Adsorption of heavy metal cations by Na-clinoptilolite: Equilibrium and selectivity studies

• The adsorption capacity of zeolite for heavy metals ions was investigated.
• The Langmuir, Freundlich and Dubinin–Radushkevich models were used to describe the adsorption process.
• The selectivity of zeolite was found to increase with a decrease in the dehydration energies of cations.
• The zeolite structure exhibits stability in the pH range 1–4.

This paper summarizes the conclusions of experiments conducted on the adsorption of Cd2+, Co2+, Cu2+, Mn2+, Ni2+, Pb2+ and Zn2+ onto zeolite. The focus of the experiments was to establish the influence of the initial pH of the contact solution as well as the selectivity of zeolite on the efficiency of the adsorption process. To this end, experimental adsorption isotherms were established for the pH values ranging from 1 to 4 by using the Na-form of clinoptilolite (particle size range 0.5–1 mm) as an adsorbent. Langmuir, Freundlich and Dubinin-Raduschkevich isotherm models were used to validate the experimental data and the Gibbs free energy was calculated based on the distribution coefficient. From the Langmuir model, correlations between the maximum adsorption capacity and selected physical–chemical parameters of the cations studied were established. The results of the experiments suggest that the selectivity of zeolite is strongly influenced by the pH of the contact solution, dehydration energy of cations, diffusion coefficient and the pH at which the precipitation of hydroxides occurs.

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