Long-Term Selective Retention of Natural Cs and Rb by Highly Weathered Coastal Plain Soils

Naturally occurring Cs and Rb are distinctly more abundant relative to K in the highly weathered upland soils of the Savannah River Site, South Carolina, than in average rock of Earth’s upper continental crust (UCC), by factors of 10 and 4, respectively. Naturally occurring Cs has been selectively retained during soil evolution, and Rb to a lesser extent, while K has been leached away. In acid extracts of the soils, the Cs/K ratio is about 50 times and the Rb/K ratio about 15 times the corresponding ratios for the UCC, indicating that relatively large amounts of natural Cs and Rb have been sequestered in soil microenvironments that are highly selective for these elements relative to K. Cation exchange favoring Cs and Rb ions, and subsequent fixation of the ions, at sites in interlayer wedge zones within hydroxy-interlayered vermiculite particles may account for the observations. The amounts of stable Cs retained and the inferred duration of the soil evolution, many thousands of years, provide new insights regarding long-term stewardship of radiocesium in waste repositories and contaminated environments. Study of natural Cs in soil adds a long-term perspective on Cs transport in soils not available from studies of radiocesium

Enrichment of Cesium and Rubidium in Weathered Micaceous Materials at the Savannah River Site, South Carolina

The enrichment of Cs and Rb relative to Ba, Sr, and K in three soils representing a range of soil maturities was determined to investigate the long-term sorption behavior of these elements in upland soils of the Savannah River Site (SRS). Elemental mass fractions normalized to upper continental crust (UCC) decreased in the order Cs > Rb > Ba > K > Sr in the soil fine fractions. Only the UCC-normalized amount of Cs was greater than unity. The UCC-normalized amounts in strong-acid extracts decreased as Cs > Rb > Ba > K ≈ Sr. In all three soil cores, the trends of the UCC-normalized amounts of acid-extractable metals were similar to trends of cation-exchange capacity (CEC) calculated from synchrotron-X-ray diffractometry measurements of soil mineralogy. Consequently, the relative enrichment of Cs and Rb is largely controlled by selective sorption to micaceous minerals, including hydroxy-interlayered vermiculite, that dominate the CEC. Where high clay content had caused retention of soil solution, amounts of acid extractable K, Sr, and Ba were enhanced. The retention of natural Cs by these three soils, which developed over many thousands of years, is a strong indicator that radiocesium will likewise be retained in SRS soils