Uranyl Hydrolysis Research Articles

Radiation-Induced Solid-State Transformations of Uranyl Peroxides.

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca2[UO2(O2)3]·9H2O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca2+ is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca2+ cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice H2O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO2(O2)2(OH)24-, that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li4[(UO2)(O2)3]·10H2O results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and H2O in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li+ and Ca2+ countercations. Highly hydratable and mobile Li+ yields to uranyl hydrolysis reactions, while Ca2+ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.

Hydrolysis of Uranyl‐, Nd‐, Ce‐Ions and their Mixtures by Thermal Decomposition of Urea

AbstractThe hydrolysis of uranyl‐, Nd‐ and Ce‐cations, induced by thermal decomposition of urea was investigated and the impact of the urea content and the experiment temperature on the reaction kinetics was evaluated. Uranyl precipitated as ammonium diuranate (ADU) with varying stoichiometry. Nd(III) and Ce(III) showed comparable pH evolutions and lanthanide carbonate hydroxide (LnCO3OH) products were identified, whereas Ce(IV) hydrolysed at lower pH and formed CeO2. The precipitation behaviour was confirmed for mixtures of uranyl and the lanthanides. Depending on the urea content, a partial co‐precipitation occurred in U/Nd mixtures. The phases formed with Ce(III) and Ce(IV) were also identified in the precipitates of binary U/Ce mixtures. In ternary U/Nd/Ce compositions, a simultaneous precipitation of Nd(III) and Ce(III) was observed and a partial incorporation of the lanthanides into the ADU phase, whereas the precipitation in the presence of Ce(IV)/CeO2 led to the formation of three individual phases.

Multidentate extracting agents based on calix[4]arene scaffold – UVI/EuIII separation studies

The development of new ligand systems for solvent extraction demands knowledge of the fundamental chemistry behind these processes. Thus, herein, we report the synthesis and structural characterization of a 6-hydroxymethylpyridine functionalized calix[4]arene (L2). Its complexation behavior towards the divalent uranyl cation as well as the trivalent europium cation was spectroscopically studied. In solution the formation of two uranyl complex species with L2 was proven by spectrophotometric titration experiments, with stability constants of log ß1:1 = 5.82 ± 0.04 and log ß2:1 = 5.57 ± 0.05, and of one europium complex species (log ß1:1 = 6.85 ± 0.03). In addition, the concept of the desired proper planar binding pocket for the linear uranyl cation has been verified in solution. Moreover, comparative UVI and EuIII extraction studies with L2 and a structure related 8-hydroxyquinoline modified calix[4]arene (L1) were performed in which the effects of various conditions such as pH of the aqueous phase, presence of relevant anions (sulfate, phosphate, carbonate) and competitive extraction with ethylenediaminetetraacetic acid (EDTA) were investigated. It has been shown that both macrocyclic ligands offer a strong chelate effect, which enables them to extract even stable uranyl hydrolysis species. With these ligands, the selective separation of UVI from EuIII-containing solutions is possible under a variety of conditions.

Complex formation of anserine with uranyl in aqueous solution: Thermodynamic studies and structural analysis

Knowledge of uranyl (UO22+) aqueous speciation and chemical interactions with natural organic matter is the basis to mode the sorption and transport behavior of uranyl. Anserine is a natural dipeptide containing β-alanine and 1-methylhistidine, which can be found in the skeletal muscle of birds and certain species of mammals. The complexation of anserine with uranyl cation was studied thermodynamically and structurally at 25 °C, I = 0.5 mol·L−1 NaCl solution. Protonation of anserine was investigated by potentiometry, microcalorimetry and NMR spectroscopy. Experimental results confirmed that there are three successive proton reaction steps for a totally deproton anserine. The order of successive protonation sites are the terminal amine nitrogen atom of alanine, amine nitrogen atoms in imidazole ring, and finally carboxylate oxygen. This trend is in agreement with exothermic enthalpies of protonation reaction. Potentiometry was used to measure stability constant of uranyl complex in solution. Thermodynamic studies identified four uranyl/anserine complexes with different degrees of deprotonation, UO2(HL)2+, UO2L+, UO2L2(aq) and UO2(OH)L2−. Coordination modes of the three complexes [UO2(HL)2+, UO2L+ and UO2L2(aq)] were investigated by NMR spectroscopy. In these complexes, anserine holds a tridentate mode. Cyclic voltammetry was performed to understand the influence of anserine exert over redox response to uranium in solution. The results confirmed that oxidation state of uranyl cation is stabilized significantly and harder to be reduced by complex with anserine. Anserine could effectively suppress the hydrolysis of uranyl in aqueous solutions.

