The potential of clinoptilolite-rich tuffs from Croatia and Serbia for the reduction of toxic concentrations of cations and anions in aqueous solutions

Natural zeolites have a great potential as low-cost, non-toxic and highly selective sorbents in pollution control, especially in waste-water management. The potential of Croatian and Serbian dinoptilolite-rich (HEU-type zeolite) tuffs from four different deposits have been evaluated for their use in metal removal from aqueous solutions by the application of adsorption equilibrium studies, structural studies of metal-loaded zeolite tuffs and their regeneration or further treatment, which were performed in our laboratories or were reported by other authors. The results showed that metals like copper could be completely removed from aqueous solutions at low metal concentrations using Serbian and/or Croatian zeolites (up to 120 mg Cu/L). At higher concentrations the efficiency of zeolites slowly decreased, also in accordance with the theoretical adsorption capacities. The studies confirmed that a higher content of zeolite in tuffs from Serbia (70-80% vs. 50% for Croatian tuff) resulted in their better adsorption performance for selected metals. Additionally, the pretreatment of zeolites with Na+ significantly enhanced the uptake of all metal cations, while the pretreatment with Fe3+, which resulted in stable iron-oxo-species on the zeolite surface and positive charge of the framework, enabled the adsorption of anions, like arsenites, arsenates and chromates. The reversibility of the metal uptake depended on the type of metal ; for example, post-treatment of the samples with HCl, NaCl or NH4Cl solutions revealed irreversible adsorption of chromium and arsenic and mostly reversible adsorption of zinc and copper. The possible strategies for the regeneration or immobilization of the used metal-loaded zeolites, like immobilization in cements or the use as catalysts, were also considered. (C) 2015 Elsevier B.V. All rights reserved