Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes

Journal Pre-proof Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes Heming Wang, Xueyong Song, Huihui Zhang, Pan Tan, Fanxin Kong

PII:

S0304-3894(19)31413-X

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121459

Reference:

HAZMAT 121459

To appear in:

Journal of Hazardous Materials

Received Date:

25 August 2019

Revised Date:

29 September 2019

Accepted Date:

10 October 2019

Please cite this article as: Wang H, Song X, Zhang H, Tan P, Kong F, Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121459

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Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes

Heming Wanga,b,*, Xueyong Songb, Huihui Zhangb, Pan Tanb, Fanxin Konga,b

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Control, China University of Petroleum, Beijing, 102249, China

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State Key Laboratory of Heavy Oil Processing, Beijing Key Lab of Oil & Gas Pollution

College of Chemical Engineering and Environment, China University of Petroleum,

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Beijing, 102249, China

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Graphical abstract

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Highlights

Chromium “removal” was carefully analyzed into three parts as reduction, adsoprtion and permeation;

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Chromium was partially removed by way of cathodic reduction; Most hexavalent chromium ions (91.1±0.7%) were adsorbed on the cathode-facing side of AEM; Permeation of chromium ions across the membrane from cathode to anode can be ignored;

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Abstract An ion exchange membrane (IEM) is an important component in dual-chamber microbial fuel cells (MFCs) to separate cathodic chromium from anode bacteria to avoid toxicity. Common used IEMs (e.g., BPM, CEM, PEM, AEM) have different ionic transfer

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abilities which could influence MFC performance and chromium removal. Additionally, to distinguish chromium “removal” or “reduction” by MFCs, the chromium removal in this

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study was further analyzed into cathodic reduction, adsorption on the membrane and

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permeation through membrane to the anode chamber. It was found that BPM achieved the best performance in removing hexavalent chromium (99.4±0.2%) and balancing pH and

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conductivity in both chambers, followed by AEM (97.9±0.8%) and CEM (95.6±0.8%),

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while PEM can not well maintain pH and conductivity leading to the worst anode performance and lowest chromium removal efficiency. However, the adsorption of chromium on the AEM accounts for 91.1±0.7%, which was much higher than the other

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three membranes. The permeation of chromium through the membrane were all lower than 0.2% which can be ignored. SEM and EDS results showed that chromium deposits and bacteria were detected on the membrane facing cahtode and anode, respectively, indicating

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that membrane scaling and fouling were inevitable and happened within 24 h operation.

Keywords: Hexavalent chromium; Removal mechanism; Ion exchange membrane; Membrane fouling; Microbial fuel cell

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1. Introduction Hexavalent chromium is widely used in electroplating, leather tanning, textile dyes, welding and wood preservation, presenting a serious threat to ecological environment and human health due to its carcinogen and mutagen [1, 2]. Hexavalent chromium is normally

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reduced to trivalent chromium which is less toxicity, mobility and solubility by

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conventional physical and chemical treatment technologies such as chemical precipitation, membrane filtration, biosorption and ion exchange [3-5]. However, conventional

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technologies are energy intensive and unsustainable with chemical requirement and hazardous waste generation [1, 3].

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A microbial fuel cell (MFC) is a promising electrochemical technology which can be

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used to treat hexavalent chromium pollution with simultaneous electricity generation. The generated current is considered as a value-added product of the system, on the other hand, the current can also indicate the progress of hexavalent chromium reduction which

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simplifies the monitoring means [6-9]. During the process, the exoelectrogens in the anode chamber oxidize organic matter to release electrons and protons, while the electrons travel through external circuit and the protons travel through internal circuit to reduce hexavalent

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chromium at the cathode. Under acidic conditions, Cr(VI) as an electron acceptor is reduced to Cr(III) based on the following equations: Cr2 O7 2− + 14H+ + 6e− → 2Cr 3+ + 7H2 O

E Θ = 1.33V

(1)

The redox potential of Cr(VI) is 1.33 V vs. SHE (standard hydrogen electrode, Eq. 1), which is higher than MFC anode potential, as well as the redox potential of oxygen (1.23V) that is commonly used in air-cathode MFCs. Due to the high redox potential, Cr(VI) is

