Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 69, Issue 16, Pages 3967-3977Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2005.03.016
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Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with Fe-55-labelled iron oxides. Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10 degrees C and 25 degrees C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe, an immediate and very rapid release of Fe-55 was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe2+ no release was observed. Fe-55 was not released from hematite, even at the higher Fe2+ concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe2+ only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of Fe-55 suggests that isotopic equilibrium will ultimately be attained. Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures. Copyright (c) 2005 Elsevier Ltd.
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