Here, a low-cost, efficient and eco-friendly capacitive deionization (CDI) technology has been introduced to simultaneously remove tetracycline (TC) and water hardness ions (Ca2+ and Mg2+) from water. In the process of CDI electrosorption, hierarchical porous carbon material derived from kelp was used as symmetric electrodes, and achieved the maximum capacity of 925.3 mg g−1 for TC removal, which is much higher than other materials reported so far. More importantly, high removal efficiency still maintained over 90% even after five CDI adsorption-desorption cycles. Meanwhile, the metal ions with opposite charges were adsorbed on the counter electrode with a preferential ion electrosorption performances as: Ca2+ (8.04 mg g−1) > Mg2+ (3.24 mg g−1) > K+ (2.59 mg g−1) > Na+ (0.75 mg g−1), proving the effective CDI technology in synchronous removal of tetracycline and water hardness ions for water purification. This work has paved a way to synchronously eliminate multiple organic and inorganic contaminants from water by using CDI technology.