bluuebunny/crossref_metadata_2025_split huggingface dataset:
UO2dissolution in the presence of hydrogen peroxide at pH>11
Authors: Meca Sandra , Marti Vincenc , de Pablo Joan , Giménez Javier , Casas Ignasi 
Published: November 2008

AbstractThe dissolution of non irradiated UO2was studied in the presence of hydrogen peroxide (10−4 mol dm−3) at alkaline pH (11, 11.5, 12, and 13). Both hydrogen peroxide and uranium concentration in solution were determined as a function of time. The H2O2consumption was modelled considering a pseudo first order reaction, the rate constants obtained were 0.126±0.004 h−1, 0.126±0.003 h−1, 0.078±0.002 h−1, and 0.056±0.002 h−1at pH 11, 11.5, 12, and 13, respectively. The uranium concentrations measured at the end of the experiments were close to the solubility of sodium uranate (Na2U2O7). X-ray photoelectron spectroscopy showed that the surface of the solid was more oxidized at lower pH, indicating that at this pH the limiting step of the oxidative dissolution process is the dissolution of the U(VI) formed on the surface. At more alkaline pH values, the rate limiting step would be the oxidation of the UO2surface.
Solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides in CaCl2solutions and the formation of ternary Ca-M(IV)-OH complexes
Authors: Altmaier Marcus , Neck Volker , Fanghänel Thomas 
Published: November 2008

AbstractThe solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides is investigated at 22±2 °C in alkaline 0.1−4.5 M CaCl2solutions. Further studies are performed with Zr(IV) over the entire pH range in NaCl and CaCl2media, and with Zr(IV) and Th(IV) in alkaline Ca(ClO4)2solutions. The comparison of Zr(IV) data in different ionic media (NaCl, NaClO4, CaCl2and Ca(ClO4)2) of similar ionic strength shows that the solubility in the acidic and neutral pH range is not affected by strong interactions between the aqueous M(IV) species and the medium ions. However, in alkaline CaCl2and Ca(ClO4)2solutions the formation of ternary Ca-M(IV)-OH complexes causes unexpectedly high solubilities of Zr(IV) at pHc=10−12 and [Ca2+]>0.05 M and of Th(IV) at pHc=11−12 and [Ca2+]>0.5 M. The dependence of the Zr(IV) and Th(IV) solubilities on the H+and CaCl2concentrations shows that the complexes Zr(OH)62−and Th(OH)84−with an unusual large number of OH−ligands are stabilized by the formation of associates or ion pairs with Ca2+ions. The SIT is used to derive equilibrium constants at zero ionic strength for the complexes Zr(OH)62−(in calcium-free solutions), Ca2[Zr(OH)6]2+, Ca3[Zr(OH)6]4+and Ca4[Th(OH)8]4+. In analogous studies with Pu(IV) hydrous oxide, the solubility increasing effect of ternary complex formation with Ca2+ions is only observed at CaCl2concentrations above 2 M.
Thermodynamic interpretation of neptunium coprecipitation in uranophane for application to the Yucca Mountain repository
Authors: Murphy William M. , Grambow Bernd 
Published: November 2008

AbstractInterpretation and modeling of recent experimental data [1] yield thermodynamic constants for the distribution of trace Np(V) between aqueous solutions and uranophane. These data indicate that neptunyl is relatively excluded from the uranyl mineral structure, but the interpretation depends on uncertain aqueous speciation and thermodynamic properties as a function of temperature. Despite Np exclusion, the low calculated solubility of uranophane at 25 °C under conditions relevant to the proposed nuclear waste repository at Yucca Mountain, Nevada, leads to Np concentrations at equilibrium with a Np-bearing uranophane solid solution that are low compared to concentrations invoked as solubility limits in Yucca Mountain performance assessments.
Synthesis, crystallographic characterization, and conformational prediction of a structurally unique molecular mixed-ligand U(VI) solid, Na6[UO2(O2)2(OH)2](OH)2·14H2O
Authors: Zehnder Ralph A. , Batista Enrique R. , Scott Brian L. , Peper Shane M. , Goff George S. , Runde Wolfgang H. 
Published: November 2008

Abstract
The first mononuclear molecular mixed-ligand U(VI) solid containing hydroxide and peroxide ligands, Na6[UO2(O2)2(OH)2](OH)2·14H2O (I), was synthesized and structurally characterized using single crystal X-ray diffraction. The crystal structure of I consisted of [UO2(O2)2(OH)2]4− molecular units, with a uranyl(VI) moiety perpendicular to 6 equatorial O atoms belonging to two side-on trans peroxo groups and two terminal trans hydroxo groups. Density functional theory (DFT) calculations determined that the trans -conformer of the [UO2(O2)2(OH)2]4− molecular unit found in I was 24 kcal/mol lower in energy than the previously proposed cis -conformer. Crystal data: monoclinic, space group P21/n with a=13.357(4) Å, b=5.8521(15) Å, c=15.948(6) Å, β=112.292(4)°, and Z=2.
Thermodynamic study on the U(VI) complexation with dicarboxylates by calorimetry
Authors: Kirishima Akira , Onishi Yuko , Sato Nobuaki , Tochiyama Osamu 
Published: November 2008