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considered as a favorable electron acceptor in acidic conditions [1, 8, 10]. To improve chromium reduction, anaerobically fermented sludge was used as anodic substrate. The rate coefficient of Cr(VI) removal increased to 0.0514 h-1, which was 36.7% higher than that in the former sludge system [11]. A novel MFC-MEC hybrid system was developed by harvesting electricity generated from Cr(VI) reduction in MFC to support

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MEC to reduce nonspontaneous-reacting metals including Pb(II) and Ni(II) [12]. Batchmode tests demonstrated that the system was able to reduce multiple metals without extral

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power supply, achieving self-sustained and cost-saving metal reduction. Besides commonly

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used chemical-cathode MFCs, some biocathode MFCs were established by utilizing electrochemically active microorganisms to reduce Cr(VI). The performances were

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enhanced by new acclimatization methods [13] and biocathode modification utilizing NaX

Fe2O3/polyaniline [17].

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zeolite [14], multi-walled carbon nanotubes (MWCNT) [15], graphene [16] and α-

However, toxic chromium in the cahode has negative effects on the anodic

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exoelectrogens, dual-chamber MFCs have to be employed in most studies. To reduce substrate crossover when treating hexavalent chromium, an ion exchange membrane (IEM) is utilized as an important semi-permeable component to separate anode and cathode.

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Various separators and designed alternative membranes for MFCs have been summarized in review papers [18-20]. However, adding IEM increases internal resistance and causes a variation of pHs between the two chambers [21]. IEM consists of the polymeric backbones fixed with functional groups carrying charges to help transport oppositely charged ions [22]. Monopolar (e.g., Dupont Nafion 117, Ultrex CMI-7000 and Ultrex AMI-7001) and bipolar (e.g., Neosepta BP-1) IEMs are commonly used in MFCs [21]. Nafion membrane is always

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known as proton exchange membrane (PEM), due to its high proton conductivity in most chemical fuel cells [23, 24]. Cation exchange membrane (CEM) is used in MFC to transport cations and reject anions [25], while anion exchange membrane (AEM) allows anions to transport. Due to ion exchanges by migrative flux and diffusive flux, monopolar membranes thus might face the problem of pH splitting, substrate loss and metal ion

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crossover [26]. A BPM which laminates both CEM and AEM is capable of dissociating water directly to H+ and OH- under the electric field. The H+ transfers through CEM to

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cathode and OH- transfers through AEM to anode, which restricts ion exchange between

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anode and cathode [27, 28]. Another concern of using either monopolar or bipolar membranes is biofouling and scaling, which deteriorates the performance of MFC and

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metal reduction efficiency.

These four membranes are commonly used in MFC for metal removal [4, 28-30], but

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each membrane has its own ionic transport mechanism which could effect removal performance. A recent study has compared CEM (CMI-7000) and AEM (AMI-7001) on

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silver recovery in bioelectrochemical reactors. It was found that better silver removal (83.73-92.50%) and columbic efficiency (11.50-19.89%) were obtained in CEM-based reactor with some diffusion of silver ions observed. In AEM-based reactor, the overall

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performance was low because of substrate loss [31]. Although univalent silver ion has opposite charges versus hexavalent chromium ion, the study indeed proved the effects of membrane type on metal removal and system performance. Four IEMs including two proton exchange (N117 and N212), a cation exchange (CMI-7000) and an anion exchange (AMI-7001) membranes on removing mixed Cr2O72-, Cu2+ and Cd2+ were evaluated, and the results showed that CEM was the most beneficial for less cross-over of Cr2O72- with

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slightly more migration of Cu2+ and Cd2+ compared to AEM [32]. A few studies have demonstrated the dependency of metal removal on membrane type, but systematically investigating their influence on MFC performance and Cr(VI) removal has not been fully understood. Furthermore, most studies only focus on the total removal efficiency of chromium at the cathode, but did not further analyze the removal pathways including

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cathodic reduction, adsorption on the membrane or the possible metal ion transfer through the membrane to the anode. In other words, make a distinction between chromium

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and help to choose the suitable membrane in the operation.

