Taking Uranium Down a Notch
International research team makes important discovery about how to clean up uranium in groundwater
The key foci of this project are rates and mechanisms of biogenic uraninite corrosion, the primary mechanism for uranium re-release to the subsurface from this material. Biogenic uraninite emplaced and retrieved from Rifle wells is being characterized using a suite of structure and reactivity measurement techniques to quantify these reactions and better understand processes that are essential to geochemical uranium transport models. Credit: John Bargar. May 1, 2009. Enlarge Image
Results: Across the country, hundreds of billions of gallons of groundwater are contaminated with uranium from early weapons development efforts and other activities. Scientists have been trying for more than two decades to find ways to safely and cost-effectively prevent that uranium from flowing toward nearby communities.
One promising technology is to use naturally occurring bacteria to change the widely spread dissolved uranium into a nanoparticulate solid called biogenic uraninite that is less likely to move with the groundwater. Unfortunately, the approach is riddled with challenges. Now an interdisciplinary, international team of researchers is applying advanced material science techniques to characterize the key properties of this biogenic uraninite that will control its stability in groundwater.
Why it matters: Scientists understand how uranium behaves under certain controlled conditions. They are less confident in predicting how it will behave in complex environmental systems, including when it interacts with bacteria or other innovative treatment approaches.
"By looking at the structure and key properties of uranium, we take it from something exotic to something familiar, with behavior that can be reliably predicted," said Dr. Eugene Ilton, a Fellow at the Pacific Northwest National Laboratory who was chosen for the team because of his extensive experience using x-ray photoelectron spectroscopy to study the geochemistry of uranium. These properties include its structure down to the nano-scale level, dissolution rate, and solubility.
Ilton's work on the team, which is led by Dr. John Bargar at the SLAC National Accelerator Laboratory, also suggested something surprising: that what was believed to be a rare form of uranium called pentavalent uranium may actually be more prevalent in nature. Although previous chemical studies showed that this form of uranium is associated with the corrosion of spent nuclear fuel, this investigation is the first to demonstrate that it is also naturally formed on the surface of tiny particles of uraninite suspended in water.
"This shows that oxidizing uranium to a more soluble form isn't the single-step process previously thought," said Ilton. "There's an intermediate step that must be taken into consideration for cleanup techniques to be effective."
Methods: The team, which also included experts in environmental engineering, materials sciences, and nuclear research from Washington University in St. Louis and the Ecole Polytechnique Fédérale de Lausanne in Switzerland, compared two types of uraninite. One type was chemically synthesized uraninite; the other was biogenic uraninite produced by bacteria.
Using advanced x-ray techniques, the team looked at the structure and chemistry of each type of the mineral, its dissolution rate both in the presence of oxygen and without, and its solubility. Combining structural and chemical investigations with studies on dissolution is crucial because understanding the structure and composition at both the molecular and nano-scales provides the necessary insight to build reaction models. These models in turn allow rates to be applied to simulate and predict how uranium will move with the groundwater.
Their studies showed that both types of uraninite, the chemically synthesized coarse-grained pure uranium oxide and the biogenic form, had similar solubility and comparable dissolution rates when oxygen was present and when it wasn't, consistent with their similar crystal core structures. Based on this information, the team developed a conceptual mechanistic model for how uraninite would react in groundwater.
What's next: The team's work provides a baseline study of how stable some forms of uranium can be in groundwater. The scientists plan to look at how adding other chemicals, such as manganese, might improve stability and prevent the mineral from dissolving in groundwater. The team is also using the results of this laboratory-based work to test the rates and mechanisms of biogenic uraninite dissolution in groundwater at the Pacific Northwest National Laboratory's Integrated Field Research Challenge site at Old Rifle, Colorado.
Acknowledgments: This work was done by Kai-Uwe Ulrich and Daniel Giammar of Washington University St. Louis, Eugene Ilton of PNNL; Harish Veeramani, Jonathan Sharp, and Rizlan Bernier-Latmani of Ecole Polytechnique Fédérale de Lausanne in Switzerland; and Eleanor Schofiel and John Bargar of SLAC National Accelerator Laboratory.
Part of the research was conducted at the Synchronton Radiation Lightsource at Stanford University, and work carried out in Switzerland was partially funded by the Swiss National Science Foundation.
Sponsor: DOE Office of Science Biological and Environmental Research's Environmental Remediation Science Program through the SLAC Science Focus Area.
Reference: Ulrich K-U, ES Ilton, H Veeramani, JO Sharp, R Bernier-Latmani, EJ Schofield, JR Bargar, and DE Giammar. 2009. "Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate." Geochimica et Cosmochimica Acta. In press. doi:10.1016/j.gca.2009.07.012.