Engineering Proteins to Mine for Rare Earths

Down at the bottom of the periodic table of elements, in the section featuring atomic numbers 57 through 71, are lanthanides. These elements, more commonly known as rare earths, are used for a variety of scientific and industrial applications, from petroleum refining to building smart phone components. Lanthanides are not particularly rare, despite the nickname, but they are dispersed in small concentrations in the ground. That can make it a challenge to mine and separate for different applications.

“Today, to separate and recover rare earths, you need to start with a very high concentration,” said Joseph Cotruvo, Jr., an associate professor of chemistry at Pennsylvania State University. “Most of the world does not have that. But even if you do, there are still a lot of processing steps that you need to do to upgrade it to at least 10 percent content or higher. Then you need to leach it with acid and expose that material to extract the molecules you want. Typically, you have to use a toxic solvent like kerosene to extract the rare earths. It’s quite complex and not very efficient.”

That’s just to extract the rare earths. Since the lanthanides like to travel together, the next step is to separate them from one another, as each is used for different applications. Now, Cotruvo and colleagues have discovered and re-engineered a special protein, lanmodulin, which can easily recover rare earths.

Cotruvo said he and his colleagues were first inspired to look at bacterial proteins for rare earth separation because biology has evolved solutions so that microbes and plants can leverage these elements for their own purposes. Once the group isolated lanmodulin, they found it had a remarkable ability to bind with lanthanide and could easily purify rare earths from other metals.

But where it fell short was in separating the rare earths from one another. They then started to look at the genetic sequences of approximately 700 other similar proteins to see if others might be better suited to selective separation. A version isolated Hansschlegelia quercus, a bacteria found in the English Oak buds, did the trick.

“We started with a well-understood microbe from a bacterium that we knew used lanthanides,” he said. “We didn’t know exactly what molecules we should be looking for, but by looking in the right place and coming at the problem with the right mindset, we were able to purify this protein that was really known to use rare earths and saw that it was a really selective lanthanide binding protein that we could use for recovery and separation.”

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The group then created a column-based proof of concept system to demonstrate how well the protein could recover and separate the different lanthanides. In this system, Cotruvo’s colleagues at the Lawrence Livermore National Laboratory tethered the lanmodulin proteins to small beads in a column. When an earth mixture was placed inside, the proteins would selectively bind with specific lanthanides, separating them from the rest of the mixture in a single step without harsh chemicals or a change in temperature. The group demonstrated the column could easily and selectively separate lanthanides like neodymium and dysprosium, which are used in magnet applications.

“As we move forward, you can imagine columns that have different properties. One material is not going to be able to separate every rare earth,” said Cotruvo. “So having different materials, some of which come from nature directly and some of which may be engineered to bind with a specific lanthanide, will allow different separations. You need this kind of specificity because lanthanides are really 17 metals that are incredibly similar.”

After publishing their results in Nature this year, Cotruvo and colleagues are now working to scale up as aspects of the process so it could be used for industrial purposes. Cotruvo’s laboratory also plan to explore the remaining 600+ proteins to see if they can amplify the selectivity between rare earths in the future.

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“We were lucky in that the first protein we found is excellent at rare earths compared to everything else but isn’t as good at discriminating between rare earths. The second protein is better between rare earths, but I still think we can do better—through scientific discovery and engineering,” he said. “Once we identify different proteins we can use, then we need to determine how to best scale the process. We need to scale up protein production, the columns, and other formats that might work better in certain applications. There’s plenty to do.”

In the meantime, Cotruvo hopes that chemists and engineers understand the power of biology in developing new approaches to solve old problems.

“We haven’t made much headway in separating rare earths in 50 years,” he said. “But biology can point us to new approaches that are potentially milder and more efficient. There’s not going to be a single solution to separate rare earths in all applications, but I think these proteins are a viable and somewhat surprising solution that can increase our ability to differentiate between these very subtly different elements. That’s huge.”

Kayt Sukel is a technology writer and author in Houston.