Exploring Crystal Engineering for Porous Uranyl–Organic Frameworks: Insight into Hydrolysis and In Situ Reaction, Structural Variation‐Dependent Physical Properties

AbstractThree porous uranyl complexes {(CH3)2NH2[(UO2)(L1)]⋅H2O}n (1), {[(UO2)5(μ2‐OH)(μ3‐OH)(L2)2(H2O)3]}n (2) and {[(UO2)4(μ3‐OH)2(L3*)3]}n (3), {H3L1 = 5‐((4‐carboxybenzyl)oxy)isophthalic acid, H4L2 = 4,4’‐oxydiphthalic acid, and H2L3 = 4,4’‐((6‐chloro‐1,3,5‐triazine‐2,4‐diyl)bis(azanediyl))dibenzoic acid}, have been designed and synthesized under solvothermal and hydrothermal reaction conditions. All of these ligands are firstly used in the syntheses of uranyl hybrid materials. We investigated the main factors on the uranyl hydrolysis systematically and discussed the in situ reaction mechanism in detail. Furthermore, physical properties of these uranyl complexes are dependent on structural variations, such as unusual solvent‐impressionable five‐peak photoluminescence for 2, surface photoelectric behavior as potential p‐type semiconductors for 2 and 3, and the remarkable superiorities toward selective removal of dye pollutant for the anionic framework 1 compared with the neutral ones 2 and 3.

Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

Gas-phase organoactinyl complexes possessing discrete An–C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C–R)3]− anion complexes in which a formally AnO22+ actinyl core is coordinated by three carboxylate ligands, with R = CH3 (methyl), CH3CC (1-propynyl), C6H5 (phenyl), C6F5 (pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An–C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastes...

One-dimensional uranium(VI) coordination polymers with pyridinecarboxylate ligands

A method to produce picolinate ligand in situ under hydrothermal conditions has been used to synthesize four uranyl hydroxyl, oxo- and oxohydroxyl picolinato complexes. (UO2)(OH)(Pic) (1) (HPic=picolinic acid) contains 7-fold coordinated uranyl hydroxyl dimers linked through μ2-bridging Pic ligands forming a one-dimensional (1D) polymer. (NH4)[(UO2)3(O)2(OH)(Pic)2] (2) consists of 7-fold coordinated uranyl oxohydroxyl trinuclear units linked through both μ2- and μ3-Pic ligands forming a 1D polymer. (NH4)[(UO2)2(O)2(Pic)] (3) is constructed with 7-fold coordinated uranyl oxo-dinuclear units linked through oxo- and μ3-Pic ligands forming a 1D polymer. (NH4)[(UO2)4(O)2(OH)2(Pic)2(INT)]·H2O (4) (HINT=isonicotinic acid) is made of 7-fold coordinated uranyl oxohydroxyl tetranuclear units linked through both Pic and INT ligands forming a 1D polymer. All four polymers are thermal robust to 350°C. Raman spectroscopy confirmed the presence of uranyl ion and Pic ligand. In addition, red enhanced fluorescence emissions have been observed for both 1 and 4. The synthesis method, with the advantage of controlling uranyl hydrolysis and generating ligand in situ, provides a more reliable way to make new coordination polymers of carboxylate ligands with various uranyl hydrolysis species.

Probing the Influence of N‐Donor Capping Ligands on Supramolecular Assembly in Molecular Uranyl Materials

AbstractThe syntheses and crystal structures of six new compounds containing the UO22+ cation, 3,5‐dichlorobenzoic acid, and a chelating N‐donor [2,2′‐bipyridine (bipy), 1,10‐phenanthroline (phen), 4,7‐dimethyl‐1,10‐phenanthroline (dimethylphen), 2,2′:6′,2″‐terpyridine (terpy), 4′‐chloro‐2,2′:6′,2″‐terpyridine (Cl–terpy), or 2,4,6‐tris(2‐pyridyl)‐s‐triazine (TPTZ)] are reported. Single‐crystal X‐ray diffraction analysis of these materials enabled the exploration of the structural relationship between the benzoic acids and the chelating N‐donor as well as providing a platform to evaluate the effects of ligand choice on uranyl hydrolysis and subsequent oligomerization. At an unadjusted pH (ca. 3), a mix of uranyl monomers and dimers are observed, dimer formation resulting from both bridging carboxylate linkers and hydroxo bridges. Assembly by halogen‐ and hydrogen‐bonding interactions as well as π–π interactions was observed depending on the experimental conditions utilized. Further, spectroscopic characterization (both vibrational and luminescence) of complexes 1, 4, and 5 to explore the effects of the electron‐donating ability of the capping ligand on the corresponding uranyl luminescence and vibrational spectra suggests that there is a relationship between the observed bathochromic shifts and the electron‐donating ability of the capping ligands.

