We usually think of radioactive contamination in terms of solid radioactive elements such as uranium and plutonium. However, there are also radioactive gases such as radon. This gas is created as naturally occurring uranium decays. In some parts of the country, natural radon can accumulate in basements and pose a health threat. When nuclear fuel is burned in a reactor, radioactive isotopes of xenon and krypton are generated. Radon, krypton and xenon are members of the noble gases in the periodic table of elements. Noble gases do not normally interact and form compounds with other elements. These three gases pose a human health threat and it is very important for us to develop an efficient and cost-effective way to remove them from air and water.
A recent research report details how a newly discovered material is more efficient at removing radioactive gases from the atmosphere and water than any other material known. The new material is called CC3. There is an artist's rendering of the CC3 molecule at the bottom of this post. The capability of CC3 to trap noble gases is surprising because of the normally non-reactive nature of these gases.
The standard method of removing noble gases from air depends on cooling the air far below freezing. Because this process is complicated and expensive, scientists have been searching for an alternative. One avenue of research has focused on metal-organic frameworks (MOFs) that might be able to trap noble gases without the cooling now necessary. They did find MOFs that could remove noble gases in low concentrations at room temperature but they continued searching for a more efficient alternative.
A different approach to capture of noble gases focused on what are called porous organic cages. These materials have a repeating three dimensional structure that forms "cages" that could hold atoms of xenon, radon or krypton. CC3 has cages that are just about the right size for these noble gas atoms. Computer simulations show that the cages open and close by about twenty percent. They are only open fully about seven percent of the time but that is enough for xenon atoms to enter. Xenon atoms are more likely to enter an empty cage than to leave a cage. This makes CC3 an excellent material to trap noble gases.
The researches took a piece of CC3 and pumped a mixture of gases through the material to test its retention of specific gases. The mix contained oxygen, carbon dioxide, argon, krypton, xenon and nitrogen. The krypton and xenon were at very low concentrations. They found that while the O2, CO2, N2 and Ar passed freely through the CC3, xenon and krypton were trapped. CC3 trapped twice as much xenon as the best MOF material tested. The CC3 also trapped twenty times as much xenon as krypton. This property is known as selectivity and it is a very important capability.
CC3 has been identified as the best noble gas trapper developed so far. The researches do not yet know exactly how CC3 functions when it traps noble gases. When they do understand the way that CC3 functions, it may be possible to fine tune this type of noble gas trapping material to trap different noble gases as desired.
Artist's rendering of the CC3 molecule: