Bioresource Technology 102 (2011) 2468–2473

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Bioresource Technology

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In-situ Cr(VI) reduction with electrogenerated hydrogen peroxide drivenby iron-reducing bacteria

Liang Liu a,b,d, Yong Yuan a, Fang-bai Li a,⇑, Chun-hua Feng c

a Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou 510650, Chinab Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Chinac School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Chinad Graduate University of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:Received 2 September 2010Received in revised form 1 November 2010Accepted 3 November 2010Available online 12 November 2010

Keywords:Cr(VI) reductionElectrogenerated hydrogen peroxideIron-reducing bacteriaMicrobial fuel cell

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.11.013

⇑ Corresponding author. Tel.: +86 20 87024721; faxE-mail address: cefbli@soil.gd.cn (F.-b. Li).

a b s t r a c t

Cr(VI) was reduced in-situ at a carbon felt cathode in an air–cathode dual-chamber microbial fuel cell(MFC). The reduction of Cr(VI) was proven to be strongly associated with the electrogenerated H2O2 atthe cathode driven by iron-reducing bacteria. At pH 2.0, only 42.5% of Cr(VI) was reduced after 12 h inthe nitrogen-bubbling-cathode MFC, while complete reduction of Cr(VI) was achieved in 4 h in the air-bubbling-cathode MFC in which the reduction of oxygen to H2O2 was confirmed. Conditions that affectedthe efficiency of the reduction of Cr(VI) were evaluated experimentally, including the cathodic electrolytepH, the type of iron-reducing species, and the addition of redox mediators. The results showed that theefficient reduction of Cr(VI) could be achieved with an air-bubbling-cathode MFC.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Hexavalent chromium (Cr(VI)) contamination is a serious envi-ronmental concern due to its high toxicity and corrosiveness(Dupont and Guillon, 2003; Rodriguez et al., 2005). Therefore, itis necessary to remove the Cr(VI) that is abundant in the wastewa-ters of many industries (Park et al., 2008) such as electroplating,leather tanning, textile dyeing, metallurgy and wood preservation.The commonly used method to treat these wastewaters is chemicalreduction of Cr(VI) to the less toxic Cr(III). This method is easy toimplement but has disadvantages such as the requirement for acontinuous feed of reducing reagents (FeSO4, NaHSO3 or FeCl2)and the production of considerable amounts of sludge (Singhet al., 2009; Erdem and Tumen, 2004; Schlautman and Han,2001). Using H2O2 as the reductant is a much cleaner process be-cause H2O2 is oxidized to oxygen (Reaction (1)) without the needfor further recycling (van Niekerk et al., 2007; Pettine et al., 2002).

2HCrO�4 þ 3H2O2 þ 8Hþ ! 2Cr3þ þ 3O2 þ 8H2O

However, at the industrial level, the storage and transportationof H2O2 is inconvenient and expensive, especially for large quanti-ties. It would be attractive if H2O2 were able to be generated in-situand immediately used for Cr(VI)-contaminated wastewater

ll rights reserved.

: +86 20 87024123.

treatment. With this in mind, we attempted to manufacture H2O2

with a bioelectrochemical system (BES). A microbial fuel cell(MFC), which allows the recovery of bioenergy from organic wastes(Rozendal et al., 2008) with the aid of microorganisms, was used.This technology enables in-situ H2O2 generation without the needfor any energy input. Briefly, in the anode compartment, oxidationof organic electron donors can occur in the presence of electro-chemically active microorganisms which transfer the electronsgenerated to the solid anode. The electrons are then transportedthrough an external circuit to the cathode compartment whereair/O2 (electron acceptor) is available and H2O2 is electrochemi-cally generated via the two-electron reaction of O2 reduction(Reaction 2).

O2 þ 2Hþ þ 2e! 2H2O2

Recently, several studies (Zhu and Ni, 2009; Rozendal et al.,2009) have demonstrated this concept of using an MFC to achievecathodic production of H2O2 on the surfaces of carbon materials.This process may have important industrial applications. Forexample, some reports (Zhu and Ni, 2009; Feng et al., 2010; Zhuanget al., 2010) have shown the utilization of H2O2 produced in such aBES as a Fenton reagent (H2O2 and Fe2+) for the production of hy-droxyl radicals, which are strong oxidants of organic pollutantsin wastewaters.

This study provides the first attempt to reduce Cr(VI) usingH2O2 that is directly generated in the MFC cathode compartment.

