Changes in Forms of Uranium in Anoxic Lake Sediments and Porewaters Near an Abandoned Uranium Mine, Bancroft, Ontario

Description

Soluble uranium (U) has been observed continuously in the porewaters of Bentley Lake,
a lake with semi-permanent anoxic sediments, despite the fact that reduced U(IV) is known to be
insoluble. To be able to predict the fate and mobility of U that has been deposited in lake
sediments, it is very important to understand the factors that determine soluble uranium in anoxic
environments. Understanding soluble U species is crucial for predicting its behavior in natural
systems as well as for the development of U remediation schemes.
To explore the factors affecting soluble U in natural environments, anoxic lake sediments
and porewaters were tested using two analytic methods, ICP-MS and ESI-HR-MS. Reduced
uranium (U(IV)) can be precipitated as U(IV)-NdF3. Using this method revealed that most of the
uranium in porewater is not able to be co-precipitated with NdF3. In addition, UO2+ was found
using ESI-HR-MS, showing uranyl ions exist in reduced porewater. However, the UO2+ might be
attached to some organic groups rather than present as free ions.
Seasonal variation and air exposure experiments on the mobility of U between sediments
and porewater were observed to test for changes of the redox state of U as a function of sample
collection and storage. The results of this study will contribute to better remediation strategies for
U tailings and will help U mining operations in the future.
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