Characterization of the state of Ni²⁺ in Ni-exchanged KAlSiO₄ by XPS, XRF, XRD, EXAFS and NMR

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 24 h. The Ni-exchanged KAlSiO₄ samples were obtained by treatment in a solution containing 10^-3-10^-1 M NiCl₂ buffered with 0.5 M 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; Ni_1-yAl_y(OH)₂X_y·nH ₂O) 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 Si04 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 ad-sorbed on imperfectly ordered KAlSiO₄.