pH-Tunable, Ligand-Free Selective Separation of Rare Earth Elements Using Silica Nanoparticles

Rare earth elements (REEs) are essential for clean energy technologies, yet their separation remains difficult due to their similar ionic radii and oxidation states. Conventional liquid-liquid extraction is energy-intensive and environmentally harmful, which motivates the development of more sustainable alternatives. Silica nanoparticles (SiO₂ NPs), widely used as supports in solid-phase extraction, offer high surface area and tunable surface chemistry. However, the direct use of unmodified SiO₂ NPs as selective REE adsorbents has been largely overlooked. In this study, we investigate the interactions between REEs and unmodified SiO₂ NPs over a range of pH conditions to uncover the underlying mechanisms governing REE adsorption and desorption and explore their use to selectively separate REEs. We identify three distinct pH-dependent interaction regimes: the negligible interaction (near the SiO₂ NPs isoelectric point), electrostatic, and hydrolysis-mediated regimes. In the negligible interaction regime, near the SiO₂ NPs' isoelectric point, electrostatic interactions are absent, and the REE cations are stable in the bulk phase, resulting in minimal REE uptake. In the electrostatic interaction regime, at intermediate pH, negatively charged SiO₂ NPs interact electrostatically with REE cations, resulting in the REE capture. Finally, in the hydrolysis-mediated regime, at high pH, neutral REE hydroxides deposit on the surfaces of the SiO₂ NP, which serve as nuclei for hydroxide deposition. These interaction modes are reversible, enabling REE capture and release from the SiO₂ NP via pH swing. Within the electrostatic regime, SiO₂ NPs exhibit clear size-dependent selectivity, favoring the adsorption of smaller, more charge-dense REEs over larger REEs. This selectivity persists under competitive conditions in both binary and ternary mixtures. Selectivity is also observed in REE desorption: lowering the pH selectively releases smaller REEs while retaining larger REEs. This work provides fundamental insight into REE-SiO₂ NP interactions and demonstrates a ligand-free, pH-responsive strategy for selective REE capture and separation using silica-based materials.