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Microbial desalination cells with ion exchange resin packed to enhance desalination at low salt concentration
Journal of Membrane Science 417–418 (2012) 28–33
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Microbial desalination cells with ion exchange resin packed to enhance desalination at low salt concentration Fang Zhang 1, Man Chen 1, Yan Zhang, Raymond J. Zeng n University of Science and Technology of China, Department of Chemistry, Hefei, Anhui 230026, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2012 Received in revised form 12 May 2012 Accepted 7 June 2012 Available online 26 June 2012
High internal resistance is one main obstacle for the application of microbial desalination cell (MDC) at low salt concentration. To enhance desalination and electricity production of MDC at low salt concentration (o 0.7 g/L NaCl), ion exchange resin (IER) was packed in a desalination chamber. The desalination profiles followed pseudo-first-order kinetics. For initial NaCl concentrations of 700 and 100 mg/L, compared to MDC without resin, the kinetic constant increased 2.5 times to 0.152 h 1 and 3.9 times to 0.363 h 1, respectively, while infinite concentration decreased 6.6 times to 7.13 mg/L and 2.6 times to 10.6 mg/L. On the other hand, the current and power density also increased with IER packing. The improved performance was due to the better transfer capacity and lower internal resistance with IER packing. The low effluent salinity would also benefit for the following deeper desalination process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microbial desalination cell Ion exchange resin Low salt concentration Pseudo-first-order kinetics
1. Introduction Freshwater (salinity below 0.5 g/L) stress is one of the main challenges for people all over the world [1]. Ultrapure water is also abundantly needed in the industrial field [2]. On the other hand, due to large water content on the surface of earth, desalination for seawater and brackish water is a promising and worthwhile technology. Till now, the main categories for water desalination are membrane (such as reverse osmosis, electrodialysis) and thermal (such as thermal distillation) based methods. Among them, reverse osmosis (RO) is the fastest growing technology, and the energy demand of RO (3.7 kW h/m3) is much lower than that of thermal distillation (from 650 to 68 kW h/m3), but these technologies are all considered to be energy and capital intensive [3,4]. Therefore, a low energy consuming technology is urgently needed for an efficient desalination process [5,6]. Recently, microbial fuel cell (MFC) is demonstrated to be one novel biotechnology used for energy recovery, wastewater treatment, bioremediation and valuable chemicals production, in which bio-convertible substrates are consumed in the anodic chamber with simultaneous electron generation, and the generated electron would transfer to cathode and be consumed by electron acceptors [7,8]. This concept has been proposed for water desalination in 2009 with anion exchange membrane close to anode while cation
n
Corresponding author. Tel.: þ86 551 3600203; fax: þ 86 551 3601592. E-mail address: [email protected] (R.J. Zeng). 1 These authors contributed equally to this work.
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.06.009
exchange membrane close to cathode, which is called microbial desalination cell (MDC) [5]. MDC is an environmental friendly desalination process and it evolves to process scale-up, hydrogen production, and pretreatment for reverse osmosis, etc. [6,9–11]. However, low solution conductivity of desalination chamber at the end of the MDC operation leads to high internal resistance, which restricts the application of MDC [5,6]. For example, Cao et al. mentioned that when the salt was removed 88%72 at an initial salt concentration of 5 g/L, the ohmic resistance of the MDC increased by 40 times from 25 O to 970 O at the end of the cycle, which was due to the conductivity decrease in desalination chamber and interface resistance of bulk solution and membrane [5]. Similarly, as demonstrated by Luo et al., 98.8% salt was removed from initial 10 g/L NaCl in microbial electrolysis and desalination cell with simultaneous hydrogen production . But as the internal resistance increased from 70–250 to 850–1100 O, both hydrogen production rate and desalination rate reduced [11]. To avoid this drawback, Mehanna et al. proposed MDC as a predesalination process for reverse osmosis, in which, the salt concentration was reduced to 10 g/L, about 50% of the initial concentration [6]. Another issue is high concentration of Cl from desalination chamber which could inhibit the activity of microbial community at anodic chamber, lots of anode solution have to be exchanged to sustain high performance of MDC [5,9], and MDC running at low salt concentration could avoid the inhibition effect from Cl . Therefore, MDC running at low salt concentration also makes sense for future application if the low conductivity can be solved. As known in practice, electro-deionization (EDI) is a substitute technology to electrodialysis (ED) on running at low salt
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Fig. 1. Configuration of microbial desalination cell with packed IER.
