A WATER PERMEABLE ION EXCHANGE MEMBRANE
FOR DESLAINATION
Hyukjin J. Kwon1,2, Bumjoo Kim,2 Geunbae Lim1 and Jongyoon Han2
1
Department of Mechanical Engineering, POSTECH, South Korea
2
Department of Electrical Engineering and Computer science, Massachusetts Institute of
Technology, USA
ABSTRACT
Desalination is energy intensive process. Therefore, energy efficiency plays an important role for
desalination process. In this work, we developed a microfluidic proof-of-concept for water permeable ion
exchange membrane which can remarkably improve energy efficiency for electrically driven desalination
system like electrodialysis.
It was verified that our water permeable ion exchange membrane (IEM) reduced the applied voltage,
under constant current condition, by about 25% compared to conventional IEM.
KEYWORDS: ion exchange membrane (IEM), desalination, electrodialysis (ED), ion concentration
polarization (ICP)
INTRODUCTION
Electrodialysis (ED) is one of the most common desalination technique using ion-exchange
membrane and electric field. It has been used for several decades especially for desalination of blackish
water due to its higher energy efficiency and recovery ratio compared to other technologies.
Recently, it was turned out that the main reason why the efficiency of electrodialysis cannot reach to
its theoretical efficiency is mainly contributed by the depletion region near the IEM which acts like a
resistance in terms of ionic current. In electrodialysis, depletion region grows steadily along with the
flow and finally cover whole channel making desalted water. Despite of considerable effort like a mixing
depletion region with bulk region using mesh structure, it remains challenging to remove depletion zone.
Therefore, resistance will become very high and even the lower concentration itself also reduce
efficiency of membrane [1].
Here, we introduce a water permeable IEM which \7ucan separate ion depletion region from both ion
path and feed flow. Consequently, the ion path, which means current path, does not pass through the
depletion region where resistance is extremely high and desalination can be processed by using the
separated depletion region. This novel approach can be applied not only to electrically driven
desalination like ED but also to ion concentration polarization (ICP) desalination that we have previously
demonstrated [2].
EXPERIMENTAL
Water permeable IEM can be simply fabricated by a salt leaching process which is one method of
making micro pore scaffold structure as shown in Figure1.(a). Figure1.(b), SEM image, clearly shows
micro pore forming on the Nafion membrane which has already been commercialized as one type of the
proton exchange membrane. This novel IEM structure has nano-pores (4nm) from inherent property of
Nafion for ion transportation and also has micro-pores which can transfer the water respectively. To
verify this concept, we fabricated the device made of three polydimethylsiloxane (PDMS) pieces made
by utilizing 3D printed mold as shown in Figure1.(c).
978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001
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19th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
October 25-29, 2015, Gyeongju, KOREA
Figure 1. (a) Simple fabrication process of water permeable IEM using salt leaching method (b) scanning electron
microscope (SEM) image of micro pores on the Nafion (commercial IEM). Nafion have nano pores to transport ion
selectively and water would pass through the fabricated micro pores respectively. (c) Schematic diagram of cross
sectional of the device and its operating process. Buffer channel and main channel are separated for fluid (out-ofplane flow) while cat-ion (in-plane flow) can pass through the CEM. ICP phenomena lead ion concentration region
and depletion region in the main channel.
RESULTS AND DISCUSSION
While cat-ion migrations make both ion concentration and depletion region under the electrical field,
ion-depleted water from depletion region pass through the hole by external pressure. As shown in Figure2.(d~f), by the fluorescence microscopic observation with fluorescence dye and particles, we confirmed
that suction flow rate through the water permeable CEM actually remove the depletion region on the CEM
and isolate the depleted water to the desalted channel as desalted water. Moreover, electrical measurement
on Figure 2.(b~c) give us direct evidence that the suction flow rate can reduce the electrical resistance and
voltage drop between the system maintaining amount of desalted water.
Figure 2. (a) Schematic of the microscopic observation (perpendicular to Figure 1. (c)) and boundary of ion depletion layer by suction flow rate through the hole on the water permeable CEM (d) Microscopic image at 0µl/min suc-
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tion flow rate (e) at 6µl/min (f) at 12µl/min. (b)~(c) Electrical effect of water permeable ion exchange membrane.
(b) Characteristics of current voltage curve at different extraction flow rate through holes. Changes are obviously
shown in over-limiting regime which start to have depletion region. (c) Measured voltage drop across the system
under the constant current at different suction flow rate. Decrease in depletion region due to suction flow rate leads
to voltage drop (resistance) decrease (about 25%).
CONCLUSION
We presented the advantage of the separation of ion path with water path and its application to desalination process for improving energy efficiency. With this validate concept, we are planning to make highthroughput device for both ED and ICP desalination as shown in Figure 3 and trying to use it as concentrator or sea mining.
Figure 3. Conceptual schematics of high-throughput device (a) for electrodialysis (ED) and (b) ion concentration
polarization (ICP)
ACKNOWLEDGEMENTS
This work is supported by ARPA-E grant (DE-AR0000294), and also by Kuwait-MIT Center for
Natural Resources and the Environment (CNRE), which was funded by Kuwait Foundation for the Advancement of Sciences (KFAS).
REFERENCES
[1] R.W. Baker, “Membrane technology and applications,” Wiley, 2005.
[2] S. J. Kim, S. H. Ko, K. H. Kang and, J. Han, “Direct seawater desalination by ion concentration polarization,” Nat. Nanotechnol. 5, 297–301, 2010.
CONTACT
* Jongyoon Han ; phone: +1 617-253-2290 ; jyhan@mit.edu
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