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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}
}