Substitution of Mg (II) in Sepiolite with Cu (II) in Aqueous Solution

Abstract

A clay mineral sepiolite is a fibrous magnesia silicate with intramolecular tunnels along the fiber axis and the Mg²⁺ along the tunnel wall which can be substituted with transition metalions. The substitution reactions of the Mg²⁺ in a sepiolite with Cu were carried out in various concentrations of CuSO₄ aqueous solution at 25°C using a natural sepiolite sample from China.The reaction rate increased with an increase of the concentration of CuSO₄ solution. The Mg²⁺ along the tunnel wall of sepiolite was substituted by Cu²⁺ about 40% for a period of 30 days. Reaction kinetics was studied and the rate of substitution (v) was represented by the equation of v=k [sep] ₀ [Cu²⁺]/[Cu²⁺] +K; where K, [sep] ₀, [Cu²⁺] and K were the rate constant, the initial concentration of sepiolite, the concentration of Cu²⁺ and a constant respectively. The well oriented fibers of the sepiolite substituted with Cu²⁺ showed an angular dependency to the applied field in the electron paramagnetic resonance (ESR) spectra. The ESR spectra for the well oriented sample which was set perpendicular or parallel to the applied magnetic field, showed different line shapes. The g_x, g_y and g_z calculated from the spectra were 2.37, 2.11 and 2.05 respectively. The hyperfine anisotropy was also observed in the ESR spectra and the hyperfine coupling constants, A_x, A_y and A_z, were found to be 8.5, 3.0, and 6.0 mT respectively. These results suggest that the coordination of Cu²⁺ is a distorted octahedral coordination and the Mg²⁺ ions along the tunnel wall are substituted by Cu²⁺ ions.