Potassium treatment from mixed monovalent solutions using Berlin green-based hybrid capacitive deionization

Potassium supply is currently dominated by mining and is geographically concentrated, creating environmental and geopolitical vulnerabilities. These constraints have motivated potassium separation from diverse potassium-rich saline streams, including agricultural, industrial, and municipal wastewaters; desalination by-products such as bittern; and other concentrated brines. Accordingly, herein, we developed a hybrid capacitive deionization system using a Berlin Green-type iron hexacyanoferrate (FeHCF) electrode synthesized by facile aqueous coprecipitation. This FeHCF electrode offered a Faradaic intercalation pathway beyond electric double-layer adsorption, as confirmed by rate-dependent electrochemical analyses. System performance was evaluated across Na + : K + concentration ratios of 1:1–49:1, applied voltages from ±0.8 to ±1.2 V, and different operation-time windows. Under these conditions, the FeHCF-based HCDI system achieved a total Na + +K + deionization capacity of 0.261 ± 0.006 mmol g −1 , approximately twice that of membrane CDI (MCDI; 0.112 ± 0.004 mmol g −1 ). Furthermore, HCDI increased charge efficiency (75.3 ± 1.0%) relative to MCDI (40.2 ± 1.3%) and reduced energy consumption (from 66.7 ± 2.2 (MCDI) to 35.6 ± 0.5 Wh mol −1 ). The system maintained a K + /Na + selectivity coefficient of 1.1–1.3 across all tested Na + :K + ratios. Together, these results establish the FeHCF-based HCDI system as an energy-efficient approach for potassium treatment from mixed monovalent solutions.