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Integrated utilization of seawater using a five-chamber bioelectrochemical system
Journal of Membrane Science 444 (2013) 16–21
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Integrated utilization of seawater using a five-chamber bioelectrochemical system Shanshan Chen, Haiping Luo, Guangli Liu n, Renduo Zhang, Haohao Wang, Bangyu Qin, Yanping Hou Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 February 2013 Received in revised form 11 May 2013 Accepted 15 May 2013 Available online 23 May 2013
To reduce membrane scaling, effectively desalinate seawater, and recover magnesium, acid and alkali from the desalination process, a novel five-chamber bioelectrochemical system (BES) was developed in this study. This development was based on a four-chamber BES proposed recently, called microbial electrolysis desalination and chemical-production cell (MEDCC). Results showed that the desalination efficiency of seawater in the five-chamber BES was two times of that in the MEDCC. Removal efficiencies of Na+, Mg2+, and Ca2+ within 18 h using the system were 657 2%, 100 70%, and 80 72%, respectively, which were 20%, 66%, and 36% higher than those in the MEDCC. With the form of Mg(OH)2 precipitation, 73% of the total magnesium in solutions was recovered from the cathodic surface. Although the removal efficiencies of Mg2+ and Ca2+ in the five-chamber BES were higher, the Mg2+ and Ca2+ scaling found on the membrane surface was only 38.5% and 18.5% of that in MEDCC, respectively. With the different removal mechanism of Mg2+/Ca2+ ions, the membrane scaling problem was better resolved in the fivechamber BES than the other desalination BESs. With reduction of membrane scaling, the production of alkali, acid, and magnesium, the five-chamber BES should be a promising way to realize an integrated utilization of seawater. & 2013 Elsevier B.V. All rights reserved.
Keywords: Bioelectrochemical system Desalination Magnesium recovery Membrane scaling Microbial electrolysis
1. Introduction The increasing demand for clean water in the world impels the development of the seawater utilization. Microbial desalination cell (MDC) [1,2] and microbial electrodialysis cell (MEDC) [3,4] have been developed recently to desalinate salt water with low energy consumption. In these cells, a desalination chamber and a pair of membranes are inserted between the anode and cathode chambers of microbial fuel cell (MFC) [5] or microbial electrolysis cell (MEC) [6]. To enhance desalination rates of the devices or produce byproducts, many kinds of multi-chambered MDCs are also designed. By inserting an acid-production chamber between the anode chamber and the anion exchange membrane (AEM) and a bipolar membrane (BPM) which could split water into H+ and OH− [7,8], a four-chamber BES system called microbial electrodialysis and chemical-production cell (MEDCC), can concurrently desalinate salt water, produce hydrochloric acid, and generate sodium hydroxide [9]. The desalination rates of the stacked MDC and MEDCC are about 1.4 times that of the single-desalinationchambered cells [10,11]. Series assembly of microbial desalination
n
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0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.05.027
cells can reduce 44% salinity of 0.06 L of synthetic seawater using only 0.12 L of anode solution [12]. Scale formation is always a serious limitation in the membrane systems because scaling should cause membrane degradation, flux decline, and elevate operating costs [13]. In the MDC system, salt removal efficiency of artificial seawater is lower than that of a pure NaCl solution because of the complex composition of seawater [14]. What is more, some compounds composed of calcium and magnesium may form precipitation and cause membrane scaling. The formation of these scaling compounds should increase the ohmic resistance of the whole reactor, and influence the electricity generation and desalination performance [15]. On the other hand, magnesium consisting in the scaling materials is a kind of valuable resource. With many advantages, such as low density, high specific strength, and good castability, magnesium itself and its alloys gradually replace aluminium and steel to be widely used in the manufacturing of automobiles, airplanes, computers, communication facilities, and e-products [16]. Generally, magnesium is produced by electrolyzing magnesium chloride from seawater, or from ores using a thermal process, such as Pidgeon process. These producing processes are energy intensive and result in relatively severe environment pollution [17]. Therefore, low-energy consumed techniques for integrated utilization of seawater (i.e., simultaneous recovery of fresh water, magnesium, acid, and alkali) are highly desired.
S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
In this paper, a five-chamber bioelectrochemical system (BES) based on the MEDCC is proposed [9]. As shown in Fig. 1A, a NaOHproduction chamber with a piece of AEM (named AEM2) as separator is inserted between the desalination and cathode chambers of the MEDCC (Fig. 1B). The additional chamber is expected to recover resources from seawater and resolve the membrane scaling problem, combined with other functions, such as to desalinate seawater and produce acid and alkali in the MEDCC [9,11]. According to the reaction O2+2H2O+4e−-4OH−, with increase of pH in the cathode chamber, the divalent Mg2+ and Ca2+ should precipitate, most of which stick to the cathode. Then the effluent of catholyte, which is almost without containing Mg2+ and Ca2+, enters the desalination chamber to be desalinated by the electrical field. The objective of this study was to demonstrate the feasibility of integrated utilization of seawater and reduction of membrane scaling using the five-chamber BES. Specifically, desalination rates of Na+, Mg2+, and Ca2+, Coulombic efficiencies, acid-/alkali-production performances, and reduction of membrane scaling in batch-fed five-chamber BESs were measured. Results were compared with those from MEDCCs.