L. Liu et al. / Bioresource Technology 102 (2011) 2468–2473 2469

Because Cr(VI) can also function as the electron acceptor, as evi-denced from previous MFC studies concerning Cr(VI) removalthrough either electrochemical (Li et al., 2008, 2009b; Wanget al., 2008) or bioelectrochemical methods (Tandukar et al.,2009), it is of interest to us to know whether the indirect Cr(VI)reduction proposed here exhibits a higher reaction rate than directCr(VI) reduction in the cathode compartment. Moreover, this studyexamines the impacts of different operational parameters on theaccumulation of H2O2 that is considered to play a key role in Cr(VI)reduction. The factors investigated include pH of catholyte, typesof bacteria and use of an external electron mediator.

2. Methods

2.1. Microorganisms and medium in the anode chamber

Two pure cultures Shewanella decolorationis S12, Klebsiellapneumoniae (K. pneumoniae) L17 and one mixed culture were usedas anodic inoculums. Culture of S. decolorationis S12 (Xu et al.,2005) was provided by the Guangdong Key Laboratory of MicrobialCulture Collection and Application, Guangzhou, China. K. pneumoniaeL17 was isolated from subterranean forest sediment by our group (Liet al., 2009a). The mixed culture was inoculated from an anaerobicactivated sludge collected from Leide wastewater plant located inGuangzhou, China. Before being transferred to the MFC reactor, thefrozen bacteria (1 mL) suspended in 100 mL of medium (10.0 g L�1

peptone, 5.0 g L�1 beef extract and 5.0 g L�1 NaCl, pH = 7.0) wereallowed to grow overnight (18 h) at 30 �C on an orbital shaker incu-bator at 150 rpm. Glucose (3 g L�1) used as the energy source wasadded to the anode chamber and was anaerobically oxidized by themicroorganism. The microbial growth medium (pH 7.0) in the anodechamber contained: 5.84 g L�1 NaCl, 0.10 g L�1 KCl, 0.25 g L�1 NH4Cl,12.00 g L�1 Na2HPO4�12H2O, 2.57 g L�1 NaH2PO4�2H2O, 10 mL vita-min solution and 10 mL mineral solution. The required vitamin andmineral solutions were prepared as previously described (Lovleyand Phillips, 1988). Prior to use, the above medium was sterilizedby autoclaving at 121 �C for 20 min.

2.2. MFC construction and operation

The MFCs consisted of two identical chambers separated by acation exchange membrane (ESC-7000, Electrolytica Corporation).Each cell chamber had an effective volume of 85 mL. Both elec-trodes were made of carbon felt (4.5 � 4.5 cm each, Panex 33160 K, Zoltek) and Ti wire was inserted inside the carbon felt toconnect the circuit. The electrode spacing was 2 cm. The cationexchange membrane was cleaned by boiling in H2O2 (3% V/V)solution and then soaking in deionized water for a day. To operatethe MFC, the microorganism growth medium containing glucose(10% v/v; medium/anode reactor value) was added to the anodechamber and the catholyte containing K2CrO4 (initial Cr(VI) con-centration of 10 mg/L) was added to the cathode chamber. Thecatholyte was continuously purged with N2 (90 ml/min) or air(90 ml/min) in different experimental arrays. To assess the impactof pH on the reduction rate of Cr(VI), precise control of the catho-lyte pH was achieved by manipulating the composition of H3PO4,NaH2PO4, and Na2HPO4 used. Open circuit experiments wereconducted as control experiments. The purpose of these controlexperiments was to confirm that the reduction of Cr(VI) was notdue to adsorption onto the electrode. All MFCs were operated intriplicate at a controlled temperature of 30 ± 1 �C in a constanttemperature incubator (HPG-280H, China) for the entire experi-ment. Unless otherwise stated, a 500 X resistor was used as theexternal load for the MFC.

2.3. Calculations and analyses

To quantify the cell performance, the resulting voltage (V)throughout the experiments was recorded every 2 min using a dataacquisition system (AD 8223 type voltage collector with 16 chan-nels, Rui Bo Hua Technology Control Corporation of Beijing, China)with a PC. The current density (I) and power density (P) normalizedto the projected surface area of the cathode were calculatedaccording to I = V R�1 and P = V2 R�1, respectively, where R is theexternal resistance. A polarization curve was obtained by adjustingthe external resistor from 20 to 50,000 X. At each fixed resistance,the voltage was measured after 10 min, when a pseudo-steadystate was approached (Logan et al., 2006). The power densitieswere normalized to the projected cathode surface area. A saturatedcalomel electrode (SCE, +0.241 V vs. standard hydrogen electrode;SHE) was used as the reference electrode. Columbic efficiency (CE)was calculated as previously described by Huang et al. (2008).

