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Rocky Mountain Section - 57th Annual Meeting (May 23–25, 2005)

Paper No. 5
Presentation Time: 4:15 PM

THE ROLE OF PCO2, PH AND GROUNDWATER COMPOSITION ON NEPTUNIUM(V) ADSORPTION TO NATURITA ALLUVIAL AQUIFER SEDIMENT: COMPARISON TO URANIUM(VI)


BUCKLEY, Patricia B.1, HONEYMAN, Bruce D.2, DAVIS, James A.3 and RANVILLE, James F.1, (1)Department of Chemistry and Geochemistry, Colorado School of Mines, Laboratory for Applied and Environmental Radiochemistry, Golden, CO 80401, (2)Environmental Science and Engineering, Colorado School of Mines, Laboratory for Applied and Environmental Radiochemistry, Golden, CO 80401, (3)United States Geological Survey, Menlo Park, CA 94025, pbuckley@mines.edu

Np(V) adsorption to untreated and treated alluvial aquifer sediment was studied in batch experiments under pH, PCO2 and groundwater conditions similar to those observed at the former uranium mill site at Naturita, Colorado (USA). Treated sediment underwent carbonate removal using sodium acetate buffered at pH 5 with acetic acid. Untreated sediment was largely composed of quartz, feldspars and calcite, with lesser amounts of magnetite and clay minerals [1]. Sediment grains exhibited illite/smectite clay coatings with occluded ferrihydrate and goethite [2].

Steady-state concentrations of added Np(V) were reached within 24 hours. Np(V) adsorption to untreated and treated sediment decreased considerably as PCO2 increased while the percent adsorbed decreased as Np(V) concentrations increased. Np(V) adsorption to treated sediment was reduced compared to adsorption to untreated sediment, possibly a result of partial dissolution of the clay coatings due to the extraction procedure. Np(V) adsorption to untreated sediment was appreciably lower in artificial groundwater than 0.1 NaClO4. This reduction was likely a consequence of competition for surface sites between Ca2+ and Mg2+ and aqueous neptunyl species. Np(V) adsorption edges for the treated sediment over a range of pH values and variable PCO2 were obtained. Consistent with the formation of aqueous neptunyl carbonato complexes, adsorption edges showed a decrease in adsorption with increasing pH and PCO2. A comparison of Np(V) adsorption to U(VI) adsorption studies using Naturita alluvial aquifer sediment will be presented [1, 3].

[1] Davis, J. A. et al. (2004) Approaches to Surface Complexation Modeling of Uranium(VI) Adsorption on Aquifer Sediments. Geochimica et Cosmochimica Acta. 68:3621-3641.

[2] Jove-Colon C. F. et al. (2004) A Combined Analytical Study to Characterize Uranium Soil Contamination: The Case of the Naturita UMTRA Site and the Role of Grain Coatings. U.S. Nuclear Regulatory Commission, in press.

[3] Sanpawanitchakit, C. (2002) The Application of Surface Complexation Modeling to the Adsorption of Uranium (VI) on Natural Composite Materials. PhD Thesis, Colorado School of Mines, Environmental Science and Engineering, Golden, CO, USA.

Session No. 6

S3. Uranium Legacy of Western Colorado and Eastern Utah
Monday, 23 May 2005: 1:00 PM-5:00 PM
Geological Society of America Abstracts with Programs. Vol. 37, No. 6, p.11
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