Under reducing conditions, electron transfer between aqueous Fe(II) and ferrihydrite rapidly accelerates ferrihydrite transformation to more stable Fe(III) oxides. The mechanism of ferrihydrite transformation by Fe(II), however, remains unclear. Several mechanisms, including dissolution-reprecipitation, topotatic transformation, and oriented aggregation have been proposed to be responsible for Fe(II)-driven ferrihydrite transformation. To probe the mechanism responsible for Fe(II)-catalyzed ferrihydrite transformation, we used Fe isotope tracers, acid extractions, X-ray diffraction, and isotope-specific ⁵⁷Fe Mössbauer spectroscopy to simultaneously track Fh recrystallization and Fe atom exchange. Two-line ferrihydrite was synthesized from natural Fe and ⁵⁶Fe and reacted with aqueous ⁵⁶Fe(II) and ⁵⁷Fe(II), respectively, to monitor Fe atom exchange between Fh and the aqueous phase. Despite no evidence for formation of lepidocrocite or goethite in XRD patterns after twenty minutes of Fe(II) exposure, Mössbauer spectroscopy showed significant changes in magnetic structure of the ferrihydrite. At longer times (> 2h) ferrihydrite transformed to lepidocrocite and goethite as expected. Notably, we observed rapid isotope exchange between the aqueous phase and the transforming ferrihydrite particles with significant mixing achieved throughout by two hours. The rapid electron transfer and isotope exchange between aqueous Fe(II) and ferrihydrite resulted in Mössbauer spectra that are remarkably similar regardless of whether the ⁵⁷Fe is initially in the Fh solids or the solution. These results provide compelling evidence that rapid dissolution and reprecipitation of Fe(II)-ferrihydrite occurs over short time scales (minutes) supporting dissolution-reprecipitation as a primary mechanism for redox-driven ferrihydrite transformation at near-neutral pH values. Isotope ratios of sequential extractions of the recrystallizing Fh further suggest that a fraction of the Fe undergoing exchange may be “buried” and no longer participating in further exchange consistent with previously proposed heterogeneous isotope exchange models for Fe(II)-catalyzed recrystallization of Fe oxides.

Drew Latta

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