Hexavalent Actinides Research Articles

Group hexavalent actinide separation from lanthanides using sodium bismuthate chromatography

Advanced used nuclear fuel (UNF) reprocessing strategies are limited by the complex radiochemical separations and engineering required to achieve the separation of actinides (An) from neutron scavenging lanthanides (Ln). The accessibility of the hexavalent oxidation state for the actinides (U – Am) provides a pathway to achieving a group hexavalent actinide separation from the trivalent lanthanides and Cm. The solid oxidant and ion exchanger, sodium bismuthate (NaBiO3), has been demonstrated to quantitatively oxidize and separate Am from trivalent Cm in a column chromatographic system. This work expands on the use of NaBiO3 chromatography to characterize the adsorption, kinetic, and elution behavior of U, Pu, and Eu. Separation factors over 200 with rapid kinetics were observed at dilute nitric acid concentrations with a complete An/Ln separation achieved in under an hour. The adsorption and chromatographic behavior of key fission products present in various reprocessing raffinates was characterized which demonstrated potential application of a NaBiO3-based separation following a TRUEX process.

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions.

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

The Uptake of Actinides by Hardened Cement Paste in High-Salinity Pore Water

The interaction of the actinides Pu(III), Am(III), Np(V), Np(VI), and U(VI) with hardened cement paste (HCP) prepared from ordinary Portland cement was investigated by batch experiments in a diluted caprock solution (I = 2.5 M) as a function of the solid-to-liquid (S/L) ratio (0.5–20.0 g L−1) and pH (10–13). Independent of the oxidation state of the actinides, strong sorption was observed with Rd values between 104 and 5 × 105 L kg−1. For the hexavalent actinides U(VI) and Np(VI), a decrease in sorption was observed with increasing pH, which could be due to the formation of the AnO2(OH)42− species. CE-ICP-MS measurements of the supernatant solution from the U(VI) batch sorption experiment at pH ≥ 10 indicate that UO2(OH)3− and UO2(OH)42− dominate the speciation. Pu LIII-edge XANES and EXAFS measurements showed oxidation of Pu(III) to Pu(IV) when interacting with HCP. Calcium silicate hydrate (C-S-H) phases effectively immobilize Pu(IV) by incorporating it into the CaO layer. This was observed in a C-S-H sample with C/S = 1.65 and HCP at pH 12.7. Compared to data published in the literature on the retention of actinides on HCP at low ionic strength, the influence of high ionic strength (I = 2.5 M) on the sorption behavior was insignificant.

Highly efficient group separation of hexavalent actinides from lanthanides through a biphasic cooperative extraction system

The implementation of actinide (An) and lanthanide (Ln) group separation is one of the most critical tasks for the GANEX process to adapt nuclear fuel cycle in the Generation IV nuclear energy system. In this work, we proposed a biphasic cooperative extraction system to fulfill this aim, achieving selective extraction of hexavalent An to the organic phase by a hydrophobic extractant, N,N-di-(2-ethylhexyl)butylamide (D2EHBA), while masking trivalent or tetravalent Ln in the aqueous phase by a hydrophilic ligand, N,N,N’,N’-tetraethyldiglycolamide (TEDGA). Efficient group separation of actinides from lanthanides was achieved with remarkable single-stage An/Ln separation factors over 102. Moreover, the mechanism of An(VI)/Ln(III/IV) selectivity was analyzed by UV–Vis, FT-IR and MALDI-TOF-MS spectroscopic analyses, indicating more hydrophobic nature of An(VI)-D2EHBA complex than Ln(III/IV)-D2EHBA complex as well as much stronger masking effect of Ln(III/IV) than An(VI) by TEDGA contributing striking An/Ln separation.