At each scheduled sampling interval, a sample (1.5 ml) waswithdrawn from the cathode chamber for Cr(VI) analysis. The con-centrations of Cr(VI) were determined via a colorimetric methodusing 1,5-diphenylcarbazide in acidic solution (Clesceri et al.,1998).

The concentration of hydrogen peroxide (H2O2) was determinedusing a H2O2 colorimeter (LOVIBOND-ET8600, Germany) at528 nm. For the determination of H2O2, separate experiments withthe same reaction conditions were conducted (without Cr(VI)). Ateach time interval, 10 mL of reaction solution was sampled andimmediately filtered through a 0.45 lm filter, followed at onceby measurement of the concentration of H2O2 in the filtrate. Allthe experiments were carried out in triplicate and only the meanvalues were reported.

3. Results and discussion

3.1. Simultaneous Cr(VI) reduction and hydrogen peroxidegeneration

Fig. 1a shows residual Cr(VI) (normalized to the initial concen-tration) as a function of time at pH 2.0 under various conditionswith S12 as the anodic inoculum. The removal rate of Cr(VI) withthe air-purged cathode in the MFC was substantially greater thanthat with the nitrogen purged one. The Cr(VI) was almost com-pletely removed after 3.5 h with the air-bubbling-cathode, whileonly 42.5% of Cr(VI) was reduced to Cr(III) with the nitrogen-purged cathode after 12 h. Previous reports have documented thesuccessful reduction process of Cr(VI) at the deoxygenated cathodeof an MFC via either an electrochemical (Li et al., 2008, 2009b;Wang et al., 2008) or bioelectrochemical (Tandukar et al., 2009)method. A slow decrease of Cr(VI) was also found in our case whenthe cathode of the MFC was purged with nitrogen gas. However,the slope of the kinetic curve increased markedly when air purgingwas applied, indicating the enhancement of Cr(VI) reduction. Therole of H2O2 in the reduction of Cr(VI) was confirmed by addingH2O2 to the nitrogen-purged cathode. As shown in Fig. 1b, Cr(VI)was only slightly reduced under the nitrogen purged condition,whereas the reduction was obviously accelerated by addingH2O2, demonstrating that the removal of Cr(VI) was primarilydue to H2O2. In such a system, the formation of H2O2 was thermo-dynamically favored due to the lack of an efficient oxygenreduction catalyst (Rozendal et al., 2009) and the reduction ofCr(VI) by H2O2 was also thermodynamically favorable in the viewof their standard electrode potentials that were +0.56 V vs. SHEfor H2O2 oxidation and +1.08 V vs. SHE for the Cr(VI) reduction atpH 2.0, respectively (van Niekerk et al., 2007; Kotas and Stasicka,2000). The removal efficiency of Cr(VI) was in accordance with

0 2 4 6 8 10 120.0

0.2

0.4

0.6

0.8

1.0

Open circuit Nitrogen Air

Ct /

C0

Time (h)

(a)

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0(b)

Stop Addition of H2O2

Addition of H2O2Addition of H2O2

Ct /

C0

Time (h)

Fig. 1. Cr(VI) reduction as a function of time under open circuit and closed circuit(nitrogen-purged and air-purged cathode) conditions at a catholyte pH of 2.0 (a)and effect of H2O2 addition on Cr(VI) reduction in nitrogen-purged cathode (b).Adding 0.15 ml H2O2 (100 mg/L) every min. Anodic inoculum: S12.

0 100 200 300 4000

10

20

30

40

50

Current density ( mA m-2

Pow

er d

ensi

ty (

mW

m )-2

Air Nitrogen

(a)

0 100 200 300 400-200

0

200

400

600

800

Pote

ntia

l (m

V) v

s. S

HE

Air,anode Air,cathode Nitrogen,anode Nitrogen,cathode

(b)

)

Current density ( mA m-2)

Fig. 2. Power densities (a) and individual electrode potentials (b) vs. currentdensity curves for nitrogen-purged and air-purged cathodes (pH 2.0). Anodicinoculum: S12.

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Con

c. o

f H2O

2 (mg/

L)

Air

Nitrogen

Time (h)

Fig. 3. Concentration of electrogenerated H2O2 at the cathodes with nitrogen or air-purging (pH 2.0). Anodic inoculum: S12.

