Stacked Microbial Desalination Cells to Enhance Water Desalination

Feb 15, 2011 - ABSTRACT: Microbial desalination cell (MDC) is a new method to obtain clean water from brackish water using electricity generated from ...
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Stacked Microbial Desalination Cells to Enhance Water Desalination Efficiency Xi Chen, Xue Xia, Peng Liang, Xiaoxin Cao, Haotian Sun, and Xia Huang* State Key Joint Laboratory of Environment Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University, Beijing, 100084, P.R. China

bS Supporting Information ABSTRACT: Microbial desalination cell (MDC) is a new method to obtain clean water from brackish water using electricity generated from organic matters by exoelectrogenic bacteria. Anions and cations, derived from salt solution filled in the desalination chamber between the anode and cathode, move to the anode and cathode chambers under the force of electrical field, respectively. On the basis of the primitive single-desalination-chambered MDC, stacked microbial desalination cells (SMDCs) were developed in order to promote the desalination rate in the present study. The effects of desalination chamber number and external resistance were investigated. Results showed that a remarkable increase in the total desalination rate (TDR) could be obtained by means of increasing the desalination cell number and reducing the external resistance, which caused the charge transfer efficiency increased since the SMDCs enabled more pairs of ions separated while one electron passed through the external circuit. The maximum TDR of 0.0252 g/h was obtained using a twodesalination-chambered SMDC with an external resistance of 10 Ω, which was 1.4 times that of single-desalination-chambered MDC. SMDCs proved to be an effective approach to increase the total water desalination rate if provided a proper desalination chamber number and external resistance.

’ INTRODUCTION The lack of clean water has become a bottleneck for the development of human society, accompanied by population booming and environmental pollution. Although total amount of water on earth is abundant, the useable freshwater is rather limited. Considering large capacities of seawater and brackish water, desalination is considered as an important approach to produce needed freshwater.1-3 The main desalination technologies that have been commercialized include electrodialysis, reverse osmosis (RO), and distillation. For the above approaches, the major concerns are the relatively intensive energy consumption in forms of heat or electricity.4,5 For example, the current RO systems usually cost as much as 3-5 kWh/m3 of electrical energy to produce drinking water.2,5 In order to reduce the traditional energy consumption, renewable energies such as solar energy and wind power are under research to drive desalination process.6 However, the costs of these technologies, even higher than conventional ones, prevent them from large-scale application. A new method called microbial desalination cell (MDC), which can desalinate water using electricity generated by bacteria from wastewater, was first proposed by Cao et al. (2009). A three chambered reactor was constructed by inserting an anion exchange membrane (AEM) next to the anode and a cation exchange membrane (CEM) next to the cathode of a microbial fuel cell (MFC), with the salt solution to be desalinated filled in the middle chamber.7 The electricity generating mechanism of MDC is similar to that of MFC. Current is generated by bacteria r 2011 American Chemical Society

on the anode from oxidizing organics, and electrons and protons are released to the anode and anolyte respectively.8,9 As cations are prevented from leaving the anode chamber by the AEM, anions (such as Cl-) move from the middle desalination chamber to the anode. In the cathode, protons are consumed in the reduction reaction of oxygen, while cations (such as Naþ) in the middle chamber transfer across the CEM to the cathode. This process leads to water desalination in the middle chamber, without any external energy source required. In addition, electricity is produced and organic matters in wastewater are degraded by the anodic exoelectrogenic bacteria. In the works of Cao et al., the concept of MDC was successfully proved using a ferricyanide catholyte, and a desalination ratio up to 90% was obtained in 24 h. The results demonstrated for the first time the great potential of MDCs as a low cost desalination process with environmental friendly benefits. Mehanna et al. further developed an air-cathode MDC, which made MDCs more useful for practical applications.10 Jacobson et al. constructed an upflow MDC for continuously operation and discussed the phenomena of bipolar electrodialysis and proton transport in the reactor.11 In practical desalination process, more importance is attached to desalination efficiency so as to reduce the economic cost. To promote the desalination rate of MDC, a possible solution is to increase electron transfer efficiency and Received: October 8, 2010 Accepted: January 27, 2011 Revised: January 15, 2011 Published: February 15, 2011 2465

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Figure 1. The photo (A) and configuration (B) of 2-SMDC (AEM: anion exchange membrane; CEM: cation exchange membrane; C: concentrated chamber; D: desalination chamber).

salt solution volume by developing a stacked desalination system that contains multiple desalination chambers, somewhat similar to electrodialysis. Another approach is to reduce the external resistance, aimed at the energy produced by anodic bacteria consumed as much as possible on the internal desalination. In the present study, on the basis of the single-desalination-chambered MDC reported by Cao et al., stacked microbial desalination cells (SMDCs) with an air-cathode were constructed to enhance the desalination rate using multidesalination chambers combined with external resistance optimized. The effects of desalination chamber number and external resistance on desalination rate were examined.

