Uptake of various cations by amorphous BaAl₂Si₂O₈ prepared by solid-state reaction of kaolin with BaCO₃

The uptake of various ions by amorphous and crystalline BaAl₂Si₂O₈ was investigated. The BaAl₂Si₂O₈ samples were prepared by solid-state reaction of kaolin ground with BaCO₃ for periods from 1 to 24 h followed by firing at temperatures of 800-1200 °C for 24 h. Uptake experiments were performed at room temperature (25 °C) using a solid/solution ratio of 0.1 g/50 ml, with a cation concentration of 10^-3 M and reaction time of 24 h. The cations used for the uptake experiments were alkaline earth ions (Mg²⁺, Ca²⁺ and Sr²⁺) and transition metal ions (Ni²⁺, Co²⁺, Cu²⁺ and Zn²⁺). The uptake of alkaline earth ions was low in all the samples while the transition metal ion uptakes were higher in the amorphous BaAl₂Si₂O₈ sample than crystalline BaAl₂Si₂O₈ samples. The high uptake of transition metal ions by the amorphous sample was further enhanced by prolonging the grinding time. Since the amorphous sample appears from the ²⁹Si and ²⁷Al NMR spectra to have a similar local structure to crystalline hexacelsian (double-layered structure consisted of alternatively corner shared AlO₄ and SiO₄ tetrahedra), the uptake of transition metal ions is suggested to occur by release of interlayer Ba ions in the vicinity of edge sites with the adsorption of transition metal ions from the solution.