Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri‐n‐butyl phosphate

We present new results on the liquid–liquid extraction of uranium (VI) from a nitric acid aqueous phase into a tri‐n‐butyl phosphate/1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (TBP/[C₄mim][Tf₂N]) phase. The individual solubilities of the ionic‐liquid ions in the upper part of the biphasic system are measured over the whole acidic range and as a function of the TBP concentration. New insights into the extraction mechanism are obtained through the in situ characterization of the extracted uranyl complexes by coupling UV/Vis and extended X‐ray absorption fine structure (EXAFS) spectroscopy. We propose a chemical model to explain uranium (VI) extraction that describes the data through a fit of the uranyl distribution ratio D_U. In this model, at low acid concentrations uranium (VI) is extracted as the cationic complex [UO₂(TBP)₂]²⁺, by an exchange with one proton and one C₄mim⁺. At high acid concentrations, the extraction proceeds through a cationic exchange between [UO₂(NO₃)(HNO₃)(TBP)₂]⁺ and one C₄mim⁺. As a consequence of this mechanism, the variation of D_U as a function of TBP concentration depends on the C₄mim⁺ concentration in the aqueous phase. This explains why noninteger values are often derived by analysis of D_U versus [TBP] plots to determine the number of TBP molecules involved in the extraction of uranyl in an ionic‐liquid phase.     

Monotonic transfer annealing kinetics associated with an oscillatory phenomenon in51Cr(III)-doped potassium chromate

Infrared absorption spectra of HNO₃ solutions in UO₂(NO₃)₂(TBP)₂ have been taken. The formation of a hydrogen bond between HNO₃ and nitrate or phosphoryl group in UO₂(NO₃)₂(TBP)₂ has been established. On extracting Pu(IV) and Np(IV) by 30% TBP-dodecane, dependence of the distribution coefficients on concentration has been found at UO₂(NO₃)₂ concentrations in the aqueous phase upwards from 0.4M. This dependence appeared in the temperature interval 0–60°C. Such effects may be caused by ordered structure of saturated uranyl nitrate solutions in TBP.     

Nitrate ion transport across TBP-kerosene oil supported liquid membranes coupled with uranyl ions

The role of nitrate ions in uranyl ions transport across TBP-kerosene oil supported liquid membranes (SLM) at varied concentrations of HNO₃ and NaNO₃ has been studied. It has been found that nitrate ions move faster compared to uranyl ions at the uranium feed solution concentrations studied. The nitrate to uranyl ions flux ratio vary from 355 to 2636 under different chemical conditions. At low uranium concentration the nitrate ions transport as HNO₃ · TBP, in addition to as UO₂(NO₃)₂ · 2TBP type complex species. The flux of nitrate ions is of the order of 12.10 · 10^–3 mol · m^–2 · s^–1 compared to that of uranium ions (4.56 · 10^–6 mol · m^–2 · s^–1). The permeability coefficient of the membrane for nitrate ions varies with chemical composition of the feed solution and is in the order of 2.5 · 10^–10 m^–2 · s^–1. The data is useful to estimate the nitrate ions required to move a given amount of uranyl ions across such an SLM and in simple solvent extraction.     

Raman studies of uranyl nitrate and its hydroxy bridged dimer

In this paper, the authors report Raman spectra obtained on imidazolium di-μ-hydroxybis[dioxobis-(nitrato)uranium(VI)], (UO₂(NO₃)₂(OH))₂.2C₃H₅N₂ (IUNH). An assignment of the Raman bands is made by comparing the spectrum of IUNH with those of uranyl nitrate hexahydrate (UNH) and imidazole (IMID). The electron charge transfer from the imidazole ring to the uranyl ion has been empirically determined.     

