ABQ Times

Rare-earth elements play a crucial role in modern technology, found in everything from smartphones to electric vehicles. However, the process of purifying these metals is complex and environmentally challenging, primarily taking place in China. Researchers at Sandia National Laboratories have been developing an eco-friendly method for separating rare-earth elements from aqueous mixtures over the past three years.

The team, led by Sandia geochemist Anastasia Ilgen, has focused on creating metal-organic frameworks (MOFs) that can selectively absorb specific rare-earth metals. Their findings were recently published in several scientific journals, including ACS Applied Materials and Interfaces.

"We synthesized MOFs with variable surface chemistry and were able to show through adsorption experiments that these MOFs can pick out rare-earth elements from a mixture of other metals," said Ilgen. "Importantly, we illustrated that their ability to pick out metals can be fine-tuned by adding chemical groups on their surfaces."

Two zirconium-based MOFs were selected for their stability and adaptability in water. Dorina Sava Gallis, a materials chemist at Sandia, explained how different chemical groups could be added within MOFs to alter their properties or create structures with missing components.

In experiments published in Chemical Communications, it was found that missing linkers increased the binding capacity for certain rare-earth elements. The addition of phosphonate improved metal adsorption significantly.

Sava Gallis commented on the project's progress: "We are seeing that both approaches we implemented effectively tune the selectivity for different ions."

To further refine the design of selective MOFs, computational scientist Kevin Leung employed molecular dynamics simulations and density functional theory modeling. His research suggested a preference for negatively charged chemicals among heavier rare-earth elements.

Ilgen's use of X-ray spectroscopy provided new insights into how rare-earth metals interact with MOFs. Her work revealed the bonding preferences of these metals within various MOF structures.

"My spectroscopy work is the first to identify the surface complexes formed by rare-earth elements in MOFs," Ilgen stated.

Looking ahead, Ilgen proposed using mixed-metal hubs within MOF sponges to enhance selectivity for specific rare-earth elements. This approach remains untested but offers potential avenues for future exploration.

"There are several possible design strategies for ion-selective MOFs," Ilgen concluded. These include tuning metal hub chemistry and adjusting pore dimensions to favor particular elements.