Abstract
The thermodynamic quantities (ΔG, ΔH and ΔS) of U(VI) complexation with oxalate, malonate, succinate, glutarate, adipate, phthalate, tartronate and malate, and of Eu(III) complexation with succinate, glutarate and adipate were determined by potentiometric and calorimetric titration techniques. The obtained thermodynamic quantities indicated that these complexation reactions are mainly driven by the entropy changes while the enthalpy changes are nearly zero or preventing the complexation. The thermodynamic quantities of 1:1 U(VI) complexes with 5 dicarboxylates having different carbon chain lengths (−OOC−(CH2)
n
−COO−) showed an interesting tendency, that is, −ΔH decreased and TΔS increased systematically with increasing the length of the carbon chain from n = 2 to 6. The comparison of TΔS values of 1:1 carboxylate formation with U(VI) and Eu(III) showed that TΔS of U(VI) were always lager than those of Eu(III). The entropies of 1:1 complex formation of U(VI) with hydroxycarboxylates were always smaller than those with carboxylates of the same carbon chain length, while the enthalpies of uranyl hydroxycarboxylates were always larger. This tendency was also observed in the protonation and the complex formation of Eu(III).
A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals
Authors: Wang Zheming , Zachara John M. , Liu Chongxuan , Gassman P. L. , Felmy Andrew R. , Clark Sue B. 
Published: November 2008

Abstract
In this work we applied time-resolved laser-induced fluorescence spectroscopy (TRLIF) at both room temperature (RT) and near liquid-helium temperature (6 K) to characterize a series of natural and synthetic minerals of uranium carbonate, phosphate and oxyhydroxides including rutherfordine, zellerite, liebigite, phosphuranylite, meta-autunite, meta-torbernite, uranyl phosphate, sodium-uranyl-phosphate, becquerelite, schoepite, meta-schoepite, dehydrated schoepite and compreignacite, and have compared the spectral characteristics among these minerals as well as our previously published data on uranyl silicates. For the carbonate minerals, the fluorescence spectra of rutherfordine showed significant difference from those of zellerite and liebigite. The fluorescence spectra of the phosphate minerals closely resemble each other despite the differences in their composition and structure. For all uranium oxyhydroxides, the fluorescence spectra are largely red-shifted as compared to those of the uranium carbonates and phosphates and their vibronic bands are broad and less resolved at RT. The enhanced spectra resolution at 6 K allows more accurate determination of the fluorescence band origin and offers a complemental method to measure the O=U=O symmetrical stretch frequency, ν
1, from the spacings of the vibronic bands of the fluorescence spectra. The average ν
1 values appear to be inversely correlated with the average pK
a values of the anions.
A spectroscopic study of the effect of ligand complexation on the reduction of uranium(VI) by anthraquinone-2,6-disulfonate (AH2DS)
Authors: Wang Zheming , Wagnon K. B. , Ainsworth C. C. , Liu Chongxuan , Rosso Kevin M. , Frederickson K. J. 
Published: November 2008

Abstract
In this paper, the reduction rate of uranyl complexes with hydroxide, carbonate, EDTA, and desferriferrioxamine B (DFB) by anthraquinone-2,6-disulfonate (AH2DS) is studied by stopped-flow kinetic technique under anoxic atmosphere. The apparent reaction rates varied with ligand type, solution pH, and U(VI) concentration. For each ligand, a single largest pseudo −1st order reaction rate constant, k
obs, within the studied pH range was observed, suggesting the influence of pH-dependent speciation on the U(VI) reduction rate. The maximum reaction rate found in each case followed the order of OH−>CO3
2−>EDTA>DFB, in reverse order of the trend of the thermodynamic stability of the uranyl complexes and ionic sizes of the ligands. The pH-dependent rates were modeled using a second-order rate expression that was assumed to be dependent on a single U(VI) complex and an AH2DS species. By quantitatively comparing the calculated and measured apparent rate constants as a function of pH, species AHDS3− was suggested as the primary reductant in all cases examined. Species UO2CO3(aq), UO2HEDTA−, and (UO2)2(OH)2
2+ were suggested as the principal electron acceptors among the U(VI) species mixture in each of the carbonate, EDTA, and hydroxyl systems, respectively.
The relationship of monodentate and bidentate coordinated uranium(VI) sulfate in aqueous solution
Authors: Hennig Christoph , Ikeda A. , Schmeide K. , Brendler Vinzenz , Moll Henry , Tsushima S. , Scheinost A. C. , Skanthakumar Suntharalingam , Wilson R. , Soderholm L. , Servaes K. , et al.  
Published: November 2008

