Study of synergistic mixtures of bis(o-trifluoromethylphenyl) dithiophosphinic acid and neutral oxygen donor ligands

The separation of minor actinides such as Am(III) from light lanthanides remains the more difficult technical challenges in the nuclear fuel cycle. These trivalent elements possess few differences in chemical characteristics that are necessary for an efficient separation process to partition these elements from each other. Current efforts have focused on the use of compounds containing soft donor (i.e. N, S) atoms which show a preference for binding Am(III). This study reports the combination of a soft donor acid, bis-(o-trifluoromethylphenyl)-dithiophosphinic acid (BFDPAH), with three neutral oxygen donors to extract these elements. The synergistic effect of the combination of TOPO and BFDPAH was significant with separation factors (SF = Am/Eu) of over 50 observed in extractions of Am(III) from 0.75 M HNO 3. Investigation of the overall extraction mechanism using these synergistic mixtures by the slope analysis method illustrated the mechanism was different for the extraction of Am(III) compared to Eu(III).;The first experiment performed was varying the synergists TOPO, TEP and TBP to determine if they enhanced the extraction capabilities of the BFDPAH. The results showed a small increase of extraction when TEP and TBP were used as the synergist, and TOPO displayed a substantial enhancement of extraction. Variation of the TOPO concentration while holding all other constituents at fixed concentrations gave a slope of 1.07 for Eu(III) on a logD vs. log[TOPO] graph. Using slope analysis it was determined that 1 TOPO molecule binds to Eu(III) at the interface of the organic and aqueous phase. A slope of 1.29 was obtained when Am(III) was used as the tracer isotope. Slope analysis determined that either 1 or 2 TOPO molecules bind to Am(III) at the interface of the organic and aqueous phase. When the BFDPAH concentration was varied while holding all other constituent concentrations constant the extraction of Eu(III) was minimal illustrating the lack of interaction of Eu(III) with the soft donor ligand. A slope of 1.40 was obtained on a logD vs. log[BFDPAH] when Am(III) was used as the radioactive tracer. This suggests that 1 or 2 molecules of BFDPAH bind to Am(III) at the interface of the organic and aqueous phase. When no synergist was added to the system a slope of 1.21 was calculated when Am(III) was used as the tracer. This data agrees with the previous analysis that 1 or 2 molecules of BFDPAH bind to Am(III) at the interface of the organic and aqueous phase. Using the calculated value of the extraction equilibrium constant, 1 molecule of NO3- was determined to bind with Am(III). The extraction complex of Am(III) was determined to be (BFDPAH) 2AmTOPO(NO3) by the slope analysis of the extraction data.