2470 L. Liu et al. / Bioresource Technology 102 (2011) 2468–2473

the power generation of the MFCs as shown in Fig. 2. The maxi-mum power density of 52.1 mW/cm2 obtained from the MFC withthe air-bubbling-cathode was much higher than that of 6.8 mW/cm2 from the MFC with the nitrogen-bubbling-cathode (Fig. 2a).The difference in the removal efficiency of Cr(VI) and power densi-ties between two cases can be essentially attributed to the differ-ent reactions involved at the cathodes. As shown in Fig. 2b, theanode potentials of the two MFCs are almost consistent while thecathode potentials vary. The cathode potential under air-bubblingwas significantly greater over the entire current range. In the caseof the nitrogen-purged cathode, the slow removal efficiency ofCr(VI) and low power density indicated that Cr(VI) was not an effi-cient electron acceptor at this cathode. However, the reduction ofCr(VI) could be accelerated and the power output could be en-hanced when air purging was applied at the cathode. In this case,both O2 and Cr(VI) can react with the electrons produced at the an-ode, which results in a higher energy output. Moreover, the two-electron reduction of O2 resulted in the formation of H2O2 whichcould be an efficient chemical reductant to further react withCr(VI). The coulombic efficiencies (CE) of the S12 inoculated MFCwas 8.3 ± 0.7% with the air-bubbling-cathode at pH 2.0. The effi-ciency of the current at ultimately reducing Cr(VI) was calculatedto be 70.3 ± 1.3%. The Cr(VI) reduction rate was 2.78 g Cr(VI)/m3 h in the air-bubbling-cathode at pH 2.0. Li et al. (2009b) re-ported a Cr(VI) reduction rate of 0.97 g/m3 h at rutile-catalyzedcathodes and Wang et al. (2008) reported a reduction rate of0.5 g/m3 h at nitrogen-purged cathodes based on the direct electro-chemical reduction in microbial fuel cells. The in-situ generation ofH2O2 in the cathode compartment was confirmed by measuring its

concentration over time. The measurement was conducted duringa separate experiment without Cr(VI) because oxidizing agentsinterfere with the spectrophotometric detection of H2O2. As re-vealed in Fig. 3, when O2 was expelled from the catholyte, noH2O2 was detected. However, the presence of dissolved O2 in thecathode led to the formation of H2O2. The H2O2 concentrationwas negligible before the MFC was initiated and rapidly increasedwith time to a relatively stable concentration of 1.38 mg/L. This is

L. Liu et al. / Bioresource Technology 102 (2011) 2468–2473 2471

consistent with other observations in a traditional electrochemicalcell (Brillas and Casado, 2002), where a constant concentration wasmaintained because the rate of H2O2 generation was equal to therate of H2O2 decomposition.

3.2. Enhancing Cr(VI) reduction with controlled anodic strains andelectron shuttle

Because the reduction of Cr(VI) took place at the cathode of theMFC by accepting the electrons transferred from the anode, theanodic biocatalysts can obviously affect the reaction rate of Cr(VI)reduction. Fig. 4a shows the effect of varying the inoculum (L17,

0 1 2 3 40.0

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Control L17 S12 Anaerobic activated sludge (AAS)

Ct/

C0

Time (h)

(a)

0 1 2 3 40.0

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Ct/

C0

Time (h)

(b)

0 1 2 3 4 50.0

0.4

0.8

1.2

1.6

2.0

L17 S12 AAS S12+0.1mM AQDS

Con

c. o

f H2O

2 (m

g/L)

Time (h)

(c)

Fig. 4. Effects of anodic iron-reducing strains (a), and the addition of AQDS (b) onthe reduction of Cr(VI) and electrogeneration of H2O2 (c) at a catholyte pH of 2.0.

