alexa Deterioration in the Cathode Performance during Operation of the Microbial Fuel Cells and the Restoration of the Performance by the Immersion Treatment | Open Access Journals
ISSN: 1948-5948
Journal of Microbial & Biochemical Technology
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Deterioration in the Cathode Performance during Operation of the Microbial Fuel Cells and the Restoration of the Performance by the Immersion Treatment

Osamu Ichihashi and Kayako Hirooka*

River Basin Research Center, Gifu University, Japan

*Corresponding Author:
Kayako Hirooka
River Basin Research Center
Gifu University, 1-1 Yanagido
Gifu City 501-1193, Gifu University, Japan
Tel: 81-58-293-2078
Fax: 81-58-293-2079
E-mail: [email protected]

Received date: July 22, 2013; Accepted date: August 16, 2013; Published date: August 19 2013

Citation: Ichihashi O, Hirooka K (2013) Deterioration in the Cathode Performance during Operation of the Microbial Fuel Cells and the Restoration of the Performance by the Immersion Treatment. J Microb Biochem Technol S6:006. doi:10.4172/1948-5948.S6-006

Copyright: © 2013 Ichihashi O, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

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Abstract

Microbial fuel cells were operated with synthetic wastewater containing phosphate as a buffer and sodium acetate as a substrate. Linear sweep voltammetry showed the deterioration in the performance of the cathodes after operation. The immersion of the deteriorated cathodes in Milli-Q water, acidic and basic buffer solutions improved the performance. The treatment in the acidic buffer solution restored the performance of the cathode to the extent almost equivalent to that of a new cathode, whereas the treatment in Milli-Q water and the basic buffer solution did not restore the performance to that extent. The improved performance by the immersion in Milli-Q water or the buffer solutions indicates that the water-soluble components are responsible for the deterioration in the cathode performance. Almost complete recovery of the performance in acidic condition suggests that salts that are highly soluble in acidic condition, and poorly soluble in basic condition are responsible for the deterioration. The analysis of the eluted substances in the immersion solution suggests that these salts contained phosphorus, magnesium and calcium in a high concentration.

Keywords

Microbial fuel cell; Cathode performance; Immersion treatment; Deposition of alkaline salts

Introduction

A microbial fuel cell (MFC) uses the metabolism of electrogenic bacteria to convert the chemical energy stored in the organic substances in wastewater to the electrical energy [1-3].

There are a variety of MFCs today. The single-chamber MFC with an air-cathode had great advantage in terms of the structure (simpler), power density (higher), and aeration (not required) [4,5], and therefore, it has been considered the most anticipated model for practical application. The generation of electricity in these singlechamber MFCs, however, has been reported to decline during the course of continuous operation [6,7]. Previous studies [6-9] indicated the deterioration in the performance of the cathode was one of the possible causes. Saito et al. [8] assumed that the formation of biofilm over the cathode was a possible cause of the performance deterioration. They washed away the biofilm formed over the cathodes of their MFC reactors after eight-month (128 cycles) operation using deionized water. The result showed the increase of maximum power density in all the reactors, compared with that before removing biofilm, but it did not restore the performance to the extent equivalent to that of the initial stage of operation. Kiely et al. [6] removed the biofilm of the cathode that had been operated for one year by washing away with deionized water, and found the increase of maximum power density as in the case of Saito et al. [8], but could not restore the performance of the cathode to the extent equivalent to that of a new cathode. These results suggested that the existence of biofilm was only part of the causes of the deterioration in the cathode performance, and there existed yet another cause or causes of the deterioration.

Jiang et al. [9] reported that gradual fouling of calcium and sodium in wastewater over the cathode surface was responsible for the increased internal resistance of the MFCs, thereby reducing the generation of electricity. When the fouling substance was removed by a paper towel, the internal resistance of the MFCs dropped from 150 Ω to 120 Ω, and the power density increased from 600 mW/m2 to 1200 mW/m2 in the 6th week of the operation. However, when the same procedure was implemented in the 9th week, although the internal resistance dropped from 260 Ω to 200 Ω, the power density did not increase, remaining at 300 mW/m2. Jiang et al. [9] suggested the possibility that the longer operation time led to stronger adhesion of the fouling substance on the cathode surface, which could not be removed by simple wiping [9].

