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.

Molecular models of cesium and rubidium adsorption on weathered micaceous minerals.

Understanding the adsorption mechanisms of metal cations onto soils and sediments is of critical importance in the protection of the environment, especially for the case of radioactive materials including the fission product (137)Cs. Mechanism-based adsorption models for the long-term interaction of chemical and radionuclide species with clay minerals are needed to improve the accuracy of groundwater reaction and flow models, as well as related simulations for performance assessment of waste sites and repositories. Toward this goal, molecular simulation using geometry optimization and molecular dynamics methods have been used to investigate the adsorption behavior of Cs(+) and Rb(+) cations at frayed edge wedges (a proxy for frayed edge sites, FES) and in the interlayer region formed as a result of the transformation of muscovite to Al-hydroxy interlayered vermiculite (HIV) during weathering and pedogenesis. Frayed edge wedges, formed both on individual smectite and illite phases and on the mica-HIV intergrade, have previously been recognized as significant sinks for the strong adsorption of Cs(+) and Rb(+). Atomic density profiles, interlayer adsorption site maps, radial distribution functions, and adsorption enthalpies derived from the equilibrated structural models are used to evaluate the optimal adsorption configurations and thermodynamics for Cs- and Rb-endmembers, a 50:50 Cs-Rb composition for the aqueous interlayer of vermiculite, and for the interlayer wedge zone as mica is transformed to HIV (i.e., HIV-mica wedge). Adsorption enthalpies for both cations are significantly larger for the frayed edge wedges (as represented by the HIV-mica wedge model) compared to values for the vermiculite and mica interlayers. Cesium cation binds more strongly than Rb(+) in the vermiculite interlayer, while Rb(+) binds more strongly than Cs(+) in the HIV-mica wedge. In all cases, the derived adsorption enthalpies for both cations indicate a preference for the wedge environment where electrostatic interaction is enhanced due to the presence of layer charge and the increased size of interlayer at the wedge accommodating cations larger than K(+).

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.

Phosphorus K-edge XANES spectroscopy of mineral standards.

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy was performed on phosphate mineral specimens including (a) twelve specimens from the apatite group covering a range of compositional variation and crystallinity; (b) six non-apatite calcium-rich phosphate minerals; (c) 15 aluminium-rich phosphate minerals; (d) ten phosphate minerals rich in either reduced iron or manganese; (e) four phosphate minerals rich in either oxidized iron or manganese; (f) eight phosphate minerals rich in either magnesium, copper, lead, zinc or rare-earth elements; and (g) four uranium phosphate minerals. The identity of all minerals examined in this study was independently confirmed using X-ray powder diffraction. Minerals were distinguished using XANES spectra with a combination of pre-edge features, edge position, peak shapes and post-edge features. Shared spectral features were observed in minerals with compositions dominated by the same specific cation. Analyses of apatite-group minerals indicate that XANES spectral patterns are not strongly affected by variations in composition and crystallinity typical of natural mineral specimens.

Sorption and desorption of radiocesium by muscovite separated from the Georgia kaolin.

Radiocesium (137Cs) sorption by moderately weathered, sand-sized muscovite, obtained as a byproduct of kaolin ore processing, was observed at various concentrations of added stable Cs (0-100 µmol/L) over a 130 d period. After 18 h of batch sorption with 1 mmol/L NaCl as background electrolyte, conditional 137Cs Kd values were near 2000 L/kg across the entire range of added stable Cs. Over four succeeding months, the Kd values increased by large factors for suspensions with little added Cs but increased only slightly for the suspensions with the most added Cs. The large grains of muscovite used in this study behaved distinctly differently than previously studied, much finer illite in that highly Cs-selective but low-abundance cation exchange sites appeared to be unavailable to the aqueous Cs during the first few days of the experiment. Diffusion pathways to highly Cs-selective sites were thought to be much longer in the muscovite than in frayed edges of illite, causing the highly Cs-selective sites to be isolated from the bulk solution. The longer diffusion pathways may be due to much greater stiffness of the material bounding interlayer wedges in the muscovite than in illite. This isolation from solution led to slow uptake at trace levels of Cs though the final Kd values (after 130 d) at those levels were comparable to those found for some illite. After 130 days, the original solutions were replaced by new electrolyte solutions containing no Cs, to observe 137Cs desorption over another 130 d period. There was no indication of desorption of 137Cs from the slowly accessible, highly Cs-selective sites apparently reached by most of the 137Cs during sorption at the low Cs levels. The byproduct mica from kaolin processing might serve effectively as a chemically stable sorbent to isolate accidently released radiocesium and to hold it until the 137Cs is virtually gone.

