United States Nuclear Regulatory Commission - Protecting People and the Environment

A Combined Analytical Study to Characterize Uranium Soil and Sediment Contamination: The Case of the Naturita UMTRA Site and the Role of Grain Coatings (NUREG/CR-6898)

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Publication Information

Manuscript Completed: August 2005
Date Published: February 2006

Prepared by:
C. F. Jové Colón1, C. Sanpawanichakit2, H. Xu3,
R.T. Cygan1, J.A. Davis4, D.M. Meece4, R.L. Hervig5

1Sandia National Laboratories
P.O. Box 5800, Mail Stop 0735
Albuquerque, New Mexico 87185-0735
2Colorado School of Mines
Environmental Science and Engineering
Golden, Colorado 80401
3Department of Earth and Planetary Sciences
University of New Mexico
Albuquerque, New Mexico 87131

4Water Resources Division
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
5Department of Geological Sciences
Arizona State University
Tempe, Arizona 87287-1404

E. O'Donnell, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6464

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Abstract

Composite sediment samples from the Uranium Mill Tailings Remedial Action site at Naturita, Colorado, were analyzed for uranium using a suite of microbeam analytical techniques encompassing Scanning Electron Microscopy—Energy Dispersive Spectrometry (SEM-EDS), Secondary Ion Mass Spectrometry (SIMS), High-Resolution Transmission Electron Microscopy (HRTEM), Micro-Synchrotron X-Ray Fluorescence (M-SXRF), and Micro-X-ray Absorption Near-Edge Spectroscopy (M-XANES). Two sets of alluvial sediment samples were considered in this study: an untreated composite sediment sampled from several uranium contaminated wells, and a carbonate-free (treated) composite sediment from an area significantly up-gradient from the contaminated portion of the site. The carbonate-free sample was treated with Na-acetate to remove carbonate material and then subsequently exposed to 10-5 molal uranyl (U6+) nitrate solution. The purpose of the treatment was to investigate uranium adsorption onto the sediment without complications resulting from carbonate complexation with uranyl ions. In all samples, SEM-EDS analysis showed the conspicuous presence of Fe-rich and Al-Si rich (clay) coating layers surrounding the periphery of soil/sediment grains. The bulk grains that serve as substrate to the overlying coatings are mostly quartz and detrital feldspar. The Fe-rich coatings are arranged conformably in both continuous and discontinuous modes, in some cases between the quartz interface and the clay-rich region. However, Fe-rich phases are also present as small scattered particles immersed in the clay layer. SIMS analysis on polished epoxy grain mounts of the naturally-contaminated composite sample reveals the presence of uranium diffusely distributed within the Al-Si rich clay layer. No clear association between Fe and uranium from materials collected in sampling wells with the highest level of uranium contamination was discerned using this analytical technique. However, M-SXRF analyses on the laboratory contaminated carbonate-free sample reveals a close association between uranium and Fe-rich domains on the grain surface. HRTEM analysis on a single grain from the carbonate-free sample characterized by this strong uranium-Fe spatial correlation indicate that these Fe-bearing phases are highly heterogeneous, composed mainly of mixed domains of hematite, goethite, and nanoporous and/or amorphous Fe-(oxy)hydroxides. These Fe-rich nanoporous and amorphous domains within larger Fe-bearing grains are identified as ferrihydrite on the basis of HRTEM observations. The arrangement of Fe-rich amorphous phase domains along with crystalline goethite resembles aggregated textural forms. However, HRTEM analysis indicates that these domains are structurally coherent and continuous, suggesting homogeneous transformation. Mixed layer illite/smectite (I/S) clays are to a great extent the main coating phases present hosting nanosized Fe and Ti oxides. On the basis of these combined analytical observations, mixed layer clays and Fe-rich coatings are the main sinks for uranium in the composite sediment material at Naturita.

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