UO2 Complexes Research Articles

Europium(III) Functionalized Covalent Organic Framework as Sensitive and Selective Fluorescent Switch for Detection of Uranium.

Uranium poses severe health risks due to its radioactivity and chemical toxicity if released into the environment. Therefore, there is an urgent demand to develop sensing materials in situ monitoring of uranium with high sensitivity and stability. In this work, a fluorescent Eu3+-TFPB-Bpy is synthesized by grafting Eu3+ cation onto TFPB-Bpy covalent organic framework (COF) synthesized through Schiff base condensation of monomers 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 5,5'-diamino-2,2'-bipyridine (Bpy). The fluorescence of Eu3+-TFPB-Bpy is enhanced compared with that of TFPB-Bpy, which is originated from the intramolecular rotations of building blocks limited by the bipyridine units of TFPB-Bpy coordinated with Eu3+. More significantly, Eu3+-TFPB-Bpy is a highly efficient probe for sensing UO22+ in aqueous solution with the luminescence intensity efficiently amplified by complexation of UO22+ with Eu3+. The turn-on sensing capability was derived from the resonance energy transfer occurring from UO22+ to the Eu3+-TFPB-Bpy. The developed probe displayed desirable linear range from 5 nM to 5 μM with good selectivity and rapid response time (2 s) for UO22+ in mining wastewater. This strategy provides a vivid illustration for designing luminescence lanthanide COF hybrid materials with applications in environmental monitoring.

Experimental and computational evidence of U(VI)–OH–Si(OH)4 complexes under alkaline conditions: Implications for cement systems

The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10−6 and 5 × 10−6 mol kgw−1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV–Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)–OH–Si(OH)4 complexes at 0.05 mol kgw−1 NaNO3. The main spectroscopic features – characteristic peak positions and decay-time – are reported for the first time for the UO2(OH)2SiO(OH)3− species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22− predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3− species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10β°(UO2(OH)2SiO(OH)3−) and log10β°(UO2(OH)2SiO2(OH)22−). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

Controlled and sequential single-electron reduction of the uranyl dication.

A flexible tripodal pyrrole-imine ligand (H3L) has been used to facilitate the controlled and sequential single-electron reductions of the uranyl dication from the U(VI) oxidation state to U(V) and further to U(IV), processes that are important to understanding the reduction of uranyl and its environmental remediation. The uranyl(VI) complexes UO2(HL)(sol) (sol = THF, py) were straightforwardly accessed by the transamination reaction of H3L with UO2{N(SiMe3)2}2(THF)2 and adopt 'hangman' structures in which one of the pyrrole-imine arms is pendant. While deprotonation of this arm by LiN(SiMe3)2 causes no change in uranyl oxidation state, single-electron reduction of uranyl(VI) to uranyl(V) occurred on addition of two equivalents of KN(SiMe3)2 to UO2(HL)(sol). The potassium cations of this new [UVO2(K2L)]2 dimer were substituted by transmetalation with the appropriate metal chloride salt, forming the new uranyl(V) tetra-heterometallic complexes, [UVO2Zn(L)(py)2]2 and [UVO2Ln(Cl)(L)(py)2]2 (Ln = Y, Sm, Dy). The dimeric uranyl(V)-yttrium complex underwent further reduction and chloride abstraction to form the tetrametallic U(IV) complex [UIVO2YIII(py)]2, so highlighting the adaptability of this ligand to stabilise a variety of different uranium oxidation states.

Dielectric Behaviour of Metal Complexes Cu (II), Ni (II) and UO2 ((VI) Of the Ligand Oxaloanilic Acid Hydrazone

Abstract: The dielectric behaviour and molecular structure of metal complexes Cu(II),Ni(II) and UO2((VI) of the ligand oxaloanilic acid hydrazine have been investigated using Time Domain Reflectometry (TDR) technique in the frequency range 10 MHz to 10GHz. Cole-Davidson model has been applied for fitting the complex permittivity spectra [ ε* (ω)]. The parameters εs –the static permittivity, τ- the dielectric relaxation time are evaluated at different temperatures from 2880K to 3330K and are discussed in respect of their molecular structural behaviours. The thermodynamic parameters, such as ΔH* - molar enthalpy, ΔS* - the molar entropy are obtained using Eyring.s rate equation. In the light of such data the possible structures of the present complexes are discussed.

