Complexation reactions of lanthanides in supercritical fluid carbon dioxide

Metal-complexation followed by supercritical fluid extraction (SFE) is a new extraction technique that involves solubility and chemical reaction phenomena in the supercritical fluid phase. In metal-complexation/SFE, metals are rendered into extractable forms by in situ derivatization with soluble complexing agents. Extraction of metals from solid surfaces and soil has been demonstrated using complexing agents in supercritical carbon dioxide (scCO ₂). Extraction efficiencies are generally high; however, variable results have been reported. Quantitative extraction of strongly bound and recalcitrant metals from soil has not been achieved. The causes for incomplete extraction of metal from soil are directly related to the complexation reaction. Whereas knowledge of solubility in SCFs is sufficiently advanced, current knowledge of metal complexation reactions in SCF solvents is shallow.; The work described in this dissertation focuses on metal-complexation reactions between lanthanides, organophosphorus reagents, and β-diketones in scCO₂. Titration reactions were performed and product stoichiometry determined. Reactions using both single-ligand systems and dual-ligand synergistic systems were studied. The products dissolved in the supercritical fluid phase were quantitatively measured using spectroscopic techniques. Spectra were examined using both univariate and multivariate analyses. Conditional extraction coefficients and conditional formation constants were calculated from the spectral data using least-squares regression and hard-equilibria models.; Reactions between neodymium, praseodymium, and tributyl phosphate in scCO₂ at 308 K and pressures greater than 27.5 MPa gave rise to complexes having metal to ligand stoichiometric ratios of 1:4. Holmium produced complexes having stoichiometric ratios of 1:2 and 1:4 depending on the amount of ligand added. Complexation of neodymium with tributyl phosphine oxide produced 1:5 complexes, whereas complexation with tributyl phosphite produced 1:8 complexes. Extraction coefficients computed for the different lanthanide-organophosphorus reactions in scCO₂ were found to be 4 to 6 orders of magnitude higher than in hexane.; Synergistic complexation of lanthanides with tributyl phosphate and several β-diketones was shown to result in increased extraction coefficients and additional displacement of coordinated water. The more acidic β-diketone hexafluoroacetylacetone resulted in production of stable, soluble complexes having a higher extraction coefficient than the thenoyltrifluoroacetone-tributyl phosphate complexes.