Extraction of Neptunium, Plutonium, Americium, Zirconium, and Technetium by Di-(2-Ethylhexyl)-Iso-Butyramide (DEHiBA) at High Metal Loadings

ABSTRACT Increased focus on carbon neutral energy has generated a resurgence of interest in nuclear power, and in particular advanced reactors which are likely to utilize high assay low enriched uranium (HALEU). This in turn could increase the economic attractiveness of recovering still partially enriched uranium from used nuclear fuel. Concomitant to development of advanced reactors, advanced reprocessing schemes should be developed which address the disadvantages to well established reprocessing schemes. The present study focuses on using di-(2-ethylhexyl)-iso-butyramide (DEHiBA) under high metal loading conditions for the reprocessing of used nuclear fuel. The elements examined in the study include the dominant transuranic actinides (Np, Pu, Am) as well as the often-problematic Tc and Zr. By increasing the concentration of the extractant from the more commonly reported 1.0 M – 1.5 M, the extraction of hexavalent actinides is substantially increased, while maintaining effective rejection of tri, tetra, and pentavalent actinides, particularly in the presence of high loadings of uranium. The extraction of Zr by 1.5 M DEHiBA is noted to be negligible by comparison to tributyl phosphate (TBP), however the coextraction of Tc with U is observed to be nominally twice the quantity that is extracted by TBP indicating a need for effective Tc management.

Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1–3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

Multiplicity of Th(IV) and U(VI) HEH[EHP] Chelates at Low Temperatures from Concentrated Nitric Acid Extractions

Organophosphorus extractants have been widely investigated for lanthanide recovery from ore and for application in the reprocessing of spent nuclear fuel, such as in Advanced TALSPEAK schemes. Determining the speciation of the extracted metal complex in the organic phase remains a significant challenge. A better understanding of the variability of HEH[EHP]-actinide complexes and the speciation of chelates for tetra- and hexavalent actinides can improve the predictability of actinide phase transfer in such biphasic systems. In this study, the extraction of Th(IV) and U(VI) from nitric acid media using HEH[EHP] in heptane is examined. The distribution ratio as a function of nitric acid concentration was quantified using UV-vis spectroscopy, and then the speciation of HEH[EHP]-metal complexes in the organic phase was investigated using Fourier transform infrared (FTIR) spectroscopy and low-temperature 31P nuclear magnetic resonance (NMR) spectroscopy. In addition to perturbation of the vibrational modes proximal to the phosphonic moiety in HEH[EHP] in the FTIR spectra, the appearance of a nitrate signal was found in the organic phase following extraction from the highest acidity conditions for U(VI). The 31P NMR spectra of the organic phase at a low temperature (-70 °C) exhibited a surprising number (n) of resonances (n ≥ 7 for Th(IV) and n ≥ 11 for U(VI)), with the distribution between these resonances changing with the initial concentration of nitric acid in the aqueous phase. These results indicate that the compositions of the inner and outer spheres of the extracted actinides in the organic phase are more diverse than initially thought.

Group Separation of Hexavalent Actinides from Lanthanides through Selective Extraction by Sterically Hindered 2-Ethylhexyl Phosphonic Acid Mono-2-ethylhexyl Ester

Group Separation of Hexavalent Actinides from Lanthanides through Selective Extraction by Sterically Hindered 2-Ethylhexyl Phosphonic Acid Mono-2-ethylhexyl Ester

A comparison on the use of DEHBA or TBP as extracting agent for tetra- and hexavalent actinides in the CHALMEX Process

The Chalmers Grouped ActiNide EXtraction process is a solvent extraction process for the homogeneous recycling of spent nuclear fuel. The use of TBP for the extraction of tetra- and hexavalent actinides can be problematic for several reasons, including troublesome degradation products causing crud formation, decreased extraction yield and the possibility of explosive red oil reactions. Here, the substitution of TBP by a N,N-dialkyl monoamide, DEHBA, is investigated. The findings suggest that DEHBA can be a suitable extracting agent for use in the CHALMEX solvent, although identified drawbacks need to be further investigated.

