Water Desalination by Capacitive Deionization - Advantages Limitations and Modification

Capacitive deionization may serve as an energy efficient method for salt removal from brackish water (<5000ppm). This method may have advantages over other competitive desalination methods. One advantage introduced here is what we call “selective desalination”. Since the salt is electro-adsorbed onto activated porous carbon electrodes, the pores can be designed and shaped by applying a chemical vapor deposition (CVD) process of carbon layers on top of surfaces of activated carbon electrodes, so, the entries of the pores become narrow enough to avoid electro-adsorption of doubly charged magnesium and calcium ions, which are important nutrients, while sodium ions can be removed selectively from aqueous solutions. Another advantage is the possibility to remove boric acid from tap water in one stage by exploiting the pH changes which occur nearby the negatively charges electrode in polarized CDI cells due to surface reactions or water reduction at the cathode side, in order to transfer boric acid into borate ion which can be further electroadsorbed onto the positively polarized electrode (see illustrations in figure 1). We also discovered and addressed one of the main limitations of the capacitive ionization method which is called herein the “Rocking Chair” problem. (namely, ions are moving between the electrodes by adsorption-desorption cycles, instead of adsorption of counter-ions on both electrodes as the only process occurring when the cell is polarized). We are able to calculate the amount of cation and anion adsorbed/desorbed onto/from the high surface area activated carbon electrodes in CDI cells, as a function of the potential difference applied. It was revealed that for wide pore size electrodes and NaCl solutions, the amounts of counter-ions and co-ions electro-adsorbed/desorbed onto/from the activated carbon electrodes in CDI processes are nearly equal within a potential domain of about 300 mV around the point of zero charge (PZC) of the electrodes.