Abstract
Lignin is a promising natural polymer for adsorbing metal from aqueous solutions. However, its application in the separation of rare earth metals-particularly adjacent elements such as Pr(Ⅲ), Nd(Ⅲ), and Sm(Ⅲ)-remains underexplored. In this work, alkali lignin was fractionated to obtain a low molecular weight fraction (F5) enriched with functional groups. Subsequently, F5 was assembled into highly hydrophilic nanoparticles (F5-NPs), which expose a greater density of surface functional groups and thereby enhance rare earth metal ion adsorption from aqueous media. 31P nuclear magnetic resonance analysis confirmed that F5 possesses a higher density of phenolic OH groups compared with the original alkali lignin. Batch adsorption experiment showed that F5-NPs achieved a total adsorption capacity of 38.7 mg/g for Pr(Ⅲ), Nd(Ⅲ), and Sm(Ⅲ), which is 3.4 times higher than that of alkali lignin. Notably, the F5-NPs adsorbent exhibited inherent selectivity for Pr(Ⅲ), Nd(Ⅲ) and Sm(Ⅲ), with a highest affinity to Sm(Ⅲ), followed by Nd(Ⅲ) and Pr(Ⅲ). These findings demonstrate the natural potential of lignin-based materials for selectively separating adjacent rare earth elements.