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Title: Microscopic Mass Transfer of Uranium in Contaminated Sediments at Hanford Site

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Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
Report Number(s):
R&D Project: ERSD 1029506
Resource Type:
Resource Relation:
Conference: Annual Environmental Remediation Science Program (ERSP) Principal Investigator Meeting, April 16-19, 2007, Lansdowne, VA
Country of Publication:
United States

Citation Formats

Chongxuan Liu, John Zachara, James McKinley, Paul Majors, and Sebastien Keristi. Microscopic Mass Transfer of Uranium in Contaminated Sediments at Hanford Site. United States: N. p., 2007. Web.
Chongxuan Liu, John Zachara, James McKinley, Paul Majors, & Sebastien Keristi. Microscopic Mass Transfer of Uranium in Contaminated Sediments at Hanford Site. United States.
Chongxuan Liu, John Zachara, James McKinley, Paul Majors, and Sebastien Keristi. Thu . "Microscopic Mass Transfer of Uranium in Contaminated Sediments at Hanford Site". United States. doi:.
title = {Microscopic Mass Transfer of Uranium in Contaminated Sediments at Hanford Site},
author = {Chongxuan Liu and John Zachara and James McKinley and Paul Majors and Sebastien Keristi},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Apr 19 00:00:00 EDT 2007},
month = {Thu Apr 19 00:00:00 EDT 2007}

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  • Microscopic and spectroscopic analysis of uranium-contaminated sediment cores beneath the BX waste tank farm at the US Department of Energy (DOE) Hanford site revealed that uranium (U) existed as uranyl precipitates primarily associated with the intragrain fractures of granitic clasts in the sediment (McKinley et al. 2005). The dissolution of the precipitates appeared to be controlled by intragrain ion diffusion coupled with the dissolution kinetics of the uranyl precipitates most likely as Na-boltwoodite. Here we presented a coupled microscopic reactive diffusion model by independently characterizing the intragrain diffusion and dissolution kinetics of Na-boltwoodite. Diffusion characterization with a nuclear magnetic resonancemore » (NMR) pulse gradient spin-echo (PGSE) technique showed that the intragrain fractures of the granitic clasts in the Hanford sediment contain two domans with distinct diffusivities. The fast diffusion domain has an apparent tortuosity of about 1.5, while the slow region has a tortuosity of two orders of magnitude larger. A two-domain diffusion model was assembled and used to infer the geochemical conditions that led to intragrain uranyl precipitation when the sediment was contaminated by U-containing wastes at the site. Rapid precipitation of Na-boltwoodite was simulated when a U-containing, alkaline caustic, and high carbonate tank waste solution diffused into intragrain fractures originally containing Si-rich solutions. The model was also used to simulate uranyl dissolution and release from the contaminant sediment to aqueous solutions. With independently characterized parameters for Na-boltwoodite dissolution, the model simulations demonstrated that diffusion could significantly decrease the rates of intragrain uranyl mineral dissolution due to diffusion-induced local solubility limitation, and the intragrain uranyl precipitates could serve as a long-term uranyl source for the vadose porewater and underlying groundwater at this site.« less
  • Uranium in DOE Hanford sediments was found to be distributed as uranyl silicate precipitates almost exclusively within interiors of sediment grains. The precipitates were minute, generally 1-3 {micro}m across in either radiating or parallel arrays in intraparticle microfractures of a few microns width and variable connectivity to particle surfaces. Grain-scale porosity, tortuosity and diffusivity of tracer (H2O) and U(VI) were measured and imaged using various spectroscopic techniques. Simulations using a microscopic reactive diffusion model suggested that diffusion-limited mass transport generated a favorable thermodynamic condition within the grain microfractures for precipitation and concentration of uranium from waste plumes. The rate andmore » extent of uranyl precipitate dissolution were studied in various electrolytes with variable pH under ambient CO2 pressure. Uranium speciation and distribution before and after dissolution were monitored by spectroscopic and imaging techniques . Experimental, spectroscopic and modeling results collectively indicated that dissolution of uranyl precipitates was controlled by diffusion-limited dissolution kinetics.« 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