Exchange mechanism of alkaliions on zinc ferrocyanides

Abstract

Abstract This work was devoted to the understanding of the fixation mechanisms of alkali ions on two types of zinc ferrocyanides in order to determine the optimal recovery conditions of cesium from radioactive liquid wastes. The studied products were prepared by direct precipitation and were analysed by several methods for their composition. The mixed ferrocyanides belong to the M2IZn3 [Fe(CN)6]2, xH2O series (MI = H, Na, K, Cs) with a rhombohedirc structure R 3 c. Pure zinc ferrocyanide, Zn2Fe(CN)6. 2H2O has a trigonal structure. The kinetics of sodium self-diffusion was studied in Na, Zn3 [Fe(CN)6]2 labelled with the 24Na isotope, and placed in an inactive sodium chloride solution. The equilibrium was reached within ten minutes. The experimental results were compared to different mathematical models related to diffusion into the crystals or through a bounding liquid film. The sodium self-diffusion seems to be controlled by the rate of transfer through the liquid film. The study of Na+ - H+ exchange on this product was performed by a potentiometric method correlated to tracer measurments. H+ fuxation is slower phenomenon and the rate of the process seems to be controlled by H+ diffusion into the particles. Sorption of cesium on Zn2Fe(CN)6 requires more than 70 h, while on M2IZn[Fe(CN)6]2 sorption equilibrium is attained within a few minutes. The capacity reaches 1.6 Cs atoms per one Fe atom in the first product and is limited to one Cs atom per Fe atom in the second product. Cesium is partly retained by cation exchange with the alkali metal or zinc ions and partly by molecular adsorption of a cesium salt. According to the kinetics studies, mixed alkali-zinc ferrocyanides are the most promising products for cesium decontamination.