Speciation of Iron and Uranium in Sediments Collected at Savannah River National Laboratory
Authors
Maxim Boyanov1,2* (mboyanov@anl.gov), Edward O’Loughlin1, Pamela Weisenhorn1, Anthony Boever3, Martial Taillefert3, Brian Powell4, Connor Parker4, Daniel Kaplan4, Kenneth Kemner1
Institutions
1Argonne National Laboratory, Lemont, IL; 2Bulgarian Academy of Sciences, Sofia, Bulgaria; 3Georgia Institute of Technology, Atlanta, GA; 4Clemson University, Clemson, SC; 5University of Georgia, Athens, GA
URLs
Abstract
The M Area Fuel Fabrication Facility at the Savannah River Site manufactured nuclear fuel and target assemblies between 1954 to 1989, resulting in significant discharge of uranium and co-contaminants (e.g., nickel, chromium, zinc, and lead) into Tims Branch (a small second-order stream) and associated riparian wetlands. The natural processes occurring in these environments are complex and control elemental dynamics through various reactions, such as adsorption, precipitation, and particle transport. The Argonne (ANL) Wetland Hydro-biogeochemistry SFA focuses on providing the molecular-level information for relevant biogeochemical reactions to enable their inclusion in predictive models of the system. To better understand the redox dynamics in wetland sediments researchers collected ~30 cores from saturated and unsaturated locations along Tims Branch and characterized them at the Advanced Photon Source. Iron (Fe) and uranium (U) were primarily in their oxidized forms in near-surface sediments that remained dry. In below-surface, organic-rich areas of saturated sediments, researchers found reduced Fe species that are present in clays, oxides, and associated with organic matter. Uranium found in the same areas was reduced to U(IV) in the form of adsorbed complexes (i.e., not mineralized). Molecular U(IV) species are not yet described in thermodynamic databases and therefore not considered in transport models. This disconnect between modeled and actual species is likely to cause significant uncertainties in the predicted behavior of U in such environments. The dynamic redox nature of wetlands further complicates predictive understanding of the system—in gaining stretches of the stream, the Fe(II) in emerging groundwater oxidizes to form visible orange flocs, which Fe extended X-ray absorption fine structure spectroscopy determined to be composed of ferrihydrite and lepidocrocite. The oxides and contaminants found in these Fe flocs can experience redox transformations, which will affect their stability and transport as particles and as dissolved species. In addition to characterizing the flocs collected at the field site, researchers are also carrying out incubation experiments in the laboratory to determine the molecular speciation of Fe and the contaminants under alternating redox conditions (see ANL SFA poster by O’Loughlin et al.).