Protein nanocages are hollow structures that can be used for a variety of applications in medical fields, including for drug and vaccine delivery. Now, groundbreaking work from the University of Pittsburgh (Pitt; www.pitt.edu) has shown that a class of protein nanocage known as ferritin is highly effective at separating and concentrating metal ions from solution, making it a promising pathway for use with critical-metal-containing leachates that result from recycling e-waste and batteries.
In a mixture of cobalt and lithium, for example, ferritin adsorbs extremely selectively to the cobalt, leaving only lithium in solution. “Ferritin has spherical structure, and it takes up metal ions from solution, concentrating them in its cavity. It is like a nano-sized concentrator that can constantly collect metal ions. We have seen that the concentration of metal ions inside of the cage can be hundreds to thousands of times higher than the concentration in the solution,” explains Meng Wang, assistant professor of civil and environmental engineering (www.mengwang.org).
Being able to cultivate such a locally high concentration also simplifies the downstream precipitation process when compared to other metal-recovery methods, notes Wang. “Because the metal ions are highly concentrated within the cage, we can do the precipitation at very benign conditions. We don’t have to go to high pH, like 10, 11 or higher, to do the precipitation, which is commonly required using other methods. We can do the precipitation at a pH of 7 or 8.” When carbonate ions are added at these mild pH values, the purity of precipitated cobalt carbonates is over 95%. This in-situ concentration could also potentially be leveraged to recover metal ions at relatively low levels.
In addition to the milder pH, Wang points out that many other metal-recovery processes rely on organic solvents or hazardous chemicals, or may require very high temperatures, none of which are necessary when using ferritin nanocages.
So far, Wang’s team has demonstrated ferritin’s effectiveness with cobalt-lithium and nickel-lithium mixtures at the laboratory scale, but he sees potential scaleup on the horizon. “Expressed proteins can self-assemble into this nanocage structure, and it requires minimal post-processing, so that means scaleup could be relatively easy using commercially available reactors and fermenters,” he says. The next steps will be material and process improvements that enhance ferritin’s selectivity and enable continuous processing, and for the protein nanocages to be re-used for more cycles, lowering costs for larger-scale processing. These findings were published in the journal Environmental Science and Technology Letters.