Synthesis and Structural Characterization of Hydrolysis Products within the Uranyl Iminodiacetate and Malate Systems

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U(6+) hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Series of Uranyl-4,4′-biphenyldicarboxylates and an Occurrence of a Cation–Cation Interaction: Hydrothermal Synthesis and in Situ Raman Studies

Three uranium(VI)-bearing materials were synthesized hydrothermally using the organic ligand 4,4'-biphenyldicarboxylic acid: (UO2)(C14O4H8) (1); [(UO2)2(C14O4H8)2(OH)]·(NH4)(H2O) (2); (UO2)2(C14O4H8)(OH)2 (3). Compound 1 was formed after 1 day at 180 °C in an acidic environment (pH(i) = 4.03), and compounds 2 and 3 coformed after 3 days under basic conditions (pH(i) = 7.95). Coformation of all three compounds was observed at higher pH(i) (9.00). Ex situ Raman spectra of single crystals of 1-3 were collected and analyzed for signature peaks. In situ hydrothermal Raman data were also obtained and compared to the ex situ Raman spectra of the title compounds in an effort to acquire formation mechanism details. At pH(i) = 4.00, the formation of 1 was suggested by in situ Raman spectra. At an increased pH(i) (7.90), the in situ data implied the formation of compounds 1 and 3. The most basic conditions (pH(i) = 9.00) yielded a complex mixture of phases consistent with that of increased uranyl hydrolysis.

Hydrolysis of Uranyl(VI) in Acidic and Basic Aqueous Solutions Using a Noncomplexing Organic Base: A Multivariate Spectroscopic and Statistical Study

In the field of actinide aqueous chemistry, this work aims to resolve some controversy about uranyl(VI) hydroxide species present in basic aqueous solutions. We revisit the Raman, IR, and UV-visible spectra with two new approaches. First, Raman, IR and UV data were recorded systematically from aqueous solutions with the noncomplexing electrolyte (C(2)H(5))(4)NNO(3) at 25 °C and 0.1 MPa ([U(total)] = 0.005-0.105 M) in H(2)O and D(2)O over a wide range of -log mH(D)(+) between 2.92 and 14.50. Second, vibrational spectra (IR and Raman) of basic solutions in H(2)O and D(2)O were analyzed using the Bayesian Positive Source Separation method to estimate pure spectra of individual species. In D(2)O solutions, the new spectroscopic data showed the occurrence of the same species as those in H(2)O. As observed for the wavenumber of the symmetric stretching mode, the wavenumber characteristic of the O═U═O antisymmetric stretching mode decreases as the number of OH(D)(-) ligands increases. These kinds of data, completed by (1) analysis of the signal widths, (2) persistence of the apparent exclusion rule between IR and Raman spectra of the uranyl species stretching modes, and (3) interpretation of the absorption UV-visible spectra, allow discussion of the chemistry, structures, and polynuclearity of uranyl(VI) species. In moderate basic solutions, the presence of two trimers is suggested. In highly basic solutions ([OH(-)] ≈ 3 M), the two monomers UO(2)(OH)(4)(2-) and UO(2)(OH)(5)(3-) are confirmed to be in good agreement with earlier EXAFS and NMR results. The occurrence of the UO(2)(OH)(6)(4-) monomer is also suggested from the more basic solutions investigated.

Uranyl adsorption at (0 1 0) edge surfaces of kaolinite: A density functional study

We report density functional investigations of kaolinite edge surfaces and uranyl adsorption thereon. Applying periodic slab models, we studied the (0 1 0) surface of kaolinite as an example of kaolinite edge facets which are expected to be highly reactive and to adsorb preferentially metal ions. Among the four terminations of the (0 1 0) surface, we selected the two most likely ones and determined their structures to be affected by solvation. On these modified surfaces, we explored bidentate inner-sphere adsorption complexes of uranyl, at single metal center sites, Al(O,OH), and sites of mixed type, AlOH–SiO. On one of the terminations hydrolysis of uranyl was found to occur. Comparison of key calculated structure parameters with available experimental data suggests an extension of the prevailing interpretation and implies that a set of uranyl complexes may coexist on edge surfaces.

