Acid-Stable Magnetic Core–Shell Nanoparticles for the Separation of Rare Earths

Core–shell Fe₃O₄@SiO₂ nanoparticles were prepared with a modified Stöber method and functionalized with <i>N</i>-[(3-trimethoxysilyl)­propyl]­ethylenediamine triacetic acid (TMS-EDTA). The synthesis was optimized to make core–shell nanoparticles with homogeneous and thin SiO₂ shells (4.8 ± 0.5 nm) around highly superparamagnetic Fe₃O₄ cores (14.5 ± 3.0 nm). The core–shell Fe₃O₄@SiO₂(TMS-EDTA) nanoparticles were then used for the extraction and separation of rare-earth ions. By comparing them with previously published results for Fe₃O₄(TMS-EDTA) and SiO₂(TMS-EDTA) nanoparticles, it was clear that the core–shell nanoparticles combine the advantage of magnetic retrieval observed for Fe₃O₄(TMS-EDTA) nanoparticles with the higher selectivity observed for SiO₂(TMS-EDTA). The advantages of the SiO₂ shell include a lower specific weight and a larger grafting density compared to Fe₃O₄ surfaces, but also the improved resistance to acidic environments required for the stripping of rare-earth ions