Carbon Dioxide Addition to Microbial Fuel Cell Cathodes Maintains

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Environ. Sci. Technol. 2010, 44, 2728–2734

Carbon Dioxide Addition to Microbial Fuel Cell Cathodes Maintains Sustainable Catholyte pH and Improves Anolyte pH, Alkalinity, and Conductivity JEFFREY J. FORNERO,† MIRIAM ROSENBAUM,‡ MICHAEL A. COTTA,§ AND L A R G U S T . A N G E N E N T * ,‡ Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, One Brookings Drive, CB 1180, St. Louis, Missouri 63130, Department of Biological and Environmental Engineering, Cornell University, 214 Riley-Robb Hall, Ithaca, New York 14853, and Fermentation Biotechnology Research Unit, United States Department of Agriculture, Agricultural Research Service (ARS), National Center for Agricultural Utilization Research (NCAUR), Peoria, Illinois 61604

Received October 20, 2009. Revised manuscript received January 27, 2010. Accepted February 10, 2010.

Bioelectrochemical system (BES) pH imbalances develop due to anodic proton-generating oxidation reactions and cathodic hydroxide-ion-generating reduction reactions. Until now, workers added unsustainable buffers to reduce the pH difference between the anode and cathode because the pH imbalance contributes to BES potential losses and, therefore, power losses. Here, we report that adding carbon dioxide (CO2) gas to the cathode, which creates a CO2/bicarbonate buffered catholyte system, can diminish microbial fuel cell (MFC) pH imbalances in contrast to the CO2/carbonate buffered catholyte system by Torres, Lee, and Rittmann [Environ. Sci. Technol. 2008, 42, 8773]. We operated an air-cathode and liquid-cathode MFC side-byside. For the air-cathode MFC, CO2 addition resulted in a stable catholyte film pH of 6.61 ( 0.12 and a 152% increase in steadystate power density. By adding CO2 to the liquid-cathode system, we sustained a steady catholyte pH (pH ) 5.94 ( 0.02) and a low pH imbalance (∆pH ) 0.65 ( 0.18) over a 2-week period without external salt buffer addition. By migrating bicarbonate ions from the cathode to the anode (with an anion-exchange membrane), we increased the anolyte pH (∆pH ) 0.39 ( 0.31), total alkalinity (494 ( 6 to 582 ( 6 as mg CaCO3/L), and conductivity (1.53 ( 0.49 to 2.16 ( 0.03 mS/cm) relative to the feed properties. We also verified with a phosphate-buffered MFC that our reaction rates were limited mainly by the reactor configuration rather than limitations due to the bicarbonate buffer.

Introduction Bioelectrochemical system (BES) technology holds promise to produce environmentally benign and sustainable energy * Corresponding author tel.: +1-607-255-2480; fax: +1-607-2554080; e-mail: [email protected]. † Washington University in St. Louis. ‡ Cornell University. § National Center for Agricultural Utilization Research (NCAUR). 2728