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2. Experimental 2.1. Reactor construction A five-chamber BES reactor consists of five polycarbonate cubic blocks (each with inner cylindrical chamber 3 cm in diameter) and four pieces of membrane (Fig. 1A). Effective liquid volumes of the anode chamber, the acid-production chamber, the desalination chamber, the NaOH-production chamber, and the cathode chamber were 30, 10, 10, 10, and 10 mL, respectively. A piece of BPM (Fumasep-FBM) was installed between the anode and acidproduction chambers. Two pieces of AEM (Ultrex AMI-7001) (AEM1 and AEM2) were used as the separators between the acid-production and desalination chambers, and between the NaOH-production and cathode chambers, respectively. The desalination and NaOH-production chambers were separated by a piece of CEM (Ultrex CMI-7000). A graphite brush (25 mm diameter 30 mm length) was used as the anode. A carbon cloth with platinum (0.5 mg cm−2) and four diffusion layers on it with the effective surface area of 7 cm2 was used as the cathode. MEDCC reactors were constructed following Chen et al. [9], with the available volume of cathode chamber of 10 mL. 2.2. Medium and operating condition Two five-chamber BESs and two MEDCCs were operated at 30 1C. Each of the reactors was inoculated with 10 mL effluent of matured MDC anolyte, which contained exoelectrogens. The anode chamber contained 1 g L−1 CH3COONa and a nutrient buffer solution containing (in 1 L deionized water): 4.0896 g of Na2HPO4, 2.544 g of NaH2PO4, 0.31 g of NH4Cl, 0.13 g of KCl, 12.5 mL of trace mineral metal solution, and 5 mL vitamin solution [18]. In the MEDCC, the acid-production and cathode chambers were filled with 10 g L−1 NaCl, while the desalination chamber was fed with Kalle's artificial seawater (consisting of 28.566 g L−1 NaCl, 3.887 g L−1 MgCl2, 1.787 g L−1 MgSO4, 1.308 g L−1 CaSO4, 0.832 g L−1 K2SO4, 0.124 g L−1 CaCO3, 0.103 g L−1 KBr, 0.0288 g L−1 SrSO4, and 0.0282 g L−1 H3BO3, pH ¼ 7.5) [19]. In the five-chamber BES, the acid-production and NaOH-production chambers were filled with 10 g L−1 NaCl, the cathode chamber with artificial seawater, and the desalination chamber with the effluent of the cathode chamber of the five-chamber BES in the previous cycle. The reactors were operated in a fed-batch operation mode and a fixed voltage of 1.0 V was applied to the reactor using power supplies (Itech, IT6700). In addition, two five-chamber BESs were operated in open circuit to determine the dialysis effects. To measure the ion balance, after each cycle, the cathode chamber block of the MEDCCs and the five-chamber BESs were dipped into 100 mL HCl solution (pH ¼0.8) for 3 h; cathode and CEM of the MEDCC, cathode and AEM2 (Fig. 1A) of the fivechamber BES were dipped into 10 mL HCl solution (pH ¼0.8) for 3 h. Then cation (Na+, Mg2+ and Ca2+) concentrations in the HCl solution were determined using inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Optima 5300dv). While these parts were dipped in the HCl solution, new cathode chamber blocks, cathodes, and membranes were used in each reactor for the next cycle. For each treatment, duplicated reactors were used and triplicate cycles were operated in each reactor, resulting in six measurements of each variable. 2.3. Analyses and calculations
Fig. 1. Configuration of (A) the five-chamber BES, and (B) the MEDCC.
A five-chamber BES reactor consists of five polycarbonate cubic blocks (each with inner cylindrical chamber 3 cm in diameter). Output voltages (U, V) were measured across an external resistor (Re) of 10 Ω, which was connected in series with the negative lead
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S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
of the power supply and the cathode of the reactor, and recorded using a data acquisition system (Keithley, model 2700). The current density (A m−2) was calculated by U/Re and then normalized with the cathode surface area. The coulomb was calculated as Chen et al. [9]. Solution conductivities and pH values were measured using a conductivity meter (Leici, DDS-11A) and a pH meter (Leici, PHS-3C), respectively. Conductivity was used here to reflect the total dissolved solid (TDS) and to calculate the desalination efficiency and rate. Based on the concentrations of Na+, Mg2+ and Ca2+ determined using ICP-OES, the removal rate (mol h−1) of each ion was calculated as follows: r ¼ ðC t1 −C t2 ÞV 0 =T
ð1Þ
where V0 is the volume of the artificial seawater (i.e., 0.01 L), T is the time interval (h), and Ct1 and Ct2 are the ion concentrations (mol L−1) at the beginning and the end of the corresponding time interval. The removal efficiency (%) of each ion was calculated by 100% (C0−Ct)/C0, where C0 (mol L−1) is the ion concentration in the influent, and Ct (mol L−1) is the ion concentration in the corresponding time point. The charge transfer efficiency (η, %) was calculated by η¼
RFV0∑ziðCdi−Cf iÞ 100% R tb 0 Udt
ð2Þ
Here R is the external resistor (Ω), F is the Faraday's constant (96485C mol−1), U is the measured voltage (V) over a time interval t (s), zi is the cation charge, and Cdi is the final concentrations (mol L−1) of corresponding cation caused by dialysis, and Cfi is the final concentrations (mol L−1) of corresponding cation caused by dialysis and electrodialysis. Results are reported in the form of mean 7standard deviation. With low variability, values of mean and standard deviation of pH were calculated based on three measurements. For other variables, five measurements were used for the statistical analysis. After the experiments, the MEDCCs and five-chamber BESs were disassembled. Cathodes, CEM of the MEDCC, and AEM2 of the five-chamber BES were taken off and air-dried for 24 h. Then the cathodes, CEM, and AEM2 were examined using scanning electron microscopy (SEM, Quanta 400 FEG), energy dispersive X-ray spectroscopy (EDS, Quanta 400 FEG), and Powder X-ray diffractometer (XRD, Rigaku D/Max-IIIA), respectively, to analyze the structure of scaling layer and the element compositions fouled membrane samples.
3. Results and discussion 3.1. Performance of the MEDCC with artificial seawater When the desalination chamber of the MEDCC was filled with seawater, the maximum current density in the system was 8.7 70.3 A m−2 (Fig. 2A) and the coulombs were 238 7 6 C. Conductivities in the desalination chamber decreased from 52 to 42 ms cm−1 within 18 h. The corresponding desalination efficiency was 19%. Within 18 h, pH values of the solution in the acid-production chamber decreased from 6.96 70.01 to 0.6 70.04 (H+ production of 0.25 mol L−1) (Fig. 2B). Within the same period, pH values in the alkali-production chamber (i.e., the cathode chamber) increased from 6.96 70.01 to 13.1 70.1 (OH− production of 0.21 mol L−1). Compared to the result with pure NaCl in the desalination chamber [9], the TDS removal of the system using artificial seawater was 7.2 ms cm−1 lower within 12 h because of the complex composition of seawater. The result was consistent with
Fig. 2. (A) Current densities and (B) pH values in the MEDCC. Values are expressed as mean 7standard deviation (n¼3).
