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July 5, 2006

Strontium speciation during reaction of kaolinite with simulated tank-waste leachate: Bulk and microfocused EXAFS analysis

S. Choi^1,2, P.A. O'Day², N.A. Rivera², K.T. Mueller³, M.A. Vairavamurthy⁴, S. Seraphin⁵, and J. Chorover^1
1Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ; ²School of Natural Sciences, University of California, Merced, CA; ³Department of Chemistry, The Pennsylvania State University, University Park, PA; ⁴Brookhaven National Laboratory, Upton, NY; ⁵ Department of Materials Science and Engineering, University of Arizona, Tucson, AZ

Radioactive strontium (⁹⁰Sr) is an important constituent of complex wastes produced from past nuclear weapons production and stored in underground tanks at DOE sites (e.g., Hanford, WA). Using bulk and microfocused EXAFS spectroscopy, we examined temporal changes in solid phase Sr speciation in kaolinite samples reacted for 1 to 369 days with high-pH, high ionic strength synthetic tank-waste leachate containing Sr²⁺ and Cs⁺. Our results and supporting characterizations show that Sr forms a transient carbonate phase at early reaction times that is replaced by incorporation into neoformed, cage-type feldspathoid aluminosilicate minerals, with Sr becoming dehydrated and non-extractable with longer reaction times. Formation of feldspathoid minerals as products of sediment alteration may help sequester contaminants at sites such as Hanford.

Authors Jon Chorover (left) and Sunkyung Choi.

The remediation of radioactive contamination at former weapons testing and production sites is a costly and protracted legacy of the U.S. post-WWII military industrial complex. ⁹⁰Sr and ¹³⁷Cs are important contaminants at a number of Department of Energy (DOE) sites, and particularly at the Hanford (WA) site, because their half-lives fall within human timescales (29 and 30 years, respectively), they are potentially mobile in groundwater, and they are bioavailable substituents for Ca²⁺ and K⁺, respectively, in organisms. Large volumes of high-level radioactive waste and contaminant metals were generated from plutonium production and separation processes, much of which was stored in underground tanks. At least 67 single-shell tanks containing high-level wastes are known to have leaked into vadose zone sediments, where caustic fluids have reacted with sediment minerals and groundwater to generate a complex history of mineral alteration, fluid evolution, and waste dispersal.

Figure 1. From left: SEM images, synchrotron Sr X-ray microprobe maps, and microfocused Sr K-edge EXAFS of three neoformed particles from a kaolinite sample reacted for 33 days with synthetic tank-waste leachate. A bulk EXAFS spectrum of the sample is shown for comparison. The bulk spectrum was collected at the NSLS (beamline X-18B) and the microfocused spectra were collected at the APS (GSECARS beamline 13-ID-C). Particles were dispersed on carbon tape and imaged with SEM to identify isolated Sr-bearing particles prior to synchrotron analyses. X-ray element maps were made prior to EXAFS data collection in order to re-locate the Sr-bearing particles.

Figure 2. (a) Atomic structure of basic sodalite, which has a single cage and cation site. Sr (yellow atoms) occupies the Na site; surrounding anions and water within cages are omitted for clarity; (b) Nitrate cancrinite has two distinct cation sites in large (sodalite-type) and small (cancrinite-type) cages; Sr is shown occupying Na sites within the sodalite cage, nitrate groups occupy the counter-anion position in the center of the cage, and Na and water are shown in the cancrinite cage. Nearest interatomic cation-cation positions in the ideal structures are indicated.

Our research team has examined the reaction of high-pH, high ionic strength synthetic tank-waste leachate (STWL) with model clay minerals and Hanford sediments in the laboratory to unravel the geochemical processes that control subsurface contaminant uptake or release. Our approach is to link quantitative macroscopic measures of contaminant partitioning to the molecular-scale mechanisms that mediate the process, particularly those that may lead to irreversible sequestration of contaminants into a solid phase. This study examined temporal changes in solid-phase Sr speciation using bulk and microfocused EXAFS analyses in samples of STWL containing Sr²⁺ and Cs⁺ (at 10^-3 m) reacted with the clay mineral kaolinite from 1 to 369 d. Bulk EXAFS spectral analyses showed that Sr initially forms a precipitate by 7 d with a local structure similar to SrCO₃(s). At 33 d, the bulk sample spectrum showed few features beyond the first shell of oxygen atoms, but microfocused EXAFS of three individual particles in the same sample indicated distinct differences (Fig. 1). Quantitative analyses revealed a mixture of hydrated and dehydrated Sr associated with neoformed sodalite-type (feldspathoid) phases. At aging times of 93 d and longer, bulk EXAFS spectra and supporting characterization methods indicated an increasing fraction of non-exchangeable Sr with a local structure consistent with incorporation into increasingly crystalline aluminosilicate particles, particularly sodalite (Fig. 2). A combination of spectroscopic and microscopic characterization studies by our group, including NMR, XRD, SEM/TEM, and FTIR, indicated the formation of both sodalite and cancrinite, with cancrinite forming at the expense of sodalite for reaction times longer than 93 d. The Sr-EXAFS analysis uniquely identified the stronger association of Sr with sodalite-type cage structures, rather than cancrinite-type structures, in these samples and showed that local dehydration of Sr within the neoformed phases was associated with increasingly irreversible sequestration. Although this experimental system is greatly simplified from field conditions, an important observation is that trapping of radionuclides can occur if they are present as cage-structure aluminosilicates form and age as alteration products of local Si-bearing sediment minerals. Sequestration of radionuclides by feldspathoid mineral trapping may be an important mechanism for retardation near waste sources because re-release to solution requires mineral dissolution rather than ion exchange.

BEAMLINE
X18B

FUNDING
U.S. Department of Energy

PUBLICATION
Choi, S., O'Day, P.A., Rivera, N.A., Mueller, K.T., Vairavamurthy, M.A., Seraphin, S., and Chorover, J., "Strontium speciation during reaction of kaolinite with simulated tank-waste leachate: Bulk and microfocused EXAFS analysis," Environmental Science & Technology, 40, 2608-2614 (2006).

FOR MORE INFORMATION
Peggy A. O'Day
School of Natural Sciences
University of California
Merced, CA
Email: poday@ucmerced.edu