Crystalline insoluble acid salts of tetravalent metals—XXX: Forward and reverse Li +/Cs +, Na +/Cs + and K +/Cs + ion exchange isotherms on α-zirconium phosphate

Abstract

The titration of α-Zr(HPO 4) 2·H 2O, prepared by the HF-procedure, with (CsCl + CsOH) solutions, leads to the formation of Zr(CsPO 4) 2·6H 2O, inter-layer distance: 14.2 Å. The exchange behaviour of this phase towards alkali metal cations has been investigated by determining the respective ion-exchange isotherms and the percentage of the solid phases at the various stages of exchange. In the experimental conditions used, Cs + is not able to replace completely Li + and Na + from Zr(LiPO 4) 2·4H 2O and Zr(NaPO 4) 2·3H 2O, the phases ZrCsLi(PO 4) 2·4H 2 O and ZrCs 1.5Na 0.5(PO 4) 2·3H 2O being obtained. On the contrary the Cs + replacing K + process leads to Zr(CsPO 4) 2·6H 2O and occurs in a single step. In the reverse processes high solubilities of the ingoing counter-ions in the already formed phases were found. The large hysteresis loop found for all the investigated systems has been related to the formation of different solid phases in the two opposite processes and to super-saturation phenomena. Since the reported isotherms have completed the set of alkali-alkali ion exchange processes on crystalline zirconium phosphate, some general comments have been drawn considering the effect of the effective ionic radius of the counter-ions and of inter-layer distance of the involved solid phases.