Effect of Ca(II) on U(VI) and Np(VI) retention on Ca-bentonite and clay minerals at hyperalkaline conditions - New insights from batch sorption experiments and luminescence spectroscopy

In deep geological repositories for radioactive waste, interactions of radionuclides with mineral surfaces occur under complex geochemical conditions involving complex solution compositions and high pH resulting from degradation of cementitious geo-engineered barriers. Ca2+ cations have been hypothesized to play an important role as mediators for the retention of U(VI) on Ca-bentonite at (hyper)alkaline conditions, despite the anionic character of both the mineral surface and the aqueous uranyl species. To gain deeper insight into this sorption process, the effect of Ca2+ on U(VI) and Np(VI) retention on alumosilicate minerals has been comprehensively evaluated, using batch sorption experiments and time-resolved laser-induced luminescence spectroscopy (TRLFS). Sorption experiments with Ca2+ or Sr2+ and zeta potential measurements showed that the alkaline earth metals sorb strongly onto Ca-bentonite at pH 8–13, leading to a partial compensation of the negative surface charge, thereby generating potential sorption sites for anionic actinyl species. U(VI) and Np(VI) sorption experiments in the absence and presence of Ca2+ or Sr2+ confirmed that these cations strongly enhance radionuclide retention on kaolinite and muscovite at pH ≥ 10. Concerning the underlying retention mechanisms, site-selective TRLFS provided spectroscopic proof for two dominating U(VI) species at the alumosilicate surfaces: (i) A ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡ Ca – OH – U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates under hyperalkaline conditions. Consequently, the present study confirms that alkaline earth elements, which are ubiquitous in geologic systems, enable strong retention of hexavalent actinides on clay minerals under hyperalkaline repository conditions.

An insight into the sequestration of tetra and hexavalent actinides by tri ethoxysilyl-amino-propyl-3-oxa-glutaramic acid (SAPOGA) functionalized titania.

Silyl-amino-propyl-3-oxa-glutaramic acid (SAPOGA) functionalized titania has been synthesized for highly efficient solid phase sequestration of thorium and uranyl ions from an aqueous acidic waste stream. The XRD pattern suggested that the grafting was performed on the anatase phase, leading to a rougher surface resulting in better interaction with actinides. The successful grafting of SAPOGA bridging was confirmed using spectroscopic methods. The Langmuir isotherm and the intraparticle diffusion-based kinetics model were found to be operative with sorption capacities of 231 mg g-1 and 458 mg g-1 and rate constants of 51 mg g-1 min-1 and 48 mg g-1 min-1 for U and Th, respectively. The entropy driven sequestration process was thermodynamically favourable (ΔGU = -6.0 kJ mol-1 and ΔGTh = -9.1 kJ mol-1) and endothermic in nature. The experimentally corroborated complexation pattern was assisted by density functional theory (DFT) calculations, which gave further insight into the metal-ligand interaction.

Group Separation of Hexavalent Actinides from Lanthanides Through Selective Extraction by 2-Ethylhexyl Phosphonic Acid Mono-2-Ethylhexyl Ester

Group Separation of Hexavalent Actinides from Lanthanides Through Selective Extraction by 2-Ethylhexyl Phosphonic Acid Mono-2-Ethylhexyl Ester

Dipicolinamide functionalized titania for highly efficient sorption of tetra and hexavalent actinide

Dipicolinamide functionalized titania particles have been synthesized for efficient decontamination of uranyl and thorium ions from aqueous acidic waste solution. Langmuir isotherm was found to be predominating with maximum sorption capacity of 125 mg/g for Th and 111 mg/g for U, respectively. The sorption was found to follow pseudo 2nd order kinetics predominately, with rate constant 8.6E−04 mg g−1 min−1 for Th and 6.1E−04 mg g−1 min−1 for U, respectively. Up to 200 kGy of gamma irradiation, not much deterioration in extraction performance of sorbent material was observed. Three contacts of 0.1 M aqueous solutions of oxalic acid and sodium carbonate were found to be effective in quantitative elution of Th4+ and UO22+ from the loaded sorbent, respectively. The sorption of U and Th on dipicolinamide functionalized titania was evidenced by SEM images, elemental mapping and the EDX. Theoretical investigation revealed that, the solution phase free energy of sorption (ΔGsol) was higher for Th4+ (−114.1 kcal mol−1) compared to UO22+ (−80.41 kcal mol−1).