’ MATERIALS AND METHODS SMDC Construction. SMDC were cubic-shaped bioreactors, which consisted of three blocks (anode chamber, cathode chamber and stacked desalination cells). The stacked desalination cells were composed of desalination chambers and concentrated chambers that were spaced by compartmental AEMs (DF120, Tianwei Membrane) and CEMs (Ultrex CMI7000, Membrane International). One desalination chamber was separated by one AEM and one CEM, respectively. For each desalination chamber, the AEM was on the side close to the anode while the CEM was on the other side close to the cathode. Between two adjacent desalination chambers laid one concentrated chamber, which collects ions moving out from the desalination chambers. One, two, and three desalination chambers, together with zero, one, and two concentrated chambers, respectively, were inserted between the anode and cathode chambers to form one-, two-, and three-desalination-chambered SMDCs (referred to as 1-SMDC, 2-SMDC, and 3-SMDC, respectively). Figure 1 shows the photo and configuration of 2-SMDC. Different chambers and ion exchange membranes (IEMs: AEM and CEM) were clamped together with gaskets

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to provide a water seal between the chambers. The inner diameter of the cross section of each chamber was 3.0 cm. The effective volumes of the anode, desalination, concentrated and cathode chambers were 21.2, 7.1, 7.1, and 7.1 mL with width of 3, 1, 1, and 1 cm, respectively. The anode chamber was filled with several pieces of carbon felt (each piece with the size of 1.0  1.0  1.0 cm),12 which served as electrode material. A graphite rod (5 mm diameter) was inserted into the felt to provide an external electrical contact. Prior to use, the carbon felt and graphite rod were soaked in 1 M HCl for 48 h, followed by rinsing with deionized water to remove trace metals. Cathode was based on 30% wet-proofed carbon cloth (projected area: 7.1 cm2; E-Tek, Type B, BASF Fuel Cell, Inc., Somerset, NJ), which was coated with 0.5 mg/cm2 platinum on the water-facing side and four layers of polytetrafluoroethylene (PTFE) on the air-facing side.13,14 The carbon cloth was connected with external circuit using a titanium wire (length: 1 cm, diameter: 1 mm). Microorganisms and Medium. The anodic carbon felt of SMDCs, which had been covered with biofilm before use, was obtained from an operating acetate-fed MFC. The anolyte was a solution of sodium acetate (1.64 g/L) in a nutrient buffer solution containing (per liter in deionized water): 4.4 g KH2PO4, 3.4 g K2HPO4 3 3H2O, 1.5 g NH4Cl, 0.1 g MgCl2 3 6H2O, 0.1 g CaCl2 3 2H2O, 0.1 g KCl and 10 mL of trace mineral metals solution,15 with conductivity of 10.8 mS/cm. The catholyte was a buffer solution containing (per liter in deionized water): 9.0 g KH2PO4 and 8.0 g K2HPO4 3 3H2O, with conductivity of 12.2 mS/cm. The desalination and concentrated chambers were filled with 20 g/L NaCl solution, which was a representative concentration of brackish water and seawater. SMDC Operation and Experimental Procedures. The reactors were operated in MFC mode before conducting desalination experiments. The anodes were acclimated by running a MFC with one single AEM between the anode and cathode chambers for more than 10 cycles until the peak voltage was stable at around 600 mV, with an external resistance of 1000 Ω. All the three types of SMDCs were transformed from MFC to MDC mode at the same time and fed with the same anolyte and catholyte, which were continuously circulated from a 500 mL and a 150 mL bottles, respectively, at a rate of 5 mL/min using a peristaltic pump (BT100-1 L, Lange, China). The desalination process started when the 20 g/L NaCl solution was injected into desalination and concentrated chambers. Since MDCs might be proposed as a pretreatment process for RO,10 one desalination cycle in this study was defined as the time needed for an SMDC to reach a desalination ratio of 70%, that is, the salt concentration in each desalination chamber was decreased to 6 g/L. The anolyte, catholyte, and salt solutions in both desalination and concentration chambers were replaced when all the three types of SMDCs reached the 70% desalination ratio. The anolyte and catholyte were replaced twice in one desalination cycle in order to ensure a sufficient supply of substrate and avoid pH change. Meanwhile, the conductivity of NaCl solution in each desalination and concentrated chambers was measured using a conductivity meter (SG3ELK, Mettler Toledo, Columbus, OH). In order to investigate the influence of external resistance on the performance of SMDCs, the external resistance was reduced from 1000 Ω to 1 Ω using a resistance box (0.1-99,999 Ω; ZX21, Tianshui, China). All the SMDCs were run at least three desalination cycles for each external resistance and operated in duplicate simultaneously under ambient temperatures (25 ( 1 °C). The results in this paper were the average of the two parallel samples. 2466

