Cation selectivity during flow electrode capacitive deionization

Efficient separation of specific ions from aqueous media is crucial for advanced water treatment and resource recovery. Flow electrode capacitive deionization (FCDI) offers potential for selective ion removal through continuous operation. This study evaluates the performance of selective cation separation using a commercial activated carbon slurry in a multi-ion solution of monovalent (Li + , Na + , K + ) and bivalent (Ca 2+ , Mg 2+ ) cations. We assess ion removal and cation selectivity under different operational parameters, such as applied potential, slurry flow rate, and feed water flow rate. Our data show that bivalent cations, namely Ca 2+ and Mg 2+ , are preferentially removal due to their higher charge-to-size ratio, aligning with hydrated ion sizes. The highest separation rate was observed for Ca 2+ (5.7 μg cm −2 min −1 ), and the lowest for Li + (0.2 μg cm −2 min −1 ). At the highest applied voltage (1.2 V), charge efficiencies reached 70 %, with an energy consumption of 41 Wh mol −1 for nearly complete cation removal. Optimal conditions were identified with a slurry flow rate of 6 mL min −1 , feed water flow rate of 2 mL min −1 , activated carbon content of 10 mass%, 1 mass% carbon black, and a cell voltage of 1.2 V. These findings highlight the importance of optimizing operational parameters to enhance ion removal. Flow capacitive deionization (FCDI) effectively enables the continuous removal of ions from aqueous solutions. Incorporating conductive carbon black significantly enhances the ion uptake efficiency of the FCDI system. Furthermore, valence and hydrated radius influence cation selectivity during the deionization process. This means that ions with different charges and sizes are absorbed at varying rates, with higher valence ions and those with smaller hydrated radii typically being more selectively removed. • FCDI enables ion removal via continuous operation. • Higher charge-to-size ratio favors bivalent cations in ion removal. • Activated carbon content at 10 % mass optimizes FCDI performance. • Conductive carbon black enhances ion uptake and separation efficiency.