In situ experimental observation and determination of speciation in phase-separated mixtures of aqueous uranyl sulfate and sulfuric acid under hydrothermal conditions by Raman spectroscopy

Liquid-liquid phase-separated aqueous solutions of [UO2SO4 + H2SO4] were investigated by quantitative Raman methods at temperatures from 300 to 425 °C and 25 MPa, using quartz-capillary optical cells. UO2SO4 concentrations ranged from 0.3 to 0.8 mol·kg-1, and the ratio of SO4/U was varied from 1 to 3.5 by the addition of sulfuric acid. Phase transition temperatures were determined by direct visual observation. The Raman spectra of the homogeneous phase solutions, at temperatures just below the lower critical solution temperature, were used as an external standard to quantify the concentrations of the species present in each of the phase-separated solutions. Deconvolution of the Raman spectra in both the dense and dilute aqueous phases showed that the uranyl ion is completely associated into the complexes UO2SO40(aq) and UO2(SO4)22−(aq). UO2SO40(aq) is the dominant species at 350 °C, and its relative concentration increases further upon heating. HSO4−(aq) was found to exist in appreciable concentrations in the dense phase only at t ≥ 400 °C. The concentration of HSO4−(aq) in the dilute phase is significant at 350 °C and increases above 375 °C. Raising the temperature to 400 °C resulted in no detectable concentration of uranium or sulfate species in the dilute phase. A solid phase formed inside the uranium-rich phase of solutions of UO2SO4 without added sulfuric acid, and is presumed to be UO3·H2O(s).

Characterization and antioxidant activity of newly synthesized N1-(2-chlorophenyl)-N2-phenyl-1,2-hydrazinedicarbothioamide and its Cu(II), Zn(II), ZrO(II), Hg(II) and UO2(II) metal complexes

Thiourea derivatives are well known class of organic compounds having diverse application in the field of medicine, agriculture, coordination, and analytical chemistry. This study reports novel N1-(2-chlorophenyl)-N2-phenyl-1,2-hydrazinedicarbothioamide and its Mn (II),Co(II), Ni(II),Cu(II), metal complexes. These synthesized compounds were characterized by H1NMR, UV-Vis, IR spectroscopy and elemental (CHN/S) analysis. Moreover, antioxidant activity was screened for synthesized compounds. All the compounds showed significant antioxidant potential with IC50 values ranges from 23.81± 1.57µM - 58.32± 1.23µM. It was revealed that synthesized compounds displayed better antioxidant activity with IC50 values between 127.10-271.41 µM to256.42 ± 0.34 µM. The ligand (IC50 =33.41 ± 1.32µM)as well as metal complex Cu(L)2 (IC50 = 28.10± 1.05µM ), Zn(L)2 (IC50 = 24.53± 1.12µM), ZrO(L)2 (IC50 = 58.32± 1.23µM), Hg(L)2 (IC50 = 23.81± 1.57µM ), UO2(L)2 (IC50 =52.14± 1.45µM) showed far better activity than the standard (IC50 = 44.7± 1.21). Among these metal complexes, Hg complex (IC50 = 23.81 ± 1.57) showed highest antioxidant activity, while Zn complex (IC50= 24.53 ± 1.12), Cu complex (28.10 ± 1.05) and Co complex (IC50= 29.76 ± 1.21), The metal complexes UO2(L)2, ZrO(L)2 showed moderate anti-oxidant activity. These compounds were found be less active than other metal complexes as well as standard. The antioxidant activity pattern of these compounds is Hg(L)2 > Zn(L)2 > Cu(L)2 > UO2(L) > ZrO(L)2. These metal complexes showed remarkable antioxidant activity which may find their importance in avoiding or reducing the development of both heart disease and some forms of cancer.

