Subsurface Conditions Controlling Uranium Incorporation in Iron Oxides: A Redox Stable Sink

Technical Report · Tue Apr 05 04:00:00 EDT 2016

DOI:https://doi.org/10.2172/1245538· OSTI ID:1245538

Fendorf, Scott ^[1]

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1. Stanford Univ., CA (United States); Stanford University

Toxic metals and radionuclides throughout the U.S. Department of Energy Complex pose a serious threat to ecosystems and to human health. Of particular concern is the redox-sensitive radionuclide uranium, which is classified as a priority pollutant in soils and groundwaters at most DOE sites owing to its large inventory, its health risks, and its mobility with respect to primary waste sources. The goal of this research was to contribute to the long-term mission of the Subsurface Biogeochemistry Program by determining reactions of uranium with iron (hydr)oxides that lead to long-term stabilization of this pervasive contaminant. The research objectives of this project were thus to (1) identify the (bio)geochemical conditions, including those of the solid-phase, promoting uranium incorporation in Fe (hydr)oxides, (2) determine the magnitude of uranium incorporation under a variety of relevant subsurface conditions in order to quantify the importance of this pathway when in competition with reduction or adsorption; (3) identify the mechanism(s) of U(VI/V) incorporation in Fe (hydr)oxides; and (4) determine the stability of these phases under different biogeochemical (inclusive of redox) conditions. Our research demonstrates that redox transformations are capable of achieving U incorporation into goethite at ambient temperatures, and that this transformation occurs within days at U and Fe(II) concentrations that are common in subsurface geochemical environments with natural ferrihydrites—inclusive of those with natural impurities. Increasing Fe(II) or U concentration, or initial pH, made U(VI) reduction to U(IV) a more competitive sequestration pathway in this system, presumably by increasing the relative rate of U reduction. Uranium concentrations commonly found in contaminated subsurface environments are often on the order of 1-10 μM, and groundwater Fe(II) concentrations can reach exceed 1 mM in reduced zones of the subsurface. The redox-driven U(V) incorporation mechanism may help to explain U retention in some geologic materials, improving our understanding of U-based geochronology and the redox status of ancient geochemical environments. Additionally, U(VI) may be incorporated within silicate minerals though encapsulation of U-bearing iron oxides, leading to a redox stable solid. Our research detailing previously unrecognized mechanism of U incorporation within sediment minerals may even lead to new approaches for in situ contamination remediation techniques, and will help refine models of U fate and transport in reduced subsurface zones.

• Research Organization: Stanford Univ., CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: SC0006772
• OSTI ID: 1245538
• Report Number(s): DOE-Stanford--SC0006772; 6507235238
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ABUNDANCE
ADSORPTION
AGE ESTIMATION
AMBIENT TEMPERATURE
AUGMENTATION
BIOGEOCHEMISTRY
CHEMICAL REACTION KINETICS
CONCENTRATION RATIO
CONTAMINATION
ENCAPSULATION
GOETHITE
IRON
IRON HYDROXIDES
IRON OXIDES
PH VALUE
RADIONUCLIDE MIGRATION
REDOX REACTIONS
REDUCTION
REMEDIAL ACTION
SEDIMENTS
SILICATE MINERALS
SINKS
SOILS
STABILITY
STABILIZATION
UNDERGROUND
URANIUM
US DOE
stabilization
subsurface
transport