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“removal” or “reduction” is beneficial for us to understand the process of Cr(VI) removal,

In this study, the performance of these four membranes on reducing hexavalent

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chromium with spontaneous electricity generation in MFCs was systematically investigated. The ability of the membranes on maintaining the reactor condition was

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measured in terms of pH and conductivity. Chromium removal was additionally recognized as reduction at the cathode, adsorption on the membrane and diffusion across the membrane

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to the anode. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS) were used to characterize and evaluate the morphological structure of

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the fouled membrane and chromium precipitates on the cathode.

2. Materials and methods 2.1 MFC construction A dual-chamber MFC was constructed by plexiglass blocks as previously described [33]. The ion exchange membrane (IEM) with a projected area of 7 cm2 was installed

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between the two chambers, resulting in the total volumes of anode chamber and cathode chamber were 0.014 L and 0.028 L, respectively. Four ion exchange membranes were examed including PEM (Dupont Nafion 117), CEM (Ultrex CMI-7000), AEM (Ultrex AMI-7001) and BPM (Neosepta BP-1). The PEM was pretreated sequentially by boiling in H2O2 (30%), deionized (DI) water, 0.5M H2SO4, and DI water again with each step for 1 h

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[34]. The CEM and AEM were preconditioned by immersion in NaCl solution (0.01 mol/L) for membrane expansion [21]. BPM was stored in DI water and rinsed before use. A

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graphite brush was used as anode for exoelectrogen enrichment and a carbon cloth (7 cm2,

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Hesen HCP330N, China) was used as cathode. Two Ag/AgCl reference electrodes (195 mV

monitor electrode potentials.

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2.2 Inoculation and operation

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vs. standard hydrogen electrode, SHE) were placed into the anode and cathode chamber to

The anode brushes used in this study were transferred from air-cathode single chamber

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MFCs, which were inoculated by anaerobic sludge from Changping Wastewater Treatment Plant in Beijing, China, and operated stably for 6 months. The anode medium contained (per liter) 1.00 g CH3COONa, 0.31 g NH4Cl, 2.772 g NaH2PO4, 4.576 g Na2HPO4, 0.31 g

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KCl, 12.5 mL minerals and 5 mL vitamins [35]. The cathode medium was synthetic wastewater contained (per liter) 0.2829 g K2Cr2O7, 13.60 g KH2PO4, 11.70 g NaCl, with Cr(VI) concentration of 100 mg/L. The addition of KH2PO4 and NaCl was to increase the buffering capacity and conductivity of the catholyte. The pH of catholyte was adjusted to 2 with H3PO4. All the chemicals used in the study were analytical grade purchased from Aladdin.

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Four two-chamber MFC reactors with comparable anodes were operated to assess different membranes. Open circuit control without electric field was conducted to identify the membrane adsorption. The difference between initial amount and final amount of the catholyte is regarded as the chromium adsorbed on the membrane. A 1000 Ω external resistor was connected to MFC to close the circuit. At the end of the cycle, both anolyte and

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catholyte were collected for chromium analysis. The chromium present in the anolyte is considered as the chromium permeated through membrane from cathode to anode. The

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chromium via cathodic reduction was calculated by subtracting the final chromium in the

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closed circuit and adsorbed chromium in the open circuit from the initial amount. The effect of electric field on the adsorption was not considered. The membranes and cathode cloth

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were replaced with new ones at the end of each cycle. All MFCs were run in batch mode

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for 24 h. The experiments were repeated three times and performed at 28 °C. 2.3 Analyses and calculations

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The voltage and electrode potentials were continuously recorded using the automatic data acquisition system at 10 min intervals. Chemical oxygen demand (COD) was measured using a Four-parameter tester (5B-6C(V8), Lianhua science and technology). The

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Coulombic efficiency (CE) was calculated as follows: 𝑡

