Structural investigation of Zn2+ sorption on clinoptilolite tuff from the Vranjska Banja deposit in Serbia

Three Zn2+-modified clinoptilolite samples from the deposit of Vranjska Banja (Serbia) were structurally studied using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXS) and X-ray photoelectron spectroscopy (XPS). Local environment of Zn ions was investigated by X-ray absorption spectroscopy (XAS). The pre-treatment of clinoptilolite tuff by using NaCl or CaCl2 solutions resulted in a higher zinc uptake comparing to the untreated zeolite sample, i.e. up to 4 wt.% of zinc in the clinoptilolite crystals in Na-pretreated form, and up to 1.5 or 0.6 wt.% of zinc in Ca-pretreated and in untreated zeolite form, respectively. The X-ray photoelectron spectroscopy depth profile analyses show that on all three samples the Zn concentrations decrease from the surfaces towards the subsurface region of crystallites suggesting that the Zn atoms are mainly located at surfaces of crystallites. EXAFS (extended X-ray absorption fine structure) analysis revealed that Zn2+ cations in untreated zeolite sample bond to four oxygens, two at 1.96 angstrom and two at 2.12 angstrom, indicating distorted tetrahedral coordination of zinc. On the other hand, EXAFS analysis of Na-pretreated sample showed that zinc atoms were coordinated to six oxygens, four at 2.01 angstrom and two at 2.15 angstrom indicating that Zn atoms in the sample were located at sites with distorted octahedral symmetry. Additionally, EXAFS fit for Ca-pretreated sample also revealed octahedral symmetry of zinc, i.e. six oxygen atoms bonded to zinc at a distance of 2.05 angstrom. Pre-treatment of zeolite tuff thus affects the sorption capacity and local structure of Zn2+. The majority of the loaded zinc could be desorbed from the sample after posttreatment with NaCl solution that indicates mostly reversible sorption of zinc in the zeolite via ion exchange mechanism. The results suggest that the studied zeolite tuff could successfully be used for the removal of zinc from wastewaters. (c) 2007 Elsevier Inc. All rights reserved.