Jacobson et al. [14]. In conjunction with the conductivity measurements, the desalination performance of the reactor was also examined based on the Mg2+, Ca2+ and Na+ mass decrements in the desalinated chamber. As shown in Fig. 3A, in the MEDCC, the Mg2+ removal efficiency was 34 76% within 18 h of operation, and the removal efficiencies of Ca2+ and Na+ were 44 79% and 45 73%, respectively. The maximum removal rates of Mg2+ (within 6 h), Ca2+ and Na+ (within 3 h) were 0.028, 0.004, and 0.27 mmol h−1, respectively (Fig. 3B). The charge transfer efficiency in the MEDCC was 56 76%. 3.2. Performance of the five-chamber BES with artificial seawater In the five-chamber BES, the maximum current density was 8.3 70.2 A m−2 (Fig. 4A), and the coulombs were 230 7 15 C. Electricity generation in the five-chamber BES was slightly lower (3%) than that in the MEDCC because the additional CEM and chamber in the five-chamber BES increased the internal resistance of the cell [10]. In terms of the desalination, the conductivities of the seawater decreased from 52 to 44 mS cm−1 in the cathode chamber, and further decreased from 44 to 32 mS cm−1 in the desalination chamber. Within 18 h of operation, the desalination efficiency of the five-chamber BES reached 38%, which was two times as that of the MEDCC. The acid-/alkali-production performance was consistent with the electricity generation performance. Within 18 h, pH values in
S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
Fig. 3. (A) Removal efficiencies and (B) removal rates of Na+, Mg2+ and Ca2+ in the MEDCC and the five-chamber BES. Values are expressed as mean7 standard deviation (n¼ 5).
the acid-production chamber decreased from 6.96 70.01 to 0.7 70.08 (H+ production of 0.20 mol L−1) (Fig. 4B). The pH values in the alkali-production chamber (i.e., the NaOHproduction chamber) increased from 6.967 0.01 to 12.8 70.2 (OH− production of 0.07 mol L−1), while 34% of OH− produced in the cell was remained in the cathode chamber and 41% combined with Mg2+ and Ca2+ as precipitates. As showed in Fig. 3A, in the five-chamber BES, the Mg2+ removal efficiency was 100 70% within 6 h of operation. The removal efficiencies of Ca2+ and Na+ within 18 h were 80 70% and 6572%, respectively. The maximum removal rates of Mg2+ (within 3 h), Ca2+ and Na+ (within 6 h) were 0.14, 0.013, and 0.39 mmol h−1, respectively (Fig. 3B), which were 5.00, 3.25, and 1.44 times of those in the MEDCC, respectively. The charge transfer efficiency in the five-chamber BES was 82 711%. 3.3. Magnesium recovery and membrane scaling reduction in the five-chamber BES In the reactors, the removal mechanisms of Mg2+ and Ca2+ from the seawater were also investigated based on the mass balance analysis. In the MEDCC, the decrements of Mg2+ and Ca2+ in the seawater within 18 h of operation were 347 5.8% (0.19 mmol) and 44 78.7% (0.05 mmol), respectively. About 14 73% and 21 74% of the total Mg2+ and Ca2+ (i.e., 42% and 47% of the removed Mg2+ and Ca2+), respectively (Table 1), were observed on the surface of
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Fig. 4. (A) Current densities and (B) pH values in the five-chamber BES. Values are expressed as mean7 standard deviation (n ¼3).
the membranes, which could potentially result in membrane scaling after long-term operation. In the five-chamber BES, the decrements of Mg2+ and Ca2+ in the seawater within 18 h (i.e., 100 70% and 80 70.3%, respectively) were higher than those of MEDCC. However, only 5.5 70.7% and 3.8 71.4% of the removed Mg2+ and Ca2+ were found on the membrane surface, which were only 38.5% and 18.5% of those in MEDCC, respectively. Most of the total Mg2+ was observed on the cathode surface (73 74.2%, 0.4 mmol) and the inner surface of the chamber block (13 7 3%, 0.07 mmol), and most of the total Ca2+ was on the inner surface of the chamber block (41 74%, 0.04 mmol) and the cathode surface (247 1%, 0.03 mmol) (Table 1). About 5% to 9% Mg2+ and Ca2+ were not balanced in the experiments. Probably in the five-chamber BES, some of the ions were precipitated on the AEM1, CEM surface, and the inner surface of NaOH-production chamber block, while in the MEDCC, some of them were precipitated on the AEM surface and the inner surface of desalination chamber block. As shown by the SEM, the AEM2 surface in the cathode chamber of the five-chamber BES was partly covered by small round grains (Fig. 5A). According to the EDS, the grains were mainly consisted of C, Na, Cl, Mg, Ca, P, and S (Fig. 6A). However, the CEM surface in the cathode chamber of the MEDCC was completely covered by blade-like crystals and some floccules, which consisted of C, O, Na, Mg, P, S, Cl, and Ca (Fig. 5B and Fig. 6B). Examined by XRD, the compounds of the scaling were Mg
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(OH)2 and CaCO3. Blowball-like crystals and some floccules, which were consisted of Mg, O, C, Na, and Ca, were formed on the cathode surface in the cathode chamber of the five-chamber BES (Fig. 5C and Fig. 6C). Based on XRD, the compound was pure Mg (OH)2. The removal efficiencies of Mg2+ and Ca2+ in the five-chamber BES were higher than those in the MEDCC, whereas much less Mg2 + and Ca2+ were precipitated on the membrane in the fivechamber BES. In the cathode chamber of the five-chamber BES, the reaction O2+2H2O+4e−-4OH− occurred on the cathode first, which resulted in pH increase near the cathode. The solubility product constants of Mg(OH)2 and Ca(OH)2 are 1.8 10−11 and 5.5 10−6, respectively. Thus, precipitates of Mg2+ were first produced in the area near the cathode and stuck to the cathode. Some SO42− and CO32− transferred from the desalination chamber to the cathode chamber. The solubility product constants of CaSO4 and CaCO3 are 9.1 10−6, and 3.36 10−9, which are smaller than that of Ca(OH)2. Therefore, most of Ca2+ became CaSO4 and CaCO3 in the cathode chamber. The formation of different precipitates of Mg2+ and Ca2+ were confirmed by the results of XRD mentioned above.Therefore,thedifferentdistributionsoftheanions(OH−, CO32−, and SO42−) in the cathode chamber caused different distributions of Mg2+ and Ca2+ precipitation. Compared to the other desalination BESs, such as the MDC, MEDC, and MEDCC, the removal mechanism of Mg2+/Ca2+ ions is
Table 1 Mass balance of Mg2+ and Ca2+ in the BES systems after 18 h of operation. The results (%) were calculated by the concentrations of Mg2+ and Ca2+ in the corresponding component divided by the initial Mg2+ and Ca2+ concentration of seawater used. Values are expressed as mean 7 standard deviation (n¼ 5). Seawater after desalination Five-chamber BES Mg2 0.0 70.0 +
Membrane in the cathode chamber
Chamber block
Cathode
Solution in the adjacent chambera
Others
5.5 7 0.7
13.2 7 2.8 73.0 7 4.2 1.3 7 1.1
7.0 7 0.1
20.0 70.3
3.8 7 1.4
41.3 7 4.0 24.07 0.9 1.7 7 0.3
9.2 7 1.4
MEDCC Mg2 66.0 75.8 +
14.3 7 2.9
8.4 7 3.0
0.0 7 0.0 5.0 7 1.8
6.3 7 3.5
20.5 7 3.6
16.5 7 0.8
0.4 7 0.2 2.0 7 0.0
4.6 7 2.2
Ca2 +
Ca2 +
56.0 78.7
a In the five-chamber BES, the adjacent chamber is the NaOH-production chamber, and in the MEDCC, the adjacent chambers are the cathode chamber and the acid-production chamber.
