N2 and O2 adsorption properties of an Iranian clinoptilolite-rich tuff in the original and pre-exchanged forms

The adsorption characteristics of an Iranian clinoptilolite-rich tuff from the Semnan region, both in the unmodified and K+, Na+, Ca2+ and Mg2+ exchanged forms has been investigated. The effect of zeolite agglomerate particle size (d < 45 μm and d = 4 mm) on the adsorption isotherms has been studied. Large ion-exchanged particles exhibit significant less saturation adsorption because of less nanopore connectivity due to minimal residues of unwashed salts. Using the Dubinin–Astakhov model, the optimum values of the structural parameter, characteristic energy, saturation capacity and pore volume have been evaluated. K-cp possesses the largest structural parameter (n = 2.3 for coarse and n=2.5 for the fine powder). Also, this form has the highest selectivity (amount N2 adsorbed/amount O2 adsorbed), i.e., 1.61 and 1.98 for the fine and coarse powders, respectively. The confinement effect is significant in O2 adsorption for the ion-exchanged forms, resulting in considerable smaller calculated pore volumes. The K+ exchanged Iranian clinoptilolite could be a potential candidate for O2/N2 separation while the Ca form is the least appropriate due to eventual blocking of the perpendicular 8 and 10 member-ring channels.