Low temperature synthesis and high temperature behavior of feldspar-like CdAl2Si2O8, a promising material for cadmium immobilization

Cadmium is an extremely toxic nonbiodegradable heavy metal, which accumulates in significant quantities in the near-surface environments, where it easily enters surrounding water and soil. In this work, we synthesized a series of cadmium silicates (CdAl₂Si₂O₈, Cd₂SiO₄, Cd₃Si₃O₉, Cd₃(SiO₄)O), which are prospective materials for cadmium immobilization. The most environmentally stable and least studied of them, CdAl₂Si₂O₈, was investigated by means of scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder and single crystal X-ray diffraction (over the temperature range of 27–1100 °C). Using alkali cations additives, we were able to obtain CdAl₂Si₂O₈ at a temperature (500 °C) half previously reported. We showed that cadmium can be successfully incorporated into a feldspar-like crystal structure and that the amount of cadmium could range depending on synthesis conditions from 21 to 37 wt % CdO. Our data indicate that CdAl₂Si₂O₈ is stable up to at least 1100 °C and does not undergo phase transitions with a change in the volume of the unit cell. We can conclude that CdAl₂Si₂O₈ is a promising material for cadmium immobilization, given its resistance to leaching and low synthesis temperatures.