Extra-framework cation release from heulandite-type rich tuffs on exchange with NH4+

The outgoing cations of Greek heulandite-rich tuff samples (heulandite type-III, 91 wt.%, mica 4 wt.%, feldspar 5 wt. %, CEC 2.22 meq/g) were analysed upon exchange with ammonium acetate using atomic absorption spectrometry (AAS). The kinetic curves of each cation were investigated over a total time of contact of 720 h with sampling at frequent intervals. The materials were examined by powder X-ray diffraction, SEM-EDS, and AAS. The sorption ability was measured using the ammonium acetate saturation method. It was found that Ca2+ presents an unexpected extra-framework release and a surprisingly high degree of exchange (90%). The exchange of Mg (57%) is also worthy of note whereas the behavior of K+ showed an expected rapid initial release. The behavior of Na+ must be similar. However, its lower concentration in the zeolitic material minimizes its overall significance somewhat. On the other hand, Ca2+ and Mg2+ release is kinetically much slower, compared to that of alkali metal ions, and this phenomenon indicates that different exchange energies are needed till final equilibrium.