Zhuang et al. [7] argued that the formation of alkali salts inside the cathode covered the catalytic surface, thereby preventing contact between oxygen and the surface of catalyst. They rinsed the cathode of the MFC that had been operated for 60 days using deionized water until the pH of the rinsed water became neutral, and determined the cathode performance before and after rinsing using linear sweep voltammetry (LSV). The LSV result confirmed that the rinsing restored the performance of the cathode to the extent close to that of the 20th day of operation.

In the previous research, the authors reported that the treatment of the wastewater containing phosphorus (P), magnesium (Mg) and ammonium (NH4) by MFCs led to the deposition of struvite crystal on the cathode [10]. Furthermore, the treatment of the wastewater containing P, Mg, but no NH4 led to the deposition of cattiite, a type of magnesium phosphate. The deposition could be explained by the following mechanism: the pH increase in the vicinity of the cathode due to the oxygen reduction reaction, there decreased the solubility of struvite and cattiite, leading to the deposition of these compounds.

The LSV detected the deterioration in the performance of the cathode where struvite deposited even though no biofilm existed, as compared with that before operation, but further confirmed that the removal of struvite by the immersion treatment in Milli-Q water (pH 7.0), and the MES buffer solution (pH 5.5) restored the performance of the cathode to the extent equivalent to that of a new cathode. The pH increase of the MES buffer solution from 5.5 to 6.3 suggested that the elution of alkaline salts had occurred. The cathode performance of the control group having no biofilm formed thereon also deteriorated, although the extent of deterioration was not as large as the one where struvite deposited. The immersion treatment of both groups in Milli-Q water (pH 7.0), and the MES buffer solution (pH 5.5) restored the performance of the cathodes to the extent equivalent to that of a new cathode. These results suggested the possibility that the deposition of the (alkaline) salt over the surface or inside of the cathode, whose solubility was decreased due to increased pH in the vicinity of the cathode, prevented the movement of substance in the cathode, thereby deteriorating the performance. Hence, removing the deposition should restore the performance of the cathode. We thought that the salt would deposit in the basic condition, and therefore we would be able to remove the salt effectively by immersing the deteriorated cathode in the acidic solution.

In this research, we examined how the conditions of the immersion treatment influenced the restoration of the deteriorated cathodes, by running the air-cathode single-chamber MFCs and immersing the deteriorated cathodes in the acidic buffer solutions having a variety of pH. We used the LSV to evaluate the cathode performance.

Materials and Methods

Preparation of the air-cathodes

Carbon paper (Toray Carbon Paper TGP-H-120, Toray Industries, Inc., Japan) was used as a base of the air-cathodes. According to the method described by Cheng and Liu [11], coating of platinum catalyst (TEC 10E50E, the amount of platinum 2.5 mg/cm2, Nafion binder) was applied to the inner surface of the reactor, and PTFE was applied to the outer surface of the reactor so that it would work as an air diffusion layer. The projected area of the air cathode was set to 47 cm2 (7.7 cm diameter).

Structure of the reactor and operation for acclimatization

Six identical air-cathode single-chamber MFCs were operated. Figure 1 shows a schematic diagram of the MFC reactor. The effective capacity of the reactor was 70 ml. The anode was made of carbon felt having a diameter of 7.7 cm (projected area 47 cm2) and a thickness of 1 cm (LFP-210, Osaka Gas Chemicals Co., Ltd., Japan), fixed by a graphite rod with its one end connected to the external circuit. A piece of polyethylene nonwoven fabric (2×3.5 cm, 0.5 cm thick) was placed over the cathode to prevent direct contact with the anode. The MFC reactor was then connected to a 300 mL glass bottle, and the internal solution was circulated at a flow speed of 20 mL/min, according to the method described by Borole et al. [12]. The bottle was replaced by a new bottle filled with the synthetic wastewater containing the materials other than the substrate every five to ten days. The external resistance was set to 10 Ω.

microbial-biochemical-technology-schematic

Figure 1: A schematic diagram of the MFC reactor.

