Article pubs.acs.org/est
Enrichment of Cesium and Rubidium in Weathered Micaceous Materials at the Savannah River Site, South Carolina Laura K. Zaunbrecher,† W. Crawford Elliott,*,† J. M. Wampler,† Nicolas Perdrial,‡,∥ and Daniel I. Kaplan§ †
Geosciences Department, Georgia State University, Atlanta, Georgia 30302-3965, United States Department of Soil, Water, and Environmental Science, University of Arizona, Tucson, Arizona 85721, United States ∥ Department of Geology, University of Vermont, Burlington, Vermont 05405, United States § Savannah River National Laboratory, Aiken, South Carolina 29808, United States ‡
S Supporting Information *
ABSTRACT: 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 UCCnormalized 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 cationexchange 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.
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INTRODUCTION From the 1950s through the 1980s, operation of nuclear reactors for the production of nuclear materials at the Savannah River Site (SRS) in South Carolina led to generation of extensive quantities of nuclear waste. Low-level waste was buried in various kinds of vaults and trenches.1 The most abundant radiological waste constituent is 137Cs.2 The key longterm radioactive risk drivers at these facilities are commonly 129 99 I, Tc, and 14C, while the short-term risk drivers are 137Cs and 3H.1 Stable and radioactive Cs and Rb exist as weakly hydrated ions in aqueous solutions and natural waters. Recent studies stress the importance of mineral sorption of radiocesium in fallout-contaminated soils from the Fukushima Prefecture, Japan.3−5 This sorption is consistent with the well-established concept that radiocesium is held strongly at frayed-edge sites (FES) of micaceous minerals.6,7 FES occur where phyllosilicate layers are splayed apart from one another, owing to soil weathering processes, to form a unique, stereoselective environment for sorption of Cs8,9 and other large monovalent cations.10 In mineral-rich soils low in organic carbon, Cs in trace amounts is sorbed primarily by illite and related minerals. Cs distribution coefficients are correlated with edge surface areas measured from illite samples of various geologic ages.11 © 2015 American Chemical Society
The release of radiocesium recently deposited on soils relates to clay content, cation-exchange capacity (CEC), organic matter content, and soil pH.12 In some situations, not all sorbed Cs is held strongly by micaceous clay minerals. Radiocesium is readily taken up by plants from soils rich in organic matter,13 which may lessen the sorption of Cs at FES.9 Nakao et al.14 noted, however, the important role of K content as an indicator of the radiocesium interception potential of the clay fractions of rice paddy soils, where the predominant clay mineral often was smectite. They considered the K content of the clay fractions to be an indicator of the amount of micaceous clay minerals. Also, when experimental Cs concentrations are high relative to natural conditions, Cs is held by micaceous minerals as partially dehydrated ions (inner sphere complexes) on planar siloxane surfaces and as hydrated ions (outer sphere complexes) but not held as strongly as at FES.15,16 At the SRS and throughout the Atlantic Coastal Plain and the Piedmont Province, muscovite has been transformed during Received: Revised: Accepted: Published: 4226
November 8, 2014 February 28, 2015 March 5, 2015 March 5, 2015 DOI: 10.1021/es5054682 Environ. Sci. Technol. 2015, 49, 4226−4234
Article
Environmental Science & Technology
fractions by standard timed settling techniques. The percentage of each size fraction was calculated from its mass after drying, except that the mass of the clay fraction was determined by difference. The amounts of fine silt and coarse silt were combined for texture analysis. One untreated portion of each soil core segment and of each sediment sample was dry-sieved to obtain the Sr ≈ K. The range of normalized values from Cs to Sr and K is largenearly 2 orders of magnitude in the two moremature cores and >1 order of magnitude in FQQAL. The amounts extracted by acid reflect the differences in behavior of these elements during pedogenesis more clearly than do the
total amounts in the fine fractions discussed in reference to Figure 3. Downward in each core, the UCC-normalized amounts of the large-ion elements extracted by acid from the fine fractions approximately parallel calculated values of fine-fraction CEC (Figure 4) with a few clear exceptions. The approximate proportionality of acid-extractable K, Sr, and Ba to calculated CEC is understandable because these more abundant or more mobile elements (of those studied) should have been sorbed largely as ordinary exchange cations. Too much K was extracted (10% or more of the calculated CEC) for it to have been held mostly by the FES, which should account for
