Characterization of cathodically deposited nickel hexacyanoferrate for electrochemically switched ion exchange

Thin films of cathodically deposited nickel hexacyanoferrate are investigated as electrochemically switched ion-exchange (ESIX) materials for the selective separation of alkali cations in aqueous nitrate solutions at room temperature. Potential cycling in the range -100 to 900 mV vs SCE is used to reversibly load and elute alkali cations. Electrochemical cyclic voltammetry is combined with thin-film energy-dispersive x-ray spectroscopy to nondestructively determine element-specific cation loading in the ion-exchange matrix. Selectivity parameters are reported for the alkali cation pair Cs+/Na+. Separation factors and distribution coefficients for Cst in the presence of excess Nai are determined over the range [Na+]/[Cs+] = 10(1) - 10(6). The distribution coefficient For cathodically deposited nickel hexacyanoferrate is greater than 2 X 10(5) mL/g for [Cs+] = 10(-6) M and [Na+] approximate to 1 M, which is comparable in magnitude to the bulk metal hexacyanoferrates used in conventional ion exchange. A comparison between cathodically deposited nickel hexacyanoferrate materials and anodically derivatized materials is described. We also show that the Cs-form of the matrix can be electrochemically regenerated to the Na-form with nearly 80% of the intercalated Cs+ removed.