29
concentration to realize high purity water production [2,12]. Packing anion and cation exchange mixed resin in the dilution chamber of electro-deionization is the key to realize deeper desalination with high current efficiency [12]. Since the conductivity of IER was higher than that of bulk solution, almost all ions were transferred through IER to ion exchange membrane [13,14]. Therefore, the added IER played two roles in desalination chamber—increase desalination chamber conductivity and enhance ion transfer from bulk solution to ion exchange membrane. Recently, the IER was also integrated in biopolar membrane electrodialysis (BMED) to decrease internal resistance and save energy [15,16]. For example, the voltage drop of BMED under constant applied current decreased about 44.3–61.4% as IER was added [16]. Therefore, in our study, MDC with IER was proposed at low salt concentration to realize better desalination performance and electricity production. 700 and 100 mg/L NaCl were chosen to represent the normal MDC effluent of Cao et al. and Luo et al. [5,11]. Meanwhile, other parameters of MDC, such as
Fig. 2. Desalination profiles of MDC without and with packed IER at initial NaCl concentrations of 700 mg/L (A and C) and 100 mg/L (B and D).
30
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
electricity production, and power density were also determined to realize energy recovery.
2. Materials and methods
Power density and polarization curves were generated via changing the external resistance of three-chamber MDC from 10,000 O to 10 O, and the system was stabilized at least for 0.5 h for each external resistance [8]. The calculation of coulombic efficiency was based on the acetate change during the experiment [8].
2.1. The configuration of MDC and operation The configuration of MDC is shown in Fig. 1. Two separators were arranged from left to right in MFCs as follows: anion exchange membrane (AMI-7001, Membranes International Inc., New Jersey) and cation exchange membrane (CMI-7000, Membranes International Inc., New Jersey). The volume of anode and cathode chambers was 130 mL while the working volume was 100 mL. The volume of desalination chamber was 15 mL, filled with IER (Fig. 1). Meanwhile, the total volume of desalination solution was 40 mL with recirculation to another flask by peristaltic pump (5 mL/min) to determine ion concentration. The carbon fiber felt (2 cm 2 cm 0.2 cm, Sanye Carbon Co. Ltd., Beijing) was used as anode and cathode. The external resistance was 75 O. The anodic chamber was filled with 90 mL of acetateladen synthetic wastewater containing (in 1 L of 50 mM phosphate buffer solution, pH 7.0) NH4Cl, 310 mg; KCl, 130 mg; CaCl2, 10 mg; MgCl2 6H2O, 20 mg; NaCl, 2 mg; FeCl2, 5 mg; CoCl2 2H2O, 1 mg; MnCl2 4H2O, 1 mg; AlCl3, 0.5 mg; (NH4)6Mo7O24, 3 mg; H3BO3, 1 mg; NiCl2 6H2O, 0.1 mg; CuSO4 5H2O, 1 mg; and ZnCl2, 1 mg. The cathodal chamber was filled with potassium ferricyanide (in 50 mM phosphate buffer solution, pH 7.0). The MDC was inoculated with 10 mL anaerobic sludge collected from local wastewater treatment plant. The MDC was operated at 30 1C, and each experiment was repeated at least three times. 2.2. The activation of anion and cation exchange resin and resin mixing For anion exchange resin (Gel type-711, Sinopharm Chemical Reagent Co., Ltd., China), three times volume of deionized water was used firstly to wash the original resin for three times; then three times volumes of NaOH (1 mol/L) and HCl (1 mol/L) were used alternatively for at least three times. Finally, 5 g/L NaCl was used to transform anion exchange resin to Cl-type. For cation exchange resin (Gel type-732, Sinopharm Chemical Reagent Co., Ltd., China), NaOH and HCl were used with reverse sequence of anion exchange resin. The ion exchange capacities (IEC) of anion exchange resin and cation exchange resin were 3.0 and 4.2 mmol/ g, respectively, and so the mixed ratio was 1.4:1 to realize the same IEC in the mixed resin. Before use, the mixed resin was washed with deionized water till the NaCl concentration of the supernatant water was below 1 mg/L.
3. Results and discussion 3.1. The desalination rate of MDC with ion exchange resin packing It is reported that the IER could act as a bridge to enhance ion transfer between bulk solution and ion exchange membrane, especially at low salt concentration [2,14]. As shown in Fig. 2A and B, the microbial desalination cell achieved the desalination from about 720 mg/L to 50 mg/L within 80 h and from 100 mg/L to 30 mg/L within 40 h, respectively. With IER packing in the dilution compartment (Fig. 2C and D), the concentration of NaCl decreased from 720 mg/L to 40 mg/L within 30 h, and from 100 mg/L to 10 mg/L within 10 h, respectively. Meanwhile, the initial and final pH in anodic, desalination and cathodic chambers did not change notably (within 7.0–7.2). When the ion concentration in desalination chamber was lower than that in cathodic chamber, ion back diffusion from cathodic chamber to desalination chamber would occur if the voltage drop of MDC was zero [17]. Therefore, in this study, the infinite concentration was defined as the salt concentration that could not be reduced anymore for the converse effect of ion back diffusion and electric migration in MDC. And the linear relationship of ln((Ct CN)/(C0 CN)) vs time was assumed to follow pseudo-first-order kinetics [18], as shown in Eq.(1), where Ct, CN and C0 are concentrations of NaCl at t, infinite and initial time, respectively,and k is the pseudo-first-order kinetics constant, h 1. ln
C t C 1 C 0 C 1
¼ kt
As shown in Table 1 and Fig. S1, for an initial concentration of NaCl at 700 mg/L, kinetic constant for normal MDC was 0.06270.006 h 1, and the infinite concentration was 47.178.3 mg/L. With packing of IER in desalination chamber, the kinetic constant increased to 0.15270.005 h 1, about 2.5 times higher, and the infinite concentration was 7.170.2 mg/L, about 6.6 times lower. Similarly, for MDC with packed IER running at the 100 mg/L initial concentration of NaCl, the kinetic constant increased from 0.09370.007 to 0.36370.030 h 1, about 3.9 times higher, and the infinite concentration decreased from 27.772.3 to 10.670.2 mg/L, about 2.6 times lower. Therefore, it is rather clear that, the IER could efficiently enhance the desalination rate, and the final concentration of NaCl was also lower. Meanwhile, it also suggests that lower the initial salt concentration, better the enhancing effect. Except the function of enhancing ion transfer, the IER was also utilized for exchanging H þ and OH with Na þ and Cl in bulk solution to reduce salt concentration [14]. But the IER regeneration to H þ and OH types would consume abundant acid and alkali. In this study, the IER was transformed to Na þ and Clbefore desalination experiments, so that the IER regeneration could be avoided. Furthermore, as shown in Fig. S2, within 20 cycles of desalination experiments of mixed salt (KCl and NaCl) without IER regeneration, the performance did not change much.
