Web Release Date: February 16,
In Situ Spectroscopic and Solution Analyses of the Reductive Dissolution of MnO2 by Fe(II)
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Department of Hydrology and Water Resources, The University of Arizona, P.O. Box 210011, Tucson, Arizona 85721-0011, and Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404
Received for review June 12, 2000
Revised manuscript received January 8, 2001
Accepted January 9, 2001
Abstract:
The reductive dissolution of MnO2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO42-) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe2O4), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.
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