Bioresource Technology 156 (2014) 14–19

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Sulfate and organic carbon removal by microbial fuel cell with sulfate-reducing bacteria and sulfide-oxidising bacteria anodic biofilm Duu-Jong Lee a,b,⇑, Xiang Liu a,c, Hsiang-Ling Weng a a

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan c Department of Environmental Science and Engineering, Fudan University, Shanghai, China b

h i g h l i g h t s  Microbial fuel cell with SRB + SOB anodic biofilm treats sulfate + lactate wastewater.  Tested MFC efficiently converted sulfate to elemental sulfur.  Electrochemical analyses showed that sulfide is dominating species for performance.  Form sulfide diffused to SOB cell for conversion to sulfur and excess electrons.  Short diffusional distance of sulfide ions between cells enhances MFC performance.

a r t i c l e

i n f o

Article history: Received 18 November 2013 Received in revised form 28 December 2013 Accepted 30 December 2013 Available online 10 January 2014 Keywords: Sulfate Sulfide Microbial fuel cell Diffusion

a b s t r a c t Biological sulfur removal can be achieved by reducing sulfate to sulfide with sulfate-reducing bacteria (SRB) and then oxidising sulfide to elemental sulfur (S0) with sulfide oxidising bacteria (SOB) for recovery. In sulfate–carbon wastewaters lacking electron acceptor for sulfide, excess sulfide will be produced and accumulated in the reactor. This study applied the microbial fuel cell (MFC) cultivated with the SRB + SOB anodic biofilm for treating the sulfate + organic carbon wastewaters. Excess sulfate ions were efficiently converted to sulfide by SRB cells in the biofilm, while the formed sulfide was diffused to the neighboring SOB cells to be irreversibly converted to S0 with produced electrons being transferred to the anode. The cell–cell sulfide transport principally determined the electron flux of the MFC. Short diffusional distance of sulfide ions between cells significantly reduced the polarization resistances, hence enhancing performance of the MFC. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Industrial effluents from sugar, alcohol, pharmaceutical products and monosodium glutamate can contain high levels of sulfate. The sulfate-redusing bacteria (SRB) can convert sulfate to sulfide in anaerobic environment (Silva et al., 2012). To remove sulfide from water, the use of sulfide-oxidising bacteria (SOB) was proved feasible (Wang et al., 2010). SRB are heterotrophic bacteria that consume organic compounds as carbon and energy sources. Conversely, SOB are commonly autotrophic so inorganic electron acceptors such as nitrate are needed for sulfide oxidising. The biological reactor with both SRB and SOB can achieve simultaneous removal of sulfide, nitrate and organic compounds (in term of

⇑ Corresponding author at: Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. Tel.: +886 233663028. E-mail address: [email protected] (D.-J. Lee). 0960-8524/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.12.129

chemical oxygen demand, COD) (Xu et al., 2013, 2014; Chen et al., 2013; Show et al., 2013; Lee et al., 2012a,b). In wastewaters lacking nitrate or other electron acceptors, simultaneous removal of sulfide and organic compounds is not achievable; while addition of electron acceptors is not a costeffective and environmental friendly option. Microbial fuel cells (MFC) can generate electricity from degradation of organic and inorganic substrates in wastewater and in sludges (Jiang et al., 2009). A few studies explored the removal of sulfide using MFC (Habermann and Pommer, 1991; Cooney et al., 1998; Wang et al., 2001; Tender et al., 2002; Ryckelunck et al., 2005; Zhang et al., 2009a,b; Nielsen et al., 2009; Rabaey et al., 2006; Sun et al., 2009, 2010; Chou et al., 2013; Lee et al., 2013). Zhang et al. (2008) proposed the use of SRB and SOB to remove sulfate from waters with MFC. Zhao et al. (2008, 2009) produced electricity from MFC with sulfate removal. Ghangrekar et al. (2010) noted that low COD/SO24 ratio would yield poor MFC performance. Rabaey et al. (2006) noted that their microbial fuel cell (MFC) could convert the dissolved sulfide to S0.