Hydrothermal Synthesis of Uranyl Squarates and Squarate-Oxalates: Hydrolysis Trends and in Situ Oxalate Formation

We report the synthesis of two uranyl squarates and two mixed-ligand uranyl squarate-oxalates from aqueous solutions under hydrothermal conditions. These products exhibit a range of uranyl building units from squarates with monomers in (UO(2))(2)(C(4)O(4))(5).6NH(4).4H(2)O (1; a = 16.731(17) A, b = 7.280(8) A, c = 15.872(16) A, beta = 113.294(16) degrees , monoclinic, P2(1)/c) and chains in (UO(2))(2)(OH)(2)(H(2)O)(2)(C(4)O(4)) (2; a = 12.909(5) A, b = 8.400(3) A, c = 10.322(4) A, beta = 100.056(7) degrees , monoclinic, C2/c) to two squarate-oxalate polymorphs with dimers in (UO(2))(2)(OH)(C(4)O(4))(C(2)O(4)).NH(4).H(2)O (3; a = 9.0601(7) A, b = 15.7299(12) A, c = 10.5108(8) A, beta = 106.394(1) degrees , monoclinic, P2(1)/n; and 4; a = 8.4469(6) A, b = 7.7589(5) A, c = 10.5257(7) A, beta = 105.696(1) degrees , monoclinic, P2(1)/m). The dominance at low pH of monomeric species and the increasing occurrence of oligomeric species with increasing pH suggests that uranyl hydrolysis, mUO(2)(2+) + nH(2)O right harpoon over left harpoon [(UO(2))(m)(OH)(n)](2m-n) + nH(+), has a significant role in the identity of the inorganic building unit. Additional factors that influence product assembly include in situ hydrolysis of squaric acid to oxalic acid, dynamic metal to ligand concentration, and additional binding modes resulting from the introduction of oxalate anions. These points and the effects of uranyl hydrolysis with changing pH are discussed in the context of the compounds presented herein.

Capturing Hydrolysis Products in the Solid State: Effects of pH on Uranyl Squarates under Ambient Conditions

Two uranyl squarates, (UO(2))(6)(C(4)O(4))(3)(OH)(6)O(2)·9H(2)O·4NH(4) (1; a = 16.6897(7) Å, cubic, I23) and (UO(2))(C(4)O(4))(OH)(2)·2NH(4) (2; a = 8.5151(4), b = 15.6822(8), c = 7.3974, orthorhombic, Pbcm), have been synthesized from ambient aqueous solutions as a function of pH. Oligomerization of the uranyl cation from monomeric pentagonal bipyramids (pH < 5) to [(UO(2))(3)O(OH)(3)] trimers (5 < pH < 8) in 1 and ultimately [(UO(2))(OH)(2)](n) chains (7 < pH < 8) in 2 is observed. This evolution of speciation versus pH is consistent with what has been observed in solution and thus may be represented by the uranyl hydrolysis equilibrium, mUO(2)(2+) + nH(2)O ↔ [(UO(2))(m)(OH)(n)](2m - n) + nH(+). Structural systematics, physical properties, and a discussion of species selectivity by squarate anions are presented.

Stoichiometry of uranyl hydrolysis reaction in acidic aqueous solutions from the evidence of oxygen exchange kinetics

The kinetics of uranyl oxygen exchange with water molecules in aqueous solutions was studied in the pH range 1–4 and uranium concentration range 10 –4–0.1 M. It was confirmed that the exchange is stimulated by hydrolyzed uranyl species. From the evidence of data on the kinetics of uranyl oxygen exchange the reaction stoichiometry of uranyl hydrolysis was determined. The scheme of uranyl hydrolysis involving formation of ( UO 2 ) 2 ( OH ) 2 2 + , ( UO 2 ) 2 ( OH ) 3 + , and other hydrolyzed species was proposed. To cite this article: L.G. Mashirov et al., C. R. Chimie 336 (2004).

