ENRICHMENT OF STABLE CS AND RB IN SAVANNAH RIVER SITE SOILS

The Savannah River Site (SRS) in South Carolina is the focus of an extensive remediation project. Cs-137 is a well-known fission product that has been produced, stored, and handled in this and other US Department of Energy facilities. Documenting the mobility of ¹³⁷Cs and its sorption by geologic materials is important to predict long-term behavior of ¹³⁷Cs migrating from contaminated sites such as the SRS.

Three uncontaminated (radionuclide free) soil cores were used to study Cs and other alkali metals in the SRS soils. Based on the mineralogy, clay content, and parent material, each core is known to represent a different stage of weathering. For each core, depth profiles of (1) mineral composition, (2) acid-extractable Cs, Rb, K, Sr, and Ba ions, and (3) clay content were collected. These data permit a test of our hypotheses that (1) Cs has been favorably enriched with respect to K in the soils as a result of long-term weathering processes and (2) that this Cs will affect the ability of soils at the SRS to adsorb ¹³⁷Cs and (3) interlayer wedge zones in highly weathered mica particles, now mostly hydroxy-interlayered vermiculite (HIV), may be responsible for selective uptake of Cs and Rb from soil solution and for their fixation.

Rietveld modeling of synchrotron powder X-Ray diffractograms provides a quantitative determination of the clay fraction (< 2um). Acid extractions of ions from soil fine fractions show that the ion concentrations vary sympathetically with depth. The peak in extracted ion concentration in each core peaks not far below the point where the clay content becomes more than 10% of the soil (going downward) and thus seems to be controlled more by clay content than by clay mineralogy.

Stable Cs is significantly enriched with respect to K in all three soil cores. During soil formation, K is increasingly removed as weathering progresses, and Cs and Rb are preferentially retained in the soils. The longer a soil has undergone weathering at the SRS, the more enriched it has become in Cs/K (and Rb/K) with respect to the average for upper continental crust. Preliminary molecular modeling results support the hypothesis that Cs is the sterically most favored ion in the interlayer wedge zone of HIV. These preliminary results suggest that Cs can be sorbed effectively at the wedge-apex sites of HIV by electrostatic attraction.