Cleaning plutonium from soil using carbon dioxide

RESEARCHERS HAVE found a way to clean soil contaminated with two radioactive elements. The method takes advantage of an industrial process called supercritical fluid extraction to clean up long- lived radioactivity that could persist well after the hills have crumbled. Potentially, this method could help the nuclear industry clean up contaminated soil.

Researchers at the Idaho National Engineering and Environmental Laboratory (INEEL) in the U.S. used pressurised, heated carbon dioxide and an added metal binding chemical compound to clean radioactively contaminated soil.

The method removed more than 69 percent of the plutonium and americium from spiked, local soil according to a report in Radiochemical Acta. Supercritical fluid extraction is already used to decaffeinate coffee, purify spices and dry clean clothes and has been shown to remove plutonium from stainless steel but this is the first time it has been used to remove plutonium from soil.

Supercritical fluid extraction is industrially safe and environment friendly. For these experiments, carbon dioxide and soil were mixed, heated and pressurized. Under these conditions, carbon dioxide flows like a gas, dissolves like a liquid, but behaves with chemical properties unlike gases or liquids.

A chemical agent added to the carbon dioxide flowed through the soil and grabbed the plutonium and americium, whisking the compound back into the fluid-like carbon dioxide.

The carbon dioxide was then shunted out of the soil and depressurised, dropping the compound into a vial on its way back into the atmosphere. In an industrial-scale setting, the carbon dioxide would be recycled. Also, the researchers added ethanol and can add different chemical agents to improve the efficiency of extraction.

Unlike harsher methods of extraction, supercritical fluid extraction leaves the soil intact, making it suitable for cleaning up plutonium-contaminated soil at DOE sites. "The DOE has the technology to isolate plutonium contaminated soils. However, there are no effective extraction technologies for removing strongly adsorbed and recalcitrant radionuclides from soil," INEEL chemist Robert Fox said.

In contrast, the supercritical extraction method is nondestructive-no soil mass is lost in the process. How effectively this supercritical fluid extraction removes radioactive elements from soil depends partly on the chemistry of the soil.

Though a handful of soil looks uniform, soil particles are made up of minerals from both rocks and clay, which react differently with the radioactive elements. Also, the plutonium that is bound near the surface of a particle is easier to remove than that bound inside the mineral lattice.

The efficiency of the process on the INEEL soil surprised the authors of the study. Said INEEL chemist Bruce Mincher, "I thought we'd get the easy plutonium. We perhaps got the plutonium that migrated into the mineral lattices of the soil, where it's almost impossible to get out."

"Our follow-up experiments removed almost 100 per cent of the americium and plutonium," said coauthor Robert Fox. "Someone needs to give us a harder problem or a harder sample."

"The obvious next step is to obtain real-world samples and demonstrate the method is effective on all manner of soils," said Fox.

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