2.3. Analysis The concentration of acetate was determined by a Gas Chromatograph (Agilant 7890, CA) with a flame ionization detector and a 10 m 0.53 mm HP-FFAP fused-silica capillary column. The concentration of NaCl was calculated from conductivity determined by a conductivity meter (DDS-11AW, Bante instrument Co., China). The pH was determined by a pH meter (PHS-3C, Shanghai Precision & Scientific Instrument Co., Ltd., China). The voltage of MDC was determined with multimeter and recorded manually. The ohmic resistance of MDC was determined via the current interrupt method using electrochemical workstation (CH Instruments CHI660C, Chenhua Instrument Co., China) with the twoelectrode-setup system [8]. In order to compare the cell work state under constant potential (0.6 V), the CHI660 was also used for fixing the potential difference between anode and cathode.
ð1Þ
Fig. 3. The profiles of internal resistance vs salt concentration in MDC.
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Table 1 Pseudo-first-order kinetic fitting parameters for MDC desalination profiles in Fig. 2. Normal MDC
MDC with packed IER
k (h 1)
CN (mg/L)
R2
k (h 1)
CN (mg/L)
R2
700 mg/L
1 2 3
0.065 0.065 0.055
37.6 50.8 53.0
0.99 0.99 0.99
0.157 0.153 0.147
7.4 7.0 7.0
0.96 0.98 0.99
100 mg/L
1 2 3
0.094 0.100 0.086
26.6 30.3 26.2
0.99 0.99 0.99
0.353 0.339 0.396
10.5 10.6 10.8
0.99 0.99 0.99
31
3.2. Comparisons of electricity production The low conductivity of desalination was one big problem for the application of microbial desalination cell. And IER could decrease internal resistance of bulk solution at low salt concentration [15,16]. In our experiment, the total internal ohmic resistance (including anode, cathode, desalination chamber and ion exchange membrane) of MDC was determined via the current interrupt method. As shown in Fig. 3, the internal ohmic resistance decreased with the increase of NaCl concentration. With packing of IER, the internal resistance decreased notably, such as from 7383769 to 1590758 O and from 64171 to 27776 O at 50 mg/L and 700 mg/L, respectively. Accordingly, the current and maximum power density of MDC with packed IER were higher. As shown in Fig. 4, with IER packing, the initial current increased from 0.7 to 1.4 mA (Fig. 4A and C) and from 0.15 to 0.65 mA (Fig. 4B and D) at 700 mg/L and 100 mg/L, respectively. On the other hand, the maximum power densities were 360 and 328 mW/m2 for MDC with and without resin packing at 700 mg/L NaCl, while
Fig. 4. The current profile of microbial desalination cell at initial NaCl concentrations of 700 mg/L (A and C) and 100 mg/L (B and D). (A) and (B) MDC without packed IER; (C) and (D) MDC with packed IER .
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F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Fig. 5. Performance of MDC under constant anode potential. (A) MDC without packed IER; and (B) MDC with packed IER. these values were 60 and 37 mW/m2 at 50 mg/L NaCl. Lower internal ohmic reistance, higher current and power density all verified the better conductivity of MDC with IER packing. Meanwhile, it should be noted that the coulombic efficiency (CE) in this study was lower than 10%. However, Cao et al. reported up to 100% CE on running at high salt concentration [5]. The big gap was due to high internal resistance in our study, resulting in most of the substrate being converted to methane at anodic chamber (data not shown). On the other hand, N2 sparging would be another solution to avoid this low CE [19].
