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Title: Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington

Abstract

An alkaline brine containing uranyl (UO22+) leaked to the thick unsaturated zone at the Hanford Site. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1-3 µm precipitates were difficult, but indicated a sodium potassium uranyl silicate, likely sodium boltwoodite. Observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a diffusion reaction model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore-space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During thismore » time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated vadose zone.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
887011
Report Number(s):
PNNL-SA-43416
Journal ID: ISSN 0016-7037; GCACAK; 4690; 8202; 4597; KP1504010; TRN: US0604154
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta, 70(8):1873-1887; Journal Volume: 70; Journal Issue: 8
Country of Publication:
United States
Language:
English
Subject:
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS; BRINES; BUFFERS; DIFFUSION; ELECTRONS; FRACTURES; HYPOTHESIS; POTASSIUM; PRECIPITATION; RESOLUTION; SEDIMENTS; SILICA; SILICATE MINERALS; SODIUM; SOLUBILITY; SUPERSATURATION; URANIUM; URANIUM MINERALS; URANYL SILICATES; Environmental Molecular Sciences Laboratory

Citation Formats

McKinley, James P., Zachara, John M., Liu, Chongxuan, Heald, Steve M., Prenitzer, Brenda I., and Kempshall, Brian. Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington. United States: N. p., 2006. Web. doi:10.1016/j.gca.2005.10.037.
McKinley, James P., Zachara, John M., Liu, Chongxuan, Heald, Steve M., Prenitzer, Brenda I., & Kempshall, Brian. Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington. United States. doi:10.1016/j.gca.2005.10.037.
McKinley, James P., Zachara, John M., Liu, Chongxuan, Heald, Steve M., Prenitzer, Brenda I., and Kempshall, Brian. Sat . "Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington". United States. doi:10.1016/j.gca.2005.10.037.
@article{osti_887011,
title = {Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington},
author = {McKinley, James P. and Zachara, John M. and Liu, Chongxuan and Heald, Steve M. and Prenitzer, Brenda I. and Kempshall, Brian},
abstractNote = {An alkaline brine containing uranyl (UO22+) leaked to the thick unsaturated zone at the Hanford Site. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1-3 µm precipitates were difficult, but indicated a sodium potassium uranyl silicate, likely sodium boltwoodite. Observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a diffusion reaction model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore-space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During this time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated vadose zone.},
doi = {10.1016/j.gca.2005.10.037},
journal = {Geochimica et Cosmochimica Acta, 70(8):1873-1887},
number = 8,
volume = 70,
place = {United States},
year = {Sat Apr 15 00:00:00 EDT 2006},
month = {Sat Apr 15 00:00:00 EDT 2006}
}
  • differences in uranium contributed by contaminated vadose sediments at two locations was investigated. At the BX tank farms, alkaline waste was accidentally released to a thick vadose zone. At the 300 Area, waste of variable acidity was released by unintended infiltration through the base of settling ponds. The waste form at the BX site was devoid of dissolved silica, and reacted with fluids trapped in microfractures to precipitate uranyl silicates. These secondary deposits were isolated physically from the vadose pore space and are not readily leached into pore fluids. At the 300 Area, the aluminum-rich waste precipitated on the surfacesmore » of sediment clasts, forming a microporous reservoir of solid-phase uranium. Interaction of this coating with water in transit through the vadose zone provides a persistent source of dissolved uranium to groundwater.« less
  • Contamination of vadose zone sediments under tank BX-102 at the Hanford site, Washington, resulted from the accidental release of 7-8 metric tons of uranium dissolved in caustic aqueous sludge in 1951. We have applied synchrotron-based X-ray spectroscopic and diffraction techniques to characterize the speciation of uranium in samples of these contaminated sediments. U LIII-edge X-ray absorption fine structure (XAFS) spectroscopic studies demonstrate that uranium occurs predominantly as a uranium-(VI) silicate from the uranophane group of minerals. XAFS cannot distinguish between the members of this mineral group due to the near identical local coordination environments of uranium in these phases. However,more » these phases differ crystallographically, and can be distinguished using X-ray diffraction (XRD) methods. As the concentration of uranium was too low for conventional XRD to detect these phases, X-ray microdiffraction (?XRD) was used to collect diffraction patterns on {approx}20 ?m diameter areas of localized high uranium concentration found using microscanning X-ray fluorescence (?SXRF). Only sodium boltwoodite, Na(UO2)(SiO3OH)?1.5H2O, was observed; no other uranophane group minerals were present. Sodium boltwoodite formation has effectively sequestered uranium in these sediments under the current geochemical and hydrologic conditions. Attempts to remediate the uranium contamination will likely face significant difficulties because of the speciation and distribution of uranium in the sediments.« less
  • The concentration and distribution of uranium (U) in sediment samples from three boreholes recovered near radioactive waste storage tanks at Hanford, Washington State, USA, were determined in detail using bulk and micro-analytical techniques. The source of contamination was a plume that contained an estimated 7000 kg of dissolved U that seeped into the subsurface as a result of an accident that occurred during filling of tank BX-102. The desorption character and kinetics of U were also determined by experiment in order to assess the mobility of U in the vadose zone. Most samples contained too little moisture to obtain quantitativemore » information on pore water compositions. Concentrations of U (and contaminant phosphate—P) in pore waters were therefore estimated by performing 1:1 sediment-to-water extractions and the data indicated concentrations of these elements were above that of uncontaminated “background” sediments. Further extraction of U by 8 N nitric acid indicated that a significant fraction of the total U is relatively immobile and may be sequestered in mobilization-resistant phases. Fine- and coarse-grained samples in sharp contact with one another were sub-sampled for further scrutiny and identification of U reservoirs. Segregation of the samples into their constituent size fractions coupled with microwave-assisted digestion of bulk samples showed that most of the U contamination was sequestered within the fine-grained fraction. Isotope exchange ( 233U) tests revealed that ~51 to 63% of the U is labile, indicating that the remaining fund of U is locked up in mobilization-resistant phases. Analysis by micro-X-Ray Fluorescence and micro-X-Ray Absorption Near-Edge Spectroscopy (μ-XRF and μ-XANES) showed that U is primarily associated with Ca and is predominately U(VI). The spectra obtained on U-enriched “hot spots” using Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) provide strong evidence for uranophane-type [Ca(UO 2) 2(SiO 3OH) 2(H 2O) 5] and uranyl phosphate [Ca(UO 2) 2(PO 4) 2(H 2O) 10-12] phases. These data show that disseminated micro-precipitates can form in narrow pore spaces within the finer-grained matrix and that these objects are likely not restricted to lithic fragment environments. Uranium mobility may therefore be curtailed by precipitation of uranyl silicate and phosphate phases, with additional possible influence exerted by capillary barriers. Consequently, equilibrium-based desorption models that predict the concentrations and mobility of U in the subsurface matrix at Hanford are unnecessarily conservative.« less