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 CDI is a complex dynamical nonlinear process, it is still possible to describe it in simple words. Desalination by CDI occurs when charging two or more pairs of high-surface area electrodes in brackish water. The electrodes are typically made of activated carbon, but other materials are also possible such as carbon nanotubes (CNTs). Anions and cations in solution are electrically adsorbed by electric fields upon polarization of the anode versus the cathode electrode by a direct current (DC) power source (Welgemoed and Schutte 2005). The removal of salt in the process not only depends on the potential applied between anode and cathode but also depend on the influent salt concentration. Electrical double layer (EDL) theory has been shown to be useful in predicting how much salt can be removed as function of salt concentration and applied cell voltage (Zhao et al., 2010).

In a real CDI experimental or commercial setup, several pairs of electrodes are compacted in one stack. In between each anode and cathode, an open spacer layer is placed in order to prevent short-cutting and to allow the passage of water. 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 the Gouy-Chapman-Stern model based on EDL theory from the beginning of the charging process to the final stage where the carbon electrodes become saturated with ions (Biesheuvel et al.,2009).

Technological Development

Membrane Capacitive deionization

Membrane capacitive deionization (MCDI) is a modification of CDI without membranes. The difference is that cation exchange membranes, anion exchange membranes, or both, are placed in front of the anode and in front of the cathode (Andelman, 2002; Lee et al., 2006; Li et al., 2008). The advantage of insertion of membranes is that coions inside the electrode material are prevented from going back into the bulk solution. This has the potential to increase the efficiency for salt removal compared to CDI without membranes.

Reversed CDI

Instead of putting energy into the CDI stack to charge the cell and thereby produce potable water from brackish water, energy can also be extracted from a CDI-cell by flowing through high-salinity water and low-salinaty water in sequence. During this reverse-CDI process, a low voltage is applied during flow-through of high salinity water, i.e. 0.5 V. When flowing through fresh water, 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 to the cell, electrical energy can be extracted (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)