3.3. MDC performance under constant potential The potential between anode and cathode in MDC decreases with time. To visually demonstrate the advantage of the MDC with packed IER, a constant potential was applied between anode and cathode (see the section ‘‘Materials and methods’’). As shown in Fig. 5A, the concentration of NaCl decreased from 700 mg/L to 50 mg/L within 40 h. With packing ofthe IER, the time reduced to 25 h (Fig. 5B). And, the desalination rate in Fig. 5B (0.191 h 1) was also faster than the one in Fig. 2C (0.152 h 1), which was due to the fact that, though the resin was packed, the voltage of MDC also decreased during the desalination process (from 100 to 20 mV).
wastewater in the anode would also promote the desalination process [6]. To realize the maximum desalination and low capital investment, much more pairs are needed, where valuable salts would also be recovered in concentrated compartments [20,24,25].
4. Conclusions Microbial desalination cell with packed IER was applied to enhance desalination at low salt concentration, which demonstrated shorter running time, lower effluent salinity and higher electricity production. The desalination profiles followed pseudo-first-order kinetics. As IER involved, the kinetic constant increased from 0.062 to 0.152 and from 0.093 to 0.363 h 1 at the initial NaCl concentration of 700 mg/L and 100 mg/L, respectively. The infinite NaCl concentration decreased from 47.1 to 7.1, and from 27.7 to 10.6 mg/L at initial NaCl concentrations of 700 mg/L and 100 mg/L, respectively. Meanwhile, the current and power density of MDC increased with IER packing.
3.4. Perspectives of MDC with packed IER MDC is commonly utilized for water desalination at high salt concentration for the benefits of higher conductivity and higher concentration difference between desalination solution and electrolytes in anodic and cathodic chambers [6]. Recently, two methods were proposed for desalination at low salt concentration [17,20]. Firstly, Kim and Logan proposed thinner stack space to reduce internal resistance of MDC [20], which demonstrated better current density and desalination efficiency at initial high salt concentration (35 g/L). But, the desalination performance reduced notably at low salt influent (about 6 g/L), and the time required rose to 50 h to reach 0.7 g/L NaCl effluent. It is expected that if the IER is added in their configuration, the performance would be enhanced. Secondly, MDC integrated with capacitive deionization (CDI) used for desalination at low salt concentration (60 mg/L) was proposed by Yuan et al. [17]. As demonstrated by Kim and Choi, on addition of ion exchange polymer (IEP) in electrode of CDI, the desalination efficiency also enhanced by 27–56%, which was mainly due to the selective ion transport between electrode surface and bulk solution [21]. Therefore, MDC packed with IER and/or IEP would be a useful supplement for desalination. Moreover, the lower effluent salinity would reduce the operational cost of RO and other desalination technologies for freshwater and ultrapure water production [22,23]. As known, MDC is an environmental friendly desalination process, which could realize energy recovery and desalination simultaneously. In this study, acetate was used as substrate in the anode, but wastewater is a much more promising substrate for MDC [5]. Simultaneously, the low salt concentration of
Acknowledgments The authors would like to acknowledge the financial support from Hundred-Talent Program of CAS and the Fundamental Research Funds for the Central Universities (Grant no. WK2060190007).
Appendix A. Supplementary material information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2012.06.009.
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[3] R. Semiat, Energy issues in desalination processes, Environ. Sci. Technol. 42 (2008) 8193–8201. [4] B. Pilat, Practice of water desalination by electrodialysis, Desalination 139 (2001) 385–392. [5] 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. [6] M. Mehanna, T. Saito, J. Yan, M. Hickner, X. Cao, X. Huang, B.E. Logan, Using microbial desalination cells to reduce water salinity prior to reverse osmosis, Energy Environ. Sci. 3 (2010) 1114–1120. [7] K. Rabaey, R. Rozendal, Microbial electrosynthesis—revisiting the electrical route for microbial production, Nat. Rev. Microbiol. 8 (2010) 706–716. ¨ [8] B.E. Logan, B. Hamelers, R. Rozendal, U. Schroder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5192. [9] 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. [10] K.S. Jacobson, D.M. Drew, Z. He, Use of a liter-scale microbial desalination cell as a platform to study bioelectrochemical desalination with salt solution or artificial seawater, Environ. Sci. Technol 45 (2011) 4652–4657. [11] 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. [12] G. Ganzi, Electrodeionization for high purity water production, AIChE Symp. Ser. 84 (1988). [13] P.B. Spoor, W.R.T. Veen, L.J.J. Janssen, Electrodeionization 1: migration of nickel ions absorbed in a rigid, macroporous cation-exchange resin, J. Appl. Electrochem. 31 (2001) 523–530. [14] Y. Tanaka, Ion Exchange Membranes—Fundamentals and Applications, Elsevier Science, 2007.