through electrodialysis, in which ions must transfer from one chamber to another through the membranes. This process may cause internal membrane scaling, which is difficult to clean. Therefore, by changing the removal mechanism of Mg2+/Ca2+ ions, the five-chamber BES resolved the membrane scaling problem. Based on the calculation, 73% of Mg2+ (0.4 mmol) were recovered from the cathode surface. In the conventional method of magnesium production from seawater, lime (Ca(OH)2) is added into the seawater to precipitate Mg2+ and then Mg(OH)2 is collected [20]. The addition of Ca(OH)2 affects the purity of Mg (OH)2. Without such effect, the purity of Mg(OH)2 produced in the five-chamber BES should be higher.
3.4. Comparison of the desalination performance Different removal mechanisms of the five-chamber BES and the MEDCC resulted in the different ion removal performances (Fig. 3A and Fig. 3B). In the five-chamber BES, Mg2+ and Ca2+ were removed by precipitation reaction in the cathode chamber. Based on calculations using the solubility product constant [21,22], Mg2+ in the artificial seawater began to precipitate when the pH value reached 9.26, while Ca2+ began to precipitate when the pH value reached 12.35. As shown in Fig. 4B, pH values in the cathode chamber of the five-chamber BES reached 11.08 70.16 within 3 h and 12.54 70.14 within 6 h. Therefore, Mg2+ could be rapidly removed within 3 h (maximum removal rate was 0.14 mmol h−1 at 3 h) and totally removed within 6 h. At 6 h, Ca2+ precipitation reached the maximum rate of 0.013 mmol h−1. In the MEDCC, Mg2 + and Ca2+ were removed by electrodialysis in the desalination chamber. The removal efficiency of Ca2+ was 10% higher than Mg2+ within 18 h in the MEDCC due to the smaller hydrated ionic radii of calcium ion than magnesium ion [23]. The MEDCC produced circa 1.6 times of desalination rates compared with other desalination bioelectrochemical systems such as MEDC [9], and the Na+ desalination efficiency in the five-chamber BES was even 20% higher than that in the MEDCC (Fig. 3A). The higher Na+ desalination rate in the five-chamber BES was probably attributable to two aspects. First, there are almost only Na+ ions in the desalination chamber of the five-chamber system, while Mg2+ and Ca2+ ions in the desalination chamber of the MEDCC should compete with Na+ during the electrodialysis process. Therefore, the precipitation process in the five-chamber BES increases the electron desalination efficiency of Na+. Secondly, the membrane scaling problem causing by Ca2+ and Mg2+ precipitations in the MEDCC, which should negatively affect the desalination performance [15], was alleviated in the five-chamber system (Table 1).
Fig. 5. Scanning electron microscopy micrographs of (A) the AEM2 surface in the cathode chamber of the five-chamber BES, (B) the CEM surface in cathode chamber of the MEDCC, and (C) the cathode surface in the cathode chamber of the five-chamber. The size bar of this photograph corresponds to 5 μm.
S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
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Fig. 6. Energy dispersive X-ray spectroscopy analyses of (A) the AEM2 surface in the cathode chamber of the five-chamber BES, (B) the CEM surface in cathode chamber of the MEDCC, and (C) the cathode surface in the cathode chamber of the five-chamber.
4. Conclusion Seawater desalination is a way to alleviate the water-shortage problem. However, the desalination process is with high energy demand and production of much concentrated salty water. Results in this study provide useful information to resolve above problems. The five-chamber BES can be used to treat wastewater in the anode chamber and desalinate seawater in the desalination chamber. During the desalination, valuable byproducts are collected, including magnesium from the cathode chamber, acid from the acid-production chamber, and alkali from the alkaliproduction chamber, with reduction of the membrane scaling. The five-chamber BES is an environment-friendly device to achieve the goal of comprehensive utilization of seawater.