The reactor was provided with the synthetic wastewater, containing sodium acetate as a substrate, a 100 mM phosphate buffer solution (NaH2PO4: 36 mM, Na2HPO4: 64 mM, NH4Cl: 12 mM, KCl: 3.5 mM, pH 6.8, electrical conductivity 11.2 mS/cm), minerals and vitamins described in previous publication [10]. The reactor was operated at room temperature. Sodium acetate was continuously added at the rate of 8.1 mmol/L-day with a syringe pump, and the synthetic wastewater containing the components other than sodium acetate was replaced by exchanging the bottle. A section about 0.5 mm square was taken away from the anode of the MFC under operation, and used for seeding.

The acclimatization operation was performed for more than 50 days, and the generation of approximately 4 A/m2 electric current was observed in all the MFC reactors.

The immersion treatment of the cathodes and installation to the reactors

Six new identical air-cathodes (C1-C6) were prepared, and three of them were treated by immersing in acidic buffer solutions. The C1, C2 and C3 cathodes were immersed in 300 mL of a 30 mM citrate buffer solution (pH 3.0), 300 mL of a 30 mM acetate buffer solution (pH 4.5), and 300 mL of a 30 mM MES buffer solution, respectively, for 24 hours, then rinsed with Milli-Q water, and finally dried in the oven at 40°C overnight (Immersion Treatment 0). The C4-C6 cathodes were not immersion-treated. The old cathodes of the reactors which had completed the acclimatization operation were replaced with the above C1-C6 cathodes, and then the reactors were operated for 50 days

Restoring the performance of the cathodes

The cathodes after operation were rinsed in Milli-Q water to remove the biofilm formed over the surface, and dried in the oven at 40°C. Thereafter, the C1 and C4 cathodes were immersed in 500 mL of a 30 mM citrate buffer solution (pH 3.0) for 24 hours. Likewise, 24-hour immersion was performed for the C2 cathode in 500 mL of a 30 mM acetate buffer solution (pH 4.5), the C3 and C5 cathodes in 500 mL of a 30 mM MES buffer solution (pH 5.5), and the C6 cathode in 500 mL Milli-Q water. All of them were then rinsed in Milli-Q water, and dried in the oven at 40°C overnight (Immersion Treatment 1). Further 24- hour immersion (Immersion Treatment 2) was done for the C1 and C2 cathodes in 500 mL of a 30 mM MES buffer solution (pH 5.5), and the C6 cathode in 500 mL of a 30 mM CHES buffer solution (pH 9.2). All of them were then rinsed in Milli-Q water, and dried in the oven at 40°C. Table 1 summarizes the treatments done for all the cathodes.

  Buffer for Immersion 0 (before MFC operation) Buffer for Immersion 1 (after MFC operation) Buffer for Immersion 2 (after Immersion 1)
Cathode 1 Citrate buffer (pH 3.0) Citrate buffer (pH 3.0) MES buffer (pH 5.5)
Cathode 2 Acetate buffer (pH 4.5) Acetate buffer (pH 4.5) MES buffer (pH 5.5)
Cathode 3 MES buffer (pH 5.5) MES buffer (pH 5.5) -
Cathode 4 - Citrate buffer (pH 3.0) -
Cathode 5 - MES buffer (pH 5.5) -
Cathode 6 - Milli-Q water (pH 7.0) CHES buffer (pH 9.2)

Table 1: Summary of the immersion treatments performed for the cathodes.

