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Formation of green rust sulfate : a combined in situ time-resolved X-ray scattering and electrochemical study.

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  • Imad A. M. Ahmed
  • Liane G. Benning
  • Gabriella Kakonyi
  • Aryani D. Sumoondur
  • Nick J. Terrill
  • Samuel Shaw
<mark>Journal publication date</mark>4/05/2010
Issue number9
Number of pages11
Pages (from-to)6593-6603
Publication StatusPublished
<mark>Original language</mark>English


The mechanism of green rust sulfate (GR-SO4) 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-SO4 was synthesized under strictly anaerobic conditions by coprecipitation from solutions with known FeII/FeIII ratios (i.e., 1.28 and 2) via the controlled increase of pH. The reaction in both systems proceeded via a three-stage precipitation and transformation reaction. During the first stage,schwertmannite (Fe8O8(OH)4.5(SO4)1.75) precipitated directly from solution at pH 2.8−4.5. With increasing pH (>5), Fe2+ ions adsorb to the surface of schwertmannite and catalyze its transformation to goethite (α-FeOOH) during the second stage of the reaction. In the third stage, the hydrolysis of the adsorbed Fe2+ ions on goethite initiates its transformation to GR-SO4 at pH >7. The GR-SO4 then continues to crystallize up to pH ∼8.5. These results suggest that with an FeII/FeIII ratio of ≤2 in the initial solution the structural FeII/FeIII of the GR-SO4 will be close to that of the starting composition.