Adsorption equilibrium and kinetics of cesium onto insoluble Prussian blue synthesized by an immediate precipitation reaction between Fe3+ and [Fe(CN)6]4−

Abstract

The adsorption equilibrium and kinetics of cesium ion (Cs+) onto insoluble Prussian blue (PB) prepared by an immediate precipitation reaction between Fe3+ and [Fe(CN)6]4− was investigated under initial Cs+ concentration of under 0.15 mmol/L. Synthesis conditions in this method were almost insensitive to the adsorption ability of insoluble PB, and this method provided one of the smallest PB crystallites among synthesis methods. Even when molar concentration of H3O+ was more than 200 times higher or molar concentration of K+ was more than 50,000 times higher than that of Cs+ in the aqueous solution, the equilibrium adsorption amount was reduced by only approximately one-half to two-third of that in the pure system; that is, the insoluble PB synthesized possessed a considerably high adsorption selectivity for Cs+. In contrast to the excellent adsorption ability under adsorption equilibrium, adsorption rate was quite slow. It took at least 2 weeks at 25 °C to completely attain the adsorption equilibrium, even though the primary particle size (crystallite size) and secondary particle size (aggregate size of the crystallites) were sufficiently small at approximately 14 nm and 53–106 μm, respectively. This slow adsorption is primarily due to the large resistance of intracrystalline diffusion; the intracrystalline diffusion coefficient was extremely small at less than 3.3 × 10−22 m2/s. We also found that increase in temperature could significantly decrease this diffusion resistance, resulting in much quicker elimination of Cs+ from the aqueous solution.