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Figure 3. Maximum currents of SMDCs under different desalination chamber numbers and external resistances.

Figure 2. Current curves of SMDCs with an external resistance of (A) 100 Ω, (B) 50 Ω, (C) 10 Ω, and (D) 5 Ω. Arrows indicate anolyte and catholyte replacement during one desalination cycle.

Analyses and Calculation. Cell voltage was automatically monitored by a data acquisition system (DAQ2213, ADLINK, China) every 10 min during experiments. The current (I) in the electrical circuit of an SMDC was determined from the cell voltage (U) and external resistance (Re) according to I = U/Re. The concentration of NaCl was experimentally confirmed having a linear correlation with the conductivity of NaCl solution within the range of NaCl concentration tested in the study. Hence, the desalination ratio (η) of an SMDC with a number of desalination chambers of N at a certain desalination time, can be obtained by the following equation:

η ¼ 1-

V1  σ1 þ V2  σ2 þ ::: þ VN  σ N ðV1 þ V2 þ ::: þ VN Þ  σ 0

ð1Þ

Here VK and σK (K = 1, 2, ..., N) stand for the volume and conductivity of salt solution in the Kth desalination chamber at a certain desalination time, respectively, and σ0 is the conductivity of the initial salt solution with a NaCl concentration of 20 g/L. When one desalination cycle terminated, both the specific desalination rate (SDR) based on salt solution volume (i.e., on the basis of desalination chamber volume (7.1 mL)) and total desalination rate (TDR) were calculated to evaluate desalination performance according to eqs 2 and 3. The former tells NaCl removal rate per unit desalinated water volume and the latter shows the total NaCl removal rate in a whole SMDC. 70%  C0 T

ð2Þ

70%  C0  ðV1 þ V2 þ ::: þ VN Þ T

ð3Þ

SDR ¼ TDR ¼

Here 70% and T are the desalination ratio and needed time respectively when one desalination cycle terminated. C0 is the concentration of the initial NaCl solution (20 g/L).

Charge transfer efficiency was calculated as the ratio Qth/Q, where Q is the coulombs harvested R through the electrical circuit over one desalination cycle (Q = I dt), and Qth stands for the theoretical amount of coulombs that is required for the movement of NaCl (Qth = 70%  0.342 mol/L  (V1 þ V2 þ ... þ VN)  F; 70%: the desalination ratio when one desalination cycle terminated; 0.342 mol/L: the initial molar concentration of salt solution (20 g/L); F: the Faraday’s constant (96485 C/mol)). Current interrupt method was employed to determine the ohmic resistance of SMDCs.16 A steep rise in cell voltage was immediately observed when the electrical circuit was cut off meanwhile the real-time data of cell voltage was recorded by the data acquisition system with a sampling frequency of 1000 Hz. The ohmic internal resistance was calculated from RΩ = ΔU/I0 , where ΔU represents the steep rise in cell voltage and I0 is the current before interruption.

’ RESULTS AND DISCUSSION Electricity Generation Performance of SMDCs. Three types of SMDCs were operated for more than two desalination cycles at different external resistances. All SMDCs exhibited a similar overall trend of current generation during one desalination cycle as typically shown in Figure 2A-D. The current rose to a peak level (maximum current) immediately at the beginning of each desalination cycle, and then dropped slowly along the whole cycle, although the peak magnitudes varied with different external resistances and desalination chamber numbers. Slight fluctuation was caused by the replacement of anolyte and catholyte during one cycle, indicating that the substrate levels (including substrate concentration and pH value) were not the limiting factors of current generation. The decline of the current was mainly attributed to the increase of ohmic resistance that resulted from changes in the conductivity of salt solution, which has been confirmed by Cao et al. using a single-desalinationchambered MDC (which indeed was referred to as 1-SMDC in the current research). For an SMDC with a fixed desalination chamber number, the maximum current varied with the change of external resistance. As shown in Figure 3, the maximum current values of 1-SMDC, 2-SMDC, and 3-SMDC increased by 716%, 622% and 491% from 0.91 mA, 0.88 mA, and 0.79 mA to 7.43 mA, 6.35 mA, and 4.67 mA, respectively, when the external resistance was reduced from 500 Ω to 10 Ω. However, when the external resistance continued to reduce from 10 Ω to 1 Ω, the maximum currents declined sharply. In terms of the maximum current, the optimal 2467

