Electricity generation by two types of microbial fuel cells using nitrobenzene as the anodic or cathodic reactants

Bioresource Technology 101 (2010) 4013–4020

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Electricity generation by two types of microbial fuel cells using nitrobenzene as the anodic or cathodic reactants Jie Li, Guangli Liu *, Renduo Zhang, Yong Luo, Cuiping Zhang, Mingchen Li School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, Guangdong 510275, China

a r t i c l e

i n f o

Article history: Received 15 September 2009 Received in revised form 20 December 2009 Accepted 29 December 2009

Keywords: Microbial fuel cell Nitrobenzene Degradation Electricity generation

a b s t r a c t The effect of nitrobenzene (NB) on electricity generation and simultaneous biodegradation of NB were studied with two types of microbial fuel cells (MFCs): a ferricyanide-cathode MFC with NB as the anodic reactant and a NB-cathode MFC. Compared to controls without NB, the presence of NB in the anode of the first MFC decreased maximum voltage outputs, maximum power densities and Coulombic efficiencies. No electricity was generated from the first MFC using NB as the sole fuel; however, the second MFC using NB as the electron acceptor generated electricity successfully with a maximum voltage of 400 mV. NB was degraded completely within 24 h in both anode and cathode chambers. Denaturing gradient gel electrophoresis (DGGE) profiles demonstrated that the presence of NB caused changes in relative abundance of the dominant bacterial species and emergence of new bacteria on the anodes. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The use of microbial fuel cells (MFCs) is a promising method for wastewater treatment and power generation. Various organic materials have been used as fuels in MFCs, including easily degradable glucose (Rabaey et al., 2003), acetate (Liu et al., 2005), monosaccharides (Catal et al., 2008), complex carbohydrates (Min et al., 2005; Shimoyama et al., 2008), and some biorefractory organics, such as cellulose (Ren et al., 2007) and petroleum contaminants (Morris and Jin, 2008). Even a few toxic and biorefractory organics can be used as the fuel and degraded in the MFC. For example, Luo et al. (2009) operated an MFC using phenol as the fuel and demonstrated that phenol was degraded effectively. With different substrates as the fuel, the MFC performs differently. Catal et al. (2008) showed that some of the hydrolysates of lignocellulosic biomass cannot be utilized directly for electricity production in the MFC in the absence of other electron donors, and even inhibit electricity generation. Some non-electrode electron acceptors in the anode solution, such as nitrate, can result in electron consumption and affect performance (Liu and Logan, 2004). Sukkasem et al. (2008) found that nitrate in single chamber air cathode MFCs did affect the maximum voltage outputs at low external resistances, but not at higher external resistances. Coulombic efficiency was greatly reduced by the addition of nitrate.

* Corresponding author. Tel.: +86 20 84110052; fax: +86 20 84110267. E-mail address: [email protected] (G. Liu). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.12.135

Nitroaromatic compounds are often found in wastewaters and can be degraded by microorganisms from active sludge (Majumder and Gupta, 2003) and contaminated water or soils (Spain, 1995; Ye et al., 1995). The biodegradation pathways include one anaerobic reductive pathway and two aerobic oxidative pathways (He and Spain, 1999; Liu et al., 2002). Attributable to the strong electron affinity of the nitro group, the oxidation of NB is difficult to achieve, and normally the reductive reaction occurs first under the natural conditions (Gurevich et al., 1993). Therefore, reductive methods including chemical reduction (Nefso et al., 2005), catalytic hydrogenation reduction, electrochemical reduction (Li et al., 2007), and anaerobic reduction (Kuscu and Sponza, 2009), have attracted more and more attention. However, the reduction potential of NB (<0.700 V) (Oliveira, 2003) is lower than that of molecular hydrogen at most electrodes and thus results in decreased current efficiency. An external power supply is necessary to carry out electrochemical reduction (Li et al., 2007). Based on its characteristics, it may be possible to use NB as the cathode substrate of the MFC without an external power supply. Such a MFC should achieve simultaneous electricity generation and NB reduction. In this study, the effect of NB on electricity generation was investigated in two types of MFCs: a ferricyanide-cathode MFC with NB as the anodic reactant and a NB-cathode MFC. We also examined NB degradation rates in the anode chamber of the ferricyanide-cathode MFC using various glucose + NB mixtures as the fuel, and NB reduction rates in the cathode chamber of the NBcathode MFC. Microbial communities on the anode of the ferricyanide-cathode MFC with different substrates were also analyzed.

