Hybrid capacitive deionization using a KCuFC electrode for selective potassium electrosorption in Na-rich brines

Selective potassium (K) recovery from saline streams remains challenging because K + is typically present as a minor component in the presence of excess sodium ions (Na + ) and competing divalent cations. In this study, we developed a copper based potassium hexacyanoferrate (KCuFC)-based hybrid capacitive deionization (HCDI) system that achieved a maximum recovery selectivity of α(des) K/Total(Na+Mg+Ca) = 15.24 ± 3.13 in simulated seawater reverse osmosis (SWRO) concentrate, highlighting its practical potential for K recovery from Na-rich brines. To clarify the origin of K selectivity, the KCuFC-based HCDI configuration was compared with conventional membrane capacitive deionization (MCDI) and asymmetric membrane capacitive deionization (ACDI). In 10 mM KCl, HCDI achieved a final K + adsorption capacity of 36.89 ± 0.66 mg/g, the lowest energy consumption of 0.49 ± 0.01 Wh/g-KCl. In an equimolar mixed-cation solution, only HCDI exhibited clear K preference, with α K/Na = 3.01 ± 0.16, α K/Mg = 2.50 ± 0.39, and α K/Ca = 2.14 ± 0.03, whereas the MCDI and ACDI showed selectivity values below unity. The most favorable operating window was identified at 1.2 V, 2 mL/min, and 5–10 mM KCl. Under binary K/Na competition, α K/Na = 3.66 ± 0.18 at K:Na = 1:100 and 1.2 V, indicating robust K preference under severe Na excess. In addition, KCuFC-mid retained 84% of its initial electrochemical capacity after 400 cycles, while maintaining a coulombic efficiency >99%. Furthermore, small-data supervised learning analysis was used as a supplementary local consistency check for the held-out binary K/Na family, yielding a q K mean absolute error (MAE) of 2.315 mg/g. Overall, this study demonstrated that KCuFC-based single-membrane HCDI is a promising and practical tunable platform for selective K recovery from Na-rich brines.