S12 or anaerobic activated sludge) in the anodic compartment onthe reduction of Cr(VI). The MFC inoculated with anaerobic acti-vated sludge had the best performance, reducing 97% of Cr(VI)within 3 h, while the L17 and S12 inoculated MFCs required longertimes to achieve this reduction. The electron transfer from themicrobial cell to the fuel cell anode, a process that links microbiol-ogy and electrochemistry, represents a key factor defining the the-oretical limits of the energy conversion (Schröder, 2007).Subsequently, the efficiency of microbial energy conversion influ-ences the cathodic reduction reactions because the electrons in-volved in the reduction reactions are produced by the microbialoxidation of organics at the anode. In other words, the cathodicreactions in MFCs are actually driven by the electrochemically ac-tive microorganisms of the anode. Iron-reducing strains are pres-ently the most common microorganisms used in MFCs to directlyproduce electricity. However, the efficiency of electron productionand transfer towards the anode varies amongst different species.For this reason we examined the efficiency of Cr(VI) reduction withdifferent iron-reducing species at the anodes. From our results forCr(VI) reduction, the anaerobic activated sludge was a betterinoculum for this purpose than the other two pure cultures – thisis also in agreement with energy generation capability of MFCs re-ported in the literature (Rabaey et al., 2004; Rabaey et al., 2005). Inaddition, the rate of Cr(VI) reduction could be enhanced by addingelectron shuttles to the anode chamber (Fig. 4b). The rate of Cr(VI)reduction was significantly increased when anthraquinone-2,6-disulfonate (AQDS), a known electron transfer mediator, was addedto the anode chamber, lowering the time for 97% Cr(VI) reductionto less than 2 h. Previous studies showed that the electron transferbetween iron-reducing bacteria and an extracellular electronacceptor could be significantly accelerated in the presence of elec-tron shuttles (Watanabe et al., 2009; Thygesen et al., 2009). Forexample, the addition of AQDS increased current production inMFCs by 24% (Bond et al., 2002). The addition of AQDS to the anodeof the MFC also led to an increased rate of Cr(VI) reduction at thecathode. As mentioned above, the Cr(VI) was mainly reduced bythe H2O2 produced in-situ in the MFCs. Varying the anodic biocata-lysts and the addition of AQDS, results in variation in the amount ofH2O2 produced in-situ, as shown in Fig. 4c. The activated sludgeinoculated MFC produced the highest concentration of H2O2 ofthe three inoculums. When AQDS was added to the anode com-partment, the concentration of H2O2 was as high as 2.0 mg/L, al-most 1.5 times that in the absence of AQDS. Obviously, thehigher amount of H2O2 produced, the faster the rate of Cr(VI)reduction achieved. This demonstrates that an optimum anode isimportant for the reduction of Cr(VI) in this bioelectrochemicalsystem.

3.3. Dependence of Cr(VI) reduction on catholyte pH

According to the Cr-Pourbaix diagram (Pourbaix, 1974), thereduction of Cr(VI) to Cr(III) is thermodynamically favored underacidic conditions due to an increase of its standard potential withproton ion concentration (Rodriguez et al., 2005). The effect ofpH on the kinetics of Cr(VI) reduction at the cathode is shown inFig. 5a. The pH has a strong influence on the Cr(VI) reduction ratein the range of pH 2.0–4.0. For example, at pH 2.0, Cr(VI) was com-pletely reduced in 3.5 h in the air-bubbling-cathode MFC, whileonly 9.0% of Cr(VI) was reduced at pH 7.0 after 6 h. In addition,as shown in Eq. (1), seven H+ ions were consumed by the reductionof each mole of Cr(VI). This reaction was thus strongly pH depen-dent, i.e. lower pH might make the reaction more favorable.Moreover, the electrogeneration of hydrogen peroxide driven byiron-reducing bacteria is also strongly dependent on the pH of thecatholyte. As depicted in Fig. 5b, at low pH the production of H2O2

was obviously favored. As the pH increased, the concentration of

0 1 2 3 4 5 6

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pH=2.0 pH=3.0 pH=3.2 pH=3.5 pH=3.7 pH=4.0 pH=5.0 pH=6.0 pH=7.0 Control

Ct /

C0

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(a)

0 1 2 3 4 50.0

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1.8 pH2.0 pH3.0 pH4.0 pH5.0 pH6.0 pH7.0

Con

c. o

f H2O

2 (m

g/L)

Time (h)

(b)

Fig. 5. Effect of cathodic pH on the reduction of Cr(VI) (a) and electrogeneration ofH2O2 (b). Anodic inoculum: S12.

