Flooded mine pits often become permanently stratified, meromictic water bodies that are comprised of an upper, mixed layer (mixolimnion) and a lower layer that does not mix seasonally (monimolimnion). The development of a permanently-reduced zone at the bottom of pit lakes is touted as a passive method to stabilize potentially toxic metals at mine sites, as the redox gradient between the two layers controls the speciation and fate of numerous redox-active metals. In this study, we investigated the cycling of U, As, and Ni in two meromictic pit lakes of different ages and with differing geochemistry located at a former uranium mining site in northwestern Saskatchewan, Canada, to uncover the mechanisms of metals cycling and sequestration. Combined use of electron microscopy, ultrafiltration, and asymmetric field flow fractionation revealed that in the water columns of both pits, U and Ni were primarily present as dissolved species or were sorbed onto small colloidal particles (&lt; 10 kDa), despite considerable differences in aqueous chemistry between the pits. In the older and shallower pit, the sharp redox gradient at the intersection of the oxygenated mixolimnion and reduced monimolimnion resulted in primarily tetravalent U species at greater depth generated primarily by Fe(II) oxidation in the water column. In both pits, we observed the “colloidal pumping” mechanisms that demonstrated that with time, colloids agglomerated into larger ones that were more likely to precipitate and transport bound metals/metalloids to the bottom sediments. Synchrotron-based Laue diffraction and XANES identified uraninite alteration products, including vandendriesscheite and gauthierite (both being U(VI) oxyhydroxides), in pit floor sediments; however, many U(IV)-rich grains in the sediment did not exhibit diffraction patterns, indicating the potential presence of non-crystalline U(IV) species. Non-crystalline U(IV) is typically easier to oxidize than solid-phase U(IV) minerals, generating U(VI) species that are more soluble and mobile in groundwater. Our results suggest that characterization of colloid-bound metals, which may pass through standard 0.45 μm membranes and be interpreted as dissolved species, is essential to understanding the cycling and fate of trace metals in mine pits that undergo decommissioning.

Mechanisms of metals cycling in cold-regions meromictic pit lakes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.