Investigation on removal of hardness ions by capacitive
deionization (CDI) for water softening applications
, Hongrae Jeon
, Jae Kwang Lee
, Gha-Young Kim
, Daewook Park
, Jaeyoung Lee
*, Seung-Hyeon Moon
Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro,
Buk-gu, Gwangju 500-712, Republic of Korea
Advanced R&D Group, Digital Appliances Division, Digital Media Business, Samsung Electronics Co., Ltd., 416 Maetan-3dong,
Yeongtong-gu, Suwon 443-742, Republic of Korea
Received 6 July 2009
Received in revised form
22 September 2009
Accepted 18 October 2009
Available online 22 October 2009
Capacitive deionization (CDI)
Activated carbon electrode
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 acti-
vated 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 mono-
valent 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.
ª 2009 Elsevier Ltd. All rights reserved.
Puriﬁcation of several contaminants in tap, ground, and river
water is needed for the drinking and industrial purposes.
Among the contaminants, hard water minerals such as
calcium, magnesium, iron, and manganese ions can react
with soap anions, decreasing the cleaning efﬁciency (Park
et al., 2007a). The minerals also induce scaling problems and
serious failures in pipelines of boilers, heat exchangers, and
electrical appliances such as washing machines, dishwashers
and steam irons (Gabrielli et al., 2006).
In order to remove the divalent ions, various methods have
been widely applied as a means of effective water softening:
chemical precipitation, ion exchange process, nanoﬁltration,
reverse osmosis, and electromembrane systems such as
electrodialysis, electrodialysis reversal, and electro-deioniza-
tion reversal. However, in case of the chemical precipitation,
the choices of additional chemicals are restricted for the
purpose of drinking water. Monovalent ions and acids
released in the regeneration of ion exchange and membrane
processes would result in harmful effects on environment.
Finally, high power consumption and expenses are required
for operation and maintenance of the equipment (
Cuda et al.,
* Corresponding author. Tel.: þ82 62 970 2440; fax: þ82 62 970 2434.
E-mail address: email@example.com (J. Lee).
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved.
water research 44 (2010) 2267–2275