Researchers at the  U.S.  Department of Energy’s Oak Ridge National Laboratory have discovered a “chameleon” chemistry effect that could improve the processing of rare earth metals for use in clean energy, healthcare, and national security applications. The research, conducted in collaboration with Vanderbilt University, is the latest in a series of efforts by ORNL’s Chemical Sciences Division to lower barriers to access to the metals, called lanthanides, which are widely used in products and applications ranging from biomedical imaging to industrial chemical manufacturing  to electronics. There are 15 lanthanides, and  they , along with two other elements, are collectively known as rare earth metals.  The research was  published in the Journal of  the American  Chemical Society .

Contrary to their name  , most rare earth metals are not actually rare; lanthanides occur naturally in mineral ore deposits, and many are as common in the environment as copper and lead. However, the metals’ strong properties make  them widely used and useful when an individual lanthanide is separated from a mixture of other metals that contain it during mining.

The metal of choice must be highly purified before it becomes useful in its intended application. Its rarity lies in the difficulty of this separation process. “This is a major challenge, because lanthanide ions are very similar in size and chemical properties ,” said Subhamay Pramanik, a former ORNL postdoc and now a radiochemist in ORNL’s nanomaterials chemistry group. “ They differ only slightly  , so  isolating  the lanthanides into pure individual compounds requires a high level of precision separation science . ”

To  isolate  and separate a metal from rare earth mineral solutions, scientists and industry  use ligands—chemical compounds that selectively bind to a specific metal in solution. These compounds are mixed with an organic solvent, which is  then  mixed with an aqueous solution of the lanthanide mixture. Like oil, the organic solvent does not mix with water, so the layers separate. If the compound is successful  in separating the target metal from the solution during mixing, it will attract the metal to the organic layer as the solvent and aqueous solution separate. The metal can then be processed and refined further.

The best current industrial separation processes  are done in stages, with the lanthanides separated in a specific order—heavy to light or light to heavy. This process is time-consuming and expensive, and produces a lot of waste that is not always environmentally friendly.

While studying an existing ligand, similar to the compounds used in the procedure mentioned above, the scientists discovered something very unique: the ligand behaved differently depending on the test conditions.

Like a chameleon that can change color to adapt to its environment, the compound also changes its behavior as its environment changes, binding to different lanthanides depending on the acidity of the solution and the amount of time the ligand interacts with it. For example, if the environment is more acidic, the ligand will preferentially bind to a heavier lanthanide.

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

Using the same compound to separate multiple lanthanides in a series could reduce the number of steps required in the current common and costly separation procedure. Furthermore, depending on the conditions, the ligand in this study could separate the heaviest, lightest, and intermediate lanthanides in any order.

This discovery has the potential to make separation processes faster, cleaner and better, providing selectivity and obtaining rare earth metals of better purity, resulting in more environmentally friendly processes.

Source: PTT  (NASATI), according to https://phys.org/news/9/2024