Monday, November 13, 2006

A Fractional Packing Concept to Describe the Rate and Extent of Uranium(VI) Adsorption in Polydispersed Systems.

Kathryn E. Draper, Andy L. Ward, Nik Qafoku, Ray E. Clayton, and Steve B. Yabusaki. Pacific Northwest National Laboratory, P.O. Box 999 MSIN K6-75, Richland, WA 99352

Field-scale models of coupled processes require defensible geochemical parameters that capture the transition from small, idealized sample geometries in the laboratory to large field domains with multiscale heterogeneities. However, the upscaling of geochemical parameters such as distribution coefficients, exchange capacity, and reactive surface area has received little attention. A major source of the discrepancy between laboratory- and field-scale reaction parameters appears to come from the fact that the characterization of processes and properties is typically based on the < 2 mm fraction rather than the entire particle size distribution found in the field. In this study, we investigate U(VI) sorption onto different sediment size fractions to better understand the role of surface area on sorption onto bulk sediments. The kinetics and equilibria of U(VI) sorption in monodispersed and polydispersed systems of increasing fine fractions were observed in stirred solutions containing 100 ppb of U(VI). Finer textured sediments reacted quickly, generally less than 1 h, but appeared to overshoot the eventual adsorption maximum and were ultimately released U(VI) over time (> 24 hrs). Adsorption maxima on coarse sediments were low but nonzero. Adsorption maxima in the polydispersed systems increased with increasing fines content, but the relationship was not linear. The maxima depended on whether the system was grain supported (fine grains fit within voids created by large grains) or matrix supported (coarse particles dispersed in the fine-grained matrix). The piece-wise linear relationship showed an inflection around the point where the volume for fine-grained particles equaled the porosity of the coarse-grained component. An upscaling methodology will be presented that describes U(VI) sorption based on a fractional packing approach. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC05-76RL01830.