When nuclear fuel gets recycled, the process releases radioactive krypton and xenon gases. Naturally occurring uranium in rock contaminates basements with the related gas radon. A new porous material called CC3 effectively traps these gases by breathing enough to let the gases in but not out.
The CC3 material could be helpful in removing unwanted or hazardous radioactive elements from nuclear fuel or air in buildings and also in recycling useful elements from the nuclear fuel cycle. CC3 is much more selective in trapping these gases compared to other experimental materials. Also, CC3 will likely use less energy to recover elements than conventional treatments, according to the authors.
"Xenon, krypton and radon are noble gases, which are chemically inert. That makes it difficult to find materials that can trap them," said coauthor Praveen Thallapally of PNNL. "So we were happily surprised at how easily CC3 removed them from the gas stream."
To test whether CC3 might be useful here, the team simulated on a computer CC3 interacting with atoms of xenon and other noble gases. The molecular structure of CC3 naturally expands and contracts. The researchers found this breathing created a hole in the cage that grew to 4.4* angstroms wide and shrunk to 3.6 angstroms. One atom of xenon is 4.1 angstroms wide, suggesting it could fit within the window if the cage opens long enough. (Krypton and radon are 3.69 angstroms and 4.17 angstroms wide, respectively, and it takes 10 million angstroms to span a millimeter.)
The computer simulations revealed that CC3 opens its windows big enough for xenon about 7 percent of the time, but that is enough for xenon to hop in. In addition, xenon has a higher likelihood of hopping in than hopping out, essentially trapping the noble gas inside.
The team then tested how well CC3 could pull low concentrations of xenon and krypton out of air, a mix of gases that included oxygen, argon, carbon dioxide and nitrogen. With xenon at 400 parts per million and krypton at 40 parts per million, the researchers sent the mix through a sample of CC3 and measured how long it took for the gases to come out the other side.
Oxygen, nitrogen, argon and carbon dioxide — abundant components of air — traveled through the CC3 and continued to be measured for the experiment's full 45 minute span. Xenon however stayed within the CC3 for 15 minutes, showing that CC3 could separate xenon from air.
In addition, CC3 trapped twice as much xenon as the leading MOF material. It also caught xenon 20 times more often than it caught krypton, a characteristic known as selectivity. The leading MOF only preferred xenon 7 times as much. These experiments indicated improved performance in two important characteristics of such a material, capacity and selectivity.
"We know that CC3 does this but we're not sure why. Once we understand why CC3 traps the noble gases so easily, we can improve on it," said Thallapally.
This story is reprinted from material from PNNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.