title: Formation of green rust sulfate: a combined in situ time-resolved X-ray scattering and electrochemical study.
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 of pH. The reaction in both systems proceeded via a three-stage precipitation and transformation reaction. During the first stage,schwertmannite (Fe(8)O(8)(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 approximately 8.5. These results suggest that with an Fe(II)/Fe(III) ratio of < or = 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.
authors.affiliation: School of Earth & Environment, University of Leeds, UK. i.ahmed@lancaster.ac.uk
authors.forename: Imad A M
authors.initials: IA
authors.lastname: Ahmed
authors.forename: Liane G
authors.initials: LG
authors.lastname: Benning
authors.forename: Gabriella
authors.initials: G
authors.lastname: Kakonyi
authors.forename: Aryani D
authors.initials: AD
authors.lastname: Sumoondur
authors.forename: Nick J
authors.initials: NJ
authors.lastname: Terrill
authors.forename: Samuel
authors.initials: S
authors.lastname: Shaw