TY - JOUR
T1 - Effects of flow on uranium speciation in soils impacted by acidic waste fluids
AU - Perdrial, Nicolas
AU - Vázquez-Ortega, Angélica
AU - Reinoso-Maset, Estela
AU - O'Day, Peggy A.
AU - Chorover, Jon
N1 - Funding Information:
Uranium LIII-edge extended X-ray absorption fine structure (EXAFS) spectra were collected for the ground bulk sediments of both batch (after 180 and 365 d of reaction) and column (after ca. 800 PV) systems in fluorescence mode on beam line 11-2 at the SSRL (3 GeV, 80–100 mA). Spectra were collected using a Si(220) crystal monochromator (beam size of 1 x 4–7 mm) with a second crystal detuned 20–30% at 17,600 eV to reject harmonic reflections (17 keV cutoff energy). Beam energy was calibrated with a Y0 standard (first inflection on the K-edge set to 17,038 eV) and a secondary UO2(s) standard (energy at the edge jump midpoint set to 17,166 eV). Sediments were packed evenly in aluminum holders, sealed with Kapton tape and held in a N2 cryostat at ∼77 K during data collection. Fluorescence absorption spectra were collected using a solid-state 100 element Ge-array detector and successive scans (2–10) were averaged. The background was subtracted using a linear fit through the pre-edge region and normalized to the post-edge step height in SIXpack (Webb, 2010). Normalization was extended into the EXAFS region (k = 0 Å−1 set to 17,175 eV) using a cubic spline. Previous work on similar systems (Perdrial et al., 2018, Vázquez-Ortega et al., 2021) demonstrated the complexity of the U speciation in the sediments, rendering non-linear least-squares shell-by-shell fitting inconclusive. Therefore, least-squares linear combinations (LC) of reference compound spectra (previously analyzed by XRD and modeled by shell-by-shell EXAFS fits; Fig. S3; Kanematsu et al., 2014) was the preferred approach to analyze the EXAFS spectra of the reacted sediments. The LC fits were performed with ATHENA (Ravel and Newville, 2005) with a data k-range consistently set from 2 to 12 Å−1 and component weights of reference spectra constrained between 0 and 1 but not forced to sum to unity. Following recommendations from Gaur and Shrivastava (2015), the number of reference spectra used in the fits was limited to the number of candidate species identified by XRD and thermodynamic modeling. The reference EXAFS spectra show distinctive features in the 7–12 Å−1 region (Fig. S3) that support a meaningful interpretation of the LC fits. The statistical reduced χ2 and R-factor values provided by Athena, visual determination, and reasonableness and consistency of the results with those derived independently from XRD, were used as criteria for selecting the final fits. Reduced χ2 values below 0.7 and R-factor values below 0.1 were considered indicative of reasonably good fits. Components with a fractional contribution of less than 10% in the fit were discarded, as they did not significantly improve fit statistics. Reported results correspond to the best statistical fits involving the lowest number of reference spectra, i.e., if adding a reference improved the goodness-of-fit to the experimental data only marginally, the reference was discarded.This research was funded by the Subsurface Biogeochemical Research program (Grant SBR-DE-SC0006781), Office of Biological and Environmental Research, Office of Science, U.S. Department of Energy.
Funding Information:
Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, a national user facility supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank the Arizona Laboratory for Emerging Contaminant for access to analytical instrumentation, and Dr. Robert A. Root (University of Arizona) for assisting with synchrotron data collection.
Funding Information:
This research was funded by the Subsurface Biogeochemical Research program (Grant SBR-DE-SC0006781 ), Office of Biological and Environmental Research, Office of Science , U.S. Department of Energy .