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[15] K. Zhang, M. Wang, D. Wang, C. Gao, The energy-saving production of tartaric acid using ion exchange resin-filling bipolar membrane electrodialysis, J. Membr. Sci. 341 (2009) 246–251. [16] Q. Li, C. Huang, T. Xu, Alcohol splitting for the production of methyl methoxyacetate: integration of ion-exchange with bipolar membrane electrodialysis, J. Membr. Sci. 367 (2011) 314–318. [17] L. Yuan, X. Yang, P. Liang, L. Wang, Z.H. Huang, J. Wei, X. Huang, Capacitive deionization coupled with microbial fuel cells to desalinate low-concentration salt water, Bioresour. Technol. 110 (2012) 735–738. [18] R.M. Asmussen, M. Tian, A. Chen, A New, Approach to wastewater remediation based on bifunctional electrodes, Environ. Sci. Technol. 43 (2009) 5100–5105. [19] B. Min, S. Cheng, B.E. Logan, Electricity generation using membrane and salt bridge microbial fuel cells, Water Res. 39 (2005) 1675–1686. [20] Y. Kim, B.E. Logan, Series assembly of microbial desalination cells containing stacked electrodialysis cells for partial or complete seawater desalination, Environ. Sci. Technol. 45 (2011) 5840–5845. [21] Y.J. Kim, J.H. Choi, Improvement of desalination efficiency in capacitive deionization using a carbon electrode coated with an ion-exchange polymer, Water Res. 44 (2010) 990–996. [22] K.S. Spiegler, Y.M. El-Sayed, The energetics of desalination processes, Desalination 134 (2001) 109–128. [23] A. Zhu, P.D. Christofides, Y. Cohen, Energy consumption optimization of reverse osmosis membrane water desalination subject to feed salinity fluctuation, Ind. Eng. Chem. Res. 48 (2009) 9581–9589. [24] M. Kumar, B.P. Tripathi, V.K. Shahi, Electro-membrane process for in situ ion substitution and separation of salicylic acid from its sodium salt, Ind. Eng. Chem. Res. 48 (2009) 923–930. [25] C. Huang, T. Xu, Y. Zhang, Y. Xue, G. Chen, Application of electrodialysis to the production of organic acids: state-of-the-art and recent developments, J. Membr. Sci. 288 (2007) 1–12.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Microbial desalination cells with ion exchange resin packed to enhance desalination at low salt concentration Fang Zhang 1, Man Chen 1, Yan Zhang, Raymond J. Zeng n University of Science and Technology of China, Department of Chemistry, Hefei, Anhui 230026, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2012 Received in revised form 12 May 2012 Accepted 7 June 2012 Available online 26 June 2012
High internal resistance is one main obstacle for the application of microbial desalination cell (MDC) at low salt concentration. To enhance desalination and electricity production of MDC at low salt concentration (o 0.7 g/L NaCl), ion exchange resin (IER) was packed in a desalination chamber. The desalination profiles followed pseudo-first-order kinetics. For initial NaCl concentrations of 700 and 100 mg/L, compared to MDC without resin, the kinetic constant increased 2.5 times to 0.152 h 1 and 3.9 times to 0.363 h 1, respectively, while infinite concentration decreased 6.6 times to 7.13 mg/L and 2.6 times to 10.6 mg/L. On the other hand, the current and power density also increased with IER packing. The improved performance was due to the better transfer capacity and lower internal resistance with IER packing. The low effluent salinity would also benefit for the following deeper desalination process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microbial desalination cell Ion exchange resin Low salt concentration Pseudo-first-order kinetics
1. Introduction Freshwater (salinity below 0.5 g/L) stress is one of the main challenges for people all over the world [1]. Ultrapure water is also abundantly needed in the industrial field [2]. On the other hand, due to large water content on the surface of earth, desalination for seawater and brackish water is a promising and worthwhile technology. Till now, the main categories for water desalination are membrane (such as reverse osmosis, electrodialysis) and thermal (such as thermal distillation) based methods. Among them, reverse osmosis (RO) is the fastest growing technology, and the energy demand of RO (3.7 kW h/m3) is much lower than that of thermal distillation (from 650 to 68 kW h/m3), but these technologies are all considered to be energy and capital intensive [3,4]. Therefore, a low energy consuming technology is urgently needed for an efficient desalination process [5,6]. Recently, microbial fuel cell (MFC) is demonstrated to be one novel biotechnology used for energy recovery, wastewater treatment, bioremediation and valuable chemicals production, in which bio-convertible substrates are consumed in the anodic chamber with simultaneous electron generation, and the generated electron would transfer to cathode and be consumed by electron acceptors [7,8]. This concept has been proposed for water desalination in 2009 with anion exchange membrane close to anode while cation
n
Corresponding author. Tel.: þ86 551 3600203; fax: þ 86 551 3601592. E-mail address: [email protected] (R.J. Zeng). 1 These authors contributed equally to this work.
