Multiphase Electrode Microbial Fuel Cell System that Simultaneously

Aug 5, 2010 - fuel cell (multiphase MFC). The multiphase MFC consisted of a floating electrode (FE), a midelectrode (ME), and a sediment electrode (SE...
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Environ. Sci. Technol. 2010, 44, 7145–7150

Multiphase Electrode Microbial Fuel Cell System that Simultaneously Converts Organics Coexisting in Water and Sediment phases into Electricity JUNYEONG AN,† HYUNSOO MOON,‡ AND I N S E O P C H A N G * ,† School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, South Korea, and School of Biological and Chemical Engineering, Yanbian University of Science and Technology, 3458 Chaoyang Street, Yanji, Jilin 133000, China

Received February 13, 2010. Revised manuscript received June 29, 2010. Accepted July 2, 2010.

Our challenge in this study was to harvest electricity from organics coexisting in two different phases (water and sediment) in an organics-contaminated benthic environment and to obtain increased current using a multiphase electrode microbial fuel cell (multiphase MFC). The multiphase MFC consisted of a floating electrode (FE), a midelectrode (ME), and a sediment electrode (SE) with no other components. The SE was embedded in sediment; the FE and ME were then overlaid in the water surface layer and in the middle of the water column of an aquarium, respectively. During continuous supply of organics at a COD loading rate of 94 mg of COD L-1 day-1 and after the cessation of organics being supplied at COD loading rates of 330 and 188 mg of COD L-1 day-1, the multiphase MFC showed the highest current production, as compared to the control MFCs [a floating-type MFC (FT-MFC) and two types of sediment MFCs (SMFC-A and SMFC-B)]. The total charges (in coulombs) of the multiphase MFC integrated from the currents, obtained under the three operating conditions mentioned above, were comparable to the sums of charges for the FTMFC and SMFC. As a result, this study found that the multiphase MFC can (1) utilize organics in the sediment similarly to SMFCs, (2) use organics in the water phase similarly to FTMFCs, and (3) obtain increased current analogous to the sum of an SMFC and a FT-MFC. Thus, it is thought that the multiphase MFC developed in this work could be suitable for use in water bodies being continuously or frequently contaminated with organic waste.

Introduction Since it was revealed that microorganisms can transfer electrons outside their cell wall to electrodes (1, 2), numerous studies pertaining to microbial fuel cells (MFCs) have focused on practical uses for this promising technology, such as for biological oxygen demand (BOD) sensing (3), wastewater * Corresponding author e-mail: [email protected]; phone: +8262-970-3278; fax: +82-62-970-2434. † Gwangju Institute of Science and Technology (GIST). ‡ Yanbian University of Science and Technology. 10.1021/es100498g

