ABSTRACT
Toxic metals enriched in the incineration residues of municipal solid waste (MSW) and sewage sludge are a substantial threat to ecosystems and human health. One example is cadmium, a toxic metal reported at concentrations ranging from 10 to 2100 mg/kg in fly ash. Various sorbents (e.g., bauxite, alumina, and calcium oxide) are usually injected into thermal treatment processes to immobilize toxic metals. However, this method has certain disadvantages including the agglomeration of sorbents, the clogging of sorption sites, and the need for additional ash stabilization. Solidification/stabilization (S/S) technologies are an alternative which aim to use physical and/or chemical mechanisms to prevent metal leaching from waste incineration residues. Common S/S technologies use sorption or cementation to immobilize metals but may not reliably control metal leaching in a variety of acidic environments. The development of a novel, economical, and reliable technology to stabilize toxic metals, such as cadmium, in waste incineration residues is a timely and important need. Gamma-alumina (γ-Al2O3) and hematite (α-Fe2O3) are common, low-cost240 industrial materials. It has been reported that γ-Al2O3 and α-Fe2O3 reacts with cadmium under thermal conditions to form crystal structures which immobilize and stabilize cadmium. However, for a reaction mechanism to become a feasible treatment technique, the optimal conditions for effectively incorporating cadmium into crystal structures using γ -Al2O3 and α-Fe2O3 precursors must be investigated in detail. In this study, γ-Al2O3 and α-Fe2O3 were employed to stabilize cadmium, and the operational parameters, such as treatment temperature and treatment time, were systematically evaluated. The chemical durability of reaction products was also evaluated using an acid leaching test to assess their metal stabilization effects. It was found that γ -Al2O3 and α-Fe2O3 were capable of incorporating cadmium into stable crystal structures under attainable thermal conditions, and the product phases, particularly ferrite spinel, showed a remarkably high acidic resistance.
