Mining Uranium from Seawater: A Coordination Chemistry Approach

Poly(acrylamidoxime) fibers are the current state-of-the-art adsorbent for mining uranium from seawater. However, the amidoxime group is not perfectly selective towards the uranyl cation, in particular, competition with transition metal cations remains a major challenge. In order for subsequent generations of chelating polymer adsorbents to be improved, the coordination chemistry of amidoxime-uranyl and -transition metal cation complexes needs to be better understood. While the coordination mode of amidoxime-uranyl complexes has been established in the literature, a number of amidoxime-transition metal cation complex binding motifs can be observed on the Cambridge Structrural Database. Likewise, the formation constants, or log K values, of a number of essential amidoxime-uranyl and -transition metal cation complexes remain largely unresolved due to the wide range of conflicting acid dissociation constants, or pK_a [pKa] values, that have been reported for representative acyclic amidoxime ligands in the literature. Therefore, in Chapter 2 we use spectroscopic titrations to resolve the pK_a values of acetamidoxime and benzamidoxime. Subsequently, we use those pK_a values to develop computational protocols for predicting the pK_a values of aqueous oxoacid ligands. In Chapters 3 and 4, we computationally investigate the binding motif of formamidoximate-dioxovanadium(V) and –oxovanadium(IV) complexes, major competing ions in seawater by utilizing density functional theory and wave-function methods in conjunction with continuum solvation calculations. Our investigations of these formamidoximate complexes universally identified the most stable binding motif to be a tautomerically rearranged imino hydroxylamine chelate formed via coordination of the imino nitrogen and hydroxylamine oxygen. In Chapter 5, we build on the design principles acquired in Chapters 2-4 to design a ligand, salicylaldoxime that is more selective towards utanyl than competing transition metal cations. Finally, in Chapter 6 we potentiometrically determine the proton affinity distribution of the classical poly(acrylamidoxime) fiber between pH 2 and pH 10 via the Stable Numerical Solution of the Adsorption Integral Equation Using Splines (SAIUS) algorithm. This work lays the foundation for resolving the metal cation affinity distribution of the poly(acrylamidoxime) fibers, which can aid in improving the uranium selectivity of subsequent generations of chelating polymers.