucose, and other organics found in wastewater. Reducible substances in the cathode compartment 106 include, for example, oxygen, organic substances (e.g., fumarate), and inorganic substances including radionuclides (e.g., uranium and technetium) and other metals (e.g., hexavalent chromium, arsenic, iron, manganese, selenium, etc.). Other inorganic substances that can be reduced include, for example, thiosulfate and other sulfur-containing substances, such as polysulfide and elemental sulfur..

[034] For reduction of electron acceptors other than oxygen, the cathode compartment is kept substantially anaerobic to essentially eliminate the competing oxygen reduction reaction. This can be accomplished, for example, by continuous sparging of nitrogen into the cathode compartment. A high cell concentration of a facultative anaerobe (such as Shewanella) will also scavenge oxygen in the compartment. A non-oxygen electron acceptor (e.g., an oxidized metal substance) present at the cathode can act as the oxidizing agent. Substances that react at the cathode to form insoluble substances are contained in the cathode compartment of the fuel cell as a solid precipitate 206, and are unable to re-enter the medium undergoing treatment. Following treatment, the fuel cell 200, and thus the precipitated form of the contaminant 206 can be removed from the medium 208.

[035] When the contaminant is chromium, for example, Cr(VI) is reduced in the cathode compartment of an MFC to Cr(III), which can react in water to form insoluble substances such as Cr(OH) ²⁺ , Cr(OH) ₃ , and Cr(OH) ₄ ^~ . Depending on the chemical composition of the contaminated medium, Cr(III) may also form complexes with sulfate, ammonium, cyanide, fluoride, chloride, and natural organic matter. Thus, the amount of solid precipitation will depend on characteristics of the contaminated medium. With MFCs described herein, precipitation of trivalent chromium substances can remove hexavalent chromium from solution, resulting in a concentration of hexavalent chromium in the cathode compartment of a microbial fuel cell, or in the contaminated medium, of less than about 2 ppb, less than about 1 ppb, less than about 0.5 ppb, or less than about 0.2 ppb.

[036] In some embodiments, oxidation of an inorganic substance occurs at the anode with metal-oxidizing bacteria, and reduction of an organic substance occurs at the cathode with the same or different bacteria. The oxidized organic substance forms a precipitate in the anode compartment. The precipitate is contained in the anode compartment, allowing the precipitated form of the contaminant to be contained in the MFC. After treatment, the MFC containing the precipitated form of the contaminant can be removed from the medium.

[037] The fuel cell depicted in FIG. 2 can be modified to increase power output, provide renewable power generation, and oxidize or reduce selected substances with desired efficiency. Modifications include, for example, choosing appropriate bacteria for the anode and cathode, reducing internal resistance by altering electrode structure, enhancing diffusion or transport, and selecting membranes suitable for particular applications.

[038] Concentrated microbial cultures in self-contained MFC systems provide high reductive rates. At high rates of reduction, a steep concentration gradient can develop proximate the fuel cell. Contaminant transport from surrounding areas by diffusion or advection is enhanced in some cases by electrokinetic assistance, in which an electric field is applied to selectively direct the migration of contaminants toward the MFC, thereby increasing contaminant concentration and remediation rates within the fuel cell. Electrical energy generated by operation of the fuel

cell can be used to support electrical demand as needed, e.g., for either the electrokinetic electrodes used in the electrokinetic process or other electrical needs of the operation.

[039] The MFCs described herein can be used for in situ as well as ex situ operation. In situ treatment of contaminated media, such as groundwater, can be accomplished by operation within a wellbore or barrier system. FIG. 3 depicts MFC 200 in wellbore 300 with medium 302. In situ treatment can be enhanced with electrokinetic assistance, as described above. Ex situ treatment systems can be incorporated as a flow-through unit into active waste processing or effluent discharge streams, including industrial or metal-working waste streams or waste streams in sewage treatment or food processing plants.

[040] Sequestration of foreign biomass and reduced substances can be accomplished with appropriate design of a fuel cell apparatus. Large, localized densities of specific bacteria can be utilized followed by removal of the bacteria (e.g., when treatment or decontamination is complete), for example, to minimize disturbance of local subsurface ecosystems. By confining the biological reactions to compartments within the MFC, introduction of non-indigenous microorganisms into environmental systems is avoided. Solid precipitates formed from reduced contaminants can be trapped in the MFC (e.g., in the cathode compartment for the case of Cr(VI)) for recovery and disposal/reuse when the fuel cell operations have been completed and the fuel cell is removed from the contaminated medium or treatment site. The potential danger of reoxidation and mobilization of contaminants associated with current in situ technologies is avoided with recovery of contaminants from affected sites.

[041] The following examples are provided to more fully illustrate some of the embodiments of the present invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute exemplary modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

[042] EXAMPLE 1. Dual compartment MFCs were used to observe the differences in current generation when using a microbial catalyst at the anode and cathode under different catholyte conditions. All MFCs were assembled using pretreated NAFION® 117 (E. I. du Pont de Nemours and Company, USA) ion exchange membranes and electrodes constructed from graphite felt bonded to platinum wire with a graphite conductive adhesive.

