Adsorption of heavy metal cations by Na-clinoptilolite: Equilibrium and selectivity studies L. Mihaly-Cozmuta a, * , A. Mihaly-Cozmuta a , A. Peter a , C. Nicula a , H. Tutu b , Dan Silipas c , Emil Indrea c a Technical University of Cluj Napoca e Romania, Scientific Research Center for Environment, Food and Health Safety, North University Center of Baia Mare, Victoriei Street 76, Baia Mare 430122, Romania b Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa c National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania article info Article history: Received 27 March 2013 Received in revised form 28 January 2014 Accepted 2 February 2014 Available online Keywords: Zeolite stability Diffusion coefficient Dehydration energy Hydrocomplex Adsorption isotherms abstract This paper summarizes the conclusions of experiments conducted on the adsorption of Cd 2þ , Co 2þ , Cu 2þ , Mn 2þ , Ni 2þ , Pb 2þ and Zn 2þ 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 0.5e1 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 physicalechemical 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 coef- ficient and the pH at which the precipitation of hydroxides occurs. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Zeolites are widely used due to their capacity to adsorb a wide range of metallic ions. They have applications in drinking water purification (Doula, 2006), acidic mine and radioactive wastewater treatment (Faghihian et al., 1999). Zeolite has also been used in combination with flotation and biological methods as a support for biomass (Zamboulis et al., 2004) The adsorption of Ag þ on the zeolite surface gives it antibacterial properties (Top and Ülkü, 2004). Utilization of zeolites in adsorption processes of metallic cations and experimental data fitting by different kinetic models and adsorption isotherms in order to establish the adsorption model and quantify the factors that strongly influence the adsorption capacity is widely described in scientific papers (Trgo et al., 2006a, b; Kocaoba et al., 2007; Sprynskyy et al., 2006; Hernández-Montoya et al., 2013). The most important of the many parameters which can impact the process of adsorption of various cations on zeolite can be broken down into three cate- gories: (i) Parameters related to work conditions: the static or dy- namic nature of the regime of adsorption, solid:liquid ratio, working temperature, initial concentration and pH of contact so- lutions, stirring intensity of the heterogenous system as well as the nature of the cation and accompanying anion; (ii) Parameters related to the characteristics of zeolite: the average diameter of particles, mineralogical and chemical composition, initial activa- tion, internal structure of macropores and micropores and (iii) Parameters related to the characteristics of adsorbed ions: hydrated radius of the ion, tendency to form hydrocomplexes in solutions, dehydration energy and ionic mobility. In general, clinoptilolite has a well-known higher preference for Pb 2þ and Cd 2þ than for Cu 2þ , Co 2þ and Ni 2þ . This happens because it prefers the cations with lower dehydration energy, lower hydration radius and higher mobility. According to Sprynskyy et al. (2006), the selectivity of zeolites is not correlated to the covalent radius, electronegativity or ionization potential. The selectivity of zeolite to adsorb various cations is the result of the complex combined effect of all the above e mentioned parameters and formulating a mathematical model taking into account all of those parameters, as well as other factors, * Corresponding author. Tel.: þ40 742588575; fax: þ40 262 276153. E-mail address: mihalyl@yahoo.com (L. Mihaly-Cozmuta). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman http://dx.doi.org/10.1016/j.jenvman.2014.02.007 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved. Journal of Environmental Management 137 (2014) 69e80