Cation exchange of cesium and cation selectivity of natural zeolites: Chabazite, stilbite, and heulandite

The sorption characteristics of Cs and competitive ion exchange with other alkali cations and Sr on three natural zeolites, chabazite, stilbite, and heulandite, were investigated. Chabazite showed the most rapid sorption of Cs, followed by heulandite and then stilbite, and this was closely related to its high cation exchange capacity (CEC) and low Si/Al ratio. The kinetic data for all zeolites were best explained by a pseudo-second-order rate model. The sorption process involved multiple steps and is influenced by the CEC values of the zeolites. The equilibrium adsorption isotherms of the three zeolites were best fitted by both the Langmuir and Freundlich models. The exchange isotherms of Cs and the other investigated cations clearly showed that chabazite showed higher selectivity for Cs than for the others. At lower equivalent fractions of Cs in solution, the selectivities for the other cations showed the following trend: Na > Li > Sr > K > Rb. However, for stilbite and heulandite, the selectivity for Cs is only higher at lower equivalent fractions, and isotherms show sigmoidal shapes mainly because there are more than two sorption sites for Cs while chabazite has one site. Those trends were confirmed by Kielland plots.