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

초록

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 alpha(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 alpha(K/Na) = 3.01 +/- 0.16, alpha(K/Mg) = 2.50 +/- 0.39, and alpha(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, alpha(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.

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