Microbial Fuel Cell using Anaerobic Respiration as an Anodic

Center for Biofilm Engineering and Department of Civil Engineering, Montana State University, Bozeman, Montana 59717 ... Several fuel cells were opera...
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Environ. Sci. Technol. 2005, 39, 4666-4671

Microbial Fuel Cell using Anaerobic Respiration as an Anodic Reaction and Biomineralized Manganese as a Cathodic Reactant ALLISON RHOADS,† HALUK BEYENAL,† AND Z B I G N I E W L E W A N D O W S K I * ,†,‡ Center for Biofilm Engineering and Department of Civil Engineering, Montana State University, Bozeman, Montana 59717

We have operated a microbial fuel cell in which glucose was oxidized by Klebsiella pneumoniae in the anodic compartment, and biomineralized manganese oxides, deposited by Leptothrix discophora, were electrochemically reduced in the cathodic compartment. In the anodic compartment, to facilitate the electron transfer from glucose to the graphite electrode, we added a redox mediator, 2-hydroxy-1,4-naphthoquinone. We did not add any redox mediator to the cathodic compartment because the biomineralized manganese oxides were deposited on the surface of a graphite electrode and were reduced directly by electrons from the electrode. We have demonstrated that biomineralized manganese oxides are superior to oxygen when used as cathodic reactants in microbial fuel cells. The current density delivered by using biomineralized manganese oxides as the cathodic reactant was almost 2 orders of magnitude higher than that delivered using oxygen. Several fuel cells were operated for 500 h, reaching anodic potentials of -441.5 ( 31 mVSCE and cathodic potentials of +384.5 ( 64 mVSCE. When the electrodes were connected by a 50 Ω resistor, the fuel cell delivered the peak power density of 126.7 ( 31.5 mW/m2.

Introduction Most researchers of microbial fuel cells focus on the anodic part of the fuel cell. Typically, they attempt to (A) determine the most efficient anodic reactions, those producing the highest number of electrons per unit weight of the reactant (1-3); (B) determine the most efficient microorganisms, those that can offer the highest rate of oxidation or are able to extract the highest number of electrons per mole of the substrate (4-6); (3) study the effectiveness of redox mediators (7, 8); and (4) select more effective electrode materials (9-11). The cathodic compartments of microbial fuel cells are less studied, and in most microbial fuel cells the cathodic reaction is abiotic, typically the reduction of oxygen or ferricyanide (1, 12-15) * Corresponding author address: Center for Biofilm Engineering, P. O. Box 173980, Room 366 EPS, Montana State University, Bozeman, MT 59717-3980; phone: 1-406-994-5915; fax: 1-406-994-6098; e-mail: [email protected]. † Center for Biofilm Engineering. ‡ Department of Civil Engineering. 4666

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O2 + 4H+ + 4e- f 2H2O Fe(CN)63- + e- f Fe(CN)64-

E° ) +1.223 VSHE

(1)

E° ) +0.355 VSHE (2)

Ferricyanide is a good choice of cathodic reactant because its concentration is not limited by solubility, as is oxygen concentration. However, ferricyanide is toxic to microorganisms and difficult to use in microbial fuel cells. Oxygen, however, is known for having notoriously slow reduction kinetics on solid electrodes and quite low solubility in water (16-18). Because oxygen is a universal oxidant, much research on abiotic fuel cells is done to improve the kinetics of oxygen reduction on solid electrodes (19-21). Improving oxygen reduction efficiency on solid electrodes would be beneficial for microbial fuel cell applications. Having in mind the difficulties with using oxygen as the cathodic reactant, we attempted to use biomineralized manganese oxides instead. Because oxygen remains the terminal electron acceptor (microorganisms reduce oxygen to oxidize manganese ions), the redox couple composed of manganese ions and manganese oxides can be considered a redox mediator in the cathodic reaction. Previously, our research group showed that biomineralized manganese oxides can be efficient cathodic reactants in microbially influenced corrosion (22-27). To use manganese oxides as cathodic reactants in microbial fuel cells, biofilms of manganese-oxidizing microorganisms were grown on the surfaces of solid-state cathodic electrodes. These oxides were then reduced by the electrons derived from the anodic reactant and delivered via an electronic conductor. We expected that biomineralized manganese oxides might be superior cathodic reactants for two reasons: (A) manganese oxides are solid state and do not change activity when the reactant is consumed, which bypasses the difficulty caused by the limited solubility of oxygen in water and (B) manganese oxides are attached to the electrode surface, which bypasses the problem with the mass transport resistance of dissolved cathodic reactants. The goal of this study was to evaluate the feasibility of using biomineralized manganese oxides as cathodic reactants in microbial fuel cells, and we tested their cathodic efficiency in many arrangements. Manganese-oxidizing bacteria (MOB)-colonizing noble metals immersed in natural waters deposit biomineralized manganese oxides on their surfaces and increase their open circuit potential (23, 26, 28-30). Because the biomineralized manganese oxides are in direct electrical contact with the electrode, the electrode exhibits the equilibrium dissolution potential of the MnO2 through the following half-reactions:

