Systematic Raman spectroscopic study of the complexation of uranyl with fluoride.

A simple aqueous complexing system of UO22+ with F- is selected to systematically illustrate the application of Raman spectroscopy in exploring uranyl(VI) chemistry. Five successive complexes, UO2F+, UO2F2(aq), UO2F3-, UO2F42-, and UO2F53-, are identified, as well as the formation constants except for the 1 : 5 species UO2F53-, which was experimentally observed here for the first time. The standard relative molar Raman scattering intensity for each species is obtained by deconvolution of the spectra collected during titrations. The results of relativistic quantum chemical first-principles and ab initio calculations are presented for the complete set of [UO2(H2O)mFn]2-n complexes (n = 0-5), both for the gas phase as well as for aqueous solution modelling bulk water using the conductor-like screening model. Electronic structure calculations at the Møller-Plesset second-order perturbation theory level provide accurate geometrical parameters and in particular reveal that k water molecules in the second coordination sphere coordinating to the F- ligands in the resulting [UO2(H2O)mFn]2-n(H2O)k complexes need to be treated explicitly in order to obtain vibrational frequencies in very good agreement with experimental data. The thermodynamics and structural information obtained in this work and the developed methodology could be instructive for the future experimental and computational research on the complexation of the uranyl ion.

Complexation of Am(III) and Eu(III) with DRAPA (N,N-dialkyl-6-amide-pyridine-2-carboxylic acid).

The similarity of the coordination chemistry of Am(III) and Eu(III) and two homologous tridentate ligands, N,N-di-2-ethylhexyl-6-amide-pyridine-2-carboxylic acid (DEHAPA, HL') in solvent extraction and N,N-dimethyl-6-amide-pyridine-2-carboxylic acid (DMAPA, HL) in aqueous solution and in the solid state, is revealed structurally and spectroscopically with complexes ML'3 (org), ML3 (aq) and ML3 (s), respectively.

A computational study on the coordination modes and electron absorption spectra of the complexes U(iv) with N,N,N',N'-tetramethyl-diglycolamide and anions.

Lipophilic N,N,N',N'-tetraalkyl-diglycolamides (TRDGAs) are promising extractants for actinides separation in spent nuclear fuel reprocessing. Usually, in the extracted complexes of actinide and lanthanide ions of various oxidation states, the metal ions are completely surrounded by 2 or 3 TRDGA molecules, and the counter anions do not directly coordinate with them. In contrast, the extracted complexes of U(iv) from different media presenting different absorption spectra indicate that the anions (Cl- and NO3-) are directly involved in the coordination with U(iv) in the first inner sphere. Based on this exceptional observation in solvent extraction, taking the coordination of U(iv) with N,N,N',N'-tetramethyl-diglycolamide (TMDGA, the smallest analogue of TRDGA) as the research object, we mimic the behaviours of counterions (Cl- and NO3-) and the water molecule during coordination of TMDGA with U(iv), especially combining with the simulation of the absorption spectra. We demonstrate that during the complexing of TMDGA to U(iv), the counterion Cl- will occupy one coordination number in the inner coordination sphere, and NO3- will occupy two by bidentate type; however, the ubiquitous water cannot squeeze in the inner coordination sphere. In addition, the coordination of Cl- and NO3- is proved to favour the extraction with the lower binding energy. Moreover, the simulation of absorption spectra is in good agreement with the observation from experiments, further verifying the aforementioned conclusion. This work in some way will provide guidance to improve the computation methods in research of actinides by mimicking the absorption spectra of actinide ions in different complexes.

Revealing the Chemical Nature of Functional Groups on Graphene Oxide by Integrating Potentiometric Titration and Ab Initio Calculations.

A new graphene oxide (GO) model with reasonable functional group types and distribution modes was proposed by integrating potentiometric titrations and ab initio calculations. Due to the complex synthesis mechanism, the atomic structure of GO has been controversial for a long time. Here, we use density functional theory calculations to mimic the oxidation process, and a series of GO fragments (GOFs) were deduced. A new pKa calculation method (RCDPKA) developed specifically in this work was further used to predict pKa values of the fragments. Then, we performed potentiometric titrations on four different GO samples to confirm the existence of these GOFs and determine the content of functional groups. Interestingly, different GO samples present the same pKa values in titration, and the results are consistent with the predicted ones. Based on the evidence from titration and calculation, prominent correlations between functional groups could be found. Groups at the edges are mainly double-interactive carboxyls (pKa1 ≈ 3.4, pKa2 ≈ 5.7) and double-adjacent phenolic hydroxyls (pKa1 ≈ 8.8, pKa2 ≈ 12.1), while groups on the plane are mainly collocated epoxies and hydroxyls (pKa1 ≈ 11.1, pKa2 ≈ 13.8) on both sides of the plane with a meta-positional hydrogen bond interaction. These findings were further validated by multiple characterizations and GO modifications. These results not only stimulate a fundamental understanding of the GO structure but also provide a quantitative analysis method for functional groups on GO.

