title: Ion exchange studies on natural and modified zeolites and the concept of exchange site accessibility.
abstract: In the present study natural and Na-rich form of clinoptilolite are examined, in respect to ion exchange of Pb2+, Cu2+, Cr3+, and Fe3+. Equilibrium and kinetic studies performed, under the same normality (0.01 N). Equilibrium studies demonstrate that Na+ enrichment of clinoptilolite is beneficial in respect to metal uptake for all metals, except Cr3+, which is shown to have the same equilibrium behavior in both materials. Kinetic study shows that diffusion coefficients are in the range of 0.16 to 9 x 10(-9) cm2/s, and are not always improved in Na-rich form of clinoptilolite. The effect of temperature on diffusion coefficients is also examined, and Arrhenius activation energy is determined to be in the range of 3.02 to 13.9 kcal/mol, for all metals and materials, except Cu2+, which have extremely low activation energy in the natural sample, equal to 0.04 kcal/mol.
authors.affiliation: Laboratory of Chemical Process Engineering, School of Chemical Engineering, Section II: Process Analysis & Plant Design, National Technical University of Athens, Zografou Campus, GR-15773 Athens, Greece.
authors.forename: Vassilis J
authors.initials: VJ
authors.lastname: Inglezakis
authors.forename: Maria M
authors.initials: MM
authors.lastname: Loizidou
authors.forename: Helen P
authors.initials: HP
authors.lastname: Grigoropoulou