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Sulfate is an electron acceptor hence cannot be directly removed by MFC. Lee et al. (2012a) utilized SRB to convert sulfate to sulfide, then MFC further oxidised sulfide to S0. Restated, the use of SRB + SOB MFC can remove sulfide and sulfate simultaneously with electricity production. This MFC study is a subsequent report to Lee et al. (2012a) to convert sulfate to S0 with an SRB + SOB anodic biofilm and with anode as the electron acceptor. In particular, the electrochemical analysis was conducted to unveil the mechanisms noted for the behaviors of the studied SRB + SOB MFC.

acceptor due to low overpotential. 50 mM ferricyanide was used in this experiment and phosphate buffer was added to regulate the pH change in the cathode. The composition of this cathodic solution is as follows: NaH2PO4 H2O, 17.77 gl 1; Na2HPO4, 32.33 gl 1; K3Fe(CN)6, 16.46 gl 1 at pH 6.9. At the end of each cycle when the cell voltage was dropped to lower than 100 mV, fresh medium was replenished to the anodic cell to start a new cycle.

2. Methods

The voltage of each MFC was recorded every 180 s using a data acquisition system developed by Advantech Co., Taipei, Taiwan. During measurements, the bioanode and another new anode were placed in the anodic chamber while the former was referred to as ‘‘biotic’’ and the latter as ‘‘abiotic’’ tests. The voltage of the MFC refers to the voltage difference across the external resistance. Precision resistors of 1000 ohms were used as external resistance in this experiment. All electrochemical tests were conducted using a potentiostat (CH Instruments Electrochemical Workstation model CHI611), including linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemcial impedance spectroscopy (EIS). And the power supply system used in this experiment was LPS 505N from MOTECH programmable DC power supply. For potentialstats, to control the anode potential, the working electrode of potentialstats was connected with the anode. The reference electrode was connected with the Ag/AgCl reference electrode, which placed on the sample hole at above side of anodic chamber. For power supply, voltage was added to the MFC system by connecting the positive lead of the power supply on the anode and the negative lead of the power supply on the reference electrode in the anode chamber. LSV experiments were carried out at a scan rate of 1 mV s 1 from the open-circuit cell voltage (OCV), where zero current is passed across the circuit (I = 0), to the short-circuit cell voltage, where current is at a maximum. From the V–I curve obtained using LSV, polarization curve and power density curve could be calculated. In CV measurement the working electrode potential is ramped linearly versus time and the working electrode’s potential ramp is inverted upon reaching a set potential (Liu et al., 2011). In EIS a sinusoidal signal with small amplitude is superimposed onto the applied potential of the working electrode. The frequency of the sinusoidal signal is from 100 kHz to 5 mHz. The plots of measured electrode impedance can be obtained to show detailed information about the electrochemical system. EIS can be used to measure the ohmic and internal resistance of an MFC. In this