Abstract
The coordination of U(VI) sulfate complexes has been investigated by uranium L
III-edge EXAFS and HEXS measurements with the aim to distinguish monodentate and bidentate coordinated sulfate in aqueous solution. UV-vis absorption spectroscopy has been used to differentiate the species and to determine the species distribution as a function of the [SO4
2−]/[UO2
2+] ratio. A monodentate coordination prevails in solutions with [SO4
2−]/[UO2
2+] ratio of 1, where UO2SO4 is the dominant species. Besides the dominating monodentate sulfate a small amount of bidentate sulfate could be observed, indicating that two isomers may exist for UO2SO4. With increasing [SO4
2−]/[UO2
2+] ratio the UO2(SO4)2
2− species becomes the main species. The uranium atom of this species is coordinated by two bidentate sulfate groups.
Hydrolysis constants of tetravalent neptunium by using solvent extraction method
Authors: Fujiwara Kenso , Kohara Y. 
Published: November 2008

Abstract
The hydrolysis constants of tetravalent neptunium (Np(IV)) were determined by solvent extraction method using thenoyltrifluoroacetone(TTA). In order to avoid colloid formation, a stock solution of carrier-free 239Np(V) was from 243Am milked. The valence of Np in the solution was then reduced to Np(IV) by using zinc amalgam. The hydrolysis constants (β

m
) of the reactions, Np4++mOH−=Np(OH)
m

(4−m)+ was evaluated by using distribution ratios at ionic strengths (I)=0.1, 0.5 and 1.0. All experiments were performed in oxygen- free 0.5% H2−N2 atmosphere (below 1.0 ppm of O2) in a glovebox at room temperature (23±2 °C) to avoid oxidation of Np(IV). The β

m
values were extrapolated to the standard state (I=0) by using the specific ion interaction theory (SIT), and the formation constants at I=0 were determined to be logβ
1°=13.91±0.23, logβ
2°=27.13±0.15, logβ
3°=37.70±0.30 and logβ
4°=46.16±0.30. The ion interaction coefficients were also evaluated to be ε(NpOH3+, ClO4
−)=0.49±0.15, ε(Np(OH)2
2+, ClO4
−)=0.35±0.11, and ε(Np(OH)3
+, ClO4
−)=0.29±0.15.
Oxidative dissolution of Tc(IV)O2·nH2O colloids by sonolysis
Authors: Sekine Tsutomu , Zakir Muhammad 
Published: November 2008

Abstract
Redox reactions of Tc in aqueous solutions by sonolysis were investigated. Aqueous pertechnetate (Tc(VII)O4
−) solutions and Tc(IV)O2· n H2O colloids solutions were sonicated (200 kHz, 200 W) in a glass cell with flat bottom under Ar or He atmosphere at 20 °C. No reduction of TcO4
− was observed in this study. However, we found that the TcO2· n H2O colloids dispersed in an aqueous solution were completely dissolved by ultrasonic irradiation within 30 min under Ar atmosphere, and TcO4
− was eventually produced. The production of TcO4
− was considerably suppressed in the presence of t-butyl alcohol (an effective scavenger of OH radicals), indicating that Tc(IV) was oxidized by OH radicals (produced by dissociation of water molecules) in hot cavitation bubbles created by ultrasound. The formation rate of TcO4
− under He atmosphere was smaller than that under Ar atmosphere. This can be attributed to a difference of the effective maximum temperature in the collapsing bubbles. Because thermal conductivity of He is much higher than Ar, thermal transport effectively occurs from the bubbles to the surrounding liquid. Thus, temperature of cavitation bubbles filled with He should be lower than those filled with Ar, resulting in the smaller OH formation rate.
Sorption of selenium on Mg-Al and Mg-Al-Eu layered double hydroxides
Authors: Curtius Hilde , Paparigas Z. , Kaiser G. 
Published: November 2008