Reactivity of a di(amidoxime) ligand in the presence of Cu(II)/Ni(II).

As the primary functional groups of amidoxime sorbents for uranium recovery from seawater, di(amidoxime) ligands can be cyclized in situ into different ligands in the presence of Cu(II)/Ni(II) at different pH values. Here we first found that a linear ligand glutardiamidoxime can be catalyzed into a cyclic ligand glutarimidedioxime by Ni(II) in acidic solution.

A Novel Functionalized Ionic Liquid for Highly Selective Extraction of TcO4.

Due to the highly acidic and high-salinity nature of the high-level radioactive waste liquid (HLLW), selective extraction of TcO4- from HLLW remains to be a challenging task. Traditional anion exchangers show low selectivity and unsatisfactory extraction performance due to the lack of functional groups that can interact strongly with TcO4-. In this work, a tailor-made binding site was constructed by decorating two acetamide functional groups on imidazolium cation to fabricate a new Tc separation material, which exhibits high selectivity. Unlike most reported Tc separation materials, which can only perform well under low acidic, neutral, or alkaline conditions, this material still has good extraction performance in highly acidic solutions. In the simulated high-level waste liquid of 3 M nitric acid, the extraction efficiency of 0.5 mol/L organic phase for Tc can reach 96.5% through the first-stage extraction. Our theoretical simulations suggest that ReO4-/TcO4- anions were adsorbed on the top of the imidazolium ring during the extraction process, with p-π and p-p interactions acting as the driving forces.

Identifying the Missing Protonated Complex Species and Revisiting the Overestimated Stability Constants of Two Known Complexes of Uranyl(VI) with Dipicolinic Acid.

The complexation of uranium(VI) [U(VI)] with dipicolinic acid was revisited with respect to the missing protonated complex species existing in acidic solutions. For the first time, the presence of the protonated complex, UO2HL+, in aqueous solutions was confirmed and the stability constant was determined by fluorescence spectroscopy. Considering the protonated species, which was missing in previous investigations, the overestimated stability constants of the two known complexes, UO2L and UO2L22-, were carefully reevaluated with potentiometry using N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid as a competing ligand. While a single crystal of the solid compound UO2(HL)2(H2O)4 with two monoprotonated HL- bonding UO22+ in a tridentate mode was successfully grown from aqueous solutions, the corresponding complex species UO2(HL)2(aq) could not yet be clearly identified and characterized.

Identification of complexes of Nd(III) with dithiophosphinic acids verifying the difference in complexation between Ln(III) and An(III).

Five complex species of Nd(III) with HA have been spectroscopically and compositionally identified as NdA3, NdA3(HA), NdA3(HA)H2O, NdA3(H2O)3, and Nd(H2O)23·3A (HA, bis(2,4,4-trimethylpentyl)dithiophosphinic acid) with the help of X-ray diffraction analysis on single crystals of Nd(H2O)9·H2O·3B (HB = bis(iso-butyl)dithiophosphinic acid.

Selective Separation of Am(III)/Eu(III) by the QL-DAPhen Ligand under High Acidity: Extraction, Spectroscopy, and Theoretical Calculations.