Evaluation of the cathode performance

The performance of the cathodes was evaluated by the LSV. MFCs having the same structure as the one used for operation were provided and filled with a 100 mM phosphate buffer solution (NaH2PO4: 36 mM, Na2HPO4: 64 mM, NH4Cl: 12 mM, KCl: 3.5 mM, pH6.8). Each of the cathodes to be evaluated was designated as a working electrode, and a carbon felt having a diameter of 7.7 cm and a thickness of 1 cm was designated as a counter electrode. Silver/silver chloride (Ag/AgCl) electrode was chosen as a reference electrode. A potentionstat (HZ- 5000, Hokuto Denko Corporation, Japan) was used to scan the electric potential over a range from the open circuit potential (OCP) to -0.2 V (vs. Ag/AgCl) at the scanning rate of 1 mV/s.

The LSV was conducted for each cathode immediately after it was assembled (C1-C6), after Immersion Treatment 0 (before operation) (C1-C3), immediately after operation of the MFCs (C1-C6), after Immersion Treatment 1 (C1-C6), and after Immersion Treatment 2 (C1-C2, C6). The pH of the solution where the immersion treatments were performed was measured, and the elemental analysis of the same solution was also conducted by ion chromatography, ICP-AES and ICP-MS.

Monitoring of the MFCs and chemical analysis

The voltage of the cell was measured by a data logger (GL820, Graphtec), and automatically recorded every 10 minutes. The current was calculated by dividing the voltage by the external resistance, and the current density was calculated by dividing the current by the projected area of the cathode.

Nitric acid was added to the solution where the immersion treatments were performed, which was then heated at 115 for 60 minutes. Thereafter, the concentration of Mg and P in the solution was measured by ICP-AES (ULTIMA 2, Horiba, Japan), and the concentration of Al, Co, Mn, Cu, Zn, and Fe was measured by ICPMS (HP7500, Agilent, Japan). The solution where the immersion treatments were performed was also filtered with a pore size of 0.45 μm without undergoing the heat treatment, and the concentration of Ca2+ was measured by ion chromatography (Ion Chromatograph Dual Flow Analysis System, Shimadzu, Japan).

Results and Discussion

Change of the performance of the unused cathodes by Immersion Treatment 0

Figure 2 shows the linear sweep voltammogram of new cathodes when they were prepared. Because the electric potential of the cathodes under operation was considered 0-0.2 V (vs. Ag/AgCl), the current density of the cathodes was evaluated primarily over this range. When the performance of the new cathodes before Immersion Treatment 0 was evaluated, the OCP fell within the range from 0.54 V to 0.60 V (vs. Ag/AgCl), and the current densities were within the range from -4.5 to -5.7A/m2 at 0.2 V (vs. Ag/AgCl), from -7.5 to -9.6A/m2 at 0.1 V (vs. Ag/AgCl), from -12.3 to -14.4A/m2 at 0 V (vs. Ag/AgCl). The OCP of the cathodes that had undergone the immersion treatment dropped by approximately 0.1 V, regardless of the type of the buffer solution used (Figure 3). The current increased in the MES-treated (pH 5.5) cathode and citrate-treated (pH 3.0) cathode at the electric potential of 0.2 V or below, but no substantial difference was observed in the acetatetreated (pH 4.5) cathode when compared with the untreated ones. The MES-treated (pH 5.5) cathode and the citrate-treated (pH 3.0) cathode exhibited similar tendencies, which were different from the one of the acetate-treated (pH 4.5) cathode. This result indicates that the effect does not have simple pH dependence.

microbial-biochemical-technology-cathode

Figure 2: The performance of the cathode at the time of assembly.

microbial-biochemical-technology-immersion

Figure 3: The performance of the cathodes before and after the immersion treatment 0 (a: cathode 1 treated in citrate buffer (pH 3.0), b: cathode 2 treated in acetate buffer (pH 4.0), c: cathode 3 treated in MES buffer (pH 5.5)).

