Sorption of Cr3+ on clinoptilolite tuff: A structural investigation

A Cr3+-modified natural zeolite sample from the deposit of Donje Jesenje (Croatia) was structurally studied using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXS), inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The analyses revealed the presence of up to 0.4 wt% of chromium in the clinoptilolite crystals. The X-ray photoelectron spectroscopy depth analysis suggested a uniform distribution of chromium from the surface of the crystallites to the depth of 50(3) nm. The study of local environment of chromium by XANES (X-ray absorption near edge structure) showed that chromium cations in the sample were predominantly in the 3+ oxidation state. EXAFS (extended X-ray absorption fine structure) analysis revealed that Cr3+ cations bond to six oxygen atoms at the distance of 1.97(2) Å and two chromium atoms at the distance of 2.54(2) Å suggesting that Cr3+ cations most probably arrange in the zeolite pores in the form of small clusters of CrO6 octahedra. The chromium could not be completely desorbed from the sample after post-treatment with NaCl or HCl solutions thus indicating mostly irreversible sorption of chromium in the zeolite.