 2010 American Chemical Society

Published on Web 08/05/2010

treatment (4, 5), and electrical power generation (6-8), among others. In particular, sediment (or benthic) microbial fuel cells (SMFCs or BMFCs) are well-known devices for generating electricity from seawater or freshwater sediment through the activities of microorganisms in the anode (2, 7-12). Electrical power from SMFCs is generated by overlaying the cathode in an oxygen-rich aqueous phase and embedding the anode in sediment (2, 7-12), such that SMFC studies have developed a variety of methods for increasing the power output (6, 7, 11, 12). For SMFC operation, oxygenrich conditions in the water phase are maintained for electron consumption at the cathode or biocathode (13). However, in a natural environment, the water phase of water bodies such as lakes, ponds, rivers, and streams is not always in an oxygen-rich condition owing to pollution or contamination caused by nonpoint or point pollution sources (14, 15). This variance implies that, if there is an overload of organics or reduced inorganic compounds in water systems where an SMFC is (or was) installed, there could be potential a lack of oxygen in the aquatic environment, resulting in a deterioration of the water quality; consequently, the SMFC is likely to be affected by this lack or depletion of oxygen caused by the increase in oxygen demand (16). To this end, recently developed floating-type microbial fuel cells (FT-MFCs) have been used to demonstrate electricity generation from organics-contaminated water phases by placing the anode in an organic-rich water phase (i.e., oxygendepleted conditions) and exposing a part of the cathode electrode to the atmosphere (17). The cathodic oxidation potential of the MFC is derived from (1) the oxygen dissolved in the surface layer of water under organic-rich conditions and (2) oxygen directly transported from the atmosphere to the cathode electrode exposed from the water surface under organics-rich conditions. Simultaneously, the anodic exchange current is produced through the interaction between electrochemically active bacteria (EAB) and the electrode (17). Based on the previous studies mentioned above, we recognized that, to date (1) no previous research has yet considered harvesting electricity from organics in the sediment of an organic-contaminated benthic environment, probably owing to the oxygen depletion in the water phase, and (2) there have yet to be any substantive suggestions on how to increase the current by simultaneously using the organics coexisting in two different phases of a benthic environment, namely, the water phase and the sediment phase. In this study, assuming that temporal or consistent organic pollution has occurred in a water body, we examine the possibility of harvesting increased current by utilizing the organic materials coexisting in the water and sediment phases. As a solution, we propose a multiphase electrode microbial fuel cell (multiphase MFC) that is a combination of an FT-MFC and an SMFC. Note that this approach for increasing the current generation from organics existing in different material phases in an organics-contaminated benthic environment using a multiphase MFC has not been previously demonstrated.

Materials and Methods Materials. 1. Electrodes of Multiphase MFCs. Figure 1 shows the dimensions and configurations of the multiphase MFCs developed in this work, which consist of a floating electrode (FE), a midelectrode (ME), and a sediment electrode (SE). Plain graphite felt (Electrosynthesis, Amherst, NY) was used as the FE (7-cm diameter, 2.54-cm thickness), ME (5-cm diameter, 2.54-cm thickness), and SE (5-cm diameter, 2.54VOL. 44, NO. 18, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Configurations and dimensions of MFCs installed in the MFC operating system. Component abbreviations: FE, floating electrode; ME, midelectrode; SE, sediment electrode; multiphase MFC, multiphase electrode microbial fuel cell; FT-MFC, floating-type microbial fuel cell; SMFC-A, sediment microbial fuel cell type A; SMFC-B, sediment microbial fuel cell type B. An overall schematic of the system is illustrated in Figure S1 of the Supporting Information. cm thickness). An acrylic pipe (3-cm length, 0.6-cm inner diameter, 0.8-cm outer diameter) was then skewered into the center of the FE and ME (but not the SE) and fixed in the middle of the electrodes. 2. Electrodes of MFC Control Sets. Three different types of control MFCs [floating-type microbial fuel cell (FT-MFC), sediment microbial fuel cell type A (SMFC-A), and sediment microbial fuel cell type B (SMFC-B), as illustrated in Figure 1] were constructed to compare the current of the multiphase MFCs with those obtained by individual operations of the control MFCs. The dimensions and configurations of the FT-MFC, SMFC-A, and SMFC-B were derived from the multiphase MFC configuration: The FT-MFC consisted of an FE and an ME having the same electrode size and materials as used in the construction of the multiphase MFCs, SMFC-A was constructed with the same ME and SE as the multiphase MFCs, and SMFC-B consisted of the same FE and SE as the multiphase MFCs. Each MFC control set was operated in duplicate. 3. Current Collector. A titanium plate (0.1 cm × 3 cm × 2.5 cm, purity 99.9%) was used as the current collector, with one edge of a sheared titanium plate spliced to the terminus of a copper wire and sealed with water-resistant epoxy; the joint was then insulated with shrinkable waterproof tubing. Finally, to ensure a solid sealing of the joint, the tube was again sealed with a water-repellent epoxy. Operating System and MFC Installations. Figure S1 of the Supporting Information shows the installed MFC operating system, containing the multiphase MFCs and control MFCs. An acrylic aquarium (56 cm × 20 cm × 43 cm) was evenly divided into 10 sections using 13-cm-long acrylic partitions that were freely adjustable; five Teflon feeding tubes (1 mm inner diameter) were then mounted on the walls of the partition to induce artificial organic pollution. Next, the aquarium was filled with reservoir sediment, collected from a local water reservoir (35° 08′ 56.74′′ N, 126° 51′ 20.26′′ E) in Gwangju Metropolitan City, South Korea; horizontally paved to a thickness of 10 cm in the aquarium. Thereafter, the SEs of the multiphase MFCs with the electron collector were buried at a depth of 2 cm from the surface layer of the sediment. Then, a cylindrical acrylic stick (43-cm length, 0.5cm diameter) was inserted into the FE and ME pipes; one end of the stick was vertically fixed at the bottom of the aquarium as shown in Figure 1, where the FE, ME, and SE are shown to be vertically aligned. After that, one end of the 7146

