Simultaneous enhancement of salt removal performance and divalent/monovalent (Ca2+/Na+) ion selectivity in flow-electrode capacitive deionization with functionalized activated carbon

초록

This study presents a continuous flow-electrode capacitive deionization (FCDI) system that simultaneously enhances salt removal performance and enables tunable monovalent/divalent ion selectivity using sulfuric acid-functionalized activated carbon (F-AC) slurry electrodes. The electrode surface functionalization introduced negatively charged sulfur- and oxygen-containing groups, enabling ion selectivity to originate directly from the electrode without additional ion-exchange membrane modification. Under mixed Ca²⁺/ Na⁺ single-pass operation, F-AC exhibited a pronounced dual effect: the average salt removal rate (ASRR) increased from 1.70 to 4.45 μmol/cm²·min (2.6-fold compared with pristine activated carbon (P-AC)) and the Ca/Na selectivity index (ρ(Ca/Na)) decreased from 3.78 to 2.65 (∼30% reduction). By tuning the operating parameters, ASRR reached up to 7.11 μmol/cm²·min and ρ(Ca/Na) decreased to as low as 1.07, indicating a substantial mitigation of Ca²⁺-preferential uptake and a relative shift in competitive selectivity toward Na⁺. Operating parameters such as slurry flow rate, electrolyte flow rate, and feed salinity further allow tuning of the throughput–selectivity balance in continuous FCDI. Sulfuric acid functionalization markedly improved electrode wettability, promoting electrolyte accessibility and more efficient electrical double-layer charging in aqueous slurry-based flow electrodes. Electrochemical analyses provided a consistent mechanistic explanation for the observed behavior. Electrochemical impedance spectroscopy revealed reduced interfacial and transport resistances for F-AC, supporting faster ion transport and higher throughput. These results indicate that electrode-level surface functionalization can effectively decouple ion selectivity from bulk textural properties under continuous flow operation. Overall, F-AC offers a scalable and low-complexity electrode platform that mitigates the selectivity–throughput tradeoff in FCDI, enabling high-rate desalination with controllable monovalent/divalent ion fractionation under continuous operation. © 2026 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

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