(538d) Stabilizing Foams for Foam Fractionation of Rare Earth Elements Using Lanthanide-Binding Peptide Surfactants

Rare earth elements (REEs) are crucial to the modern economy as advanced technologies rely heavily on their unique chemical, magnetic and luminescent properties. However, their selective separation from ores is extremely challenging because of the common trivalent charge and similar ionic radii. Current separation methods make use of multiple iterations of solvent extraction steps that are energy intensive and involve large volumes of hazardous chemicals, such as organophosphates. Hence, there is a need to focus on alternative chemistries that enable an environmentally friendly REE separation. Recently, we have been developing a bioinspired green separation process that utilizes REE-binding peptide surfactants (PEPS) for selective recovery of rare earths at the air-water interface by ion foam flotation (IFF). The PEPS molecule consists of a lanthanide binding tag that binds selectively with REE cations and a hydrophobic sequence that confers surface activity for subsequent recovery by IFF. In this work, we study the interfacial adsorption, foam generation, and stability of the PEPS:REE complexes at the air-water interface by coupling microfluidic foam generation, inductively coupled plasma optical emission spectroscopy (ICP-OES), and solution-state nuclear magnetic resonance (NMR) spectroscopy. Furthermore, we investigate the effect of co-added glutaraldehyde as a crosslinking agent for enhanced foam stabilization.

Foams were generated in microfluidic cells for two PEPS molecules (LBT1LLA & LBT1) with and without REE³⁺ cations to examine the foam stability. Coalescence and coarsening was studied over time for the generated foams at varying concentrations of the PEPS:REE complexes. Complexed PEPS molecules were found to improve foam stability as net charge reduction enhanced surface adsorption. Additionally, it was observed that negatively charged residues outside the binding loop caused peptide bridging via trivalent lanthanide cations forming a network at the interface, which further promoted foam stabilization. Co-addition of glutaraldehyde crosslinks adsorbed peptide complexes and results in stable interfacial layers of high elasticity as evidenced in microfluidic experiments.

To validate the process of foam fractionation for recovery of REEs, additional experiments were conducted in which foam was created using PEPS samples containing an equimolar binary mixture of rare earth cations (La³⁺, Tb³⁺) and co-added glutaraldehyde. The REE content in these foam samples were subsequently analyzed by ICP-OES. A higher Tb³⁺/La³⁺ molar ratio was observed in the foam samples compared to the bulk, establishing selective complexation of REE³⁺ by PEPS molecules.

Finally, the 3D molecular structures of PEPS:REE complexes were solved using multi-dimensional solution-state NMR spectroscopy experiments, which were used to establish ¹H, ¹⁵N, and ¹³C signal assignments and chemical shifts. The structures were calculated using through-space distance constraints with the CYANA program and were refined via MD simulations with the GROMACS software. The amino acid residues responsible for crosslinking PEPS complexes upon addition of glutaraldehyde were also examined by NMR spectroscopy. Lastly, the molar ratios of PEPS bound to different REEs in the bulk solution was quantified by ¹H solution-state NMR measurements, yielding insights into competitive binding. Overall, the diverse techniques used here result in rational design principles aimed at designing PEPS for targeted REE complexation and foam stability.