Acknowledgment This work was partly supported by grants from the Chinese National Natural Science Foundation (Nos. 51039007, 51179212 and 51278500), the Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2011K0001), and the program of Guangzhou Science & Technology Department (No. 2012J4300115). References [1] X. Cao, X. Huang, P. Liang, K. Xiao, Y. Zhou, X. Zhang, B.E. Logan, A new method for water desalination using microbial desalination cells, Environ. Sci. Technol. 43 (2009) 7148–7152. [2] K.S. Jacobson, D.M. Drew, Z. He, Efficient salt removal in a continuously operated upflow microbial desalination cell with an air cathode, Bioresour. Technol. 102 (2011) 376–380. [3] M. Mehanna, P.D. Kiely, D.F. Call, B.E. Logan, Microbial electrodialysis cell for simultaneous water desalination and hydrogen gas production, Environ. Sci. Technol. 44 (2010) 9578–9583. [4] H. Luo, P.E. Jenkins, Z. Ren, Concurrent desalination and hydrogen generation using microbial electrolysis and desalination cells, Environ. Sci. Technol. 45 (2010) 340–344. [5] B.E. Logan, J.M. Regan, Microbial fuel cells-challenges and applications, Environ. Sci. Technol. 40 (2006) 5172–5180.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Integrated utilization of seawater using a five-chamber bioelectrochemical system Shanshan Chen, Haiping Luo, Guangli Liu n, Renduo Zhang, Haohao Wang, Bangyu Qin, Yanping Hou Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 February 2013 Received in revised form 11 May 2013 Accepted 15 May 2013 Available online 23 May 2013
To reduce membrane scaling, effectively desalinate seawater, and recover magnesium, acid and alkali from the desalination process, a novel five-chamber bioelectrochemical system (BES) was developed in this study. This development was based on a four-chamber BES proposed recently, called microbial electrolysis desalination and chemical-production cell (MEDCC). Results showed that the desalination efficiency of seawater in the five-chamber BES was two times of that in the MEDCC. Removal efficiencies of Na+, Mg2+, and Ca2+ within 18 h using the system were 657 2%, 100 70%, and 80 72%, respectively, which were 20%, 66%, and 36% higher than those in the MEDCC. With the form of Mg(OH)2 precipitation, 73% of the total magnesium in solutions was recovered from the cathodic surface. Although the removal efficiencies of Mg2+ and Ca2+ in the five-chamber BES were higher, the Mg2+ and Ca2+ scaling found on the membrane surface was only 38.5% and 18.5% of that in MEDCC, respectively. With the different removal mechanism of Mg2+/Ca2+ ions, the membrane scaling problem was better resolved in the fivechamber BES than the other desalination BESs. With reduction of membrane scaling, the production of alkali, acid, and magnesium, the five-chamber BES should be a promising way to realize an integrated utilization of seawater. & 2013 Elsevier B.V. All rights reserved.
Keywords: Bioelectrochemical system Desalination Magnesium recovery Membrane scaling Microbial electrolysis
1. Introduction The increasing demand for clean water in the world impels the development of the seawater utilization. Microbial desalination cell (MDC) [1,2] and microbial electrodialysis cell (MEDC) [3,4] have been developed recently to desalinate salt water with low energy consumption. In these cells, a desalination chamber and a pair of membranes are inserted between the anode and cathode chambers of microbial fuel cell (MFC) [5] or microbial electrolysis cell (MEC) [6]. To enhance desalination rates of the devices or produce byproducts, many kinds of multi-chambered MDCs are also designed. By inserting an acid-production chamber between the anode chamber and the anion exchange membrane (AEM) and a bipolar membrane (BPM) which could split water into H+ and OH− [7,8], a four-chamber BES system called microbial electrodialysis and chemical-production cell (MEDCC), can concurrently desalinate salt water, produce hydrochloric acid, and generate sodium hydroxide [9]. The desalination rates of the stacked MDC and MEDCC are about 1.4 times that of the single-desalinationchambered cells [10,11]. Series assembly of microbial desalination
n
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0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.05.027
cells can reduce 44% salinity of 0.06 L of synthetic seawater using only 0.12 L of anode solution [12]. Scale formation is always a serious limitation in the membrane systems because scaling should cause membrane degradation, flux decline, and elevate operating costs [13]. In the MDC system, salt removal efficiency of artificial seawater is lower than that of a pure NaCl solution because of the complex composition of seawater [14]. What is more, some compounds composed of calcium and magnesium may form precipitation and cause membrane scaling. The formation of these scaling compounds should increase the ohmic resistance of the whole reactor, and influence the electricity generation and desalination performance [15]. On the other hand, magnesium consisting in the scaling materials is a kind of valuable resource. With many advantages, such as low density, high specific strength, and good castability, magnesium itself and its alloys gradually replace aluminium and steel to be widely used in the manufacturing of automobiles, airplanes, computers, communication facilities, and e-products [16]. Generally, magnesium is produced by electrolyzing magnesium chloride from seawater, or from ores using a thermal process, such as Pidgeon process. These producing processes are energy intensive and result in relatively severe environment pollution [17]. Therefore, low-energy consumed techniques for integrated utilization of seawater (i.e., simultaneous recovery of fresh water, magnesium, acid, and alkali) are highly desired.
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In this paper, a five-chamber bioelectrochemical system (BES) based on the MEDCC is proposed [9]. As shown in Fig. 1A, a NaOHproduction chamber with a piece of AEM (named AEM2) as separator is inserted between the desalination and cathode chambers of the MEDCC (Fig. 1B). The additional chamber is expected to recover resources from seawater and resolve the membrane scaling problem, combined with other functions, such as to desalinate seawater and produce acid and alkali in the MEDCC [9,11]. According to the reaction O2+2H2O+4e−-4OH−, with increase of pH in the cathode chamber, the divalent Mg2+ and Ca2+ should precipitate, most of which stick to the cathode. Then the effluent of catholyte, which is almost without containing Mg2+ and Ca2+, enters the desalination chamber to be desalinated by the electrical field. The objective of this study was to demonstrate the feasibility of integrated utilization of seawater and reduction of membrane scaling using the five-chamber BES. Specifically, desalination rates of Na+, Mg2+, and Ca2+, Coulombic efficiencies, acid-/alkali-production performances, and reduction of membrane scaling in batch-fed five-chamber BESs were measured. Results were compared with those from MEDCCs.
