Uptake of various cations by amorphous CaAl₂Si₂O₈ prepared by solid-state reaction of kaolinite with CaCO₃

The uptake of various ions by amorphous and crystalline CaAl₂Si₂O₈ was investigated. The CaAl₂Si₂O₈ samples were prepared by solid-state reaction of kaolinite with CaCO₃, ground for 0-24 h and fired at 600-1000°C for 24 h. Uptake experiments were performed both at room temperature (25°C) and 60°C at a solid:solution ratio of 0.002 g ml°¹ with cation concentrations ranging from 10^-4-10^-2 M. The cations examined were alkali ions (Na⁺ and K⁺), alkaline earth ions (Mg²⁺, Sr²⁺ and Ba²⁺) and transition metal ions (Ni²⁺, Co²⁺, Cu²⁺ and Zn²⁺). The uptake of transition metal ions by amorphous CaAl₂Si₂O₈ was found to be higher than by crystalline CaAl₂Si₂O₈. The uptake of transition metal ions by amorphous CaAl₂Si₂O₈ was higher than the uptake of alkali and alkaline earth ions, indicating a high selectivity for transition metal ion and their uptake was further enhanced at higher solution temperatures. Uptake of the various ions showed strong pH dependence and indicated high uptake ability at neutral pH. For the heavy metal ions the uptake selectivity was found to be Cu > Zn > Ni ≥ Co, suggesting a correlation with their ion products. The effectiveness of amorphous CaAl₂Si₂O₈ as an uptake agent for heavy metal ions was confirmed by experiments with real river water.