Activated sludge was collected from a bakery factory in Taoyuan County. The sludge was filtered using coarse screen and incubated anaerobic to enrich sulfate reducing consortium for one month in a medium of composition as follows: Na2SO4, 1.15 gl 1; sodium citrate, 5.0 gl 1; NH4Cl, 1.0 gl 1; K2HPO4, 0.5 gl 1; sodium lactate, 5.0 gl 1; Fe(NH4)2(SO4)2, 1.0 gl 1; Wolf’s vitamin solution, 1 ml; Wolf’s mineral solution, 1 ml. The pH of the medium was adjust to 7.5 by 1 M HCl or 1 M NaOH. The Wolf’s vitamin solution has the following composition (gl 1): biotin, 0.2; folic acid, 0.2; pyridoxine HCl, 1.0; riboflavin, 0.5; thiamin, 0.5; nicotinic acid, 0,5; pantothenic acid, 0.5; B-12, 0.01; p-aminobenzoic acid, 0.5; thioctic acid, 0.5. The Wolf’s mineral solution contained (gl 1): NTA, 1.5; MgSO4, 3.0; MnSO4 H2O, 0.5; NaCl, 1.0; FeSO4 7H2O, 0.1; CaCl2 2H2O, 0.1; CoCl2 6H2O 0.1; ZnCl2, 0.13; CuSO4 5H2O, 0.01; AlK(SO4)2 12H2O, 0.01; H3BO3, 0.01; Na2MoO4, 0.025; NiCl2 6H2O 0.024; Na2WO 2H2O, 0.025. Before inoculation, the enriched sulfate-reducing bacteria consortia were incubated in the same medium as in medium mentioned above for more than 3 days, then the enriched sulfatereducing bacterial consortia were fed into the MFC anodic chamber for cultivation with syntheses sulfate-laden wastewater, which composition is as follows: Na2SO4, 1.15 gl 1; NH4Cl, 1.0 gl 1; K2HPO4, 0.5 gl 1; sodium lactate, 5.0 gl 1; Wolf’s vitamin solution, 1 ml; Wolf’s mineral solution, 1 ml. The pH of the medium was adjusted to 7.5 by 1 M HCl or 1 M NaOH. For brevity sake, the syntheses sulfate-laden wastewater will be called ‘‘fresh media’’ in the following discussion relative to the ‘‘old media’’, which has been operated in the MFC for 3 days (18 cycles) and contained some metabolite excreted by the biological consortium. 2.2. MFC design Dual MFC comprising anode and cathode cylindrical chambers (inside diameter 5 cm; length 4 cm each) were connected to a cation exchange membrane (CEM) (Ultrex CMI-7000; Membrane (International, Inc., Glen Rock, NJ, USA). Red rubber stoppers were placed in the sampling holes through which samples were taken and medium was replaced. Anode was made of carbon felt (C0S3002; CeTech Co., Taichung, Taiwan). Cathode was made of carbon cloth (W0S1002; CeTech Co., Taichung, Taiwan). The sizes of the carbon cloth in both experiments were 3 cm  3 cm. Before inoculation, all the electrodes were first immersed in 1 M NaOH then in 1 M HCl for one-hour each to remove microbial residues on the electrodes surface. The MFC reactor was started up by directly inoculated 10 ml pre-cultured SRB into 100 ml MFC reactor fed with 90 ml artificial waste water containing lactate and sulfate. Dual carbon felt electrodes are placed in the same anodic chamber to compare the difference of electrochemical performance in the same reactor. Ferricyanide solution is often used in cathodic medium in bench-scale MFC test to substitute the oxygen as cathodic electron

300 Sulfide

250

Sulfate-S Thiosulfate-S Sulfite-S

S-Species (ppm)

2.1. Inoculation and medium

2.3. Electrochemical analysis

200

150

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50

0 10

11

12

13

14

Time (Day) Fig. 1. Metabolite distributions.

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(a)

0.4

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1

1

1

(b) 0.4

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Voltage (V)

Voltage (V)

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1 SRB O Abiotic

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Time (Day)

Time (Day)

Fig. 2. Potential versus time for MFC. (a) 1: lactate + sulfide; 2: lactate only. (b) 1: lactate + sulfide; 2: sulfide only; abiotic: no biofilm.

Table 1 OCV and maximum power densities under different testing conditions.

6 biotic, old media biotic, fresh media abiotic, old media abiotic, fresh media

5

Current (mA)

4

3

Condition

OCV (mV)

Pmax (mW m

Biotic, old media Biotic, fresh media Abiotic, old media Abiotic, fresh media

730 730 659 442

61.2 62.9 1.05 0.97

2

)

2

5 mHz with an AC signal of 5 mV amplitude. Anode impedance spectra were recorded using the anode as the working electrode and the cathode as the counter electrode. During these measurements, the Ag/AgCl reference electrode in the compartment of the working electrode was used as the reference electrode.

1

0

-1 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2.4. Chemical analysis

Voltage (V) Fig. 3. Cyclic voltammetry under different conditions: biotic, abiotic, fresh media and old media.

experiment, EIS measurements were carried out for the anode, the cathode and the whole MFC in a frequency range of 100 kHz to

70 biotic, old media biotic, fresh media abiotic, old media abiotic, fresh media

Power density (mW/m -2)