Complexation of uranyl with chelidamic acid: Crystal structures, binding strength, and electrochemical redoxes

Chelidamic acid (denoted as H3L in this paper) is a naturally occurring derivative of dipicolinic acid that functions as a versatile ligand. The complex formation of chelidamic acid with UO22+ has been investigated by potentiometry in a 0.1 M NaClO4 solution. The potentiometry identified 1:1 uranyl–chelidamate complexes (UO2HL, UO2L2−), 1:2 uranyl–chelidamate complex (UO2L24−), and a hydrolytic complex UO2(OH)2L2−. The possible UO2H2L22− structure was determine by crystal analysis. Chelidamate molecules coordinate to uranyl mainly through the pyridine–carboxylate chelation pocket (O, N, O), and 4-pyridinol–OH group is protonated. The presence of hydrolysate UO2(OH)2L2– was further confirmed by nuclear magnetic resonance spectroscopy (NMR). In an alkaline aqueous medium under anaerobic conditions, the complex UVIO2(OH)2L2−can be electrochemically reduced to pentavalent uranyl.

Exploring the Redox Properties of Bench-Stable Uranyl(VI) Diamido–Dipyrrin Complexes

The uranyl complexes UO2(OAc)(L) and UO2Cl(L) of the redox-active, acyclic diamido–dipyrrin anion L– are reported and their redox properties explored. Because of the inert nature of the complexes toward hydrolysis and oxidation, synthesis of both the ligands and complexes was conducted under ambient conditions. Voltammetric, electron paramagnetic resonance spectroscopy, and density functional theory studies show that one-electron chemical reduction by the reagent CoCp2 leads to the formation of a dipyrrin radical for both complexes [Cp2Co][UO2(OAc)(L•)] and [Cp2Co][UO2Cl(L•)].

Mobilization and health risk assessment of fertilizer induced uranium in coastal groundwater

Uranium (U) in groundwater is hazardous to human health, especially if it is present in drinking water. The semiarid regions of southern India chiefly depend on groundwater for drinking purposes. In this regard, a comprehensive sampling strategy was adopted to collect groundwater representing different lithologies of the region. The samples were collected in two different seasons and analysed for major and minor ions along with total U in the groundwater. Two samples during pre monsoon (PRM) and seven samples during post monsoon (POM) had U > 30 μgL−1, which is above the World Health Organization's provisional guideline value. The high concentration of U (188 μgL−1) was observed in the alluvial formation though a few samples showed the release of U near the pink granite (39 μgL−1) and the concentration was low in the lateritic formation (10 μgL−1). The uranyl carbonato complexes UO2(CO3)22− and UO2(CO3)34− were associated with high pH which facilitated the transport of U into groundwater especially during POM. U3O8 is the major form observed in groundwater compared to either UO2 or UO3 in the both seasons. The uranium oxides were observed to be more prevalent at the neutral pH. Though U concentration increases with pH, it is mainly governed by the redox conditions. The principal component analysis (PCA) analysis also suggested redox conditions in groundwater to be the major process facilitating the U release mechanism regardless of the season. The POM season has an additional source of U in groundwater due to the application of nitrogenous fertilizers in the alluvium region. Furthermore, redox mobilization factor was predominantly observed near the coastal region and in the agricultural regions. The process of infiltration of the fertilizer-induced U was enhanced by the agricultural runoff into the surface water bodies in the region. Health risk assessment was also carried out by determining annual effective dose rate, cancer mortality risk, lifetime average daily dose and hazard quotient to assess the portability of groundwater in the study area. Artificial recharge technique and reducing the usage of chemical based fertilizers for irrigation are suggested as sustainable plans to safeguard the vulnerable water resource in this region.