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copper wires from the ME and SE were stranded and joined outside the system, with the other end connected to the electron collector of the ME to complete the electric circuit with the FE. The distances between the FE, ME, and SE in the multiphase MFC were each approximately 13 cm (Figure 1). Two of the four multiphase MFCs were operated under closed-circuit mode (CCM) using a 10 Ω load. The other two cells were used to monitor the open-circuit potential (OCP) of the electrodes: FE, ME, and SE. In addition, each electrode of the FT-MFCs, SMFC-A, and SMFC-B was installed at the same height and depth as the corresponding electrode of the multiphase MFC shown in the figure. Finally, 28 L of source water with no added acetate [chemical oxygen demand (COD) ) 10.3 ( 0.42 mg/L, dissolved oxygen (DO) concentration ) 5.3 ( 0.2 mg/L] was poured into the aquarium. Operating Conditions. In our previous study, the first version of the FT-MFC was inoculated with an anaerobic digestion fluid and operated using artificial wastewater (17). In this study, to simulate a real aquatic environment contaminated with organic waste, reservoir water was used as the inoculation source of the MFCs, as well as the feed solution for inducing pollution. However, the COD of the reservoir water (10.3 ( 0.42 mg/L) was too low to be directly used as the pollution source; thus, sodium acetate (Sigma Chemical Co., St. Louis, MO) was added to the source water as an organic pollution source. To observe the behavior of the MFCs under various organic strengths, autoclaved reservoir water containing sodium acetate (5, 3, and 1.5 mM) was fed into the systems at COD loading rates of 330, 188, and 94 mg of COD L-1 day-1, respectively, at a hydraulic retention time (HRT) of 26.7 days; these samples are referred to as C330, C188, and C94, respectively. Before the COD loading rates were changed to the next level, the MFCs installed in the system were operated under corresponding batch operating modes (B330, B188, and B94), the sequence of which was as follows: C330 f B330 f C188 f B188 f C94 f B94. Analyses and Instrumentation. The electrical wire from the MFCs was connected to a multimeter for voltage measurement (Keithley Instruments Inc., Cleveland, OH) on a personal computer; the current (I) was then calculated based on Ohm’s law, I ) V/R, where V is the voltage difference between the anode and cathode and R is the external circuit resistance used during experiments. In addition, the dissolved oxygen (DO) concentration, oxidation/reduction potential

FIGURE 2. (A) Current during batch operation at B330. Changes in (B) COD and (C) DO at FE and ME during operation.

FIGURE 3. (A) Current during batch operation at B188. Changes in (B) COD and (C) DO at FE and MF during operation. (ORP), and pH were measured using a 4-Star DO pH meter (Thermo Fisher Scientific Inc., Waltham, MA). Conductivity was measured using a 3-Star desktop conductivity meter (Thermo Fisher Scientific Inc., Waltham, MA), and the soluble chemical oxygen demand (SCOD) of the solutions where the FEs and MEs were placed (denoted as the FE level and ME level, respectively) was measured using a COD analysis system (HS-COD-LR&MR, Humas Co. Ltd., South Korea). The content of organic matter in the sediment [as volatile solids (VS)] was analyzed by drying the sediment (105 °C, 48 h) and then combusting the dried sample (550 °C, 5 h); the organic content of the sediment was 7.3 ( 0.6% (w/w). Finally, to monitor the electrode potentials, Ag/AgCl reference electrodes (MF-2052; Bioanalytical Systems Inc., West Lafayette, IN) were placed as close as possible (around 0.2 cm) to the cathode electrodes.

