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 non-REEs (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 exchange, electrostatic adsorption, and surface complexation. Overall, the novel findings reported here provide new perspectives for the design of absorbents with highly selective recovery of REEs from mining wastewater.