MnO2(s) + H+ + e- w MnOOH(s) E0 ) +0.81 VSCE E’pH)7.2 ) +0.383 VSCE (3) MnOOH(s) + 3H+ + e- w Mn2+ + 2H2O E0 ) +1.26 VSCE

E’pH)7.2 ) +0.336 VSCE (4)

The overall reaction is

MnO2(s) +4 H+ + 2e- w Mn2+ + 2H2O E0 ) +1.28 VSCE E’pH)7.2 ) +0.360 VSCE (5) The mechanism of the cathodic reaction involves an initial deposition of manganese dioxide on the electrode surface and its subsequent reduction by two electrons from the 10.1021/es048386r CCC: $30.25

 2005 American Chemical Society Published on Web 05/14/2005

Materials and Methods

FIGURE 1. Schematic diagram of manganese deposition and reoxidation used as the cathodic reaction (24, 25).

FIGURE 2. Schematic of designed MFC. Glucose was oxidized in the anodic compartment and the electrons were transferred via an electronic conductor to the cathodic compartment, where they reduced microbially deposited manganese oxides. To facilitate the electron transfer from the glucose to the graphite electrode in the anodic compartment, we used a redox mediator, 2-hydroxy-1,4naphthoquinone. In the cathodic compartment, manganese was microbially deposited on the graphite electrode as manganese oxides and reduced directly, without a redox mediator, by the electrons derived from the anodic reaction. SCE and RVC refer to the saturated calomel electrode and the reticulated vitreous carbon, respectively.

anodic reaction, which results in the release of manganese ions. Because the release of the divalent manganese occurs in close proximity to the MOB-colonized electrode surface, the divalent manganese is immediately reoxidized to manganese dioxide by the MOB, and the cycle continues (Figure 1). To demonstrate that biomineralized manganese oxides can serve as efficient cathodic reactants in microbial fuel cells (MFCs), we constructed and operated MFCs with anodic compartments utilizing anaerobic respiration by Klebsiella pneumoniae (ATCC no. 700831) and cathodic compartments using biomineralized manganese oxides deposited by Leptothrix discophora SP-6 (Figure 2). In the anodic compartment, glucose was oxidized anaerobically by Klebsiella pneumoniae. To facilitate the electron transfer to the graphite electrode, we used 2-hydroxy-1,4naphthoquinone (HNQ), added after a visible microbial growth was observed, following literature findings that suggest that using HNQ delivers a high columbic output (1) and that HNQ is chemically more stable than other redox mediators (2). In the cathodic compartment, we used Lepthothrix discophora SP-6 to oxidize manganese ions, Mn2+, to manganese dioxide MnO2 in an aerated solution of Mn2+. L. discophora SP-6 deposited MnO2 on the electrode surface.

