INEEL researchers used pressurized, 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, report two chemists from the U.S. Department of Energy's Idaho National Engineering and Environmental Laboratory in the October 2001 issue of Radiochemica 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 environmentally 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 depressurized, 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. "They tried nitric acid extraction, but that dissolved 25 percent of the soil mass. They weren't left with anything resembling soil after the extraction." In fact, he explained, dissolving soil in nitric acid creates a radioactive sludge that must still be disposed of.
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, "We didn't think we'd get such high percentages right off the bat. Plutonium is fairly difficult to remove sometimes. 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."
In fact, Mincher said the amount of radioactivity left on the soil is below the levels required by the government for clean up, in spite of the fact that the spiked sample started out with more radioactivity than is commonly found on INEEL soil.
"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."
Since actual contaminated INEEL soil was unavailable for this initial work, the chemists spiked clean INEEL soil with the two radioactive materials. They verified that the spiked soil resembled real contaminated soil by subjecting the spiked soil to the same characterization method that has been used on contaminated INEEL soil previously.
Based on their characterization of the spiked soil, the researchers found that it was a suitable surrogate for a real-world sample. However, weather and age can affect the chemistry of the materials bound to the soil, a criticism the two chemists readily acknowledge.
"The obvious next step is to obtain real-world samples and demonstrate the method is effective on all manner of soils," said Fox. "We also want to have a fundamental understanding of the chemistry that occurs -- why does it work that way, and what is inhibiting it from working faster and better."
Mincher and Fox were funded to investigate plutonium extraction by the BNFL Group. BNFL Group's subsidiary BNFL, Inc. is the company contracted by DOE to run INEEL's Advanced Mixed Waste Facility. BNFL, Inc. is always looking for inexpensive, practical technologies to clean up contaminated materials such as those found at the Radioactive Waste Management Complex at INEEL's desert site.
Use of supercritical carbon dioxide in industry goes back to the 1920s, when it was first used to extract asphalt from oil. Since then, it has been used to remove the caffeine from coffee beans and as an alternative to hazardous dry cleaning solvents. Since the methods are industrially mature, Fox and Mincher expect scaling up the method would be relatively easy.
The INEEL is a science-based, applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in national security, environment, energy and science. The INEEL is operated for the DOE by Bechtel BWXT Idaho, LLC, in partnership with the Inland Northwest Research Alliance.
Technical contact: Bruce Mincher, 208-533-4449
Robert Fox, 208-526-7844
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