Uranium ion reacts
Uraniumâ??oxygen bond can do chemistry after all.
Uranium, the heaviest naturally occurring element, prevalent in nuclear waste, has been forced into a chemical reaction.
The chemical achievement won't yet allow researchers to clean uranium out of the environment. But it is big step forwards in understanding how the element works — which might one day lead to uranium 'mops' or even new catalysts for industrial processing.
The uranyl ion, [UO2]2+, is the default form that uranium takes in water the environment. The metal–oxygen bonds in this linear molecule are incredibly strong, making the ion very unreactive. But because the substance is very soluble in water, and so very mobile, it is difficult to clean up. If the uranium ion is radioactive, this presents a problem.
The US Department of Energy estimates that in the United States 2,500 billion litres of groundwater are contaminated with uranium from nuclear-weapons production. Researchers have looked at using bacteria or plants to soak up the element, but another solution might be to get the uranyl ion to react with something that makes it insoluble, so that it drops out of water. That would make it easier to collect and get out of the environment.
Polly Arnold at the University of Edinburgh, UK, and her colleagues, got around the ion's stubborn unreactivity by trapping it in the mouth-shaped cavity of a large, 'pacman'-shaped organic molecule. This arrangement slightly bends the usually rigid uranyl molecule, so that the oxygen atom sticking out of the top of the large complex becomes reactive, and grabs silicon-containing groups chucked into the mixture. The oxygen then binds to silicon as well as uranium, forming a strong silicon–oxygen bond.
This particular reaction doesn't work in the presence of air or water, so it won't be much use in the environment. "We can’t use these molecules to clean up nuclear waste," says Arnold.
But it stands as proof of principle that the ion can react — which might be turned to use in cleaning in future.
Put to use
A better understanding of uranium chemistry is fundamental to waste-disposal techniques, Arnold says. “Everything that challenges our preconceptions about bonding helps us understand the bigger picture,” she says. The system should also help researchers to understand the chemistry of the similar element plutonium, which is too dangerous to study in most labs.
Arnold is also excited by the longer-term prospect of using uranium as a catalyst, like the other transition metal oxo-compounds that are used widely in industry to make everything from fertilizer to pharmaceuticals.
"Metals from the rest of the periodic table have been doing this sort of chemistry for ever," says Arnold. But as the heaviest natural element, uranium might add something that the other, lighter metals don’t have. "It provides a very different set of electronic properties than other metals," says Arnold.
- Arnold, P. L., Patel, D., Wilson, C. & Love, J. B. Nature 451, 315-317, (2008).