(368ah) Lanthanide Binding Tags for Eco-Friendly Separation of Rare Earth Elements

Rare Earth Elements (REEs) encompassing scandium, yttrium and the 15 lanthanides are crucial to diverse technological applications such as electronics, catalysts, wind turbines, various alloys etc. However, the separation of individual REEs from feedstocks constitutes a major difficulty due to underlying similarities in physicochemical properties. Traditional separation methods of REEs such as solvent extraction are not environmentally benign due to hazardous nature of extractants, large volumes involved, extensive energy consumption and necessity of multiple extraction stages for complete purification. Current research efforts are heavily focused on addressing these issues, propelling the development of sustainable REE separation methods.

Recently, there has been growing interest in bimolecular approaches utilizing Ln³⁺ binding proteins or peptides for REE recovery, driven by their high affinity, selectivity, adoption of an all-aqueous separation platform, and minimal energy demands. Specifically, Lanthanide Binding Tags (LBTs) which are artificial peptides engineered from EF hand motifs of metalloproteins (e.g. Calmodulin, Troponin C) have shown great affinity for Ln³⁺ cations and the ability to selectively distinguish them. Leveraging these capabilities of LBTs could advance the development of new eco-friendly REE separation technologies. We aim to exploit the surface activity of LBT:Ln³⁺ complexes to facilitate selective separation of REEs at the air-water interface. The LBT:Ln³⁺ complexes can adsorb to the rising air/aqueous bubble interfaces and can be recovered by a foam fractionation process. The surface activity and interfacial compositions of the LBTs and Ln³⁺ cations were characterized by Pendant drop Tensiometry, X-ray reflectivity and X-ray Fluorescence measurements. We show that the sequence composition of the peptide can be tuned to promote surface activity without detrimental effects to the Ln³⁺ affinity. Additionally, we were able to demonstrate an enhancement in the selective adsorption of LBT:Tb³⁺ complexes to the air-water interface in an equimolar binary mixture containing Tb³⁺ and La³⁺.

LBTs also provide us with a tunable amino acid sequence space that can be exploited for targeted Ln³⁺ capture. A molecular-level insight into the structure selectivity relationship of LBT:Ln³⁺ binding loop could provide design guidelines for rational sequence modifications. To elucidate the underlying dynamics that dictate selectivity, we investigated the structural characteristics of LBT3 peptide with La³⁺ and Lu³⁺ cations using a combination of multidimensional solution state NMR and Molecular Dynamics (MD) simulations. We reveal that LBT:Ln³⁺ complexes are very dynamic in nature exhibiting populations with diverse coordination environments. For larger cations like La³⁺ we observe dissociation of weaker binding residues like ASN with simultaneous infiltration of a water molecule into the binding loop. However, for smaller cations like Lu³⁺ the ASN dissociation is followed by an intramolecular hydrogen bond with a GLU residue in the binding loop interlocking the pocket from bulk water molecules. We further validate these observations by measuring dissociation constants (K_d) of LBT3:Ln³⁺ complexes by luminescence spectroscopy.

Finally, we propose an alternate interfacial REE separation methodology using LBTs in conjugation with aminated silica nanoparticles (Am-SiO₂). Here we electrostatically adsorb negatively charged LBT:Ln³⁺ complexes onto the positively charged surface of the nanoparticle. The nanoparticles aid in enhancing the surface adsorption of LBT:Ln³⁺ complexes and improve foam stability. Additionally, employing nanoparticles can assist in low energy sedimentation separation of the LBT:Ln³⁺ complexes. We demonstrate that this approach effectively isolates and concentrates REEs, selectively removing undesired elements such as calcium validated by ICP-MS.

In conclusion, these LBT-based methods promise a sustainable and efficient REE separation process, representing a significant advancement in eco-friendly extraction technologies.