Ion exchange behaviour of two synthetic phillipsite-like phases

Two phillipsite-like phases have been synthesized at 80°C treating a rhyolitic pumice in a mixed Na+-K+ alkaline environment, having different Na+/K+ molar ratios. The synthesized samples, displaying distinct x-ray diffraction patterns, have been characterized as regards their ion exchange properties. Ion exchange isotherms for the cation pairs Na/NH4, Na/Ba, Na/K and Na/Ca have been obtained and the relevant thermodynamic equilibrium constants calculated with the help of a computer program. Both phillipsite-like phases revealed an excellent to good selectivity for Ba, NH4 and K and a substantial unselectivity for Ca, in substantial accordance with previous results obtained with a sedimentary phillipsite. Nevertheless, the different shapes of some isotherms and therefore the different occupancy of peculiar ion exchange sites indicate that the two synthetic phillipsites are structurally distinct, even if this supposition should be confirmed by a structural analysis