Capacitive deionization (CDI) for removal of water hardness was investigated for water
softening applications. In order to examine the wettability and pore structure of the activated
carbon cloth and composites electrodes, surface morphological and electrochemical
characteristics were observed. The highly wettable electrode surface exhibited faster
adsorption/desorption of ions in a continuous treatment system. In addition, the stack as
well as unit cell operations were performed to investigate preferential removal of the
hardness ions, showing higher selectivity of divalent ions rather than that of the monovalent
ion. Interestingly, competitive substitution was observed in which the adsorbed Na
ions were replaced by more strongly adsorptive Ca and Mg ions. The preferential removal
of divalent ions was explained in terms of ion selectivity and pore characteristics in
electrodes. Finally, optimal pore size and structure of carbon electrodes for efficient
removal of divalent ions were extensively discussed