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Journal of Hazardous Materials 176 (2010) 759–764

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

lectricity generation from indole and microbial community analysis in theicrobial fuel cell

ong Luo, Renduo Zhang, Guangli Liu ∗, Jie Li, Mingchen Li, Cuiping Zhangchool of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, China

r t i c l e i n f o

rticle history:eceived 28 September 2009eceived in revised form8 November 2009ccepted 18 November 2009vailable online 24 November 2009

a b s t r a c t

Indole is a typical refractory and inhibitory compound present in coking wastewater. The aim of thisstudy was to investigate possible electricity generation with indole degradation in the microbial fuelcell (MFC). Experiments were conducted in two types of the MFC: a continuous-fed MFC (C-MFC) and abatch-fed MFC (B-MFC). In the C-MFC, the maximum power densities reached 45.4, 51.2, and 2.1 W/m3,respectively, from using 1000 mg/L glucose, a mixture of 1000 mg/L glucose and 250 mg/L indole, and250 mg/L indole as the fuel. When using 250 mg/L indole as the fuel, the removal efficiency of indole

eywords:ndole

icrobial fuel cellegradationlectricity generation

was up to 88% within 3 h. Increasing indole concentrations from 250 to 1500 mg/L resulted in decreaseof the maximum power densities from 2.1 to 0.8 W/m3, and average degradation rates from 41.7 to8.9 mg/(L h). Compared with the C-MFC, the B-MFC increased the maximum power densities from 2.1 to3.3 W/m3 and the coulombic efficiencies from 0.7% to 81.5%. Microbial community analyses showed thatthe addition of indole obviously changes the microbial community of the anode electrode, including thechanges of relative abundance and emergence of new species. The results should be useful for treatment

indo

of wastewater containing

. Introduction

Coking wastewater, one of extremely toxic industrial effluentso the environment, is generated from the processes of coal car-onization and fuel classification in the iron and steel industry [1,2].t present, coking wastewater is treated mainly with the conven-

ional activated sludge system [3]. However, several representativeoxic and refractory compounds, including indole, are frequentlyound in the coking wastewater [4–7]. These refractory compoundsave a great inhibition on the performance of the biological treat-ent method, resulting in relatively high chemical oxygen demand

COD) in the effluents [4]. Therefore, it is important to study thereatment processes to decompose these toxically refractory com-ounds efficiently [8].

The microbial fuel cell (MFC) has recently drawn wide interestss a new method of directly generating electricity from wastewa-ers, and simultaneously treating the wastewaters [9]. It has beeneported that microorganisms in the MFC can convert many kinds of

rganics, such as acetate, glucose, cysteine, and xylose, to generatelectricity [10–13]. Recently, the use of the MFC for the biodegra-ation of and simultaneous power generation from recalcitrantontaminants in the coking wastewater has made a significant

∗ Corresponding author. Tel.: +86 20 84110052; fax: +86 20 84110267.E-mail address: liugl@mail.sysu.edu.cn (G. Liu).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.11.100

le.© 2009 Elsevier B.V. All rights reserved.

progress. Luo et al. [14] demonstrate that phenol can be utilizedas the fuel to produce electricity in the MFC. They also show thatthe degradation efficiencies of phenol in the MFC are more than95%. Pyridine and quinoline can also be quickly degraded in theMFC, although the organics may not make a great contribution toelectricity generation [15,16]. However, indole, a typical recalci-trant contaminant present in the coking wastewater, has not beentested as the fuel in the MFC.

The objective of this study was to investigate the feasibilityof the biodegradation of indole and simultaneous power genera-tion through a continuous-fed MFC (C-MFC) and a batch-fed MFC(B-MFC). We also examined the effect of different indole con-centrations on electricity generation and analyzed the microbialdiversity of the anodic biofilm using the denaturing gradient gelelectrophoresis (DGGE).

