| Prussian blue and its transition-metal analogs display excellent candidate for ECIS because of its unique stoichiometry and structural properties. In the ECIS process, an electroactive ion exchange film is deposited onto a high surface area electrode, ion loading and unloading can be easily controlled directly by modulating the redox states of ion exchange thin films, to perform the separation of ions from mixed solution and regeneration of the matrix. ECIS is being developed for both anion and cation separations, of particular interest here are selective alkali cation separations. Nickel hexacyanoferrate (NiHCF) is an excellent candidate for ECIS separation of alkali cations because of its different affinity for alkali cations (Cs+>Rb+>K+>Na+>Li+). Especially in remove radioactive wastes 137Cs. Considering 90Sr is a radioactive isotope, developing ECIS for separate alkaline-earth metal ions will enlarge the application field of ECIS.In this paper, MHCF(M=Ni,Co,Cu) films were synthesized on platinum/quartz crystal electrode by electrochemistry deposition and the film-electrode systems were investigated as electrochemically controlled ion separation (ECIS) materials for the selective separation of alkaline earth ions in aqueous nitrate solutions. In 0.1 mol·L-1 Mg(NO3)2, Ca(NO3)2, Sr(NO3)2 and Ba(NO3)2 solutions, cyclic voltammetry (CV) was used to reversibly intercalate and deintercalate alkaline earth cations from the matrix and to investigate the electroactivity, electrochemical behavior and the loading mechanism of MHCF film electrodes. CV and electrochemical quartz crystal microbalance (EQCM) were used to investigate the ion selectivity of the MHCF films in 0.1 mol·L-1 [Mg(NO3)2+Ba(NO3)2], [Mg(NO3)2+Sr(NO3)2] mixture solution. Choronoamperometry analysis the solvent transport during potential stepped. Besides, the stability of MHCF films are compared through charge-decrease.Experimental results show that MHCF films have reversible electrochemical behavior in alkaline earth metal solution and display a high Ba2+,Sr2+selectivity in Mg2+/Ba2+, Mg2+/Sr2+mixtures. In 0.1M Sr(NO3)2, MHCF films have good stability. In a word, the alkaline earth metal ions can be separated effectively by ECIS processes.To further confirm MHCF films suitable for ECIS, the chemical composition and elemental valence of NiHCF films in reduced and oxidized form were also characterized by X-ray photoelectron spectroscopy (XPS). XPS gave direct evidence that the iron element existed in the form of Fe(Ⅲ) in oxidized films and was reduced to Fe(Ⅱ) during X-ray scanning. Fe is the active-center. In 0.1 mol·L-1 Mg(NO3)2,Sr(NO3)2,Ba(NO3)2, alkaline-earth region of XPS shows divalent cation intercalation MHCF matrix to maintaincharge neutrality during electrochemical reduction. |
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