Deterioration in the cathode performance during operation

Figure 4 shows the change of the current density over time during operation of the MFCs. Six MFCs exhibited almost identical behavior in the fluctuation of the current density, that is, the current density dropped to approximately 2.5 A/m2 immediately after the replacement of the cathodes, then gradually increased, and reached about 4 A/m2 around the 15th day, and became stable without any noticeable decline of the amount of power generation until the end of operation.

microbial-biochemical-technology-operation

Figure 4: The performance of the cathodes before and after operation (a: cathode 1 treated in citrate buffer (pH 3.0), b: cathode 2 treated in acetate buffer (pH 4.0), c: cathode 3 treated in MES buffer (pH 5.5), d: cathode 4 not treated, e: cathode 5 not treated, f: cathode 6 not treated). The “before-operation” curves of a-c in this Figure are the same as the “after-immersion” curves in Figure 3.

Figure 5 and 6 show the results of the LSV of each cathode before and after operation. The current density of all the cathodes declined over the range of all measurements (Figure 5). The result indicates that their performance was deteriorated. The performance after operation was almost identical in all the cathodes, and no influence of the immersion treatments before their use was detected (Figure 6). However, although the effect of the immersion treatments was minor compared with the variation they had when they were initially prepared, the cathodes that had undergone the immersion treatment in the acetate buffer solution before their use had lower current density over the range of all measurements than did the other cathodes. This observation might suggest the possibility of some negative effect of the immersion treatment in the acetate buffer solution on the oxygen reduction capability of the cathode.

microbial-biochemical-technology-performance

Figure 5: Comparison of the performance of the cathodes after operation. (C1: cathode 1 treated in citrate buffer, C2: cathode 2 treated in acetate buffer, C3: cathode 3 treated in MES buffer, C4: cathode 4 not treated, C5: cathode 5 not treated, C6: cathode 6 not treated).

microbial-biochemical-technology-citrate

Figure 6: The performance of the cathodes after operation, after the immersion treatment 1, and after the immersion treatment 2 (a: cathode 1 treated in citrate buffer (pH 3.0)→citrate buffer (pH 3.0)→MES buffer (pH 5.5), b: cathode 2 treated in acetate buffer (pH 4.0)→acetate buffer (pH 4.0)→MES buffer (pH 5.5), c: cathode 3 treated in MES buffer (pH 5.5)→MES buffer (pH 5.5), d: cathode 4 not treated→citrate buffer (pH 3.0), e: cathode 5 not treated→MES buffer (pH 5.5), f: cathode 6 not treated→Milli-Q water (pH 7.0)→CHES buffer (pH 9.2)).

Restoration of the cathode performance by the immersion treatments after operation

Figure 7 shows the LSV result of the cathodes that had undergone the immersion treatments after operation. The first immersion treatment of the cathodes after operation (Immersion Treatment 1) increased the current density of all the cathodes over the range of all measurements, and improved the performance. However, the extent of recovery of the cathode immersed in the acetate buffer solution (C2) was small compared with the others, and the extent of recovery of the one immersed in Milli-Q water (C6) was further less. This might suggest that the immersing in the acidic buffer solution is important for the restoration of the cathode performance. However, the immersion treatment by the pH 3.0 citrate and the pH 5.5 MES exhibited similar restoring effect of the cathodes, and the treatment by the pH 4.5 acetate exhibited the restoring effect inferior to the other two (Figure 8). These results suggest that the restoration of the cathode performance by the immersion treatment has no simple dependence on the pH: it is not such that the lower pH the greater the recovery, it has an optimum value of pH, or it is greater below certain value of pH; rather, it has a more complicated mechanism.

microbial-biochemical-technology-acetate

Figure 7: Comparison of the performance of the cathodes after the immersion treatment 1. (C1: cathode 1 treated in citrate buffer (pH 3.0)→citrate buffer (pH 3.0), C2: cathode 2 treated in acetate buffer (pH 4.0)→acetate buffer (pH 4.0), C3: cathode 3 treated in MES buffer (pH 5.5)→MES buffer (pH 5.5), C4: cathode 4 not treated→citrate buffer (pH 3.0), C5: cathode 5 not treated→MES buffer (pH 5.5), C6: cathode 5 not treatedMilli-Q water (pH 7.0)).

