A new material developed at Northwestern University permanently traps the radioactive ion caesium, but not other harmless ions such as sodium.
The synthetic material – made from layers of a gallium, sulphur and antimony compound – is very selective. The Northwestern researchers found it to be extremely successful in removing caesium – found in nuclear waste but very difficult to clean up – from a sodium-heavy solution that had concentrations similar to those in real liquid nuclear waste.
It is, in fact, the caesium itself that triggers a structural change in the material, causing it to snap shut and trap the caesium ions within. The material sequesters 100 per cent of the caesium ions from the solution while ignoring all the sodium ions.
‘A new class of materials that takes advantage of the mechanism could lead to a much-needed breakthrough in nuclear-waste remediation,’ said Mercouri G Kanatzidis, professor of chemistry at Northwestern University.
Capturing only caesium from vast amounts of liquid nuclear waste is like looking for a needle in a haystack, according to Kanatzidis. The waste has a much higher concentration of sodium compared to caesium, with ratios as great as 1,000:1. This difficult-to-achieve selectivity is why there is currently no decent solution for caesium removal.
The Northwestern material is porous, with its atoms arranged in an open and layered framework structure with many windows to promote rapid ion exchange. Initially, organic cations reside in the material. When the material comes into contact with the liquid, the cations leave the material and the caesium ions enter. In the end, the material contains only caesium ions and no organic cations.
The caesium ions are attracted to the material, specifically the sulphur atoms, and together form a weak bond. This interaction causes the material to change shape and trap the caesium.