Researchers have discovered a versatile chemical agent—referred to as a ‘chameleon’—that could enhance the purification processes of rare-earth metals. These metals play key roles in clean energy, healthcare, and national security fields.
A team from Oak Ridge National Laboratory, part of the Department of Energy, has unveiled a chemical “chameleon” that may revolutionize how we purify rare-earth metals essential for clean energy, medical applications, and national security.
This research, conducted in partnership with Vanderbilt University, is part of ongoing work by ORNL’s Chemical Sciences Division aimed at simplifying the extraction of lanthanides. These metals are present in many products, from medical imaging tools to industrial chemicals and electronics. Lanthanides, numbering 15, together with two other elements, form what are collectively known as rare-earth metals.
Despite their name, rare-earth metals are often not rare at all; lanthanides occur naturally in mineral deposits and can be as common as copper and lead. However, to harness the unique properties of these metals for practical use, they must be separated from other metals found in their ores. This purification process is challenging and resource-intensive, which is where the true rarity lies.
“The challenge arises because lanthanide ions are remarkably similar in size and chemistry,” noted Subhamay Pramanik, a former ORNL postdoctoral researcher now working as a radiochemist with ORNL’s Nanomaterials Chemistry group. “They differ only minutely, making precise separation crucial to isolating pure lanthanides.”
To extract specific metals from solutions containing rare-earth minerals, scientists and industry professionals utilize ligands—chemical compounds that latch onto specific metals. These ligands are combined with an organic solvent and then mixed with a water-based solution of the lanthanide mixture. Similar to how oil doesn’t mix with water, the mixture separates into layers, where successful ligands pull the target metal into the organic layer, allowing for subsequent processing.
Currently, efficient industrial separation methods require multiple stages to extract lanthanides in a specific order—either from heavy to light or vice versa. This approach is lengthy, costly, and generates significant waste, often harmful to the environment.
That’s where the chameleon comes in. While investigating an existing ligand that closely resembles commonly used compounds, scientists made an intriguing discovery: this ligand displays different behaviors based on experimental conditions. Much like a chameleon adapts its color, this ligand changes its binding preferences according to factors like the solution’s acidity and the duration of its interaction. For instance, a more acidic environment leads the ligand to bind more readily with heavier lanthanides.
“In typical separation methods, ligands generally prefer either lighter or heavier lanthanides,” explained Santa Jansone-Popova from ORNL, who co-led the study. “However, we found that the same ligand can be employed for various separations, which is both exciting and novel. We’ve also pinpointed the mechanisms behind these behaviors.”
Utilizing a single compound for multiple lanthanide separations could significantly reduce the complexity of this costly process. Additionally, depending on the environmental conditions, the ligand could isolate the heaviest, lightest, or mid-weight lanthanides in any sequence.
Unlike other ligands, this one exhibits behavior that was previously unrecognized. It shares structural similarities with established ligands but operates quite differently. Now that the potential for such functionalities has been identified among lanthanide-binding compounds, further studies could unveil more ligands with similar capabilities.
“Just because one ligand looks like another, it doesn’t mean they will behave the same way. This new understanding expands the frontiers of our knowledge,” stated Ilja Popovs from ORNL, who co-led the study. “This discovery holds promise for making separation processes faster, greener, and more efficient by reducing the number of steps, enhancing selectivity and purity, and improving environmental outcomes.”
This research received funding from the DOE’s Office of Science, specifically its Separation Science and Materials Chemistry programs. Parts of this study utilized facilities supported by the DOE Office of Science, including Argonne National Laboratory’s Advanced Photon Source and Brookhaven National Laboratory’s National Synchrotron Light Source II.
