Numerical Modeling Unlocks Remarkable Ion Selectivity of Capacitive Deionization
Authors: Suss Matthew , Shocron Amit N. , Guyes Eric N. , Uwayid Rana 
Published: November 2022

Ion-selective water treatment is an important frontier in water research, as for many applications removing all ions indiscriminately leads to significant extra energy and downstream re-ionization costs. Capacitive deionization (CDI) is under intensive investigations for ion-selective treatment of polluted feedwaters [1]. CDI has the remarkable feature of being not only highly selective, but additionally dynamically tuneable to adjust, real-time, to varying feedwater composition and produced water targets [1]. However, we will show that to further develop CDI technologies to achieve desired separations, in feedwaters of several competing anions and cations, requires detailed numerical models. Such models couple ion transport theory to nanopore electrosorption, and often include pH dynamics. We here describe our recent work exploring the limits of ion-ion selectivity by capacitive deionization with inexpensive nanoporous carbon electrodes. We show how theory enabled us to achieve in the lab remarkable selectivity and a diverse set of separations, such as “perfect” divalent cation selectivity[2], monovalent ion selectivity[3], and removal of amphoteric pollutant species such as boric acid and nutrient species[4]. We show using strong-acid functionalized electrodes that the same two-electrode system can be used for either excellent divalent selectivity, or long-lasting monovalent ion selectivity, depending on cell operational parameters [2,3]. Our work furthers the argument that membraneless CDI, based on inexpensive and easily-scalable porous carbon electrodes, can address a wide variety of important applications in water treatment.

References:

[1] J.G. Gamaethiralalage, et al. “Recent advances in ion selectivity with capacitive deionization”, Energy & Environmental Science, 2021.
[2] R. Uwayid, E.N. Guyes, A.N. Shocron, J. Gilron, M. Elimelech, M.E. Suss. “Perfect divalent cation selectivity with capacitive deionization”, Water Research, 2022.
[3] E.N. Guyes, A. Shocron, Y. Chen, C. Diesendruck, M.E. Suss. “Long-lasting, monovalent selective capacitive deionization electrodes.” NPJ Clean Water, 2021.
[4] A.N. Shocron, E.N. Guyes, H.H. Rijnaarts, P.M. Biesheuvel, M.E. Suss, J.E. Dykstra, “Electrochemical removal of amphoteric ions”, Proceedings of the National Academy of Sciences, 2021.

Figure 1: From Ref. 3. CDI concept for treatment of water for direct use towards irrigation. a) Schematic of a CDI cell fed with water containing excessive Na+ which must be removed for direct use in irrigation. The cell is charged at an applied cell voltage Vch
at or above 1 V and for a time tch
significantly shorter than the time to reach equilibrium, teq
. b) The cathode nanopore is functionalized with strong-acid sulfonic groups, which enhances the preferential storage of monovalent Na+ over divalent Ca2+ at short charging times. c) The treated water has significantly reduced sodium absorption ratio (SAR) and ionic conductivity, rendering it suitable for direct use in irrigation.

Figure 1