First Report on the Complexation of Uranyl Ion with Two Diglycolamide Ligands in a Room Temperature Ionic Liquid: Optical Spectroscopy and Calorimetric Studies

AbstractThe complexation of UO22+ with N,N,N′,N′‐tetraoctyl diglycolamide (TODGA) and N,N,N′,N′‐tetra‐2‐ethylhexyl diglycolamide (T2EHDGA) was investigated in ionic liquid using UV−vis absorption spectrophotometry and isothermal titration calorimetry. Spectrophotometric titration suggested the formation of two successive complexes, [UO2L]2+ and [UO2L2]2+ where L=TODGA or T2EHDGA. The formation constant values obtained in ionic liquid medium were several orders of magnitude higher than those observed with the analogous DGA ligands in aqueous medium. The stability constant (logβ) for UO22+/TODGA complex was at least an order of magnitude higher than those observed for UO22+/T2EHDGA complex. The speciation of UO22+/DGA complex obtained by spectrophotometric titration was in conformity with those observed in biphasic solvent extraction. Energetically the UO22+/DGA complexation was mainly driven by negative enthalpy and large positive entropy changes as a result of the release of entire water molecules from the solvation sphere of the hydrated UO22+ ions. The results from this work gave an insight understanding of the complexation behaviour of UO22+ ions with DGA ligands in ionic liquid. It is anticipated that such understanding will give valuable information for understanding the extraction behavior of uranyl ions by DGA ligands in ionic liquid.

Uranyl Complexes with Selenium or Tellurium Containing Chelate Ligands

AbstractThe syntheses and characterization of uranyl complexes containing the OP(Se)(OEt)2− or OP(Te)(OEt)2− ligands are described. The U(VI) complexes UO2(OP(Se)(OEt)2)2(OP(NMe2)3)2 (1) and UO2(OP(Te)(OEt)2)2(OP(NMe2)3)2 (4) were prepared by the reaction of UO2(NO3)2(OP(NMe2)3)2 with the corresponding NaOP(E)(OEt)2 salt in THF (E=Se, Te). The complexes UO2(OP(Se)(OEt)2)2(OPPh3)2 (2) and UO2(OP(Se)(OEt)2)2((OAsPh3)(HOMe) (3) were prepared by the reaction of [UO2Cl2(THF)2]2 with NaOP(Se)(OEt)2 in THF solution in the presence of OPPh3 (2) or OAsPh3 (3) and recrystallised from THF (2) and HOMe (3) respectively. Although the OP(E)(OEt)2− (E=Se, Te) ligands may be able to act as bidentate chelate ligands, evidence for the weak coordination of the Se atom to the uranium(VI) centre was only observed in complexes 2 and 3. No coordination of the Te atom to the U(VI) centre in complex 4 was observed. Complexes 2–3 were characterized using vibrational (IR, Raman) spectroscopy. In addition, the structures of compounds 1–4 were determined using single crystal X‐ray diffraction techniques: Compounds 2 and 3 are examples of structurally characterized compounds containing significant U(VI)−Se interactions.

Alkynyl-Based sp 2 Carbon-Conjugated Covalent Organic Frameworks with Enhanced Uranium Extraction from Seawater by Photoinduced Multiple Effects

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

Effect of temperature on the complexation of triscarbonatouranyl(VI) with calcium and magnesium in NaCl aqueous solution.

