New Material Sops Up Radioactive Cesium

By Lisa Grossman, Science News

A new material is like a roach motel for radiation: Cesium checks in, but it can’t check out.

The substance, described online January 24 in Nature Chemistry, could provide a better way to safely store radioactive isotopes of the element cesium, which is produced in nuclear power plants

“This material points to a new mechanism with which we can go after these very hard-to-get ions,” says Mercouri Kanatzidis of Northwestern University, a coauthor of the new study.

Kanatzidis and his colleagues designed the substance — made from layers of a gallium, sulfur and antimony compound — with its atoms in an open framework arrangement, resembling windows in an apartment building.

“This compound demonstrates a clever design,” comments Omar Yaghi of the University of California, Los Angeles.

Such substances, when they have a charged particle, or ion, balanced in each window, are known to trade that ion for a similarly charged particle in the environment.

“It’s like when you’re going through a door and at the same time someone else is trying to exit the same door, and you say, ‘Excuse me!’” Kanatzidis says.

Usually the newly trapped ion can escape from the lattice by swapping places with another, nearby ion. This meant that previous nuclear cleanup substances based on the open framework arrangement allowed radioactive cesium to leach back into the environment.

But the new material trapped cesium ions for good. “It grabbed the cesium and wouldn’t let it go,” Kanatzidis says.

What’s more, the material seemed to specifically trap cesium and not other ions that have the same charge, such as sodium or potassium. Kanatzidis compared the lattice to a Venus flytrap, which ignores small rocks and leaves but snaps shut on nutritious bugs.

To find out why the substance has such an appetite for cesium, the researchers used X-ray crystallography to investigate the atomic structure of the lattice. Crystallography showed that the lattice windows, which start out wide enough for cesium to slip through, shrink when the large atom enters.

“As soon as cesium went in, somehow the material recognized this, and triggered a window-closing response which encapsulated and incarcerated the cesium permanently,” Kanatzidis says.

The gatekeepers are the sulfur atoms in the window frame, an arrangement previous ion traps lacked. Sulfur and cesium both form “soft” ions — their outer electrons are loosely bound and far from their nuclei. Soft ions like to bond to other soft ions, Kanatzidis says. When cesium ions pass through the window, the sulfur ions reach out and bond to the cesium, changing the angle of chemical bonds between sulfur and its neighbors and pulling the window shut.

“The recognition event is this interaction between cesium and sulfur, which then closes the window permanently,” Kanatzidis says. “Cesium is the only thing that does this.”

The researchers say that the material, which is in powder form, could be used to filter contaminated water from a power plant. The resulting cesium-saturated residue could be safely contained in a piece of dense glass until the radioactive element decayed to a safe background level, Kanatzidis says, which takes at least 90 years.

The new material may be too expensive to use commercially, Kanatzidis points out. But now that the researchers know the Venus flytrap mechanism is possible, the group is working to design similar materials with cheaper elements.

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