Characterization of the State of Ni2+ in Ni-Exchanged KAlSiO4 by XPS, XRF, XRD, EXAFS and NMR.

Abstract

Imperfectly ordered KAlSiO4 shows a greatly increased uptake of Ni2+ from solution which motivated this study of the state of Ni2+ in Ni-exchanged KAlSiO4 using X-ray photo-electron spectroscopy (XPS), X-ray fluorescence (XRF), powder X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and solid state nuclear magnetic resonance spectroscopy (NMR). The imperfectly ordered KAlSiO4 was synthesized by solid-state reaction of kaolinite with K2CO3 at 550°C for 24h. The Ni-exchanged KAlSiO4 samples were obtained by treatment in a solution containing 10-3-10-1M NiCl2 buffered with 0.5M NaCl at 60°C. Comparison of the surface and bulk chemical compositions analyzed by XPS and XRF indicated that the Ni2+ sorbed on the KAlSiO4 is largely concentrated at the surface. The XRD pattern of the Ni-exchanged KAlSiO4 showed weak broad peaks corresponding to a phase with a layered structure and a change in the halo pattern of the KAlSiO4. EXAFS analysis confirmed the presence of a Ni-containing layered double hydroxide (LDH; Ni1-yAly(OH)2Xy⋅nH2O) as indicated by the interatomic Ni-Ni distance and Ni coordination number. The 29Si MAS NMR showed shift and peak splitting corresponding to an increase in the degree of polymerization of the SiO4 tetrahedra forming the framework of the tridymite-type structure and the 27Al MAS NMR showed an increase of six coordinated Al in the Ni-exchanged KAlSiO4. It is concluded, from all these results, that the Ni2+ in Ni-exchanged KAlSiO4 forms a Ni-Al LDH-like phase on the surface of the KAlSiO4. This is considered to be the main reason for the higher Ni2+ uptake compared with various other cations adsorbed on imperfectly ordered KAlSiO4.