Advances in Cadmium Detoxification/Stabilization by Sintering with Ceramic Matrices

Cadmium (Cd) is often found in industrial sludge, fly ash, slag, and flue gas, and it presents serious risks to the environment and biota. Stabilizing and detoxifying Cd in waste streams is thus of great importance. The use of various low-cost and attainable ceramic matrices (amorphous SiO2, γ-Al2O3, α-Fe2O3, and Fe3O4) to interact with Cd-containing waste is a promising method of Cd stabilization. Heating mixtures of cadmium oxide (CdO) and ceramic matrices at various molar ratios and temperatures (600–1000 °C) for 3 h could achieve the goal of Cd incorporation. Phase transformation was assessed using X-ray diffraction (XRD), and the efficiency of Cd incorporation was quantified through Rietveld analysis of the obtained XRD patterns. The XRD results show that Cd can be crystallochemically incorporated into CdSiO3, Cd2SiO4, Cd3SiO5, CdAl4O7, and CdFe2O4 phases. The treatment temperature greatly affected the Cd incorporation reactions. The Cd incorporation efficiency was quantified and expressed as a transformation ratio according to the weight fractions of crystalline phases in the sintering products. To evaluate the metal stabilization effect of the Cd detoxification process, a series of constant-pH leaching tests was conducted. A remarkable reduction in Cd leachability was achieved by forming different Cd-hosting crystalline products, particularly spinel phase CdFe2O4. Overall, the efforts to stabilize Cd by sintering with ceramic matrices suggest a promising strategy for the detoxification of Cd in wastes.