2. Materials and methods

2.1. MFC set up

The B-MFC consisted of an anode and a cathode, which were sep-

arated by a proton exchange membrane (PEM, Nafion 212, DupontCo., USA). The electrodes were made of carbon fiber brush with 7 cmin length and 3.6 cm in width. The cathode compartment of theMFC contained 50 mM ferricyanide, used as the electron acceptor.The net volumes of the anode and cathode chambers were 18 mL,

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espectively. Copper wires were used to connect the circuit with anxternal resistance of 1000 � and all wire contacts were sealed withpoxy material. The MFC was operated at a constant temperature30 ± 1 ◦C).

The C-MFC also consisted of an anode chamber and a cath-de chamber, each of which was connected with a brown bottle250 mL capacity) as the anode (or cathode) groove. A flow rate of0 mL/min was controlled by a peristaltic pump for solution circula-ion between each pair of the brown bottle and the chamber. Othereactor configuration and operation conditions were the same ashe B-MFC described above.

.2. Microbial inoculum and operation

Anaerobic and aerobic sludge (1:1, 10 mL) was taken from Liedeunicipal Wastewater Treatment Plant, Guangzhou City, and inoc-

lated in the C-MFC with a glucose solution of 1000 mg/L and annodic solution. The anodic solution contained (in 1 L deionizedater): 4.0896 g Na2HPO4, 2.544 g NaH2PO4, 0.31 g NH4Cl, 0.13 gCl, and 12.5 mL trace metal solution and 12.5 mL vitamin solu-

ion [17]. After stable voltage outputs were achieved and kept forore than two cycles, a mixture of 1000 mg/L glucose + 250 mg/L

ndole was used to replace the solution in the MFC. Again aftertable voltage outputs were achieved and kept for more than twoycles, we used an indole solution of 250 mg/L as the fuel to con-inue operating the MFC. Before the operation of each cycle, thenode compartment of the MFC was flushed with N2 for 10 min tonsure an anaerobic environment.

The B-MFC was inoculated with active microorganisms takenrom the anode of the C-MFC, which was incubated with the sub-trate of 1000 mg/L glucose. When a stable power output wasbtained from the MFC, we used an indole solution of 250 mg/Ls the fuel continuously to incubate the active microorganisms.

.3. Analyses and calculation

Samples from the anode solutions in the B-MFC and C-MFC werereated by filtered through a membrane (with a pore diameterf 0.22 �m) to remove cells. Glucose concentrations of the sam-les were determined using the anthrone method [18]. Chemicalxygen demand (COD) was measured according to the standardethod [19]. Indole concentrations were analyzed using HPLC (Agi-

ent 1100), in which 60% methanol and 40% water were used as theobile phase and a flow rate of 1 mL/min was maintained. A UV

pectrophotometric detector was employed with a wavelength of85 nm.

Voltages of the MFCs were measured at a time interval of 30 scross the external resistance (1000 �) using a data acquisition sys-em (DT50). The current was calculated based on the voltage andhe eternal resistance. The volumetric power density (PV, W/m3)as calculated as follows:

V = UI

V(1)

here I is the current (A), U is the voltage (V), V (m3) is thevailable volume of anodic compartment. The volumetric powerensity indicates how much power is generated from unit volumef wastewater.

Coulombic efficiency (CE) is defined as the ratio of totaloulombs actually transferred to the anode from the substrate to

he maximum possible coulombs if all substrate removal producesurrent. The CE (%) is calculated by:

E = 100%

∑ni=1Uiti

RFb�SVM = 100%

EM

Fb�SV(2)

aterials 176 (2010) 759–764

Here Ui is the output voltage of MFC at time ti, R is the external resis-tance (1000 �), F is Faraday’s constant (96,485 C/mol electrons), bis the number of moles of electrons produced per mol of the COD(4 mol e−/mol COD), �S 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), and E is the coulombic number.

Polarization curves were generated by varying the externalresistances from 20 to 8000 �. For each resistance, at least twocycles of each of the MFCs were operated to ensure that repeatablevoltage outputs were achieved. Averaged voltages from the outputswere used to calculate the power density (W/m3).

