Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies

Introducing Phosphate Ester into DAPhen by Propyl Enhanced the Selectivity for UO22+ over Th4

A new type of phenanthroline carboxamide(DAPhen)-phosphate ester ligand (L1/L2) was synthesized for the selective separation of U(VI) over Th(IV). Liquid-liquid extraction experiments showed that the introduction of phosphate ester could increase the extraction ability of ligands for U(VI), especially L2, which showed high selectivity for the separation of U(VI) over Th(IV). The slope analysis indicated that L1 could form 1:1 and 1:2 complexes with U(VI) and 1:1 complexes with Th(IV). NMR titration revealed that the DAPhen unit of ligands combined with one U(VI) to form 1:1 complexes, and then the phosphate ester unit of the 1:1 complexes further combined with another U(VI) to form 1:2 complexes. Ligands provide only the DAPhen unit to Th(IV) to form 1:1 complexes. The crystal structures found 1:2 complexes of L1 and U(VI), 1:1 complexes of L2 and U(VI), and 1:1 complexes of L1 and Th(IV). The larger stability constant (log β) of the 1:1 complexes of L2 with U(VI) than that of the 1:1 complexes of L1 with U(VI) showed that the binding ability of U(VI) with the DAPhen unit of L2 is stronger than that of U(VI) with the DAPhen unit of L1. This study provides new ideas for designing extractants with excellent properties.

Molecular and Supramolecular Study of Uranium/Plutonium Liquid-Liquid Extraction with N,N-Dialkylamides

In the context of the separation of uranium and plutonium from spent fuel allowed by N,N-dialkylamides, three regioisomers of N,N-di(2-ethylhexyl) butyramide (DEHBA or ββ) and the diastereopure isomers of N-(2-ethylhexyl)-N-(oct-3-yl)butyramide (EHOBA or αβ) were synthesized to assess their extraction performance and to study the mechanisms at the origin of the differences observed between the stereo- and regioisomers. The N,N-dialkylamides showed differences in extraction, with a greater effect of regio- than stereoisomerism. A mechanistic study at both the molecular and supramolecular scales was initially applied to explain these effects. X-ray absorption and UV-vis spectroscopy showed that uranium is extracted by a UO2(NO3)2L2 complex, which is not very sensitive to steric hindrance, while plutonium is extracted by two complexes, Pu(NO3)4L2 and Pu(NO3)6(HL)2, which are differently affected by stereo- and regioisomerism. Investigations at the supramolecular scale also showed that Pu(NO3)4L2 complexes are disadvantaged by the bulkiness of the extractants, while Pu(NO3)6(HL)2 is favored by the preformation of larger supramolecular aggregates.

Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(L_I) and HONTA(L_II), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The L_I/L_II exist as monomers (aggregarte radius for L_I: 6.0 Å; L_II:7.4 Å) in the presence of 1-decanol, whereas L_II forms dimers (aggregarte radius for L_II:9.3 Å; L_I does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO₃ and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M L_I/L_II dissolved in 20% isodecanol +80% n-dodecane. The interaction of 1-decanol with L_II and HNO₃/Th(IV) with amidic oxygens of L_I/L_II results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-L_I/L_II complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th⁴⁺ ions with L_I than L_II, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-L_II aggregate in the nonpolar media.

Membrane-Based Electrochemical Detection of Uranium: A Review

The determination of uranium in environmental samples has always been a crucial environmental issue due to its adverse impacts on human life. Electrochemical detection is one of the most suitable methods for directly determining uranium because of its portable instrument and quick response characteristics. The ion-selective membrane in the working electrodes is selectively responsible for uranium transport and separation. This mini-review provides a general overview of the membrane-based ion-selective electrodes in detecting uranium ions reported in the literature. The ion-selective membranes are classified according to their ionophore categories. Furthermore, the limits and outlook are also discussed to provide a reference for further developing membrane-based electrochemical uranium sensors.

