Cadmium stabilization via silicates formation: Efficiency, reaction routes and leaching behavior of products

Stabilizing cadmium by incorporating it into crystalline products is an effective approach to detoxify cadmium-containing wastes. In this study, two Si-rich matrices in amorphous and crystalline forms (i.e., silica fume and α-quartz, respectively) were employed to incorporate Cd. The processing parameters, namely the type of Si-rich matrix, Cd/Si molar ratio (Г) and sintering temperature, were thoroughly investigated using quantitative X-ray diffraction technique. Cd incorporation was more energetically favored when silica fume was used rather than when α-quartz was used because of the lower Gibbs free energy of formation for silica fume. The sintering temperature and Г values substantially affected the formation of three cadmium silicates, namely monoclinic CdSiO₃, orthorhombic Cd₂SiO₄, and tetragonal Cd₃SiO₅. CdSiO₃ formed only in Г = 1.0 systems. Cd₂SiO₄ was dominant in all reactive systems. In Г = 3.0 systems, Cd₃SiO₅ rather than Cd₂SiO₄ was the predominant Cd-hosting product at temperatures above 850 °C. Leaching test results demonstrated that CdSiO₃ possessed the highest acid resistance among the cadmium silicates. The leachability of Cd₂SiO₄ was very similar to that of Cd₃SiO₅. CdSiO₃ preferred incongruent dissolution, whereas Cd₂SiO₄ and Cd₃SiO₅ favored near-congruent dissolution. This study delineated the feasibility of cadmium incorporation by Si-rich matrices, identifying a promising approach for cadmium detoxification.