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Fabrication and process

Fabrication and process

Metal ion separation speeds up

06 Jun 2019 Isabelle Dumé
A faster method to purify elements
A faster method to purify elements. (Courtesy: Marilyn Chung/Berkeley Lab)

A new method to purify actinide elements that is much more efficient than conventional approaches could revolutionize metal ion separation. The method relies on siderophore-inspired ligands that can bind, with unprecedented selectivity, metallic cations based on the size and the charge of the metal.

Producing pure isotopes is crucial for many fields, including nuclear medicine, waste recycling, space exploration and, of course, fundamental research. All these applications call for high product purity, which means highly-efficient and cost-effective separation techniques. Purifying the target element by separating out contaminants can be difficult and time-consuming, however. These contaminants are often the target’s neighbour elements in the periodic table.

Researchers at the Lawrence Berkeley National Laboratory have been studying a class of hydroxypyridinone (HOPO) chelators for their ability to selectively bind metals with high efficiency. These molecules show promise for use in separation science thanks to their unique combination of properties; These are: their solubility in water; their structure consisting solely of H, C, N and O atoms; their ability to control metal oxidation states without the need for additional redox-active species; their extremely high charge-based selectivity; and their stability when bound to metals, even in strong acids.

Siderophore-based compounds

Gauthier Deblonde, Abel Ricano and Rebecca Abergel of Berkeley Lab have been focusing on synthetic siderophore-based compounds. Researchers have known about this class of ligands, comprising HOPO and catecholamide (CAM) derivatives, for decades but they have rarely, if ever, studied them for separation applications.

The team looked at the model compound 343HOPO, which is better than any known chelator in terms of charge-specific selectivity, and in particular for binding tetravalent ions. This means that it can be used to isolate charged ions – something that could come in useful for separating actinium/thorium ions (Ac3+/Th4+) or plutonium/americium ion (Pu4+/Am3+) mixtures.

The researchers began by testing 343HOPO on Ac-225. This isotope shows promise for targeted radiotherapy (alpha therapy), but its development and use are being hampered for lack of availability. They found that the separation factor (SF) is as high as 106 for isolating Ac from metal impurities. The SF is a measure of how well an element can be separated from a mixture. A high SF means that fewer steps and less solvents are needed in the separation process, making it faster and more cost-effective.

An alternative to current chemical processes

Using 343HOPO could be an alternative to current chemical separation processes under development. “With any production process, you need to purify the final isotope,” explains Abergel, who heads Berkeley Lab’s Heavy Element Chemistry group and is an assistant professor in the Nuclear Engineering Department of UC Berkeley. “Our method could be used right after production, before distribution.”

The researchers also purified two other actinides in their study, Pu and berkelium (Bk). An isotope of plutonium, Pu-238, is used for powering robots being sent to explore Mars. Pu isotopes are also present in waste from nuclear power plants, where they must be separated out from uranium. Bk, for its part, is important for fundamental research and as a target for discovering new elements.

Again, the researchers found extremely high SF values: an SF of 10for purifying Pu from uranyl ions and trivalent actinides or fission products, and an SF of more than 3 x 10for isolating Bk from adjacent actinides and fission products.

“Our proposed process appears to be much more efficient than existing processes, involves fewer steps and can be done in aqueous environments, and therefore does not require harsh chemicals,” adds Abergel.

The team, reporting its work in Nature Communications 10.1038/s41467-019-10240-x, will now try using its process on medical isotopes other than Ac-225. The method could be generalized as long as there are different charges on the metals being separated, explains Abergel. “Having a good separation process available could make everything easier in terms of post-production processing and availability,” she says.

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