60

50

40

30

All the pH measurements were conducting using WTW pH meter pH-315. Before tests, the pH meter is calibrated with pH 4.0 and 7.0 calibration solution; the pH meter probe was rinsed with deionized water before and after its use to remove impurities. Samples containing sulfide ions (S2 ) were added to Zn(NH3)6 (OH)2 solution to form Zn(OH3)S complex. Then N,N-dimethyl-1– 1,4-phenylene diammonium dichloride, DPDA Solution and Fe(III) chloride solution (1% FeCl3) were added to form a methylene blue color solution. The samples were then quantitatively analyzed using a spectrophotometer at 665 nm. An ion chromatography (Dionex ICS-3000) measured the concentration of sulfate (SO24 ), thiosulfate (S2O23 ) and sulfite (SO23 ) in the collected liquor samples following 0.45-lm filtration. Samples were prepared prior to SEM examination. Samples were first immersed in glutaraldehyde (2.5%, 60 min). Samples were then washed with phosphate buffer (0.1 M, pH 7.0, 3 times). The samples were then immersed in ethanol (30%, 50%, 70% and 90%, 10 min). Finally, the samples were treated with critical point drying to dehydrate the biological tissues and coated with gold.

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3. Results and discussion

10

3.1. Cell performances

0 0

100

200

300

400

500

600

Current density (mA m -2) Fig. 4. Comparison of power density curves under different conditions.

700

After cultivation period, the biofilm was established on the carbon felt (Fig. S1). The sulfur metabolites over two typical cycles were presented for demonstration (Fig. 1). Sulfate concentration

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300 abitoc ,fresh media

-80

abitoc ,fresh media abiotic, old media

P h a s e a n g le (d e g re e )

logZ (ohm)

biotic, old media fitting 1 fitting 2 fitting 3 fitting 4

3.0 2.5

abiotic, old media

250

biotic , fresh media

biotic , fresh media

3.5

abitoc, fresh media

abiotic, old media

2.0 1.5

biotic, fresh media

biotic, old media -60

biotic, old media 200

fitting 1 fitting 2

-Zimg (ohm)

4.0

fitting 3 fitting 4 -40

fitting 1 fitting 2 fitting 3 fitting 4

150

100 -20

50

1.0 0.5

0

0

-4

-2

0

2

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6

-4

-2

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2

logf (Hz)

logf (Hz)

(a)

(b)

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6

0

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200

300

400

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600

Zreal (ohm)

(c)

Fig. 5. Comparison of experimental and fitted impedance spectra for MFC with biotic or abiotic electrodes.

Table 2 Fitting parameters for the spectra of MFC under different conditions. Electrode electrolyte Element

Abiotic fresh media Value

Electrode electrolyte Element

Abiotic old media Value

R1 CPE1-T CPE1-P R2 CPE2-T CPE2-P R3

11.4 (ohm) 1.64  10 4 (F) 0.727 (F) 103 (ohm) 3.59  10 3 (F) 0.952 (F) 296000 (ohm)

R1 CPE1-T CPE1-P R2 CPE2-T CPE2-P R3

13.7 (ohm) 7.36  10 5 (F) 0.771 (F) 84.7 (ohm) 4.51  10 3 (F) 0.66 (F) 448 (ohm)

Electrode electrolyte Element

Biotic fresh media Value

Electrode electrolyte Element

Biotic old media Value

R1 CPE1-T CPE1-P R2 CPE2-T CPE2-P R3

13.7 (ohm) 1.35  10 4 (F) 0.771 (F) 128 (ohm) 3.50  10 3 (F) 0.691 (F) 120 (ohm)

R1 CPE1-T CPE1-P R2 CPE2-T CPE2-P R3

13.4 (ohm) 1.21  10 4 (F) 0.874 (F) 120 (ohm) 4.96  10 3 (F) 0.753 (F) 17.2 (ohm)

2013). The overall conversions for sulfate to S0 in these two demonstrative cycles were respectively 77.9% and 47.6%. Fresh medium was fed into MFC and was operated for several cycles to make sure the MFC had similar and stable voltage outputs. Then the fresh media without sulfate (lactate only) were fed to the reactor (Fig. 2a and b). The voltage dropped rapidly from maximum voltage 0.320–0.052 V. After 1 day operation, the fresh media with sulfate (lactate + sulfate) were fed to the reactor, and the voltage was quickly recovered from 0.040 V to 0.3 V. This occurrence demonstrated that the presence of sulfate ions enhanced the cell performance.