[043] Pure culture biomass was utilized as the anode and cathode catalyst on the fuel cell depicted in FIG. 1. S. oneidensis MR-I cultures were obtained from stable chemostat operation started from frozen stocks stored at -80 ⁰ C. MFCs were sterilized by autoclave followed by addition of sterile PIPES buffer and MR-I from stable chemostats. The planktonic MR-I strain was cultivated aerobically in Shewanella Federation Minimal Media using lactate as the electron donor. The harvested MR-I was injected into both MFC compartments to achieve an optical density of 0.20 (at 600 nm).

[044] Resistance across the MFC electrodes was set to 10 ω and voltage monitored throughout the experiment. Addition of lactate to the anode compartment and appropriate electron acceptor (either oxygen or fumarate ) to the cathode compartment marked the zero time point. Sampling was performed at regular intervals to monitor suspended cell concentration and organic acid concentrations.

[045] Oxygen was provided to the cathode of the first MFC by continuously flowing air through the cathode compartment at a rate of 20 mL/min. Sodium fumarate was injected into the cathode compartment of the second MFC to achieve 20 mM starting concentrations. All MFC experiments began with lactate (20 mM) as the fuel at the anode. The anolyte and catholyte solutions were sampled periodically and analyzed using high-pressure liquid chromatography to monitor the metabolic products present in the anode and cathode compartments.

[046] Current density vs. time data were calculated using the measured cell potential across a 10 ω resistor. FIGS. 4A and 4B depict current densities as a function of time for the lactate/oxygen system and the lactate/fumarate system, respectively. As seen in FIGS. 4A and 4B, the maximum current densities for the lactate/oxygen MFC were approximately four times

higher than the lactate/fumarate system. Additionally, as seen in FIGS. 5 A and 5B, for the lactate/oxygen system and the lactate/fumarate system, respectively, each MFC system showed unique lactate 500, acetate 502, pyruvate 504, and formate 506 oxidation rates, indicating that the cathodic reduction was the rate limiting reaction when MR-I are utilized at the anode and cathode with both oxidants.

[047] Anode, cathode, and membrane samples were taken from fuel cells operating at peak current production and at current cessation. Samples were fixed in 2.5% glutaraldehyde, and dehydrated using an increasing series of ethanol concentrations. Critical point drying was performed as the final step (Tousimis AUTOS AMDRI ® 815 Series A, Tousimis; Rockville, MD) before scanning electron micrographs were generated utilizing a Cambridge 360 scanning electron microscope operating at 15kV.

[048] Minimum biofilm development at the anode was observed in both the lactate/oxygen and the lactate/fumarate MFCs based on SEM micrographs. However, cathode images showed a well-developed biofilm (up to 6 μm thick) at the electrode surface of the lactate/oxygen system and very little development on the cathodes form the lactate/fumarate systems. These results indicated that the cathode oxidant had a strong impact on the metabolic activity and growth of the MR-I at the anode and cathode electrodes.

[049] EXAMPLE 2. Microbial chromium reduction was performed with four Shewanella ssp.: S oneidensis MR-I, S. amazonensis SB2B, S. putrefaciens SP200, and S. species PV4. Cell cultures were grown from frozen stock in defined minimal media under aerobic conditions for 24-48 hours. Cultures were then diluted with minimal media to achieve a biomass concentration of about 35 mg volatile suspended solids (VSS) per liter. These cells were then transferred to an anaerobic glove box for 1-2 hours. Cultures were then spiked with stock Cr(VI) solution to achieve a final concentration of 5 ppm Cr(VI) for each culture. As shown in FIG. 6, the Cr(VI) concentration was reduced to less than 40 ppb in less than 5 hours for S. putrefaciens SP200 600, S oneidensis MR-I 602, S. species PV4 604, and S. amazonensis SB2B 606. Within 48 hours, Cr(VI) concentrations were significantly below 2 ppb. Fitting the data to a first order rate

dependence on chromium concentration show rate of removal for S putrefaciens SP200 to be greater than that of other strains tested (TABLE I).

[050]

TABLE I. First order decay constants for Cr(VI) reduction

[051] Similar batch culture experiments were performed to examine the effects of different carbon sources on Cr(VI) removal rates. The results indicated that reduction favored the utilization of substances such as lactate or molasses, an inexpensive agricultural waste, as electron donors.

[052] EXAMPLE 3. A fuel cell utilizing S. oneidensis MR-I as biocatalyst on both the anode and cathode electrodes was set up to utilize lactate as the electron donor in the anode compartment and fumarate as the electron acceptor in the cathode. Once fumarate was consumed, as indicated by a voltage drop, chromium spikes were introduced into the cathode compartment. As shown in FIG. 7, the MFC system exhibited voltage generation responses for each spike of concentrated Cr(VI) 700, 702, 704, indicating the occurrence of a reductive process at the cathode.

[053] EXAMPLE 4. Fuel cells utilizing S. oneidensis MR-I as biocatalyst on the cathode only, the anode only, and both the anode and cathode were set up to utilize lactate as the electron donor in the anode compartment and fumarate as the electron acceptor in the cathode. FIG. 8 shows current vs. time for one spike 800 of concentrated Cr(VI) for MFCs with S. oneidensis MR-I at the cathode 802, at the anode 804, and at the cathode and anode 806, 808, 810. No detectable current was generated with S. oneidensis MR-I at the cathode only 802. While some

current was generated with S. oneidensis MR-I at the anode only 804, a greater initial response was seen with S. oneidensis MR-I at both the cathode and anode 806, 808, 810.

[054] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.