Evaluating circulation-type MCDI as a dual-function system for ion removal and enrichment

Conventional membrane capacitive deionization (MCDI) is structurally constrained because adsorption and desorption are carried out on the same feed stream, resulting in low product yield and the continuous generation of low-salinity brine during regeneration. We previously introduced the concept of circulation-type MCDI (C-MCDI) and demonstrated its potential for improving water yield. However, research on C-MCDI has largely focused on enhancing water recovery or modifying cell architecture, with little attention to the mechanisms of ion enrichment and no experimental validation of C-MCDI as an ion enrichment system. Therefore, we systematically evaluated the enrichment performance and full-cycle energy demands of C-MCDI. We conducted 20-cycle experiments on MCDI and C-MCDI using NaCl solutions (222–4000 μS/cm) to comparatively assess their performance and clarify the dual-function capability of C-MCDI under cyclic operation. We found that ion removal efficiencies were consistently maintained above 80 % in both systems. Specific energy consumption (SEC) during adsorption was 0.17–2.87 kWh/m³ for MCDI and 0.14–4.88 kWh/m³ for C-MCDI, confirming the low-energy potential of both configurations. In contrast, the SEC during desorption in C-MCDI increased up to 6.53 kWh/m³, indicating an additional energy burden associated with enrichment. To quantitatively assess this trade-off, we propose ΔC (absolute concentration difference), defined as the conductivity difference between the recycle tank and the feed stream. Overall, this study provides the most comprehensive experimental evaluation to date of C-MCDI as a dual-function platform capable of both desalination and enrichment, while highlighting energy reduction during desorption and long-term stability as critical challenges for future practical applications.