(464c) Surface Rheology of Peptide Surfactants (PEPS) for Rare Earth Element Recovery

Rare earth elements (REEs), crucial to modern and green technologies, are notoriously difficult to separate from each other owing to the similar diameters of REE cations and the fact that they are typically present in the (+3) oxidation state. We are developing an environmentally friendly REE separation process, inspired by biology, which exploits peptide surfactants (PEPS) that bind selectively to REEs and brings them to the air-water interface for recovery via a froth flotation process. We adopt a known lanthanide binding tag as our initial PEPS structure. The PEPS bind to REE cations in bulk solution to form PEPS:REE complexes in which the REE is wrapped in a binding loop designed to sequester the cation. PEPS:REE complexes adsorb at the air water interface and form films with surface rheology that depends on the presence of REE that are not in the PEPS binding pocket. The films are viscous at dilute REE concentrations and form highly elastic networks in the presence of REE greater than the PEPS concentration. We quantify the rheology of these layers using particle tracking methods and simulation, and discuss elastic network formation as evidence for an undesired non-specific REE interaction. We find that this non-specific REE interaction is mitigated by appropriate mutation of the peptide sequence to eliminate residual charge in the PEPS:REE complex. To deepen our understanding of the structure of PEPS complexes at the air-water interface, we have investigated other ions: Al(+3) and Ca(+2) and find cation-specific interactions that confirm the importance of binding loop integrity and elimination of residual charge in PEPS design. These results suggest that the PEPS binding loop remains intact in the air-aqueous interface, and that non-specific interactions with REE in solution can be avoided, essential features to our envisioned process.