Iron nanoparticles synthesized using Euphorbia cochinchinensis leaf extracts exhibited highly selective recovery of rare earth elements from mining wastewater: Exploring the origin of high selectivity

Iron nanoparticles synthesized using Euphorbia cochinchinensis leaf extracts (Ec-FeNPs) showed high selectivity for rare earth elements (REEs) recovery from mining wastewater. REEs recovery efficiencies were > 90 %, with distribution coefficients ranging from 2483.9 to 37500 mL/g, which were consistently much higher than nonREEs (15.0 - 234.8 mL/g). Moreover, even after 5 consecutive reuse cycles, Ec-FeNPs effectively adsorbed > 60 % of REEs. Application of advanced characterization techniques found that the high selectivity of Ec-FeNPs for REEs was mainly due to the biomolecules present in the Ec extract. During the synthesis of FeNPs, these biomolecules are modified on the surface of Ec-FeNPs, giving Ec-FeNPs an enhanced ability to separate REEs from non-REEs. The biomolecule capping layer, which is modified on the surface of Ec-FeNPs, constitutes a primary source of high selectivity. LC-MS identified amino acids, carbohydrates, and organic acids as the major biomolecule categories in the capping layer. Density functional theory (DFT) confirmed that the biomolecule capping layer of Ec-FeNPs had the strongest interaction with REEs; an association confirmed by Spearman's correlation analysis. The adsorption mechanism of REEs by Ec-FeNPs mainly involved a combination of ion