Insights into the Coordination Behavior of Methyl-Substituted Phosphinic Acids with Actinides

The electronic structure and complexation behavior of methyl-substituted phosphinic acids with U(VI) and Pu(IV) were explored by applying quantum chemical methods. In contrast to Ingold's classification, our results indicate that the methyl group is electron-withdrawing, reducing the phosphoryl group electron density in substituted phosphinic acids. The magnitude of the computed complexation energy values increases along with the series, PA → MPA → DMPA, and MP → MMP → MDMP, implying an increasing complexation tendency upon methyl group substitution for both U(VI) and Pu(IV) complexes. One of the nitrate groups in UO₂(NO₃)₂•2L complexes (L = PA, MPA, and DMPA) is in monodentate coordination mode due to the additional stability gained from O₂N-O···H hydrogen bonding interactions with the acidic H atoms of respective ligands. The calculation indicates marginally stronger metal-ligand interactions in Pu(IV) complexes compared to that in U(VI), which is supported by the computed complexation energies, M-O_P bond lengths, ν(P═O), the extent of metal-ligand charge transfer, and properties of M-O_P bond critical points. The energy landscape of substituted phosphinic acid ligands is further analyzed within the framework of the activation strain model to explain the energetic preference of certain conformers.

The pervasive impact of critical fluctuations in liquid-liquid extraction organic phases

Liquid-liquid extraction is an essential chemical separation technique where polar solutes are extracted from an aqueous phase into a nonpolar organic solvent by amphiphilic extractant molecules. A fundamental limitation to the efficiency of this important technology is third phase formation, wherein the organic phase splits upon sufficient loading of polar solutes. The nanoscale drivers of phase splitting are challenging to understand in the complex hierarchically structured organic phases. In this study, we demonstrate that the organic phase structure and phase behavior are fundamentally connected in a way than can be understood with critical phenomena theory. For a series of binary mixtures of trialkyl phosphate extractants with linear alkane diluents, we combine small angle x-ray scattering and molecular dynamics simulations to demonstrate how the organic phase mesostructure over a wide range of compositions is dominated by critical concentration fluctuations associated with the critical point of the third phase formation phase transition. These findings reconcile many longstanding inconsistencies in the literature where small angle scattering features, also consistent with such critical fluctuations, were interpreted as reverse micellar-like particles. Overall, this study shows how the organic phase mesostructure and phase behavior are intrinsically linked, deepening our understanding of both and providing a new framework for using molecular structure and thermodynamic variables to control mesostructure and phase behavior in liquid-liquid extraction.

Selective Separation of HNO3 and HCl by Extraction: The Investigation on the Noncovalent Interaction between Extractants and Acids by Density Functional Theory

There is a huge demand for the highly selective separation of HNO₃ and HCl in many industries, and solvent extraction is considered a feasible method. In this article, DFT calculations were performed to investigate the interactions between acids and extractants including alcohols, ketones, phosphorus, and amines. One of the significant findings to emerge from this study is that amines bind to acids through ion association. Nevertheless, the interaction between acids and alcohols, ketones, and phosphorus with a (RO)₃P═O structure is mainly dominated by hydrogen bonds. The change of Gibbs free energy in the extraction process shows that the phosphorus ((RO)₃P═O) is superior to other types of extractants in the selective separation of HNO₃ and HCl. Furthermore, after the alkoxyl group (RO-) in phosphorus ((RO)₃P═O) is replaced by RN- or R- with less electronegativity, the interaction between HCl and the substituted extractants transitions from a hydrogen bond to ion association, but there are still strong hydrogen bonds between them and HNO₃. That will lead to a decrease in the selectivity of phosphorus due to the change in interaction types. This new understanding should help the design and screening of efficient extractants for the separation of mineral acids.

Speciation analysis the complexation of uranyl nitrate with tri-n-butyl phosphate in supercritical CO2

The complexation of solid uranyl nitrate with tri-n-butyl phosphate (TBP) in supercritical CO₂ is quite different from that of a liquid–liquid extraction system because fewer water molecules are involved. Here, the complexation mechanism was investigated by molecular dynamics simulation, emphasising on speciation distribution analysis. In the anhydrous uranyl nitrate system, poly-core uranyl-TBP species [UO₂(NO₃)₂]₂·3TBP and [UO₂(NO₃)₂]₃·3TBP were formed in addition to the predominant [UO₂(NO₃)₂]·1TBP and [UO₂(NO₃)₂]·2TBP species. The poly-core species was mainly constructed via the linkage of U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O⋯U contributed by pre-developed [UO₂(NO₃)₂]·1TBP species. However, in the hydrated uranyl nitrate system, TBP·[UO₂(NO₃)₂]·H₂O species form, preventing the formation of the poly-core species. The complexation developed differently depending on the TBP to the uranyl nitrate ratio, the solute densities and the participation of water. It suggested that the kinetically favoring species would gradually convert into the thermodynamically stable species [UO₂(NO₃)₂]·2TBP by ligand exchange.

Uranyl nitrate complex with TBP in supercritical CO₂ could form 1 : 1 and 1 : 2 species, and further generate 2 : 3 and 3 : 3 poly-core uranyl species.

Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes

A pair of neptunium Schiff base coordination complexes, Np^VIO₂L(MeOH) and Np^IVL₂ {H₂L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, ¹H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that Np^VIO₂L(MeOH) and Np^IVL₂ are isomorphous with the previously reported U^VIO₂L(MeOH) and M^IVL₂ (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of Np^VIO₂L(MeOH) in situ or direct synthesis from a (Np^VO₂)⁺ source shows evidence of a pentavalent neptunyl (Np^VO₂L)_x^n- species as determined by UV/vis/NIR and ¹H NMR spectroscopy. The synthesis of (Np^VO₂L)_x^n- directly from a (Np^VO₂)⁺ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np-L covalent interactions in Np^IVL₂ are similar to those in the Np^VIO₂L(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np-L bonds.

Extraction Behavior of Acidic Phosphorus-Containing Compounds to Some Metal Ions: A Combination Research of Experimental and Theoretical Investigations

To provide feasible methods for the extraction of valuable metals from spent batteries or low-grade primary ores, the extraction behavior of some representative acidic phosphorus-containing compounds (APCCs) as extractants is evaluated from the perspective of experimental and theoretical investigations in this work. Aqueous solutions containing five metal ions, Ca(II), Co(II), Mg(II), Mn(II), and Ni(II), were made to simulate leaching liquids, and the extraction of these metals was investigated. A simplified calculated model was used to evaluate the interaction between each extractant and metal ions. The calculation results agree well with the experimental tests in trend. This work not only provides potential extractants for the extraction of valuable metals from spent batteries or low-grade primary ores but also demonstrates the practicability of the simplified calculation model.

Binding affinity of pyridines with AmIII/CmIII elucidated by density functional theory calculations

In recent decades, N-heterocyclic ligands have been extensively used in the separation of lanthanides/actinides, whereas the selective extraction of amercium or curium has been very challenging. Using density functional theory calculations, this study is devoted to the investigation of the binding affinity of a series of modelling pyridine ligands with Am^III and Cm^III. The structure-property correlations between the amercium and curium systems and the binding affinity were obtained, and promising strategies for efficient separation of Am^III/Cm^III were proposed.

The coordination chemistry of lanthanide and actinide metal ions with hydroxypyridinone-based decorporation agents: orbital and density based analyses

In the context of the mitigation of the biological effects of internal radionuclide contamination and for efficient decorporation, the design and development of efficient chelators for lanthanide and actinide metal ions has become a central issue. The pioneering work of Raymond and coworkers (Chem. Rev., 2003, 103, 4207-4282) led to the development of siderophore-related hydroxypyridinonate ligands for possible treatment of internalized radionuclides. However, the structure-function relationship of Ln/An bound to these ligands, particularly the bonding and coordination aspects are not clearly understood at the atomic level. Here, we have investigated the structure, binding and energetics of trivalent and tetravalent Ln/An (Sm3+, Eu3+, Am3+, Cm3+, Th4+, Pu4+) ions with spermine-based octadentate hydroxypyridinonate chelators, namely 3,4,3-LI(1,2-HOPO) and its 3,3,3 variant, using relativistic density functional theory (DFT). Furthermore, we have performed orbital and density based analyses to elucidate the nature of bonding in these complexes. In accordance with the experimental stability constant, we found the maximum binding free energy for An4+ (Pu4+, Th4+) as compared to trivalent metal ions. CDA and ECDA analyses along with orbital-based population analyses confirmed the higher ligand to metal charge transfer for An4+ than for trivalent metal ions. Furthermore, the aromaticity index analysis suggested the presence of crucial chelatoaromatic stabilization for all these metal ions with the maximum for An4+. QTAIM descriptors indicated that the binding of An/Ln with the hard oxygen donor of the ligands is of the donor-acceptor type but a higher degree of covalency exists for actinides as compared to lanthanides. Furthermore, QTAIM and molecular orbital analysis confirmed that such covalency is of the energy-driven type and strictly originates from the orbital mixing event of An-5f orbitals with the ligand orbitals.

Inner to outer-sphere coordination of plutonium(iv) with N,N-dialkyl amide: influence of nitric acid

N,N-Dialkylamides are extensively studied as alternative organic ligands to achieve the extraction and separation of uranium(vi) and plutonium(iv). We report here the coordination structures of the plutonium(iv) ion with N,N-di(2-ethylhexyl)-n-butanamide as a function of nitric acid concentration in the aqueous phase. The coordination structure of Pu(iv) evolves gradually with increasing nitric acid concentration from an inner-sphere with two coordinated amide ligands toward an outer-sphere hexanitrate complex with only nitrate ions in the first coordination sphere and protonated amide ligands in the outer shell.

