Correspondence Comment on “Sustainable Power Generation in Microbial Fuel Cells Using Bicarbonate Buffer and Proton Transfer Mechanisms” The recent research paper by Fan et al. (1) on investigating sustainable power generation in microbial fuel cells (MFCs) using bicarbonate buffer and proton transfer mechanisms was quite impressive and presented many interesting results on the role of bicarbonate as a pH buffer and proton carrier in MFC performance. The description on the impact of a bicarbonate buffer during an MFC operation is, however, still unclear. The authors used a bicarbonate buffer (200 mM) and a phosphate buffer (50, 100, 200 mM) for their comparative experiment on MFC performance. MFCs adopt an anaerobic oxidation process in the anode compartment and operate in the neutral pH range, typically. Low pH conditions in MFCs as well as a conventional anaerobic process may be caused by two sources of acidity, H2CO3 and volatile fatty acids (VFAs), which are generated from microbial metabolism. In well-operated MFCs, the major requirement for alkalinity is neutralization of the H2CO3 which results from the high partial pressure of CO2 in the reactor. Because bicarbonate is the dominant species in the reactor, the metabolism-mediated carbonate buffer system plays an important role in the MFC performance. In their work, the effect of the carbonate buffer system generated from anaerobic microbial metabolism was not estimated and also not included in the proposed proton transfer mechanisms. Therefore I would like to comment on the effect of metabolism-mediated bicarbonate buffer in terms of pH and alkalinity in MFC. MFC is a bio-electro-chemical process that produces electricity from anaerobic oxidation of ready biodegradable organic substrate. Microbes in the anodic compartment produce electrons and protons in the dissimilative oxidizing process of the organic substrate, and CO2 as a final product. A typical electrode reaction occurring in the MFC can be expressed in terms of half-cell reactions using acetate as simple substrate (2, 3). Theoretically, the anaerobic oxidation of 1 mol of acetate produces 2 mol of CO2, corresponding to 2 mol of bicarbonate alkalinity. The concentration of bicarbonate in MFC reactor is significantly dependent upon carbon dioxide (CO2) partial pressure, pH, ionic strength, conductivity (or salinity), etc. Anode reaction C2H4O2 + 2H2O f 2CO2 + 8H+ + 8e-
(1)
+ C2H3O2 + 4H2O f 2HCO3 + 9H + 8e
(2)
or
Cathode reaction 2O2 + 8H+ + 8e- f 4H2O
(3)
C2H4O2 + 2O2 f 2CO2 + 2H2O + Electrical Energy
(4)
Overall reaction
As described by the Authors, use of a phosphate buffer in laboratory MFC experiments is common to control pH for electricity-generating bacteria and/or to increase the solution conductivity. However, CO2 produced from the anaerobic 10.1021/es800780d CCC: $40.75
Published on Web 07/09/2008
2008 American Chemical Society
oxidation automatically activates the carbonate buffer system in the reactor. Due to having a high Henry’s constant (kH,CC ) Caq/Cgas ) 0.8317 @ 298 K), CO2 in the reactor is converted into carbonic acid (H2CO3) that can quickly turn into bicarbonate (HCO3-) as an important component of the pH buffering system of water. The metabolism-generated alkalinity can produce from degradable cation-releasing organic components (such a protein), the salts of organic acids or soap, and sulfate/sulfite reduction (4). The carbonate ion is shown by these equilibrium reactions (5). + + 2(5) H2CO3 + 2H2O S HCO3 + H3O + H2O S CO3 + 2H3O
CO32- + 2H2O S HCO3- + H2O + OH- S H2CO3 + 2OH- (6) Figure 1 shows the distribution of carbonic acid fractions with temperatures of 5 and 25 °C and for the salinities of 0 and 35‰ as a function of the pH (6). Salinity is typically expressed as conductivity. The authors operated MFC reactors in the pH range of 7-9.5 with 200 mM bicarbonate. As shown in Figure 1, however, bicarbonate distributions are distributed in a wider range according to the pH value, illustrating the dependence of the carbon distribution on salinity and temperature. In the pH range tested by the authors, the peak fraction of bicarbonate moves toward pH 7.5 when temperature and salinity increase. Figure 2 represents the relationship between pH and alkalinity for water in equilibrium with the CO2 partial pressure of 0.01-1.0 atm for two ionic strengths, 0.0 and 0.2. The result represents that even a CO2 level as low as 1-3% in the gas (0.01-0.03 atm of partial pressure) has a significant impact on the pH of water in equilibrium. A doubling of the CO2 partial pressure results in a pH decrease of 0.3 units, whereas doubling of the alkalinity results in a pH increase of 0.3 units (4). In the author’s experiment, the alkalinity of the phosphate buffer (based on mass ratio of H2PO42-:HPO4- )1:1.87) is calculated to 3.4 g CaCO3/L (for 50 mM phosphate buffer) to 13.5 g CaCO3/L (for 200 mM phosphate buffer). The authors also used a 200 mM bicarbonate buffer, corresponding to 10 g CaCO3/L of alkalinity. The metabolism-mediated bicarbonate