Transformation Mechanisms and Kinetics of Fe-(oxyhydr)oxide Reduction and their Impact on Uranium Sequestration
EMSL Project ID
40109
Abstract
Redox cycling of Fe-(oxyhydr)oxides such as ferrihydrite in naturally or artificially reduced sediments has a major impact on the speciation and stability of uranium. Ferrihydrite is a poorly crystalline Fe-oxyhydroxide, ubiquitous at redox transition zones in aquifers as grain coatings and colloids, that strongly adsorbs U(VI). Previous work suggests that U(VI) adsorbed to ferrihydrite becomes incorporated directly into the structures of Fe-(oxyhydr)oxide reduction products. The finding that magnetites from reduced sediments at the Rifle IFRC site contain ~100 ppm U supports this conclusion. Incorporation of reduced U into Fe-(oxyhydr)oxide products is anticipated to dramatically lower U mobility. Moreover, even if U has a low solubility in Fe-(oxyhydr)oxide reduction products, their abundance in the subsurface suggests that they could sustain a large and recalcitrant pool of uranium. Reduction of ferrihydrite by Fe(II) is known to produce a variety of products, including magnetite, goethite, and lepidocrocite, however, the mechanistic transformation pathways are not known. This inability constitutes a major knowledge gap that hinders the development of biogeochemical models. The structural mechanisms by which uranium is accommodated into the structures of Fe-(oxyhydr)oxide reduction products also is not known, hindering our ability to rationalize its stability in groundwater. This proposal seeks to address key questions regarding Fe-(oxyhydr)oxide reductive transformation pathways and the importance of U sequestration into the reaction products, including: (1) What are the reaction mechanisms and kinetics of reductive Fe-(oxyhydr)oxide transformations driven by Fe(II)? Is Fe(II) incorporated in ferrihydrite (FHY) and its transformation products, e.g., goethite and lepidocrocite, magnetite (as excess Fe2+ in the latter phase)? Does dissolution occur via the recently reported surface reaction/internal electron transport/re-dissolution mechanism reported for goethite? What is the impact important groundwater solutes (Ca2+, HCO3-, Si4+) on FHY transformation mechanisms and rates? (2) Under which conditions is U structurally incorporated into transformation products, and what is the role of reductive transformation by Fe(II)? What is the oxidation state of incorporated U and neighboring Fe cation sites? We propose to use Mossbauer spectroscopy and transmission electron microscopy to address these questions. This work will be complemented by ongoing synchrotron-based X-ray PDF, XAS and powder diffraction studies by our group. Expected outcomes from this research include improved process models of Fe-(oxyhydr)oxide reduction mechanisms and their impact on natural and engineered U attenuation, and improved natural and engineered in situ reductive stabilization of U in contaminated groundwater.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2011-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members