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from wastewaters, replace energy intensive wastewater treatment processes, and produce chemical products (1-6). A BES can be configured as a microbial fuel cell (MFC), or as a microbial electrolysis cell (MEC) by adding an externally applied potential (7-9). With either configuration, microbes oxidize organic substrates to supply electrons to the anode. The electrons then travel through an external circuit to the cathode and participate in reduction reactions (2, 4, 5, 10). Associated with these reactions is the generation of protons and hydroxide ions (with an oxygen reduction reaction [ORR]) in the anolyte and catholyte, respectively, which creates a BES pH imbalance in systems with membranes. The ion migration, which satisfies the electroneutrality constraint, does not relieve the pH imbalance because cations or anions other than protons or hydroxide ions (dependent upon ionexchange membrane selection) mediate the charge balance (11-15). And since the pH imbalance produces a 0.059 V/pH BES potential loss, minimizing the pH imbalance is necessary for maximizing BES power densities (14). In most lab-scale BESs with liquid catholytes, these potential losses are omitted by using nonsustainable phosphate buffer electrolytes. However, with the intention of developing realistic BESs, novel sustainable solutions have to be developed. Air-cathode BESs may be even more susceptible to pH imbalance losses than liquid-cathode BESs because of the small liquid volume that permeates across an air-cathode membrane (16). Within the limited catholyte volume (i.e., catholyte film), ORR reactions more quickly increase the catholyte hydroxide ion concentration, and therefore the pH, compared to liquid-cathode BESs. To mitigate an air-cathode pH increase, Torres et al. performed a ground-breaking study to demonstrate the concept of adding carbon dioxide (CO2) to an air-cathode to buffer the pH shift and increase the power density (17). The CO2 reacted with hydroxide ions to form carbonate species (CO32- and HCO3-), which became the highest concentration anions in the catholyte. With an anion exchange membrane (AEM), the carbonate species were transported from the cathode to the anode to maintain electroneutrality and, in essence, served as the vehicle to transfer hydroxide ions and this stabilized the catholyte pH. While the pH of the air-cathode liquid film was not reported, Torres et al. did find that CO32was the primary carbonate species migrating (17). On the basis of carbonate species equilibrium, we estimated the catholyte pH to be g10.5, and therefore, the cathode pH remained much higher than the anolyte pH (7.3 to 7.5), resulting in a BES pH imbalance of ∼3.1 (17, 18). The authors identified the slow rate of CO2 absorption into the catholyte as a limitation factor for the use of carbonate species as hydroxide ion carriers (17). Under conditions of sufficient CO2 absorption, we calculated that the catholyte pH could theoretically be maintained between 3.92 and 5.63 with bicarbonate as the dominating ion species (details in the Supporting Information [SI]) (19). Since this would reduce the pH imbalance, increase the ORR potential, and therefore increase the power density; we hypothesized that a CO2/bicarbonate buffered catholyte would improve MFC performance in comparison to the CO2/ carbonate buffered catholyte system described by Torres et al. (17). Our objective was, thus, to determine if continuous CO2 addition to an air-cathode and a liquid-cathode MFC with an engineered system to provide sufficient CO2 absorption would create a CO2/bicarbonate buffered catholyte system that could theoretically diminish the pH imbalance. In addition, we examined the influence of the bicarbonate ion migration on anolyte properties (pH, alkalinity, and 10.1021/es9031985

 2010 American Chemical Society

Published on Web 02/24/2010

FIGURE 1. MFC-LC catholyte circulation with CO2 contactor column. The catholyte is at CO2 equilibrium ([H+] ) 1.15 × 10-6; [HCO3-] ) 8.0 × 10-3) before entering the MFC cathode chamber. Within the cathode chamber, the oxygen reduction reaction generates eight hydroxide ions (per mol acetate), which increases the catholyte hydroxide ion concentration. To maintain electroneutrality, eight bicarbonate ions migrate from the cathode to anode. The catholyte exiting the MFC, therefore, has a higher hydroxide ion and lower bicarbonate ion concentration than the catholyte entering the cathode. Within the CO2 contact column, the catholyte is exposed to 100% CO2 gas while passing over a high surface area packing to ensure sufficient CO2 absorption. After absorption, the eight CO2 molecules hydrate and dissociate to produce eight protons, which combine with the hydroxide ions, and eight bicarbonate ions as a reaction product. Thus, the cathode hydroxide ion production is neutralized, the bicarbonate ions lost to migration are replenished, CO2 equilibrium is restored, and the pH remains stable. conductivity) and identified the rate-limiting step in achieving higher power densities. To test our hypotheses, we used two 10-L MFCs with AEMs; one operated as an air-cathode and the other one as a liquid-cathode. The air-cathode MFC, which only had a small volume of water in the cathode chamber that permeated from the anolyte, was operated with an air/CO2 mixture (MFC-AC) and with air-only (MFC-AO). The liquid-cathode MFC, which was filled with water, was operated with a CO2/bicarbonate buffered catholyte (MFCLC), nonbuffered air-only catholyte (MFC-LO), and conventional phosphate-buffered catholyte (MFC-LP).

Materials and Methods Experimental Setup. Two identical tubular, upflow MFCs (UMFCs) were used. Each UMFC consisted of an inner cathode chamber (total and net volume of 1.96 and 1.64 L, respectively) and an outer anode chamber (total and net volume of 8.49 and 5.80 L, respectively) separated by a tubular AEM. The cathode (with platinum catalyst) and anode were made of graphitized (unsized) carbon fabric cloth. Details of the experimental setup and figures are included in the SI. The liquid-cathode MFC included a catholyte circulation system with two small liquid degassers that were open to the atmosphere, and a clear PVC CO2 contact column (75 cm long × 6 cm ID) filled with high surface area packing (Figure 1 and Figure S2 of the SI). Anodic gas production was measured with a gas meter (Ritter MGC-1 Milligas Counter, Bochum, Germany). Operation. The anolyte inoculum was a homogenized granular sludge from an upflow anaerobic bioreactor at a brewery (Anheuser Busch-Inbev, Inc., St. Louis, MO). Acetate synthetic wastewater (∼480 mg/L total chemical oxygen demand [TCOD] concentration) was fed to each of the MFCs at a continuous flow rate of ∼12.7 L/day (∼11-h anode hydraulic retention time). A mechanical agitator (Model 5vb, EMI Inc.; Clinton, CT) slowly mixed the feed container. The synthetic wastewater consisted of (per liter of deionized water; all chemicals, unless noted, were from Sigma Aldrich, St.

