Experiments and data processing of ion exchange equilibria involving Italian natural zeolites: a review

After a historical survey, the present status of research on thermodynamic aspects of ion exchange in Italian natural zeolites, namely phillipsite, chabazite and clinoptilolite, is reported.Equilibrium data relative to exchange reactions between these zeolites and various toxic and noxious cations, namely Cd2+, Pb2+, Cu2+, Zn2+, Cs+, Sr2+, Ba2+, Co2+, NH4+, and two representatives of alkaline and alkaline-earth cations, such as K+ and Ca2+, are critically reviewed. Exchange isotherms and the most relevant thermodynamic quantities, such as the equilibrium constant Ka and the standard Gibbs free energy ΔG0, are reported. Furthermore, the exchange isotherms are interpreted in terms of selectivity of one cation over another, and the selectivity series are analyzed in the light of the Eisenman–Sherry theory and on the basis of the structural features of the various zeolitic terms. Eventually, a thermodynamic model recently proposed as a tool to predict either the ion exchange behaviour of a zeolite for varying compositions of the aqueous phase, or the equilibrium behaviour of multicomponent systems based on the experimental data relative to binary systems, is discussed.