Growth of thermophilic and hyperthermophilic Fe(III)-reducing microorganisms on a ferruginous smectite as the sole electron acceptor.

Recent studies have suggested that the structural Fe(III) within phyllosilicate minerals, including smectite and illite, is an important electron acceptor for Fe(III)-reducing microorganisms in sedimentary environments at moderate temperatures. The reduction of structural Fe(III) by thermophiles, however, has not previously been described. A wide range of thermophilic and hyperthermophilic Archaea and Bacteria from marine and freshwater environments that are known to reduce poorly crystalline Fe(III) oxides were tested for their ability to reduce structural (octahedrally coordinated) Fe(III) in smectite (SWa-1) as the sole electron acceptor. Two out of the 10 organisms tested, Geoglobus ahangari and Geothermobacterium ferrireducens, were not able to conserve energy to support growth by reduction of Fe(III) in SWa-1 despite the fact that both organisms were originally isolated with solid-phase Fe(III) as the electron acceptor. The other organisms tested were able to grow on SWa-1 and reduced 6.3 to 15.1% of the Fe(III). This is 20 to 50% less than the reported amounts of Fe(III) reduced in the same smectite (SWa-1) by mesophilic Fe(III) reducers. Two organisms, Geothermobacter ehrlichii and archaeal strain 140, produced copious amounts of an exopolysaccharide material, which may have played an active role in the dissolution of the structural iron in SWa-1 smectite. The reduction of structural Fe(III) in SWa-1 by archaeal strain 140 was studied in detail. Microbial Fe(III) reduction was accompanied by an increase in interlayer and octahedral charges and some incorporation of potassium and magnesium into the smectite structure. However, these changes in the major element chemistry of SWa-1 smectite did not result in the formation of an illite-like structure, as reported for a mesophilic Fe(III) reducer. These results suggest that thermophilic Fe(III)-reducing organisms differ in their ability to reduce and solubilize structural Fe(III) in SWa-1 smectite and that SWa-1 is not easily transformed to illite by these organisms.

Freundlich and dual Langmuir isotherm models for predicting 137Cs binding on Savannah River Site soils.

Distribution of 137Cs and stable cesium between aqueous solution and near-surface soil samples from five locations at the Savannah River Site was measured in order to develop a predictive model for 137Cs uptake by the soils. Sorption of 137Cs in these soils appears to be mostly by hydroxy-interlayered vermiculite. Batch sorption studies with 4 d for equilibration were conducted at three cesium concentrations and at two backing electrolyte (NaNO3) concentrations. The soil-solution mixtures were pH-adjusted to evaluate the effects of pH on cesium sorption. Sorbed cesium was related to the equilibrium aqueous cesium concentrations by a Freundlich isotherm model. Model fits on logarithmic scales have a common slope of 0.60 +/- 0.03 for acidic mixtures and 0.69 +/- 0.04 for neutralized mixtures but have unique intercepts that are influenced by backing electrolyte concentration and pH. An ion-exchange model is proposed that pertains to all five soils and relates the Freundlich isotherms to the cation exchange capacity of soil and the aqueous concentrations of cesium, sodium, and a third ionic species that was hydrogen in the acidic mixtures and potassium in the neutralized mixtures. Model fits are consistent with Kd values in the entire range of 5-2,300 L kg(-1) determined for the five soil types. As an alternate model, dual Langmuir isotherms were fitted to the data. The results suggest cesium sorption by (1) relatively few interlayer-wedge sites, highly selective for cesium, and (2) much more abundant but less selective sites on internal and external planar surfaces.