alkalinity through the anaerobic oxidation of substrate can be estimated by using Figure 2 (4). If the MFC had been operated under a continuous steady state condition, CO2 partial pressure in the headspace of MFC would be maintained at almost 1 atm. Then, the alkalinity at pH 7 would be produced in the range of approximately 9-17 g CaCO3/L for two ionic strengths, 0.0 and 0.2. Under the constant CO2 partial pressure, alkalinity is proportional to a pH level. Therefore, it could be inferred from these data that the author’s experiment was performed under considerably high bicarbonate concentrations in even MFC using a phosphate buffer. These experimental designs make it difficult to define the effect of bicarbonate in MFCs. The authors explained that as compared to that of pH 7 (0.7 atm), the CO2 partial pressure was much lower at pH 9 (0.005 atm), which could result in slowing down the CO2 loss through the cathode. In addition, they described that the increase of bicarbonate concentration might be a major reason for the reduced internal resistance and increased power density of MFCs at pH 9. However it is not clear if the CO2 partial pressure was actually measured. The author’s comparative description on the releasing rate of CO2 through the cathode under different pH conditions (pH 7 and 9) related to power density seems to be quite unreasonable for VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Distribution of carbonic acid fractions as percentages of the total carbon contents, CT. The values are calculated for temperatures of 5 and 25 °C and for the salinities of 0 and 35‰ as a function of the pH (6).
FIGURE 2. pH vs alkalinity for CO2 in contact with water (4). the continuous mode MFC if the MFC had well behaved under a steady state for a long-term period. The CO2 partial pressure of the MFC must be maintained to considerably high level, approximately 1 atm. In the proposed proton transfer mechanism, the authors describe that the monobasic phosphate/dibasic phosphate ion-pair accounts for a major mechanism for a proton transfer in air-cathode MFCs using the phosphate buffer solution, while bicarbonate-carbonate is a major proton carrier for MFCs using the bicarbonate buffer solution. In the article, there is, however, no evidence on the competitive availability of a buffer type as compared to a metabolism-mediated bicarbonate buffer. Therefore, further research on proton transfer mechanisms in MFC needs to be done to improve the understanding of the metabolism-generated bicarbonate alkalinity, which is not included in the proposed mechanism. In an anaerobic process under a typical operational condition, alkalinity problems are uncommon due to the metabolism-mediated bicarbonate buffer system and can also be minimized by a proper operational procedure. 6304
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Actually, the concentration of bicarbonate in wastewater significantly depends on the characteristics of substrate, from a few hundreds of mg/L of alkalinity as found in the domestic wastewater to several thousand mg/L as found in the piggery waste (7, 8). Actually there is little in the literature on the alkalinity deficit problem in MFCs, treating real wastewater due to the potential for developing metabolism-mediated alkalinity. A better understanding of a bicarbonate buffer system may allow exploiting MFC with higher performance. Consequently, to reach a conclusion on the effect of bicarbonate on MFC performance is premature without the detailed estimation of parameters such as bicarbonate, CO2 partial pressure, pH, salinity (conductivity), etc.
Literature Cited (1) Fan, Y.; Hu, H.; Liu, H. Sustainable power generation in microbial fuel cells using bicarbonate buffer and proton transfer mechanisms. Environ. Sci. Technol. 2007, 41, 8154–8158. (2) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder, U.; Keller, J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Microbial fuel cells: methodology and technology. Environ. Sci. Technol.
2006, 40, 5181–5192. (3) Rabaey, K.; Verstraete, W. Microbial fuel cells: novel biotechnology for energy generation. Trends Biotechnol. 2005, 23, 291– 298. (4) Speece, R. E. Anaerobic Biotechnology for Industrial Wastewaters; Archae Press: Nashville, TN, 1996. (5) http://en.wikipedia.org/wiki/Bicarbonate. (6) IAEA. Chemistry of Carbonic Acid in Water; 2003;cited 5 March 2008, available from http://www.iaea.org/programmes/ripc/ ih/volumes/vol_one/chi_i_09.pdf. (7) Ahn, Y. H.; Bae, J. Y.; Park, S. M.; Min, K. Anaerobic digestion elutriated phased treatment (ADEPT) of piggery waste. Water
Sci. Technol. 2004, 49 (5-6), 181–189. (8) Ahn, Y. H.; Logan, B. E. Low solids production using microbial fuel cells for power generation and domestic wastewater treatment. In Microbial Fuel Cells: First International Symposium; May 2008; pp 27-29.
Youngho Ahn School of Civil and Environmental Engineering, Yeungnam University, Gyungsan, 712-749, South Korea ES800780D
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