The effect of cell dimensions of hydrous mixed metal oxides with a pyrochlore structure on the ion-exchange properties

The relationship between the lattice parameter of the cubic unit cell and the ion-exchange properties was studied for a group of mixed metal oxides with the cubic pyrochlore structure, which had been prepared hydrothermally and by precipitation. The materials included binary oxides antimony silicate, antimony titanate, and titanium tungstate and related compounds, which contained a third metal (W6+, Sb5+, Nb5+) in the framework structure that had been incorporated in an attempt to improve their ion-exchange properties. The lattice parameters were found to vary between 10.2440 and 10.4793 Angstrom depending on the element combination and the cationic form of the material. A clear increase in the trace strontium (Sr-89) uptake in 0.1 M HNO3 and 0.1 M NaNO3 was observed when the lattice parameter was > 10.34 Angstrom. A similar increase was found for alkaline earth metals at macro concentrations, but the increase was transcient as the cubic unit cell dimensions were affected by the nature of the in-going cation. Some of the materials showed high selectivity for the alkaline earths against the H+ ion making them suitable for removal of these cations from acidic solutions. However, the relative selectivity between the different alkaline earth cations was rather low in general, which does not allow for their selective individual separation. The three-dimensional tunnel structure of the pyrochlore showed clear ion sieve property in the separation of the relatively large Cs+ cation from the other alkali metals.