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Formation of Green Rust Sulfate: A Combined in Situ Time-Resolved X-ray Scattering and Electrochemical Study

Authors

Ahmed,  I. A. M.
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Benning,  Liane G.
0 Pre-GFZ, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Kakonyi,  G.
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Sumoondur,  A. D.
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Terrill,  N. J.
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Shaw,  S.
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Citation

Ahmed, I. A. M., Benning, L. G., Kakonyi, G., Sumoondur, A. D., Terrill, N. J., Shaw, S. (2010): Formation of Green Rust Sulfate: A Combined in Situ Time-Resolved X-ray Scattering and Electrochemical Study. - Langmuir, 26, 9, 6593-6603.
https://doi.org/10.1021/la903935j


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_806894
Abstract
The mechanism of green rust sulfate (GR-SO(4)) formation was determined using a novel in situ approach combining time-resolved synchrotron-based wide-angle X-ray scattering (WAXS) with highly controlled chemical synthesis and electrochemical (i.e., Eh and pH) monitoring of the reaction. Using this approach,GR-SO(4) was synthesized under strictly anaerobic conditions by coprecipitation from solutions with known Fe(II)/Fe(III) ratios (i.e., 1.28 and 2) via the controlled increase or pH. The reaction in both systems proceeded via a three-stage precipitation and transformation reaction. During the first stage,schwertmannite (Fe(8)O(5)(OH)(4.5)(SO(4))(1.75)) precipitated directly from solution at pH 2.8-4.5. With increasing pH ( >5), Fe(2+) ions adsorb to the surface of schwertmannite and catalyze its transformation to goethite (alpha-FeOOH) during the second stage of the reaction. In the third stage, the hydrolysis of the adsorbed Fe(2+) ions on goethite initiates its transformation to GR-SO(4) at pH >7, The GR-SO(4) then continues to crystallize up to pH similar to 8.5. These results suggest that with an Fe(II)/Fe(III) ratio of <= 2 in the initial solution the structural Fe(II)/Fe(III) of the GR-SO(4) will be close to that of the starting composition.