The fuel cell (Figure 3) was made of polycarbonate, as described by Allen and Bennetto (1993) (2). It had two compartments, each 250 mL in volume, separated by the proton exchange membrane. The influent and effluent were delivered and removed using tubing made of Neoprene (ColeParmer 148441). Flow breakers were used to prevent back flow and contamination from the inlet or outlet tubing. The electrodes placed in the anodic and cathodic compartments were made of Reticulated Vitreous Carbon, RVC, which are porous plates made of graphite (80 PPI, (The Electrosynthesis Co. 1-716-684-0513). To facilitate electrical connections, the RVC electrodes were attached to the carbon rods (Thermadyne Arcair Plain Pointed Electrodes) by direct insertion of the rods into the RVC; the electrical resistance of the connection was less than 1 Ω. The potentials of the electrodes were measured against the saturated calomel reference electrode (SCE) (Fisher, cat. no. 13-620-51.) The compartments of the fuel cell were separated by a proton exchange membrane: ESC-7000 (The Electrosynthesis Co.). To prevent mechanical damage to the membrane and to minimize microbial growth on the membrane, we placed a J-cloth (First Brands Corporation) between the membrane and the RVC electrodes. It was cut to the size of the cation exchange membrane and glued to the surface of a rubber gasket sealing the compartments using a silicone sealant. We recycled the cation exchange membranes and used them more than once. After each run (typically 500 h), the MFC were taken apart, the proton membranes were rinsed gently to remove surface deposits, and then the membranes were placed in a solution of 1 M NaCl for 24 h to recharge, as recommended by the vendor. Before installing in the fuel cells, we carefully inspected the membranes for mechanical damage; defective membranes were discarded. Growth Medium, Microorganisms, and Inoculum. In the anodic compartment of the fuel cell, we grew Klebsiella pneumoniae (ATCC no. 700831). The growth medium was composed of tryptone, 10 g/L; yeast extract, 5 g/L; NaCl, 5 g/L; Na2HPO4, 1.825 g/L; KH2PO4, 0.35 g/L; and glucose, 1 g/L, known as a Luria-Bertani medium with glucose (LBG). As inoculum, we used 1 mL of frozen stock culture per 100 mL of the autoclaved growth medium in a 250 mL flask. The flask was placed in a rotary shaker at room temperature, operated at 150 rpm, and the microorganisms were grown for ∼18 h prior to inoculating the anodic compartment. In the cathodic compartment, we grew Leptothrix discophora SP-6 using the ATTC Culture Medium 1917 MSVP. The medium was prepared by mixing together the following: 0.24 g of (NH4)2SO4, 0.06 g of MgSO4‚7H2O, 0.06 g of CaCl2‚2 H2O, 0.02 g of KH2PO4, 0.03 g of Na2HPO4, 2.383 g of HEPES, and 15.0 g of Agar Noble (Difco 0142) (for plates only). It was added to 984 mL of distilled water, and the pH was adjusted to 7.2 using 6 N NaOH. The medium was autoclaved at 121 °C for 15 min and then cooled to approximately 50 °C before adding the following filter-sterilized solutions aseptically: 1 mL vitamin solution, 1.0 mL from 10 mM FeSO4, 5.0 mL from 20% sodium pyruvate, and 4.0 mL from 0.05 M manganese stock solution. The vitamin solution contained (in 1 L of distilled water) 20.0 mg of biotin, 20.0 mg of folic acid, 50.0 mg of thiamine HCl, 50.0 mg of D-(+)-calcium pantothenate, 1.0 mg of vitamin B12, 50.0 mg of riboflavin, 50.0 mg of nicotinic acid, 100.0 mg of pyridoxine hydrochloride, and 50.0 mg of p-aminobenzoic acid. To prepare the inoculum, a MSVP plate (prepared with Agar Noble (Difco 0142)) was streaked with a frozen stock culture of Leptothrix discophora SP-6 as described by Yurt et al. (31). Once the bacteria formed colonies, a single colony was transferred aseptically to 100 mL of MSVP medium in a flask. The flask was placed on a rotary shaker at room VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Microbial fuel cell, (A) general view, (B) side plates. Port 1: outlet, port 2: air or nitrogen, port 3: media feed line. (C) Growth chamber for anodic or cathodic compartment. (D) Top view of the cell. Port 4: salt bridge for reference electrode, ports 5 and 6 for graphite rods connected to RVC. (E) Electrode configurations used in the compartments. temperature operated at 150 rpm. The microorganisms grew for ∼72 h, and then the solution was used to inoculate the cathodic compartment of the MFC. After assembling the MFC, two rubber stoppers (size 00) were placed in the center hole at the top of the MFC (see Figure 3D and E; Port 4) (where the salt bridge would eventually be located). Both the anode and cathode compartments were filled with deionized water. The MFC and all of the components (tubing, flow breakers, rubber stoppers etc.) were autoclaved for 20 min with the rubber stoppers off the top to allow for ventilation. Once autoclaved, the stoppers were immediately placed in port 4. Microbial Fuel Cell Operation. To start operating the microbial fuel cell, we followed the following steps: (a) assembled the polycarbonate fuel cell; (b) filled the fuel cell with deionized water; (c) autoclaved the polycarbonate fuel cell with connected tubing, flow breakers, and rubber stoppers; (d) replaced the deionized water with MSVP medium in the cathodic compartment and inoculated it with Leptothrix discophora SP-6; (e) operated the cathodic compartment until the cathode reached a steady potential of ∼350mVSCE; (f) replaced the deionized water with the growth LBG medium in the anodic compartment and inoculated it with Klebsiella pneumoniae (ATCC no. 700831); and (g) operated the entire fuel cell 4668