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2. Methods

acquisition system (DT85). The volumetric power density (PV, W/ m3) is calculated as follows:

2.1. MFC configuration

PV ¼ A dual-chambered MFC was constructed with an anode chamber and a cathode chamber, which were separated by a proton exchange membrane (PEM, Nafion 212, Dupont Co., USA). To collect solution samples easily from the anode chamber during the experiments, an anode receptacle was set up, which was a brown bottle (250 mL capacity) connected with the anode chamber. Similarly, a cathode receptacle was set up to balance the flow rates in the two chambers. The anode receptacle was filled with solutions of 200 mL throughout the experiments. Fifty mmol/L ferricyanide was used as terminal electron acceptor in the cathode. A peristaltic pump was used to cycle the flow at a constant flow rate of 20 mL/ min in the receptacles. The electrodes were made of carbon fiber brush of the same size (7.0  3.5 cm). The effective liquid volumes of the anode and cathode chambers were 27.0 mL. A copper wire was used to connect the circuit with an external resistance of 1000 X, and all exposed metal surfaces were sealed with nonconductive epoxy resin. The MFC was operated at a constant temperature (30 ± 1 °C). To examine electricity generation of the MFC with NB as the final electron acceptor in the cathode chamber, another dual-chambered MFC was constructed. The cathode electrode was made of carbon cloth (TGP-H-060, Toray, Japan) with an area about 24.5 cm2. The flexible carbon-cloth electrode contained around 0.5 mg/cm2 Pt catalyst. Other operation conditions of the NB-cathode MFC were the same as those of the ferricyanide-cathode MFC. 2.2. MFC inoculation and operation For quick start-up of the MFCs, the MFCs were inoculated with 200 mL of mixed aerobic activated sludge and anaerobic sludge (1:1, v/v) with diverse electrochemically active bacteria. The sludge inocula were collected from the Liede Municipal Wastewater Treatment Plant of Guangzhou City, China. Substrates used in the ferricyanide-cathode MFC included glucose, glucose + NB mixtures, and NB. At first, the MFC was operated with 1000 mg/L glucose as the fuel. After obtaining steady electrical outputs, the MFC was operated sequentially using 1000 mg/L glucose + 50 mg/L NB, 1000 mg/L glucose + 150 mg/L NB, 1000 mg/L glucose + 250 mg/L NB, 250 mg/L NB, and 1000 mg/L glucose as the fuel. For each type of the substrates, the MFC ran for 3–4 cycles. In the NB-cathode MFC, 1000 mg/L glucose was used as the sole anode fuel and 250 mg/L NB as the electron acceptor in the cathode. Besides the substrates, the anodic solution also contained (per liter of deionized water): 4.0896 g Na2HPO4, 2.544 g NaH2PO4, 0.310 g NH4Cl, 0.130 g KCl, trace metal solution 12.5 mL, vitamin solution 12.5 mL, and the initial pH of all solutions was adjusted to 7.0 (Lovley and Phillips, 1988). To the NB-cathode MFC, the initial pH of the cathode NB solution was adjusted to 3.0 with H2SO4. In all operations, once the voltage outputs obtained were below 100 mV, the substrate was replaced with a new solution for the next cycle. Before starting the experiment, the anode compartments of the two MFCs were flushed with N2 for 10 min to ensure an anaerobic environment, and the cathode compartment of the NB-cathode MFC was flushed with N2 for 10 min to eliminate the electrons competition between O2 and NB. 2.3. Electrical and chemical analyses Cell voltages across the external resistance were measured using a multimeter and data were automatically recorded by a data

