Capacitive deionization (CDI) has been getting attention as an alternative process for removing the hardness ion species. Recently, ion-exchange materials have been introduced in a CDI system to improve the deionization performance of the CDI. Many scientists have researched in the development of low-cost, highly efficient and environmentally benign ion-exchange materials. In this paper, we propose Ca-alginate as a cation exchange coating material on a negative electrode and assemble a CDI and membrane CDI (MCDI) system for hardness control. Ca-alginate was facilely coated onto a carbon electrode under ambient condition. The CDI with the Ca-alginate coated-electrode showed superior ion removal performance for hardness species such as calcium ions (44% more) than that of the conventional CDI. Additionally, the degree of enhancement in CDI performance was well correlated with the selectivity over the electric resistance of the Ca-alginate (S-c/R-ratio) Furthermore, MCDI with a Ca-alginate "coated electrode had comparable performance to a conventional MCDI system with good recyclability and selectivity for hardness species. The results indicate that the Ca-alginate coated-electrode significantly developed a superior de ionization performance and showed great potential of this facile material in the CDI and MCDI systems. (C) 2016 Elsevier B.V. All rights reserved.