Although 1,10-phenanthroline-based ligands have recently shown vast opportunities for the separation of trivalent actinides (Ans(III)) from lanthanides (Lns(III)), the optimization and design of the extractant structure based on the phenanthroline framework remain hotspots for further improving the separation. Following the strategy of hard and soft donor atom combination, for the first time, the quinoline group was attached to the 1,10-phenanthroline skeleton, giving a lipophilic ligand, 2,9-diacyl-bis((3,4-dihydroquinoline-1((2H)-yl)-1),10-phenanthroline (QL-DAPhen)), for Am(III)/Eu(III) separation. In the presence of sodium nitrate, the ligand can effectively extract Am(III) over Eu(III) in HNO3 solution, with the separation factor (SFAm/Eu) ranging from 29 to 44. The coordination chemistry of Eu(III) with QL-DAPhen was investigated by slope analysis, NMR titration, UV-vis titration, Fourier transform infrared spectroscopy, electrospray ionization-mass spectrometry, and theoretical calculations. The experimental results unanimously confirm that the ligand forms both 1:1 and 1:2 complexes with Eu(III), and the stability constants (log ß) of each of the two complexes were obtained. Density functional theory calculations show that the Am-N bonds have more covalent characteristics than the Eu-N bonds in the complexes, which reveals the reason why the ligand preferentially bonds with Am(III). Meanwhile, the thermodynamic analysis reveals that the 1:1 complex is more thermodynamically stable than the 1:2 complex. The findings of this work have laid a solid theoretical foundation for the application of phenanthroline-based ligands in the separation of An(III) from practical systems.

Highly Efficient Uptake of TcO4 - by Imidazolium-Functionalized Wood Sawdust.

99Tc is a radioactive fission product, mainly in the form of TcO4 -, with good solubility and mobility in the environment. The development of effective and inexpensive materials to remove TcO4 - from nuclear industry wastewater or contaminated water is significant. Wood sawdust is a byproduct of the wood processing industry and is an abundant, low-cost, and sustainable material. The mesostructure of wood consists of numerous hollow cells that are joined endwise to form an interconnected channel matrix capable of rapid transfer of ions. Imidazolium-functionalized wood sawdust (IM-WS) was synthesized using natural wood sawdust by a two-step reaction. It has excellent properties of TcO4 -/ReO4 - adsorption including rapid adsorption dynamics (30 s to equilibrium), good adsorption stability (pH 3-9), high selectivity (adsorption of 45.4 Re % in 1000 times excess of NO3 - ions, 76.6 Re % in 6000 times excess of SO4 2- ions, and 92.2 Tc % in a simulated mixed solution; after adsorption, the concentration of TcO4 - decreased to 0.056 ppb from the initial concentration of 12.09 ppb in 1000 times excess of SO4 2-), and in particular low production costs. These characteristics give it great prospects for low-level radioactive wastewater treatment and environmental remediation.

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[double bond, length as m-dash]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.

Hydrophilic Sulfonated 2,9-Diamide-1,10-phenanthroline Endowed with a Highly Effective Ligand for Separation of Americium(III) from Europium(III): Extraction, Spectroscopy, and Density Functional Theory Calculations.

The design and development of a water-soluble heterocyclic ligand are believed to be an alternative way for improving the separation efficiency of actinides from lanthanides. Herein, we designed and synthesized a novel hydrophilic multidentate ligand: disulfonated N,N'-diphenyl-2,9-diamide-1,10-phenanthroline (DS-Ph-DAPhen) with soft and hard donor atoms, as a masking agent in aqueous solutions for Am(III) separation. The combination of N,N,N',N'-tetraoctyldiglycolamide in kerosene and DS-Ph-DAPhen in aqueous phases could separate Am(III) from Eu(III) across a range of nitric acid concentrations with very high selectivity. The coordination behaviors of Eu(III) with DS-Ph-DAPhen in aqueous solutions were studied by UV-vis titration, electrospray ionization mass spectrometry, and Fourier transform infrared spectra. The results indicated that Eu(III) ions could form both 1:1 and 1:2 complexes with the DS-Ph-DAPhen ligand in aqueous solution. Density functional theory calculation suggests that there are more covalent characters for Am-N bonds than that for Eu-N bonds in the complexes, which supports the better selectivity of the DS-Ph-DAPhen ligand toward Am(III) over Eu(III). This work demonstrates a feasible alternative approach to separating trivalent actinides from lanthanides with high selectivity.

Selective Separation and Coordination of Europium(III) and Americium(III) by Bisdiglycolamide Ligands: Solvent Extraction, Spectroscopy, and DFT Calculations.