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Because we could not sterilize the reference electrode, to prevent contamination of the fuel cell we connected the reference electrode with the solution via a salt bridge, which was made of glass tubing with porous glass affixed to one end, as described by Geiser et al. (26). The other end of the tubing was equipped with a rubber stopper and was fixed to the top of the MFC. The autoclave-sterilized tubing was connected to the fuel cell. The filling solution for the salt bridge was made of 1 g/100 mL of R2A agar (DIFCO 1826-17-1) and 1 mL of 0.1 M Na2SO4. The filling solution and syringe apparatus were autoclaved at 121 °C and 1 atm before filling the glass tubing. The filling solution was then added to the glass tubing using a syringe. Once the filling solution was in place, a connector was added to the glass tubing and the remainder was filled with 1 N Na2SO4 solution. Start-Up Procedure. The cathodic compartment of the reactor was initially filled with 150 mL of MSVP medium and operated as a batch reactor. Fifty mL of L. discophora inoculum (prepared according to the procedure described above) was then added to the cathodic compartment of the reactor. The cathodic compartment was aerated continuously at a rate of 2.5 mL/s through port 2 (Figure 3B) using air filtered through a 0.2-µm filter (PALL Corporation PN no. 4210). Once the electrode in the cathodic compartment reached a steady potential of 350 mVSCE, the reactor was

operated as a continuous flow reactor and the fresh sterile MSVP medium was pumped into the reactor at a flow rate of 0.6 mL/hr. Then, the anodic compartment of the fuel cell was drained to remove the sterile water, filled with ∼150 mL of the Luria-Bertani with glucose growth medium, and inoculated with 1 mL of K. pneumoniae. The anodic compartment was purged continuously with nitrogen at a rate of 1.25 mL/s to remove oxygen through port 2 in Figure 3B; the nitrogen was filtered using a 0.2-µm filter (Corning 431219) to prevent contamination of the growth medium. After 18 h of batch growth of K. pneumoniae, the anodic compartment was operated as a continuous flow reactor and the fresh sterile Luria-Bertani with glucose (LBG) growth medium was pumped into the reactor at a flow rate of 0.6 mL/hr. Once the anode reached a constant potential, we measured the current by connecting the anode and cathode trough electrical resistors. We used a 510 Ω resistor in the absence of HNQ and 50 Ω resistor in the presence of HNQ. To establish the baseline for further measurements, we first measured the current in the absence of HNQ, and then added an appropriate amount of HNQ aseptically to reach the final concentration of HNQ in the fuel cell equal to 0.05 mM. The concentrated solution of HNQ was added through a 0.2-µm filter (Corning, no. 431219) to prevent microbial contamination. Measurements. Electrode potentials were measured every 60 min using a multimeter (Hewlett-Packard data logger, 34970A). When we measured the current, we connected the electrodes through a 510 οr 50 Ω resistor, which was comparable to the resistors used in the literature (1, 32). The current was measured using a Fluke multimeter (model no. 189). Once the multimeter was connected, the peak current was recorded. By definition, the peak current is the maximum amount of current produced immediately after a load (resistor) is applied to the microbial fuel cell. From this measurement, the peak power of the fuel cell is calculated by multiplying the measured current by the cell potential. We realize that the peak values are not representative of the sustainable current or of the sustainable power delivered by the fuel cell. However, for the purpose of testing the fuel cell they were sufficient, and the measurement was consistent with other such measurements reported in the literature (1, 5, 7, 33). Chemical Analysis. The effluent from the cathodic compartment was sampled daily, and the concentration of the divalent manganese ion was evaluated using the methods of Goto et al. (34) with the detection limit of 4 ppm. In a similar fashion, the glucose concentration was measured daily in the effluent from the anodic compartment using the one touch basic glucose monitoring system made by Lifescan, a Johnson and Johnson Company. Cathodic Polarization. To compare the cathodic efficiency of manganese oxides with that of oxygen, we measured cathodic polarization curves using three cathodic reactants: (1) biomineralized manganese deposited on a graphite electrode, (2) manganese oxides electrochemically deposited on a graphite electrode, and (3) dissolved oxygen reduced on a graphite electrode. To deposit biomineralized manganese oxides on the graphite electrode, we used Aldrich no. 496545-60G graphite rods (diameter of 3 mm, with 2 cm of the rod exposed and the rest covered with silicone tubing) placed in flasks filled with the MSVP growth medium. The assemblage was first autoclaved, then the filtered solution of manganese ions was added to the medium, and the medium was inoculated with a single colony of Leptothrix discophora that had been grown on a MSVP nutrient agar plate. The flask was placed on a rotating shaker at 150 rpm for 4 days, during which time the microorganisms biomineralized the divalent manganese and deposited manganese oxides on the graphite rod.

