Researchers develop mussel-inspired coating that can extract rare earth elements

RESEARCHERS at Penn State University, US, have developed a mussel-inspired nanocellulose coating (MINC) that can extract neodymium – a critical element used in clean energy technologies – from secondary sources such as industrial wastewater without using a high amount of energy.  

Playing a vital role in the production of lightweight, efficient batteries, and as essential powerful magnetic components in wind turbines, rare earth elements (REEs), including neodymium, are in high demand.  

Despite their name, many REEs are actually common in the Earth's crust. The problem is they are found in low concentrations in minerals, and they are difficult to obtain. Ironically, the methods normally used to extract these elements are anything but “environmentally friendly”.  

Along with requiring high-energy processes that typically use fossil fuel, mining is often criticised for the habitat destruction it can cause, and for the toxic chemicals that are released into the environment. Figures suggest that processing 1 t of rare earth metals produces 2,000 t of toxic waste, as well as vast quantities of wastewater – another commodity that is in short supply in parts of the world. 

In an effort to circumvent these negative environmental impacts, some scientists have turned to extracting REEs from waste streams such as industrial and municipal wastewater sources. This too, however, can have various disadvantages such as high consumption of chemicals, high operational costs, and low purity of the extracted elements. 

But, after finding inspiration from under the sea, Penn State researchers have discovered what they say is an eco-friendly solution to the problem: anionic hairy cellulose nanocrystals, or more simply, mussel stickiness.

Dubbed MINC (mussel-inspired nanocellulose coating), the adsorbent mimics the natural glue mussels use to keep themselves attached to rocks.  

To develop MINC, cellulose fibrils from delignified softwood kraft pulp are converted to bifunctional hairy cellulose nanocrystals (BHCNC), which contain dialdehyde and dicarboxylate molecules, via oxidation reactions with periodate and chlorite. 

Cellulose is an ideal base for an adsorbent as along with being the most abundant biopolymer on Earth, hairy cellulose nanocrystals have a high specific surface area, excellent mechanical properties, and good biocompatibility. And, when combined with certain reactants, hydroxyl groups on cellulose can be converted into carboxyl groups. 

These BHCNC then stick to a substrate, such as silica gel (though any substrate would do, note the researchers), via the reaction of dopamine with the dialdehyde groups of BHCNC and the polymerisation of dopamine on the substrates. Meanwhile, the dicarboxylate on the needle-like nanoparticles use electrostatic interaction to help filter out neodymium in a solution within minutes.  

According to the researchers, the MINC is to neodymium what a magnet is to iron, pulling the REE out of water, even when the element is only present as parts per million. That’s because neodymium is normally found in compounds in the ⁺³ oxidation state. The hairy cellulose nanocrystals are anionic, however, so the two are drawn to one another. 

To recover the adsorbed neodymium from the BHCNC, the solution pH was decreased. Lowering the pH increases acidity and causes the dicarboxylate groups to undergo protonation, which liberates the captured neodymium. 

MINC is also partially selective and doesn’t significantly pull out other undesired elements like sodium and calcium, which would require additional time and energy if they had to be filtered to further refine the neodymium. 

Amir Sheikhi, assistant professor of chemical engineering and biomedical engineering, is lead author of the research. He explained that other rare earth extraction methods which have used adsorbents such as alginate gels, phosphorus sol-gel materials, nanotubes and porous carbon, have limited efficiency, but MINC overcomes these problems.

“The public and society will benefit from this work through the potential for increased availability of neodymium, a crucial element for not just developing clean energy technologies, but also for creating new medical and electronic devices,” Sheikhi said. 

Sheikhi plans to investigate how the MINC method may work to extract other REEs by further improving the selectivity and would eventually like to scale up MINC for commercial use. “We are very excited about this technology,” he said.