Diglycolamide-based ligands have recently received increased attention due to their outstanding affinity for trivalent actinides and lanthanides. The structure optimization of the ligands, however, still remains a hot topic to achieve better extraction performance. In this work, we prepare and investigate three multidentate diglycolamide ligands for the selective separation of Eu(III) over Am(III) from a nitric acid solution to explore the effect on the extraction of alkyl groups on the nitrogen atoms in the center of the BisDGA ligands. The introduction of ethyl or isopropyl groups on the central nitrogen atoms greatly increased the distribution ratios of trivalent metal ions and enhanced the separation factor of Eu(III) over Am(III). The complexation behaviors of Eu(III) and Am(III) ions were studied by slope analyses, electrospray ionization mass spectrometry (ESI-MS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The results indicated that the trivalent metal ions were extracted as 1:2 and 1:3 complexes for all three BisDGA ligands during the extraction. Density functional theory (DFT) calculations verified the relevant experimental conclusion that the selectivity of THEE-BisDGA for Eu(III) is better than that for Am(III). The metal-DGA bonds in the ML3(NO3)3 complexes seem to be stronger than those in ML2(NO3)3 complexes.

The spatio-temporal features of chicken mitochondrial ND2 gene heteroplasmy and the effects of nutrition factors on this gene.

Mitochondrial heterogeneity is the presence of two or more types of mitochondrial (mt)DNA in the same individual/tissue/cell. It is closely related to animal health and disease. ND2 is a protein-coding gene in mtDNA, which participates in mitochondrial respiratory chain and oxidative phosphorylation. In previous studies, we observed that the mt.A5703T and mt.T5727G sites in the ND2 gene were the heteroplasmic variation sites. We used pyrophosphate sequencing technology to examine chicken mt.A5703T and mt.T5727G heteroplasmic sites in the ND2 gene, in different tissues and at different development stages in chickens. We also investigated whether nutritional factors could affect the mt.A5703T and mt.T5727G heteroplasmy. Our results showed that chicken mt.A5703T and mt.T5727G heteroplasmy had clear spatio-temporal specificities, which varied between tissues/development stages. The mtDNA heterogeneity was relatively stable upon nutrition intervention, 30% dietary energy restriction (from 18 to 48 days old) and different types of dietary fats (at 5% concentration, from 1 to 42 days old) did not change the breast muscle heteroplasmy of broilers at the mt.A5703T and mt.T5727G sites. In addition, multiple potential heteroplasmic sites were detected by clone sequencing in the ND2 region, which potentially reflected abundant heteroplasmy in the chicken mitochondrial genome. These results provide an important reference for further research on heteroplasmy in chicken mitochondria.

Selective Extraction of Light Lanthanides(III) by N,N-Di(2-ethylhexyl)-diglycolamic Acid: A Comparative Study with N,N-Dimethyl-diglycolamic Acid as a Chelator in Aqueous Solutions.

The complexation and selectivity of N,N-di(2-ethylhexyl)-diglycolamic acid (HDEHDGA/kerosene, HA) toward the light lanthanides, La(III), Ce(III), Pr(III), and Nd(III), are presented for the extraction from chloride media. In the low pH region (pH 1.8-2.8), the obtained data reveal that the extraction of Ln(III) is governed by cation-exchange mechanism and is driven by the negative change in enthalpy. The results from the slope analysis method suggest the formation of LnA3·(HA)1or2 in the extraction process. As major extracted species with a core of LnA3 in the first coordination sphere, LnA3 might connect with one or two additional HA molecules in the second coordination sphere by hydrogen bonding. The LnA3 core might share similar coordination geometry to those of 1:3 Ln(III) complexes (LnA'3) with water-soluble N,N-dimethyl-diglycolamic acid (HDMDGA, HA') formed in aqueous solutions or in solid-state compounds. The correlation between the extraction with HDEHDGA (HA) as an extractant and the complexation with HDMDGA (HA') as a chelator has been explored by interpreting the separation factors for HA with the difference in the stability constants for HA'. Consequently, the ratios of the stability constants of the corresponding 1:3 complexes (LnA'3) with HDMDGA could be reasonably translated to the separation factors (SFs) with HDEHDGA, providing a valuable approach for understanding the origin of the extraction/separation mechanism. By comparing the extraction selectivity of HDEHDGA with that of the currently deployed extractants in the industry such as P204, P507, and Cyanex 272, HDEHDGA offers outstanding selectivity with considerable SFs (SFCe/La = 6.68, SFPr/Ce = 2.79, and SFNd/Pr = 2.65) for light Ln(III) pairs under conditions of low acid concentrations.

Raman spectral titration method: an informative technique for studying the complexation of uranyl with uranyl(vi)-DPA/oxalate systems as examples.

