Capacitive deionization

Background

Capacitive deionization (CDI) is a new technology for desalination and water treatment. The idea was exploited in the 1960s. By taking advantage of the excess ion adsorption in the electrical double layer (or polarization layers) of porous carbon electrodes, it is possible to remove salt from aqueous solutions by charging the electrodes. There are already some pre-commercial tests of this technology. ENPAR Technologies INC., is a Canadian company which announced a small-scale CDI based technology called DesEL which was shipped to the University of Montreal for testing. A mobile CDI system which was presented in a document by the Missile Defense Agency, Advanced Systems Technology Application Program (MDA/AS) (2004) was announced being used in disaster areas for water treatment. The ENPAR CDI technology has been used to remove nitrate from a drinking water source with an average NO3-concentration of 155mg/l to less than 50mg/l. CDI has also been used for high ammonia/nitrate contaminated groundwater at an industrial site in southwestern Ontario where ammonia-N and nitrate-N compounds were reduced in concentration from 500mg/l to below the target level of 10mg/l. Additionally, CDI was used to reduce calcium and chloride in intake process water by 80% and 90%, to reduce TDS (total dissolved solid) by 92%, and to recover clean water from mine wastewater up to 90% in northern Quebec, Ontario, Canada (Oren, 2008).

Working Principle

Although the CDI is a complex dynamical nonlinear process, it is still possible to describe it in simple words. Desalination by CDI occurs when charging the two or more pairs of high-surface area carbon electrodes in brackish water. The high-surface area carbon electrodes in other words is activated carbon electrode or porous carbon electrode, which has a huge surface area for adsorption. Carbon nanotubes (CNTs) can also be used as the electrode material. Anions and cations in solution are electrically adsorbed by electric fields upon polarization of both anode and cathode electrode in each pair by a direct current (DC) power source (Welgemoed and Schutte 2005). The removal of salt in the procedure not only depends on the potential applied between anode electrode and cathode electrode but also depend on salt concentrations of the bulk solution. Electrical double layer(EDL) theory could be used to predict and explain how much salt could be removed as function of salt concentration and applied cell voltage(R.Zhao, 2010).

In a real CDI experimental or commercial setup, several pairs of electrodes are compacted in one stack. In between of anode and cathode, a spacer is placed in order to preventing from short-cutting. Brackish water is pumped from one side of the stack to the other. When charging up the cell, desalination takes place. The entire CDI process is possible to be simulated with Gouy-Chapman-Stern model based on EDL theory from the begining of charging to the ending as the carbon electrodes are saturated with ions (Biesheuvel, P.M.,2009).

Technological Development

Membrane Capacitive deionization

Membrane capacitive deionization (MCDI) is a modification of the normal CDI without membrane. The difference is inserting cation exchage membrane, anion exchange membrane or both in between of the anode electrode and the cation electrode (M.D. Andelman,2002; J.B.Lee,2006; H. Li,2008). The advantage of membrane inserted is to prevent coions at the electrode side going back to the bulk solution. Thus the efficiency of salt removal is much higher than normal CDI.

Reversed CDI

Instead of putting energy into the CDI stack to charge, energy could be extracted from the CDI cell by flow though high salinated water and low salinated water in sequence. During the reversed-CDI process, a low voltage is need to adsorb ions when flowing through high salinated water, i.e. 0.5 V. After flow though high salinated water, salt ions are adsorbed in the carbon electrode. When flowing through low salinated water, the salt ions diffuse away from the carbon electrode against the electrostatic force. Therefore, the electrostatic energy of the system increases. If a resistance or an electrical appliance is connected in due time, the increased energy is able to be extracted (D.Brogioli,2009).

Reference

1. Arnold, B. B. and G. W. Murphy (1961). "Studies on electrochemistry of carbon and chemically modified carbon surfaces." Journal of Physical Chemistry 65(1): 135-&.
2. Biesheuvel, P. M. (2009). "Thermodynamic cycle analysis for capacitive deionization." Journal of Colloid and Interface Science 332(1): 258-264.
3. Biesheuvel, P. M., B. van Limpt, et al. (2009). "Dynamic Adsorption/Desorption Process Model for Capacitive Deionization." Journal of Physical Chemistry C 113(14): 5636-5640.
4. Brogioli, D. (2009). "Extracting Renewable Energy from a Salinity Difference Using a Capacitor." Physical Review Letter 103, 05801.
5. H. Li, Y. Gao, L. Pan, Y. Zhang, Y. Chen, and Z. Sun, “Electrosorptive desalination by carbon nanotubes and nanofibres electrodes and ion-exchange membranes,” Water Research 42 4923 (2008).
6. J.-B. Lee, K.-K. Park, H.-M. Eum, and C.W. Lee, “Desalination of a thermal power plant wastewa-ter by membrane capacitive deionization,” Desalination 196 125 (2006).
7. M.D. Andelman, “Charge barrier flow-through capacitor,” Can. Patent CA 2444390 (2002).
8. Oren, Y. (2008). "Capacitive delonization (CDI) for desalination and water treatment - past, present and future (a review)." Desalination 228(1-3): 10-29.
9. Postel, S. (1992). Last Oasis: Facing Water Scarcity. New York, W.W. Norton & Company.
Welgemoed, T. J. and C. F. Schutte (2005). "Capacitive Delonization Technology (TM) : An alternative desalination solution." Desalination 183: 327-340.
10. R. Zhao, P.M.Biesheuvel, M. Miedema, H. Bruning, A. van der Wal. "Charge efficiency: a functional tool to probe the double layer structure inside porous electrodes, and application in the modeling of capacitive deionization" J. Phys. Chem. Lett., accepted (2010)