2024 Theses Doctoral

Integrated CO₂ utilization and structured ligand design for the sustainable separation of critical elements from unconventional resources

Ooi, Whai Shin

This thesis investigates innovative and sustainable methods for the extraction and recovery of critical metals from various waste streams, with the goal of reducing reliance on primary ores and minimizing the associated environmental impact. As global demand for these essential materials grows, finding effective alternatives becomes increasingly urgent. This research is structured around four main chapters, each addressing different aspects of metal recovery in the hydrometallurgical process and focusing on integrating environmentally friendly processes. By exploring advanced extraction techniques and the use of novel materials, this work aims to contribute to the development of greener technologies in the field of materials recovery.

Chapter 1 introduces a new framework that combines critical element recovery from waste-to-energy fly ash (WTE FA) with carbon sequestration, addressing environmental concerns related to the growing demand for materials in green technologies. This study integrates electrochemical Zn recovery with carbon capture, utilization, and storage (CCUS), demonstrating the potential for carbon-neutral Zn recovery. Using renewable acids (HNO₃), Ca and Zn were leached undermild conditions (pH 3), followed by electrochemical separation for high-purity Zn recovery. The unique morphology of the feedstock facilitated rapid metal extraction, while water wash pretreatment removed Ca-rich salts for subsequent carbonation, converting the remaining Ca into high-purity calcite.

Chapter 2 develops new ligand systems that selectively extract and release critical elements, such as lanthanides, from solutions containing competing metal ions. A tunable molecular scaffold based on tris(2-aminoethyl)amine was functionalized with salicylaldehydes to create imine ligands that effectively extracted Ce, even in the presence of Mg and Ca. The study employed CO₂ as a stimulus for re-extraction, producing cerium carbonate and high-purity ceria. This pHswing mechanism, driven by controlled CO₂ partial pressure, enables efficient recovery of energy-relevant elements from unconventional resources, demonstrating the potential of green chemistry in metal recovery.

Chapter 3 develops silica gel functionalized with polyethyleneimine (PEI), known as μOHMs, for the efficient and selective capture of heavy metal ions on both batch and continuous scales. Maximum adsorption capacities for Cu, Zn and Ni ions are 63.5, 43.1, and 36.2 mg/g, respectively, at pH 5.5. The adsorption process follows pseudo-second-order kinetics and the Langmuir isotherm model. Thermodynamic experiments indicate spontaneous, exothermic removal governed by monolayer chemisorption. Performance tests demonstrate a consistent removal rate of 33.5 mg/g for Cu after fifty cycles, highlighting the effectiveness of amine group complexation in heavy metal capture.

Chapter 4 examines 2-hydroxyaryloximes as effective candidates for metal separation, forming stable, size-selective pseudo-macrocyclic dimers through hydrogen-bonding networks. These ligands exhibit pH-dependent coordination properties influenced by the phenolic oxygen's protonation state. The chapter explores the structure-property relationships of seven 2-hydroxyaryloximes in liquid-liquid extraction schemes for Ni and Co. It presents a generalizedmultigram synthesis and concentration- and solvent-dependent characterization. Steric and electronic effects from peripheral substituent modifications on the dimerization constant and pKa values were systematically investigated using NMR and potentiometric titrations. These findings demonstrate the potential of these ligands for "pH-swing" separations of energy-relevant metals.

Overall, this thesis addresses critical challenges in sustainable metal recovery by employing interdisciplinary approaches that encompass chemistry, materials science, and chemical engineering. By integrating innovative techniques and materials, the research identifies effective strategies for extracting valuable metals from unconventional waste sources while minimizing environmental impact. The findings contribute significantly to the development of greener processes and materials, presenting viable alternatives to traditional extraction methods. Furthermore, this work supports sustainability in metal recovery by promoting the efficient use of resources, reducing waste, and enhancing the overall viability of circular economy principles in the field of materials recovery.