Selective Separation of the Rare Earth Elements Dysprosium and Neodymium via Tailoring Nanocellulose Chemical Structure

Abstract

Dysprosium(III) (DyIII) is one of the widely used heavy rare earth elements (HREE) for developing advanced magnetic devices and clean energy technologies because of temperature stability and coercivity. DyIII separation from light rare earth elements (LREE), such as neodymium(III) (NdIII), is however extremely difficult as a result of subtle differences in ionic radius and coordination number. Here, we report preferential DyIII adsorption by dicarboxylate-modified disordered cellulose chains (hairs) in anionic hairy cellulose nanocrystals (AHCNC) and show that monocarboxylate-modified celluloses do not have such selectivity. The AHCNC selectivity (ion distribution factor) of DyIII is ∼ 16.65 times that of NdIII, supported by the binding affinity for DyIII which is ∼ 6.8 times that for NdIII, measured using isothermal titration calorimetry. Molecular dynamics simulation provides insights into the molecular basis for the preferential DyIII binding to AHCNC over other monocarboxylate-modified cellulose derivatives, highlighting a collective contribution from inter- and intra-molecular hydrogen bonding, ionic interactions between dicarboxylate groups and metal ions, and the compressive strain-induced shrinkage of dicarboxylate “hairs”. The computational interaction energy scores and normalized probability of ion distribution trends corroborate with the experimental adsorption selectivity. This study shows how a facile chemical modification of cellulose renders it a selective biopolymer for DyIII recovery. The experimental and theoretical findings of this work may lay the foundation for developing highly selective bio-derived sorbents, enabling sustainable, selective HREE recovery from complex ion mixtures.