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 ( K d ) 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 Mg 2+, Ba 2+, and Sr 2+ . These findings establish KZnHCF as an efficient electrode material for the selective removal of Cs +, offering a promising solution for Cs + removal from wastewater.