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

Abstract

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.