CE =

8 ∫0 I dt

FVAn ∆COD

× 100%

(2)

where 8 is a constant based on the molecular weight of O2 (32) and the number of

electrons exchanged per mol of O2 (4); 𝐼 is the current (A); 𝑡 is the reaction time (h); 𝐹 is Faraday’s constant (96485 C/mol e-); 𝑉𝐴𝑛 is the volume of liquid in the anode chamber (0.028 L). At the end of each experiment, pH and conductivity of the effluents were 8

monitored. The Cr(VI) concentration was analyzed by 1,5-diphenylcarbazide spectrophotometer from the absorbance at 540 nm. Reactor internal resistance was measured by electrochemical impedance spectroscopy (EIS, CHI600E, Chenhua) with the anode as the working electrode, and the cathode as the counter electrode and reference electrode. The scan range was from 105 Hz to 0.005 Hz with a small sinusoidal perturbation

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of ±10 mV. The surface morphology of the cathode and membranes were observed by the scanning electron micrographs (SEM, Hitachi SU8010). The metal deposits on the cathode

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and membranes facing the cathode were identified by the Energy Dispersive Spectroscopy

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(EDS) attached to the SEM. Prior to observing the bacteria morphologies of the membranes facing the anode, membranes were cut into small pieces and fixed overnight at 4 °C with

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glutaraldehyde (2.5%, pH=7.0), washed three times in phosphate buffer (0.1 M, pH=7.0), and then dehydrated stepwise in a series of water/ethanol solutions with increasing ethanol

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concentration (50, 70, 80, 90, 100%). Samples were then kept in a desiccator prior to Pd/Pt sputtering and SEM observation.

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3. Results and discussion

3.1 Effects of different membranes on pH and conductivity Fig. 1 shows the average pH and conductivity of anolyte and catholyte in closed and

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open circuits. BPM showed the best performance of maintaining pH and conductivity, followed by CEM and AEM, while PEM was the worst. There was no significant difference between closed circuit and open circuit due to strong electrolyte buffers and the big pH gap between anode and cathode chambers, indicating that the effect of concentration diffusion contributed much more than electric field on ion transfer, especially when using PEM. BPM generated H+ to catholyte and OH- to anolyte in the closed circuit. The comparable

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pH to that in open circuit was due to buffer solutions of both anolyte and catholyte, which were able to accommodate H+ and OH- ions produced by BPM. The other reason was because H+ generated during organic degradation in the anode can be neutralized with OHproduced by BPM; similarly, H+ in the catholyte was consumed during Cr(VI) reduction but would be replenished in time by H+ produced by BPM. The initial pH of anolyte was 7

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which was favorable for microbial oxidation, while the initial pH of catholyte was 2 for metal reduction. In the closed circuit, the final pH of anolyte decreased to 6.62±0.03,

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6.51±0.10, 6.49±0.06 and 3.14±0.09, and final pH of catholyte increased to 2.21±0.02,

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2.46±0.02, 2.49±0.01 and 2.84±0.09 for BPM, CEM, AEM and PEM, respectively (Fig. 1A). This is consistent with the previous studies that oxidation reaction at the anode

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generated H+ which is known to be responsible for pH decrease, while reduction reaction at the cathode consumed H+ leading to pH increase.

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A BPM has a great advantage in supplementing OH- to the anode chamber and H+ to the cathode chamber. Therefore, the pH and ionic conductivity were well maintained by the

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application of BPM. When protons were released from organics in the anode chamber tying to migrate through CEM, because the concentrations of other cations (e.g., Na+, K+, NH4+, Mg2+ and Ca2+) were usually 105 times higher than that of protons in the anode chamber, so

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most of the cations attached on the sulfonate groups due to higher affinity than protons [25]. The buffer solution had to accommodate some of the H+ ions, tying to maintain the pH of electrolytes. When using AEM as the separator, the dibasic phosphate and monobasic phosphate were responsible for proton transfer from anode to cathode, which was able to alleviate H+ concentration in the anode [20, 36], since phosphate buffer solution (PBS) is commonly used during operation. It is also possible that substrates, such as acetate or other

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negatively charged intermediate metabolites after biological degradation, were lost to the cathode by transportation through AEM. As for Nafion membrane, due to its excellent ability of proton transfer and a big pH gap between the two chambers, the diffusive fluxes of protons from cathode to anode resulted in H+ accumulation in the anode chamber, so PEM was the worst in maintaining pH.