Structural investigations on uranium(vi) and thorium(iv) complexation with TBP and DHOA: a spectroscopic study

Spectroscopic studies were carried out to understand the complexation of U(vi) and Th(iv) with tri-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) in different non-aqueous solvents.

Speciation of uranium-mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory

RATIONALE

Mandelic acid is a complexing agent employed for the liquid chromatographic separation of actinides. However, the types of species and the structural details of the uranyl-mandelate complexes are still unknown. Understanding the nature of these complex species would provide better insight into the mechanism of their separation in liquid chromatography.

METHODS

Formation of different species of the uranyl ion (UO₂ ) with mandelic acid was studied using electrospray ionization mass spectrometry (ESI-MS) with a quadrupole time-of-flight analyzer. The different species of uranyl nitrate with mandelic acid (MA) at ligand (L) to metal ratios in the range 1-10 were examined in both positive and negative ion modes. The stability of different species with the possible pathways of formation was scrutinized using density functional theory (DFT) calculations.

RESULTS

In negative ion mode, nitrate-containing UO₂ (MA)₁ , UO₂ (MA)₂ and UO₂ (MA)₃ species were found in good abundance. In positive ion mode, under-coordinated uranyl-mandelate species, and solvated (S) species of types UO₂ (MA)₁ (S), UO₂ (MA)₁ (S)₂ and UO₂ (MA)₂ (S), were observed whereas nitrate-containing species were absent. Interestingly, doubly and singly charged dimeric species were also identified in positive ion mode. The theoretically computed energetics of the various species are in close agreement with their experimentally observed intensities in ESI-MS.

CONCLUSIONS

The most intense peak observed in ESI-MS, UO₂ (MA)₃ , was found to be the energetically most favorable amongst different UO₂ (MA)_n type species. Metal-ligand equilibria studied in the two modes yielded similar results. The combined experimental and quantum chemical investigations predict that T-shape complexes may be formed even in the gas phase. Copyright © 2016 John Wiley & Sons, Ltd.

Complexation Behavior of the Tri-n-butyl Phosphate Ligand with Pu(IV) and Zr(IV): A Computational Study

Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu(IV) over a large number of fission products including Zr(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu(NO3)4·2TBP and Zr(NO3)4·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M-O{TBP} and M-O{NO3} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be -73.1 and -57.6 kcal/mol for Pu(IV) and Zr(IV), respectively. The large extraction energy of Pu(IV) system is in agreement with the observed selectivity in the extraction of Pu.

Trends in small angle neutron scattering of actinide–trialkyl phosphate complexes: a molecular insight into third phase formation

SANS as a molecular probe was used to investigate and quantify the aggregation tendency of metal complexes, facilitating the prediction of third phase formation.

Elucidating the structures and cooperative binding mechanism of cesium salts to the multitopic ion-pair receptor through density functional theory calculations

Quantum chemical studies predict the binding of Cesium salts to multitopic ion-pair receptor is through cooperative mechanism.

Effect of successive alkylation of N,N-dialkyl amides on the complexation behavior of uranium and thorium: solvent extraction, small angle neutron scattering, and computational studies

The effect of successive alkylation of the Cα atom adjacent to the carbonyl group in N,N-dialkyl amides (i.e., di(2-ethylhexyl)acetamide (D2EHAA), di(2-ethylhexyl)propionamide (D2EHPRA), di(2-ethylhexyl)isobutyramide (D2EHIBA), and di(2-ethylhexyl)pivalamide (D2EHPVA)) on the extraction behavior of hexavalent uranium (U(VI)) and tetravalent thorium (Th(IV)) ions has been investigated. These studies show that the extraction of Th(IV) is significantly suppressed compared to that of U(VI) with increased branching at the Cα atom adjacent to the carbonyl group. Small angle neutron scattering (SANS) studies showed an increased aggregation tendency in the presence of nitric acid and metal ions. D2EHAA showed more aggregation compared to its branched homologues, which explains its capacity for higher extraction of metal ions. These experimental observations were further supported by density function theory calculations, which provided structural evidence of differential binding affinities of these extractants for uranyl cations. The complexation process is primarily controlled by steric and electronic effects. Quantum chemical calculations showed that local hardness and polarizability can be extremely useful inputs for designing novel extractants relevant to a nuclear fuel cycle.