(From left) Former University of Chicago Pritzker School of Molecular Engineering (UChicago PME) Ph.D. student Siqi Zou and Assoc. Prof. Chong Liu led a team of researchers from UChicago PME and Northwestern University that developed a cleaner method to separate rare earth elements from each other, which could affect technology manufacturing.
Photo Credit: John Zich
Scientific Frontline: Extended "At a Glance" Summary: Electrochemical Separation of Rare Earth Elements
The Core Concept: A novel, water-based processing method that purifies rare earth elements without the use of organic solvents by utilizing electrochemical intercalation to separate similar lanthanides based on the size of their hydration shells.
Key Distinction/Mechanism: Traditional extraction relies on custom molecules and copious amounts of toxic acid to separate chemically similar elements. In contrast, this new mechanism forces raw mixtures into angstrom-sized channels within layered manganese oxide, using a magnesium ion scaffold to "pin" the channel spacing and differentiate elements based on the exact size of the water shell surrounding each dissolved ion.
Major Frameworks/Components:
- Electrochemical Intercalation: Squeezing specific ions into the layered mineral matrix using an electric current.
- Hydration Shells: The layers of water molecules naturally enveloping dissolved rare earth ions, which dictate the effective physical size of each element in a solution.
- Manganese Oxide Channeling: Engineered mineral structures featuring precisely spaced gaps to filter elements by minute differences in atomic behavior.
- Magnesium Pinning: The strategic introduction of magnesium ions to serve as a rigid scaffold, preventing the mineral channels from expanding and forcing a higher binding selectivity between nearly identical elements.
- Density Functional Theory: Quantum mechanical simulations used to predict atomic arrangements and validate how rare earth elements position their hydration shells inside the confined channels.
Branch of Science: Materials Science, Inorganic Chemistry, and Chemical Engineering.
Future Application: The technique offers a scalable blueprint for building localized, environmentally friendly processing facilities that rely solely on water and electricity, effectively removing the need to ship ores to overseas toxic solvent processing plants.
Why It Matters: Rare earth elements, such as neodymium, lanthanum, and dysprosium, are critical components for manufacturing electric motors, LED lighting, and medical imaging devices like MRI machines. A cleaner extraction process mitigates the severe ecological damage and supply chain vulnerabilities associated with traditional purification.
Rare earth elements such as lanthanum, neodymium, and dysprosium are used to build the electric motor in your car, the LED lights in your house, and the MRI machine at your doctor’s office.
But first, they must be mined and separated from one another. Historically, that purification has been a costly, difficult process, relying on huge amounts of toxic chemicals.
Now, researchers in the lab of Associate Professor Chong Liu at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME), working with colleagues at Northwestern University and Argonne National Laboratory, have discovered a cleaner method to separate rare earth elements from one another.
The new approach relies on a layered form of manganese oxide—a mineral material with appropriately sized layers to allow ions to slip in and out and to differentiate rare earth elements.
“This is the first time that people have used electrochemical intercalation and harnessed the structural characteristics to separate similar lanthanides, which are intrinsically very hard to separate,” said Liu, senior author of the new study, which was published in Nature Chemical Engineering. “What’s also valuable is that we provided a lot of new understanding of how rare earth ions are interacting with this material and how we can manipulate it for better selectivity.”
“This kind of separation is competitive with other rare earth separation methods, but it’s done in water, without organic solvents,” said George Schatz, professor of chemistry at Northwestern and a co-author of the study. “That’s a difference that could actually matter at manufacturing scale.”
“This is the first time that people have used electrochemical intercalation and harnessed the structural characteristics to separate similar lanthanides, which are intrinsically very hard to separate.” —Associate Professor Chong Liu, senior author of the study
Squeezing Elements Through Channels
The 17 rare earth elements—including the 15 lanthanides, plus scandium and yttrium—rarely occur alone. They are almost always mined together, and chemically they are nearly identical, with only tiny differences in ion size and acidity differentiating each one. Pulling them apart typically requires custom-built molecules and large amounts of acid, which is used to strip each element off those molecules.