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.06.009
exchange membrane close to cathode, which is called microbial desalination cell (MDC) [5]. MDC is an environmental friendly desalination process and it evolves to process scale-up, hydrogen production, and pretreatment for reverse osmosis, etc. [6,9–11]. However, low solution conductivity of desalination chamber at the end of the MDC operation leads to high internal resistance, which restricts the application of MDC [5,6]. For example, Cao et al. mentioned that when the salt was removed 88%72 at an initial salt concentration of 5 g/L, the ohmic resistance of the MDC increased by 40 times from 25 O to 970 O at the end of the cycle, which was due to the conductivity decrease in desalination chamber and interface resistance of bulk solution and membrane [5]. Similarly, as demonstrated by Luo et al., 98.8% salt was removed from initial 10 g/L NaCl in microbial electrolysis and desalination cell with simultaneous hydrogen production . But as the internal resistance increased from 70–250 to 850–1100 O, both hydrogen production rate and desalination rate reduced [11]. To avoid this drawback, Mehanna et al. proposed MDC as a predesalination process for reverse osmosis, in which, the salt concentration was reduced to 10 g/L, about 50% of the initial concentration [6]. Another issue is high concentration of Cl from desalination chamber which could inhibit the activity of microbial community at anodic chamber, lots of anode solution have to be exchanged to sustain high performance of MDC [5,9], and MDC running at low salt concentration could avoid the inhibition effect from Cl . Therefore, MDC running at low salt concentration also makes sense for future application if the low conductivity can be solved. As known in practice, electro-deionization (EDI) is a substitute technology to electrodialysis (ED) on running at low salt
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Fig. 1. Configuration of microbial desalination cell with packed IER.
29
concentration to realize high purity water production [2,12]. Packing anion and cation exchange mixed resin in the dilution chamber of electro-deionization is the key to realize deeper desalination with high current efficiency [12]. Since the conductivity of IER was higher than that of bulk solution, almost all ions were transferred through IER to ion exchange membrane [13,14]. Therefore, the added IER played two roles in desalination chamber—increase desalination chamber conductivity and enhance ion transfer from bulk solution to ion exchange membrane. Recently, the IER was also integrated in biopolar membrane electrodialysis (BMED) to decrease internal resistance and save energy [15,16]. For example, the voltage drop of BMED under constant applied current decreased about 44.3–61.4% as IER was added [16]. Therefore, in our study, MDC with IER was proposed at low salt concentration to realize better desalination performance and electricity production. 700 and 100 mg/L NaCl were chosen to represent the normal MDC effluent of Cao et al. and Luo et al. [5,11]. Meanwhile, other parameters of MDC, such as
Fig. 2. Desalination profiles of MDC without and with packed IER at initial NaCl concentrations of 700 mg/L (A and C) and 100 mg/L (B and D).
30
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
electricity production, and power density were also determined to realize energy recovery.
2. Materials and methods
Power density and polarization curves were generated via changing the external resistance of three-chamber MDC from 10,000 O to 10 O, and the system was stabilized at least for 0.5 h for each external resistance [8]. The calculation of coulombic efficiency was based on the acetate change during the experiment [8].
2.1. The configuration of MDC and operation The configuration of MDC is shown in Fig. 1. Two separators were arranged from left to right in MFCs as follows: anion exchange membrane (AMI-7001, Membranes International Inc., New Jersey) and cation exchange membrane (CMI-7000, Membranes International Inc., New Jersey). The volume of anode and cathode chambers was 130 mL while the working volume was 100 mL. The volume of desalination chamber was 15 mL, filled with IER (Fig. 1). Meanwhile, the total volume of desalination solution was 40 mL with recirculation to another flask by peristaltic pump (5 mL/min) to determine ion concentration. The carbon fiber felt (2 cm 2 cm 0.2 cm, Sanye Carbon Co. Ltd., Beijing) was used as anode and cathode. The external resistance was 75 O. The anodic chamber was filled with 90 mL of acetateladen synthetic wastewater containing (in 1 L of 50 mM phosphate buffer solution, pH 7.0) NH4Cl, 310 mg; KCl, 130 mg; CaCl2, 10 mg; MgCl2 6H2O, 20 mg; NaCl, 2 mg; FeCl2, 5 mg; CoCl2 2H2O, 1 mg; MnCl2 4H2O, 1 mg; AlCl3, 0.5 mg; (NH4)6Mo7O24, 3 mg; H3BO3, 1 mg; NiCl2 6H2O, 0.1 mg; CuSO4 5H2O, 1 mg; and ZnCl2, 1 mg. The cathodal chamber was filled with potassium ferricyanide (in 50 mM phosphate buffer solution, pH 7.0). The MDC was inoculated with 10 mL anaerobic sludge collected from local wastewater treatment plant. The MDC was operated at 30 1C, and each experiment was repeated at least three times. 2.2. The activation of anion and cation exchange resin and resin mixing For anion exchange resin (Gel type-711, Sinopharm Chemical Reagent Co., Ltd., China), three times volume of deionized water was used firstly to wash the original resin for three times; then three times volumes of NaOH (1 mol/L) and HCl (1 mol/L) were used alternatively for at least three times. Finally, 5 g/L NaCl was used to transform anion exchange resin to Cl-type. For cation exchange resin (Gel type-732, Sinopharm Chemical Reagent Co., Ltd., China), NaOH and HCl were used with reverse sequence of anion exchange resin. The ion exchange capacities (IEC) of anion exchange resin and cation exchange resin were 3.0 and 4.2 mmol/ g, respectively, and so the mixed ratio was 1.4:1 to realize the same IEC in the mixed resin. Before use, the mixed resin was washed with deionized water till the NaCl concentration of the supernatant water was below 1 mg/L.