U 2 =R V

ð1Þ

where U is the voltage (V), R is the resistance (X), and V is the effective volume (i.e., the volume of the liquid media) of the anode chamber (m3). The volumetric power density indicates how much power is generated from unit volume of wastewater. The Coulombic efficiency (CE) is defined as the percent of electrons recovered as current vs. those in the starting organic substrate. For complex substrates, it is more convenient to use the removal of COD as a measure of the substrate concentration, which is used in the CE calculation (Logan, 2008). The CE reflects the efficiency of chemical energy conversion into electricity and is calculated by:

CE ¼ 100%

P M ni¼1 U i t i RFbDSV

ð2Þ

Here Ui is the output voltage (V) of MFC at time ti (s), R is the external resistance (X), F is Faraday’s constant (96,485 C/mol), b is the number of moles of electrons produced per mol of COD (4 e mol/mol), DS is the removal of COD concentration (g/L), V is the liquid volume (L), and M is the molecular weight of oxygen (32 g/mol) (Liu et al., 2005). The polarization curve was measured by changing the external resistances from 10,000 to 20 X. For each resistance, the MFC ran at least twice to ensure to achieve repeatable power outputs, and then voltages were recorded (typically 5–10 min per resistance) (Logan et al., 2006). The power density was calculated for each resistance as a function of the current using the maximum voltage output. The internal resistance of the cell (Rint) was calculated from the slope of plots of U vs. I as follows:

U ¼ Ecell  IRint

ð3Þ

where Ecell is the electromotive force of the cell (Logan et al., 2006). Samples (2.0 mL each) were taken at 0, 0.5, 1.0, 2.0, 3.0, 6.0, 12.0, 24.0, 48.0 h, filtered through a syringe with 0.22 lm filter unit to remove cells, and diluted 5 or 10 times using deionized water. The samples were used for the following chemical analyses. The COD was measured with the dichromate method (Clesceri et al., 1998). Glucose content was measured using the anthrone method (Raunkjer et al., 1994). Concentration of NB was measured using HPLC with an acetonitrile–water (7:3) mixture as the mobile phase, maintaining a flow rate of 1 mL/min. A UV spectrophotometric detector was employed with a wavelength of 242 nm. 2.4. Microbial community analysis Biofilm samples were scratched from the anode of the ferricyanide-cathode MFC fed with 1000 mg/L glucose, 1000 mg/L glucose + 250 mg/L NB, and 250 mg/L NB, respectively. For each substrate, the bacterial samples were collected after operation of the MFC for four cycles. Bacterial genomic DNA was extracted from the biofilm samples using the FastDNAÒ SPIN Kit for Soil (MP Biomedicals, Santa Ana, CA) according to the manufacturer’s instructions. The universal primer sets V3-2 (50 -ATTACCGCGGCTGCTGG-30 ) and V3-3 (50 -CGCCCGCCGCGCGGCGGGCGGGGCGGGGGCACGG GGGGCCTACGGGAGGCAGCAG-30 ) (Invitrogen Biotechnology Co., Ltd.) were used to amplify the V3 region of bacteria 16S ribosomal DNA (rDNA) from the extracted genomic DNA. PCR amplification was performed in a 25 lL reaction volume, including 17.37 lL ddH2O, 2.5 lL 10  PCR Buffer, 2.0 lL DNTP mixture, 0.5 lL V3-2, 0.5 lL V3-3, 0.13 lL Taq polymerase, and 2 lL template. The amplification involved an initial denaturation at 94 °C for 5 min;

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followed by 10 cycles, each of which included 30 s of denaturation at 94 °C, 30 s of annealing at 61 °C (the temperature of anneal decreased 0.5 °C after each cycle), and 1 min of extension at 72 °C; then 25 cycles, each of which included 30 s of denaturation at 94 °C, 30 s of annealing at 55 °C, and 1 min of extension at 72 °C, with a final 7 min extension at 72 °C. The DGGE analysis of the PCR products was carried out in a denaturing gradient gel electrophoresis system (C.B.S. Scientific, Del Mar, CA, USA). The 8% (w/v) polyacrylamide gels (16 cm  16 cm gel, thickness of 0.75 mm) contain 40–60% denaturing gradients (urea and formamide). Electrophoresis was conducted using a 1  TAE buffer at 200 V and 60 °C for 5 h. After electrophoresis, the gel was stained with 5 lL/(100 mL) ethidium bromide (EB) in 1  TAE buffer for 15 min and destained in 1  TAE buffer for 10 min. The fragments were visualized under a UV transilluminator. 16S rDNA gene fragments cut out from the DGGE gel were triturated, added into 20 lL TE, then centrifugalized at 15,496g for 2 min after 30 min water bath at 50 °C. The supernatant fluid was used for PCR amplification, and the PCR program was the same

