Doctor of Philosophy
Dr. Zhifeng Ding
Over the past 30 years electrochemistry at a liquid|liquid interface has been used to observe and quantify simple ion transfer (IT) as well as ligand assisted, or facilitated ion transfer (FIT) reactions. Liquid|liquid electrochemistry has developed to where valuable thermodynamic constants – for example, the metal ion to ligand stoichiometry and overall complexation constant, β, in FIT - can be evaluated using cyclic voltammetry (CV). Recently, ionic liquids (ILs) have shown greater metal ion extraction efficiencies in water-IL biphasic separations relative to conventional molecular organic solvents. In this way, they are of interest to the nuclear industry for applications in spent nuclear fuel (sometimes called nuclear waste) recycling. Herein, liquid|liquid electrochemistry has been used to investigate FIT of metal ions typically found in SNF at traditional water|organic solvent (w|o) and novel water|ionic liquid (w|IL) interfaces. Initially, the hydrophobicity of 8 commercially available ILs were evaluated and the data obtained, combined with valuable insight from the literature, was used to select the cation and anion components of an IL that was prepared in-house; trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate (P66614TB). P66614TB possessed a suitable w|IL polarizable potential window of ~0.9 V – comparable to other ILs found in the literature, but at a cost 10× cheaper than that found commercially. The formal ion transfer potential, formal ion transfer potential, of metal ions is a point of reference for electrochemically induced FIT and was evaluated for the first time at a w|IL interface. The alkali metals Li+, Na+, K+, Rb+, and Cs+ where found to have formal IT potentials equal to 0.565, 0.548, 0.521, 0.531, and 0.518 V, respectively, which agrees well with the trend of increasing atomic radius and thus increasing hydrophobicity. With a suitable IL in hand, FIT of UO22+, Sr2+, Rb+, and Cs+ were examined at w|o and w|IL micro-interfaces. Ligands for contemporary SNF recycling, such as octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide (CMPO) were employed. The w|IL interface demonstrated overall complexation constants, β, several orders of magnitude higher than that observed at w|o interface. For example, [SrCMPO3]+ had β equal to 5.5 × 1025 and 1.3 × 1034 for the w|o and w|IL interfaces, respectively. Indicating a higher extraction efficiency using ILs versus traditional organic solvents.
Stockmann, Thomas J., "Electrochemical Evaluation of Ionic Liquids for Biphasic Extraction of Metal Ions Common to Spent Nuclear Fuel" (2013). Electronic Thesis and Dissertation Repository. 1540.