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Apatite can absorb and contain uranium for millennia - study

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(Kitco News) - A team of researchers from Sandia, Lawrence Berkeley and Pacific Northwest national laboratories today announced the results of a test of a "sponge-like" mineral that can absorb uranium and hold it for tens of thousands of years.

For the test, the researchers have selected a contaminated uranium mill site near Rifle, which is about 180 miles west of Denver. Since 2002, the DOE's Office of Legacy Management has used the site to test a variety of different uranium-remediation technologies.

In a nutshell, the researchers found that the mineral, calcium apatite, soaks up and binds uranium from the groundwater, reducing it by more than ten-thousandfold. In addition, the results of the study suggest that the uranium will remain contained within the apatite mineral for tens of thousands of years, which is crucially important for cleaning contaminated groundwaters.

Calcium apatite is a mineral commonly used in fertilizer and is also a major component of bones and teeth. The researchers formed a "sponge" in the ground by injecting two inexpensive and nontoxic chemicals, calcium citrate and sodium phosphate, into a well especially designed for injecting solutions underground at the former uranium mill.

According to the study, once in the ground, helpful soil bacteria ate the calcium citrate and excreted calcium in a form that allows it to rapidly react with the sodium phosphate to form calcium apatite, which coated sand and soil particles underground, forming the sponge.

The apatite sponge captures contaminants, such as uranium, as it forms on the soil particles around the injection well, and afterward as the groundwater flows through the rough sponge. Once formed, the apatite is incredibly stable, and can hold onto captured contaminants for millennia, the researchers established.

"The apatite technology has successfully reduced the concentration of uranium, vanadium and molybdenum in the groundwater at the Rifle site," said Mark Rigali, the Sandia geochemist leading the project. "Moreover, the levels of uranium have remained below the Department of Energy's target concentration for more than three years."

The apatite technology is being considered for use at several other contaminated locations, both federally managed and privately owned, said Rigali. Also increasing the potential applicability of apatite remediation is the fact that it can be "tuned" to capture different contaminants of concern including lead and arsenic.

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