2.4. Microbial community analysis from anode electrode of theC-MFC

2.4.1. DNA extraction and polymerase chain reaction (PCR)amplification

Amplification biofilms were scratched from the anode elec-trodes of the C-MFC fed with different substrates (glucose,a mixture of glucose and indole, and indole). DNA was iso-lated from these bacteria using the FastDNA SPIN for soilkit (MP BIO, IIIKirch, France) and stored in −20 ◦C. Theuniversal primer sets V3-2 (5′-ATTACCGCGGCTGCTGG-3′) andV3-3 (5′-CGCCCGCCGCGCGGCGGGCGGGGCGGGGGCACGGGGGG-CCTACGGGAGGCAGCAG-3′) (Invitrogen Biotechnology Co., Ltd.)were used to amplify the V3 region of 16S ribosomal DNA (rDNA)from the extracted genomic DNA. The PCR amplification was per-formed in a 25 �L reaction volume, which was contained thefollowing ingredients: 17.37 �L dd H2O, 2.5 �L 10× PCR Buffer;2.0 �L DNTP mixture; 0.5 �L V3-2; 0.5 �L V3-3; 0.13 �L Taq poly-merase; and 2 �L template. The amplification involved an initialdenaturation at 94 ◦C for 5 min; followed by 10 cycles, each of whichincluded 30 s of denaturation at 94 ◦C, 30 s of annealing at 61 ◦C (thetemperature of anneal decreased 0.5 ◦C after each cycle), and 1 minof extension at 72 ◦C; then 25 cycles, each of which included 30 sof denaturation at 94 ◦C, 30 s of annealing at 55 ◦C, and 1 min ofextension at 72 ◦C, with a final 7 min extension at 72 ◦C.

2.4.2. Denaturing gradient gel electrophoresis (DGGE)The DGGE analysis of the PCR products was carried out in a dena-

turing gradient gel electrophoresis system (C.B.S. SCIENTIFIC, DelMar, CA, USA). The 8% (w/v) polyacrylamide gels (16 cm × 16 cm gel,thickness of 0.75 mm) contain 40–60% denaturing gradients (ureaand formamide). Electrophoresis was conducted using a 1× TAEbuffer at 200 V and 60 ◦C for 5 h. After the test, the gel was stainedusing 5 �L/(100 mL) ethidium bromide (EB) in 1× TAE buffer for15 min and destained in 1× TAE buffer for 10 min. The fragmentswere visualized under an UV transilluminator at 254 nm. The bandsthat shared identical migration position were considered as thesame species.

2.4.3. 16S rDNA gene sequencing and analysis16S rDNA gene fragments cut out from the DGGE gel were

triturated, added into 20 �L TE, and then centrifugalized at12,000 rpm for 2 min after 30 min water bath at 50 ◦C. The super-natant fluid was used for PCR amplification, and the PCR programwas the same as that mentioned above, but using the univer-

sal primer sets of V3-1 (5′-CCTACGGGAGGCAGCAG-3′) and V3-2(5′-ATTACGCGGCTGCTGG-3′). The PCR products were used forsequencing, and then the sequences were compared directly to allknown sequences deposited in the GenBank databases using thebasic local alignment search tool (BLAST).

Y. Luo et al. / Journal of Hazardous Materials 176 (2010) 759–764 761

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Fig. 2. Electricity voltage outputs of the C-MFC using 250 mg/L and 500 mg/L indoleas the fuel. The arrows show the time of anode solution replacement.

ig. 1. Electricity voltage outputs of the C-MFC using 1000 mg/L glucose and a mix-ure of 1000 mg/L glucose + 250 mg/L indole as the fuel. The arrows show the timef anode solution replacement.

