Selective bonding: Up until now, the rare earth metals that are coveted as raw materials can only be separated from one another with great effort. But in the future, a protein isolated from bacteria could simplify this separation considerably, as researchers report in “Nature”. In the test, the lanmodulin protein was able to separate the important raw materials neodymium and dysprosium from one another with a good 98 percent yield and purity – in one step. By specifically modulating its binding sites, it could also be adapted to other rare earths.
Whether neodymium, scandium, ytterbium or praseodymium: rare earth metals are indispensable for many modern technologies and are therefore in demand. But above all the processing of ores with rare earths is extremely complex. Because the 17 rare earth elements are chemically and physically very similar, they are difficult to separate from one another using common extraction methods.
The separation is the problem
Although the largest rare earth deposit in Europe was only recently discovered in Sweden, it is still unclear how and where these ores are to be processed. It is true that rare earth metals can be non-specifically enriched from solutions of other metals by physical processes such as flash Joule heating or with the help of cyanobacteria. However, separating the individual elements from each other and recovering them in their pure form is time-consuming:
“Conventional hydrometallurgical separation processes for rare earths use organic solvents such as kerosene and toxic phosphonates, and they require dozens, sometimes even hundreds of steps to recover highly pure single rare earth oxides,” said Joseph Mattocks of Pennsylvania State University and his colleagues. Therefore, scientists have long been looking for organic natural substances that could enable a more efficient and simpler separation.
Bacterial protein binds rare earth metals
Now Mattocks and his team have found what they are looking for: they have discovered a natural protein that naturally has a particularly high affinity for certain rare earth metals. This lanmodulin protein is produced by the bacterial species Hansschlegelia quercus, which was only recently discovered in oak buds. The lanmodulin has four hand-like binding pockets with which it can bind metal atoms, including rare earths.
As the researchers discovered, the newly discovered lanmodulin has an affinity for rare earth metals that is around 100 million times higher than for other metals such as calcium. Binding to the rare earths triggers a conformational change in the protein, which causes two molecules to combine and enclose up to three rare earth ions. In this regard, the new lanmodulin already significantly outperforms previously known versions of this protein, the team reports.
Separation of neodymium and dysprosium in the test
The decisive factor, however, is that lanmodulin does not react in the same way to all rare earths, but prefers to bind to the lighter elements of this group. As a result, it can be used to separate the rare earth mixtures typical of many ores. “We therefore investigated how well lanmodulin can separate the two rare earth metals neodymium and dysprosium, which are important for permanent magnets,” say Mattocks and his team.
To do this, the researchers first had larger amounts of the protein produced by genetically manipulated Escherichia coli bacteria. They then bound these proteins to the surface of agarose nanospheres, which acted as support and filter material. For the actual test, the scientists then ran a solution with five percent dysprosium and 95 percent neodymium, which is typical for electronic scrap recycling, through a column filter with these beads.
The result: “After just one cleaning step, we were able to recover 98 percent of the neodymium and with a purity of 99.8 percent,” reports the team. The lanmodulin nanospheres had selectively bound the neodymium ions and let the dissolved dysprosium through. A version of lanmodulin slightly modified in its amino acid sequence even achieved a purity of more than 98 percent and a yield of 99 percent for both rare earth metals – in just one step.
Worthwhile starting point for further optimizations
According to the researchers, these results demonstrate that bacterial lanmodulin proteins are not only suitable for extracting rare earths from metallic solutions – they could also facilitate and shorten the separation of different rare earth elements. Rinsing with 0.1 molar hydrochloric acid was enough to remove the neodymium from the lanmodulin nanospheres, for example.
Mattocks and his team therefore consider it worthwhile to continue researching lanmodulins as rare-earth absorbers. Because they have studied in detail the binding of these molecules to these elements, their study also opens up further possibilities to change and optimize the selectivity of these proteins. (Nature, 2023; doi: 10.1038/s41586-023-05945-5)
Source: Nature