Natural zeolites for heavy metals removal from aqueous solutions: Modeling of the fixed bed Ba2+/Na+ ion-exchange process using a mixed phillipsite/chabazite-rich tuff

The efficiency of Ba2+ removal from aqueous solutions by means of an ion exchange process was investigated. More specifically, a natural zeolitic tuff, namely a Campanian ignimbrite containing both chabazite and phillipsite as exchanger phases was used, and the effect of Ba2+ concentration was studied. The selected material, previously converted into its Na+ form, was used in a laboratory scale fixed bed column, and the kinetics of Ba2+ removal were studied at different concentrations, ranging from 50 to 500 g/m3. In all the experiments a very high selectivity of the tuff material towards Ba2+ was observed. The experimental data were interpreted using a diffusional model, based on the Linear Driving Force (LDF) approximation. The model accounted for the presence of two zeolitic phases in the tuff and for mass transfer limitations in both the liquid phase and tuff particles. The model interpreted the data satisfactorily, enabling it to be established that both the internal and external mass transfer resistances played a key role in the kinetics of the ion exchange process.