Results Electricity Generation of Multiphase MFCs under BatchMode Operation. The current generation of the multiphase MFC was compared to that of the control MFCs under batchmode operation at B330 and B188. After 1.7 days of batch operation (day 26), the current of the multiphase MFC started to become higher than that of the control MFCs, as shown in Figure 2A for a 2-day period. There was no current generation from SMFC-A, probably as a result of oxygen depletion at around the ME level due to COD supplied until the acetate supply stop (Figure 2B,C) (16); SMFC-B initiated current production at day 25. In addition, once the continuous acetate supply was stopped at C188 in Figure 3A, the current of the multiphase MFC gradually increased and peaked at

FIGURE 4. (A) Current during continuous acetate supply at C94. Changes in (B) COD and (C) DO at FE and ME during operation. 0.25 mA at day 54.7 [unless otherwise indicated, the standard deviations (mean ( SD, n ) 2) of all average currents are within less than 0.5% in this article]. Then, the current of the multiphase MFC horizontally aligned with that of SMFC-A at 0.080 ( 0.003 mA. Meanwhile, the current of the FT-MFC declined as a result of the stoppage of the acetate supply, which led to a limiting of the mass transfer of the substrate due to a drop in the COD, resulting in a DO increase in the ME level. After day 56, the current of the FT-MFC reached almost zero, and the current of the multiphase MFC declined and intersected with that of SMFC-B at 0.15 ( 0.01 mA (Figure 3A). In contrast, after the acetate supply stopped, the current of SMFC-A began to increase as a result of the COD and DO profile change; the multiphase MFC showed its highest current production during this 3-day period. These results clearly suggest that the multiphase MFC could be a viable solution for simultaneously increasing the current using organics that exist in two different phases. After 4.5 days of batch-mode operation at B188, the system was converted to a continuous acetate supply at C94. Electricity Generation of Multiphase MFCs under Continuous Operation. Figure 4A shows the current developed by the MFC during continuous acetate supply at C94. At day 59.7, the current of the multiphase MFC jumped and was produced in cycles. The peak current of the MFC was 0.28 mA. When compared to the multiphase MFC, but with a lower current, the FT-MFC showed a similar tendency in terms of production pattern; this behavior could possibly be due to the mass-transfer limitations incurred by substrate depletion (acetate). In this case, as the distance between the end of the feeding line and the MEs was 10 cm, the MEs might be competing with aerobes for the substrate. Figure 4B shows that, after 1.6 days of continuous acetate supply, the COD values increased from 7.2 ( 0.2 to 25.1 ( 0.9 mg/L (FE level) and from 7.8 ( 0.3 to 20.4 ( 0.2 mg/L (ME level); VOL. 44, NO. 18, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (A) Schematics of the closed circuits converted from the multiphase MFC. (B) Comparison of the current development of (1) closed circuits of the multiphase MFC [(a) FE-ME, (b, d, and g) multiphase MFC, (c and e) FE-ME, (f) FE-SE] with those of the control (9) FT-MFC and (b) SMFC-B. (h,i) Points at which acetate was supplied at C188, for 30 min each. for the same period, the DO concentrations of the FE (2.01 ( 0.04 mg/L) and ME (1.87 ( 0.04 mg/L) levels dropped to 1.78 ( 0.04 and 1.02 ( 0.03 mg/L, respectively. By changing the DO and COD profiles, SMF-A, having previously shown a current generation of 0.1 mA at B188, declined to -29 ( 3 µA (at day 63.2). During operation at C94, SMFC-B generated 0.12 ( 0.01 mA of stabilized current. Based on these results, it could be inferred that the increased current of the multiphase MFC during continuous acetate supply at C94 could be affected as a function of ME in the MFC. The multiphase MFC showed a higher production rate in terms of current than the control during a 6-day period. Electric Circuit Conversion of Multiphase MFCs. To establish the role of each electrode of the multiphase MFC, a closed-circuit conversion test for the MFC was sequentially conducted under the conditions of B188. During the test, the circuits for the FE, ME, and SE of the multiphase MFC were disconnected and promptly (within 3 s) rearranged to make closed circuits of FE-ME or FE-SEsthe same as in the control FT-MFC and SMFC-B, respectivelysunder a 10 Ω load. Figure 5A presents the schematic for the converted closed circuits of the multiphase MFC. The closed circuit of FT-MFC and SMFC-B converted from the multiphase MFC behaved remarkably similarly in terms of current production rate to the control sets of FT-MFC and SMFC (Figure 5B). When the closed circuits of the multiphase MFC were converted into FE-SE (a) and FE-ME (c and e), the current of each closed circuit immediately dropped to be comparable to that of the control MFC relevant to the converted circuit (i.e., the current of the converted circuit FE-SE was similar to that of the control SMFC-B). In addition, at b and d in Figure 5, upon conversion of the FE-SE and FE-ME circuits into a multiphase MFC circuit, the currents immediately caught up to the expected course being developed from the initial current of the multiphase MFC being tested. Thereafter, the FT-ME circuit (e) converted from the multiphase MFC was switched into an FE-SE circuit (f); 2 h later, the FE-SE was converted into a multiphase MFC (g)sthe pattern in current production was similar to that in the control MFC. After sequential switches in circuits, when acetate was supplied to C188 at h and i, for 30 min at each point, the multiphase MFC showed a slight increase in current as a function of acetate supply, relative to the control FT-MFC. In contrast, the current from SMFC-B decreased. These results, based on electric circuit change tests, show that the multiphase MFC amplifies the current to levels corresponding to the sum of the control FT-MFC and SMFC-B. Hence, it is posited here that electrons produced in the ME and SE of the multiphase MFC are coupled and consumed in the FE of the MFC. In this case, the FE of the combined MFC acts as a 7148