3. Results and discussion 3.1. The desalination rate of MDC with ion exchange resin packing It is reported that the IER could act as a bridge to enhance ion transfer between bulk solution and ion exchange membrane, especially at low salt concentration [2,14]. As shown in Fig. 2A and B, the microbial desalination cell achieved the desalination from about 720 mg/L to 50 mg/L within 80 h and from 100 mg/L to 30 mg/L within 40 h, respectively. With IER packing in the dilution compartment (Fig. 2C and D), the concentration of NaCl decreased from 720 mg/L to 40 mg/L within 30 h, and from 100 mg/L to 10 mg/L within 10 h, respectively. Meanwhile, the initial and final pH in anodic, desalination and cathodic chambers did not change notably (within 7.0–7.2). When the ion concentration in desalination chamber was lower than that in cathodic chamber, ion back diffusion from cathodic chamber to desalination chamber would occur if the voltage drop of MDC was zero [17]. Therefore, in this study, the infinite concentration was defined as the salt concentration that could not be reduced anymore for the converse effect of ion back diffusion and electric migration in MDC. And the linear relationship of ln((Ct CN)/(C0 CN)) vs time was assumed to follow pseudo-first-order kinetics [18], as shown in Eq.(1), where Ct, CN and C0 are concentrations of NaCl at t, infinite and initial time, respectively,and k is the pseudo-first-order kinetics constant, h 1. ln
C t C 1 C 0 C 1
¼ kt
As shown in Table 1 and Fig. S1, for an initial concentration of NaCl at 700 mg/L, kinetic constant for normal MDC was 0.06270.006 h 1, and the infinite concentration was 47.178.3 mg/L. With packing of IER in desalination chamber, the kinetic constant increased to 0.15270.005 h 1, about 2.5 times higher, and the infinite concentration was 7.170.2 mg/L, about 6.6 times lower. Similarly, for MDC with packed IER running at the 100 mg/L initial concentration of NaCl, the kinetic constant increased from 0.09370.007 to 0.36370.030 h 1, about 3.9 times higher, and the infinite concentration decreased from 27.772.3 to 10.670.2 mg/L, about 2.6 times lower. Therefore, it is rather clear that, the IER could efficiently enhance the desalination rate, and the final concentration of NaCl was also lower. Meanwhile, it also suggests that lower the initial salt concentration, better the enhancing effect. Except the function of enhancing ion transfer, the IER was also utilized for exchanging H þ and OH with Na þ and Cl in bulk solution to reduce salt concentration [14]. But the IER regeneration to H þ and OH types would consume abundant acid and alkali. In this study, the IER was transformed to Na þ and Clbefore desalination experiments, so that the IER regeneration could be avoided. Furthermore, as shown in Fig. S2, within 20 cycles of desalination experiments of mixed salt (KCl and NaCl) without IER regeneration, the performance did not change much.
2.3. Analysis The concentration of acetate was determined by a Gas Chromatograph (Agilant 7890, CA) with a flame ionization detector and a 10 m 0.53 mm HP-FFAP fused-silica capillary column. The concentration of NaCl was calculated from conductivity determined by a conductivity meter (DDS-11AW, Bante instrument Co., China). The pH was determined by a pH meter (PHS-3C, Shanghai Precision & Scientific Instrument Co., Ltd., China). The voltage of MDC was determined with multimeter and recorded manually. The ohmic resistance of MDC was determined via the current interrupt method using electrochemical workstation (CH Instruments CHI660C, Chenhua Instrument Co., China) with the twoelectrode-setup system [8]. In order to compare the cell work state under constant potential (0.6 V), the CHI660 was also used for fixing the potential difference between anode and cathode.
ð1Þ
Fig. 3. The profiles of internal resistance vs salt concentration in MDC.
