A Stepwise Engineered PEI–Phosphonic Acid Functional Layer on Activated Carbon: Synthesis and Enhanced Rare Earth Adsorption Efficacy

The escalating demand for rare earth elements (REEs) necessitates efficient adsorbents for REE(III) recovery. Herein, a low-cost composite (OAC–PEI–MP) is engineered by grafting polyethyleneimine (PEI) and phosphonic acid ligands onto oxidized activated carbon, creating highly abundant REE(III) binding sites. Comprehensive characterization (SEM, FTIR, XPS, ζ-potential, N2 physisorption) confirms successful functionalization. Adsorption evaluations for Nd(III) (light REE) and Yb(III) (heavy REE) demonstrate pH-/temperature-/concentration-dependent capacities, with isothermal and kinetic behaviors fitting Langmuir and pseudo-second-order models, respectively. Remarkable selectivity is observed in binary systems (La/Ni, Sm/Co). OAC–PEI–MP shows high preferential enrichment toward REEs from the leachate of the NdFeB magnet, highlighting its potential for industrial REE separation.