The complex formation of triscarbonatouranyl(VI) UO2(CO3)34- with the alkaline earth metal ions Mg2+ and Ca2+ in 0.10 mol kgw-1 NaCl was studied at variable temperatures: 5-30 °C for Mg2+ and 10-50 °C for Ca2+. Under appropriate conditions, the ternary complexes (MnUO2(CO3)3(4-2n)- with n = 1 for Mg, n = {1; 2} for Ca) were identified by time-resolved laser-induced luminescence spectrometry. Their pure spectral components at 50 °C for CanUO2(CO3)3(4-2n)- and 30 °C for MgUO2(CO3)32- were recovered by multivariate curve resolution alternating least-squares analysis. Approximation models were tested to fit the experimental data-the equilibrium constants of complexation measured at different temperatures-and deduce the thermodynamic functions, i.e., enthalpy, entropy, and heat capacity. The weak influence of temperature on complexation constants induces large uncertainties in terms of thermodynamic functions. Assuming the enthalpy is constant with temperature using the Van't Hoff equation, the first stepwise complexation of UO2(CO3)34- by Ca2+ is estimated to be slightly endothermic, with , while the second stepwise complexation of CaUO2(CO3)32- by Ca2+ with is slightly exothermic, . In contrast to Ca2+, the complexation of UO2(CO3)34- by Mg2+ is slightly exothermic, with . These values are not significantly different from zero inasmuch as the uncertainties are important due to a weak dependence of log10 K° values. The entropic character of the complexation is verified as for the first stepwise complexation of UO2(CO3)34- by Ca2+, for the second stepwise complexation of CaUO2(CO3)32- by Ca2+, and for the complexation of UO2(CO3)34- by Mg2+. The energetics of complexation and sensitivity analysis of the model estimates with temperature are discussed. The uranium speciation in the case of the safety of nuclear waste management, using the present thermodynamic functions, provides support to the assessment of underground nuclear waste repositories.

Light-up photoluminescence sensing of a nerve agent simulant by a bis-porphyrin-salen-UO2 complex.

A luminescent bis-porphyrin–salen–UO2 complex, showing a significant fluorescence light-up response upon reacting with DMMP (a simulant of nerve agents), is reported. The fluorescence change of this complex by excitation at 365 nm can be clearly observed with the naked eye, and this complex was successfully employed to construct a test paper to detect nerve agents.

Modeling and Speciation Study of Uranium(VI) and Technetium(VII) with TBP

ABSTRACT Among the fission products present in the spent nuclear fuel, technetium exhibits a singular behavior in reprocessing operations performed by solvent extraction. Indeed, this strong acid readily dissociates to form the oxo-anion TcO4 − that may interfere with uranium(VI), plutonium(IV), and zirconium(IV) in the extraction cycles of the PUREX process. This paper focuses on the uranium-technetium complex with TBP and on its non-radioactive rhenium surrogate. Despite the large set of distribution data available for rhenium and technetium extraction with TBP, the structures of the co-extracted complexes remain largely unknown. However, it is important to understand clearly the extraction mechanism of technetium with TBP in the PUREX process to optimize the separation process and to model its behavior during the extraction steps. Based on distribution data available in the literature, a thermodynamic model was developed for the extraction of technetium with TBP for a large excess of uranium(VI) in organic phase. A good representation of uranium and technetium distribution data was thus obtained when considering the formation of (HTcO4)(TBP)n complexes, as well as mixed UO2(NO3)(TcO4)(HNO3)x(TBP)n, complexes. In the complex UO2(NO3)2(HNO3)x(TBP)n., one pertechnetate anion replaces one nitrate in the uranium coordination sphere. Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) with organic phases containing a large excess of technetium(VII) or rhenium(VII) enabled full characterization of the limit mixed uranium−technetium species and also of mixed uranium-rhenium species. Details on the coordination of the uranium-technetium complex are provided with the help of DFT calculations and XAS measurements.

Luminescence studies of uranyl-aliphatic dicarboxylic acid complexes in acetonitrile medium

Abstract The uranyl (UO2 2+)-aliphatic dicarboxylic acid complexes are studied by luminescence and UV-Vis spectroscopy in acetonitrile (MeCN) medium. The ligands used are malonic acid (MA), succinic acid (SA), glutaric acid (GA), adipic acid (AA) and pimelic acid (PA). The complexes of UO2 2+ with the above ligands showed well resolved luminescence spectra at pH 4.0 with M/L = 5. Both luminescence and UV-Vis spectra indicated the formation of 1:2 and 1:3 complexes of UO2 2+ with MA and GA, AA, PA, respectively. DFT computations indicated the formation of 1:2 chelate complex of UO2 2+ with MA and two types of 1:3 complexes of UO2 2+ with SA, GA, AA and PA. Furthermore, the effect of solvent (water and acetonitrile) on the UO2 2+-ligand complexes has been performed using COSMO model. The present study demonstrates, for the first time, the formation of tris complexes of uranyl with these ligands in acetonitrile medium.

