Chemical chameleon reveals new way to separate rare earth metals

Researchers at the Department of Energy’s Oak Ridge National Laboratory have discovered a chemical “chameleon” that could improve the process used to purify rare earth metals used in clean energy, medical and national security applications.

The study, conducted in collaboration with Vanderbilt University, is the latest in a series of efforts by ORNL’s Division of Chemical Sciences to reduce barriers to accessing metals called lanthanides, which are widely used in a variety of products and applications, from biomedical imaging to industrial chemical production to electronics. There are 15 lanthanides, and they are, along with two other elements, known collectively as rare earth metals.

The research is published in the Journal of the American Chemical Society.

Contrary to what their name suggests, most rare earths are not rare. Lanthanides occur naturally in ore deposits, and many are as common in the environment as copper and lead. However, the powerful properties of the metals that make them so widely used are only functional if an individual lanthanide is separated from the mixture of other metals in which it is present during its extraction. The chosen metal must be purified to a high level before it can be used in its intended application. The rarity lies in the difficulty of this process.

“It’s a big challenge because the lanthanide ions are very similar in size and chemical properties,” says Subhamay Pramanik, a former ORNL postdoctoral fellow and now a radiochemist in ORNL’s Nanomaterials Chemistry Group. “They only differ by a tiny amount, so isolating pure individual lanthanides requires very precise separation science.”

To isolate a selected metal from rare earth mineral solutions, scientists and industry use ligands, chemical compounds that selectively bind to a particular metal in the solution. These compounds are mixed with an organic solvent and then mixed with an aqueous solution of the lanthanide mixture.

Like oil, organic solvents do not mix with water, so the layers separate. If the compound manages to capture the target metal from the solution during mixing, it pulls the metal into the organic layer as the solvent and aqueous solution separate. The metal can then be further processed and purified.

The best current industrial separation processes are stepwise, with the lanthanides separated in a particular order: from heavy to light or from light to heavy. This process is long and expensive, and produces a lot of waste that is not always environmentally friendly.

This is where the chameleon comes in. By studying an existing ligand, similar to the compounds used in the aforementioned method, the scientists discovered something unique: a ligand that behaves differently depending on the experimental conditions.

In the same way that a chameleon changes color to match its environment, the compound changes behavior when the environment around it changes, binding to different lanthanides depending on the acid concentration of the solution and the amount of time the ligand is allowed to interact with it. For example, if the environment is more acidic, the ligand preferentially binds to a heavier lanthanide.

“In conventional separation systems, a ligand typically shows a preference for lighter or heavier lanthanides,” said ORNL’s Santa Jansone-Popova, who co-led the study. “We found that it’s possible to use the same compound to perform multiple different separations, which is exciting and unique. And we identified the mechanisms by which it does this.”

Using the same compound to separate several different lanthanides in the series could reduce the number of steps required in this common and expensive process. Furthermore, depending on the conditions, the ligand used in this study could separate the heaviest, lightest, and middle-weight lanthanides, in any order.

Other ligands do not exhibit this same behavior. However, until now, scientists did not know that this one would either. The chameleon ligand is similar to other well-known ligands, but it has a completely different behavior. Now that it is known that such abilities and systems exist for lanthanide-binding compounds, the ligand can be studied further and other compounds with similar behavior could be discovered.

“Just because one ligand is very similar in structure to another doesn’t mean it necessarily behaves the same way. This understanding can help move things forward and push the boundaries of what’s known,” said ORNL’s Ilja Popovs, who co-led the study. “It has the potential to make separation processes faster, cleaner and more efficient, by reducing the number of steps, providing better selectivity and purity, and leading to more environmentally friendly processes.”