Separation and Recovery of Uranium and Plutonium from Oxalate Supernatant

The separation of uranium and plutonium from oxalate supernatant, obtained after precipitating plutonium oxalate, containing ~10 g/l uranium and 30–100 mg/l plutonium in 3M HNO₃ and 0.10–0.18M oxalic acid solution has been carried out. In one extraction step with 30% TBP in dodecane: ~92% of uranium and ~7% of Pu is extracted. The raffinate containing the remaining U and Pu is extracted with 0.2M CMPO+1.2 M TBP in dodecane and near complete extraction of both the metal ions is achieved. The metal ions are back extracted from organic phases using suitable stripping agents. The recovery of both the metal ions separately is >99%. The uranium species extracted into the TBP phase from the HNO₃+oxalic acid medium was identified as UO₂(NO₃)₂·2TBP.     

NMR studies of uranyl benzenesulfonate solvation and phase diagram of uranyl benzenesulfonate-water-tributyl phosphate system

Uranyl hydration and solvation numbers of uranyl benzenesulfonate (BSU) aqueous-organic solutions have been determined by means of dynamic NMR spectroscopy technique. Three aqua-complexes have been found to exist in aqueous-acetone solution: [UO₂ (U₆H₅SO₃)₂·4H₂O] and [UO₂(C₆H₅SO₃)₂·(H₂O)_n] where n=1 or 2 and anions are bridging bidentate. Transition from [UO₂ (C₆H₅SO₃)₂·4H₂O] to the higher aqua complexes begins at P>40. There is a disolvate in the non-aqueous solution of BSU in tri-n-butyl phosphate (TBP). Composition of aqueous and organic phase of the BSU-water-TBP ternary system has been determined at room temperature, allowing to produce the phase diagram of the system. The binodale position is related to the anion amphiphilicity. The solvation number determined for BSU in the organic phases corresponds exactly to the low temperature data and allows to observe BSU dehydration and desolvation in the region of mutual dissolution of water and the organic phase, as well as TBP and the aqueous phase.     

Extraction of pertechnetate anion as a ligand in metal complexes with tributylphosphate

The extraction of the pertechnetate anion has been investigated in the systems tributylphosphate (TBP)—solvent (carbon tetrachloride, n-heptane, chloroform)—metal salt (uranyl nitrate and chloride, thorium nitrate)—ammonium salt. In the absence of a metal, the solvates HTeO₄. iTBP (i=4) are extracted, while in the presence of uranium and thorium, the distribution of technetium corresponds to the formation of the mixed complexes: UO₂(NO₃)(TeO₄)·2TBP, UO₂Cl(TcO₄)·2TBP and Th(NO₃)₃ (TcO₁)·2TBP. The effective constants of the reactions H⁺+TcO ₄ ⁻ +i(TBP)_org←(HTcO₁·iTBP)_org, and (ML_n·2TBP)_org+TcO ₄ ⁻ ←(ML_n−1TcO₄·2TBP)_org+L were established in the above systems. The extraction of pertechnetate ion is more effective when it is coordinated to a cation solvated by TBP than the extraction in the form of pertechnetate acid solvated by TBP.     

Complexation of uranium(VI) with acetohydroxamic acid

The advanced separation extraction process based on tri-n-butyl phosphate organic phase called UREX is being developed to separate uranium from fission products and other actinides, and the acetohydroxamic acid (AHA) is employed to reduce and complex plutonium and neptunium in order to decrease their distribution to the TBP-organic phase. In this study, the extraction of uranium was performed from various aqueous matrices with different concentrations of HNO₃, LiNO₃, and AHA. Extraction of uranium increases with increasing both initial HNO₃ and total nitrate concentration. UV-VIS spectrophotometry confirmed that AHA is involved in the complex of uranium with TBP.     