Complexation of U(VI) with BiPDA, DmBiPDA, and PhenDA: Comparison on Structures and Binding Strengths in Aqueous and DMSO/20%(v)H2O Solutions.

The stability constants (log β) of 1:1 uranyl complexes with three N,O-mixed donor ligands (L = 2,2'-dipyridyl-6,6'-dicarboxylate, 3,3'-dimethyl-2,2'-bipyridine-6,6'-dicarboxylate, and 1,10-phenanthroline-2,9-dicarboxylate, denoted as BiPDA, DmBiPDA, and PhenDA, respectively) in aqueous and DMSO/20%(v)H2O solutions were determined by spectrophotometry in 0.1 M tetraethylammonium perchlorate. The effects of ligand preorganization, steric hindrance, and solvation on the binding strength of U(VI) with the three ligands were discussed. In aqueous solution, PhenDA forms stronger complexes with U(VI) than BiPDA due to its well-preorganized structure. In DMSO/20%(v)H2O solution, in contrast, the strong solvation effect of DMSO on the ligands reduces the energy gap between the trans- and cis-conformations of BiPDA, resulting in log β(UO2(BiPDA)) > log β(UO2(PhenDA)). The steric hindrance of methyl groups on DmBiPDA makes the complex UO2(DmBiPDA) of the lowest stability in both aqueous and DMSO/20%(v)H2O solutions. Single-crystal structural data of U(VI) complexes with the three ligands indicate that the ligand coordinates with UO22+ via aromatic nitrogen atoms and carboxylate oxygen atoms. There is no clear correlation between the trend of the stability constants in solutions and the U-N/O bond lengths of the three crystal complexes. Nevertheless, DmBiPDA coordinates to UO22+ in a high-strain fashion as a result of the steric hindrance of methyl groups while BiPDA in a low-strain fashion, which is in accordance with the relative complexation strength of the two respective complexes. The results from this work help us understand the effect of ligand preorganization and solvation on the binding strength of actinides with multidentate N,O-mixed ligands in solid and solutions, which is of importance in designing ligands for the partitioning of actinides from nuclear wastes.

Genesis of the South Zhuguang uranium ore field, South China: Fluid inclusion and H–C–O–S–Sr isotopic constraints

The South Zhuguang uranium (U) ore field in South China, which was discovered in the 1950s, contains an estimated >20,000 t of recoverable U. The U ore is hosted by the Zhuguang granite massif, which is located in the South China Block. The main uranium-bearing mineral, pitchblende, is found mainly in veins along high-angle normal faults, associated with hematite, quartz, fluorite, calcite, and locally pyrite. The hydrothermal alteration is generally silicification, illitization, hematization, and chloritization that developed adjacent to the ore-bearing faults and outwards over hundreds of meters into the unaltered granites.The ore-forming fluids in the South Zhuguang U ore field can be classified into two stages. Stage I fluids were acidic and oxidizing, with fluoride and sulfate complexes of UO22+ being the dominant species in the fluid. Stage II fluids were alkaline and oxidizing, with carbonate and hydroxide complexes of UO22+ being the dominant species. δ18O values of syn-ore quartz range from +6.29‰ to +14.30‰, with calculated δ18OW-SMOW values from −1.85‰ to +5.35‰. δDW-SMOW values of the ore-forming fluids were −104.4‰ to −23.1‰. δ18O values of the altered granites increase gradually from ∼+4‰ close to ore, to ∼+12‰ in the nearly unaltered granites. The δ13C and δ18O values for syn-ore calcite range from −10.3‰ to −4.3‰ and −22.41‰ to −16.8‰ relative to Pee Dee Belemnite, respectively, with calculated δ18O and δ13C values for water and CO2 of −2.97‰ to +2.83‰ and −11.2‰ to −5.2‰, respectively. The δ34SV-CDT values of syn-ore pyrite range from −17.1‰ to −3.4‰. Initial 87Sr/86Sr ratios of syn- and post-ore fluorite vary from 0.719250 to 0.721327 and 0.714598 to 0.716299, respectively.The ore-forming fluids in the South Zhuguang U ore field were sourced primarily from Cretaceous–Paleogene redbed basins in the south, and initially derived from meteoric waters. Hydrothermal alteration associated with stage I fluids released considerable amounts of Ca2+ and F− into the ore-forming fluids, increased the fluid pH, decreased the stability of the dominant fluoride complexes, and lowered the saturation solubility of the uranyl ions. Reduction of the ore-forming fluid converted U6+ into U4+, which resulted in the deposition of pitchblende–fluorite–quartz veins of the stage I ore in fractures along with pyrite. During stage II, the U mineralization was controlled mainly by CO2 degassing, which resulted in decomposition of the carbonate complex UO2(CO3)2–, and deposition of U in pitchblende–calcite–quartz veins as the ore-forming fluid was reduced.

