Rare earth elements like scandium and yttrium are the backbone of industries that rely on unique properties such as luminescence, magnetism, and catalytic ability.
However, REEs are difficult to extract and even harder to separate. These elements, despite their name, aren’t actually rare in terms of abundance. What makes them “rare” is their dispersion throughout the Earth’s crust and their chemical similarities, which make them incredibly challenging to isolate from one another. Current separation methods—largely reliant on toxic solvents like kerosene—are not only inefficient, but also harmful to people and the environment.
“Current separation methods use kerosene and extractants-molecules that bind the REE cations, a positively charged particle, that create issues, both environmentally and in terms of efficiency,” says Stebe. “The separation process is not selective enough to efficiently separate lanthanides, meaning that it has to be repeated many times to achieve REEs in sufficient purity. The whole method is cumbersome and creates unnecessary waste.”
Stebe, along with a team of researchers from Penn, the City College of New York, the University of Illinois Chicago, Northwestern University, and the University of Chicago, look to human biology to find the molecule best suited for the job of separation: peptides.
In nature, organisms have evolved proteins that selectively bind to specific ions, despite their similar properties; one example is the calcium-binding proteins in the human body, which can distinguish between calcium and magnesium ions, even though both have the same charge.
“We are applying this concept to create a similar level of selectivity for rare earth elements,” says collaborator E. James Petersson, professor of chemistry, biochemistry, and biophysics at Penn’s School of Arts & Sciences. “By using peptide-based molecules—specifically, a truncated version of the EF-hand motif, which is naturally found in calcium-binding proteins—we are designing molecules that can selectively bind to specific rare earth elements.”
“The structure resembles a hand,” says Stebe, “and each ‘finger’ of the hand is laden with a carboxyl or carbonyl group that binds to cations floating around in solution. It’s a beautiful and complex structure that can recognize the nuanced and subtle differences between two nearly indistinct cations, and then capture and hold onto whichever cation it is ‘looking’ for. This is extremely important for separating REEs, which differ in size by only one-tenth of an Angstrom.”
In the team’s recent study published in PNAS, they found that EF-hand-containing peptides could bind to the peptide-cation complex and capture it at the aqueous-air interface. The team’s next steps in this research will be investigating how to scale this process, allowing them to isolate target REEs and collect them at usable quantities in a way that is much more efficient and environmentally friendly.
Read more at Penn Engineering Today.