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cathode; the SE and ME could function as anodes. However, categorizing the role of the ME in the multiphase MFC can be considered somewhat controversial. As probable roles, we suspect that (1) the ME of the multiphase MFC might function as an electrode for both electron consumption and production and (2) the ME might work as an anode (not cathode) against the FE [i.e., two anodes (ME and SE) for a single cathode (FE)]. Calculation of Coulomb Generation of Multiphase MFCs and Control MFCs. The results reported above indicate that the electrons of the ME and SE of the multiphase MFC contribute to increasing the current during operation at B330, B188, and C94. Thus, we posited that, if the electrons produced in the ME and SE of the multiphase MFC cause an increase in the current, the electrons produced in the MFC could be conserved. Consequently, the total numbers of coulombs transferred in the multiphase MFC are thought to be equal to the sum of the total numbers of coulombs in the FT-MFC and SMFC. Based on this assumption, the coulombic sums of the pairs FT-MFC/SMFC-A and FT-MFC/SMFC-B were calculated for subsequent comparison with the total numbers of coulombs of the multiphase MFC. The total numbers of coulombs (C) of the MFCs were integrated to investigate the current produced over the days of the integrated sanctions, as shown in Figures 1A, 2A, and 3A. The total charges obtained from the multiphase MFC at B330, B188, and C94 were comparable to the sums of the charges of the control FT-MFC and SMFC-B (Table 1). The SMFC-A produced positive charges of 73.6 and 7.8 C at B188 and C94, respectively; however, the total charges transferred in FTMFC were greater than those for SMFC-A alone. In addition, the fact that the sum of FT-MFC and SMFC-A at B330 did not approximate the total charge of the multiphase MFC could be further evidence that the electrons of the ME and SE were associated.

