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Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U­(IV)], a form once considered largely immobile. Changing h...

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Bibliographic Details
Published in:Environmental science & technology 2020-02, Vol.54 (3), p.1493-1502
Main Authors: Bone, Sharon E, Cliff, John, Weaver, Karrie, Takacs, Christopher J, Roycroft, Scott, Fendorf, Scott, Bargar, John R
Format: Article
Language:English
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Summary:Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U­(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U­(IV) is essential to understand the release mechanism, yet the relevant U­(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U­(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.
ISSN:0013-936X
1520-5851
DOI:10.1021/acs.est.9b04741