3.2. Electrochemical analysis

Fig. 6. Proposed pathways in MFC.

was decreased from 248 mg l 1 to 29.0 mg l 1 on day 2. Correspondingly, sulfide concentration was increased from 0 to 172.3 mg l 1 on day 2. In addition, thiosulfate was increased slightly in concentration with no sulfite being detected. These results indicate that the SRB cells in biofilm efficiently converted sulfate to sulfide in biofilm. After day 2, sulfide concentrations were decreased continuously from peak value to a low value. The formed S0 was noted to deposit on the electrode (Rabaey et al., 2006; Sun et al., 2009) or being utilized again (Nanda et al.,

3.2.1. Cyclic voltammetry analysis The cyclic voltammetry analysis revealed that old medium with biotic or abiotic electrodes had strong oxidising peaks at 0.35 or 0.48 V, respectively, while the fresh medium had no such corresponding peaks (Fig. 3). Hence, some metabolites in the old medium could be oxidised. The CV result did not reveal reduction peaks, suggesting that the oxidation of metabolites was an irreversible reaction. Fig. 3 also demonstrated the test with 150 ppm sulfide only feed, leading to the sudden drop of voltage from 0.2 V to 0.045 V. Since the latter was still much higher than the control with abiotic anode (0.012 V), certain SOB as reported by Lee et al. (2012b) that used sulfide as an electron donor were active in the biofilm, just did not have a dominating role.

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3.2.2. LSV analysis Fig. 4 shows the LSV results, with corresponding data listed in Table 1. Both biotic experiment sets reveal similar maximum power densities (61–63 mW m 2) and OCV (730 mV). Conversely, the maximum power densities of abiotic tests were much lower. The corresponding OCV was also decreased. Hence, the electricity produced by the cell was primarily via biological activities rather than via pure chemical reactions. 3.2.3. ESI analysis A basic discussion on the EIS analysis, including the Bode plot (log f versus log Z) and the Nyquist plot (Zreal versus Zimg) diagram, is shown in Supplementary Materials (Figs. S2 S6). The circuit model used for the present study is shown in Fig. S7. Fig. 5 shows the result of EIS, including the experimental result and fitting result based on equivalent circuit model in Fig. S7. The values of model elements are listed in Table 2. The fit values for R1, CPE1-T, CPE1-P, R2, CPE2-T and CPE2-P for abiotic and biotic tests did not change much, demonstrating that neither biofilm nor metabolites significantly affect the behavior of the circuit except R3. The value of R3 was strongly affected by the presence of biofilm and the formation of metabolites. To replace fresh medium by old medium with abiotic electrode led to drop in polarization resistance R3 from 296,000 ohms to only 448 ohms. Replacing abiotic electrode by biotic electrode in fresh medium decreased charge transfer resistance R3 from 296,000 ohms to only 120 ohms. When abiotic electrode was placed in old medium, R3 was further decreased to only 17.2 ohms. Both biofilm and metabolite(s) in the suspension could accelerate electron transfer rate to anode surface. 3.3. Discussion Desulfurization of sulfate wastewaters needs electron donor to convert sulfate to sulfide, then electron acceptor to convert formed sulfide to S0. In denitrifying sulfide removal (DSR) process nitrate is a promising electron acceptor with high free energy gain. However, in sulfate–carbon wastewaters, the end product of sulfate is sulfide that is harmful to live forms and reactor and pipelines. We applied the anodic surface as the electron acceptor for the oxidising reaction of sulfide. The testing showed that the MFC can produce much more electricity with sulfate than with sulfide (Fig. 3), also the oxidising reaction was not inhibited by the formed sulfide. Additionally, the metabolite(s) in old media led to strong oxidising peak compared with the fresh media. The presence of biofilm only shifted the position of the oxidation peak to low voltage regime, rather than affected the occurrence of the oxidation peak. The fact that there were no reduction peaks for the old media suggested that oxidation of the metabolite(s) was irreversible. These observations suggested that in the present MFC system the sulfate ions were efficiently decreased by SRB in the biofilm, then the formed sulfide was rapidly converted to S0 by SOB with the anodic surface receiving the electrons. The formed S0 was not converted back to sulfide, likely since the formed S0 was converted to an inert form, perhaps with an organic adsorbing layer or S0 nanoparticles were suspended in the anolyte hence could not be easily accessed by the SOB. The possible mechanisms in the studied MFC are proposed (Fig. 6). Restated, the metabolites noted in the EIS results (Fig. 5) were sulfide ions in the solution, the sulfide flux to the anodic surface controlled the polarization resistance, and the electrical resistance between SOB cells and the anodic surface was negligible. For abiotic electrode with fresh medium (sulfate + lactate), the polarization resistance was very high (296,000 ohms), indicating negligible electricity production. When old medium was applied,