Enhancement of hydrolysis through the formation of mixed hetero-metal species

In order to analyze the formation of hetero-metal polynuclear hydrolytic species, in this paper, we reported some results of an investigation (at I = 0.16 mol L −1 in NaNO 3, at t = 25 °C by potentiometry, ISE-H +, glass electrode) on the hydrolysis of several mixtures (in different ratios) of two couples of cations: dioxouranium(VI)/copper(II) and dioxouranium(VI)/diethyltin(IV). The elevated total concentrations of cations 0.005 ≤ Σ C M mol L −1 ≤ 0.05) adopted in these measurements induced us to study again the hydrolysis of uranyl, for which no suitable literature data are available in these particular experimental conditions. All measurements were performed by two different operators, using completely independent instruments and reagents. Many different speciation models were considered in the calculations, including the simultaneous refinement of homo- and hetero-metal species, and a statistical analysis of obtained results was proposed too. Main results can be summarized as follows: U O 2 2 + and Cu 2+ form three hetero-metal polynuclear hydrolytic species [(UO 2)Cu(OH) 3+, (UO 2)Cu ( OH ) 2 2 + and (UO 2) 2Cu ( OH ) 4 2 + , with log β pqr = −2.93 ± 0.01, −7.34 ± 0.03 and −13.78 ± 0.03, respectively], all those common to their simple speciation without the other cation; U O 2 2 + and (C 2H 5) 2Sn 2+ form seven mixed hydrolytic species [(UO 2)(DET)(OH) 3+, (UO 2)(DET) ( OH ) 2 2 + , (UO 2) 2(DET) ( OH ) 4 2 + , (UO 2)(DET) 2 ( OH ) 4 2 + , (UO 2) 2(DET) ( OH ) 5 + , (UO 2)(DET) 2 ( OH ) 5 + and (UO 2) 2(DET) ( OH ) 7 − , with log β pqr = −2.5 ± 0.2, −4.74 ± 0.02, −10.70 ± 0.06, −10.34 ± 0.03, −15.70 ± 0.06, −15.58 ± 0.06 and −27.9 ± 0.1, respectively] that are of the same kind of those formed by uranyl; formation of mixed hydrolytic species causes a significant enhancement of the percentage of hydrolyzed metal cations, modifying the solubility and, therefore, the bioavailability of these cations. We also determined, for dioxouranium(VI)/copper(II) system, the corresponding complex formation enthalpies and entropies by direct calorimetric measurements. We obtained Δ H 112 = 47.9 ± 0.6 and Δ H 214 = 92.9 ± 0.5 kJ mol −1, TΔ S 112 = 6 ± 1 and TΔ S 214 = 14 ± 1 kJ mol −1 (±S.D.), respectively, for the formation of (UO 2)(Cu) ( OH ) 2 2 + and (UO 2) 2(Cu) ( OH ) 4 2 + species (according to reaction 2). We also calculated the single enthalpic and entropic contributes to the extra-stability that these species show with respect to the corresponding homo polynuclear ones.

Speciation study on uranium(VI) hydrolysis at high temperatures and pressures

Hydrolysis behavior of U(VI) has been examined at elevated temperatures (up to 100 °C) and pressures (0.1–40 MPa) using time-resolved laser-induced fluorescence spectroscopy. Emission spectra and lifetimes of 5 × 10 −5 M uranium(VI) in 0.5 M NaClO 4 at pH 3.8–6.0 were measured as a function of temperature and pressure. With increasing temperature, the emission intensities and lifetimes of uranium(VI) hydrolysis species decreased rapidly. On the other hand, the pressure effect on the luminescence properties was quite small as compared with the effect of temperature. The temperature dependence of the lifetimes followed well the Arrhenius law, enabling the activation energies to be estimated for the luminescent species. These experimental results were compared with the speciation calculations carried out at several temperatures by using the DQUANT equation together with the thermodynamic data in the literature. From this comparison, each component of luminescence could be assigned to the hydrolysis species such as UO 2(OH) +, (UO 2) 2(OH) 2 2+, (UO 2) 3(OH) 5 + or uranyl(VI) ion indicating that the Arrhenius plot of luminescence lifetime is an effective method for in-situ speciation of uranyl hydrolysis product at elevated temperatures.

