Title: Molecular-scale Controls on Rare Earth Element (REE) Occurrences in Highly-Weathered Environments

Abstract: 

Rare earth elements (REEs) are essential to advanced and clean energy technologies. Sustainable REE resource development requires both a fundamental understanding of the geochemical processes governing resource formation and advances in environmentally compatible extraction and recovery strategies. Highly weathered, clay-rich deposits represent important natural REE resources. However, the molecular-scale mechanisms by which solution chemistry, mineral surfaces, and naturally occurring ligands regulate REE behavior remain incompletely understood. Meanwhile, conventional REE extraction and recovery often involve high chemical and energy consumption and substantial waste generation. Bio-assisted approaches, including fungal bioleaching, offer promising alternatives that operate under milder conditions and may reduce chemical consumption and environmental impacts. However, the mechanistic links among cultivation conditions, microbial metabolism, and REE mobilization remain poorly understood.

This dissertation investigates the key geochemical controls on REE speciation, mobility, secondary phase formation during chemical weathering, and explores fungal bioleaching as a promising approach to REE extraction. Controlled laboratory experiments integrate solid-state nuclear magnetic resonance spectroscopy, synchrotron X-ray absorption spectroscopy, microscopy, aqueous-phase analyses, and extracellular metabolomics to connect molecular-scale reaction mechanisms controlling REE behavior at mineral-water interfaces and extraction performance. Using yttrium (Y) as a probe for heavy REEs, phosphate and citrate as representative ligands, kaolinite as a mineral substrate, this work identifies complementary mechanisms governing REE retention and release during weathering. Phosphate promotes Y immobilization at kaolinite-water interfaces through ternary surface complex and secondary Y-phosphate precipitation. In contrast, citrate can promote either Y retention or remobilization, depending on ligand concentration, solution pH, and reaction sequence. Reactant concentrations and solution pH further regulate the crystallinity of newly formed REE-bearing secondary phase. Extending this investigation to biologically mediated mobilization, experiments with Aspergillus niger and natural REE-bearing mineral sands suggest that initial pH act as a signaling cue shaping subsequent metabolic response and leaching trajectories. These findings suggest that cultivation conditions could be used to guide microbial metabolism and regulate the balance between REE dissolution and precipitation.

Together, this work advances the mechanistic understanding of REE speciation, mobility, redistribution, and secondary-phase formation in weathering systems and provides a basis for tuning bioleaching systems to achieve desired extraction outcomes.

Committee members: Dr. Yuanzhi Tang (Advisor), Dr. Martial Taillefert, Dr. Jennifer B. Glass, Dr. Christopher T. Reinhard, and Dr. Brian L. Phillips

Date & Time: Friday November 13th, 2:00 pm 

Location: ES&T L1114