A density functional theory study on the interaction between UO22+ and the carbamoylphosphoramidic acid ligand for uranium extraction from seawater

Phosphorylurea molecules, which contain both phosphoryl and carbonyl groups, are considered efficient extractants for UO22+. This study aims to explain the complexation of UO22+ with carbamoylphosphoramidic acid (CPO), a simple model for phosphorylurea, for ligand design for uranium recovery from seawater using density functional theory calculations, natural bond order analysis, and the quantum theory of atoms in molecules. The results showed that, when CPO acts as a monodentate ligand, the affinity of phosphoryl for UO22+ is stronger than that of carbonyl, and CPO coordinates with UO22+ through the phosphoryl oxygen atom. When CPO serves as a bidentate ligand, both the phosphoryl and carbonyl oxygen atoms connect to UO22+, and the U–O(carbonyl) bond plays a more important role than the U–O(phosphoryl) bond in the interaction between UO22+ and CPO. This paradox may be caused by the significant charge transfer from the U–O(carbonyl) π bond orbital to the C–N σ antibond orbital of the bidentate CPO. The NH spacer between the phosphoryl and carbonyl groups could ensure the delocalization of the electron system of the molecule. The bidentate binding motif is favored by entropy and opposed by enthalpy, while the monodentate binding motif is favored by enthalpy and opposed by entropy. Ultimately, the bidentate binding motif is more favorable than the monodentate one. As expected, the interaction between UO22+ and the deprotonated CPO is stronger than that between UO22+ and the neutral CPO. Comparing the interaction between UO22+ and CPO with that between UO22+ and N-phenylcarbamoylphosphoramidic acid (PhCPO), formed by replacing one hydrogen atom from the terminal nitrogen atom of CPO with a phenyl group, the phenyl substituent at the terminal nitrogen atom of PhCPO shows a slightly negative effect on the interaction between UO22+ and PhCPO.

Thermodynamic, Structural, and Computational Investigation on the Complexation between UO22+ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution.

The stability constants (log β), enthalpies of complexation (ΔH), and entropies of complexation (ΔS) for the complexes of uranium(VI) with a series of amine-functionalized diaetamide ligands, 2,2'-benzylazanediylbis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-methylazanediylbis(N,N'-dimethylacetamide) (MABDMA), in aqueous solution were determined by potentiometry and calorimetry. Electronspray ionization mass spectrometry was used to verify the presence of uranium(VI) complexes in solution. The thermodynamic data indicate that the binding strengths of the three ligands with UO22+ follow the order BnABDMA < ABDMA < MABDMA, parallel to the order of the protonation constants as well as the order of the stability of the Nd3+ complexes, suggesting that the complexation of UO22+ with the ligands consist predominantly of electrostatic interactions. Denisty functional theory calculations were conducted to reveal the structures, electronic charge distribution, and energetics of the uranium(VI) complexes, providing insight into the thermodynamic trends of the complexation. Extended X-ray absorption fine structure spectroscopy was used to identify the structures of the uranium(VI) complexes in aqueous solution.