Formation and leaching behavior of ferrite spinel for cadmium stabilization

Highly mobile and toxic cadmium (Cd) is often found in waste incinerator residues. To investigate the effectiveness of immobilizing Cd-bearing residues, two iron oxide precursors, hematite (alpha-Fe3O4) and magnetite (Fe3O4), were reacted with cadmium oxide (CdO) at 600-850 degrees C for 3 h. The XRD results showed that cadmium ferrite spinel (CdFe2O4) was the only Cd-hosting product phase in both systems. Both of the iron oxides were found to be efficient in incorporating Cd into the CdFe2O4 spinel structure, but Fe3O4 was more reactive in the Cd incorporated at low temperatures. The weight factions of the crystalline phases in the sintered products were quantified by the Rietveld refinement method, and the transformation ratio (TR) index was used to reveal the Cd incorporation efficiencies of the iron oxides. A TR of 100% was achieved at 850 degrees C, indicating the potential for efficient stabilization of Cd through thermal reactions with iron oxide precursors. The Cd stabilization effects were further assessed by leaching CdFe2O4 and CdO in constant pH (2.0, 3.0 and 4.0) leaching tests. The results revealed the incongruent dissolution of CdFe2O4. The amount of Cd leached from the CdFe2O4 was more than three orders of magnitude smaller than that from the CdO, signifying a highly promising and reliable strategy for Cd stabilization. To our knowledge, this study is the first work to systematically report the roles of different Fe-rich precursors in Cd stabilization mechanism, and indicates the significant reduction of Cd leachability via CdFe2O4 formation. These findings have provided further understanding on the stabilization of cadmium by sintering with low-cost and attainable Fe-rich precursors at relatively low temperatures.