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A Venus flytrap for nuclear waste
Not every object is food to a Venus flytrap. Like the carnivorous plant, a new material developed at Northwestern University permanently traps only its desired prey, the radioactive ion caesium, and not other harmless ions like 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. (The solution had concentrations similar to those in real liquid nuclear waste.)
It is, in fact, caesium itself that triggers a structural change in the material, causing it to snap shut its pores, or windows, and trap the caesium ions within. The material sequesters 100 percent of the caesium ions from the solution while at the same time ignoring all the sodium ions.
The results are published online by the journal Nature Chemistry.
'Ideally we want to concentrate the radioactive material so it can be dealt with properly and the nonradioactive water thrown away,' said Mercouri G. Kanatzidis, Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences and the paper's senior author. 'A new class of materials that takes advantage of the flytrap mechanism could lead to a much-needed breakthrough in nuclear waste remediation.'
Capturing only caesium from vast amounts of liquid nuclear waste is like looking for a needle in a haystack, Kanatzidis said. The waste has a much higher concentration of sodium compared to caesium, with ratios as great as 1,000-to-1. This difficult-to-achieve selectivity is why currently there is no good 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 by going through the windows, and the caesium ions come in. In the end, the material contains only caesium ions and no organic cations. (The presence of organic cations in the liquid is not an issue as the cations are not radioactive.)
The snap-shut Venus flytrap mechanism occurs because 'soft' materials like to interact with each other. A caesium ion is big and soft, and the metal-sulphide material is soft, too. 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, close its windows and trap the caesium - like a juicy insect in a flytrap. Sodium, which is clothed in water molecules, can't trigger the response.
Kanatzidis and Nan Ding, then a doctoral student in Kanatzidis' research group and an author of the paper, did not set out to discover the flytrap mechanism. Instead, they were investigating different structures of the material, wondering if they could act as ion exchangers.
'Seeing the windows close was completely unexpected,' Kanatzidis said. 'We expected ion exchange - we didn't expect the material to respond dynamically. This gives us a new mechanism to focus on.'
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