Quantitative X-Ray Diffraction Technique for Evaluating the Stabilization of Hazardous Metals in Ceramic Products

A sustainable waste management system requires the beneficial uses of waste residues, such as sludge and incineration ashes, generated from environmental treatments. Among the use strategies, the practices of mixing hazardous metal-bearing solids with clay materials to produce ceramic products are often found with significant improvement in reducing the metal leach ability from products. However, relatively much fewer studies have clearly answered the questions of "What are the metal stabilization mechanisms other than simply the dilution effect and the encapsulation of metals?"; "What are the mineral phases of metals and how much are they in the products?"; and "How thermal conditions can effetely promote the transformations of metal-hosting mineral phases?". As many sustainability movements have increasingly promoted the adoption of those products generated from the beneficial use of waste materials, quantitative understandings of the metal incorporation efficiencies are important to facilitate the design of safe and reliable waste-to-resource strategies. Current findings on the metal incorporation mechanisms between common alumino silicates and hazardous metals (nickel, copper, and zinc) under different thermal conditions will be presented, and the results show the important role of forming aluminates and ferrites to significantly reduce the metal leach ability from the products. In the study, the technique of quantitative X-ray diffraction (QXRD) was applied to report the metal incorporation efficiencies through a 3-hour sintering process, aiming to stabilize the hazardous metals and also to turn the waste residues for usable ceramic products. Prolonged leach tests for potential metal-containing phases were carried out in acidic environments to evaluate the durability of thermally treated products. Both aluminate and ferrite spinels proved superior for thermomobilization of hazardous metals. With the information reported, this study has identified the key mechanisms of stabilizing the hazardous metals when thermally treated with common ceramic raw materials, and also demonstrates the importance of quantitative understanding in the development of a safe waste-to-resource strategy.