Hydrolysis and chemical speciation of dioxouranium(VI) ion in aqueous media simulating the major ion composition of seawater

The hydrolysis and chemical speciation of the dioxouranium(VI) ion at 25 °C was studied in a number of binary electrolytes (LiCl, NaCl, MgCl 2, CaCl 2, Na 2SO 4) and some mixtures (NaCl–Na 2SO 4, NaNO 3–Na 2SO 4, CaCl 2–MgCl 2) as well as artificial seawater (SSWE) as a function of ionic strength. The results in LiCl, CaCl 2 and MgCl 2 solutions confirmed the formation of (UO 2) 2(OH) 2 2+, (UO 2) 3(OH) 4 2+, (UO 2) 3(OH) 5 + and (UO 2) 3(OH) 7 − species (at I=0 mol l −1: log T β 22=−5.76, log T β 34=−11.82, log T β 35=−15.89 and log T β 37=−29.26). For NaNO 3, NaCl and artificial seawater the hydrolysis constant for the formation of the UO 2(OH) + species was also determined (at I=0 mol l −1: log T β 11=−5.19). The results in Na 2SO 4, Na 2SO 4/NaNO 3 and Na 2SO 4/NaCl required the formation of UO 2(SO 4) 0, UO 2(SO 4) 2 2−, UO 2(OH)SO 4 −, (UO 2) 2(OH) 2SO 4 0, (UO 2) 3(OH) 4SO 4 0 and (UO 2) 3(OH) 5SO 4 −, whose estimated values of each complex formation constant at I=0 mol (kg H 2O) −1 are (log T β pqr ±standard deviation, species in parenthesis): 3.32±0.02 [UO 2SO 4 0], 4.26±0.04 [UO 2(SO 4) 2 2−], −2.30±0.01 [UO 2(OH)(SO 4) −], −2.64±0.04 [(UO 2) 2(OH) 2(SO 4) 0], −8.45±0.04 [(UO 2) 3(OH) 4(SO 4) 0], −13.58±0.04 [(UO 2) 3(OH) 5(SO 4) −]. All the results were examined using the Pitzer model by considering the interactions of the cation hydrolytic species with Cl − and NO 3 − and anion hydrolytic species with Li +, Na +, Ca 2+ and Mg 2+ and, in addition, the “same sign” and “triple” interaction parameters. The resulting Pitzer parameters give an adequate representation of all the hydrolysis constants measured in the binary, ternary and artificial seawater solutions. Alternatively to the interpretation of the dependence of uranyl hydrolysis constants on ionic strength and on ionic medium in terms of variations of activity coefficients of ions, the formation of ion pairs was considered and some complex formation constants among dioxouranium(VI) species and different ions of background salts were calculated. Interactions of uranyl with major components of seawater were taken into account using the “single salt” BA approximation according to which SSWE is considered as a single sea salt (BA) where cation B and anion A, having charge z=± I/C BA =±1.117 , are representative of all major cations (Na +, K +, Ca 2+, Mg 2+) and anions (Cl − and SO 4 2−) of seawater, respectively. Pitzer parameters were also calculated for both the interactions of uranyl with B z+ and A z− ions and for the internal BA interactions. The last ones are: β (0)=0.1081 and β (1)=0.4238 for B 1.117+–A 1.117− interactions, and β (0)=−0.3134 and β (1)=1.5375 for H +–A 1.117− interactions. Literature data were collected carefully, and a critical analysis and accurate comparisons with results presented here were made.

Raman spectroscopic determination of formation constant of uranyl hydrolysis species (UO 2) 2(OH) 22+

Raman spectra of uranyl species in 10 −2 M uranium(VI) perchlorate solutions were measured in pH range 2.88–3.46. The bands of ν 1 symmetrical stretching vibration were observed at 872 and 853 cm −1 which were attributed to UO 2 2+ and (UO 2) 2(OH) 2 2+, respectively. Under the coexistence of (UO 2) 2(OH) 2 2+, concentration of UO 2 2+ was accurately determined using the Raman intensity. The formation constant of (UO 2) 2(OH) 2 2+ was estimated, and this constant was discussed with the values determined by other methods. The validity of this method was discussed in conjunction with the covalent-bond character of UO.

The hydrolysis of uranium(VI) in sulfate media

The hydrolysis of uranium(VI) has been investigated in 0.10 and 1.0 mol dm -3 Na2SO4 using the potentiometric titration technique. Analysis of the data indicates the presence of the following uranyl hydrolysis species: (UO2)2(OH)2 2+, (UO2)3(OH)4 2+, (UO2)4(OH)7 + and (UO2)5(OH)8 2+ in both media, and additionally, UO2OH+ and (UO2)3(OH)5 + in the lower ionic strength only. The data were used to calculate stability constants for ternary uranyl hydroxo-sulfate complexes. These constants were found to be in excellent agreement with those found in an earlier study at a higher Na2SO4 concentration (1.5 mol dm-3).