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Final PhD Defense
May 3, 2019 @ 9:00 am - 10:00 am
Cecilia Eiroa Lledo (Wall Group)
Zoom video link: https://wsu.zoom.us/j/826851685
A THERMODYNAMIC INVESTIGATION OF AQUEOUS TECHNETIUM(IV) COMPLEXES
Technetium-99 occurs as a high yield fission product of U-235. It exists in nature in two main oxidation states; Tc(VII) an environmentally mobile ion, and Tc(IV) an environmentally immobile ion which can be found in naturally reducing environments, such as soils or underground water repositories. It is important to investigate the interactions of Tc(IV) with environmentally relevant ligands to better understand Tc(IV) behavior in solution. Studies have determined the stability constants of Tc(IV) with several ligands including citrate, acetate, oxalate, EDTA and several polyamino-polycarboxylates. These ligands are sterically hindered, and have several binding sites. A lack of knowledge exists for the interactions of tetravalent technetium with smaller ligands such as F−, Cl−, and Br−. These anions are abundant in natural water and soil systems, leading to a high probability of interaction with Tc(IV) in contaminated areas. Previous research on Tc(IV) has focused on the interactions with larger ligands, ignoring the interactions that Tc(IV) has with the background electrolytes used on their systems, which often consist of halides. The purpose of this work is to determine the stability constants of Tc(IV)-halide and Tc(IV)-oxalate complexes, and to determine the thermodynamic parameters for the complex formation. This work begins by determining the stability constants of Tc(IV) with Ox2-, F−, Cl−, and Br− through the use of solvent extraction techniques. The thermodynamic parameters of the reaction for ligand addition was attained using van’t Hoff analysis. It was concluded that Tc(IV) formed the strongest complex in the following order; Ox2- > F- >> Cl- > Br-. The thermodynamic parameters for oxalate and fluoride gave characteristics associated with inner-sphere complexes (positive enthalpy with large entropy terms), whereas chloride and bromide yielded parameters that more closely mimicked outer-sphere complexes (negative enthalpy with negative entropy terms). Overall, the stability constant of oxalate and fluoride with Tc(IV) are large compared to the other halides, and provide a stable and energetically favored complex. The interactions between chloride and bromide with Tc(IV) result in weak complexation; nonetheless, the interactions of these later halides with Tc are significant when found at high concentrations.