Crystallization purification of undivided u-pu-np mixture

A full-scale semi-industrial crystallization and refining stand was created on the basis of the Siberian Chemical Combine to develop and optimize the hydrometallurgical technology. The main purpose of this stand was to study the crystallization refining of uranium, plutonium and neptunium nitrates at a full-scale stand using significant amounts of nuclear materials. A bench experiment on crystallization refining of an undivided U-Pu-Np mixture was carried out. As a result of crystallization purification, mixed crystals of hexahydrate nitrates of hexavalent actinides were obtained. The yield of U, Pu, Np into the crystalline phase was 60, 85, and 60%, respectively. The degree of purification of the target product from elements of stable isotopes imitating fission products was about 102.

PSresin for the analysis of alpha-emitting radionuclides: Comparison of diphosphonic acid-based extractants

The analysis of radionuclides is complex, with high economic and time costs. For this reason, there is a need to develop new methods and strategies to reduce these costs. One important group in the analysis of radionuclides is the actinides, which are the main constituents assessed in the total gross alpha together with radium and radon test used to measure radioactivity in drinking water. Moreover, in nuclear dismantling processes, the possible spread of the released radionuclides has to be controlled, which is measured by many techniques, depending on the radionuclides, through scintillation. This work presents a new method to analyse actinides using plastic scintillation resins (PSresins) packed in a solid-phase extraction cartridge. The proposed method combines chemical separation and sample measurement into a single step, reducing the effort, time and reagents required for analysis as well as decreasing the amount of waste generated. The PSresins compared in this study contained three selective extractants based on methylenediphosphonic acid with different radicals, which has a high affinity for tri-, tetra-, and hexavalent actinides in dilute acids. These extractants were immobilised on plastic scintillation microspheres at a ratio of 1/1:6, producing a retention and detection efficiency of 100% for 241Am, 230Th, Uranium and 238Pu. The retention and detection efficiency were 20% and 100%, respectively, for 210Po and low for 226Ra.

Carboxylated UiO-66 Tailored for U(VI) and Eu(III) Trapping: From Batch Adsorption to Dynamic Column Separation

U(VI) and Eu(III), as representative elements of the hexavalent actinide and trivalent lanthanides (always as a chemical analogue for trivalent actinide), respectively, have attracted more and more attentions due to the widespread use of nuclear energy. Much effort has been focused on developing versatile materials for their uptake from aqueous solution. For the first time, we report here UiO-66 and its mono- (UiO-66-COOH) and di-carboxyl (UiO-66-2COOH) functional derivatives as robust adsorbents for efficient U(VI) and Eu(III) removal. It is found that the introduction of carboxyl groups greatly reduces the surface charge of UiO-66, thus guaranteeing excellent adsorption capacity at low pH. At pH = 3, for example, the adsorption capacity of UiO-66-2COOH for U(VI) and Eu(III) is more than 100 and 60 mg/g, respectively, while almost no adsorption occurs for pristine UiO-66. At pH = 4, both UiO-66-COOH and UiO-66-2COOH show high performance on U(VI) and Eu(III) removal. UiO-66-COOH has adsorption capacities of 80 and 43 mg/g for U(VI) and Eu(III), respectively, while the values for UiO-66-2COOH reach 150 and 80 mg/g, respectively. Also, all these materials achieve adsorption equilibrium within 100 min. More importantly, combining the needs of practical applications and the characteristics of high stability, high porosity, and excellent adsorption performance of UiO-66-2COOH, dynamic adsorption column experiments were successfully conducted; ∼99% U(VI)/Eu(III) can be efficiently adsorbed, and >90% adsorbed U(VI)/Eu(III) can be re-collected with dilute nitric acid solution, even after four adsorption-desorption cycles. The findings of this work demonstrate the application potential of metal-organic framework materials to remove radionuclides from environmental samples or nuclear waste liquids.

