GEOC 40 |
| Tao Cheng1, Mark O. Barnett1, and Eric E. Roden2. (1) Department of Civil Engineering, Auburn University, 238 Harbert Engineering Center, Auburn, AL 36849, (2) Department of Biological Sciences, University of Alabama, A122 Bevill Bldg 7th Ave, Tuscaloosa, AL 35487-0206 |
| The complex pH-dependent interactions of U(VI) with ligands and surfaces are typical of the complex processes that control actinide mobility in the subsurface. The adsorption of U(VI) by Fe(III)-oxides such as goethite can dominate the partitioning of U(VI) and control uranium mobility in oxic environments. Geochemical conditions, particularly pH and complexing ligand concentrations, have profound and sometimes unexpected impacts on U(VI) surface complexation. Among the inorganic ligands, the effects of dissolved inorganic carbon have been studied extensively. Phosphate is a much stronger ligand, which has been shown to control the subsurface mobility of U(VI) at sites that are used as natural analogues of nuclear waste repositories. However, there have been few studies on the effects of phosphate on U(VI) adsorption. We studied U(VI) adsorption on goethite-coated sand (to mimic natural Fe-coated subsurface materials) as a function of pH in systems closed to the atmosphere, in the presence and absence of added phosphate. Below pH 5, the adsorption of U(VI) increases in the presence of 10-4 mol/L total phosphate. The phosphate is strongly bound by the goethite surface in this low pH range and the increased uptake of U(VI) is attributed to the formation of ternary surface complexes involving both UO22+ and PO43-. In the high pH range, the adsorption of U(VI) decreases in the presence of phosphate and the majority of phosphate is in the dissolved form. This decrease in U(VI) adsorption is attributed to the formation of dissolved complexes between UO22+ and PO43-. A surface complexation model (SCM) that incorporates the interactions involving UO22+ and PO43- can successfully predict U(VI) adsorption in the presence of phosphate under a range of conditions. |
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Chemistry of Metals in Terrestrial and Aquatic Systems
1:30 PM-5:10 PM, Monday, March 29, 2004 Marriott -- Marquis NE, Oral
Division of Geochemistry |