Discussion Applicability. For operation of SMFCs, oxygen-rich conditions in the water phase are required because the DO concentration affects the performance of SMFCs (16) (i.e., the higher the DO concentration, the higher the current). In contrast, FT-MFCs could be used as an alternative for producing electricity from a water body where waste organic is always abundant, in which oxygen levels are depleted (17). However, no previous reports have suggested an effective means for how to simultaneously harvest electrical energy using the organics of the sediment and the water phase from an organic-contaminated benthic environment. Accordingly, assuming that contamination of a water body occurs continuously or temporally, in this study, we operated benthic

TABLE 1. Total Charge (in Coulombs) of the MFCs, Integrated with the Current Obtained in the Integrated Sections of Figures 1A, 2A, and 3A, along with Coulombic Sums (FT-MFC Plus SMFC-A, FT-MFC Plus SMFC-B) for Comparison operating mode

FT-MFC (C)

SMFC-A (C)

SMFC-B (C)

multiphase MFC (C)

sum of SMFC-B and FT-MFC (C)

sum of SMFC-A and FT-MFC (C)

batch (B330) 79.9 -9.6 100.0 181.0 180.0 70.3 batch (B188) 125.0 73.6 443.0 578.0 568.0 198.0 continuous (C94) 371.0 7.8 886.0 1170.0 1260.0 378.7 *Figures S2 and S3 of the Supporting Information illustrate development of current under continuous acetate supply at C330 and C188.

MFCs in a simulated microcosm using source reservoir water without addition of nutrients. In addition, the FT-MFC used in this study has a totally different configuration from the first-version FT-MFC, which consisted of electrodes, an anode compartment, and a membrane (17). The reason we simplified the configuration of the FT-MFC was to minimize the number of electrodes used and to facilitate its construction and application to natural water bodies. As a result, we found that the current generation of the FT-MFC and SMFC could be simultaneously limited by a lack of substrate and oxygen depletion, respectively. Based on our results illustrated in Figure 2, when the COD and DO concentration of the water phase in the aquarium were maintained at approximately 15-40 and 0.2 mg/L, respectively, in the ME level [20-40 mg/L (COD) and 0.7 mg/L (DO) in the FE level], the current generation of the FT-MFC and SMFC was limited because of insufficient DO and organics, respectively. The multielectrode MFC, nevertheless, produced an increased current corresponding to the sum of the currents obtained from the FT-MFC and SMFC. The coupled current from the multiphase MFC indicates that a decrease in current generation of the SMFC due to oxygen depletion is offset by the FE of the FT-MFC and that a decrease in the current generation of the FT-MFC due to a lack of organics is offset by electrons produced in the SE of the SMFC. In a natural water environment, numerous water bodies suffer from organic pollution throughout the world; the DO concentration in the water phase changes with organic contamination as time passes. Thereby, electricity generation using an FT-MFC and SMFC would be naturally affected by the concentration of organics changing the concentration of DO in water phase. To overcome these defects in MFCs, the multiphase electrode MFC developed in this work could be suitable for application in all water bodies being continuously or frequently contaminated with organic waste. Hence, advantages of multiphase MFCs include their ability to (1) utilize organics in the sediment of an organicscontaminated benthic environment similarly to an SMFC, (2) use organics of the water phase similarly to an FT-MFC, and (3) obtain an increased current analogous to the sum of those of the SMFC and FT-MFC, by complementing the defects of FT-MFCs and SMFCs. However, for a wider application of MFCs to various concentrations of organics in the water phase, improving the SE and FE should proceed first (as suggested in the next section). Limitations and Improvement. In this study, during operation at B330, B188, and C94, the multiphase MFC simultaneously treated organics existing in water and sediment phases. However, the multiphase MFC did not always produce an increased current. Figures S2A and S3A and Table S1 of the Supporting Information show that the multiphase MFC generated a lower current than the FT-MFC during operation at C330 or C188. One of the causes of this limited production could be that the ME of the multiphase MFC overlaid in the solution had a greater reducing power and faster kinetic activity than the SEs buried in the sediment. Under the operating conditions at C330 and C188, the electrons produced in the ME of SMFC-A

