Use of Computerized Microtomography to Examine the Relationships of Sorption Sites in Alluvial Soils to Iron and Pore Space Distributions (NUREG/CR-6784)

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Publication Information

Manuscript Completed: June 2002
Date Published: July 2002

Prepared by:
A.A. McLain¹, SJ. Altman¹, M.L.Rivers², and R.T. Cygan³

¹Geohydrology Department, Sandia National Laboratories, Albuquerque, NM 87185-0735
²CARS, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637
³Geochemistry Department, Sandia National Laboratories, Albuquerque, NM 87185-0750

Sandia National Laboratories
Albuquerque, NM 87185-0750

E. O'Donnell, NRC Technical Monitor

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 W6811

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Abstract

This report presents results from computerized microtomographic (CMT) imaging of soils from Naturita, Colorado, a Uranium Mill Tailings Remedial Action (UMTRA) site. The objective is to better understand the mineralogical and morphological controls of the sorption of uranium in the sediment. The samples were prepared and imaged with three goals in mind: 1) to visualize iron-rich mineral distribution in fine-grained materials (<355 µm grain size) and aggregate grains, 2) to visualize pore space in the aggregates, and 3) to visualize cesium sorption sites as a proxy for uranium. Two sets of images were obtained with voxel sizes of 6.7 µm and 3.3 µm on a side, respectively. Bright rims with a thickness of 4 to 15 µm were observed on fine-grained materials and aggregate grains. Porous, iron-rich interstitial material was also observed within the aggregates. It is speculated that these interstitial materials are clay minerals closely associated with some iron oxyhydroxide phases or iron oxyhydroxides that are porous at the sub-micron scale. However, to verify this identification, the CMT data needs to be post-processed using calibration materials to minimize noise and error. A subtraction technique using images obtained just above and below the X-ray absorption edges of iodine and cesium successfully illustrates the pore space distribution in the KI-saturated samples and sorption sites in CsCl-treated samples. The cesium sorption sites appear to be associated with accessible pore space observed in the untreated and KI-saturated samples. The cesium sorption sites are also associated with iron-rich areas both as exterior coatings and in the interstitial areas of the aggregate grains.