Structure Investigation of As(III)- and As(V)-Species Bound to Fe-Modified Clinoptilolite Tuffs

The structure of ($AsO_{2})^{−}$- and $(HAsO_{4})^{2−}$-modified clinoptilolite samples from the two deposits (Vranjska Banja, Serbia and Donje Jesenje, Croatia) was studied using powder X-ray diffraction (XRPD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXS) and X-ray photoelectron spectroscopy (XPS). Local environment of As ions was investigated by X-ray absorption spectroscopy (XAS). The pretreatment of clinoptilolite tuff by using first NaCl and later $FeCl_{3}$ solutions resulted in an arsenic uptake of up to 1.2 wt.%, whereas no As-uptake could be detected in the untreated zeolite. The X-ray photoelectron spectroscopy depth profile analyses of ($AsO_{2})^{−}$- and $(HAsO_{4})^{2−}$-modified samples show that As concentrations decrease from the surfaces towards the subsurface region of crystallites suggesting that the As atoms are mainly located at surfaces of crystallites. EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) analysis revealed that arsenic cations in the samples arrange in the form of oxo-complexes attached to the iron cations on the surface of the clinoptilolite. The posttreatment of the samples with three different solutions of HCl confirmed irreversible sorption of arsenic in the zeolite by the chemisorption mechanism