Development of Mathematical Model of H2O-HNO3-UO2(NO3)2-TBP-diluent System

The mathematical model of H₂O-HNO₃-UO₂(NO₃)₂-TBP-dodecane system has been elaborated. Mole fractions and volume ones and rational activity coefficients have been used in order to create the system of equations on the base of mass action law. Method for calculating activity coefficients was provided. The formation constants of uranyl nitrate di-solvate and mono-solvate and di-solvate of acid have been determined. Interaction between uranyl nitrate di-solvate and dodecane and between TBP and dodecane was taken into account. Activity coefficients of nitric acid and uranyl nitrate in mixed solutions were considered. Errors of adequacy have been determined for the systems containing 30% and 12% TBP concentrations.     

Actinide interactions with microbial chelators: the dioxobis[pyridine-2,6-bis(monothiocarboxylato)]uranium(VI) ion

The title complex, bis­(tetra­phenyl­phospho­nium) dioxobis(py­ridine-2,6-dicarbo­thio­ato-O,N,O′)­uranium(VI), (C₂₄H₂₀P)₂[UO₂(C₇H₃NO₂S₂)₂], was prepared by reacting two equivalents of ­pyridine-2,6-bis­(mono­thio­carboxyl­ate) (pdtc) with uranyl nitrate. The geometry of the eight-coordinate U atom is hexagonal bipyramidal, with the uranyl O atoms in apical positions. This is the first reported complex in which this ligand binds a metal through the O and not the S atoms. Principal bond lengths include uranyl lengths of 1.774 (2) Å, U—O distances of 2.434 (2) and 2.447 (3) Å, and two U—N distances of 2.647 (3) Å. The anion lies on an inversion centre.     

Salicylate of Uranium

The reactions of uranium pentaethoxide with salicylic acid have been carried out in different stoichiometric ratios, yielding products of the type, U(OEt)₃(C₇H₄O₃), U(OEt)(C₇H₄O₃)₂, U(C₇H₄O₂)₂(C₇H₄O₃), and U₂(C₇H₄O₃)₃. A detailed study of the complexation reactions of uranyl ion with salicylic acid have also been made by potentiomertic and conductometric methods. The formation of 1:1 and 1:2 complexes have been confirmed by preparative studies. Two compounds, viz. (C₅H₆N)₂UO₂(C₇H₄O₃)₂ (NH₄)₂UO₂(C₇H₄O₃)₂ have been isolated.     

Extraction of uranium(VI) from nitric acid medium by 1.1M tri-n-butylphosphate in ionic liquid diluent

Summary A systematic study on the extraction of U(VI) from nitric acid medium by tri-n-butylphosphate (TBP) dissolved in a non-traditional diluent namely 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) ionic liquid (IL) is reported. The results are compared with those obtained using TBP/n-dodecane (DD). The distribution ratio for the extraction of U(VI) from nitric acid by 1.1M TBP/bmimPF₆ increases with increasing nitric acid concentration. The U(VI) distribution ratios are comparable in the nitric acid concentration range of 0.01M to 4M, to the ratios measured using 1.1M TBP/DD. In contrast to the extraction behavior of TBP/DD, the D values continued to increase with the increase in the concentration of nitric acid above 4.0M. The stoichiometry of uranyl solvate extracted by 1.1M TBP/IL is similar to that of TBP/DD system, wherein two molecules of TBP are associated with one molecule of uranyl nitrate in the organic phase. Ionic liquid alone also extracts uranium from nitric acid, albeit to a small extent. The exothermic enthalpy accompanying the extraction of U(VI) in TBP/bmimPF₆ decreases with increasing nitric acid and with TBP concentrations.     

Determination of liquid-liquid partition data for hydrazoic acid between tributylphosphate-alkane/nitric acid solutions using gas-liquid chromatography

The gas chromatographic technique of elution by characteristic point (ECP) has been used to determine partition data for HN₃ at finite concentrations with tributyl phosphate (TBP) in hydrocarbon (hexadecane) solution in the presence of nitric acid and uranyl nitrate. The data are used to derive predictive equations for calculating gas-liquid and liquid-liquid partition coefficients for varying temperature and varying concentrations of TBP, HNO₃, UO₂(NO₃)₂, and HN₃ in hydrocarbon solvents simulating nuclear fuel reprocessing flow sheets. The chromatographically derived partition data presented, being based on more precise measurements than were previously possible using conventional methods, allowed demonstration and quantification of the logarithmic temperature effect expected, but previously unobservable.     