flowed toward the SE of the MFC (Figures S2A and S3A of the Supporting Information). In addition, the OCP of the ME, monitored under open-circuit mode during continuous operation at C188, was always more negative, by an average -30 ( 7 µV, than that of the SE (Figure S3B of the Supporting Information). As such, the SE had a relatively higher equilibrium potential and slower kinetic activity compared to ME, inferred to be due to the mass-transfer limitations of the SE in the MFC (7, 10, 18). Consequently, during operation at C330 and C188, a portion of the electrons produced in the ME of the multiphase MFC flowed in a reverse direction toward the SE, and thus the total charge in the FT-MFC was observed to be higher than that of the multiphase MFC (Table S1 of the Supporting Information). Thus, the total charge transferred in the multiphase MFC was more comparable to the sum of the charges of the FT-MFC and SMFC-A (not SMFC-B) (Table S1 of the Supporting Information). Second, it is thought that limitations could be due to the cathodic reaction of the FE in the multiphase MFC. In this work, SMFC-A did not produce electricity under most operating conditions. Although the MFC generated 0.1 mA of current at B188 for 3 days, this generation was due to the temporal increase in the ME-level DO concentration to 1.86 ( 0.03 mg/L when the acetate supply was stopped (Figure 3A). In addition, SMFC-B temporarily produced a current for 8 days (Figure S2A and Table S1 of the Supporting Information), but after day 8, there was no current production from the MFC owing to a continuous COD influx, as the DO concentration at the FE level of the system was maintained at less than 0.10 ( 0.02 mg/L (Figure S2B of the Supporting Information). To confirm whether the cathodic reaction limitation of the FE in MFCs at C330 was due to a lack of oxygen, 1.27 cm of the FEs (tested using the multiphase MFC, FT-MFC, and SMFC-B, not SMFC-A) in the system were exposed to the atmosphere (Figure S4 of the Supporting Information). This exposure led to a considerable increase in current of the multiphase MFC and FT-MFC, thus clearly showing that the cathodic reaction of the tested MFCs was limited by oxygen depletion caused by an influx of organic materials (17). Therefore, for the application of a multiphase MFC to a natural body of water contaminated with organic waste, improving the FE and SE by using different electrode materials or chemical reformations is recommended. To date, noble metals such as platinum have been widely used as catalysts to increase the kinetic activities of the cathode electrode of MFCs during the oxidation-reduction reaction (ORR) (19). In addition, binary alloys of transitional metals with platinized carbon (e.g., a Pt-Co alloy) have been reported as potential ORR catalysts for MFCs (19, 20). To further enhance the kinetic activity of the SE, chemical reformation using anthraquinone1,6-disulfonic acid (AQDS) could be considered (7). In addition, to increase the equilibrium potential of the SE, selection of electrode materials might be worth consideration. Recently, a number of studies related to SMFCs have attempted to obtain electrical energy from organics in the sediment of oxygen-rich benthic environments (6-12). Nevertheless, an extensive study of harvesting electrical VOL. 44, NO. 18, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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energy using SMFCs from an organic-contaminated benthic environment has yet to be reported. Herein, we developed a multiphase MFC having a dual-function FT-MFC and SMFC. This new concept might allow the application of MFC technology to be extended to various water conditions in the natural environment, as a complementary MFC system to FT-MFCs and SMFCs.

Acknowledgments This work was supported by a grant from Doyak (previously, the NRL Program) funded by the National Research Foundation (NRF) of the South Korean government (MEST).

Supporting Information Available Figure S1 illustrates the multiphase MFC operating system; Figure S2 shows the current development from MFCs and the change of DO in the aquarium during continuous acetate supply at C330; Figure S3 contains the developments of current and electrode potential from MFCs, and the change of DO in the aquarium during continuous acetate supply at C188; Figure S4 presents the effect of exposing the floating electrodes of the MFCs to the atmosphere; Table S1 illustrates the total coulombs of MFCs and Coulombic sums, obtained during continuous acetate supply at C330 and C188. This material is available free of charge via the Internet at http:// pubs.acs.org.

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