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The initial conductivities of anolyte and catholyte were 7.14 and 30.00 mS/cm, respectively. In the closed circuit, the final conductivities of anolyte were 7.37±0.03,

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6.68±0.09, 8.36±0.04 and 10.17±0.90, and final conductivities of catholyte were

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28.27±0.31, 27.87±0.61, 25.70±0.36 and 21.47±0.15 for BPM, CEM, AEM and PEM,

OH- and other ionic migration.

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respectively (Fig. 1B). The trend was comparable to that of pH change as a result of H+,

3.2 Cr(VI) removal through cathodic reduction, adsorption and permeation

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The total removal of Cr(VI) was analyzed into three pathways including reduction by cathode, adsorption on the membrane and permeation through the membrane from cathode

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to anode. The results showed that BPM achieved the highest total removal efficiency of 99.4±0.2%, while the removal efficiencies of AEM and CEM were 97.9±0.8% and 95.6± 0.8%, respectively (Fig. 2). The highest Cr(VI) removal efficiency of BPM was closely

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related to the MFC condition that BPM was able to well maintain pH and conductivity of both chambers. The proper MFC condition can facilitate efficient microbial metabolism in the anode and reduction reaction in the cathode, which were both beneficial to Cr(VI) removal. The removal efficiency of PEM gradually declined in the three successive cycles from 80.4% to 47.3%, due to the pH drop at the anode causing the deterioration of MFC performance. The color of catholyte after 24 h operation in the closed circuit confirmed the

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results that the catholyte changed from yellow to light green for BPM, CEM and AEM comparing to yellowish green for PEM (Fig. S1B). It was found that cathodic reduction and adsorption were the two main pathways for Cr(VI) removal. The average cathodic reduction efficiencies of BPM and CEM were 69.8±6.3% and 71.1±1.5%, respectively, which was much higher than that of AEM

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(6.8±0.4%). The cathodic reduction of PEM decreased from 48.1% to 43.9% and 9.2% during three cycles, influencing decrease of total removal efficiencies as mentioned above.

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The adsorption of chromium on the AEM achieved 91.1±0.7%, much higher than BPM

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(29.6±6.4%), CEM (24.3±1.5%) and AEM (32.2±5.2%). The catholyte of AEM after 24 h in the open circuit became light yellow (Fig. S1A), which was lighter than that in the closed

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circuit, because Cr(VI) was simply adsorped on the membrane in the open circuit versus some of the Cr(VI) was reduced to Cr(III) in the closed circuit. The color change further

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verified that adsorption played a major role for AEM to remove chromium. The transportation of AEM is influenced by various factors including the size, charge and

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hydrated radius of the anion. The thermodynamic radius of Cr2O72- is 0.297 nm, which caused a slow transportation through the membrane because Cr2O72- is larger than other ions in the solution [37]. On the other hand, a high concentration of polychromate ions

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were detected within the membrane by Raman spectroscopy [38]. It was also found that the permeation through the membrane were all lower than 0.2% for BPM, AEM and CEM, while the permeation for PEM gradually increased during the three cycles from 0.07% to 0.64%, indicating that permeation from cathode to anode was not a significant pathway to remove Cr(VI) and chromium loss by permeation can be ignored. Green deposits can be observed on the carbon cloth after 24 h operation at the end of

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the first cycle (Fig. S2). It is consistent with the cathodic reduction efficiency that BPM and CEM obtained higher Cr(VI) reduction efficiency, so more products were deposited on the cathode carbon cloth. The reduction efficiency of PEM was lower than BPM and CEM, so less green deposits were observed. Cr(VI) removal mainly due to adsorption for AEM, so limited products were deposited on the the carbon cloth. Scanning electron micrographs

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(SEM) of carbon cloth were taken before and after MFC operation. Fig. 3 shows a distinct morphologic difference among these carbon cloth cathodes. The new carbon cloth surface

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clearly displayed carbon fiber network, while the carbon cloth cathodes for BPM and CEM

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were covered by abundant organized crystal particles. The carbon cloths for PEM and AEM were covered by less particles that carbon fiber network can easily be seen. The green

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deposits covering carbon cloth could reduce the active cathode surface area like chemical scales therefore exhibited adverse effects on power generation [39]. The deposited green

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particles were further analyzed by EDS to be chromium. According to the green color, the deposits should be trivalent chromium, confirming the reduction of Cr(VI) to Cr(III).

