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

Abstract

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.

Molecular modeling of the effects of 40Ar recoil in illite particles on their K–Ar isotope dating

“…laboratory cation exchange, that is supported by e.g. Cs + fixation in illite (Wampler et al, 2012;Fuller et al, 2014). Removal of potassium from the illite fringes is expected to be more difficult by common cation exchange procedures than from the surface of illite crystallites due to strong electrostatic forces retaining potassium in the site.…”

“…The cation exchange procedure with low-hydration (high attraction to the K + position) cations that can easily substitute interlayer K + , i.e. Cs + or Rb + (Wampler et al, 2012;Fuller et al 2014) may thus be suggested for the most complete potassium removal from the low-Ar retention sites. Such experiments could verify the results of this study.…”

“…Derkowski et al, 2009). Some potassium is always removed from the illite crystallite fringes, but there is always some K left, strongly attached, despite the procedure involved (Aronson and Douthitt, 1986;Wampler et al, 2012).…”

Identification of Multiple Cs+ Adsorption Sites in a Hydroxy-interlayered Vermiculite-like Layered Silicate through 133Cs MAS NMR Analysis

“…This experiment suggests that Cs + ions adsorb on two sides of the hydroxylated aluminium interlayers: the base side near hydrated Al 3+ and the apex side near the mica zone of the acute isosceles triangle-like wedge zone. 7 In conclusion, the hydroxy-interlayered vermiculite (HIV)-like structures formed when phlogopite is altered with hydrated Al 3+ affect the adsorption of Cs + ions. Cesium ions in HIV-type layered silicate reside at sites not only in mica zones but in interlayer wedge zones of Al-Phl.…”

“…The Al 3+ in these Al-Phl structures underwent minimal replacement with Cs + ions, but the Al-Phl exhibited high partition coefficients (K d ) at lower Cs concentrations, indicating significant selectivity for Cs. 5,7 When the remnant mica transforms to vermiculite, the wedges in the silicate interlayers are splayed. These vermiculite interlayer wedges have been considered equivalent to those frayed edges of less thoroughly weathered mica particles, and should be similarly selective for large-ion alkali metals such as Cs + (Figure 1).…”