Researchers at the US Department of Energy’s Idaho National Engineering and Environmental Laboratory (INEEL) have used pressurised and heated carbon dioxide in a technique known as supercritical fluid extraction to remove plutonium and americium from contaminated soil, removing more than 69% of the two elements. “Our follow-up experiments removed almost 100% of the americium and plutonium,” said INEEL chemist and co-researcher Robert Fox. “Someone needs to give us a harder problem or a harder sample.”
Supercritical carbon dioxide was first used in the 1920s, to extract asphalt from oil, and has since been used to remove caffeine from coffee beans, to purify spices and as an alternative to hazardous dry cleaning solvents. It is regarded as industrially safe, and environmentally friendly. In the past, the technique has been used to remove plutonium from stainless steel, but this is the first time that it has been used on soil, the researchers say.
In the INEEL work, carbon dioxide and soil were mixed, then heated and pressurised, under which conditions carbon dioxide flows like a gas and dissolves substances like a liquid. A metal binding chemical is added to the carbon dioxide, flowing through the soil and attaching itself to the plutonium and americium, taking them back into the fluid-like gas. Carbon dioxide is then removed from the soil and depressurised, dropping the new compound into a vial. The gas can then be reused.
Unlike harsher methods of decontaminating soil, supercritical fluid extraction leaves the soil intact. “The DOE has the technology to isolate plutonium contaminated soils,” said Fox. “However, there are no effective extraction technologies for removing strongly adsorbed and recalcitrant radionuclides from soil. They tried nitric acid extraction, but that dissolved 25% of the soil mass. They weren’t left with anything resembling soil after the extraction.” What did remain was a radioactive sludge that still had to be disposed of, he explained.
The effectiveness of the technique depends partly on the chemistry of the soil, say the researchers, with particles of rocks and clay reacting differently with the radioactive elements, and plutonium that is bound near the surface of a particle is easier to remove than that bound inside the mineral lattice. Weather and age can also affect the chemistry of the materials bound to the soil, the researchers admit.
“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.”