Time-resolved laser fluorescence spectroscopy of UO2(CO3)3(4-)

Identification and Quantification of Multiphase U(VI) Speciation on Gibbsite with pH Using TRLFS and PARAFAC of Excitation Emission Matrices

The significant abundance of uranium in radioactive waste inventories worldwide necessitates a thorough understanding of its behavior. In this work, the speciation of uranyl(VI), (UO22+) in a gibbsite system under ambient conditions has been determined as a function of pH by deconvolution and analysis of luminescence spectroscopic data. Uniquely, a combined experimental and statistical approach utilizing time-resolved luminescence spectroscopy and parallel factor analysis (PARAFAC) of excitation emission matrices has been successfully utilized to identify four separate luminescent U(VI) species in the uranyl-gibbsite system for the first time. The speciation of all luminescent U(VI) species in an environmentally relevant system over a pH range of 6-11 is discerned through the analysis of emission fingerprints at low temperature (20 K). Comparison of the deconvoluted luminescence spectra with mineral standards and geochemical models of the system allows the assignment of the luminescent chemical species as metaschoepite, Na-compreignacite, surface adsorbed ≡AlO2-UO2(OH) and ≡AlO2-UO2(CO3)24- complexes, with assignments supported by fitting of extended X-ray absorption fine structure data. The combined spectroscopic techniques in this study show that assignment and quantification of uranyl(VI) species in a sorption system over a large pH range can be accurately achieved using PARAFAC to deconvolute a three way emission spectroscopic data set.

Progress of energy-related radiochemistry and radionuclide production in the Republic of Korea

The field of radiochemistry in the Republic of Korea has expanded greatly over the last three decades to meet the rapid growth of technological demands in various areas such as nuclear energy and nuclear technologies for human health and environmental protection. Major research activities, which were initially centered at the Korea Atomic Energy Research Institute (KAERI), have gradually spread to major universities and the commercial sector. In this review, progress and recent research trends in nuclear and radiochemistry in Korea are summarized. The main research outcomes achieved by KAERI scientists are highlighted, with emphasis on basic actinide chemistry in nuclear fuel cycles, the radioanalytical chemistry of various radionuclides from radioactive waste and the environment, and medical radionuclide production. In addition, recent efforts to promote radiochemical education and future perspectives are briefly outlined.

Structural and spectroscopic studies of spontaneously formed crystalline Eu(iii)-aliphatic dicarboxylates at room temperature

Complexation of actinides and lanthanides with carboxylic organic ligands is a critical issue affecting radionuclide migration from deep geological disposal systems of spent nuclear fuel. A series of Eu(iii)-aliphatic dicarboxylate compounds, as chemical analogs of radioactive Am(iii) species, Eu2(Ox)3(H2O)6, Eu2(Mal)3(H2O)6, and Eu2(Suc)3(H2O)2, were synthesized and characterized using X-ray crystallography and time-resolved laser fluorescence spectroscopy to examine the ligand-dependent binding modes and the corresponding changes in spectroscopic properties. Powder X-ray crystallography results confirmed that all of the compounds presented a crystalline polymer structure with a trigonal prism square-face tricapped polyhedron geometry centered on Eu(iii) in a nine-coordinate environment involving nine oxygen atoms. This study captures the transition of the coordination modes of aliphatic dicarboxylate ligands from side-on to end-on binding as the carbon chain length increases. This transition is illustrated in malonate bindings involving a combination of side-on and end-on modes. Strongly enhanced luminescence, especially for the hypersensitive peak, indicates a low site symmetry in the formation of solid compounds. The number of remaining bound water molecules was estimated from the resultant increased luminescence lifetimes, which were in good agreement with crystal structures. The excitation-emission matrix spectra of these crystalline polymers suggest that Ox ligands promote the sensitized luminescence of Eu(iii), especially in the UV region. In the case of Mal and Suc ligands, charge transfer occurs in the opposite direction from Eu(iii) to the ligands under UV excitation, resulting in weaker luminescence.

Influence of Alkaline Earth Metal Ions on Structures and Luminescent Properties of NamMnUO2(CO3)3(4-m-2n)- (M = Mg, Ca; m, n = 0-2): Time-Resolved Fluorescence Spectroscopy and Ab Initio Studies