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Environmental Science & Technology external resistance for all the three types of SMDCs was about 10 Ω. These results elucidated that the current generated by SMDCs could be promoted by reducing the external resistance.17,18 Nevertheless, exoelectrogenic bacteria could not produce enough electrons and even behaved unstable when the external resistance became too low. Consequently, the maximum current went downward. For an SMDC with a fixed external resistance, the maximum current decreased with the increase of desalination chamber number. Taking the case of 10 Ω external resistance for an example (Figure 2C), the maximum currents were 7.43 mA, 6.35 mA and 4.67 mA for 1-SMDC, 2-SMDC, and 3-SMDC, respectively. The desalination chamber number also influenced the current curve evolution in one desalination cycle. As shown in Figure 2C, the currents decreased by 6.81 mA, 4.24 mA, and 2.40 mA from their maximum values to 0.62 mA, 2.11 mA, and 2.27 mA for 1-SMDC, 2-SMDC, and 3-SMDC after 12 h of desalination, respectively, indicating that more desalination chambers induced a slower decreasing rate of the current. In addition, the effect of the desalination chamber number on electricity generation was also related to the external resistance. The desalination chamber number had little effect on electricity generation performance when the external resistance was large enough. For example, when the external resistance was higher than 100 Ω, only small differences in the maximum current production (Figure 3) and current curves between the three SMDCs (Figure 2A) were observed. In contrast, the desalination chamber number showed more remarkable influence under the condition of relatively low external resistance such as 10 Ω (Figure 2C and Figure 3). Since a pair of AEM and CEM, together with salt solution of two chambers (one desalination chamber and one concentrated chamber), were increased into the system when another desalination cell was added, it is reasonable for the decrease of the maximum current due to the increased internal resistance caused by the extra added desalination cell. Meanwhile, as will be discussed in later sections, the SDR decreased with the increase of desalination chamber number, leading to a slower rising in ohmic resistance and correspondingly a slower decrease in current. The total resistance in an SMDC consisted of internal resistance and external resistance. The former was affected by the desalination chamber number. Therefore, the variation of internal resistance, resulting from the change of the desalination chamber number, would have a significant impact on current generation when the external resistance was relatively low. Desalination Performance of SMDCs. The conductivity of salt solution in each desalination chamber of SMDCs was measured during one desalination cycle. The average desalination ratio was calculated according to eq 1. As shown in Supporting Information (SI) Figure S1, the desalination ratios of all the three types of SMDCs went up along with one desalination cycle and increased slower when more desalination cells were set in an SMDC. In the case of 50 Ω external resistance, the desalination ratios for 1-SMDC, 2-SMDC, and 3-SMDC were 99.4%, 85.6%, and 72.1% after 18 hours of operation, respectively (SI Figure S1B). In addition, the time needed to reach a certain desalination ratio was shortened with the external resistance reduced. The SDR values of the three types of SMDCs during one desalination cycle were calculated according to eq 2 (Figure 4A). The SDR increased with the decrease of external resistance when the desalination chamber number was fixed. The SDR for

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Figure 4. (A) Specific desalination rate (SDR) based on the salt solution volume and (B) total desalination rate (TDR) of SMDCs under different desalination chamber numbers and external resistances.