FIGURE 4. Typical temporal variations in the anode and cathode potentials against saturated calomel electrode (SCE) in our microbial fuel cells. To electrochemically plate manganese oxides on the graphite electrodes, we used Aldrich no. 496545-60G graphite rods (diameter of 3 mm, with 2 cm of the rod exposed, as above). Each graphite electrode was polarized anodically at 3 mA/cm2 for 20 s in a solution of 5 mM MnSO4 and 0.1 M Na2SO4 at a pH of 6.4 (22). Cathodic polarization of the samples was performed using a potentiostat/ galvanostat (EG&G Princeton Applied Research, model 273A) with a graphite auxiliary electrode and a saturated calomel electrode (SCE) in a 125 mL electrochemical cell made by the Ace Glass Corporation (Vineland, NJ). In all of the measurements, the distance between the working electrode and the auxiliary electrode was approximately 1 mm. The open circuit potentials (OCP) of the samples versus the SCE reference electrode were measured using a handheld multimeter (Wavetek DM23XT) to establish the initial potential for the voltage scan. Scans were initiated 50 mV above the OCP, and the samples were scanned at a rate of 0.167 mV/sec. The scan was completed at -800 mVSCE. The measurements were conducted in sterile, oxygensaturated MSVP medium with 0.2 mM Mn2+ at a pH of 7.2 (22). Each measurement was repeated at least four times using new electrodes. Comparing the Effectiveness of Biomineralized Manganese Oxides and Oxygen as Cathodic Reactants. We operated two microbial fuel cells to compare the cathodic efficiency of biomineralized manganese oxides with that of oxygen used as cathodic reactants in microbial fuel cells. In the anodic compartments of both reactors, glucose was oxidized anaerobically by Klebsiella pneumoniae (ATCC no. 700831). As cathodic reactions, we used the reduction of biomineralized manganese oxides deposited by Leptothrix discophora SP-6 in the first fuel cell (Figure 2, eq 5), and the reduction of oxygen in the second fuel cell (eq 1).