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2. Experimental 2.1. Reactor construction A five-chamber BES reactor consists of five polycarbonate cubic blocks (each with inner cylindrical chamber 3 cm in diameter) and four pieces of membrane (Fig. 1A). Effective liquid volumes of the anode chamber, the acid-production chamber, the desalination chamber, the NaOH-production chamber, and the cathode chamber were 30, 10, 10, 10, and 10 mL, respectively. A piece of BPM (Fumasep-FBM) was installed between the anode and acidproduction chambers. Two pieces of AEM (Ultrex AMI-7001) (AEM1 and AEM2) were used as the separators between the acid-production and desalination chambers, and between the NaOH-production and cathode chambers, respectively. The desalination and NaOH-production chambers were separated by a piece of CEM (Ultrex CMI-7000). A graphite brush (25 mm diameter 30 mm length) was used as the anode. A carbon cloth with platinum (0.5 mg cm−2) and four diffusion layers on it with the effective surface area of 7 cm2 was used as the cathode. MEDCC reactors were constructed following Chen et al. [9], with the available volume of cathode chamber of 10 mL. 2.2. Medium and operating condition Two five-chamber BESs and two MEDCCs were operated at 30 1C. Each of the reactors was inoculated with 10 mL effluent of matured MDC anolyte, which contained exoelectrogens. The anode chamber contained 1 g L−1 CH3COONa and a nutrient buffer solution containing (in 1 L deionized water): 4.0896 g of Na2HPO4, 2.544 g of NaH2PO4, 0.31 g of NH4Cl, 0.13 g of KCl, 12.5 mL of trace mineral metal solution, and 5 mL vitamin solution [18]. In the MEDCC, the acid-production and cathode chambers were filled with 10 g L−1 NaCl, while the desalination chamber was fed with Kalle's artificial seawater (consisting of 28.566 g L−1 NaCl, 3.887 g L−1 MgCl2, 1.787 g L−1 MgSO4, 1.308 g L−1 CaSO4, 0.832 g L−1 K2SO4, 0.124 g L−1 CaCO3, 0.103 g L−1 KBr, 0.0288 g L−1 SrSO4, and 0.0282 g L−1 H3BO3, pH ¼ 7.5) [19]. In the five-chamber BES, the acid-production and NaOH-production chambers were filled with 10 g L−1 NaCl, the cathode chamber with artificial seawater, and the desalination chamber with the effluent of the cathode chamber of the five-chamber BES in the previous cycle. The reactors were operated in a fed-batch operation mode and a fixed voltage of 1.0 V was applied to the reactor using power supplies (Itech, IT6700). In addition, two five-chamber BESs were operated in open circuit to determine the dialysis effects. To measure the ion balance, after each cycle, the cathode chamber block of the MEDCCs and the five-chamber BESs were dipped into 100 mL HCl solution (pH ¼0.8) for 3 h; cathode and CEM of the MEDCC, cathode and AEM2 (Fig. 1A) of the fivechamber BES were dipped into 10 mL HCl solution (pH ¼0.8) for 3 h. Then cation (Na+, Mg2+ and Ca2+) concentrations in the HCl solution were determined using inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Optima 5300dv). While these parts were dipped in the HCl solution, new cathode chamber blocks, cathodes, and membranes were used in each reactor for the next cycle. For each treatment, duplicated reactors were used and triplicate cycles were operated in each reactor, resulting in six measurements of each variable. 2.3. Analyses and calculations
Fig. 1. Configuration of (A) the five-chamber BES, and (B) the MEDCC.
A five-chamber BES reactor consists of five polycarbonate cubic blocks (each with inner cylindrical chamber 3 cm in diameter). Output voltages (U, V) were measured across an external resistor (Re) of 10 Ω, which was connected in series with the negative lead
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S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
of the power supply and the cathode of the reactor, and recorded using a data acquisition system (Keithley, model 2700). The current density (A m−2) was calculated by U/Re and then normalized with the cathode surface area. The coulomb was calculated as Chen et al. [9]. Solution conductivities and pH values were measured using a conductivity meter (Leici, DDS-11A) and a pH meter (Leici, PHS-3C), respectively. Conductivity was used here to reflect the total dissolved solid (TDS) and to calculate the desalination efficiency and rate. Based on the concentrations of Na+, Mg2+ and Ca2+ determined using ICP-OES, the removal rate (mol h−1) of each ion was calculated as follows: r ¼ ðC t1 −C t2 ÞV 0 =T
ð1Þ
where V0 is the volume of the artificial seawater (i.e., 0.01 L), T is the time interval (h), and Ct1 and Ct2 are the ion concentrations (mol L−1) at the beginning and the end of the corresponding time interval. The removal efficiency (%) of each ion was calculated by 100% (C0−Ct)/C0, where C0 (mol L−1) is the ion concentration in the influent, and Ct (mol L−1) is the ion concentration in the corresponding time point. The charge transfer efficiency (η, %) was calculated by η¼
RFV0∑ziðCdi−Cf iÞ 100% R tb 0 Udt
ð2Þ
Here R is the external resistor (Ω), F is the Faraday's constant (96485C mol−1), U is the measured voltage (V) over a time interval t (s), zi is the cation charge, and Cdi is the final concentrations (mol L−1) of corresponding cation caused by dialysis, and Cfi is the final concentrations (mol L−1) of corresponding cation caused by dialysis and electrodialysis. Results are reported in the form of mean 7standard deviation. With low variability, values of mean and standard deviation of pH were calculated based on three measurements. For other variables, five measurements were used for the statistical analysis. After the experiments, the MEDCCs and five-chamber BESs were disassembled. Cathodes, CEM of the MEDCC, and AEM2 of the five-chamber BES were taken off and air-dried for 24 h. Then the cathodes, CEM, and AEM2 were examined using scanning electron microscopy (SEM, Quanta 400 FEG), energy dispersive X-ray spectroscopy (EDS, Quanta 400 FEG), and Powder X-ray diffractometer (XRD, Rigaku D/Max-IIIA), respectively, to analyze the structure of scaling layer and the element compositions fouled membrane samples.
3. Results and discussion 3.1. Performance of the MEDCC with artificial seawater When the desalination chamber of the MEDCC was filled with seawater, the maximum current density in the system was 8.7 70.3 A m−2 (Fig. 2A) and the coulombs were 238 7 6 C. Conductivities in the desalination chamber decreased from 52 to 42 ms cm−1 within 18 h. The corresponding desalination efficiency was 19%. Within 18 h, pH values of the solution in the acid-production chamber decreased from 6.96 70.01 to 0.6 70.04 (H+ production of 0.25 mol L−1) (Fig. 2B). Within the same period, pH values in the alkali-production chamber (i.e., the cathode chamber) increased from 6.96 70.01 to 13.1 70.1 (OH− production of 0.21 mol L−1). Compared to the result with pure NaCl in the desalination chamber [9], the TDS removal of the system using artificial seawater was 7.2 ms cm−1 lower within 12 h because of the complex composition of seawater. The result was consistent with
Fig. 2. (A) Current densities and (B) pH values in the MEDCC. Values are expressed as mean 7standard deviation (n¼3).