Cr(III)

Cr(VI)Products

Glucose

-e +e

AH2DS

AQDSProducts

Glucose

-e +e -e +e

Cr(III)

Cr(VI)

Products

Glucose H2O2

O2

-e +e -e +e

Cr(III)

Cr(VI)

AH2DS

AQDSProducts

Glucose H2O2

O2

-e +e -e +e -e +e

Cr(III)

Cr(VI)

edonA

Bioelectrochemical

C6H12O6 + 6H

6CO2 + 24 H

S. decolorationi

(a) (b)

Fig. 6. Possible electron transfer pathways for the reduction of Cr(V

2472 L. Liu et al. / Bioresource Technology 102 (2011) 2468–2473

H2O2 continuously decreased. The concentration of H2O2 producedin-situ was up to 1.38 mg/L at pH 2.0, a factor of 23 times higherthan the 0.06 mg/L at pH 7.0. In summary, acidic conditions in thecatholyte benefited both the electrogeneration of H2O2 and thereduction reaction of Cr(VI), resulting in the fast removal rate ofCr(VI).

3.4. Cr(VI) reduction mechanism

We have demonstrated the concept of utilizing Cr(VI) as theelectron acceptor for anaerobic respiration in MFC technology.The electrons donated through the respiration of an iron-reducingmicroorganism can be successfully transferred to Cr(VI), leading tothe reduction of Cr(VI). Fig. 6a describes the mechanism of electrontransfer for the reduction of Cr(VI). In such an MFC, there are threetypes of reactions available in the process for electron production,transfer and consumption – bioelectrochemical, electrochemicaland chemical reactions. The bioelectrochemical reactions occurredin the anode chamber, where the electrons were produced duringmicrobial metabolism. Subsequently, the electrons collected in theanode pass through an external load and arrive at the cathodechamber where the electrochemical reactions happen. The electro-chemical reactions include the electrochemical reduction ofdissolved oxygen to H2O or H2O2 and direct electrochemical reduc-tion of Cr(VI). The chemical reaction that is the reduction of Cr(VI)by electrogenerated H2O2 also occurs in the cathode compartment.Thus, the actual reduction of Cr(VI) was achieved by two differentpathways: direct electrochemical reduction at the cathode andindirect chemical reduction through the in-situ generated hydro-gen peroxide as shown in Fig. 6b. As mentioned above, the fact thatthe reduction rate of Cr(VI) with the air-bubbling-cathode MFCwas much faster than that with the nitrogen-bubbling-cathodeMFC indicates that the reduction of Cr(VI) by the in-situ generatedhydrogen peroxide is more favorable than the direct electrochem-ical reduction. Furthermore, under both pathways, the reduction of

edohtaC

Electrochemical reactions Chemical reactions

Cr6+ + 3e Cr3+

O2 + 4H+ + 4e 2H2O

O2 + 2H+ + 2e H2O2

2HCrO4- + 3H2O2 + 8H+

2Cr3+ + 3O2 + 8H2O

reactions

2O

+ + 24 e

s S12

I) (a) and the reactions involved at both anode and cathode (b).

L. Liu et al. / Bioresource Technology 102 (2011) 2468–2473 2473

Cr(VI) could apparently be accelerated by adding extracellularmediators, such as AQDS, into the anode of the MFC. This increaseis largely as a result of an increase in the electron transfer rate be-tween iron-reducing microorganism and electrode in the presenceof AQDS. In such an MFC, the oxidized AQDS is first reduced byelectrons from microorganisms, and then reoxidized at the elec-trode to convey the electrons. Subsequently, the electrons weretransferred to the cathode via the external circuit and Cr(VI)directly or indirectly accepted the electrons to be reduced to Cr(III).Since the iron-reducing microorganism is capable of recoveringelectrons directly from various wastes, the MFC is expected to becapable of treating two wastewaters at the same time, in whichthe iron-reducing microorganisms break down the organics inwastewaters and the cathode reduces Cr(VI) to Cr(III). Meanwhile,the MFC technique might also be expected to treat other contam-inants that could react with H2O2 to decrease their toxicity.

4. Conclusions

In this study, a significant acceleration of Cr(VI) reduction wasobserved when the catholyte was purged with air rather thannitrogen in an dual-chamber MFC. By monitoring the accumulationand consumption of H2O2 at the cathode, it was suggested thatCr(VI) was mostly reduced to the less toxic Cr(III) by electrogener-ated H2O2 at the carbon felt cathode of the MFC. The efficiency ofCr(VI) reduction was strongly related to the catholyte pH, theanodic iron-reducing species, and the addition of extracellularmediators. This study demonstrated that the MFC technology withan aerated cathode was promising for Cr(VI)-contaminated waste-water treatment.

Acknowledgements

The authors appreciate the financial support of the NationalScience Foundation of China (No. 40771105) and the GuangdongInnovative Technique Foundation (No. 2006A36703003,2007B080401019 and 2008A080401008).

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