New capacitive deionization (CDI) materials based on using nanoporous silicon dioxide or alumina surface-doped with magnesium were shown to remove a wide range of cations having different valent states and hydrated radii. The use of the asymmetric insulating oxide nanoparticles as CDI materials increased removal rates due to their small size and opposite surface potentials. This system can be regenerated electrochemically by reversing the potential of the electrodes in order to drive the ions off the electrodes. Unique to this system is that ions do not adsorb onto the opposite electrode during regeneration due to the inherent surface potential of the nanoporous oxide which repels the ions under the conditions employed for these tests. As might be expected, the maximum allowable regeneration voltage was directly related to the zeta potential of the nanoporous coating on the working electrode under the specific test conditions. (C) 2009 Elsevier Ltd. All rights reserved.