C.M.B. Gallagher, W.R. Archer, M.D. Schulz
Virginia Tech,
United States
Keywords: rare earth elements, isothermal titration calorimetry, polymer synthesis, polymer structure
Summary:
The purification of rare-earth elements (REEs: La-Lu, Y, Sc) from (un)conventional sources is a critical component of transitioning to a green energy-based future. Metal-chelating polymers can improve and/or supplant technologies used in REE separation (i.e. solvent extraction or ion exchange), but the impact of polymer structure on metal-chelation is relatively underexplored. Over the past 7 years, our group has used isothermal titration calorimetry (ITC), a technique widely used to study the binding of biological molecules, to directly investigate the impact of all aspects of polymer structure, beyond and in addition to the chelating group, on the underlying binding thermodynamics. Using various poly(carboxylates) as a model system, we’ve shown that binding is always entropically driven and is consistent across the lanthanides. Additionally, binding thermodynamics remain independent of polymer molecular weight and copolymer composition. However, small changes to the local environment around the binding site can have a profound impact on the binding thermodynamics. For example, as the polymer becomes more syndiotactic, the absolute value of both the enthalpy and entropy decreases, while the overall binding affinity remains uniform. Similarly, by modulating the hydrophobicity around the binding site, large changes in the heat capacity are observed by ITC, consistent with changes in desolvation behavior. Because the binding interaction is entropically dominated, the solution environment around the polymer and metal plays a key role in binding. Recent efforts have focused on the role of the solution environment in REE binding as well as how solution phase thermodynamics can inform solid phase materials. In total, these results lead directly to new design principles that will improve materials used in environmental remediation applications—including mining waste processing and water treatment—as well as extraction and purification of scarce metals from (un)conventional sources.