Interactions of Metal Hydrous Oxides with Chelating Agents. VI. Dissolution of a Nickel Ferrite in EDTA Solutions Available to Purchase

The dissolution of a nickel ferrite, Ni_0.53Fe_2.47O₄, in EDTA solutions of different pH values and containing various additives has been studied at 79.5 C. The leaching of metal ions from this oxide is at first a rather slow process, but the rate of dissolution gradually increases. Initially, nickel is leached preferentially to iron, indicating an uneven internal composition of the solids with respect to these two metals. Enrichment of nickel in the surface region of the particles was detected by chemical means. At the early stages, the dissolution is dominated by the adsorption of the complexing molecules at the surface/solution interface. At longer reaction times, the leaching is controlled in part by reductive dissolution of the ferrite particles; this is due to the presence of Fe(II)-EDTA solute complexes generated from released Fe²⁺ ions. Both steps are greatly influenced by pH; the dissolution was found more effective at pH values lower than 6. The presence of hydrazine drastically increases the rate of dissolution; in contrast, the process is slower in EDTA solutions containing KNO₃ or H₂O₂.