Transformation of AkaganÉite into Goethite and Hematite in Alkaline Media

R. M. Cornell1 and R. Giovanoli2
1 ETHZ Zürich, Laboratory of Inorganic Chemistry CH-8092 Zürich, Switzerland
2 University of Berne, Laboratory for Electron, Microscopy Freiestrasse 3, CH-3000 Berne 9, Switzerland

Abstract: The conversion of akaganéite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH-] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution of akaganéite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganéite.

Silicate species retarded the formation of goethite + hematite principally by inhibiting dissolution of akaganéite; to a lesser extent, they interfered with the nucleation of goethite. Silicate modified the morphology of goethite, but not hematite.

Comparison of the transformation behavior of akaganéite with that previously observed for ferrihydrite indicated that the composition of the reaction product depended strongly on the transformation conditions, i.e., pH and the presence of foreign species. The nature of the solid precursor was important insofar as its degree of crystallinity governed the dissolution kinetics and its surface properties influenced interaction with any foreign species in the system.

Key Words: Akaganéite • Dissolution • Goethite • Hematite • Iron oxides • Transmission electron microscopy • X-ray powder diffraction

Clays and Clay Minerals; October 1990 v. 38; no. 5; p. 469-476; DOI: 10.1346/CCMN.1990.0380502
© 1990, The Clay Minerals Society
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