Capacitive Deionization (CDI) is a water purification process that offers promise in the deionization of salt and brackish water. The process can create potable water out of salt laden water by using electric currents to separate the ions in the solution and attract them to cathode and anode plates. These plates act as filters, drawing salt and other ionized metals out of the source water. When salt is mixed in water, it does not remain in its molecular state; rather it ionizes into Na+ cations and Cl- anions. Other metals also ionize in water, making it unsafe to drink. If this ionized, or hard, water is passed through a set of electrodes connected to a direct current, the polarized electrodes will “grab” the anions and cations in solution.
This technology has been around for nearly 50 years, but it has not been commercialized for seawater desalination to this point due to the major drawback that the recovery rate of feed water is low. New advancements in electrode design utilizing nanotechnology hold promise. The increase surface area of nanoparticles allows more ions to be captured with less current and less material mass. Recently researchers were able to use self-assembled polymerized Mesoporous Carbon (MC) gels that were cured on roughened graphite electrodes and covered with carbon fiber for structural support. These electrodes could then be stacked together to be used in the CDI process. This new electrode design produced mesopore sizes in the electrode of 6-10nm and macropores ranging from 50-300nm.
Using MC electrodes greatly enhanced the surface area over previous CDI advancements that relied on carbon aerogels, another nanotechnology, as a coating for electrodes. The results were salt removal rates of 21 mg/g of MC material at concentrations of 35000 ppm (about seven times the concentration of ocean water). This is much higher than carbon aerogels, which were only able to recover 5.8mg/g at a maximum salt concentration of 5000 ppm. For comparison purposes, activated carbon filters, those used in water bottles and the likes, only remove 0.275 mg/g of filter material.
If this process to increase the recovery rate of feed water can be commercialized, CDI will be able to desalinate water with significant reductions in energy input, membrane fouling, brine output, capital costs, and maintenance costs over currently preferred Reverse Osmosis (RO) technology. This breakthrough in CDI electrode technology will potentially allow for the cost effective desalination of salt water on scales ranging from personal use to industrial and utility use.
- . Mesoporous Carbon for Capacitive Deionization of Saline Water. Environmental Science & Technology. 2011 ;45(23):10243 - 10249.
If this process to increase the recovery rate of feed water can be commercialized, CDI will be able to desalinate water with significant reductions in energy input, membrane fouling, brine output, capital costs, and maintenance costs over currently preferred reverse Osmosis (RO) technology. This breakthrough in CDI electrode technology will potentially allow for the cost effective desalination of salt water on scales ranging from personal use to industrial and utility use. Additionally, the direct current requirements of the CDI process has drawn interest in coupling CDI systems with mature solar photovoltaic technology for desalination in rural, sunny, and developing regions of the world, making the technology a true zero emissions clean tech.
There would be no risk for environmental contamination since the materials used in the membrane are only porous sheets and are contained between layers of environmentally benign graphite. The only identifiable risks at this point lie in the manufacture and polymerization of the MC gel. The preparation of the Polymerized MCs use formaldehyde for solution separation and argon gas for high temperature carbonization-setting of the materials into a hard electrode. Risks would be minimal as long as precautionary lab procedures were adhered to, and chemicals used are common in industrial and chemical processes.