When phones or computers are recycled, small amounts of important materials get discarded. Those minute amounts of cobalt, nickel and lithium add up quickly, and separating and recovering these "critical materials" for reuse is a dirty, energy intensive job.
New Pitt research has found there are proteins that may be able to do the same job without the use of harsh chemicals and rigorous energy demands. Meng Wang , an assistant professor of environmental and civil engineering in the Swanson School of Engineering, has used ferritin to successfully recover some of these same critical metals from a liquid solution.
In 2019, the United States generated nearly 7 million tons of electronics, but only 15% of their critical materials are recovered. "That's lower than the world average of 17%," Wang said, citing research published in 2020 and 2021 .
And those unrecovered metals are worth about $7 billion. "If they can be recycled, the critical metals can be used to complement the supply chain," Wang said.
Wang's lab focuses on using proteins for environmental remediation and, generally, to aid in sustainability efforts. He turned to ferritin, a protein with porous walls on its surface which leads to a hollow inside. Proteins like this are called nanocages because of their ability to trap smaller materials inside.
Less than 10 years ago, researchers started to use a different protein, lanmodulin (LanM), to trap rare earth materials. "We were inspired by this study," Wang said.
He and his team were already working with ferritin when he saw a call for a biomining tool — a microorganism that can be used to extract metals from rocks or other materials. They turned to the protein to see if it might work similarly to LanM.
The team had reason to be hopeful: The inside of ferritin nanocages carry dense negative charges. Earlier studies had shown that ferritin nanocages could sequester metals, so Wang decided to find out if the protein was in any way selective about which metals it sequestered and which it left undisturbed.
Most of the critical metals in a lithium-ion battery are in its cathode, usually a solid, and its electrolyte, which is liquid. To recover them, the cathode metals are leached into a liquid solution. The result is a liquid mixture of cobalt, nickel and lithium ions, instead of a relatively simple solution containing one metal, which would be easier to recover.
"That's why selectivity is key," Wang said. "You want the protein to selectively separate or recover the metals."
The team ran several experiments to test ferritin's selectivity. They added it to a solution with cobalt ions and found that ferritin was not only selective for the metal, but had such strong affinity that the concentration of cobalt inside the nanocages was thousands of times higher than that left in the solution. This affinity created hot spots of metal ions which precipitated out of the liquid and sank to the bottom, where they were easily recovered.
Ferritin also had a strong affinity for nickel ions, although not quite as strong as it had for cobalt, but had barely any affinity for lithium — which was good news.
"Ultimately, we want to recover metals like cobalt and nickel through precipitation," Wang said. "Then we leave the lithium in the solution. If you get a relatively pure lithium solution, that's much easier for downstream processing.
This process also has the benefit of taking place in benign, neutral conditions. Current practices, such as solvent extraction, require using harsh chemicals that require careful disposal.
The next step for Wang is to investigate why ferritin has an affinity for some metals, but not others.
Part of the reason for the discrepancy, Wang said, is to do with charge. The interior of the nanocages have strong negative charges, while the three metals are positive, but the cobalt and nickel ions have a stronger positive charge (+2) than lithium (+1).
That can't be the whole story, though. "Even though cobalt and nickel are both +2, we still observed a significant adsorption difference between the two," Wang said. "That part we don't yet understand."
The team hopes a better understanding of ferritin's selectivity will allow them to refine the nanocages, engineering one that selects cobalt only and one that selects nickel.
"So, you'd have three tanks," Wang said of his long-term goal. "The first tank uses one ferritin to recover nickel, the second tank uses a second ferritin to recover cobalt, and then we have a lithium solution for downstream processing."