2-SMDC increased from 0.25 g/(L 3 h) to 1.96 g/(L 3 h) when the external resistance decreased from 500 Ω to 10 Ω. However, the SDR decreased by 6.2% when the external resistance was further reduced from 10 Ω to 5 Ω. On the other hand, the SDR decreased with the increase of desalination chambers. For an external resistance of 10 Ω, the SDR values were 2.54 g/(L 3 h), 1.96 g/(L 3 h), and 1.24 g/(L 3 h) for 1-SMDC, 2-SMDC, and 3-SMDC, respectively. The increase in internal resistance caused by the increase of desalination chambers is one of main reasons for the decrease of SDR. Meanwhile, the electric potential created by the salt gradient between desalination chamber and concentrated chamber may contribute the decrease of SDR since the electric potential is impeditive to ion transfer from desalination chambers to concentration chambers and will increase along with the desalination process. The effects of the external resistance and desalination chamber number on the SDR were consistent with those on the current. Since the current determined the number of ions that transferred across each membrane per second and the volume of each single desalination chamber clipped between a pair of IEMs was fixed at 7.1 mL, the current also mainly determined the SDR. Figure 4B shows the TDR of SMDCs under different conditions, which were obtained from eq 3. The influence of the external resistance on the TDR was the same as that on the SDR, since the TDR depended only on the current when the desalination chamber number was fixed. Taking the case of 2-SMDC for an example, the maximum TDR of 0.0252 g/h was obtained under an external resistance of 10 Ω, meanwhile the maximum current of 7.43 mA was also generated (see Figure 3). The influence of the desalination chamber number on the TDR was associated with the external resistance. For an external resistance larger than 10 Ω, the TDR rose with the addition of desalination chambers. However, when the external resistance was not larger than 10 Ω, the TDR first increased then decreased with the increasing desalination chamber number. Taking the external resistance of 20 Ω as an example, the TDR rose from 0.014 g/h to 0.0187 g/h while the desalination chamber number was 2468

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Figure 5. The charge transfer efficiencies of SMDCs under different desalination chamber numbers and external resistances.

increased from one to three. For an external resistance of 10 Ω, the maximum TDR of 0.0252 g/h, however, occurred in 2-SMDC, which was 1.4 times that of 1-SMDC (0.0178 g/h) and 1.3 times that of 3-SMDC (0.0196 g/h). In an SMDC, when one electron passed through the external circuit, a pair of cation and anion would be separated from each desalination chamber simultaneously to accomplish a closed loop. Hence, the TDR depended not only upon the current, which determined the desalination rate of one single desalination chamber, but also upon the desalination chamber number, which determined the number of ion-pairs separated at the same time along with each electron that flows through the external circuit. This exactly was why the current decreased with the increase of desalination chambers but the TDR still went up when the external resistance was above 10 Ω. Meanwhile, when the external resistance was no larger than 10 Ω, the current reduction caused by the increase of desalination chambers would be so large that the simultaneous separation of ions in multiple desalination chambers could not compensate the reduction of the TDR caused by current decrease. All the discussion above proved the existence of an optimal desalination chamber number along with an external resistance optimized for salt removal. In the present study, the optimal operation condition was two desalination chambers with an external resistance of 10 Ω, under which a maximum TDR of 0.0252 g/h was obtained. Charge Transfer Efficiency. As expected, charge transfer efficiency was increased significantly by increasing desalination chambers between anode and cathode chambers (Figure 5), since multiple ion-pairs were separated along with one electron passing through the external circuit. For the external resistance of 10 Ω, the charge transfer efficiencies were 120%, 223% and 283% for 1-SMDC, 2-SMDC, and 3-SMDC, respectively. Moreover, the charge transfer efficiency increased when the external resistance declined, despite a few peculiar values. For example, as for 3-SMDC, the charge transfer efficiency rose from 170% to 323% when the external resistance decreased from 500 Ω to 5 Ω. This indicated that the reduction of the external resistance would benefit the charge transfer efficiency. It was proposed that the enlarged current, resulting from the decline in external resistance, offset the reverse diffusion of ions from concentrated chambers to desalination chambers. Moreover, the above electron transfer efficiencies of SMDCs, larger than their theoretical maximum value (100% for 1-SMDC, 200% for 2-SMDC, and 300% for 3-SMDC), may be attributed to the diffusion of ions from desalination chambers to anode and cathode caused by the salt

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Figure 6. Ohmic resistance in one desalination cycle of SMDCs with an external resistance of 10 Ω.