Results and Discussion Microbial Fuel Cell Operation: Cell Potentials and Currents. Even though we operated the fuel cells several times, the results were similar, and in this paper we present only two representative results (Figure 4). From the measurements repeated several times, we concluded that the anodic potentials in our cells reached a potential of -441.5 ( 31 mVSCE, whereas the cathodic potentials showed a slow drift in time and reached, after 500 h operation, 384.5 ( 64 mVSCE. In repeated experiments, the average anode and cathode potentials were always within the ranges of the reported standard deviations. The variation in the cathodic potential may have been caused by the mixed potential of the electrode resulting from the presence of two cathodic reactants, manganese oxides and oxygen. Once the cell potential stabilized, the Mn2+ concentrations in the effluent decreased below our detection limit (4 mg/L). VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Temporal variation of power density generated by the microbial fuel cell. The average power density was 126.7 ( 31.5 mW/m2. We did not measure the power before 180 h because the microbial growth and the anodic and cathodic potentials were not at a steady state. In the effluent from the anodic compartment, the glucose concentration was less then 0.5 mg/L versus the inlet concentration of 1 g/L. The average potential difference between the electrodes was 809.30 ( 19 mVSCE, somewhat higher than most of the potentials reported in the literature (2, 5-7, 35) because the biomineralized manganese oxides in the cathodic compartment produced a higher potential than the potential generated by reducing oxygen, which was used by other authors as the cathodic reactants. The microbial fuel cell potentials reported in the literature vary widely depending on the type of anodic and cathodic reaction used and depending on the anodic and cathodic efficiencies of the cell used. The anode reactions reported in the literature vary between -300 and -500 mVSCE (14, 32, 36), and cathode potentials vary between +100 and +300 mVSCE (14, 36). We found the highest reported microbial fuel cell potential equal to 800 mV (36), which is close to the one typically measured in our cell. The results in Figure 4 show that as time progresses the potentials of the cathode and anode tend to decrease, and we do not have explanation for that, except for hypothetical changes in the properties of the electrodes’ surfaces resulting from biofilm buildup. For example, in pristine waters Leptothrix discophora deposits manganese oxides in close proximity to the metal surface and the potential of the metal is about +350 mVSCE (28), whereas in heavily polluted waters, Leptothrix discophora is out competed by other microorganisms, deposits manganese oxides away from the metal surface, and the metal’s potential is only about +200 mVSCE (27). The average potential decrease of the cathode was -0.29 ( 0.24 mV/h and, the decrease of the anode was -0.13 ( 0.19 mV/h. Because the potential of the cathode and the anode were changing in the same direction, these changes resulted in negligible overall variations in the cell potential. Because the cell potential was approximately constant, the power density remained approximately constant. Figure 5 shows the power generation in the microbial fuel cells. The average power density generated by the microbial fuel cell was 126.7 ( 31.5 mW/m2. It is important to note that after HNQ addition, the power density increased 10 times; the current reading taken prior to HNQ addition was 1.5 mA (through a 510 Ω resistor) and that taken after HNQ addition was 16.8 mA (through a 50 Ω resistor). The addition of a redox mediator increased the transport efficiency of electrons between the microorganisms and the electrode, as expected. We believe that the power density oscillations shown in Figure 5 were caused by the biofilm attachment and regrowth on the electrode surfaces. Comparing the Cathodic Efficiency of Biomineralized Manganese and Oxygen. The cathodic polarization curves 4670

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FIGURE 6. Cathodic polarization curves for the following samples: (1) biomineralized manganese on a graphite electrode, (2) electrochemically deposited manganese on a graphite electrode, (3) oxygen reduction on a graphite electrode. For the same electrode potential (e.g., -0.35 VSCE), reducing manganese oxides can deliver a current density up to 2 orders of magnitude higher than that delivered by reducing oxygen.

FIGURE 7. Variations in the cathode and anode potentials measured against the saturated calomel electrode (SCE). are shown in Figure 6. As expected, the chemically deposited manganese oxides and microbially deposited manganese oxides had similar cathodic polarization curves. However, the cathodic polarization curves measured for oxygen were different from those measured for manganese oxides: reducing manganese oxides could deliver a current density up to 2 orders of magnitude higher than the current delivered by reducing oxygen. Dickinson et al. (22) measured almost identical cathodic polarization curves using 316L stainless steel coupons in 0.01 M Na2SO2 as the supporting electrolyte. Our measurements were done in the growth medium. Figure 7 shows that when oxygen is used as the cathodic reactant the equilibrium potential of the graphite electrodes is 50.6 ( 30 mV, and when biomineralized manganese oxides are used as the cathodic reactant the equilibrium potential increases to 382 ( 58 mVSCE. Biomineralized manganese oxides increase the cell potential by approximately 300 mV as compared to the equilibrium potential reached in the presence of oxygen. The power density generated was 3.9 ( 0.7 mW/m2 when oxygen was used as the cathodic reactant and 126.7 ( 31.5 mW/m2 when manganese oxides and oxygen were used; the biomineralized manganese oxides increased the power density generated by the fuel cell. The results demonstrate that using biomineralized manganese oxides as the cathodic reactant in microbial fuel cells is superior to using oxygen. That may be caused by at least two factors, both related to the fact that manganese oxides are in the solid state and are attached to the electrode’s surface. Therefore: (1) manganese oxides are not subject to mass transport limitations, as oxygen is, and (2) their activity is always equal to one and does not change when the reactant is being used.

