Highly efficient recovery of light, medium and heavy rare earth elements using magnetic core-shell nanoparticles

The adsorption of rare earth elements (REEs) from wastewater is vital for environmental protection and resource utilization. Adsorbents with magnetic properties are easy to separate but incorporating magnetic particles can reduce adsorption capacity by decreasing the surface area or blocking active sites. Herein, an efficient magnetic adsorbent (i.e., Fe₃O₄@PDAPEI), consisting of an Fe₃O₄ core, a polydopamine (PDA) intermediate layer and a polyethylenimine (PEI) outer layer, was designed to extract Gd³⁺, Nd³⁺, Ho³⁺, and Y³⁺ from low-concentration solutions with adsorption capacities of 168.3, 168.5, 179.7, and 180.3 mg/g, respectively. The adsorption capacities exceed those of most reported magnetic REE adsorbents in the literature. The adsorption behavior could be fitted to the pseudo-second-order model, intraparticle diffusion model, and Langmuir model. Fe₃O₄@PDAPEI exhibited good reusability, with the adsorption capacity remaining above 90% of the initial value after five reuse cycles. In addition, despite the presence of competing ions (i.e., Na⁺, Mg²⁺, and Al³⁺) in model wastewater, the adsorption capacity could be maintained above 100 mg/g for all four REEs. The adsorption mechanism was investigated via density functional theory calculations, zeta potential measurements, and surface force measurements via atomic force microscopy. REEs could adsorb on Fe₃O₄@PDAPEI through binding to primary amines and electrostatic interactions. This work presents a highly efficient magnetic adsorbent and evaluates the underlying interaction mechanism from both theoretical and experimental perspectives, shedding light on facile and efficient REE recovery in various engineering processes.