The Raman band at about 870 cm-1 originating from the symmetric stretch vibration (ν1) of uranyl, UO22+, has proven to be very informative for investigating the complexation of uranyl using perchlorate or nitrate of known concentration as internal standards. The concentration of uranyl can be conveniently calculated by using the ratio of the directly read band intensities of uranyl and the added reference, ClO4-, with a factor of 1.72. While with NO3- of concentration lower than 1.8 M as the reference, a factor of 0.85 should be used. Furthermore, with added internal standards, the linear relationship between the Raman intensity and the concentration of the corresponding species is illustrated by the spectral titration of U(vi) with a very strong ligand, dipicolinic acid (DPA); and the application of a spectral titration method with Raman spectroscopy in studying the complexation of uranyl is demonstrated by the titration of U(vi) with oxalate. The stepwise changes in the Raman shift of 18, 17, and 6 cm-1, corresponding to the three oxalate anions successively bonding to UO22+, imply that the coordination modes are different. In the 1 : 1 and 1 : 2 ratios of metal to ligand complexes, the oxalate anions bond to the uranyl ion in side-on bidentate mode, but in the 1 : 3 complex the third oxalate bonds in head-on mode, which is much weaker than the first two.

A fluorescence study on the complexation of Sm(iii), Eu(iii) and Tb(iii) with tetraalkyldiglycolamides (TRDGA) in aqueous solution, in solid state, and in solvent extraction.

Tetraalkyldiglycolamide (TRDGA) ligands have potential utilization for the separation of actinide and lanthanide ions in the nuclear industry as chelates in aqueous solution if water-soluble or as extractants in organic solvents if water-insoluble. Here, a spectral titration method is extensively applied to investigating the complexation of fluorescent Sm(iii), Eu(iii), and Tb(iii) with TRDGA ligands in aqueous solutions and a solvent extraction system. In aqueous solutions using N,N,N',N'-tetramethyldiglycolamide (TMDGA, LI) as chelate, three successive complex species of Ln(iii), including [LnLI]3+, [LnL]3+, and [LnL]3+, are identified for each Ln(iii) (Ln = Sm, Eu, and Tb), and their stability constants are determined with fluorescence spectral titration method at 25 °C in 1 M NaNO3. The coordination mode in [LnL]3+ is illustrated by single-crystal structures of the solid compounds LnL(ClO4)3 (Ln = Sm, Eu, Tb, and LI = TMDGA) grown from aqueous solutions by slow evaporation. The crystal structures show that in the complexes Ln(iii) ions are coordinated by nine oxygen atoms from three tridentate LI ligands in a distorted tricapped trigonal prism geometry. To provide parallels to solvent extraction chemistry, the extracted Ln(iii) complexes with N,N'-dimethyl-N,N'-dioctyldiglycolamide (DMDODGA, LII, a lipophilic analogue of TMDGA) are prepared, and the fluorescence spectra are collected as well for comparison. The fluorescence spectra of the extracted Ln(iii) complexes with LII in an organic phase of 40-60% (v/v) n-octanol-kerosene are nearly identical to the corresponding deconvoluted spectra of [LnL]3+ in aqueous solution. The similarity in fluorescence spectra suggests that Ln(iii) ion in the extracted complexes is also coordinated by three tridentate LII ligands and that the nitrate anions acting just as counterions do not directly bond to Ln(iii) in the organic phase of solvent extraction.

Np(v) complexation with carbonate in aqueous solutions studied by spectrophotometric titration at various temperatures.

The complexation of neptunium(v) with carbonate has been studied at temperatures from 10 to 70 °C in 0.1 M LiClO4 by spectrophotometry. Three NpO2(+)-CO3(2-) complex species, NpO2(CO3)n((2n-1)-) (n = 1, 2, 3), are identified and the stability constants are calculated by using the absorption spectra in the near-IR region collected from titrations at varying temperatures. The enthalpies and entropies are calculated with van't Hoff equations in the temperature range of 10 to 70 °C, indicating that the formation of all NpO2(+)-CO3(2-) complexes is mainly entropy driven. The structures of the NpO2(+)-CO3(2-) complex species in aqueous solutions are also reviewed. Based on the molar absorptivity of Np(v) in the near-IR region the structure of NpO2(CO3)2(3-) is re-constructed as NpO2(CO3)2(H2O)(3-)of low symmetry but not as NpO2(CO3)2(H2O)2(3-)of high symmetry as suggested in a previous study.