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Further analysis of the green deposit was not conducted in this study, but it should be amorphous trivalent chromium oxide particles detected by X-ray photoelectron spectroscopy (XPS) according to the previous studies [27]. The same result was obtained

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that the deposits were crystalline Cr2O3 nanoparticles, which was analyzed by FTIR (Fourier Transform Infrared) and XRD (X-ray diffraction) [40]. As we know, the pH of the catholyte was around 2-3 which was normally employed to promote Cr(VI) reduction, but the finding was that green chromium oxide was deposited on the cathode in the acid condition, which seems contradictory to the fact that chromium precipitates usually happen under high pH values. Kim et al. explained that the MFC condition may facilitate growth of

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crystal nucleation on the porous and heterogeneous cathode in the pH range of 1-4 to form precipitation [27]. Another study found that most of the Cr(III) precipitated in the form of Cr(OH)3 on the electrode surface in a plant-MFC system [41]. In our opinion, the formation of Cr(III) deposits (Cr2O3 or Cr(OH)3) was probably because of the high local pH when consuming protons on the cathode. The formed specific deposit should be related to ionic

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concentration, pH, cathodic potential, and some other conditions in the system. However, fine brown deposits were also observed on the cathodic carbon cloth at the end of the

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Cr(VI) reduction. The deposits adsorbed on the cathode were analyzed to be CrCl3 by XPS

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[42]. Although MFC cathode has been determined reducing Cr(VI) to Cr(III) in different studies, the reduced deposits and how the products generated in the electrochemical system

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need more investigations.

3.3 Power production under different membranes

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Voltage and electrode potentials were determined to evaluate electricity generation using different membranes. Fig. 4A shows that the voltage increased rapidly and

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subsequently fell in the first hour when refilling the reactors. The same trends were observed for anode potential (Fig. 4B) and cathode potential (Fig. 4C). Although the initial catholyte were the same for different membranes, the different internal resistance and ionic

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transfer resulted in different cathode potential. The highest average voltage during three cycles were 451±19.1 mV for AEM, which was higher than that of 432±4.6 mV and 327±6.8 mV for CEM and BPM, respectively. It is consistent with the result of electrode potential that their anode potentials were comparable, while the variation of cathode potential caused the voltage difference. The potential loss across the membrane between the two chambers should contribute to voltage difference as well [34]. However, the highest

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voltage of Nafion decreased from 396 mV in the first cycle to 262 mV in the second cycle and 134 mV in the third cycle, which was due to the pH decrease in the anode that suppressed biological metabolism. The increased pH of cathode leading to an enhanced cathode potential (Fig. 4C), but the final voltage was low because increased anode potential made the potential difference small (Fig. 4B). AEM achieved the highest CE (20.9±8.7%)

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and the maximum power density (431.8 mW/m2), followed by CEM (16.8±3.9%; 388.0 mW/m2), BPM (12.8±3.0%; 320.0 mW/m2) and PEM (9.8±3.6%; 19.6 mW/m2), as shown

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in Fig. S3. When anode and cathode were both filled with nutrient medium of pH 7 buffer

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solution, the reactors with PEM achieved the highest maximum power density, comparing to AEM and CEM [21]. The lower power density and output voltage obtained by PEM

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could be attributed to the larger charge transfer resistance and diffusion resistance [43], although the solution resistances of the four membranes were within the range of 32.4-51.0

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Ω, which was further verified by EIS results (Fig. S4 and Fig. S5). The MFC with BPM (261.1 Ω) exhibited higher total resistance than those with AEM (181.4 Ω) and CEM (200.4

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Ω) because of its high charge transfer resistance hindered ion transfer, while the MFC with PEM showed the highest total resistance (2017.8 Ω) which was likely due to the deterioration of bioanode in the decreased pH condition that raised charge transfer

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resistance and diffusion resistance.

