Plant-based material offers sustainable method of recovering rare earth element

UNIVERSITY PARK, Pa. — Despite rare earth elements’ importance in manufacturing cell phones, magnets and a host of other consumer and commercial electronics, the lack of a sustainable, environmentally friendly approach to obtaining these metals has led to a global shortage, according to Amir Sheikhi, associate professor of chemical engineering.

Sheikhi is the principal investigator on a paper, recently published in Advanced Functional Materials, that proposes a novel technology of isolating and recovering dysprosium, a rare earth element used to manufacture semiconductors, engines, generators and more. The team used cellulose — an abundant and critical building block found in the walls of plant cells — to selectively separate dysprosium from other elements. According to the researchers, the approach is more environmentally friendly, as well as more sustainable and efficient than other commercial approaches.

The 17 metals classified as rare earth elements can be further categorized as heavy or light depending on their chemical makeup, according to Sheikhi, who also holds the title of Dorothy Foehr Huck and J. Lloyd Huck Early Career Chair in Biomaterials and Regenerative Engineering. His team had previously used cellulose-based compounds to successfully salvage neodymium, a light rare earth element used to develop powerful magnets, from e-waste like recycled circuit boards from computers.

However, this approach had not yet been used to isolate and recover dysprosium specifically, a heavy rare earth element that can be used to do things like keep nuclear control rods thermally stable.

“As technology advances, manufacturers will need more and more dysprosium — some forecasts estimate the demand for this material may surge over 2,500% in the next 25 years,” said Sheikhi, founding director of the Bio-Soft Materials Laboratory. “Having a sustainable and environmentally friendly way to recover this material will strategically help the U.S. stay competitive with countries like China.”

Commercialized approaches to separating rare earth elements primarily use solvents, dissolved liquids or solids that can break apart minerals, and require rooms full of machinery and chemicals to function, according to Sheikhi.

“Separating rare earth elements from one another has been extremely difficult, due to the metals' very similar chemical structures,” Sheikhi said. “We have been looking for a reliable way to separate heavy elements like dysprosium from lighter elements like neodymium, while avoiding the negative environmental side effects that come from current separation approaches."

To improve this inefficient and pollutant process, the team turned to cellulose. They adjusted the cellulose’s molecular structure to create a very small, crystalline material, only about 100 nanometers long — 1,000 times smaller than the width of a human hair. This nanocellulose is covered with tiny, hair-like cellulose chains at both ends – known as anionic hairy cellulose nanocrystals (AHCNC).

The team then added their nanocellulose to a water-based solution of neodymium and dysprosium, observing how the nanocellulose was able to separate the dissolved metals through a process called adsorption, where a surface collects and holds ions from a liquid or dissolved solid. When exposed to the solution, AHCNC behaved differently from other cellulose-based materials — the chemically-modified chains in its hairs distinctively shrank, indicating a specific sensitivity to dysprosium.

“This is, to my knowledge, the first example of a cellulose-based adsorbent that can selectively filter between heavy and light rare earth elements,” Sheikhi said. “On top of that, our process is very straightforward and efficient. We just add our nanocellulose to a solution and separate the metals.”

Further study of the samples revealed how the hairs found on AHCNC can essentially act as a filter to target and separate dysprosium ions specifically. Sheikhi said this surprised the team, who had initially thought adjusting the functional group type, or specific sets of atoms that determine how elements will chemically react with one another, of the cellulose would be key to optimizing separation.

“After comparing this behavior side-by-side with other cellulose-based platforms, we determined it's not just the functional group type of the material that facilitates this selectivity,” Sheikhi explained. “It’s the structure of the material itself and the position of the functional groups, which showcases the unique properties of these hairy nanostructures.”

With more development, the team said they believe this approach could offer a faster, cleaner and commercially practical way to recycle dysprosium and other rare earth elements. Moving forward, the researchers plan to test their method’s viability isolating other rare earth elements and critical minerals. They also plan to further optimize the cellulose, with the goal of preparing the technology to scale for practical use in factories and laboratories around the U.S.

Other co-authors affiliated with Penn State include Roya Koshani, chemical engineering postdoctoral scholar; Shang-Lin Yeh and Mica L. Pitcher, chemical engineering doctoral candidates at the time of the research who have since graduated and are now both working at PPG; and Dawson Alexander, undergraduate student in chemical engineering at the time of the research who has since graduated and is now working at Syensqo. Additional co-authors include Karuna Anna Sajeevan, a postdoctoral researcher at Iowa State University; and Ratul Chowdhury, assistant professor of chemical and biological engineering at Iowa State University.

Sheikhi also holds courtesy appointments at the departments of biomedical engineering and chemistry.

The work is supported by Penn State and the Dorothy Foehr Huck and J. Lloyd Huck Early Career Chair; the U.S. Department of Energy; the Office of Energy Efficiency and Renewable Energy; the Advanced Materials and Manufacturing Technologies Office; and Iowa State University.

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