Of all the radioactive isotopes left over from nuclear weapons testing and nuclear power plants, cesium-137 is among the most dangerous. The soft, silvery-white metal has a half-life of 30 years, enters the body quickly, and can trigger cancer even decades after exposure. Removing cesium-137 from the environment has proven difficult, but researchers say they have a promising new way to clean it up: a flexible, porous solid that grabs cesium ions much like a Venus flytrap ensnares its prey.
The new material is part of a class of materials made of spongelike frameworks of inorganic elements. Over the years, these materials have been used for everything from catalyzing chemical reactions to capturing carbon dioxide from the air. Mercouri Kanatzidis, a chemist at Northwestern University in Evanston, Illinois, and his student Nan Ding--now an assistant professor at Claflin University in Orangeburg, South Carolina--were working to create one of these inorganic frameworks, possibly one that could be used to capture environmental contaminants.
The researchers made their framework from a mixture of gallium, tin, and sulfur, which formed sheets with holes. They also added dimethylammonium (DMA) ions. The sheets stacked atop one another with the holes running up and down through the material and with the DMA ions sitting in between the layers.
Judging from the size of the holes in the framework, Kanatzidis and Ding suspected that cesium-137 ions might be able to wiggle through the holes into the heart of the solid and trade places with the DMA, which, like the cesium's ions, are positively charged. And that's what happened. But then to the researchers' surprise, when they attempted to flush out the cesiums with other charged ions, such as lithium and sodium, the cesium didn't exchange places as expected and instead remained locked in the solid.
To find out why, Kanatzidis and Ding took an x-ray snapshot of their material. They found that when the cesium enters, it not only displaces DMA, it also binds to a sulfur atom in the lattice. This tugs on the framework and pulls the holes closed, thereby trapping the cesium inside, a bit like a molecular Venus flytrap. Further studies also showed that the material preferentially bound cesium ions even in the presence of chemically similar alkali metals. That suggests that the material might work in a chemically complex environment such as one found at a nuclear cleanup site, the team reports online this week in Nature Chemistry.
"Binding metal ions is not an easy task. And being able to do that selectively is difficult," says Omar Yaghi, a chemist and open framework materials designer at the University of California, Los Angeles. Kanatzidis says the new material is not likely to head straight for nuclear cleanup sites. That's because it uses gallium, an expensive metal. So he and his colleagues are now looking to replace the gallium with cheaper components for use in similar cesium trapping materials.