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Access Type

WSU Access

Date of Award

January 2022

Degree Type


Degree Name




First Advisor

Matthew J. Allen


This thesis outlines projects pertaining to the extraction, enrichment, and use of rare-earth elements in the trivalent and divalent states through the modification of coordination chemistry. Modulating the coordination environment can impact the selectivity of rare-earth elements in solid-liquid enrichment through the adjustment of donor atoms, denticity and pKa values as detailed in Chapter 1. Changes in coordination environment, such as to the identity of donor atoms, can lead to major differences in properties such as luminescence. The studies reported in this thesis contribute to the body of knowledge surrounding the use of coordination chemistry for both the solid–liquid extraction of rare-earth elements and the luminescence properties of divalent europium.Chapter 2 describes the analysis of a multidentate, pH-dependent solid-phase media for rare-earth element extraction. Separation experiments showed selectivity for the mid and heavy rare-earth elements and attachment of the ligand to the solid phase had similar thermodynamic affinities to trends in unmodified ligands. The pH-dependent nature of the ligand was characterized, and efficiency was retained for at least six cycles of reuse. The solid-phase media retained the selectivity for mid and heavy rare-earth elements from fly ash leachate, even in competition with much higher concentrations of competing ions. The results of this study expand the body of research surrounding solid–liquid extraction of rare-earth elements towards creating an organic solvent-free method of extraction. Chapter 3 focused on the interactions between the noncovalently attached ligand and resin in the solid-phase media described in Chapter 2. By adjusting the length of the hydrophobic group that interacts with the hydrophobic resin, we found that the ligand featuring the butyl hydrophobic groups showed less loading onto the resin than the ligand featuring the hexyl hydrophobic groups. We also determined that the ligand featuring butyl groups showed less wash off at pH 5.5 than the ligand with hexyl groups, suggesting that longer hydrophobic groups lead to less wash off and therefore more efficient solid-phase media than solid-phase media with shorter hydrophobic groups. This study outlines preliminary results towards the rational design of reusable noncovalently attached solid-phase media for the enrichment of rare-earth elements. Chapter 4 aimed to distinguish the experimental luminescent differences between two divalent europium cryptates using computational analyses. Crystal structures were optimized in methanol and time-dependent density functional theory calculations were used to calculate excitation and emission spectra of both complexes. Natural-transition orbitals revealed that similar orbitals were involved in the excitation and emission of both complexes. Therefore, the bright yellow luminescence observed experimentally with the octaaza-cryptate was attributed to a greater splitting of the 5d orbitals of the octaaza-cryptate relative to the 2.2.2-cryptate. This study provided foundational knowledge for the calculation of emission spectra for solvated divalent europium complexes. This thesis outlines the general ligand design for rare-earth elements taking into consideration the charge density of the metal, ligand donor identity, ligand denticity, and the range of pKa values on the ligand. Chapter 5 discusses how these reports are expected to provide tools for the rational design of ligands for rare-earth element enrichment and modulation of photophysical properties of low-valent rare-earth element complexes using coordination chemistry.

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