Charge-Density-Dependent Partitioning of Cs+ and K+ into Nickel Hexacyanoferrate Matrixes

Abstract

Nickel hexacyanoferrate (NiHCF) is a zeolite-like Prussian Blue analogue, where cyanide bonding between nickel and iron creates a cubic unit cell with intercalated cations that balance the negative charge on the matrix. Partitioning of Cs+ and K+ into electrodeposited NiHCF thin films is examined as a function of solution composition and the charge density on the matrix. Potential cycle and hold sequences in the range −25 to +1125 mV versus saturated calomel electrode are used to reversibly intercalate Cs+ and K+ mixtures into NiHCF matrixes possessing variable charge densities. Energy-dispersive X-ray spectroscopy is used to quantify the intercalated cation content in the matrix, and Raman spectroscopy is used to confirm the oxidation state of the matrix. Cs+/K+ partitioning is investigated over the range 10-2 < [K+]/[Cs+] < 105. Oxidized NiHCF, with its modest matrix charge density (≈ −190 C/cm3) and low concentration of intercalated alkali cations, is cesium-selective for all solution compositions examined. In contrast, reduced NiHCF, with its larger charge density (≈ −560 C/cm3) and higher concentration of intercalated cations, is selective for cesium over potassium for low [Cs+] but exhibits a selectivity reversal as [Cs+] increases. An equilibrium isotherm developed for ion partitioning in zeolites shows that high-charge-density reduced NiHCF matrixes exhibit strong repulsive Cs+−Cs+ interactions when the intercalated alkali cation fraction exceeds 60% Cs+ in the solid. These repulsive interactions give rise to a stable solid composition possessing approximately 75% Cs+ and 25% K+ over a range of solution compositions.