2025 Theses Doctoral

Thermoswitchable Hydrophilicity Solvents for Selective Mineral Recovery from Brines

Dach, Elizabeth May

Many critical minerals for clean energy systems, such as lithium and rare earth elements, are present in industrial “waste” brines, including reverse osmosis concentrate and postflash geothermal brines. The selective extraction of minerals from these saline resources is critical for developing a sustainable supply chain.

This dissertation proposes a novel technology for selective ion separations using thermally switchable hydrophilicity solvents. Chapter 2 provides an overview of solvent-driven extraction processes, detailing state-of-the-art applications in desalination and zero liquid discharge. Gaps in our understanding of salt transport motivate further research into the behavior of ions in these systems. Chapter 3 combines computational chemistry with experimental studies to probe the molecular-level differences in salt partitioning between common monovalent cations and anions. Molecular dynamics simulations agree with experimental insightsinto water and solvent mutual solubility. However, simulations face limitations in capturing the partitioning of salt into switchable solvents, likely due to polarization and charge transfer effects.

Chapter 4 delves deeper into behavior of ions in water-solvent-salt ternary systems. The chapter investigates the properties of ions that govern partitioning into switchable solvents. Salt partitioning is shown to be highly ion-specific and its dependence on water co-extraction is quantitatively described by ion properties, most notable polarizability and charge density. Chapters 5 and 6 demonstrate that ion-specific partitioning persists in mixed electrolyte systems, enabling ion-selective separations. We establish a proof-of-concept for switchable solvent selective extraction (S3E) as a direct lithium extraction technology, achieving robust lithium/sodium and lithium/potassium selectivity and practical lithium recoveries.

Finally, Chapter 7 highlights the role of solvent structure in thermally switchable hydrophilicity. Trends in the temperature- and water-dependence of switchable solvents’ polarity are compared to macroscopic phase behavior, and comparative analysis of structurally isomeric solvents reveals significant performance differences. Overall, this dissertation advances the mechanistic understanding of salt transport in aqueous-organic biphasic systems and provides a foundation for the rational design of next-generation lithium extraction processes to promote a sustainable, circular economy for critical minerals.