Gabriel B. Hall

An Advanced TALSPEAK Concept for Separating Minor Actinides. Part 1. Process Optimization and Flowsheet Development

ABSTRACT A solvent extraction system was developed for separating trivalent actinides from lanthanides. This “Advanced TALSPEAK” system uses 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) to extract the lanthanides into an n-dodecane-based solvent; the actinides are retained in a citrate-buffered aqueous phase by complexation to a polyaminocarboxylate ligand. Several aqueous-phase ligands were investigated, and N-(2-hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid (HEDTA) was chosen for further study. Batch distribution measurements indicate that the separation of americium (Am) from the light lanthanides increases as the pH increases. However, previous investigations indicated that the extraction rates for the heavier lanthanides decrease with increasing pH. Therefore, a balance between these competing effects is required. An aqueous phase at pH 2.6 was chosen for further process development, because this offered optimal separation. Centrifugal-contactor single-stage efficiencies were measured to characterize the system’s performance under flow conditions, and an Advanced TALSPEAK flowsheet was designed.

Nitric Acid and Water Extraction by T2EHDGA in n-Dodecane

ABSTRACT Liquid–liquid distribution behavior of nitric acid (HNO3) and water by a diglycolamide (DGA) ligand, N,N,N’,N’-tetra-2-ethylhexyldiglycolamide (T2EHDGA), into n-dodecane diluent was investigated. Spectroscopic Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) characterization of the organic extraction solutions indicate the T2EHDGA carbonyl coordinates with HNO3 and progressively aggregates at high acid conditions. Water extraction increases in the presence of HNO3. The experimentally observed distribution of HNO3 was modeled using the computer program SXLSQI. The results indicated that the formation of two organic-phase species—HNO3·T2EHDGA and (HNO3)2·T2EHDGA—satisfactorily describes the acid distribution behavior. Temperature-dependent solvent extraction studies allowed for the determination of thermodynamic extraction constants and ΔH and ΔS parameters for the corresponding extractive processes.

Theoretical Modeling of 99Tc NMR Chemical Shifts

Technetium-99 (Tc) displays a rich chemistry due to its wide range of accessible oxidation states (from -I to +VII) and ability to form coordination compounds. Determination of Tc speciation in complex mixtures is a major challenge, and (99)Tc nuclear magnetic resonance (NMR) spectroscopy is widely used to probe chemical environments of Tc in odd oxidation states. However, interpretation of (99)Tc NMR data is hindered by the lack of reference compounds. Density functional theory (DFT) calculations can help to fill this gap, but to date few computational studies have focused on (99)Tc NMR of compounds and complexes. This work evaluates the effectiveness of both pure generalized gradient approximation and their corresponding hybrid functionals, both with and without the inclusion of scalar relativistic effects, to model the (99)Tc NMR spectra of Tc(I) carbonyl compounds. With the exception of BLYP, which performed exceptionally well overall, hybrid functionals with inclusion of scalar relativistic effects are found to be necessary to accurately calculate (99)Tc NMR spectra. The computational method developed was used to tentatively assign an experimentally observed (99)Tc NMR peak at -1204 ppm to fac-Tc(CO)3(OH)3(2-). This study examines the effectiveness of DFT computations for interpretation of the (99)Tc NMR spectra of Tc(I) coordination compounds in high salt alkaline solutions.

Extraction Behavior of Ln(III) Ions by T2EHDGA/n-Dodecane from Nitric Acid and Sodium Nitrate Solutions

ABSTRACT The diglycolamide extractant T2EHDGA has proven to be promising for the separation of lanthanides and minor actinides in high-level nuclear waste reprocessing. This neutral extractant has shown significant extraction capacity for HNO3 into the nonpolar organic phase, along with hyper-stoichiometric nitrate dependence on extraction of trivalent f-elements. The transport behavior of T2EHDGA/n-dodecane toward trivalent lanthanides is not well understood. This work found a significant increase in distribution ratios for Eu(III) extracted from aqueous HNO3 media compared with that from NaNO3. The extraction of Eu(III) from HNO3 results in a different thermodynamic product than predicted by classic solvent extraction of 3:1 ligand–metal complex as observed with NaNO3 in FTIR and UV-vis spectroscopy. Experimental distribution measurements in conjunction with mass-action modeling using the solvent extraction modeling program SXLSQI suggest participation of 1 to 2 HNO3 molecules in the Eu(III)/T2EHDGA complex upon extraction from HNO3 media, indicative of a mechanism change responsible for the enhanced extraction behavior toward lanthanides in the presence of HNO3.

