On the selectivity regulation of K2ZrSi3O9•H2O-type ion exchangers

A series of mixed potassium zirconium-titanium silicates of the general formula K2MSi3O9. H2O (where M = Zr, Ti), containing different amounts of titanium and zirconium (from 0 to 100% at.) were synthesized hydrothermally. All these compounds have a framework structure containing two types of channels the size of which smoothly changes, namely, decreases with the increase of titanium content in the compound. Both channels, small channels with a 22-membered ring opening and large ones with a 16-membered ring opening, run along the c-axis and they are connected to each other via a 14-membered ring opening in the perpendicular direction. This creates three-dimensional openings for the total channel system that enables easy cation migration. The ion exchange behavior of K2MSi3O9. H2O and protonic forms of two end members of the family, K0.3H1.7TiSi3O9. 2H(2)O and K0.5H15ZrSi3O9. 2H(2)O, towards alkali metal ions was studied and a distinct correlation between the exchanger composition (channels size) and selectivity to certain ions was found. Zirconium-rich silicates with large channels exhibit affinity for rubidium and cesium ions, whereas titanium-rich compounds with much smaller channels show a preference for potassium ion. (C) 1998 Elsevier Science B.V. All rights reserved.