microbial-biochemical-technology-treatedMilli

Figure 8: Comparison of the performance of the cathodes after the immersion treatment 1. (C1: cathode 1 treated in citrate buffer (pH 3.0)→citrate buffer (pH 3.0), C2: cathode 2 treated in acetate buffer (pH 4.0)→acetate buffer (pH 4.0), C3: cathode 3 treated in MES buffer (pH 5.5)→MES buffer (pH 5.5), C4: cathode 4 not treated→citrate buffer (pH 3.0), C5: cathode 5 not treated→MES buffer (pH 5.5), C6: cathode 5 not treatedMilli-Q water (pH 7.0)).

As for the cathodes that had undergone the immersion treatment before operation (C1, C2, and C3), the performance of the cathodes that had undergone the first immersion treatment after operation (Immersion Treatment 1) was almost identical to that of the cathode that has undergone the immersion treatment before operation (Immersion Treatment 0) (Figure 9). Hence, the immersion treatments in these buffer solutions indeed restored the cathode performance almost completely. However, no substantial difference was observed in the performance after Immersion Treatment 1 between the immersiontreated cathode before operation (Immersion Treatment 0) and the untreated cathode (Compare C1 and C4, and C3 and C5 in Figure 8).

microbial-biochemical-technology-cathodes

Figure 9: The amount of P, Mg, Mn, Co, and Ca2+ eluted from the cathodes in the immersion treatments.

When the cathode (C2), which had been immersed in the acetate buffer solution after operation was further immersed in the MES buffer solution, the performance of the cathode was improved to the extent almost equivalent to that of C1, C3, C4, and C5, which had undergone the immersion treatment (Figure 7b). This result suggests that the components which had not been removed by the immersion in the acetate buffer solution were removed by the immersion in the MES buffer solution, which contributed toward restoring the performance of the cathode.

When C6 (untreated before operationimmersed in Milli-Q water after operation) was further immersed in the CHES solution (pH 9.2), the current density decreased over the range from the OCP to 0.05 V (vs. Ag/AgCl), but increased elsewhere. Furthermore, after the CHES immersion treatment, the current density decreased over the range from the OCP to 0.2 V (vs. Ag/AgCl), as compared with the density at the end of operation (Figure 7f).

In summary, the immersion of the cathode in Milli-Q water alone contributed to the restoration of the cathode performance to some extent. This result suggests that some water soluble components are responsible for the deterioration in the cathode performance. Furthermore, the immersion of the cathode in the acidic solution contributed more to the restoration of the cathode performance than did the immersion in water. This result suggests that some acid-soluble components (such as alkaline salts) are responsible for the deterioration in the performance. These ideas agree with the hypothesis that the localized increase of pH in the vicinity of the cathode contributes to the deposition of the alkaline salts on the cathode.

Eluted components in the solution

Figure 10 shows the concentration of P, Mg, Mn, Co, and Ca2+ in the solution where the immersion treatment of the cathodes was performed.

P, Mg, Mn, and Co was detected in a higher concentration in the solution, where the immersion treatment of the cathodes was performed after operation (Immersion Treatment 1) than in the solution where the immersion treatment was performed before operation (Immersion Treatment 0). Among these elements, Mg, Mn, and Co was detected in a higher concentration in the acidic solution than in Milli-Q water (pH 7.0) or the basic solution (pH 9.2). These results suggest that Mg, Mn and Co accumulated as alkaline salts during operation, and the removal of these salts by elution due to the immersion treatments restored the performance of the cathode. The dissolving concentration of P remained almost constant, having no dependence on pH of the solution. Therefore, P might have deposited as a water-soluble salt that was soluble under pH 9.2. Though the concentration of Ca2+ ion could not be quantified in the acetate buffer solution because of the higher background concentration in measurement, Ca2+ ion was detected in a higher concentration in the acidic solution than in Milli-Q water or the basic solution. This result suggests that Ca also accumulated over the cathode as an alkaline salt like Mg, Mn, and Co.

