Rare-earth elements (REEs: La–Lu, Y, and Sc) are integral to modern technology. As technological development continues, world demand for these metals will increasingly outpace supply. Consequently, new and sustainable sources are needed, as well as more efficient methods of extraction and purification. Many waste streams—mining effluents, desalination brines, e-waste, and wastewater from semiconductor fabrication plants—contain high concentrations of REEs, which could be extracted. Metal-chelating polymers have great potential in REE extraction and separation applications due to their relatively low cost and high affinity for target elements. However, the effect of key polymer structural parameters on REE-binding interactions has long been underexplored. To investigate the interplay between polymer structure and metal chelation, we synthesized metal-chelating polymers with systematic variations in hydrophobicity and tacticity. We then used isothermal titration calorimetry to directly measure the binding affinity, enthalpy changes, and stoichiometry of the interactions between a series of REEs in solution and these metal-chelating polymers. These measurements enabled us to characterize the complete thermodynamic profile of these polymer-metal interactions. Further measurements of changes in heat capacity, along with computational data, reveal the key role of water during metal binding as well as the effect of polymer chain flexibility. This combination of systematic polymer synthesis and advanced calorimetry measurements will guide future development of metal-chelating polymers for REE extraction and separation.

Elucidating the effect of polymer hydrophobicity and tacticity on rare-earth element chelation thermodynamics

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