Understanding aggregation of solvating extractants in applied separations

Abstract

Solvent extraction is an efficient and effective method for the industrial-scale recovery and purification of metals from mixed raw materials. Understanding the molecular-scale forces driving extraction remains an area of active research despite over 70 years of experience with industrial-scale solvent extraction processes. This thesis presents a series of studies on the extraction chemistry of two extractants relevant to applied separations. The extractant tributyl phosphate (TBP) is used in the Plutonium Uranium Reduction Extraction (PUREX) process for recovering uranium and plutonium from used nuclear fuel. The extractant N,N,N',N'-tetraoctyl diglycolamide (TODGA) is being considered for use in the Actinide Lanthanide Separation (ALSEP) process for separation and purification of lanthanides and minor actinides from PUREX process wastes. Distribution data collected for the extraction of trace amounts of fission and corrosion products by TBP in the presence of bulk uranium were mostly consistent with extraction through a traditional solvation mechanism. The exeception was the decreased extraction of low valence transition metals with increasing uranium concentration, which suggests another extraction mechanism such as reversed micelle formation. Distribution data collected for lanthanides in a TODGA solvent extraction system were also consistent with a traditional solvation mechanism. The selectivity trend for TODGA across the lanthanide series was found to follow the amount of water co-extracted with each lanthanide, suggesting that the extraction of lanthanides by TODGA is impacted by species in the outer coordination sphere of the extracted complexes. Diffusion NMR spectroscopy was used to determine the sizes and interactions of colloidal TBP aggregates in 20% TBP samples containing nitric acid, zirconium, and uranium. The aggregate sizes calculated from diffusion experiments were similar to those found previously using small angle neutron scattering (SANS). However, diffusion experiments suggested the presence of repulsive interactions between aggregates, while prior SANS experiments suggested attractive interactions. Diffusion NMR spectroscopy, rheology, and SANS experiments with 30% TBP samples containing nitric acid, zirconium, and uranium also produced conflicting results when the TBP aggregates were assumed to be colloidal particles. These results suggest that understanding the extraction of metals by TBP on a molecular-scale requires treating TBP aggregates as molecular species.