Extraction of uranium into a third phase of thorium nitrate—tributyl phosphate

Extraction of 0.05–0.25M uranyl nitrate into 30% tributyl phosphate (TBP) in dodecane from nitric acid solutions of thorium nitrate at equilibrium with its salt has been studied. Under investigated conditions a third (second organic) phase is formed. As the heavy organic phase extracts uranium, the calculated ratio of TBP to thorium and uranium sum decreases from 2.7 to less than 7. Electronic spectra show that in heavy organic phase approximately 80% of uranium is found as trinitrate complex, while in the light organic phase this complex is not detected. The measurements of dielectric constant () of the heavy phase reveal a frequency dependence of . The data obtained point to the existence of an ordered structure in the heavy organic phase.     

Extraction of Pm(III) from acidic solutions using dihexyl N,N-diethylcarbamoylmethyl phosphonate

Benzene solution of dihexyl N,N-diethyl-carbamoylmethyl phosphonate (CMP) has been used for the extraction of Pm(III) from 0.2 to 6.0M HNO₃. High extraction of Pm(III) was observed between 2 to 4M HNO₃. The species extracted in the organic phase were Pm(NO₃)₃.3CMP and Pm (NO₃)₃ (3-n) CMP.nTBP when the extractants were CMP and CMP+TBP, respectively. Pm could be efficiently backextracted from both organic phases by pH 2.0 HNO₃ solution.     

Liquid-liquid extraction of uranium(VI) from aqueous acidic solutions using Alamine, TBP and CYANEX systems

Liquid-liquid extraction of uranium(VI) (UO₂ ²⁺) from aqueous acidic (HCl and HNO₃) solutions into a co-existing organic phase containing Alamine 308 (triisooctyl amine), TBP (tri-n-butyl phosphate) or CYANEX 302 (bis(2,4,4-trimethylpentyl) monothiophosphinic acid) and diluent (toluene) was studied at isothermal conditions (298.2 K) at aqueous phase acidity varying in the range 0.5-6 mol/dm³. All solvent systems exhibit a maximum distribution ratio restricted in the acidity range 3-4 mol/dm³. An obvious difference in extraction behavior through amine system has been observed for two acids, HCl and HNO₃, distinguishing the divergent interactions attributed to the different mechanism of complexation depending on the acidic medium. The high degree of separation of UO₂ ²⁺ from HNO₃ solution is feasible through a complex formation with extractants ranging in the order CYANEX 302 > TBP > Alamine 308. The results were correlated using various versions of the mass action law, i.e., a chemodel approach and a modified version of the Langmuir equilibrium model comprising the formation of one or at least two U(VI)-extractant aggregated structures.     

Extraction of U(VI) by cyanex-272

Extraction of U(VI) from HNO₃, HCl and HClO₄ media using cyanex-272 (bis[2,4,4 trimethyl pentyl] phosphinic acid)/n-dodecane has been carried out. In the case of HNO₃ and HClO₄ media, the distribution ratio (D) value first decreases and then increases, whereas from HCl medium it first decreases and then remains constant with increase in H⁺ ion concentration. At lower acidities, U(VI) was extracted as UO₂(HA₂)₂ by an ion exchange mechanism, whereas at higher acidities as UO₂(NO₃)₂ ^.2(H₂A₂) following a solvation mechanism. The D for U(VI) by cyanex-272, PC-88A and DEHPA at low acidities follows the order cyanex-272 > PC-88A > DEHPA. Also, cyanex-272 was found to extract U(VI) more efficiently than TBP at 2M HNO₃. The effect of diluents on the extraction of U(VI) by cyanex-272 followed the order cyclohexane > n-dodecane > CCl₄ > benzene. The loading of U(VI) into cyanex-272/n-dodecane from 2M HNO₃ has shown that at saturation point, cyanex-272 was 78% loaded. No third phase was observed at the saturation level. The stripping of U(VI) from the loaded organic phase was not possible with water, it was poor with acetic acid and sodium acetate but quantitative with oxalic acid, ammonium carbonate and sodium carbonate.     