as that mentioned above, but using the universal primer sets of V31 (50 -CCTACGGGAGGCAGCAG-30 ) and V3-2 (50 -ATTACGCGGCTGCTGG-30 ). The PCR products were used for sequencing, and then the sequences were compared directly to all known sequences deposited in the GenBank databases using the basic local alignment search tool (BLAST).

3. Results and discussion 3.1. Effect of NB on electricity generation in the ferricyanide-cathode MFC After the ferricyanide-cathode MFC was operated using 1000 mg/L glucose as the fuel to obtain stable outputs of electric voltage, mixtures of 1000 mg/L glucose + 50 mg/L NB, 1000 mg/L glucose + 150 mg/L NB, 1000 mg/L glucose + 250 mg/L NB, and 250 mg/L NB, and 1000 mg/L glucose sequentially were used in the anode as the substrates. Some representative curves of voltage outputs are shown in Fig. 1A. The MFC using the glucose-NB mix-

A 700 600 0 mg/L NB 50 mg/L NB 150 mg/L NB 250 mg/L NB

Voltage (mV)

500 400 300 200 100 0 0

B

10

20

30 Time (h)

40

50

30 0 mg/L NB 50 mg/L NB 150 mg/L NB 250 mg/L NB

25 3

Power density (W/m )

60

20 15 10 5 0 0

20

40 60 80 3 Current density (A/m )

100

120

140

Fig. 1. Influence of nitrobenzene (NB) on (A) voltage outputs (at the external resistance of 1000 X), and (B) power density in the ferricyanide-cathode MFC, calculated based on the voltages marked with the circles in (A).

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tures as the fuel was operated successfully; however, the addition of NB affected electricity generation of the MFC. With the NB concentrations increasing from 0 to 250 mg/L, the maximum voltage outputs decreased from 670 to 489 mV, the maximum volumetric power densities decreased from 28.57 to 8.47 W/m3, and the internal resistances increased from 400 to 1000 X. The CE and electric charge of the MFC decreased with the NB concentrations linearly with the coefficients of determination (R2) 0.71 and 0.79, respectively (Table 1). As shown in Fig. 1A, the maximum voltage outputs decreased and the operation periods became shorter with increasing NB concentrations. In addition, the voltage outputs were not able to return to their original level immediately after the replacement of 250 mg/L NB by 1000 mg/L glucose as the fuel at the final stage. The effect of NB on electricity generation of the MFC was more profound than that of some other electron-withdrawing compounds. Sun et al. (2009) reported that a concentration of active brilliant red X-3B (ABRX3) of 1500 mg/L resulted in a decrease of the maximum voltage about 25%, whereas 250 mg/L NB in the anode chamber resulted in a decrease of the maximum voltage output 27%. Sukkasem et al. (2008) showed that a nitrate concentration of 496 mg/L did not affect the maximum voltage outputs at all. The decreasing voltage outputs and shortened operation periods are possibly attributable to electron consumption by NB reduction, rapid consumption of the available carbon source by microbes at a higher NB concentration, and/or inhibition of electricity-generating microbes in the MFC because of the NB toxicity.