. Results and discussions

.1. Power generation from the C-MFC and the B-MFC

When 1000 mg/L glucose was used as the fuel in the C-MFC,lectricity generation began to increase noticeably and reached aaximum voltage output of approximately 670 mV (Fig. 1). Cor-

espondingly, the maximum power density reached 45.4 W/m3. Itas showed that the bacteria easily adapted to glucose, and the

ultivated time, i.e., the period from adding glucose to generatingtable power, was less than 5 h.

Voltages were rapidly generated when using a mixture of50 mg/L indole and 1000 mg/L glucose as the fuel. The power out-ut was similar to that using glucose alone as the fuel. A maximumoltage of 660 mV and a maximum power density of 51.2 W/m3

ere obtained with the mixture in the MFC. However, the operationeriod from the mixture fuel was prolonged to 70 h.

Typical voltage outputs vs. time are shown in Fig. 2 for using 250nd 500 mg/L indole as the fuel, respectively. The maximum volt-ges were in the range from 110 to 145 mV and operation periodsere in the range from 20 to 30 h. The cultivated time for indoleas about 13 h.

After the stable power generation was achieved with 1000 mg/Llucose, 250 mg/L indole was used in the B-MFC as the fuel. One rep-esentative cycle is shown in Fig. 3. The maximum voltage reached20 mV within 18 h and the operation period was up to 110 h. Fig. 4hows the relationships between the cell potential and power den-ity vs. the current density. A maximum power density of 3.3 W/m3

3

as obtained with indole as the fuel at a current density of 9.5 A/mn the MFC.

To ascertain the effect of indole concentrations on the char-cteristics of the MFC, indole concentrations increased from 250o 1500 mg/L in the C-MFC. As shown in Table 1, the maximum

Table 1The maximum power densities and indole degradation rates changing with th

Concentration (mg/L) Maximum power density (

250 2.1 ± 0.12a

500 2.3 ± 0.11750 1.7 ± 0.09

1000 1.2 ± 0.131250 1.1 ± 0.111500 0.8 ± 0.14

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

Fig. 3. A representative cycle of electricity voltage output from the B-MFC using250 mg/L indole as the fuel.

power densities slightly increased from 2.1 to 2.3 W/m3 whenthe indole concentrations increased from 250 to 500 mg/L. Fur-ther increase in indole concentrations led to sharp reduction ofthe maximum power densities. The maximum power densityobtained with 1500 mg/L indole was only 0.8 W/m3. The resultsshowed that the lower indole concentrations (250–500 mg/L) didnot affect power density significantly, while the higher concentra-tions (750–1500 mg/L) severely inhibited the power generation.

The time-average indole degradation rates decreased with theindole concentrations exponentially (R2 = 0.97) as follows:

D = 58 exp(−0.0 015C) (3)

e indole concentrations in the C-MFC.

W/m3) Indole degradation rate (mg/(L h))

41.7 ± 0.925.0 ± 0.316.9 ± 0.713.2 ± 1.110.6 ± 0.9

8.9 ± 0.8

762 Y. Luo et al. / Journal of Hazardous Materials 176 (2010) 759–764

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Table 2Biodegradation rates (%) of glucose, indole, and COD (with the initial value of1805 mg/L) in the C-MFC using a mixture of 1000 mg/L glucose and 250 mg/L indoleas the fuel.

Time (h) Glucose Indole COD

0 0.0 0.0 0.03 88.4 ± 0.3a 21.2 ± 1.5 59.2 ± 0.56 100.0 ± 0.0 82.0 ± 1.5 87.6 ± 0.2

10 100.0 ± 0.0 100.0 ± 0.0 89.5 ± 0.3

ig. 4. Power density and cell potential as functions of current density for the B-MFC.

n which D is the time-average indole degradation rate and C is thendole concentration used in the MFC.

