Network analysis and percolation transition in hydrogen bonded clusters: nitric acid and water extracted by tributyl phosphate.

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)/HNO3/H2O/dodecane system for the first time. The distribution of local topologies of the TBP/HNO3/H2O hydrogen bonded clusters is shown to be consistent with an equilibrium binding model. Mixed TBP/HNO3/H2O 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-HNO3-HNO3 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/HNO3 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 : HNO3 ratio reported in the experimental literature for TBP/HNO3/H2O 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.

Hexavalent Americium Recovery Using Copper(III) Periodate.

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 Cu3+ 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 Cu3+ periodate autoreduction in acidic media to aid in development of the solvent extraction system. Following characterization of the Cu3+ 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 241Am were suppressed, suggesting some oxidizing capacity of the Cu3+ periodate is significantly consumed by other redox active metals in the simulant. The manuscript demonstrates Cu3+ 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.

Tributyl Phosphate Aggregation in the Presence of Metals: An Assessment Using Diffusion NMR Spectroscopy.

Diffusion nuclear magnetic resonance (NMR) spectroscopy was used to find the interaggregate interactions and sizes of tributyl phosphate (TBP) aggregates containing varying concentrations of uranium or zirconium and HNO3 in an n-dodecane diluent. The average diffusion coefficients of TBP species were measured using a pulsed-field gradient stimulated echo experiment with a longitudinal eddy-current delay (STE-LED). Interaggregate interactions were determined by measuring the diffusion coefficient of TBP in a sample after a series of dilutions with n-dodecane. The interaction-independent infinite dilution diffusion coefficient was also calculated from these measurements. The sizes of TBP aggregates were calculated from the infinite dilution diffusion coefficient using the Wilke-Chang equation. Interactions between TBP aggregates were observed to correspond to a hard sphere potential with a repulsive component. Aggregate sizes found by NMR were comparable to literature values found using small-angle neutron scattering. The diffusion of TBP in heavy organic third phases indicates that the third phase may be a bicontinuous structure like that found in traditional surfactant systems.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state.

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.

A Molecular Dynamics Study of Tributyl Phosphate and Diamyl Amyl Phosphonate Self-Aggregation in Dodecane and Octane.

A molecular dynamics model for tributyl phosphate (TBP) and diamyl amyl phosphonate (DAAP) is presented using the Generalized AMBER Force Field (GAFF) and the AM1-BCC method for calculated atomic charges with a modification to the phosphorus-containing dihedral parameters. The density and average molecular dipole in a neat liquid simulation, and dimerization in dodecane and octane diluents, compare favorably to experimental values. At low extractant concentration, investigation of the dimer structure reveals the offset "head-to-head" orientation as the predominant structure over a range of TBP and DAAP concentrations with a phosphoryl oxygen separation distance between dimerized extractants of 3-5.5 Å. At high extractant concentrations, a graph analysis of extractant aggregates was used to determine concentrations of each aggregate size and the average coordination number, which gives a measure of the linearity of the aggregates. For aggregates up to 7 extractant molecules, the mean free energy of association per molecule was found to be 0.55-0.59 and 0.72 kcal/mol for TBP and DAAP, respectively. In both diluents, TBP formed large aggregates more frequently than DAAP, and those aggregates were more nonlinear than their DAAP equivalents. This finding anticipates differences in aggregation chemistry between TBP and DAAP in PUREX extraction systems.

Separating and stabilizing phosphate from high-level radioactive waste: process development and spectroscopic monitoring.

Removing phosphate from alkaline high-level waste sludges at the Department of Energy's Hanford Site in Washington State is necessary to increase the waste loading in the borosilicate glass waste form that will be used to immobilize the highly radioactive fraction of these wastes. We are developing a process which first leaches phosphate from the high-level waste solids with aqueous sodium hydroxide, and then isolates the phosphate by precipitation with calcium oxide. Tests with actual tank waste confirmed that this process is an effective method of phosphate removal from the sludge and offers an additional option for managing the phosphorus in the Hanford tank waste solids. The presence of vibrationally active species, such as nitrate and phosphate ions, in the tank waste processing streams makes the phosphate removal process an ideal candidate for monitoring by Raman or infrared spectroscopic means. As a proof-of-principle demonstration, Raman and Fourier transform infrared (FTIR) spectra were acquired for all phases during a test of the process with actual tank waste. Quantitative determination of phosphate, nitrate, and sulfate in the liquid phases was achieved by Raman spectroscopy, demonstrating the applicability of Raman spectroscopy for the monitoring of these species in the tank waste process streams.