Jacobson et al. [14]. In conjunction with the conductivity measurements, the desalination performance of the reactor was also examined based on the Mg2+, Ca2+ and Na+ mass decrements in the desalinated chamber. As shown in Fig. 3A, in the MEDCC, the Mg2+ removal efficiency was 34 76% within 18 h of operation, and the removal efficiencies of Ca2+ and Na+ were 44 79% and 45 73%, respectively. The maximum removal rates of Mg2+ (within 6 h), Ca2+ and Na+ (within 3 h) were 0.028, 0.004, and 0.27 mmol h−1, respectively (Fig. 3B). The charge transfer efficiency in the MEDCC was 56 76%. 3.2. Performance of the five-chamber BES with artificial seawater In the five-chamber BES, the maximum current density was 8.3 70.2 A m−2 (Fig. 4A), and the coulombs were 230 7 15 C. Electricity generation in the five-chamber BES was slightly lower (3%) than that in the MEDCC because the additional CEM and chamber in the five-chamber BES increased the internal resistance of the cell [10]. In terms of the desalination, the conductivities of the seawater decreased from 52 to 44 mS cm−1 in the cathode chamber, and further decreased from 44 to 32 mS cm−1 in the desalination chamber. Within 18 h of operation, the desalination efficiency of the five-chamber BES reached 38%, which was two times as that of the MEDCC. The acid-/alkali-production performance was consistent with the electricity generation performance. Within 18 h, pH values in
S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
Fig. 3. (A) Removal efficiencies and (B) removal rates of Na+, Mg2+ and Ca2+ in the MEDCC and the five-chamber BES. Values are expressed as mean7 standard deviation (n¼ 5).
the acid-production chamber decreased from 6.96 70.01 to 0.7 70.08 (H+ production of 0.20 mol L−1) (Fig. 4B). The pH values in the alkali-production chamber (i.e., the NaOHproduction chamber) increased from 6.967 0.01 to 12.8 70.2 (OH− production of 0.07 mol L−1), while 34% of OH− produced in the cell was remained in the cathode chamber and 41% combined with Mg2+ and Ca2+ as precipitates. As showed in Fig. 3A, in the five-chamber BES, the Mg2+ removal efficiency was 100 70% within 6 h of operation. The removal efficiencies of Ca2+ and Na+ within 18 h were 80 70% and 6572%, respectively. The maximum removal rates of Mg2+ (within 3 h), Ca2+ and Na+ (within 6 h) were 0.14, 0.013, and 0.39 mmol h−1, respectively (Fig. 3B), which were 5.00, 3.25, and 1.44 times of those in the MEDCC, respectively. The charge transfer efficiency in the five-chamber BES was 82 711%. 3.3. Magnesium recovery and membrane scaling reduction in the five-chamber BES In the reactors, the removal mechanisms of Mg2+ and Ca2+ from the seawater were also investigated based on the mass balance analysis. In the MEDCC, the decrements of Mg2+ and Ca2+ in the seawater within 18 h of operation were 347 5.8% (0.19 mmol) and 44 78.7% (0.05 mmol), respectively. About 14 73% and 21 74% of the total Mg2+ and Ca2+ (i.e., 42% and 47% of the removed Mg2+ and Ca2+), respectively (Table 1), were observed on the surface of
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Fig. 4. (A) Current densities and (B) pH values in the five-chamber BES. Values are expressed as mean7 standard deviation (n ¼3).
the membranes, which could potentially result in membrane scaling after long-term operation. In the five-chamber BES, the decrements of Mg2+ and Ca2+ in the seawater within 18 h (i.e., 100 70% and 80 70.3%, respectively) were higher than those of MEDCC. However, only 5.5 70.7% and 3.8 71.4% of the removed Mg2+ and Ca2+ were found on the membrane surface, which were only 38.5% and 18.5% of those in MEDCC, respectively. Most of the total Mg2+ was observed on the cathode surface (73 74.2%, 0.4 mmol) and the inner surface of the chamber block (13 7 3%, 0.07 mmol), and most of the total Ca2+ was on the inner surface of the chamber block (41 74%, 0.04 mmol) and the cathode surface (247 1%, 0.03 mmol) (Table 1). About 5% to 9% Mg2+ and Ca2+ were not balanced in the experiments. Probably in the five-chamber BES, some of the ions were precipitated on the AEM1, CEM surface, and the inner surface of NaOH-production chamber block, while in the MEDCC, some of them were precipitated on the AEM surface and the inner surface of desalination chamber block. As shown by the SEM, the AEM2 surface in the cathode chamber of the five-chamber BES was partly covered by small round grains (Fig. 5A). According to the EDS, the grains were mainly consisted of C, Na, Cl, Mg, Ca, P, and S (Fig. 6A). However, the CEM surface in the cathode chamber of the MEDCC was completely covered by blade-like crystals and some floccules, which consisted of C, O, Na, Mg, P, S, Cl, and Ca (Fig. 5B and Fig. 6B). Examined by XRD, the compounds of the scaling were Mg
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(OH)2 and CaCO3. Blowball-like crystals and some floccules, which were consisted of Mg, O, C, Na, and Ca, were formed on the cathode surface in the cathode chamber of the five-chamber BES (Fig. 5C and Fig. 6C). Based on XRD, the compound was pure Mg (OH)2. The removal efficiencies of Mg2+ and Ca2+ in the five-chamber BES were higher than those in the MEDCC, whereas much less Mg2 + and Ca2+ were precipitated on the membrane in the fivechamber BES. In the cathode chamber of the five-chamber BES, the reaction O2+2H2O+4e−-4OH− occurred on the cathode first, which resulted in pH increase near the cathode. The solubility product constants of Mg(OH)2 and Ca(OH)2 are 1.8 10−11 and 5.5 10−6, respectively. Thus, precipitates of Mg2+ were first produced in the area near the cathode and stuck to the cathode. Some SO42− and CO32− transferred from the desalination chamber to the cathode chamber. The solubility product constants of CaSO4 and CaCO3 are 9.1 10−6, and 3.36 10−9, which are smaller than that of Ca(OH)2. Therefore, most of Ca2+ became CaSO4 and CaCO3 in the cathode chamber. The formation of different precipitates of Mg2+ and Ca2+ were confirmed by the results of XRD mentioned above.Therefore,thedifferentdistributionsoftheanions(OH−, CO32−, and SO42−) in the cathode chamber caused different distributions of Mg2+ and Ca2+ precipitation. Compared to the other desalination BESs, such as the MDC, MEDC, and MEDCC, the removal mechanism of Mg2+/Ca2+ ions is
Table 1 Mass balance of Mg2+ and Ca2+ in the BES systems after 18 h of operation. The results (%) were calculated by the concentrations of Mg2+ and Ca2+ in the corresponding component divided by the initial Mg2+ and Ca2+ concentration of seawater used. Values are expressed as mean 7 standard deviation (n¼ 5). Seawater after desalination Five-chamber BES Mg2 0.0 70.0 +
Membrane in the cathode chamber
Chamber block
Cathode
Solution in the adjacent chambera
Others
5.5 7 0.7
13.2 7 2.8 73.0 7 4.2 1.3 7 1.1
7.0 7 0.1
20.0 70.3
3.8 7 1.4
41.3 7 4.0 24.07 0.9 1.7 7 0.3
9.2 7 1.4
MEDCC Mg2 66.0 75.8 +
14.3 7 2.9
8.4 7 3.0
0.0 7 0.0 5.0 7 1.8
6.3 7 3.5
20.5 7 3.6
16.5 7 0.8
0.4 7 0.2 2.0 7 0.0
4.6 7 2.2
Ca2 +
Ca2 +
56.0 78.7
a In the five-chamber BES, the adjacent chamber is the NaOH-production chamber, and in the MEDCC, the adjacent chambers are the cathode chamber and the acid-production chamber.
