ENRICHMENT OF STABLE CESIUM AND RUBIDIUM IN SAVANNAH RIVER SITE SOILS

The mineralogy, soil texture, and chemical enrichment of elements (Cs, Rb, K, Ba, and Sr) are examined in three uncontaminated (radionuclide–free) soils cores from the Savannah River Site (SRS), SC. The SRS is the focus of an extensive remediation project by the US Department of Energy; due the production and storage of ¹³⁷Cs and other radionuclides at this facility. Documenting the mobility of ¹³⁷Cs and its sorption by geologic materials is important to predict long-term behavior of ¹³⁷Cs migrating from the SRS and other contaminated sites.

Unique depth profiles of (1) mineral composition, (2) soil texture, and (3) acid-extractable alkalis were collected. Mineralogical characterization of the soils suggests that each core represents a different stage of weathering progression. These data enable a test of our hypotheses that (1) Cs has been favorably enriched wrt K in the soils as a result of long-term weathering, (2) this Cs will affect the ability of soils at the SRS to adsorb ¹³⁷Cs and, (3) interlayer wedge zones in 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.

Clay (<2µm) and fine size (<62µm) fractions of the soils were separated for mineralogical and chemical analyses, respectively. Rietveld modeling of synchrotron diffraction patterns quantified the minerals HIV, illite, gibbsite, kaolinite, and quartz. Acid extractions show ion concentrations vary sympathetically with depth. A strong correlation is observed with alkali metals and the clay percentage in the soil, rather than mineralogy. The peak in extracted ion concentration corresponds to where soil texture becomes >10% clay.

Stable Cs is significantly enriched with respect to K in all three soil cores. K is increasingly removed as weathering progresses, and Cs and Rb are preferentially retained. The Cs/K and Rb/K ratio of acid-leachates are interpreted to show that SRS soils have sorbed and retained Cs and Rb in non-exchangeable sites in clay minerals during pedogenesis.

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.