F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Table 1 Pseudo-first-order kinetic fitting parameters for MDC desalination profiles in Fig. 2. Normal MDC
MDC with packed IER
k (h 1)
CN (mg/L)
R2
k (h 1)
CN (mg/L)
R2
700 mg/L
1 2 3
0.065 0.065 0.055
37.6 50.8 53.0
0.99 0.99 0.99
0.157 0.153 0.147
7.4 7.0 7.0
0.96 0.98 0.99
100 mg/L
1 2 3
0.094 0.100 0.086
26.6 30.3 26.2
0.99 0.99 0.99
0.353 0.339 0.396
10.5 10.6 10.8
0.99 0.99 0.99
31
3.2. Comparisons of electricity production The low conductivity of desalination was one big problem for the application of microbial desalination cell. And IER could decrease internal resistance of bulk solution at low salt concentration [15,16]. In our experiment, the total internal ohmic resistance (including anode, cathode, desalination chamber and ion exchange membrane) of MDC was determined via the current interrupt method. As shown in Fig. 3, the internal ohmic resistance decreased with the increase of NaCl concentration. With packing of IER, the internal resistance decreased notably, such as from 7383769 to 1590758 O and from 64171 to 27776 O at 50 mg/L and 700 mg/L, respectively. Accordingly, the current and maximum power density of MDC with packed IER were higher. As shown in Fig. 4, with IER packing, the initial current increased from 0.7 to 1.4 mA (Fig. 4A and C) and from 0.15 to 0.65 mA (Fig. 4B and D) at 700 mg/L and 100 mg/L, respectively. On the other hand, the maximum power densities were 360 and 328 mW/m2 for MDC with and without resin packing at 700 mg/L NaCl, while
Fig. 4. The current profile of microbial desalination cell at initial NaCl concentrations of 700 mg/L (A and C) and 100 mg/L (B and D). (A) and (B) MDC without packed IER; (C) and (D) MDC with packed IER .
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F. Zhang et al. / Journal of Membrane Science 417–418 (2012) 28–33
Fig. 5. Performance of MDC under constant anode potential. (A) MDC without packed IER; and (B) MDC with packed IER. these values were 60 and 37 mW/m2 at 50 mg/L NaCl. Lower internal ohmic reistance, higher current and power density all verified the better conductivity of MDC with IER packing. Meanwhile, it should be noted that the coulombic efficiency (CE) in this study was lower than 10%. However, Cao et al. reported up to 100% CE on running at high salt concentration [5]. The big gap was due to high internal resistance in our study, resulting in most of the substrate being converted to methane at anodic chamber (data not shown). On the other hand, N2 sparging would be another solution to avoid this low CE [19].
3.3. MDC performance under constant potential The potential between anode and cathode in MDC decreases with time. To visually demonstrate the advantage of the MDC with packed IER, a constant potential was applied between anode and cathode (see the section ‘‘Materials and methods’’). As shown in Fig. 5A, the concentration of NaCl decreased from 700 mg/L to 50 mg/L within 40 h. With packing ofthe IER, the time reduced to 25 h (Fig. 5B). And, the desalination rate in Fig. 5B (0.191 h 1) was also faster than the one in Fig. 2C (0.152 h 1), which was due to the fact that, though the resin was packed, the voltage of MDC also decreased during the desalination process (from 100 to 20 mV).
wastewater in the anode would also promote the desalination process [6]. To realize the maximum desalination and low capital investment, much more pairs are needed, where valuable salts would also be recovered in concentrated compartments [20,24,25].
4. Conclusions Microbial desalination cell with packed IER was applied to enhance desalination at low salt concentration, which demonstrated shorter running time, lower effluent salinity and higher electricity production. The desalination profiles followed pseudo-first-order kinetics. As IER involved, the kinetic constant increased from 0.062 to 0.152 and from 0.093 to 0.363 h 1 at the initial NaCl concentration of 700 mg/L and 100 mg/L, respectively. The infinite NaCl concentration decreased from 47.1 to 7.1, and from 27.7 to 10.6 mg/L at initial NaCl concentrations of 700 mg/L and 100 mg/L, respectively. Meanwhile, the current and power density of MDC increased with IER packing.
3.4. Perspectives of MDC with packed IER MDC is commonly utilized for water desalination at high salt concentration for the benefits of higher conductivity and higher concentration difference between desalination solution and electrolytes in anodic and cathodic chambers [6]. Recently, two methods were proposed for desalination at low salt concentration [17,20]. Firstly, Kim and Logan proposed thinner stack space to reduce internal resistance of MDC [20], which demonstrated better current density and desalination efficiency at initial high salt concentration (35 g/L). But, the desalination performance reduced notably at low salt influent (about 6 g/L), and the time required rose to 50 h to reach 0.7 g/L NaCl effluent. It is expected that if the IER is added in their configuration, the performance would be enhanced. Secondly, MDC integrated with capacitive deionization (CDI) used for desalination at low salt concentration (60 mg/L) was proposed by Yuan et al. [17]. As demonstrated by Kim and Choi, on addition of ion exchange polymer (IEP) in electrode of CDI, the desalination efficiency also enhanced by 27–56%, which was mainly due to the selective ion transport between electrode surface and bulk solution [21]. Therefore, MDC packed with IER and/or IEP would be a useful supplement for desalination. Moreover, the lower effluent salinity would reduce the operational cost of RO and other desalination technologies for freshwater and ultrapure water production [22,23]. As known, MDC is an environmental friendly desalination process, which could realize energy recovery and desalination simultaneously. In this study, acetate was used as substrate in the anode, but wastewater is a much more promising substrate for MDC [5]. Simultaneously, the low salt concentration of
Acknowledgments The authors would like to acknowledge the financial support from Hundred-Talent Program of CAS and the Fundamental Research Funds for the Central Universities (Grant no. WK2060190007).
Appendix A. Supplementary material information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2012.06.009.
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