The luminescence spectra of triscarbonatouranyl complexes were determined by experimental and theoretical methods. Time-resolved laser-induced fluorescence spectroscopy was used to monitor spectra of uranyl and bicarbonate solutions at 0.1 mol kg_w^-1 ionic strength and pH ca. 8. The concentrations of Mg²⁺ and Ca²⁺ in the samples were chosen in order to vary the proportions of the alkaline earth ternary uranyl complexes MgUO₂(CO₃)₃^2-, CaUO₂(CO₃)₃^2-, and Ca₂UO₂(CO₃)₃. The luminescence spectrum of each complex was determined by decomposition in order to compare it with the simulated spectra of model structures Na_mM_nUO₂(CO₃)₃^(4-m-2n)- (M = Mg, Ca; m, n = 0-2) obtained by quantum chemical methods. The density functional theory (DFT) and time-dependent (TD)-DFT methods were used with the PBE0 functional to optimize the structures in the ground and excited states, respectively, including relativistic effects at the spin-free level, and water solvent effects using a continuum polarizable conductor model. The changes in the structural parameters were quantified with respect to the nature and the amount of alkaline earth counterions to explain the luminescence spectra behavior. The first low-lying excited state was successfully computed, together with the vibrational harmonic frequencies. The DFT calculations confirmed that uranyl luminescence originates from electronic transitions from one of the four nonbonding 5f orbitals of uranium to an orbital that has a uranyl-σ (5f, 6d) character mixed with the 2p atomic orbitals of the carbonate oxygens. Additional single-point calculations using the more accurate TD-DFT/CAM-B3LYP allow one to determine the position of the luminescence "hot band" for each structure in the range 467-476 nm and compared fairly well with experimental reports at around 465 nm. The complete luminescence spectra were built from theoretical results with the corresponding assignment of the electronic transitions and vibronic modes involved, mainly the U-O_ax stretching mode. The resulting calculated spectra showed a very good agreement with experimental band positions and band spacing attributed to MgUO₂(CO₃)₃^2-, CaUO₂(CO₃)₃^2-, and Ca₂UO₂(CO₃)₃. The evolution of luminescence intensities with the number of alkaline earth metal ions in the structure was also correctly reproduced.

Determination of formation constants and specific ion interaction coefficients for CanUO2(CO3)3(4−2n)− complexes in NaCl solution by time-resolved laser-induced luminescence spectroscopy

The formation constants of CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃(aq) were determined in NaCl medium at ionic strengths between 0.1 and 1 mol kg_w⁻¹ using time-resolved laser-induced luminescence spectroscopy (TRLS).

Complexation of UO2(CO3)34- with Mg2+ at varying temperatures and its effect on U(vi) speciation in groundwater and seawater

The ternary alkaline earth metal uranyl tricarbonate complexes, MnUO2(CO3)32n-4 (M = Mg and Ca), have been considered to be the major U(vi) species contributing to uranium mobility in natural water. Although MgUO2(CO3)32- can account for a substantial portion of U(vi) in a Mg2+-rich aqueous system and most processes regarding uranium are subjected to variable temperatures, chemical thermodynamic data for the prediction of the formation of MgUO2(CO3)32- at variable temperatures are still unknown. To fill the knowledge gap in the current chemical thermodynamic database, ultraviolet/visible (UV/Vis) absorption spectroscopy was employed to determine the formation constants (log K') of MgUO2(CO3)32- at varying temperatures of 10-85 °C in 0.5 mol kg-1 NaCl. The formation constants at infinite dilution, log K°, were obtained with specific ion interaction theory (SIT), and an increasing tendency of log K° with temperature was observed. Using calorimetric titration, the endothermic molar enthalpy of reaction (ΔrHm) of Mg2+ complexation with UO2(CO3)34- was determined at 25 °C. According to the chemical thermodynamic data obtained in this work, approximation models for the prediction of the temperature-dependent formation constant at a given temperature were examined and the constant enthalpy approximation with modification to the isoelectric reaction showed a satisfactory agreement with our experimental results. Finally, the effects of temperature on U(vi) speciation in Mg2+-rich groundwater and U(vi) extraction from seawater by amidoxime derivatives were examined. For the first time, this work provides important chemical thermodynamic data of MgUO2(CO3)32n-4 to assess the impact of temperature on U(vi) behaviour in groundwater and seawater.

Spectroscopic speciation of aqueous Am(iii)–oxalate complexes

Speciation, thermodynamic stability, and structural information of aqueous oxalato-Am(iii) complexes were resolved by combinatorial use of UV-Vis-LWCC, TRLFS, and DFT calculations.

Formation of CaUO2(CO3)32− and Ca2UO2(CO3)3(aq) complexes at variable temperatures (10–70 °C)

The ternary complexation of calcium uranyl tricarbonate species, CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃(aq), which are the predominant U(vi) complexes in groundwater and seawater, was investigated at variable temperatures from 10 to 70 °C.

The oxidation of borohydrides by photoexcited [UO2(CO3)3]4−

The photochemistry of [U^VIO₂(CO₃)₃]⁴⁻ has not been well-explored because of its barely luminescent nature. Herein, we have demonstrated the oxidation of borohydrides by photoexcited [U^VIO₂(CO₃)₃]⁴⁻.

Formation, stability and structural characterization of ternary MgUO2(CO3)3 2− and Mg2UO2(CO3)3(aq) complexes

The formation of ternary Mg-UO2-CO3 complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO2-CO3 complexes. The presence of two different Mg-UO2-CO3 complexes was identified by means of two distinct fluorescence lifetimes of 17±2 ns and 51±2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO2-CO3 complexes forming. For the first time, the formation of both MgUO2(CO3)3 2− and Mg2UO2(CO3)3(aq) species was confirmed and the corresponding equilibrium constants were determined as log β0 113=25.8±0.3 and log β0 213=27.1±0.6, respectively. Complementarily, fundamental structural information for both Ca-UO2-CO3 and Mg-UO2-CO3 complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 Å) compared with U-Ca distance (4.15 Å). These results confirmed the inner-sphere character of the Ca/Mg-UO2-CO3 complexes. The formation constants determined for MgUO2(CO3)3 2− and Mg2UO2(CO3)3(aq) species indicate that ternary Mg-UO2-CO3 complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.