(337aq) Lanthanide Binding Tag Peptides for Rare Earth Elements Selective and Environmentally Friendly Separation Processes

Rare earth elements (REEs) possess unique chemical and physical properties that are applicable in a wide range of industrial applications, such as electronics, catalysis, clean energy, batteries and magnetics. Separation and purification of REEs have relied on solvent extraction because of the extensive advantages of the process, such as simple fast and continuous operation, mild process conditions, and inexpensive handling of large quantities of materials. Nevertheless, solvent extraction process for the separation of REEs faces problems such as high energy consumption, high pollution and an excessive use of solvents due to the high viscosity of the extractants use for the collection of metals.

Lanthanide Binding Tag (LBT) peptides that coordinate selectively with Ln³⁺ ions can be used to replace the energy intensive processes used for the separation and purification of these valuable metals. These surface-active biomolecules once selectively complexed with the trivalent REE cations adsorb to air/aqueous interfaces of bubbles for foam fractionation and recovery. Moreover, separation of REEs in bulk solution can be achieved via self-assembly aggregation of LBT-Ln³⁺ complexes by neutralization of charges, electrostatic bridging, and/or by using crosslinking reagents. We aim to exploit the high affinity of these biomolecules, that coordinate selectively with Ln3+ ions for their use in bioinspired/eco-friendly separation processes. The surface activity, interfacial arrangement, and interfacial rheological properties of the biomolecules was characterized by using pendant drop tensiometry, x-ray reflectivity (XR), x-ray fluorescence neat total reflection (XFNTR), and dilatational and shear rheology. Additionally, anomalous small angle x-ray scattering (ASAXS) measurements were taken to determine the distribution of trivalent cations on the LBT-Ln³⁺ self-assembly structures, which was confirmed by results obtained by inductively coupled plasma optical emission spectroscopy (ICP-OES) along with ultraviolet absorption spectroscopy.

We found that Lanthanide Binding Tag peptides are surface active and maintain REEs cations in binding pocket at air-aqueous interface. Moreover, the adsorption of REEs to the air-aqueous interface, and the selectivity at the surface can be tuned by modifying the hydrophobicity and the charges of the LBT molecule. Additionally, foam stabilization and REEs adsorption can be enhanced by cross-linking individual LBT-REEs complexes in bulk solution that then can adsorbed to the air-aqueous interface. Furthermore, we showed that individual LBT-REEs can self-assembled via cationic bridging and hydrophobic interactions with affinity that mirror the ones observed for molecules at the air-aqueous interface. Likewise, crosslinking reagents can induce assembly and sedimentation of peptide rich in REEs, as well as allow the separation for lower concentration of peptide and undersaturated regimes.

The ability to tune the amino acid sequence of these surface-active molecules to either improve their adsorption and/or selectivity with REEs, along with the capability of controlling the foam stability with the use of crosslinkers validate the principles for a green, eco-friendly, foam fractionation selective separation method of REEs. Furthermore, the capability of the micron-sized structures rich in trivalent cations to sediment makes possible the use of low energy consumption separation methods such as sediment separation and microfiltration to isolate the desire REEs from impurities or undesired elements present in the surrounding stock solutions.