As shown in Fig. 1B, NB affected power density significantly. Both the maximum power density and the ranges of current density decreased quickly with the NB concentrations. The power density was strongly reduced by NB and the internal resistance increased correspondingly. The presence of NB as alternative electron acceptor and its toxic effects on the microbial physiology were the dominant factors for the reduction in power output and the increase of the internal resistance. The competition for electrons by NB could lead to a reduction in electron supply. In addition, the proton–electron transfer process in the MFC might be impeded because of NB toxicity. The addition of NB might change the community structure and decrease the population of certain microbes such as Hansenula anomala, which could transfer electrons directly to electrode surfaces in presence of glucose as the carbon source (Prasad et al., 2007). Compared to previous studies (Bond and Lovley, 2003; Chaudhuri and Lovley, 2003), CEs of the MFC were lower in this study. Many factors might attribute to low CEs, including substrate utilization for methanogenesis and the electron transfer from substrate to other non-electrode electron acceptors in the solution, such as nitrate and oxygen (Liu and Logan, 2004). The consumption of electrons for anaerobic degradation of NB may be the most important reason for the low CEs since under anaerobic conditions, the reductive reaction of NB occurs first due to the strong electron affinity of the nitro group (Gurevich et al., 1993). To examine if NB could be used as the fuel of the MFC, the concentration of NB was maintained at 250 mg/L and the concentrations of glucose were set up at 1000, 750, 500, 250, and 0 mg/L,

Table 1 Comparison of the MFC performance with the different substrates: the mixtures of 1000 mg/L glucose with different concentrations of nitrobenzene (NB). Maximum voltage (mV)

Maximum power density (W/m3)

Internal resistance (X)

CE (%)

Electric charge (C)

0 50 150 250

670.0 ± 7.07a 597.7 ± 24.75 507.7 ± 7.78 489.0 ± 7.07

28.57 ± 0.704 20.24 ± 0.750 9.29 ± 0.493 8.47 ± 0.551

400.0 ± 70.71 600.0 ± 0.00 950.0 ± 70.71 1000.0 ± 0.00

2.70 ± 0.096 1.48 ± 0.042 1.24 ± 0.078 1.00 ± 0.156

65.10 ± 2.322 43.50 ± 11.456 35.48 ± 0.706 30.32 ± 0.588

The values are mean ± standard deviation (n = 3).

Removal (%)

a

NB concentrations in the mixture (mg/L)

140 120 100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0

250 mg/L NB

150 mg/L NB

50 mg/L NB

glucose

NB

COD

0

50

100

150 Time (h)

200

250

Fig. 2. Removal rates of glucose, nitrobenzene (NB), and COD in the ferricyanide-cathode MFC.

300

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respectively. When 250 mg/L NB was used as the sole fuel, a voltage about 70 mV was produced, but it gradually decreased to 30 mV within 140 h. The result showed that NB was not suitable as a sole fuel of the MFC because NB was a strong electron acceptor and NB degradation proceeded by a partial reductive pathway in the anaerobic anode chamber. 3.2. Chemical removal in the ferricyanide-cathode MFC The NB degradation and organic matter removal in the ferricyanide-cathode MFC were investigated with different initial NB concentrations (50–250 mg/L) using glucose as a co-substrate. Fig. 2 shows the removal rates of glucose, NB, and COD in the ferricyanide-cathode MFC. Using the traditional anaerobic degradation method in a hybrid reactor, Majumder and Gupta (2003) showed that the removal rates of NB and COD were 80–90% and 60–96%, respectively. In our experiment, the removal rates of NB were nearly 100% for all the NB initial concentrations and the COD removal rates were 87–98% in the MFC. The result showed that NB could be biodegraded successfully in the MFC with glucose together. NB anaerobic degradation relies on the addition of biodegradation organic compounds as electron donors (Li et al., 2007). The higher removal rate of NB in the MFC in this study might be attributed to the presence of glucose, which was easily biodegraded to supply electrons to increase the metabolic rate of anaerobic bacteria with sufficient anaerobic terminal electron acceptors (Morris et al., 2009).

