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RESEARCHERS at Northwestern University, US, have developed a synthetic material made from gallium, sulphur and antimony compounds which selectively traps caesium ions.

Radioactive caesium is commonly found in nuclear waste and is particularly problematic to clear up. The ratio of caesium to sodium can be as low as 1:1000, so selectively capturing it is extremely difficult. The Northwestern chemists, led by chemistry professor Mercouri Kanatzidis, found that their new material shows extremely high selectivity in capturing caesium from solutions containing similar ratios of caesium and sodium to nuclear waste.

The material consists of layers of [(CH₃)₂NH₂]⁺ and [Ga₂Sb₂S₇]^2-. The structure is open and porous, which encourages ion exchange. The pores, or windows, are initially occupied by organic cations, however these are easily exchanged for large caesium ions in aqueous solutions. The size and flexibility of the pores is the reason for the high selectivity towards caesium.

The caesium ions form weak bonds with the sulphur in the material, and this interaction causes the material to change shape, closing the pores and permanently trapping the caesium ions, much like  – as the researchers say – a venus fly trap catching a fly. Sodium ions cannot trigger the same response as they interact with water molecules.

“Seeing the windows close was completely unexpected. We expected ion exchange – we didn't expect the material to respond dynamically. This gives us a new mechanism to focus on,” says Kanatzidis, adding: “Ideally we want to concentrate the radioactive material so it can be dealt with properly and the non-radioactive water thrown away.”