The TALSPEAK process is an established option for lanthanide/minor actinide separations using solvent extraction. In this process, selective extraction of lanthanides is achieved by contacting a water-soluble aminopolycarboxylate complexant in a concentrated carboxylic acid buffer with a liquid cation exchanging extractant in an immiscible organic diluent. Although TALSPEAK process development has been successful on several levels, studies of the detailed fundamental chemistry have revealed undesirable complex interactions between aqueous and organic solute species. These complications threaten to impair process modeling and could impact engineered operations. In the present work, results are reported describing equilibrium partitioning and phase transfer kinetics trends for trivalent lanthanide ions and americium into bis-2-ethyl(hexyl) phosphoric acid (HDEHP) or structural analog 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) organic phases from aqueous lactate solutions containing diethylenetriamine-N,N,N′,N′′,N′′-pentaacetic acid (DTPA), triethylenetetramine-N,N,N′,N′′,N′′′,N′′′-hexaacetic acid (TTHA), or N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA). The undesirable partitioning of Na+, lactic acid, and water into the organic phase is greatly reduced when HEH[EHP] replaces HDEHP as the extractant. TTHA appears to offer little advantage over DTPA in conventional TALSPEAK, but both DTPA and TTHA are too strong for use in combination with HEH[EHP]. The combination of HEDTA with HEH[EHP] achieves good balance and exhibits a nearly flat pH dependence between 2.5 and 4.5, in contrast with conventional TALSPEAK. The latter combination demonstrates more predictable performance than is seen in conventional TALSPEAK, while providing acceptable americium/lanthanide separation factors. The HEDTA/HEH[EHP] combination offers the additional advantage of more rapid phase transfer kinetics for the heavier lanthanides without the need for high concentrations of a lactate buffer.
Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f(7) configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium's only available isotope, (249)Bk, has hindered in-depth studies of the element's coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium's complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium's multiconfigurational ground state.
Extraction of polar molecules by amphiphilic species results in a complex variety of clusters whose topologies and energetics control phase behavior and efficiency of liquid-liquid separations. A computational approach including quantum mechanical vibrational frequency calculations and molecular dynamics simulation with intermolecular network theory is used to provide a robust assessment of extractant and polar solute association through hydrogen bonding in the tributyl phosphate (TBP)/HNO/HO/dodecane system for the first time. The distribution of local topologies of the TBP/HNO/HO hydrogen bonded clusters is shown to be consistent with an equilibrium binding model. Mixed TBP/HNO/HO clusters are predicted that have not been previously observable in experiment due to limitations in scattering and spectroscopic resolution. Vibrational frequency calculations are compared with experimental data to validate the experimentally observed TBP-HNO-HNO Chain structure. At high nitric acid and water loading, large hydrogen-bonded clusters of 20 to 80 polar solutes formed. The cluster sizes were found to be exponentially distributed, consistent with a constant average solute association free energy in that size range. Due to the deficit of hydrogen bond donors in the predominantly TBP/HNO organic phase, increased water concentrations lower the association free energy and enable growth of larger cluster sizes. For sufficiently high water concentrations, changes in the cluster size distribution are found to be consistent with the formation of a percolating cluster rather than reverse micelles, as has been commonly assumed for the occurrence of an extractant-rich third phase in metal-free solvent extraction systems. Moreover, the compositions of the large clusters leading to percolation agrees with the 1 : 3 TBP : HNO ratio reported in the experimental literature for TBP/HNO/HO third phases. More generally, the network analysis of cluster formation from atomic level interactions could allow for control of phase behavior in multi-component solutions of species with a variety of hydrogen bond types.
Separation of americium from the lanthanides is considered one of the most difficult separation steps in closing the nuclear fuel cycle. One approach to this separation could involve oxidizing americium to the hexavalent state to form a linear dioxo cation while the lanthanides remain as trivalent ions. This work considers aqueous soluble Cu periodate as an oxidant under molar nitric acid conditions to separate hexavalent Am with diamyl amylphosphonate (DAAP) in n-dodecane. Initial studies assessed the kinetics of Cu periodate autoreduction in acidic media to aid in development of the solvent extraction system. Following characterization of the Cu periodate oxidant, solvent extraction studies optimized the recovery of Am from varied nitric acid media and in the presence of other fission product, or fission product surrogate, species. Short aqueous/organic contact times encouraged successful recovery of Am (distribution values as high as 2) from nitric acid media in the absence of redox active fission products. In the presence of a post-plutonium uranium redox extraction (post-PUREX) simulant aqueous feed, precipitation of tetravalent species (Ce, Ru, Zr) occurred and the distribution values of Am were suppressed, suggesting some oxidizing capacity of the Cu periodate is significantly consumed by other redox active metals in the simulant. The manuscript demonstrates Cu periodate as a potentially viable oxidant for Am oxidation and recovery and notes the consumption of oxidizing capacity observed in the presence of the post-PUREX simulant feed will need to be addressed for any approach seeking to oxidize Am for separations relevant to the nuclear fuel cycle.
Recently, efforts towards closing the nuclear fuel cycle have considered oxidizing americium to a hexavalent, linear-dioxo cation for co-recovery with hexavalent U, Np, and Pu from molar nitric acid using solvating extraction ligands such as tri-n-butyl phosphate or diamyl amylphosphonate. This work assesses solvent extraction recovery of sodium bismuthate oxidized americium by N,N-di-(2-ethylhexyl)butyramide (DEHBA), N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA), and N,N-dihexyloctanamide (DHOA). Extraction efficiency between the monoamides was found to increase in the order of DEHiBA < DHOA < DEHBA. For all monoamides, oxidized americium extraction was less than 50% from 4 M HNO 3 and below.Extraction efficiency above 50% was obtained using concentrations of 5 M HNO 3 or higher.The DEHBA extractant provided the highest distribution value of 5.4 at 7 M HNO 3 . Distribution values were found to be stable for up to 45 seconds aqueous/organic phase mixing times and indicated decreased reduction of hexavalent americium relative to separations completed with 2 organophosphorus extractants. The simultaneous co-extraction of U, Np, Pu, and Am was demonstrated using DEHiBA, and was found to decrease with increasing atomic number (D U > D Np > D Pu > D Am ). Interestingly, a break in recovery was observed where ligher actinides, U and Np, were better recovered relative to the heavier actinides, Pu and Am, in this study. This observation seems to be related to differences in the extracted metal complex for light actinides
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