A team of researchers from Washington University in St. Louis has helped discover a new chemical method to immobilize uranium in contaminated groundwater, which could lead to more precise and successful water remediation efforts at former nuclear sites.

Calcium and phosphate work together chemically to immobilize uranium, an element that has been shown to increase risk of cancer and liver damage in humans when ingested. Past field studies, including one at the Hanford site in the state of Washington, have focused on an in situ solution that injected phosphates directly into contaminated groundwater. Remediation efforts were not fully successful because the scale of overlap for the calcium, uranium and phosphates was limited.

Past efforts to inject phosphates directly into contaminated water were not fully successful. Image credit: Washington University in St. Louis.“A challenge with subsurface remediation is finding the right way to bring the necessary ingredients together in a poorly mixed system,” says Daniel Giammar, professor of environmental engineering. “In the field-scale test, much of the added phosphate never reached the uranium because it precipitated out near the injection well. The solution is to figure out scenarios where it is possible to send the phosphate to where the uranium is—and other scenarios where the phosphate can be added to a location where the natural groundwater flow will bring the uranium into contact with it.”

In a series of experiments on water containing uranium, researchers in Giammar’s lab first determined the exact level of calcium in the water. They were then able to add specific amounts of phosphate to form calcium phosphate, chemically neutralizing and structurally incorporating the uranium. The exact combination of calcium and added phosphate rendered the uranium inert and trapped it in the groundwater.

Giammar’s lab will continue this research, with the goal of developing a technique to tailor the location of phosphate injection that would be used in conjunction with the groundwater’s existing calcium to remediate the uranium also present.

“The results of this work suggest that there will not be a one-size-fits-all approach to using phosphate to remediate uranium-contaminated groundwater,” Giammar says. “With knowledge of the location of the uranium contamination and the composition of the groundwater, we can decide whether to inject phosphate directly into a plume of uranium-contaminated groundwater or to inject phosphate downstream of the uranium to form a calcium phosphate barrier.”