Pillared and open-framework uranyl diphosphonates

The hydrothermal reactions of uranium trioxide, uranyl acetate, or uranyl nitrate with 1,4-benzenebisphosphonic acid in the presence of very small amount of HF at 200 °C results in the formation of three different uranyl diphosphonate compounds, [H₃O]₂{(UO₂)₆[C₆H₄(PO₃)(PO₂OH)]₂[C₆H₄(PO₂OH)₂]₂[C₆H₄(PO₃)₂]}(H₂O)₂ (Ubbp-1), [H₃O]₄{(UO₂)₄[C₆H₄(PO₃)₂]₂F₄}·H₂O (Ubbp-2), and {(UO₂)[C₆H₂F₂(PO₂OH)₂(H₂O)}₂·H₂O (Ubbp-3). The crystal structures of these compounds were determined by single crystal X-ray diffraction experiments. Ubbp-1 consists of UO₇ pentagonal bipyramids that are bridged by the phosphonate moieties to form a three-dimensional pillared structure. Ubbp-2 is composed of UO₅F₂ pentagonal bipyramids that are bridged through the phosphonate oxygen atoms into one-dimensional chains that are cross-linked by the phenyl spacers into a pillared structure. The structure of Ubbp-3 is a three-dimensional open-framework with large channels containing water molecules with internal dimensions of approximately 10.9×10.9 Å. Ubbp-1 and Ubbp-2 fluoresce at room temperature.     

Kinetics of rhodium extraction from nitric acid solutions with a mixture of dihexyl sulfide and alkylanilinium nitrate

The manifold enhancement of the rhodium extraction efficiency from nitric acid solutions of triaquatrinitrorhodium is discovered with the use of two-component mixed extractants based on alkylaniline (AA), dihexyl sulfide (DHS), dihexyl sulfoxide (DHSO), tributyl phosphate (TBP), and oxime ACORGA P5100. A mixture of unimolar solutions of alkylanilinium nitrate and DHS is found to be the most efficient extractant: at 35°C, this mixture quantitatively extracts rhodium within 5 min from aqueous solutions containing 0.06 to 3 mol/L HNO₃. The extraction kinetics are studied. The following two-stage extraction mechanism is substantiated: the equilibrium of formation of a colloidal-chemical intermediate involving [Rh(H₂O)₃(NO₂)₃], HNO₃, and (BHNO₃)_p (an associated form of the alkylanilinium salt) and the reaction of the intermediate with DHS (the rate-controlling stage).     

Syntheses and structures of two new uranyl complexes [UO2(DPDPU)2(NO3)2](C6H5CH3) and [UO2(PMBP)2(DPDPU)](CH3C6H4CH3)0.5

Two new uranyl complexes [UO₂(DPDPU)₂(NO₃)₂](C₆H₅CH₃) (1) and [UO₂(PMBP)₂ (DPDPU)](CH₃C₆H₄CH₃)_0.5 (2), (DPDPU?=?N,N′-dipropyl-N,N′-diphenylurea, HPMBP?= 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5) were synthesized and characterized. The coordination geometry of the uranyl atom in 1 is distorted hexagonal bipyramidal, coordinated by two oxygen atoms of two DPDPU molecules and four oxygen atoms of two bidentate nitrate groups. The coordination geometry of the uranyl atom in 2 is distorted pentagonal bipyramidal, coordinated by one oxygen atom of one DPDPU molecule and four oxygen atoms of two chelating PMBP molecules.