Louis, MO) the following: glacial acetic acid, 0.268 mL; 4 M sodium hydroxide, 0.265 mL; yeast extract (Difco Laboratories, Inc., Detroit, MI), 0.025 g; NH4Cl, 0.03 g; K2SO4, 0.006 g; FeCl2 · 4H2O, 0.033 g; K2HPO4, 0.033 g; NaHCO3, 1.0 g; iron citrate, 0.011 g; NaCl, 0.25 g; KCl, 0.1 g; CaCl2, 0.1 g; MgCl2 · 6H2O, 0.1 g; and trace elements, 1.0 mL modified from refs 20 and 21. Feed to the MFC anodes was distributed through six nozzles (for each MFC), evenly spaced and placed on alternating opposing sides along the length of the MFCs (Figures S2 and S3 of the SI). The anolyte for each MFC was recirculated (58 ( 0.3 L/day) from the top to the bottom of the bioreactor. The anolyte effluent flowed through a liquid degasser to separate the biogas for measurement prior to anolyte discharge. The cathode of the air-cathode MFC was supplied directly either with a humidified air/CO2 mixture (79% air/21% CO2 mixture for the MFC-AC or solely air for the MFC-AO) at a rate of 410 L/day. The air-cathode had a drain line and collection flask that was used to collect cathode permeate water ( ∼6.0), the cell potential began to decline because of a decreasing ORR potential (data not shown). Therefore, daily water replenishments (∼2.5% of the catholyte volume) with RO water (pH 5 at atmospheric CO2 levels) to lower OH- concentrations in the catholyte were established to stabilize the pH at 5.94 and maximize the MFC-LC potential. The polarization tests, which were performed at the lower and the higher pH, further 2732

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revealed a need for cation transfer at a low pH of 5.25, because the bicarbonate concentrations were too low to fulfill the required ion migration to maintain electroneutrality. These curves show prominent mass-transport losses at higher current densities for the pH 5.25 plot, whereas the pH 5.94 plot does not indicate such losses (Figure S5 of the SI). At a pH of 5.94, the bicarbonate ion concentration was sufficient to maintain electroneutrality ([HCO3-] ) 8.0 × 10-3 M) at the rate demanded by the system, and the ohmic losses dominated the polarization curve. This information indicates that cations were transported from the anode to the cathode (0.004% of current, calculations in the SI) to counterbalance sluggish bicarbonate transport, especially during periods with a relatively low bicarbonate concentration (i.e., at a pH of 5-6; pKA1 ) 6.37). We ensured by leak tests that no anolyte to catholyte leakage occurred and calculated the rate of CO2 consumption vs the rate of CO2 hydration to show that the hydration rate was not limiting our buffer system (calculations in the SI). Sufficient CO2 was absorbed to the catholyte to buffer OHformation, but the resulting bicarbonate concentration was still too low to be efficiently transported to the anolyte to balance the pH. Therefore, cations moved from the anolyte to catholyte to maintain electroneutrality, as described above. We consider both passive cation diffusion and active cation “forcing” as driving forces for this cation transport across the AEM. Even though the AEM is designed to retard cation diffusion due to positive charge sites within the membrane pores, some diffusion of cations is likely, especially since the cation concentration gradient is favoring diffusion. At the pH of 5.25, the catholyte bicarbonate ion concentration was relatively low ([HCO3-] ) 6.6 × 10-4 M) and insufficient to support the migration flux demand with higher current densities. In this situation, active cation “forcing” may have been the stronger driving force compared to diffusion to transport cations from the anode to the cathode to maintain electroneutrality (we did not quantify diffusion and forcing). Regardless of the relative importance of diffusion and forcing, cation transfer increased the pH and, thus, the catholyte bicarbonate ion concentration (pKA1 ) 6.37). Cation forcing would have become less important when the pH increased further toward the pKA1 (but we prevented a pH increase beyond 5.94). BES Performance Trade-offs with a CO2/Bicarbonate Buffered System. From this analysis, it becomes clear that changes in catholyte pH influence the CO2/bicarbonate buffered system both positively and negatively and, therefore, performance trade-offs must be considered when selecting for an optimum catholyte pH. Compared to a balanced anolyte/catholyte (i.e., ∆pH ) 0), a catholyte pH greater than the anolyte pH creates a pH imbalance loss and decreases the ORR potential; whereas a catholyte pH lower than the anolyte pH (MFC-AC; MFC-LC; MFC-LP [Table 1]) creates a pH imbalance loss (0.059 V/pH) that is offset by an equivalent ORR potential increase (0.059 V/pH) (24). In addition, the catholyte bicarbonate ion concentration affected the bicarbonate ion concentration gradient across the AEM, and consequently the bicarbonate ion diffusion. Since the catholyte bicarbonate ion concentration was lower than in the anolyte (8.0 × 10-3 vs 1.7 × 10-2 M), an anode to cathode bicarbonate ion concentration gradient existed. The bicarbonate ion diffusion, therefore, was moving in opposite direction to the bicarbonate ion migration, which increased ohmic losses in our system (23). A higher catholyte pH and bicarbonate concentration would align the bicarbonate ion diffusion with migration. In addition, a higher catholyte pH and bicarbonate ion concentration would increase the catholyte conductivity, which would decrease the ohmic resistance, however, at the cost of ORR potential losses. Thus, it is critical for an engineer to optimize the pH in this CO2/