TABLE 1. Average Power Densities Obtained using Biomineralized Manganese Oxides and Oxygen as the Cathodic Reactants

(15)

cathodic reaction

power density

(16)

biomineralized MnO2 reduction oxygen reduction

126.7 ( 31.5 mW/m2 3.9 ( 0.7 mW/m2

(17)

Even though it is evident that microbial fuel cells produce less power than commercial abiotic fuel cells, they can offer distinct advantages. These advantages include their ability to generate power at the ambient temperature and in harvesting energy from the environment. As for possible applications, microbial fuel cells can power chemical sensors deployed at remote and inaccessible locations and telemetry systems to wirelessly transmit the data to remote receivers. The microbial fuel cell technology is at the early stages of development, and several imminent problems have to be solved before it can be implemented. One of these problems is selecting efficient metabolic reactions for the anodic and cathodic reactions in microbial fuel cells deployed in natural waters. We have demonstrated that manganese-oxidizing bacteria can produce an excellent cathodic reactant for microbial-fuel-cell manganese oxides. Manganese-oxidizing microorganisms are omnipresent in natural waters, and using manganese oxides as cathodic reactants in microbial fuel cells seems to be superior to using dissolved oxygen.

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Acknowledgments We gratefully acknowledge the financial support provided by the United States Office of Naval Research, contract N00014-02-1-0567.

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Literature Cited (1) Lee, S. A.; Choi, Y.; Jung, S. H.; Kim, S. Effect of initial carbon sources on the electrochemical detection of glucose by Gluconobacter oxidans. Bioelectrochemistry 2002, 57, 173-178. (2) Allen, R. M.; Bennetto, H. P. Microbial Fuel-Cells - Electricity Production from Carbohydrates. Appl. Biochem. Biotechnol. 1993, 39, 27-40. (3) Logan, B. E. Extracting hydrogen electricity from renewable resources. Environ. Sci. Technol. 2004, 38, 160A-167A. (4) Tsujimura, S.; Wadano, A.; Kano, K.; Ikeda, T. Photosynthetic bioelectrochemical cell utilizing cyanobacteria and watergenerating oxidase. Enzyme Microb. Technol. 2001, 29, 225231. (5) Kim, H. J.; Park, H. S.; Hyun, M. S.; Chang, I. S.; Kim, M.; Kim, B. H. A mediator-less microbial fuel cell using a metal reducing bacterium, Shewanella putrefaciense. Enzyme Microb. Technol. 2002, 30, 145-152. (6) Choi, Y.; Song, J.; Jung, S.; Kim, S. Optimization of the performance of microbial fuel cells containing alkalophilic Bacillus sp. J. Microbiol. Biotechnol. 2001, 11, 863-869. (7) Park, D. H.; Zeikus, J. G. Electricity generation in microbial fuel cells using neutral red as an electronophore. Appl. Environ. Microbiol. 2000, 66, 1292-1297. (8) McKinlay, J. B.; Zeikus, J. G. Extracellular iron reduction is mediated in part by neutral red and hydrogenase in Escherichia coli. Appl. Environ. Microbiol. 2004, 70, 3467-3474. (9) Katz, E.; Willner, I.; Kotlyar, A. B. A noncompartmentalized glucose vertical bar O-2 biofuel cell by bioengineered electrode surfaces. J. Electroanal. Chem. 1999, 479, 64-68. (10) Schroder, U.; Niessen, J.; Scholz, F. A generation of microbial fuel cells with current outputs boosted by more than one order of magnitude. Angew. Chem., Int. Ed. 2003, 42, 2880-2883. (11) Gregory, K. B.; Bond, D. R.; Lovley, D. R. Graphite electrodes as electron donors for anaerobic respiration. Environ. Microbiol. 2004, 6, 596-604. (12) Chaudhuri, S. K.; Lovley, D. R. Electricity generation by direct oxidation of glucose in mediatorless microbial fuel cells. Nat. Biotechnol. 2003, 21, 1229-1232. (13) Choi, Y.; Jung, E.; Kim, S.; Jung, S. Membrane fluidity sensoring microbial fuel cell. Bioelectrochemistry 2003, 59, 121-127. (14) Liu, H.; Logan, B. E. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence

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Received for review October 15, 2004. Revised manuscript received April 6, 2005. Accepted April 20, 2005. ES048386R VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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