The results suggest that the presence of NB caused changes with respect to the dominant bacterial species on the anodes, including changes in relative abundance and emergence of new bacteria. The DNA gene sequences represented by bands 1–4 were submitted to the GenBank of the NCBI and the accession numbers obtained from the GenBank database were GQ870291–GQ870294. Based on a comparative analysis of the 16S rRNA gene sequences, several groups of bacterial species were identified. The BLAST results are summarized in Table 2. The BLAST results indicated that the dominant bacterial species on the anodes of the MFC with glucose as the fuel shared 100% and 88% 16S rDNA sequence homology with Enterobacter sp. (band 1) and Geobacter sp. (band 4), respectively. Geobacter sp. has been frequently reported to be responsible for power generation in the MFC (e.g., Bond and Lovley, 2003; Kaufmann and Lovley, 2001). Addition of NB resulted in decreased intensities of bands 1 and 4, and increased intensities of bands 2 and 3. Band 2 might belong to Pseudomonas sp. or Comamonadaceae bacteria, both of which are able to degrade NB (He and Spain, 1999). Band 3 shared 97% 16S rDNA sequence identity with Desulfovibrio sp., a sulfur-reducing bacterium that can oxidize acetate in a two-electrode fuel cell (Bond et al., 2002). Above results suggested that the addition of NB changed the electricity-generating bacteria structure and led to rapid increase of NB degrading bacteria. Therefore, the changes in the microbial community structure should be another factor that affects electricity generation of the MFC. 3.4. Power generation from the NB-cathode MFC

3.3. Microbial community Fig. 3 shows the DGGE profiles of the 16S rDNA gene fragments amplified from the extracted DNA of the biofilms on the anodes.

To examine if NB was involved in the electricity generation process by accepting electrons from circuit without any external power input, the dual-chamber NB-cathode MFC was operated

Fig. 3. PCR-DGGE analysis of 16S rDNA extracted from the electrodes of the MFC fed with different substrates (G: 1000 mg/L glucose, G + NB: 1000 mg/L glucose + 250 mg/L nitrobenzene mixture, NB: 250 mg/L nitrobenzene).

Table 2 Characterization of isolated 16S rRNA gene fragments derived from the anodes of the MFCs with different substrates.

a b

Sequence designation

Accession number

BLAST search results

Identity (%)

Band 1a

GQ870291

GQ247734.1Enterobacter sp. FW17a GQ465230.1 Enterobacter sp. IPB54

100 100

Band 2b

GQ870292

GQ129889.1 Uncultured Pelomonas sp. clone GI8-sp-D04 AF368755.1 Pseudomonas saccharophia FJ786064.1 Pseudomonas sp. III-116-17 EU723136.1 Pseudomonadales bacterium kmd_137 EU112283.1 Uncultured Comamonadaceae bacterium B00055B41

98 98 97 97 97

Band 3b

GQ870293

GQ503869.1 Desulfovibrio sp. enrichment culture clone Ecwsrb032 FJ393114.1 Desulfovibrio sp. clone MFC-B162-E12 EF592815.1 Uncultured sulfate-reducing bacterium clone 2R2V06

97 96 96

Band 4a

GQ870294

GQ390422.1 Uncultured Geobacter sp. clone C12 EU704648.1 Uncultured bacterium clone MFC-GIST 439 FJ262598.1 Uncultured Geobacter sp. clone MFC-A36

88 88 84

Samples from the anodes of the MFC with glucose as the fuel. Samples from the anodes of the MFC with mixture of glucose and nitrobenzene, nitrobenzene as the fuel.

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using 1000 mg/L glucose as the anode fuel and 250 mg/L NB as the cathode electron acceptor. Some typical curves of electrical voltage outputs are presented in Fig. 4A. The MFC was operated for three cycles continuously for a total of about 60 h, the operation period per cycle varied from 18 to 25 h, and the maximum voltage output in each cycle was about 400 mV (387–410 mV). The voltage output reached the maximum value rapidly within 1 h after replacing the new substrate, and then decreased to about 280 mV within 6 h, and slightly increased to 285 mV for about 5 h, and finally gradually decreased. Fig. 4B shows the relationships between the cell potential and power density vs. the current density. The cell potential decreased linearly with the current density. Based on Fig. 4B and Eq. (3), an internal resistance of 626 X was obtained. The maximum power density achieved from the NB-cathode MFC was 6.30 W/m3 (69.442 mW/m2) at the current density of 19.72 A/m3 (0.2173 A/ m2). The performance of the NB-cathode MFC was worse than the ferricyanide-cathode MFC using 1000 mg/L glucose as the fuel reported above, which should be closely related to the negative redox potential of NB. Nevertheless, the result indicated that in the tested NB-cathode MFC system, NB could accept electrons through the circuit and involve in the electricity generation process.

