Capacitive deionization (CDI) has attracted significant attention for next generation water treatment due to its low energy consumption and small environmental impact in comparison to widely established methods. For CDI technology to move forward, however, the development of carbon electrodes having superb electrosorption behavior is essential. In this study, we demonstrate the functionalization of carbon nanotubes (CNTs) via acid treatment and show its improved electrochemical characteristics as a carbon electrode in a CDI process. Our results suggest that the activation effectively improves the wettability of the acid-treated CNTs (a-CNTs) via the addition of oxygen functional groups, leading to a higher electric double layer capacitance.