SEA WATER DESALINATION BY MANIPULATING ELECTRICAL
DOUBLE LAYER OVERLAP INSIDE ELECTROSTATICLY CHARGED AAO
NANOCHANNELS
Chia-Jung Chang1, Yu-Sheng Huang1, Shin-Ming Lin2, Yu-Lun Chueh2 and Fan-Gang Tseng1,3
1
Department of Engineering and System Science, National Tsing Hua University, Taiwan
Department of Materials Science and Engineering, National Tsing Hua University, Taiwan
3
Division of Mechanics, Research Center for Applied Sciences, Academia Sinica, Taiwan
2
ABSTRACT
This paper proposes to selectively remove ions from water by the manipulation of electric-double-layer (EDL) inside
nanochannels through uni-polar electrical static charges. Based on electrical static working principle, this device is more efficient in power consumption than reversal osmosis (RO) and even electrodialysis (ED) methods for water purification. It has
been successfully demonstrated the EDL controlling technique can effectively reduce the concentration of NaCl from
0.1514M to 0.01M.
KEYWORDS: electric-double-layer, desalination, uni-polar, HfO2
INTRODUCTION
The Organization of Economic Cooperation and Development (OECD) and the United Nations have reported that
0.35billion people are currently suffering from water shortage[1][2]. Converting sea water into fresh water could provide the
solution to the worldwide water shortage problem because 97% total water resource is sea water and only 0.5% is fresh water.
Traditional desalination processes usually consume much energy, such as RO and ED methods[3]. In this work we propose to
purify water by controlling EDL overlap inside nanochannels as a virtual filter to selectively remove either cations or anions
from water compared to RO or ED methods, the filter size of the EDL device is in hundreds nm range, hundreds times larger
than RO or ED, thus the water flow resistance can be reduced by 4 orders of magnitudes so as the power consumption for
pressuring water through filters. On the other hand, only 0.01V is sufficient for the EDL filter operation, making this device
easily portable in remote area.
EXPERIMENTAL
The setup of the desalination system is shown in Fig. 1 two chambers separated by the EDL water purification membrane
and the upper chamber is loaded with salt water while the lower chamber is loaded with pure DI water. Electric-chemicalelectrodes were arranged in the lower chamber for measuring the water conductivity.
Here the water conductivity is measured by electrical-chemical method by applying very little potential on the electrodes
and measured the conductive current. The higher the conductive current shows the higher NaCl concentration. The 1M NaCl
water solution is almost close to the original sea water concentration. This sea water is forced to pass through this desalination
membrane by osmosis process generated by water gravity itself. When the sea water pass through this membrane the concentration of NaCl in bottom chamber will raise continuous. So by measuring the concentration in the bottom chamber can show
the ability for this membrane to do the sea water desalination.
400nm
Figure 1: The sea water purification testing system and the EDL water purification membrane with nanochannel structures
inside. The pore size is around 200nm.
978-0-9798064-4-5/µTAS 2011/$20©11CBMS-0001
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15th International Conference on
Miniaturized Systems for Chemistry and Life Sciences
October 2-6, 2011, Seattle, Washington, USA
Figure 2. The working principle for AAO based sea water purification system. When HfO2 layer is induced partial positive
charge on the surface, negative EDL effect will occur. By controlling the density of the induced charges, the EDL thickness
can be further controlled inside nanochannels
The water purification membrane consists of Anodic-Alumina-Oxide (AAO) nanochannels coated with high dielectric
constant HfO2 by atomic-layer-deposition (ALD). The operation principle of water purification is controlling EDL-overlap in
the nanochannels (Fig. 2). When sufficient electrostatic charges are applied to the AAO nanochannels, the EDL on the HfO2
surface will overlap with one another. As a result, co-ions are repulsed from the filter region and kept in the upper chamber
while counter-ions can flow freely through the filter into the lower chamber. The fabrication process of the device (Fig. 3) includes sputtering conductive Al metal on the nanochannels in AAO membrane, and then coating HfO2 by ALD to ensure a uniformly dielectric thin film without defect.
RESULTS AND DISCUSSION
Figure 4. The calibration curve for conductive current versus salt concentration.
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3.00E-008
Conductive current (A)
0.1514M
2.00E-008
0.01M
1.00E-008
W/O Applied Voltage (Pure AAO)
With Applied 0.01 V
0.00E+000
0
4
8
12
Desalination time (HR)
Figure 5. When 0.01V (positive potential) applied to the purification membrane, the conductive current shows significantly
reduction from the effect of EDL overlap inside nanochannles. The concentration can be reduced from 0.1514M to 0.01M.
Fig. 4 shows the calibration curve of the conductive current versus different NaCl solution concentrations from 1M to
0.125M. And the calibration equation is shown below.
S = 98.22 X+153.42
(1)
Where the S is the conductance (μScm-1) for NaCl water solution and X is the concentration (mM). By this calibration curve it
is very easy to analysis the real concentration in the bottom chamber which is the desalinated water.
The desalination testing result of the EDL device for NaCl solution is shown in Fig. 5. When the EDL-overlap off, both
coions and counterions can diffuse from the upper chamber to lower chamber quickly, and the conductive current rises from
10-9A to 2.6x10-8A in 12hours. However when the EDL-overlap is on by applying +0.01V in the aluminum film, EDL will be
induced on the HfO2 surface thus the co-ions cannot pass through and the conductive current shows significant reduction from
2.6x10-8A to 8.2x10-9A in 12hours, representing a concentration reduction from original 0.1514M to 0.01M.
CONCLUSION
This work demonstrated the use of high dialectical strength material HfO2 coated inside the AAO nanochannel and
controlling the EDL thickness by applying very small potential to control it and make this EDL overlapped. By using this
EDL overlapped technology can do the desalination and do the sea water purification. It has been successfully demonstrated
the EDL controlling technique can effectively reduce the concentration of NaCl from 0.1514M to 0.01M.
ACKNOLEWEDGE
This work is supported by National Science Council Taiwan. The Project number is NSC 98-2120-M-007-001. And
very thank for the Department of Material Science and Engineering for atomic layer deposition process in National Tsing Hua
University, Taiwan.
REFERENCES
[1] “Population and Sustainable Development-Five Years After Rio 1-36” United Nation Population Fund (UNFPA, 1997)
[2] “Sustainig Water: Population and the Future of Renewable Water Supplies 7-47” Robert Engelman, L. P. (Population Action International, 1993)
[3] “Science and technology for water purification in the coming decades” Shannon, M. A. et al. Nature 452, 301-310 (2008)
CONTACT
*F-G Tseng E-mail: [email protected]
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