New material traps radioactive ions using 'Venus flytrap' method

February 26, 2010 By Louise Lerner
New material traps radioactive ions using 'Venus flytrap' method
Venus flytraps are selective in their prey—just as a new chemical material from Argonne and Northwestern selectively picks up radioactive cesium ions from contaminated water. Photo Credit: Jeremy Hiebert. / CC BY-NC 2.0

( -- Like a Venus flytrap, a newly discovered chemical material is a picky eater -- it won't snap its jaws shut for just anything. Instead of flies, however, its favorite food is radioactive nuclear waste.

Mercouri Kanatzidis, a scientist at the U.S. Department of Energy's (DOE) Argonne National Laboratory, and Nan Ding, a chemist at Northwestern University, have crafted a sulfide framework that can trap radioactive cesium ions. This mechanism has the potential to help speed clean-up at power plants and contaminated sites.

Nuclear waste from contains both non-toxic and highly radioactive cesium isotopes. For example, one of the main contaminants in the Chernobyl disaster zone is cesium-137, which leaches into soil and water and—with a half-life of 30 years—can remain in the environment, still dangerously radioactive, for decades.

Excising the few deadly isotopes from waste has proved difficult; most materials don't distinguish between the toxic ions and the harmless ones.

"The name of the game in cleaning up nuclear waste is to concentrate the dangerous isotopes as efficiently as possible," Kanatzidis said. "That's where this new material does its job."

The new material, a rigid frame composed of metal sulfides, has a negative charge. Its pores, therefore, attract positively charged ions. This makes it a good candidate for ion exchange—when immersed in a solution with other positive ions, the ions tucked inside the pores switch places with the ions outside.

A metal sulfide framework traps cesium isotopes inside its structure. Image courtesy Mercouri Kanatzidis. Image by Mercouri Kanatzidis / courtesy Argonne National Laboratory.

Sodium ions do this dance freely, switching as many times as they're immersed. However, when the team filled the material with cesium , they refused to move out of the material.

To find out why the material trapped cesium but not sodium, Kanatzidis and Ding had to come up with an image of the material's itself. They found that sodium bonded strongly to the water in the solution, which prevented it from becoming trapped by the framework; but a ion doesn't form strong bonds with water molecules, so it has less protection. The ion binds to several sulfur atoms in the rings of the framework, causing the rings to change shape, and the hole is sealed shut.

"Imagine the framework like a ," Kanatzidis said. "When the plant jaws are open, you can drop a pebble in and the plant won't close—it knows it isn't food. When a fly enters, however, the plant's jaws snap shut."

"As far as we know, this Venus-flytrap process is unique," Kanatzidis said. "It also works over a large range of acidities—an essential property for cleanup at different sites around the world, where pH can range considerably."

The paper, published in Nature Chemistry, is available online.

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5 / 5 (1) Feb 26, 2010
Cesium 137 decays into Barium 137m. Ba-137m decays with a half-life of about 2.55 minutes into the ground state. Ba-137m is responsible for all of the gamma radiation produced from Ce-137 decay.

I wonder if the trapping structure is damaged when the Ce-137 and/or Ba-137m decays. I am also curious what the retention rate of Ba is. In particular, could you trap a quantity of Ce-137, then do some sort of chemical treatment that would release the Ce-137 and Ba-137 (the stable Ba isotope)?

If so, you could then make a reversible filter that would accept a huge amount of very dilute solution of Ce-137, then when the filter has absorbed all of the Ce-137 it can, release it and the Ba137 in a concentrated form for disposal, and it would then be ready for reuse.

That would be a very cool product.

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