(115a) Interfacial Selectivity of Peptide Surfactants at the Air-Water Interface for REE Recovery

We are developing peptide surfactants (PEPS) to capture and retain rare earth elements (REEs) at air-water interfaces to enable green recovery and separation of these critical elements. Selective separation of REE is particularly challenging among the lanthanide cations, which are present predominantly in a +3-oxidation state and have ionic radii that differ only by approximately 0.2 Å between the lightest and heaviest lanthanide. This contraction is exploited to generate selectivity among extractants used in liquid-liquid extraction (LLE). In this process, extractants in an organic phase (e.g. kerosene) are exposed to aqueous mixtures of REE cations. The extractants and REE cations form complexes at the aqueous-organic phase interface and subsequently partition into the organic phase. This process is weakly selective, with separation factors close to 1 for neighboring REE, so stages of LLE are required.

PEPS designed to bind selectively to particular REE cations are promising platforms for a new, green approach based on froth flotation. PEPS should bind selectively to form PEPS:REE complexes in bulk, and adsorb to interfaces of air bubbles sparged through solution for collection. Once adsorbed, each PEPS must retain its complexed REE, and cannot attract additional REE via non-selective Coulombic interactions that would compromise their selectivity. Here, we adopt lanthanide binding tags (LBTs) designed to bind selectively to Tb³⁺ as our PEPS platform. We use rheological characterization of LBT variants complexed with Tb³⁺ at air-water interfaces, supported by molecular dynamics and metadynamics simulations, to develop requirements for interfacial selectivity.

1. Interfacial Rheology of Lanthanide Binding Peptide Surfactants at the Air-Water Interface Stephen A. Crane, Felipe Jimenez-Angeles, Yiming Wang, Luis Ortuno Macias, Jason G. Marmorstein, Jiayi Deng, Mehdi Molaei, E. James Petersson, Ravi Radhakrishnan, Cesar de la Fuente-Nunez, Monica Olvera de la Cruz, Raymond S. Tu, Charles Maldarelli, Ivan J. Dmochowski, Kathleen J. Stebe, in preparation