NOTE THE INFLUENCE OF SOME DIVALENT CAT IONS ON THE TRANSFORMATION OF FERRIHYDRITE TO MORE

In natural systems iron oxides and hydroxides are frequently associated with a variety of cations which may adsorb on the mineral surfaces or replace aproportion of Fe in the oxide structure (Schwertmann & Taylor, 1977). A number of laboratory studies have established that foreign cations can influence the kinetics of iron oxide formation and determine the nature of the end-product (Detournay et al., 1975; Lewis & Schwertmann, 1979; Cornell & Giovanoli, 1987, 1988). These studies have provided further details of the mechanisms by which iron oxides form. The present report compares the effects of a series of divalent metal ions on the kinetics and products of the transformation of ferrihydrite (5Fe203.9H20) to more crystalline minerals. The ions investigated, Mn 2+, Cu 2+, Ni 2+, Co 2+ and Zn z+, have been found in association with iron oxides in soils, sediments and, in some cases, in ferromanganese nodules. Materials and methods Metal-ferrihydrite coprecipitates (1 g/l) were precipitated with 5-6 ml 1 M KOH fro