3.5. Cathodic reduction of NB in the NB-cathode MFC In the NB-cathode MFC, NB was degraded almost completely. As shown in Fig. 5, the removal efficiency of 250 mg/L NB was up to 73% when the voltage output reached the maximum value (about 400 mV). Then, after about 10 h of operation the voltage output increased slightly to 285 mV, and the removal efficiency reached 88% correspondingly. The removal efficiency reached 98% at the end of the cycle. These results can be explained by the production of intermediates during NB reduction that allowed maximum voltage output. These intermediates could include phenylhydroxylamine, nitrosobenzene, azoxybenzene, and azobenzene (Li et al., 2007; Oliveira, 2003). These intermediates were then reduced to aniline and voltage output was maintained. It is known that aniline is the end product of NB reductive degradation at the cathode of bioelectrochemical systems (Mu et al., 2009), but in our reactor system, Pt was used as catalyst and the solution was strongly acidic. Under these conditions, aniline was prone to polymerization to polyaniline (Gangopadhyay and Amitabha, 2000). Further research is needed to confirm this hypothesis. Cathodic reduction of NB in the MFC was more efficient than conventional electrochemical reduction (Li et al., 2007). In our

A 500 450

Voltage (mV)

400 350 300 250 200 150 100 50 0

B

10

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30 Time (h)

40

50

7

700 Power density Voltage

6

600

5

500

4

400

3

300

2

200

1

100

0

Voltage (mV)

Power density (W/m3)

60

0 0

5

10

15

20

25

30

35

40

3

Current density (A/m ) Fig. 4. (A) Electricity generation (at the external resistance of 1000 X), and (B) polarization and power density of the nitrobenzene-cathode MFC, calculated based on the voltage marked with the circle in (A).

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100

400 Voltage Removal

80 60

200

40 100

Removal (%)

Voltage (mV)

300

20 0 0 0

10

20

Time (h)

30

40

50

Fig. 5. Removal rates of nitrobenzene (NB) in the nitrobenzene-cathode MFC.

experiment, the average removal rate within the initial 1 h reached 3.04 mg/(L min), which was higher than the average removal rate (2.34 mg/(L min)) reported by Li et al. (2007). Furthermore, the conventional electrochemical reduction required an external power input, whereas the cathodic reduction of NB in the MFC did not consume but produced electricity power. The removal efficiency obtained in this study is comparable to that of some studies on anaerobic degradation. For example, Zhang et al. (2007) reported that with Fe2+/Fe3+ concentrations of 100–200 mg/L, the NB anaerobic degradation increased, resulting in a NB removal efficiency of 96%. Liu et al. (2007) observed that the NB removal efficiency reached up to 99% at concentrations ranging from 50 to 180 mg/L in a sequencing batch reactor (SBR). Therefore, our results may provide a novel approach for simultaneous electricity generation and treatment of wastewaters containing NB, and even other nitroaromatic compounds. In this study, NB was degraded efficiently in the anode and cathode chambers of the ferricyanide-cathode MFC and the NBcathode MFC, respectively. In the anode and cathode chambers, the removal rates of NB reached 99% and 98% within 24 h, and the average removal rates within the initial 1 h were 2.35 and 3.04 mg/(L min), respectively. However, the degradation mechanisms in the anode and cathode chambers of the two MFCs were obviously different. In the anode chamber of the ferricyanide-cathode MFC, NB was degraded by anaerobic microorganism through a partial reductive pathway. In the cathode chamber of the NB-cathode MFC, the NB reduction was just a pure electrochemical reduction process, no microorganism involving.

4. Conclusions The effect of NB on the performance of the ferricyanide-cathode MFC and simultaneous electricity generation and reduction in the NB-cathode MFC were assessed. The addition of NB significantly inhibited electricity generation by the ferricyanide-cathode MFC primarily due to electron competition between NB and the anode. The presence of NB resulted in changes of the dominant bacterial species on the anodes possibly because of NB toxicity. In the NBcathode MFC, NB participated in the electricity generation process. In addition, NB was degraded efficiently in the anode and cathode chambers of the ferricyanide-cathode MFC and the NB-cathode MFC, respectively.

Acknowledgements This work was partially supported by grants the Natural Science Foundation of China (Nos. 50608070 and 50779080).

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