The maximum power densities achieved from the B-MFC and-MFC in our study were higher than the maximum power densityf 1.7 W/m3 generated from pyridine as the fuel in the similar MFC15]. However, comparison of the maximal voltage outputs usinghe mixture of indole + glucose and indole only as the fuel (Fig. 5)howed that indole restrained the microbial activities to generatelectricity in the absence of glucose. Furthermore, the results fromable 1 showed that the higher initial indole concentrations (e.g.,xceeding 500 mg/L) further caused substrate inhibition of the exo-lectrogenic or fermentative bacteria or affected the metabolismsf these bacteria [15], and consequently reduced the maximumower density. In the literature, some substrates, such as xylose andyridine, were well known to inhibit bacterial activity and growth13,15].

The co-substrate (indole with glucose) could enhance powereneration because more electrons might be generated in unit timerom the synchronous degradation of organics and glucose by theame or different consortia of microbes [14,15]. For example, Zhangt al. [15] reported that the coulombic number from using theyridine–glucose mixture as the fuel was 3.7 C higher than the sum

f coulombic numbers using from individual pyridine and glucoses the fuel. Surprisingly, in our experiment, the coulombic numberrom using the indole–glucose mixture as the fuel was about 70 Cigher than the sum of coulombic numbers using individual indole

ig. 5. Comparison of voltage output for the C-MFC using 1000 mg/L glucose, aixture of 1000 mg/L glucose + 250 mg/L indole, and 250 mg/L indole as the fuel.

40 100.0 ± 0.0 100.0 ± 0.0 90.4 ± 0.570 100.0 ± 0.0 100.0 ± 0.0 90.8 ± 0.5

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

and glucose as the fuel. The result showed that compared with pyri-dine, indole under the co-substrate situation had a stronger abilityto stimulate the activity of electricity-generate bacteria so that thebacteria could recover more electrons from the mixture of indoleand glucose than those from the mixture of pyridine and glucose.

3.2. Substrate utilization in the MFCs

Biodegradation rates of glucose, COD, and indole in one typicalelectrical cycle are listed in Table 2 for using a mixture of 1000 mg/Lglucose and 250 mg/L indole as the fuel in the C-MFC. Within 3 h, thebiodegradation rates of glucose and indole reached 88.4% and 21.2%,respectively, and the corresponding COD removal was 59.2%. Indolewas completely removed within 10 h, while 1000 mg/L glucose wascompletely removed within 6 h (Table 2). At the end of the cycle,the COD removal rate in the MFC was 90.8%.

The removal rates of indole and COD measured from the secondcycle in Fig. 2 are shown in Fig. 6 for using 250 and 500 mg/L indoleas the fuel in the C-MFC. For using 250 mg/L indole as the fuel, theremoval rates of indole reached up to 100% within 6 h. Correspond-ingly, the COD removal was about 95% at the end of the cycle. Whenusing 500 mg/L indole as the fuel, it took about 30 h for indole tobe completely degraded, and the corresponding COD removal wasabout 87% (Fig. 6). In the B-MFC with 250 mg/L indole as the fuel,the removal rate of indole was 87% at the end of a cycle, and thecorresponding COD removal was 70%.

It is demonstrated that co-substrate with glucose often enhance

organic degradation [14,15]. However, in our study, when using250 mg/L indole as the sole fuel in the C-MFC, indole was com-pletely degraded within 6 h (Fig. 6), while it took 10 h to degrade250 mg/L indole completely when the glucose–indole mixture was

Fig. 6. The removal rates of indole obtained from the C-MFC with 250 mg/L and500 mg/L indole as the fuel.

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sed as the fuel (Table 2). It was also observed from Table 2 thatithin 3 h, the biodegradation rate of glucose reached 88.4%, while

t the time the biodegradation rate of indole was only 21.2% for theixture as the fuel. The experimental results showed that the co-

ubstrate with glucose did not enhance indole degradation becausehe microorganisms in the MFC firstly consumed readily biodegrad-ble glucose, and then utilized the recalcitrant indole to generatelectricity. Compared with toxic and refractory compound, such asndole, glucose is easier biodegraded to supply electrons to increasehe metabolic rate of anaerobic bacteria with sufficient anaerobicerminal electron acceptors [20].