Effect of alkyl chain length on the extraction properties of U and Th using novel CnmimNTf2/isophthalamide systems

An attempt was made to investigate the structural modification of CnmimNTf2 ionic liquid in combination with a novel ligand, 5-bromo-N1,N3-bis(1-methyl-1H-pyrazol-3-yl)isophthalamide on the extraction properties of tetra and hexavalent actinides. In C4mimNTf2, C6mimNTf2, the extraction processes predominantly followed the ‘cation exchange’ mechanism involving the species: [UO2(NO3)L]+ and [Th(NO3)2L]2+, whereas, in C8mimNTf2, the ‘solvation’ mechanism was found to be predominant involving neutral species UO2(NO3)2L2 and Th(NO3)4L3. The speciation was further confirmed by investigating the participation of imidazolium cation in extraction. In ionic liquid, the extraction processes were slower compared to that in the molecular diluent. The time required to attain the equilibrium D values followed the trend: C4mimNTf2 < C6mimNTf2 < C8mimNTf2. This was attributed to the viscosity induced slow mass transfer. At 1 M HNO3, the D values for uranyl and thorium follow the trend C4mimNTf2 > C6mimNTf2 > C8mimNTf2, which was ascribed to the enhancement in organophilicity of the cation on increasing the length of the side chain and hence the effect on their dissolution in water. Three cycles of 1 mM aqueous sodium carbonate solution and four cycles of 1 mM oxalic acid solution were sufficient for almost quantitative back extraction of U and Th, respectively.C8mimNTf2 based systems were better radio-resistant compared to the lower homologue.

Extraction of Actinides with Tributyl Phosphate in Carbonates of Fluorinated Alcohols

ABSTRACT New non-inflammable compounds – carbonates of fluorinated alcohols were proposed as potential diluents for tributyl phosphate (TBP). These compounds are commercially available, have high boiling point, high density, and low solubility in water solutions. Extraction of actinides from nitric acid by TBP solutions in fluorinated carbonates was studied. TBP solutions in fluorinated carbonates show lower extraction ability comparing with TBP solutions in hydrocarbon diluents. In spite of this, it is possible to extract more than 99.9% of tetra- and hexavalent actinides presented in spent nuclear fuel solutions. The advantage of proposed solvent is possibility to obtain uranium strip product with uranium concentration higher than 100 g/l.

Optical Spectroscopic Investigation of Hexavalent Actinide Ions in n-Dodecane Solutions of Tri-butyl Phosphate

ABSTRACT The extraction of hexavalent actinides An(VI) by tri-butyl phosphate (TBP) was investigated by electronic absorption and vibrational spectroscopies. Through a series of spectral subtractions, vibrational spectra associated with TBP, TBP–HNO3 adducts, and An(VI)–TBP complexes could be isolated. Investigation of U(VI) extracts indicated spectral features consistent with the formation of the expected [UO2(NO3)2(TBP)2] complex, but spectral features of other species were clearly evident. Likewise, multiple species were evident in the electronic absorption and vibrational spectra of TBP phases generated by extraction of Pu(VI). Although definitive characterization of the additional species formed could not be achieved in this work, it is hypothesized that they contain 3:1 TBP-to-An(VI) stoichiometry.

Extraction of Some Actinide Ions from Nitric Acid Feeds Using N, N-di-n-hexyloctanamide (DHOA) in an Ionic Liquid

Liquid–liquid extraction studies of $${\text{UO}}_{2}^{{2 + }} $$ , $$ {\text{NpO}}_{2}^{{2 + }} $$ , Pu(IV), Np(IV), and Pu(III) were carried out from nitric acidic feeds using N,N-di-n-hexyloctanamide (DHOA) in 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (C4mim·NTf2). The distribution studies indicated strikingly different trends in the extraction of tetra- and hexa-valent actinide ions with varying nitric acid concentration. While the distribution ratio values for $${\text{UO}}_{2}^{{2 + }} $$ and $$ {\text{NpO}}_{2}^{{2 + }} $$ were found to decrease with increasing HNO3 concentration up to 1 mol·L−1 HNO3 and did not change thereafter, those obtained for Pu4+ and Np4+ showed monotonic increases over the entire range of acidity. On the other hand, Pu3+ was poorly extracted in the lower acidity range up to 4 mol·L−1, beyond which a sharp increase in the extraction was seen. The nature of the extracted species was obtained using several studies where the ligand and nitrate ion concentrations were varied while keeping the others constant using slope analysis.