“Rare earths always come mixed together, whether they’re in an ore or in a waste stream, and separating them from each other is a second, very challenging step even after you’ve pulled them away from everything else,” said UChicago PME graduate student Jiadong Liu, a co-first author of the new paper.
Chong Liu and her colleagues knew that one of the differences between rare earth ions was the size of the water shell surrounding each one when they are dissolved in solution. Lighter rare earths, such as lanthanum, have a larger first water shell, while heavier rare earths, such as dysprosium, have a smaller first shell.
Taking advantage of that size difference, Chong Liu’s group engineered manganese oxide so that the gaps between its stacked layers were only a few water molecules wide. Then, they squeezed raw mixtures of rare earth elements inside.
The approach divided the elements into two groups. Heavier lanthanides with smaller water shells became stuck in the channels more tightly. Lighter lanthanides with larger shells pushed the layers apart, loosening their grip.
To confirm what was happening at a molecular level, the UChicago PME team collaborated with Schatz’s group at Northwestern to run quantum mechanical simulations using a method called density functional theory, which predicts how atoms arrange themselves and interact based on the underlying physics. They also worked with Argonne scientists to obtain experimental X-ray data.
“It was incredibly rewarding to see how closely our density functional theory calculations matched the synchrotron X-ray measurements,” said co-first author Woo Cheol Jeon, who conducted the research as a postdoctoral researcher in Schatz’s lab at Northwestern. “The calculations let us see, atom by atom, how each rare earth element arranges its hydration shell inside the confined channel, which experiments couldn’t resolve directly.”
Fine-Tuning the Purification
While the new process separated the heaviest and lightest rare earths, some of the most useful elements still behaved too similarly to separate. To fine-tune the purification so that it could differentiate similar pairs of rare earth elements, the research team used an electric current and added magnesium ions.
The magnesium acted as a scaffold, holding the manganese oxide channels to their designed spacing even when rare earth elements tried to expand it—an effect called pinning. Now, rare earth ions showed differences in binding to the layered material even when they were extremely similar.
“Even elements that behave almost identically will still try to expand the material to make room for their water molecules,” said Siqi Zou, PhD ’24, co-first author of the study and former UChicago PME graduate student. “By pinning the channel so it can’t expand at all, we forced that small difference in behavior to become a much bigger difference in how strongly each element binds.”
With the addition of magnesium, the enrichment of neodymium over lanthanum jumped from a 1.6-fold difference to a 5.4-fold difference. After two cycles of purification, researchers could obtain a neodymium sample that was 97% pure. Similar improvements were seen for other rare earth elements.
“This kind of separation is competitive with other rare earth separation methods, but it’s done in water, without organic solvents. That’s a difference that could actually matter at manufacturing scale.” —George Schatz, professor at Northwestern and co-author of the study
A Cleaner Future
The new purification method could ultimately point toward different ways of thinking about where rare earth processing happens, the researchers said.
“Right now, rare earth ores get mined all over the world, but almost all of them end up being sent overseas for processing,” said Schatz, co-author of the study. “A method like this, that just uses water and electricity instead of organic solvents, is the kind of technology that could actually change how and where that processing gets done.”
The method isn’t yet ready to be scaled up to replace industrial purification. Still, it points to a broader design principle: tuning a channel’s width can determine which ions a material prefers, even for ions that differ by a fraction of an angstrom.
Chong Liu’s group is now testing the approach against more of the 15 lanthanides, while Schatz is refining his computational models, aiming to explain the pinning effect more quantitatively. He is also using the same modeling approach developed for this work to study other materials.
Published in journal: Nature Chemical Engineering
Title: Pinning ångström-size solid ionic channels for rare-earth element separation
Authors: Siqi Zou, Jiadong Liu, Woo Cheol Jeon, Maoyu Wang, Ronghui Wu, Yu Han, Gangbin Yan, Grant T. Hill, Xiaolin Yue, Hua Zhou, George C. Schatz, and Chong Liu
Source/Credit: University of Chicago | Sarah C.P. Williams
Edited by: Scientific Frontline
Reference Number: ms081026_01