concentration gradient, which was proved by Mehanna et al.10 and Jacobson et al.11 The quantitative analysis on the diffusion of ions needs further study. Change in Ohmic Resistance in a Desalination cycle. The ohmic resistance of an SMDC was made up of anodic ohmic resistance, cathodic ohmic resistance and ohmic resistance of the desalination section. The first two components were relatively constant by frequent replacement of anolyte and catholyte. The ohmic resistances of SMDCs with an external resistance of 10 Ω were measured along one desalination cycle. The comparison of the variation of ohmic resistances (Figure 6) and currents (Figure 2C) reflected a reverse coordination, indicating the internal resistance of an SMDC was significantly affected by the ohmic resistance. At the beginning of one desalination cycle, the ohmic resistances were 21 Ω, 39 Ω, and 56 Ω for 1-, 2-, and 3-SMDC, respectively. The addition of one desalination cell resulted in an increase of about 18 Ω of the ohmic resistance, which was equivalent to the ohmic resistance of a pair of AEM and CEM and two chambers of 20 g/L NaCl solution. Ohmic resistance increased along the cycle, and got a sharp rise in the last 4 h. The ohmic resistance increased the fastest for 1-SMDC (from 21 Ω to 312 Ω), medium for 2-SMDC (from 39 Ω to 256 Ω) and the slowest for 3-SMDC (from 56 Ω to 117 Ω). As the desalinating process continued, the concentration of salt solution in desalination chambers decreased, leading to the increase of ohmic resistance. The SDR determined the change in ohmic resistances. For 1-SMDC, the SDR was the fastest, the change in ohmic resistance was correspondingly the most remarkable. Similarly, the change in the ohmic resistance of 3-SMDC was the slowest due to its least SDR.

’ OUTLOOK In this study, the TDR of MDCs was remarkably enhanced by constructing a stacked MDC. Although the current and corresponding SDR declined with the increase of desalination chamber number, both electron transfer efficiency and salt solution volume increased, which proved beneficial to promote the TDR. Nevertheless, the desalination chamber number should not be increased infinitely, otherwise the current would be very low due to the increase of internal resistance caused by addition of desalination chambers, leading to the decrease of TDR. As the effects of the desalination chamber number and external resistance on TDR were mutually influenced with each other, there should be an optimal condition for TDR. Expanding desalination chamber number together with reducing external resistance 2469

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Environmental Science & Technology proved the feasible approaches to elevate salt removal rate of MDC. Therefore, exploring IEMs of smaller resistance and optimizing the structure of SMDCs, such as the dimension and configuration of desalination and concentrated chambers, are the issues needed to be investigated in future works. As the results shown in this study, desalination ratio of SMDCs increased higher (SI Figure S1C) and ohmic resistance increased faster (Figure 6) in the later period of desalination, which would cause a low desalination rate and a poor electricity generation performance in this period. To determine the degree of desalination, a study on the relationship between desalination performance and ohmic resistance would be of great help. As recommended by Mehanna et al.,10 MDCs may be used as a pretreatment technology for electrodialysis or RO, or be used to produce water that needs not to be completely desalinated.

’ ASSOCIATED CONTENT

bS

Supporting Information. One additional figure. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86 10 62772324; e-mail: [email protected].

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’ ACKNOWLEDGMENT This work was supported by the National High Technology Research and Development Program (863 Program) (No. 2009AA06Z306) and the Program of Introducing Talents of Discipline to Universities (the 111 Project). ’ REFERENCES (1) Mohtada, S; Toraj, M Treatment of sea water using electrodialysis: Current efficiency evaluation. Desalination. 2009, 249, 279–285. (2) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature 2008, 452, 301–310. (3) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J. M. Advances in seawater desalination technologies. Desalination 2008, 221, 47–69. (4) Zhou, Y.; Tol, R. S. J. Evaluating the costs of desalination and water transport. Wat. Resour. Res. 2005, 41, 1–16. (5) Veerapaneni, S.; Logan, B. E.; Bond, R. Reducing energy consumption for seawater desalination. J. Am. Water Works Assoc. 2007, 99, 95–106. (6) Mathioulakis, E.; Belessiotis, V.; Delyannis, E Desalination by using alternative energy: Review and state-of-the-art. Desalination. 2007, 203, 346–365. (7) Cao, X. X.; Huang, X.; Liang, P.; Xiao, K.; Zhou, Y. J.; Zhang, X. Y.; Logan, B. E. A new method for water desalination using microbial desalination cells. Environ. Sci. Technol. 2009, 43, 7148–7152. (8) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder, U.; Keller, J.; Freguia, S.; Alterman, P.; Verstraete, W.; Raraey, K. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 2006, 40, 5181– 5192. (9) Lovley, D. R. The microbe electric: conversion of organic matter to electricity. Curr. Opin. Biotechnol. 2008, 19, 564–571. (10) Mehanna, M.; Saito, T.; Yan, J.; Hickner, M.; Cao, X.; Huang, X.; Logan, B. E. Using microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy Environ. Sci. 2010, 3, 1114–1120. 2470

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