Asymmetric Membrane Capacitive Deionization System for Improved Cesium Ion Removal Using Potassium Zinc Hexacyanoferrate Electrodes

Cesium (Cs+), a highly toxic ion in nuclear waste, poses serious risks to human health by replacing potassium ions in the body. Membrane capacitive deionization (MCDI) is a promising technique for removing trace ions from aqueous streams. Here, we synthesize potassium zinc hexacyanoferrate (KZnHCF) as a cathode material with high Cs+ selectivity. Activated carbon (AC) serves as the anode, and a graphite sheet acts as the current collector in a custom-built MCDI stack. Ion-exchange membranes─polyethylene–polystyrene (PE–PSt) for cations and polyethylene–poly(p-methylstyrene) (PE–PMSt) for anions─are incorporated to minimize co-ion effects during the CDI process. The distribution coefficient (Kd) analysis identifies Na+ as the most competitive ion, with similar results when AC is used for both electrodes. However, replacing AC with KZnHCF at the cathode significantly enhances Cs+ selectivity, achieving a selectivity coefficient of 5.1. This enhancement is attributed to the favorable ionic radius of Cs+ (1.81 Å) and its low hydration energy (−276 kJ mol–1), which promote strong ion-exchange interactions with KZnHCF. This modification also improves the salt adsorption capacity (SAC), reaching 34.5 mg g–1 substantially higher than that for Mg2+, Ba2+, and Sr2+. These findings establish KZnHCF as an efficient electrode material for the selective removal of Cs+, offering a promising solution for Cs+ removal from wastewater.