In the previous research, the authors reported that when the wastewater containing P, Mg, and NH4 was treated in the MFC, struvite crystal deposited on the cathode, and when the wasterwater containing P and Mg but no NH4 was treated, cattiite crystal deposited on the cathode [10]. In this research, P and Mg were also detected in a higher concentration in the cathode than were any other inorganic mineral elements. However, the molar ratio of Mg to P became 3.4-4.5 in the acidic solution, which was higher than the molar ratio of struvite to P (1) or cattiite to P (1.5). The higher molar ratio of Mg suggests that the magnesium salts containing no phosphorus, such as magnesium carbonate and magnesium hydroxide, accounted for most of the magnesium deposition. In the meantime, Ca has been reported to be a main component of the deposition on the cathode to our knowledge to date [9,13,14], and found in a form of calcium carbonate [9,15]. In this research, because a great amount of Ca2+ also eluted into the solution, and a great amount of carbonate ion was assumed to be present in the internal solution of the MFCs as a result of decomposition of acetate, Ca is suspected to have accumulated in a form of calcium carbonate as reported in the literature.

The C1 and C2 cathodes underwent the immersion treatment twice after operation (Immersion Treatment 1, Immersion Treatment 2), but a small amount of P, Mg, Mn, Co, and Ca2+ eluted into the solution in Immersion Treatment 2. The performance of C2 was improved more after Immersion Treatment 2 than after Immersion Treatment 1. Therefore, the deterioration of the cathode performance may not be caused only by the accumulation of P, Mg, Mn, Co and Ca2+.

Furthermore, in the solution where Immersion Treatment 1 was performed under acidic environment with three different pH (pH 3.0, 4.5, 5.5), no substantial difference was observed in the eluted amount of P, Mg, Mn, Co, and Ca2+. However, when treated with the pH 3.0 and pH 5.5 solutions, the performance of the cathode was restored to the extent almost equivalent to that of a new cathode, but it was not restored to such extent when treated with the pH 4.5 solution. This result suggests that there can be yet another cause or causes for the deterioration in the performance of the cathode, in addition to the accumulation of P, Mg, Mn, Co, and Ca2+ .

Conclusion

When the MFCs were operated with the synthetic wastewater containing a 100 mM phosphate buffer solution, the decline of the power output was detected in all the six reactors due to the deterioration in the cathode performance. When these cathodes were removed and immersed in the buffer solutions with a variety of pH, the performance was improved in all of them. The extent of improvement was greater in the immersion treatment in the acidic buffer solution than in the neutral or basic buffer solution; particularly, in the immersion treatment in the citrate (pH 3.0) and MES (pH 5.5) buffer solution, the performance was restored to the extent almost equivalent to that of an unused cathode. The improved performance by the immersion in the water or the buffer solution indicates that water-soluble components are responsible for the deterioration in the performance of the cathode. Also, almost complete restoration of the cathode performance by the immersion under the acidic environment suggests that the components responsible for the deterioration are salts which are highly soluble in the acidic environment and poorly soluble in the basic environment.

Furthermore, the analysis of the components eluted in the immersing solution suggests that the removal of P, Mg, Mn, Co, and Ca deposited on the cathode by the immersion treatment contributed to the restoration of the cathode performance. However, there was a difference in the extent of restoration of the cathode performance regardless of the little difference in the eluted amount in the first treatment, and there were cathodes which had very little amount of elution, but none the less restored the performance. These results suggest that, in addition to the accumulation of these salts, there can be yet another cause or causes for the deterioration. Hence, further research on the mechanism of cathode deterioration is needed.

Acknowledgements

This research was financially supported by the Funding Program for Next Generation World-Leading Researchers.

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