Publisher Copyright:
© 2022 Elsevier Ltd
PY - 2022/10
Y1 - 2022/10
N2 - Radioactive acidic liquid waste is a common byproduct of uranium (U) and plutonium (Pu) enrichment and recycling processes whose accidental and planned release has led to a significant input of U into soils and sediments across the world, including at the U.S. DOE's Hanford site (WA, USA). Because of the particularly hazardous nature of U, it is important to predict its speciation when introduced into soils and sediments by acidic waste fluids. Of fundamental importance are the coupled effects of acid-driven mineral transformation and reactive transport on U speciation. To evaluate the effect of waste-fluid residence time and co-associated dissolved phosphate concentrations on U speciation in impacted soils and sediments, uncontaminated surface materials (from the Hanford Site) were reacted with U-containing synthetic acidic waste fluids (pH 2) amended with dissolved phosphate concentrations in both batch (no flow) and flow-through column systems for 7–365 days. By comparing dissolved U behavior and solid phase speciation as a function of flow regimen, we found that the availability of proton-promoted dissolution products (such as Si) to sequester U into uranyl silicates was dependent on waste fluid-sediment contact time as uranyl silicates were not detected in short contact time flow-through systems but were detected in no-flow, long contact time, reactors. Moreover, the dominance of uranyl phosphate as neoprecipitate U scavenger (principally in the form of meta-ankoleite) in phosphate amended systems confirmed the importance of phosphate amendments for an efficient sequestration of U in the soils and sediments. Overall, our experiments suggest that the formation of uranyl silicates in soils impacted by acidic waste fluids is likely to be limited unless reaction products are allowed to accumulate in soil pores, highlighting the importance of investigating soil U speciation in flow-through, transport-driven systems as opposed to no-flow, batch systems. This study provides insights into uranium speciation and its potential changes under acidic conditions for better prediction of risks and subsequent development of efficient remediation strategies.
AB - Radioactive acidic liquid waste is a common byproduct of uranium (U) and plutonium (Pu) enrichment and recycling processes whose accidental and planned release has led to a significant input of U into soils and sediments across the world, including at the U.S. DOE's Hanford site (WA, USA). Because of the particularly hazardous nature of U, it is important to predict its speciation when introduced into soils and sediments by acidic waste fluids. Of fundamental importance are the coupled effects of acid-driven mineral transformation and reactive transport on U speciation. To evaluate the effect of waste-fluid residence time and co-associated dissolved phosphate concentrations on U speciation in impacted soils and sediments, uncontaminated surface materials (from the Hanford Site) were reacted with U-containing synthetic acidic waste fluids (pH 2) amended with dissolved phosphate concentrations in both batch (no flow) and flow-through column systems for 7–365 days. By comparing dissolved U behavior and solid phase speciation as a function of flow regimen, we found that the availability of proton-promoted dissolution products (such as Si) to sequester U into uranyl silicates was dependent on waste fluid-sediment contact time as uranyl silicates were not detected in short contact time flow-through systems but were detected in no-flow, long contact time, reactors. Moreover, the dominance of uranyl phosphate as neoprecipitate U scavenger (principally in the form of meta-ankoleite) in phosphate amended systems confirmed the importance of phosphate amendments for an efficient sequestration of U in the soils and sediments. Overall, our experiments suggest that the formation of uranyl silicates in soils impacted by acidic waste fluids is likely to be limited unless reaction products are allowed to accumulate in soil pores, highlighting the importance of investigating soil U speciation in flow-through, transport-driven systems as opposed to no-flow, batch systems. This study provides insights into uranium speciation and its potential changes under acidic conditions for better prediction of risks and subsequent development of efficient remediation strategies.
KW - Boltwoodite
KW - EXAFS
KW - Hanford site
KW - Phosphate
KW - Radionuclides
KW - Thermodynamic modeling
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U2 - 10.1016/j.jenvrad.2022.106955
DO - 10.1016/j.jenvrad.2022.106955
M3 - Article
C2 - 35772319
AN - SCOPUS:85133634412
SN - 0265-931X
VL - 251-252
JO - Journal of Environmental Radioactivity
JF - Journal of Environmental Radioactivity
M1 - 106955
ER -