Ren et al. [21] reported that more than 99% of indole wasegraded by White rot fungus (Pleurotus ostreatus) within 216 h,nd the initial concentration of indole was 200 mg/L. Katapodist al. [22] reported that a bacteria stain S. thermophile could com-letely degrade 1000 mg/L indole after 6 days. In our current study,50 mg/L and 500 mg/L indole were removed completely in the-MFC within 6 and 30 h, respectively (Fig. 6). The quick degrada-ion of indole might be attributed to the following three aspects.irst, after acclimation for more than 2 years, the mixed bacte-ia in the anode culture from the anaerobic and aerobic sludgead strong degradation characteristic. It was demonstrated thathe mixed bacteria could also quickly degraded other recalcitrantontaminants, such as phenol, pyridine, and quinoline [14–16]. Sec-ndly, compared with the conventional carbon paper, the brushber material selected as the anode electrode in the MFC adsorbedore microbes because of its larger surface area [23]. Thirdly, theFC with ferricyanide as catholyte used in our study could enhance

rganic degradation as compared to the open-circuit control, inhich the normal anaerobic metabolism was prevailed [14].

The COD removal efficiency of the C-MFC was 25% higher thanhat of the B-MFC when using 250 mg/L indole as the fuel. Theigher COD removal efficiency achieved from the C-MFC has alsoeen demonstrated by our previous study [24]. However, the CE

n the C-MFC was only 0.7%, while the CE achieved from the-MFC was up to 81.5%. The low CE from the C-MFC might beainly attributed to the non-electricity-generating microbes (e.g.,ethanogens) existing in the anode grove connected with the

node compartment [14], which consumed the majority of COD25,26].

The previous and current studies indicate that it is indeed possi-le to efficiently utilize the toxic and refractory compounds existing

n the coking wastewater, such as indole, phenol, pyridine, anduinoline, in the MFC. The results are of great significance on thereatment of coking wastewater with the MFC technology.

.3. Microbial community analysis in the C-MFC

The microbial community analysis from different substrates

s helpful to understand the microbial reactions occurring at thenode and their roles for power generation. Fig. 7 shows that theGGE profiles of the 16S rDNA gene fragments amplified fromxtracted DNA of the biofilms on the anodes of the MFC fed withhree different substrates: (A) 1000 mg/L glucose, (B) a mixture of

able 3haracterization of isolated 16S rRNA gene fragments derived from the electrodes of the

Closest identification (Genebank accession numbers) Isolate gro

Enterobacter sp. FW17a (GQ247734.1) GQ925720Uncultured Geobacter sp. clone MFC-A31 (FJ262593.1) GQ925721Uncultured bacterium MW-13A (AY559762.1) GQ925722Desulfovibrio sp. RPf35E1 (AY548772.1) GQ925723Uncultured bacterium gene for 16S ribosomal RNA (AB106406.1) GQ925724

a Samples from the electrodes of the C-MFC with 1000 mg/L glucose as the fuel.b Samples from the electrodes of the C-MFC with a mixture of 1000 mg/L glucose + 250c Samples from the electrodes of the C-MFC with 250 mg/L indole as the fuel.

Fig. 7. PCR-DGGE analysis of 16S rDNA extracted from the C-MFC using glucose (A),a mixture of glucose and indole (B), and indole (C) as the carbon sources.

1000 mg/L glucose + 250 mg/L indole, and (C) 250 mg/L indole. Eachband on the DGGE profile represents a specific species in the micro-bial community and the staining intensity of a band represents therelative abundance of the corresponding microbial species. Amongthe detectable bands in the DGGE profiles of the samples with thethree substrates, only one band (band 2) was common in all thesamples. However, the staining intensities of band 2 were high-est and lowest in the samples with glucose and indole as the fuel,respectively. For glucose (A) as the fuel, bands 1 and 2 showed highintensities. When using the mixture of indole and glucose as thefuel, band 3 appeared and showed high intensity. When indole wasused as the fuel, bands 4 and 5 appeared and showed very highintensities.