Surprising formation of quasi-stable Tc(VI) in high ionic strength alkaline media

Despite decades of research, the reduction behavior of pertechnetate, TcO4−, is not well understood and represents one of the largest knowledge gaps in the transition metal series. Conventional wisdom presumes the reduction of TcO4− in aqueous systems to predominantly follow either a rapid two electron, or three electron process to form either a TcV or TcIV species. This study for the first time demonstrates the importance of the TcVI species through systematic characterization by multiple spectroscopic techniques. The reduction of TcO4− was examined in a matrix of 5 M NaNO3 at 0–2 M NaOH. Results of the cyclic voltammetric evaluation of reduction to TcVI, Nernstian analysis of the visible spectroelectrochemical signal, and structural spectroscopic EPR and XPS analysis of the reduction product generated by bulk electrolysis consistently support an electron transfer stoichiometry corresponding to a 1e− reduction. This TcVI species is markedly more stable than had been previously considered, with decomposition kinetics that correspond to a half-life of 1.91 ± 0.07 days, fully 6 orders of magnitude longer than previously reported for an aqueous solution. These results reveal the importance of a TcVI intermediate species in high ionic strength alkaline solutions and significantly contribute to the understanding of the mechanism of TcO4− reduction and formation of low-valent Tc species.

Spectroscopic Characterization of Aqua [fac-Tc(CO)3]+ Complexes at High Ionic Strength

Understanding fundamental Tc chemistry is important to both the remediation of nuclear waste and the reprocessing of nuclear fuel; however, current knowledge of the electronic structure and spectral signatures of low-valent Tc compounds significantly lags behind the remainder of the d-block elements. In particular, identification and treatment of Tc speciation in legacy nuclear waste is challenging due to the lack of reference data especially for Tc compounds in the less common oxidation states (I-VI). In an effort to establish a spectroscopic library corresponding to the relevant conditions of extremely high ionic strength typical for the legacy nuclear waste, compounds with the general formula of [ fac-Tc(CO)3(OH2)3- n(OH) n]1- n (where n = 0-3) were examined by a range of spectroscopic techniques including 99Tc/13C NMR, IR, XPS, and XAS. In the series of monomeric aqua species, stepwise hydrolysis results in the increase of the Tc metal center electron density and corresponding progressive decrease of the Tc-C bond distances, Tc electron binding energies, and carbonyl stretching frequencies in the order [ fac-Tc(CO)3(OH2)3]+ > [ fac-Tc(CO)3(OH2)2(OH)] > [ fac-Tc(CO)3(OH2)(OH)2]-. These results correlate with established trends of the 99Tc upfield chemical shift and carbonyl 13C downfield chemical shift. The lone exception is [ fac-Tc(CO)3(OH)]4 which exhibits a comparatively low electron density at the metal center attributed to the μ3-bridging nature of the -OH ligands causing less σ-donation and no π-donation. This work also reports the first observations of these compounds by XPS and [ fac-Tc(CO)3Cl3]2- by XAS. The unique and distinguishable spectral features of the aqua [ fac-Tc(CO)3]+ complexes lay the foundation for their identification in the complex aqueous matrixes.

Inorganic Ba–Sn nanocomposite materials for sulfate sequestration from complex aqueous solutions

Selective sequestration of sulfate (SO42−) in the form of barite (BaSO4) from alkaline solutions of high ionic strength containing carbonate is problematic due to the preferential formation of BaCO3. Incorporation of sulfate into the insoluble and thermally stable BaSO4 phase can potentially benefit radioactive waste processing by reducing operational challenges and suppressing volatilization of other waste components such as technetium-99. To enhance selectivity of SO42− sequestration, a series of Ba–Sn nanocomposite materials was prepared using simple hydrothermal synthesis from different Sn(II) and Sn(IV) precursors. Structural characterization indicated that all obtained products predominantly contained BaSn(OH)6 and Ba2SnO2(OH)4·10H2O nanocrystalline phases which were disrupted upon exposure to SO42− due to formation of BaSO4. Performance of the Ba–Sn materials was tested using complex alkaline solutions simulating radioactive waste containing 0.094 M SO42− and 0.5 M CO32− among other constituents. About 54–66% of SO42− was converted to BaSO4 when a quantity of Ba–Sn material containing approximately a stoichiometric amount of Ba2+ relative to SO42− was used. In comparison, previous studies indicate negligible BaSO4 formation under similar conditions when a simple Ba2+ salt is used. This improvement is attributed to the selective replacement of the stannate by SO42−. Thermal stability of the sulfate-loaded product material up to 1100 °C was demonstrated. The obtained materials promise a convenient and economical option for the selective sequestration or removal of SO42− from complex carbonate containing solutions.

Correlative Microscopic, Spectroscopic, and Computational Analysis of the Nucleation and Growth of Europium (III) Oxalate Nanoparticles

The early stages of nanoparticle nucleation are not well understood, and yet increasing our understanding of these processes is fundamental to the production of monodisperse nanoparticles with targeted properties. Many technical challenges exist when trying to study nucleation events due to both the nuclei’s small size and rapid formation, and subsequent rapid growth into post-nucleation structures. Additionally, meaningful characterization of nucleation processes must be performed on samples in their in situ solution state. A multi-pronged characterization approach is critical, and experiments are ideally paired with computational modeling to form a more robust picture of these early stages of nanoparticle growth.