1999


From: DOE/Idaho National E & E Laboratory

INEEL, universities collaborate on subsurface strontium-90 remediation efforts

Kitty litter has been one of the most effective methods of cleaning up in the history of humankind--sopping up moveable messes and sequestering them. Using a naturally forming mineral as their kitty litter, researchers at the Department of Energy's Idaho National Engineering and Environmental Laboratory are working on a way to sop up a radioactive element and immobilize it until the element's radioactivity is spent.

Funded by DOE's Environmental Management Science Program, the researchers are trying to trap strontium-90 and other similar elements in calcite. INEEL geochemist Robert W. Smith leads the multi-university collaboration that includes INEEL microbiologist Rick S. Colwell, University of Toronto geomicrobiologist F. Grant Ferris, INEEL analytical chemist Jani Ingram, and Portland State University microbiologist Anna-Louise Reysenbach. Together, the researchers received a 3-year, $0.9 million grant that was announced by Under Secretary of Energy Ernest Moniz today. The award was part of a larger $25 million grant for 31 projects at 20 universities and eight DOE laboratories.

Strontium-90 is a radioactive element produced in nuclear reactors. In the past decades, some waste disposal methods have left pockets of low-level radioactive contamination in the vadose zone (the region between the surface and the groundwater) and in the groundwater. The vadose zone under INEEL ranges from 100 feet to 600 feet deep. Getting pollutants out of the subsurface is difficult to do with engineering efforts.

Strontium and similar elements can migrate through the vadose zone and aquifer, said Smith. With the radioactive strontium-90 trapped in the calcite crystal, however, it will be unable to move and will be effectively removed from the groundwater.

All radioactive elements eventually decay to inert elements, and Strontium-90 will take 300 years to decay away to nothingness. "If we can retain it, tie it up for that length of time," said Smith, "and if we do it where it already is in the ground, then we won't have to dig it up or transport it, or have a waste stream or a final disposal site."

Calcite mineral has several properties that the researchers find useful: Elements such as strontium-90 can replace the calcium in the mineral, calcite is formed in water-based environments with high pH, and it is stable in the arid desert soil at INEEL.

Calcite mineral is made when calcium carbonate precipitates from the water in the aquifer (like when deposits form on your water pipes) when the pH of the water in which calcium carbonate is dissolved is high. While calcite forms naturally in arid environments, the researchers want to speed up the process by locally increasing the pH in regions rich in Strontium-90.

One efficient way to do this underground is to get the resident bacteria to do it. Many species of bacteria are quite happy to degrade urea in a process called urea hydrolysis, which results in an increase in pH.

"We're already experimenting with organisms from the aquifer that might hydrolyze urea," said Colwell. "We are looking for the specific ones that can be stimulated or are responsible for urea hydrolysis. We also have to determine if we need to add, in addition to the urea, a carbon source for food or other nutrients."

The crystallized mineral will stay put if the environmental conditions are right, as they are under the INEEL site in the arid Idaho desert. "The strontium gets taken up because it takes the place of calcium in the mineral," said Smith. "After the strontium is removed, more layers of calcium carbonate form on the mineral. As time goes on, the strontium gets buried inside the crystal and it's no longer accessible to the water."

Colwell and Ferris need to determine precise conditions for optimal urea hydrolysis. Colwell said, "Some organisms won't hydrolyze urea unless they are growing. If those are the organisms under INEEL, we may need to feed them an energy source and nutrients."

The researchers will spend almost two years in the laboratory before trying small-scale experiments in the field. Their first field experiments will focus on how well the calcite forms beneath the Idaho desert and in characterizing the mineral.

Laboratory experiments will focus on the chemistry of calcite formation in simulated subsurface conditions, looking for ways to increase the rate of that formation. While Smith will examine the geochemistry of the reaction, Colwell, Ferris, and Reysenbach will determine ways to characterize and stimulate the naturally occurring bacteria to help the precipitation along. Ingram will examine the resulting mineral composition using analytical chemistry techniques in the laboratory.

The laboratory research will use artificial surfaces called coupons, upon which bacteria and calcite will be deposited. Ultimately, coupons will be placed in deep holes, such as INEEL wells, for a period of time and then removed and analyzed for mineral deposition. The researchers may use a slab of basalt rock or other material as their coupon to transport their experiments to the subsurface.

Analyzing the potentially complicated mineral and coupon surfaces will be Ingram's responsibility. At the INEEL, she currently studies ways to examine surfaces whose heterogeneity makes their study difficult, such as the surfaces of basalt, a rock common to several DOE sites.

Smith brought together a large and diverse group of researchers because the natural world doesn't usually behave like isolated experiments in the lab. He said, "It is nontrivial to reproduce a 20-year process in a 6-month experiment. And that's why collaboration is so important to the success of this project."

The Environmental Management Science Program (EMSP) funds basic research through a partnership between the Department of Energy Office of Environmental Management (EM) and the Office of Science (SC). The intent of the EMSP is to develop the scientific knowledge needed to create new technologies and cleanup approaches that will solve the technically complex problems facing the government's largest environmental cleanup program. Additional information on the EMSP can be found at http://www.doe.gov/em52/

INEEL celebrates its 50 year anniversary in 1999. The national laboratory is operated for the U.S. Department of Energy by Lockheed Martin Idaho Technologies Company.

Note to editors: This proposal was funded in the field of geochemistry with the title Calcite precipitation and trace metal partitioning in groundwater and the vadose zone: remediation of strontium-90 and other divalent metals and radionuclides in arid western environments.




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