Capacitive Deionization:

Obtaining a reliable and plentiful supply of clean water is becoming much more difficult. The arid regions of the Western United States are already depleting their groundwater supplies because surface water rights are completely committed, and the population of the affected area is expected to double in the next 25 years. The Middle East and Africa are currently experiencing serious shortages of potable water, and population growth is rapidly making the problem more serious. There are large reserves of brackish (saline) water that could be used to ease these supply problems; if there was a way to cost effectively desalinate them.

Capacitive deionization technology (CDT) is an exciting new method of desalinating brackish water. The figure at right illustrates the principle of operation of CDT during the active desalination half cycle. In CDT, a brackish water stream flows between pairs of high surface area carbon electrodes that are held at a potential difference of 1.3 V. The ions and other charged particles (such as microorganisms) are attracted to and held on the electrode of opposite charge. The negative electrode attracts positively charged ions (cations) such as calcium (Ca), magnesium (Mg) and sodium (Na), while the positively charged electrode attracts negative ions (anions) such as chloride (Cl), nitrate (NO³) and silica (SiO²). The figure at right illustrates the principle of operation of CDT during the active desalination half cycle.

Eventually the electrodes become saturated with ions and must be regenerated. The applied potential is removed, and since there is no longer any reason for the ions to remain attached to the electrodes the ions are released and flushed from the system, producing a more concentrated brine stream. In practice, for a gallon of brackish water fed to the a CDT process, more than 80% emerges as fresh, deionized potable water, and the remainder is discharged as a concentrated brine solution containing virtually all of the salts in the feed. The figure at left shows the overall schematic of a CDT system. The primary advantage of CDT is its low operating cost, which is about one third that of the main competitor, reverse osmosis (RO). This is important because operating costs dominate the cost of desalination.

Electrodes from carbon aerogels are well suited for CDT because they can be prepared as monolithic sheets, eliminating the need for the porous separator used with electrodes based on carbon powders. Rather than flowing through a packed bed, the solution flows through channels between adjacent electrodes, eliminating high-pressure drops. The main drawbacks with carbon aerogel based electrodes is their low salt capacity and high cost.

TDA has recently invented a method of making porous carbon electrodes that addresses all of these problems. The figure at right illustrates a typical CDT electrode weighing 15 g (18 cm x 18 cm). We have developed inexpensive electrodes using simple production methods and low cost starting materials. The carbon has large pores to allow the ions to rapidly access the high surface area micropores giving increased ion capacity.

This work is supported by National Science Foundation grant number DMI-0216299 and U.S. Department of Energy grant number DE-FG36-04GO-14326.

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