Ion-exchange properties of zirconium phosphate-phosphite, a layered compound with asymmetrical layers

Zirconium phosphate-phosphite of composition Zr(HPO₄)_0.7(HPO₃)_1.3 · 0.5H₂O possesses a peculiar layer structure in which there is a regular alternation of an interlayer region of zirconium phosphate type with another of zirconium phosphite type. The ion-exchange properties of this particular inorganic layered compound towards alkali metal ions have been examined and many comparisons with analogous properties of α-zirconium phosphate are reported. It was found that, although only the phosphate regions are involved in the ion exchange processes, the presence of the adjacent phosphite regions strongly affects both the selectivity and rate of exchange as well as the phase transitions occurring in the salt conversion. A discussion of the different ion-exchange processes taking place in the phosphate regions of zirconium phosphate-phosphite and in the same regions of the pure α-zirconium phosphate is made on the basis of the particular layered structure of α-zirconium phosphate phosphite. Although the ion-exchange capacity of this latter exchanger is less than one half of that of pure α-Zr(HPO₄)₂ · H₂O, it can be considered a better exchanger for large cations.