Membrane arrangement influences time to steady state in FCDI with multi-ionic salt solutions

Flow-electrode capacitive deionization (FCDI) is a promising technology for energy-efficient desalination, offering the potential to selectively remove ions from multi-ionic salt solutions. This study presents a comparative investigation of two FCDI modules with distinct ion-exchange membrane (IEM) arrangements, using cation-exchange membranes (CEM) and anion-exchange membranes (AEM): CEM-AEM-CEM (C ∣ A ∣ C) and AEM-CEM-AEM (A ∣ C ∣ A). Both modules achieved similar levels of desalination and concentration in steady state, but the time required to reach steady state varied significantly due to differences in the IEM arrangement and the ionic buffer-capacity of the flow electrode. The results revealed that membrane selectivity is critical in systems with multiple ion species, particularly before achieving a steady state, where conductivity alone proves insufficient for characterizing salt composition. A higher selectivity for sulfate over carbonate was observed for AEMs, causing delayed stabilization of concentrations in the A ∣ C ∣ A module. Strategies to reduce the ionic buffer-capacity of the flow electrode, such as decreasing its volume as well as recognizing which ion should follow the flow-electrode path were identified as potential means to accelerate steady-state achievement. Furthermore, experiments with varying carbonate-to-sulfate molar ratios in the diluate feed confirmed the inherent selectivity of AEMs for sulfate and established an upper limit for normalized transport ratios. These findings emphasize the need for complementary approaches, such as membrane coatings or tailored flow-electrode materials, to enhance process selectivity and efficiency. This work provides critical insights into the interplay of IEM arrangement and flow-electrode behavior, offering a foundation for optimizing FCDI systems in desalination applications involving complex salt solutions. • Modules with IEM arrangements of two CEMs and one AEM (C | A | C) or vice versa (A | C | A). • Similar levels of desalination and concentration are reached regardless of IEM arrangement. • Time to steady state varies drastically depending on IEM arrangement. • Identified flow-electrode buffer and ion pathway as important process parameters in FCDI with multi-ionic salt solutions.