Organic Phase Inter-Ligand and Metal-Ligand Speciation in TALSPEAK and ALSEP Solvent Extraction Systems Public Deposited

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  • Partitioning of trivalent actinides (in particular, americium and curium) from the fission produced light lanthanides is a major concern of used nuclear fuel reprocessing for the purposes of waste disposal. Several solvent extraction processes have been developed to address these chemically difficult separations. The historically employed TALSPEAK Process utilizes di-2-ethylhexyl phosphoric acid (HDEHP), a cation exchange extractant as an organic phase extractant, while the Advanced TALSPEAK Process utilizes 2- ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]), its phosphonic acid analog as an organic extractant to alleviate processing problems. The newly developed mixed extractant ALSEP Process utilizes either HDEHP or HEH[EHP] in conjunction with either tetraoctyldiglycolamide (TODGA) or tetra-2-ethylhexyldiglycolamide (T2EHDGA), collectively DGA, to perform two separation steps in a single process. The nature of organic phase chemical interactions in these systems must be clarified in order to effectively model and operate them on an engineering scale. Neodymium speciation in HDEHP organic phases, the organic extractant used in the TALSPEAK Process, have been subject to multiple interpretations necessitating further investigations. UV-Visible spectroscopy coupled with Karl-Fischer titrations reveal that water in the inner neodymium coordination sphere results in an increase in the metal coordination number. Furthermore, the extraction of water was found to be independent of metal concentration, but was dependent on the HDEHP concentration. Investigating across the lanthanides revealed that light f-elements (americium, praseodymium, and neodymium) can contain inner-coordination sphere water, while heavier lanthanides (samarium, holmium, and erbium) cannot, presumably due to their smaller ionic radii. For HEH[EHP] organic phases, the Advanced TALSPEAK extractant, water extraction was found to be similarly independent of metal concentration and dependent on HEH[EHP] concentration, although a weaker water extractant. Compared the HDEHP, only the lightest lanthanide measured (praseodymium) revealed a change in UV-Vis spectra resembling a speciation change, suggesting that only the lightest of lanthanides can occupy water in the inner-metal coordination sphere. Characterization of the organic phase of the ALSEP Process is necessary to advise scaling efforts and to identify any potential difficulties which may exist due to inter-ligand interactions. The ALSEP ligand combinations tested were HDEHP - T2EHDGA, HEH[EHP] - TODGA, and HEH[EHP] - T2EHDGA. IR spectroscopy shows that the ability of the ALSEP ligand combinations to form intermolecular adducts in metal-free organic phases are relatively similar, although approximately an order of magnitude lower for the HDEHP - CMPO combination proposed for the previously developed TRUSPEAK system. Metal loaded IR spectroscopy shows that, when trivalent lanthanides are extracted from an aqueous phase which should not allow for the formation of neutral DGA solvates, the nature of the DGA carbonyl bond is shifted for the HDEHP - T2EHDGA system and the HEH[EHP] - TODGA system, but not for the HEH[EHP] - T2EHDGA system. Job’s Method further reveals that, for the ligand combinations which are measured to form ternary species, a 1:6:1 ratio of the cation exchange extractant to the DGA forms with metals in the organic phase. UV-Vis titrations further reveal that HDEHP - T2EHDGA forms ternary species the strongest with extracted metals, followed by HEHEHP] - TODGA and HEH[EHP] - T2EHDGA. Furthermore, americium forms ternary species the strongest with ALSEP ligand combinations, followed by neodymium and holmium. Finally, the dependence of organic phase nitrate on the complexation of metals in the ALSEP process was investigated for advising process operations. IR spectroscopy revealed that titration of an organic phase containing a DGA loaded with nitric acid shifts the metal coordination from the cation exchange ligand to the DGA similar to extraction of metal from a high concentration nitric acid aqueous phase. UV-Vis titration addition of nitrate to a metal loaded organic phase reveals that, in general, TODGA is more able to accommodate metal-nitrate complexes than T2EHDGA and the neodymium more strongly complexes with DGAs in the presence of nitrate than holmium. Also seen is that holmium exists in a complicated coordination environment at higher organic phase nitrate concentration, potentially indicating some process issues.
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