The nuclear butterfly effect

Materials World magazine
,
7 May 2012

A previously undiscovered ‘butterfly’ uranium molecule could inform clean-up operations of nuclear waste, say scientists at the University of Edinburgh, UK.

‘We’ve made a molecule that shouldn’t really exist,’ says Dr Jason Love, Reader in Inorganic Chemistry at the University of Edinburgh. Love’s team has discovered a new reaction between the uranyl ion (the most common form of uranium in the environment) and an organic framework capable of holding uranium, which causes the compound to change its structure.

The research builds on earlier work into uranium compounds that the researchers claim resemble iconic computer game character ‘Pacman’, but this time, Dr Love says that the uranium-oxygen motif in the new compound has a ‘central uranium-oxygen bridge, so it looks like a butterfly’.

With its linear oxygen-uranium-oxygen structure, normally the uranyl ion compound is thought of as stable and unreactive – which can render it a problematic radioactive contaminant if it gets into watercourses. Using the organic framework, two uranyls are forced very close together, which left no room to have the oxygen atoms bound in a linear arrangement. Instead, one of the oxygen atoms was forced into an unusual, 90-degree position off to one side, like a bridge, prompting the researchers to give it the ‘butterfly’ moniker.

‘We are directing the chemistry by using very specific organic frameworks that enable us to then isolate compounds that are really quite unusual,’ says Love. ‘This [bridge-shaped] motif wouldn’t normally be stable. If you removed it from the framework, it would presumably decompose. Certain compounds exhibit enough thermodynamic stability to be isolated and characterised, while others don’t and can be transient. Fortunately we have managed to find the right organic framework and reaction environment to synthesise this new compound and have the techniques available to properly characterise it.’

He adds that the unconventional, butterfly shape had been proposed in academic journals, but this was the first time it has been seen experimentally. Although the team made the discovery after investigating the fundamental structure and reactivity of uranium compounds, he claims that it could inform nuclear waste research. ‘One of the issues in radioactive waste remediation is to stop unwanted precipitation occurring. This has unwanted precipitation. When you modify the framework around the uranyl ion, you can make these oxygen atoms reactive and when that happens you get aggregation occurring and this can cause precipitation.’

Love thinks that the bridge-shaped structure of the molecule could be shared with other uranium-containing materials associated with radioactive waste. ‘Part of the problem of dealing with radioactive waste is not knowing what species are present if you want to separate the waste and target certain elements in it. This discovery gives us a little insight into what could be happening in nuclear waste, especially when you can get problems with nuclear waste remediation with an aggregation of these types of molecules or oxocompounds.’

He adds that the research could also provide some clues for studying more radioactive elements such as neptunium and plutonium, but any research would need to be handled by collaborators in Germany and France, as the UK lacks the general research facilities to handle these highly radioactive elements. ‘The UK used to be strong in this area, but there has been a loss of expertise and other countries, such as France, have taken the lead,’ he says.