Advective Removal of Intraparticle Uranium from Contaminated Vadose Zone Sediments, Hanford, USA
A column study on U contaminated vadose zone sediments from the Hanford Site, WA, was performed in order to aid the development of a model for predicting U(VI) release rates under a dynamic flow regime and for variable geochemical conditions. The sediments of interest are adjacent to and below tank BX-102, part of the BX tank farm that contained high level liquid radioactive waste. Two sediments, with different U(VI) loadings and intraparticle large fracture vs. smaller fracture ratios, were reacted with three different solutions. The primary reservoir for U(VI) appears to be a micron-sized nanocrystalline Na-U-Si phase, possibly Na-boltwoodite, that nucleated and grew on plagioclase grains that line fractures within sand-sized granitic clasts. The solutions were all calcite saturated and in equilibrium with atmospheric CO2, where one solution was simply DI-water, the second was a synthetic ground water (SGW) with elevated Na, and the third was the same SGW but with both elevated Na and Si. The latter two solutions were employed, in part, to test the effect of saturation state on U(VI) release. For both sediments and all three electrolytes, there was an initial rapid release of U(VI) to the advecting solution followed by a plateau of low U(VI) concentration. U(VI) effluent concentration increased during subsequent stop flow (SF) events. The electrolytes with elevated Na and Si appreciably depressed U(VI) concentrations relative to DI water. The effluent data for both sediments and all three electrolytes was simulated reasonably well by a three domain model (the advecting fluid, fractures, and matrix) that coupled U(VI) dissolution rates, intraparticle U(VI) diffusion, and interparticle advective transport of U(VI); where key transport and dissolution processes had been parameterized in previous batch studies. For the calcite-saturated DI-water, U(VI) concentrations in the effluent remained far below saturation with respect to Na-boltwoodite and release of U(VI) to the advecting domain was limited, in large part, by intraparticle diffusion of U(VI). In contrast, for the electrolytes with elevated Na and Si, the release of U(VI) to the advecting domain was shown to be limited by the solubility of Na-boltwoodite. The fact that one model, with the same key parameters, is able to simulate such a diverse geochemical and hydrodynamic conditions indicates that the approach is sound and that the model is robust.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 927276
- Report Number(s):
- PNNL-SA-52135; KP1504010
- Journal Information:
- Environmental Science & Technology, 42(5):1565-1571, Journal Name: Environmental Science & Technology, 42(5):1565-1571 Journal Issue: 5 Vol. 42; ISSN ESTHAG; ISSN 0013-936X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ANORTHOSITES
CALCITE
DIFFUSION
DISSOLUTION
ELECTROLYTES
FRACTURES
GROUND WATER
HYDRODYNAMICS
RADIOACTIVE WASTES
REMOVAL
SATURATION
SEDIMENTS
SOLUBILITY
STORAGE FACILITIES
TANKS
TRANSPORT
URANIUM
WATER
advection
diffusion
dissolution
na-boltwoodite