Crystallization of Metal Substituted Ferrihydrites

The effects of a series of divalent, first row transition elements, i.e. Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ on the crystallization of ferrihydrite have been compared. With the exception of Mn 2+ , the metal ions considered, stabilized ferrihydrite and enhanced the amount of haematite in the reaction product. The stabilizing ability of these ions could be related to the increase in covalency of these metals along the series. With more than 15 mole% divalent metal ion present, ferrihydrite transformed to a spinel phase by a dissolution/reprecipitation mechanism. These metals can replace some Fe 3+ in the structures of the crystallization products. Factors that influence the extent of isomorphous substitution are the match between the radii and charges of Fe 3+ and the substituent ions and also, the congruency of dissolution of the M/ferrihydrite coprecipitate.