3.4 Characterization of membrane fouling and scaling after operation Membrane fouling in open and closed circuits was visible on the surfaces of these

IEMs after 24 h operation (Fig. S6). Membrane facing anode and cathode sides were both characterized by SEM (Fig. 5) and EDS (Fig. S7). The results showed that biological fouling mainly happened on the anode-facing side and chemical scale was on the cathode-

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facing side, which was easily understandable that different conditions were between anode and cathode chambers. The microorganisms fouled the anode-facing side of the membrane were mostly rod shape bacteria with the uniform morphology, which was consistent with the former study that the fouling layer of the membrane was reported to be microorganisms dominating with rod shape bacteria from the anode chamber [44].

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The surfaces of fresh BPM and PEM have smooth compact morphology, comparing to the uneven surfaces of CEM and AEM. Abundant round-shape particles were precipitated

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on the cathode-facing sides of BPM and PEM, while the particles were less on the cathode-

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facing sides of CEM and AEM. It might be that the particles were filled in the sunken space of the uneven surface, so distinction from the image was difficult. Based on the EDS

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analysis, it is found that chromium was detected on the surface of membranes, the finding

deposited on the membranes.

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is consistent with section 3.2 that a certain amount of chromium was adsorped and

Compared to fresh membranes, the EDS spectra revealed that potassium appeared on

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the both sides of the membranes except anode-facing sides of PEM and AEM, but the anode-facing side of AEM was precipitated by some phosphorus. Buffer solutions were used in anode chamber and cathode chamber to maintain pH and conductivity, so it was not

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surprising to have some potassium or phosphorus deposited on the membrane. Since the operation time was only 24 h before membrane fouling characterization, the four membranes displayed different membrane fouling stages. On the one hand, membrane fouling and scaling is an unavoidable issue in practical applications; on the other hand, it is believed that this issue will become severer during long term operation [45]. Concentration difference and electric field effected the formation of biological fouling and chemical scale.

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Potassium, phosphorus and chromium were the main constituents of chemical scale and the microorganisms from anode caused biofouling. The fouling of membrane acted as a physical barrier for ion transfer which led to the increase of internal resistance and decrease of electricity generation [44, 46]. Choi et al. studied biofouling of Nafion on ion transport and found that the membrane electrical resistance (MER) only increased from 15.7 Ω cm2

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(fresh membrane) to 19.1 Ω cm2 (biofouled membrane) when the biofilm was as thin as 15.5±4.6 μm, suggesting that the biofouling itself was not the main reason to cause internal

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resistance increase, but the biofouled membrane decreased CE from 59.3% to 45.1% [47].

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4. Conclusions

In this study, common used ion exchange membranes including BPM, CEM, PEM and

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AEM were selected to assemble two-chamber MFCs, in order to investigate the influence

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of their different ionic transfer mechanisms on hexavalent chromium removal. It was found that BPM, CEM and AEM can well maintain pH and conductivity in both anode and cathode chambers, comparing to PEM which showed pH and conductivity splits after 24 h

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operation. Although highest output voltage and maximum power density were achieved when using AEM, most hexavalent chromium ions (91.1±0.7%) adsorbed on the cathodefacing side of the membrane leading to a high total chromium removal efficiency

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(97.9±0.8%). Hexavalent chromium was removed through cathodic reduction, adsorption on the membrane and permeation to the anode, but permeation across the membrane from cathode to anode can be ignored for all the IEMs in the study. Membrane fouling and scaling is a serious issue when utilizing these membranes for chromium removal, since fouling and scaling can be observed within 24 h operation. Therefore, BPM and CEM were the two possible options when removing chromium by MFCs in a short-time operation.

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Long-term studies are required to exam the influence of these membranes on balancing pH and conductivity, and the performance of chromium removal. In addition, only four common IEMs have been discussed in this study, since proper functioning of the membrane is of importance regarding the overall MFC performance, to design and fabricate cost-

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effective and highly efficient membranes without less fouling issues are still necessary.

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Acknowledgement

This research was supported by Beijing Natural Science Foundation (No. 8184085),

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Beijing Excellent Talents (No. 2016000020124G117), Natural Science Foundation of China (No. 21806185) and Science Foundation of China University of Petroleum-Beijing

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(No. 2462015YJRC016).

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