Scientists study how nature cleans uranium from aquifer

both microbiological and chemical — could be used to speed the process. It turns out that naturally existing bacteria can “eat” hexavalent uranium, chemically reducing it to tetravalent uranium, which has gained back two of the missing six valence electrons. Those two regained electrons make tetravalent uranium much less soluble in water, so it does not enter groundwater as a contaminant.

Certain minerals containing tetravalent uranium, such as the uranium ore uraninite, are stable in the subsurface — and, considering that life and uranium have co-existed for billions of years, these minerals are pretty safe.

“We wanted to know what chemical and physical form of tetravalent uranium was being made in the aquifer,” Bargar said. “For example, is uraninite being created? Or are other less stable molecular forms of tetravalent uranium being created?” A major scientific objective of the program is to create a complete model of the biological and chemical paths taken by uranium in the aquifer, all the way from its initial dangerous, water-soluble form to more stable ones, with all its branches and dead ends, and all its interactions with oxide and sulfide and carbonate species present in aquifers (uranium is quite a promiscuous metal).

To answer these questions, the researchers transformed a piece of river bank into an outdoor laboratory. “We put a bit of the aquifer under control” by suspending columns of native sediment in wells drilled into the alluvial soil of the Old Rifle site, Bargar said. That way, they could adjust conditions in the sediment by, as Bargar put it, “feeding the bugs” — injecting organic carbon substances like vinegar or ethanol as “bug food” to stimulate the growth of naturally occurring bacteria already known to act on uranium.

Then, Bargar and his colleagues brought the samples to SLAC and used SSRL’s powerful X-rays to start mapping the paths uranium took.

Bargar has been encouraged by the success of the approach. “It’s shed new light on the behavior of tetravalent uranium in aquifers,” he said. As might be expected, neither the biological nor the chemical path dominates; both provide complicated intersecting byways that not only transform soluble uranium to a more stable form, but also draw in other substances that prevent the uranium from reverting back to soluble form. There were surprises: the tetravalent uranium doesn’t immediately precipitate out as uraninite, but ends up in sulfide-rich coatings on other minerals. Whether the further transformation of tetravalent uranium to uraninite can be — or even should be — speeded up remains to be seen.

While these experiments took place at the Old Rifle site, Bargar emphasizes the general nature of the effort. “This is a model for at large and problematic uranium contamination problems at more than sixteen other sites in the Western U.S.,” he said — sites where a river flows directly past uranium-contaminated soil. Ultimately, the researchers hope to vastly improve the cleanup of subsurface uranium, and help make all such areas as beautiful below ground as they are above.

The SSRL Environmental Remediation program is a Scientific Focus Area (SFA) study funded by the Department of Energy’s Office of Biological and Environmental Research, Climate and Environmental Sciences Division.