Comparison of Ion Selectivity in Electrodialysis and Capacitive Deionization

Abstract

Ion-selective removal is an important frontier in water purification technologies. For many emerging applications, removing all ions indiscriminately can lead to excessive energy consumption, high levelized cost of water, poor effluent water quality, and increased waste brine volume. Electrodialysis and capacitive deionization are two electrochemical water purification technologies which are promising toward tunable, ion-selective purification. These technologies have fundamentally different ion removal mechanisms, as electrodialysis leverages electrodiffusion through ion-exchange membranes while capacitive deionization utilizes electrosorption into charged electrodes. We here provide a direct comparison of ion selectivity achieved by these two technologies, focusing on several important ion pairs. We highlight distinct differences in achieved selectivity between these technologies and provide theory results to connect such observations to ion removal mechanisms. Based on the experimental literature, we find that capacitive deionization demonstrates a wider range of achieved ion selectivities than electrodialysis for competing cations such as Na+ vs Ca2+ and Li+ vs Na+, while a wider range is observed for electrodialysis when separating anion pairs such as Cl– vs SO4 2– and Cl– vs NO3 –. We conclude with reviewing “knobs” that can be adjusted to tune the achieved selectivities by both technologies, and emphasize important questions that should be answered in future studies to improve the selectivity of both technologies.

Boosted brackish water desalination and water softening by facilely designed MnO2/hierarchical porous carbon as capacitive deionization electrode

“…4 c). One reason could be the slower kinetics of Mg 2+ intercalation/deintercalation into the tunnel structure of α-MnO 2 than that of Ca 2+, as the hydrated ionic radius of Mg 2+ (0.43 nm) is larger than that of Ca 2+ (0.41 nm) ( Singh et al., 2021 ; Shocron et al., 2022 ). The relatively higher hydration energy of Mg 2+ (−1830 kJ mol −1 ) compared to that of Ca 2+ (−1505 kJ mol −1 ) could also limit Mg 2+ intercalation/deintercalation ( Fig.…”

“…Therefore, integrating α-MnO 2 into the HPC surface will counteract their drawbacks and derive a synergistically improved hybrid electrode for CDI applications. In addition, although some carbon or battery electrodes have been used for divalent ions removal ( Cerón et al., 2020 ; Chen et al., 2022 ; Singh et al., 2020 ; Shocron et al., 2022 ; Uwayid et al., 2022 ; Zhao et al., 2012 ), information regarding the use of hybrid electrodes for water softening and divalent cation selectivity in CDI systems remains limited.…”

Assessing the suitability of desalination techniques for hydraulic barriers

“…Comparative desalination techniques and case studies NF, RO, and ED are standard brackish water desalination technologies and are therefore included in our comparative analysis. Even though, CDI is examined mostly in laboratory scale and is controversy discussed in the literature 61,[68][69][70] , we included CDI and MCDI as potentially suitable technology in our study due to a high retention of sodium chloride 13,71,72 , recent economic analyses 52,62,63 and increasing interest in this technology 73 . A literature study for costs and SEC for brackish water desalination demonstrate the suitability of the preselected desalination technologies according to the before described requirements according quantity (>100 L d −1 ), quality (25% < R NaCl < 90%) and SEC (generally <2.5 kWh m −3 ).…”