through electrodialysis, in which ions must transfer from one chamber to another through the membranes. This process may cause internal membrane scaling, which is difficult to clean. Therefore, by changing the removal mechanism of Mg2+/Ca2+ ions, the five-chamber BES resolved the membrane scaling problem. Based on the calculation, 73% of Mg2+ (0.4 mmol) were recovered from the cathode surface. In the conventional method of magnesium production from seawater, lime (Ca(OH)2) is added into the seawater to precipitate Mg2+ and then Mg(OH)2 is collected [20]. The addition of Ca(OH)2 affects the purity of Mg (OH)2. Without such effect, the purity of Mg(OH)2 produced in the five-chamber BES should be higher.
3.4. Comparison of the desalination performance Different removal mechanisms of the five-chamber BES and the MEDCC resulted in the different ion removal performances (Fig. 3A and Fig. 3B). In the five-chamber BES, Mg2+ and Ca2+ were removed by precipitation reaction in the cathode chamber. Based on calculations using the solubility product constant [21,22], Mg2+ in the artificial seawater began to precipitate when the pH value reached 9.26, while Ca2+ began to precipitate when the pH value reached 12.35. As shown in Fig. 4B, pH values in the cathode chamber of the five-chamber BES reached 11.08 70.16 within 3 h and 12.54 70.14 within 6 h. Therefore, Mg2+ could be rapidly removed within 3 h (maximum removal rate was 0.14 mmol h−1 at 3 h) and totally removed within 6 h. At 6 h, Ca2+ precipitation reached the maximum rate of 0.013 mmol h−1. In the MEDCC, Mg2 + and Ca2+ were removed by electrodialysis in the desalination chamber. The removal efficiency of Ca2+ was 10% higher than Mg2+ within 18 h in the MEDCC due to the smaller hydrated ionic radii of calcium ion than magnesium ion [23]. The MEDCC produced circa 1.6 times of desalination rates compared with other desalination bioelectrochemical systems such as MEDC [9], and the Na+ desalination efficiency in the five-chamber BES was even 20% higher than that in the MEDCC (Fig. 3A). The higher Na+ desalination rate in the five-chamber BES was probably attributable to two aspects. First, there are almost only Na+ ions in the desalination chamber of the five-chamber system, while Mg2+ and Ca2+ ions in the desalination chamber of the MEDCC should compete with Na+ during the electrodialysis process. Therefore, the precipitation process in the five-chamber BES increases the electron desalination efficiency of Na+. Secondly, the membrane scaling problem causing by Ca2+ and Mg2+ precipitations in the MEDCC, which should negatively affect the desalination performance [15], was alleviated in the five-chamber system (Table 1).
Fig. 5. Scanning electron microscopy micrographs of (A) the AEM2 surface in the cathode chamber of the five-chamber BES, (B) the CEM surface in cathode chamber of the MEDCC, and (C) the cathode surface in the cathode chamber of the five-chamber. The size bar of this photograph corresponds to 5 μm.
S. Chen et al. / Journal of Membrane Science 444 (2013) 16–21
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Fig. 6. Energy dispersive X-ray spectroscopy analyses of (A) the AEM2 surface in the cathode chamber of the five-chamber BES, (B) the CEM surface in cathode chamber of the MEDCC, and (C) the cathode surface in the cathode chamber of the five-chamber.
4. Conclusion Seawater desalination is a way to alleviate the water-shortage problem. However, the desalination process is with high energy demand and production of much concentrated salty water. Results in this study provide useful information to resolve above problems. The five-chamber BES can be used to treat wastewater in the anode chamber and desalinate seawater in the desalination chamber. During the desalination, valuable byproducts are collected, including magnesium from the cathode chamber, acid from the acid-production chamber, and alkali from the alkaliproduction chamber, with reduction of the membrane scaling. The five-chamber BES is an environment-friendly device to achieve the goal of comprehensive utilization of seawater.
Acknowledgment This work was partly supported by grants from the Chinese National Natural Science Foundation (Nos. 51039007, 51179212 and 51278500), the Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2011K0001), and the program of Guangzhou Science & Technology Department (No. 2012J4300115). References [1] X. Cao, X. Huang, P. Liang, K. Xiao, Y. Zhou, X. Zhang, B.E. Logan, A new method for water desalination using microbial desalination cells, Environ. Sci. Technol. 43 (2009) 7148–7152. [2] K.S. Jacobson, D.M. Drew, Z. He, Efficient salt removal in a continuously operated upflow microbial desalination cell with an air cathode, Bioresour. Technol. 102 (2011) 376–380. [3] M. Mehanna, P.D. Kiely, D.F. Call, B.E. Logan, Microbial electrodialysis cell for simultaneous water desalination and hydrogen gas production, Environ. Sci. Technol. 44 (2010) 9578–9583. [4] H. Luo, P.E. Jenkins, Z. Ren, Concurrent desalination and hydrogen generation using microbial electrolysis and desalination cells, Environ. Sci. Technol. 45 (2010) 340–344. [5] B.E. Logan, J.M. Regan, Microbial fuel cells-challenges and applications, Environ. Sci. Technol. 40 (2006) 5172–5180.
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