Peptide surfactants (PEPS) for the Green Separation of Rare Earth Elements
April 23rd, 2024
We have been developing functional peptide surfactant (PEPS) to meet an urgent societal need. Rare earth elements (REEs) are crucial to modern technologies. These elements are notoriously difficult to separate from each other owing to the similar diameters of the REE cations and the fact that they are typically present in the +3-oxidation state. They are currently commonly separated via liquid-liquid extraction in which oil-soluble extractants complex with the cations at aqueous/oil interfaces and pull them into the organic phase. These LLE processes are poorly selective and require multiple stages to isolate cations with the requisite purity.We are developing an environmentally friendly REE separation process which exploits PEPS that bind selectively to REEs to form PEPS:REE complexes that adsorb to the air-water interface for recovery via a froth flotation process. PEPS are ‘green’ molecules amenable to design for REE selectivity, interfacial activity, and scalable production. The success of this approach requires that PEPS’ ability to bind selectively to REE cations is retained in the highly anisotropic environment of the fluid interface. As an initial PEPS structure, we have studied a known surface-active lanthanide binding tag peptide. This peptide was designed to coordinate via multidentate interactions with REE cations in a binding loop inspired by the highly conserved EF-hand binding sequence in calcium binding proteins. Using a variety of surface characterization and molecular simulation methods, we show that PEPS:REE complexes are surface active and adsorb with intact binding loops. By rational variation of the initial PEPS sequence, we design PEPS:REE complexes that form monolayers with 1:1 ratios of REE and PEPS, essential to the success of the envisioned separation process. We further show that PEPS can bind and adsorb at fluid interfaces with selectivity among selected pairs of REE cations.Ongoing work focuses on PEPS with sequences designed for strong selectivity among neighboring lanthanides, on preserving bulk selectivity at the interface, and on the design of foams to recover and re-use these functional molecules. This work is performed by a team of researchers spanning four institutions supported by Basic Energy Sciences at the Department of Energy grant number DE-SC0022240.