An experimental and modeling investigation of UO{sub 2}{sup 2+} interaction with smectite clays

Conference · Fri Dec 31 04:00:00 EST 1993

OSTI ID:143136

McKinley, J P; Zachara, J M; Smith, S C ^[1]

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1. Pacific Northwest Laboratory, Richland, WA (United States)

Smectite clays are a common and abundant component of soils and sediments, and thus comprise an important determinant of uranium mobility in ground water systems. Experimental observations of uranyl (UO{sub 2}{sup 2+}) interaction with a series of natural smectite clays in carbonate free electrolyte (0.1, 0.01, 0.001 M NaClO{sub 4}) over a range in pH of 4-8, were modeled using the computer modeling program FITEQL. A conceptual model was developed that was consistent with the experimental data and that was parameterized, in part, using UO{sub 2}{sup 2+} sorption data on silica and gibbsite. The model included: (1) aqueous uranyl speciation as a control of surface-bound species, (2) competitive exchange of uranyl at fixed charged sites (X-); and (3) complexation of aqueous species by two amphoteric edge sites (SOH). At low pH, the ionic strength-dependent sorption behavior of uranyl was explainable by competitive cation exchange of uranyl and its hydrolyzed species with sodium on fixed-charge sites. At high pH, where ionic strength does not significantly affect aqueous/solid partitioning, adsorption was modeled to include cation exchange and complexation by edge sites. Edge-site complexation included UO{sub 2}{sup 2+} and (UO{sub 2}){sub 3}(OH){sub 5+}, the dominant aqueous species at pH>6.0; adsorption of UO{sub 2}{sup 2+} alone could not adequately account for observed results. The proportional concentrations of edge (SOH) and fixed charge site (X-), as determined by their particle morphology, accounted for differences in sorption between the different smectites. Adsorption of uranyl can thus be successfully modeled using relatively simple and logical assumptions concerning the chemical and physical conditions of the adsorbate/adsorbent system.

• OSTI ID: 143136
• Report Number(s): CONF-930304--
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
CHEMICAL REACTIONS
COMPUTERIZED SIMULATION
GEOCHEMISTRY
GROUND WATER
SMECTITE
SORPTION
URANIUM OXIDES
URANYL COMPLEXES