Separated DGGE bands were excised from the gels, purifiedusing the PCR Purification Kit to determine the sequence, andassigned to a specific group based on a combination of Blastsearches. As shown in Table 3, the dominant bacterial species on theMFC electrode with glucose as the fuel (band 1) shared 100% 16SrDNA sequence homology with Enterobacter sp., and the next mostabundant sequence type (band 2) was 84% similar to the UnculturedGeobacter sp. clone MFC-A31. Band 3 from the electrode with themixture of glucose and indole exhibited 95% 16S rDNA sequencehomology with the uncultured bacterium MW-13A. Band 4 and 5developed from indole exhibited 97% and 99% 16S rDNA sequencehomology with Desulfovibrio sp. RPf35E1 and uncultured bacterium

gene for 16S ribosomal RNA, respectively. Sequences derived fromthe analysis were deposited in GeneBank under accession numbersGQ925720 to GQ925724 (Table 3).

C-MFC with different substrates.

up representatives (Genebank accession numbers) Homology (%)

a 100a,b,c 84b,c 95c 97c 99

mg/L indole as the fuel.

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Catal et al. [27] reported that the anode microbial popula-ions were obviously different when using different kinds ofolyalcohol as the fuels of the MFC. Furthermore, for somelectricity-generating bacteria, only certain substrates can be useds the carbon sources [28,29]. For example, the Pseudomonas sp.solated from a MFC with glucose as a carbon source could not fur-her utilize the fermentative products, such as acetate, to generatelectricity [29]. In our study, the addition of indole changed theicrobial community in the electrode of the C-MFC. Under the dif-

erent substrate conditions, the dominate bacteria were obviouslyifferent (Fig. 7).

Although the 16S rDNA sequences from the MFC sampleshowed 84–100% similarity to the known bacteria (Table 3), veryittle is known about the role and performance of the dominateacteria from the electrodes. As shown from the fifth cycle in Fig. 2,he voltage outputs returned rapidly to the maximum when theuspended bacteria were removed completely and fed with freshndole as the sole carbon sources. This rapid returning of the voltageutput indicated that the bacteria attached to the electrode sur-ace were primarily responsible for the electricity generation [30].lthough some of these bacteria attached to the electrode might notirectly involve in the electricity generation, they were involved inegrading indole into simpler products that could be further uti-

ized by exoelectrogens [31]. Therefore, it is necessary to detect theole and performance of each of these dominates bacteria accordingo isolating and culturing pure bacteria from the C-MFC.

. Conclusions

This study clearly showed that indole could be utilized as theuel in the C-MFC and B-MFC for power generation. When 250 mg/Lndole was used as the fuel, the maximum power densities achievedrom the C-MFC and B-MFC were 2.1 and 3.3 W/m3, respectively. Inurn, this recalcitrant contaminant could be biodegraded efficientlyn the C-MFC. Indole concentrations of 250 mg/L and 500 mg/L

ere removed completely in the C-MFC within 6 and 30 h, respec-ively. Increase of indole concentrations from 250 to 1500 mg/Lesulted in decrease of the maximum power densities from 2.1 to.8 W/m3 and reduction of average degradation rates from 41.7 to.9 mg/(L h) in the C-MFC. Microbial community analysis showedhat the present of indole resulted in the obvious changes of theominant bacterial species on the electrode of the C-MFC. Theesults from this study may offer new information in enhancingiodegradation of recalcitrant contaminants in wastewaters andower generation from the biodegradation processes.

cknowledgements

This work was partially supported by grants from the Naturalcience Foundation of China (nos. 50608070 and 50779080) and theesearch Fund Program of Guangdong Provincial Key Laboratory ofnvironmental Pollution Control and Remediation Technology (no.006K0007).

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