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Transformation of Akaganéite Into Goethite and Hematite in the Presence of Mn

Published online by Cambridge University Press:  02 April 2024

R. M. Cornell
Affiliation:
ETH Zentrum Zürich, Laboratory for Inorganic Chemistry, CH-8092 Zürich, Switzerland
R. Giovanoli
Affiliation:
University of Bern, Laboratory for Electron Microscopy, Freiestrasse 3, 3000 Bern 9, Switzerland
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Abstract

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The interaction of Mn and akaganéite in neutral to alkaline media has been investigated using X-ray powder diffraction and transmission electron microscopy. Akaganéite transformed into goethite and/or hematite, whereas Mn precipitated as hausmannite and birnessite at pH > 12 and as manganite at pH 7.5–8.5. Mn influenced the kinetics of the transformation of akaganéite: the rate-determining step, i.e., the dissolution of akaganéite, was retarded by adsorbed Mn species. Hematite formation was not suppressed. By catalyzing the air oxidation of adsorbed Mn(II), akaganéite promoted the formation of birnessite. Akaganéite did not retard recrystallization of the Mn phases. The incorporation of Mn in the structure of goethite formed in this system was negligible, and jacobsite (MnFe2O4) did not form. The formation of mixed Mn-Fe phases appeared to require a ratio of Mn2+: Fetotal > 0.02; this ratio was not achieved due to the oxidation of Mn2+ at the akaganéite surface.

Type
Research Article
Copyright
Copyright © 1991, The Clay Minerals Society

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