Theory and practice of the removal of heavy-metal ions by their precipitation as ferrite-type compounds from aqueous solution at ambient temperature

The removal of iron and various heavy-metal ions by their incorporation into stable ferrite-type precipitates at 25°C is a promising alternative to clean up large volumes of polluted aqueous effluents. This paper presents the general overview of the conditions leading to the formation of ferrite compounds directly from aqueous solutions at room temperatures. The background information concerning the stability of the intermediate phase 'green rust-II' and its role in the ferrite-forming reactions are reviewed. The experimental work investigated the conditions to produce magnetite and various M-bearing ferrites (where M: Zn, Cu, Co, Ni, Cd, etc...) directly from aqueous solutions at 25°C by simultaneous control of the oxidizing conditions and pH. The formation of the solids was followed by monitoring the oxidation-reduction potential (ORP) and rate of proton release during the aerial oxidation of the suspensions at constant pH (contact stage). Furthermore, the ferrite formation at low temperature permits the elimination of iron and co-existing heavy metal ions to sufficiently low concentration. The correspondence between the reaction conditions of formation of the ferrite in sulfate medium and its structural and magnetic characteristics is also discussed. The formation of a Zn-bearing ferrite was considered as a first-case study. The precipitates were characterized by XRD, FT-IR, HRTEM analyses and magnetization measurements. Furthermore, the analysis of the local structure of Fe and Zn atoms by EXAFS was also undertaken.