Abstract:

Rare earth elements (REEs) are essential for clean energy technologies including batteries for electrical vehicles, wind turbines and LED screens due to their unique magnetic, optical, and electronic properties. Efficient methods for their recovery and separation are urgently needed to meet the growing demands of a rapidly evolving green economy worldwide. However, current separation technologies, such as liquid-liquid extraction (LLE), suffer from harmful environmental impacts, scalability limitations and high energy costs, due to the similar physiochemical properties of REEs and the dependence on organic solvents. These environmental, operational, and economic challenges motivate the development of sustainable, selective and scalable REE separations.

Nanostructured materials, such as those incorporating silica nanoparticles (SiO 2 NPs), are promising materials to incorporate into REE separations due to their high surface area, tunable surface chemistry and environmental compatibility. Their scale offers several advantages, including high surface-to-volume ratio, integration into dynamic architectures, and stabilization of complex structures. These features offer new opportunities for designing alternative methods for REE recovery and separation methods which do not have the drawbacks of existing approaches.

This thesis focuses on developing a SiO 2 NP-based platform for REE separation, building from fundamental understanding to practical applications. We investigate the fundamental interactions between SiO 2 NPs and REEs across the full pH range of pH 3 – 10, identifying and mapping the transition of three distinct interaction modes with a combinational of technical tools. We demonstrate the intrinsic, size dependent selectivity, with SiO 2 NPs favoring smaller, more charge dense heavy REEs (HREEs) over larger light REEs (LREEs) in both binary and ternary mixtures under competitive conditions. Our results also show reversible adsorption of REEs on SiO 2 NP surfaces, enabling ligand-free separation processes.

Building upon this mechanistic understanding, we integrate the SiO 2 NP-based platform into three separation processes: (i) solid phase extraction, where SiO 2 NPs act as active adsorbents that enable size-dependent selectivity and reversible capture and release via simple pH-swings, (ii) froth flotation, where SiO 2 NPs serve as REE carrier and foam stabilizer, and (iii) bicontinuous interfacially jammed emulsion gels, where the nanoparticle-stabilized interfaces enable high interfacial area for REE adsorption and extractant loading. This work provides a foundation for developing sustainable REE separation strategies with nanoparticles.