bicarbonate buffered system to balance potential losses and gains. With our work, we have shown that the daily addition of RO water was sufficient to maintain this optimized pH for maximized power production. Bicarbonate Migration Influences Anolyte Properties. Increased pH and TALK. Under steady-state operating conditions, the MFC-LC bicarbonate ion migration increased the anolyte pH (6.57-6.96, ∆pH ) 0.39) and TALK (494-582 as mg CaCO3/L) relative to the feed solution (Table 1), which is beneficial for BES wastewater treatment. During the anodic biocatalytic oxidation of organic substrates, typically more protons than bicarbonate ions are produced. The proton generation decreases the anolyte pH and lowers the alkalinity, which can increase pH imbalance potential losses, or, at an extreme, negatively affect the anode microbial community (25). Thus, increasing the anolyte pH and alkalinity with bicarbonate can decrease the pH imbalance, increase the alkalinity, and help maintain a healthy microbial community. Increased Conductivity. The MFC-LC bicarbonate ion migration also increased the anolyte conductivity (1.53-2.16 mS/cm) relative to the feed solution (Table 1). Because some wastewaters have a high organic content and a low conductivity, BES treatment is impaired because of a high anolyte ohmic resistance (26, 27). Thus, an intrinsic increase in anolyte conductivity is desirable. Furthermore, the addition of bicarbonate ions to wastewater is compatible with wastewater treatment objectives, whereas phosphate additions may require subsequent removal to ensure compliance with effluent discharge permit specifications. Using a CO2/ bicarbonate instead of a phosphate buffered catholyte, therefore, complements wastewater treatment objectives. Practical Impacts of This Study. By adding CO2 to an air-cathode MFC with sufficient cathode surface area and a liquid-cathode MFC with sufficient gas/liquid exchange, we controlled and sustained the catholyte pH. Bicarbonate migration improved anolyte qualities, which are favorable for decreasing BES potential losses, increasing power densities, and improving BES wastewater treatment. In industrial settings (e.g., breweries), CO2 is available as a waste gas, and an increase in TALK in the wastewater effluent is desirable in anaerobic bioreactors for brewery wastewater treatment. This would make an implementation of this CO2 buffered MFC system very promising due to a significant cost reduction by offsetting external alkalinity sources (e.g., MgOH2). Calculations at a desired current density of 10 A/m2 (see the SI) showed that CO2 dissolution will not be limiting at higher reactor performance. We believe pressurizing the cathode system (higher gas partial pressure) to increase the ORR potential and bicarbonate ion concentration while minimizing pH imbalance losses (20, 28) will help overcome the bicarbonate migration rate-limitation and increase the power density.

Acknowledgments The financial support for this work was provided through a specific collaborative agreement between Largus Angenent and the Bioenergy Research Unit, USDA, Agricultural Research Service, Peoria, Illinois, and the National Science Foundation through grant no. 0645021. We thank Brian Wood of Arelco for the provision of the tubular membranes, Pat Harkins for his significant fabrication contributions, Dan Shannon of Zoltek for the provision of the carbon cloth, and Vipul Borkar for his lab support.

Supporting Information Available Calculations for the catholyte pH based on the carbon dioxide (CO2) partial pressure, catholyte carbon dioxide gas-liquid transfer rates, cation transport estimate calculations, experimental set-up details, MFC fabrication photos, experimental data, and the question of whether a CO2/bicarbonate

buffer can support high current densities. This information is available free of charge via the Internet at http:// pubs.acs.org.

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