A free-standing flow-through capacitor (FTC) is constructed by concentrically winding two electrodes and two dividers into a hollow-center roll. A liquid-feeding pipe is inserted to the central opening for delivering fluids to the FTC. Nanoparticles of hydrated iron compound with Fe.sub.3 O.sub.4 as the main component or its composite powders are used as the active materials for the electrodes. With channels crated by the dividers assembled in the roll, fluids injected from the feed pipe are confined inside the FTC, and flow outwardly and transversely through the entire length of the electrodes. Under an application of a low DC voltage to the electrodes, charged species are adsorbed and removed from the treated liquids as soon as they are in contact with the electrodes. Capacitive deionization using FTC of the present invention is applicable to waste-streams reduction, water purification and desalination at low costs and easy operation.