Uptake properties of Ni²⁺ by nCaO·Al₂O ₃·2SiO₂ (n = 1-4) prepared from solid-state reaction of kaolinite and calcite

A series of nCaO·Al₂O₃·2SiO₂ samples (n = 1-4) were prepared by solid-state reaction of mechanochemically treated mixtures of kaolinite and calcite fired at 600-1000°C for 24 h. All the samples were X-ray amorphous after firing at 600-800°C but had crystallized by 900°C. The main crystalline phases were anorthite (n = 1), gehlenite (n = 2 and 3) and larnite (n = 4). The uptake of Ni²⁺ by nCaO·Al₂O₃·2SiO₂ samples fired at 800 and 900°C was investigated at room temperature using solutions with initial Ni²⁺ concentrations of 0.1-50 mmol/l. Amorphous samples (fired at 800°C) showed a higher Ni²⁺ uptake capacity than crystalline samples (fired at 900°C). Ni²⁺ uptake was found to increase with increasing of CaO content. Amorphous 4CaO·Al ₂O₃·2SiO₂ showed the highest Ni ²⁺ uptake capacity (about 9 mmol/g). The Ni²⁺ uptake abilities of the present samples are higher than those of other materials reported in the literature. Since the sorbed Ni²⁺/released Ca ²⁺ ratios of these samples are close to unity, ion replacement of Ni²⁺ for Ca²⁺ is thought to be the principal mechanism of Ni²⁺ uptake by the present samples.