R3 was decreased to only 448 ohms but Pmax was very low (1.05 mW m 2), suggesting that chemical oxidation of sulfide at anodic surface was feasible, but did not occur probably owing to the large diffusional resistance from bulk solution to the anodic surface. Biotic electrode with fresh medium or old medium significantly decreased R3 from 296,000 ohms to 120 ohms or 17.1 ohms and high power densities by CV tests (Fig. 4). Assume that the biofilm is a stationary porous medium with constant diffusivity of sulfide in water. Then the electron flux is reversely proportional to the diffusional distance (d) of sulfide which can be of order of 100 lm in the anodic chamber. In the present MFC with biotic anode the diffusional distance between SRB cell and neighboring SOB cell is about 4 lm (double the cell size). Hence the diffusional fluxes of the two cases have a ratio of about 25 times, correlating with the noted ratio of polarization resistances (448 ohms/17.1 ohms = 26 times). After the SOB cell received the excess electron, the passage to anodic surface will be conducted by direct contact or indirect contact mechanism with minimum resistance. 4. Conclusion The MFC with SRB + SOB anodic biofilm was adopted to treat sulfate + lactate wastewaters. The cell efficiently converted sulfate to S0 at OCV of 730 mV and Pmax of about 62 mW m 2. Sulfide ions produced by SRB from sulfate were the key metabolite that determined the cell performance. Without biofilm, anodic surface cannot efficiently oxidise sulfide. With biofilm, SRB converted sulfate to sulfide and then the formed sulfide diffused to neighboring SOB for oxidation and release of excess electrons. Short diffusional distance of sulfide ions between cells significantly decreased the polarization resistances, hence enhancing performance of the MFC. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51278128) and National Science Council (NSC). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013.12. 129. References Chen, C., Ho, K.L., Liu, F.C., Ho, M.N., Wang, A.J., Ren, N.Q., Lee, D.J., 2013. Autotrophic and heterotrophic denitrification by a newly isolated strain Pseudomonas sp. C27. Bioresour. Technol. 145, 351–356. Chou, T.Y., Whiteley, C.G., Lee, D.J., Liao, Q., 2013. Control of dual-chambered microbial fuel cell by anodic potential: implications with sulfate reducing bacteria. Int. J. Hydrogen Energy 38, 15580–15589. Cooney, M.J., Roschi, E., Marison, I.W., Comninellis, C., Stockar, U.V., 1998. Physiologic studies with the sulfate-reducing bacterium Desulfovibrio desulfuricans: evaluation for use in a biofuel cell. Enzyme Microbiol. Technol. 18, 358–365. Ghangrekar, M.M., Murthy, S.S.R., Behera, M., Duteanu, N., 2010. Effect of sulfate concentration in the wastewater on microbial fuel cell performance. Environ. Eng. Manage. J. 9, 1227–1234. Habermann, W., Pommer, E.H., 1991. Biological fuel cells with sulphide storage capacity. Appl. Microbiol. Biotechnol. 35, 128–133. Jiang, J.Q., Zhao, Q.L., Zhang, J.N., Zhang, G.D., Lee, D.J., 2009. Electricity generation from bio-treatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100, 5808–5812. Lee, D.J., Lee, C.Y., Chang, J.S., 2012a. Treatment and electricity harvesting from sulfate/sulfide-containing wastewaters using microbial fuel cell with enriched sulfate-reducing mixed culture. J. Hazard. Mater. 243, 67–72. Lee, C.Y., Ho, K.L., Lee, D.J., Su, A., Chang, J.S., 2012b. Electricity harvest from nitrate/ sulfide-containing wastewaters using microbial fuel cell with autotrophic denitrifier, Pseudomonas sp. C27. Int. J. Hydrogen Energy 37, 15827–15832.

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