Abstract
Salt domes represent deep geological formations which are under consideration as final repositories for irradiated research reactor fuel elements. For long-term safety aspects the mobilisation of the radionuclides due to a water ingress is intensively investigated. At the Institute of Energy Research (IEF-6), leaching experiments were performed in a hot cell facility with UAl
x
-Al and U3Si2-Al dispersed research reactor fuel elements in repository-relevant MgCl2-rich salt brines under anaerobic conditions. The fuel plates corroded completely within one year and a Mg-Al-layered double hydroxide (LDH) with chloride as interlayer anion was identified as one crystalline phase component of the corrosion products (secondary phases). This Mg-Al-LDH was synthesized, characterized, and the ability to retard europium by an incorporation process was investigated. Europium, as a representative for lanthanides, was identified to be one of the radionuclides which were found in the corrosion products. We could show that europium was incorporated in the lattice structure. LDHs have high anion exchange capacities that enhance their potential to remove anionic contaminants from aqueous systems.
In this work the sorption behaviour of selenium in the chemical form as selenite (SeO3
2−) on Mg-Al-LDH and on Mg-Al-Eu-LDH was investigated. Especially the influence of the larger europium-III ion was of interest. It represents in the Mg-Al-Eu-LDH about 10% of the molar aluminium amount. The sorption has been experimentally studied in a wide range of pH, ionic strength, radionuclide and sorbent concentration. Both LDHs with chloride as interlayer anion were synthesized by a coprecipitation method under controlled conditions, and their main physico-chemical properties were analyzed prior to the sorption experiments.
The sorption kinetics of selenite on the LDHs in water and in MgCl2-rich brine were rapid using a LDH concentration of 10 g/L. Equilibrium, indicated by stable pH values, was obtained within 5 h for an initial selenite concentration of 4.28×10−13 mol/L. Selenite in water sorbed quantitatively on both LDHs. In MgCl2-rich brine a different behaviour to water was observed. The sorption was not quantitatively, due to the presence of high amounts of chloride anions. Chloride, as competing anion, affected the sorption behaviour of selenite. Using a LDH concentration of 0.1 g/L, the sorption of selenite was negligible. The effect of pH on selenite sorption was investigated, too. The sorption was relatively constant when the initial pH values were adjusted between 4 and 8, because both LDHs exhibit a high pH buffering capacity. For a selenite concentration range between 5.43×10−14 and 3.24×10−12 mol/L sorption isotherms were obtained and the data could be fitted to Freundlich and Dubinin–Radushkevich equations. The sorption capacities of the LDHs and the energies of sorption were determined. The sorption of selenite occurred via an ion exchange mechanism and the sorption behaviour was similar for both LDHs.
Studies of the ternary systems humic substances – kaolinite – Pu(III) and Pu(IV)
Authors: Buda R. , Banik N. L. , Kratz Jens Volker , Trautmann N. 
Published: November 2008

AbstractThe behaviour of plutonium with respect to its migration in the aquifer has been studied under conditions close to nature. Most relevant under these conditions are Pu(III) and Pu(IV) in contact with humic substances (HS) and minerals. As a model for the host rock, kaolinite (KGa-1b) was chosen. The complexation of Pu(III) and Pu(IV) with Aldrich humic acid (AHA) in aqueous solution at ionic strength 0.1 M was investigated by the ultrafiltration method. The sorption of Pu(III) and Am(III) onto kaolinite (K) as a function of pH and metal-ion concentration was studied under aerobic and anaerobic conditions. The pH edge was found at pH∼5.5 independent of the metal-ion concentration and working atmosphere. The influence of HS on the sorption of Pu(III) and Pu(IV) onto kaolinite was investigated in the ternary systems Pu(III)-K-HS and Pu(IV)-K-HS and for comparison, in the system Th(IV)-K-HS. The dependence on pH, contact time, concentration of HS (for Pu(IV)-K-HS) was studied as well as the sequence in which the components were added. Generally, it was found that HS tend to enhance the sorption onto kaolinite below pH 6 and to decrease sorption at higher pH depending in detail on the sequence in which the components were added. An identification of the species sorbed on the surface of the kaolinite by X-ray absorption spectroscopy, as well as chemically, was attempted and preliminary results are discussed.
Polarized EXAFS characterization of the sorption mechanism of yttrium on hectorite
Authors: Schlegel Michel 
Published: November 2008

AbstractYttrium uptake in 2 g/L hectorite suspensions at high ionic strength (0.5 M NaCl) as a function of pH (6.5 and 7) and total Y concentration (10 and 50 μM) was investigated by solution chemistry and Polarized EXAFS (P-EXAFS) spectroscopy. At pH 6.5, Y uptake readily occurred, but equilibrium was not attained within 96 h of reaction. A greater fraction of Y was sorbed at pH 7, and near-equilibrium conditions were attained after 48 h of reaction.Polarized EXAFS data indicated that the environments of Y sorbed at the two pH values were structurally similar. Yttrium was surrounded by a split first coordination sphere of four O atoms at 2.27−2.28 Å and four O at 2.43 Å. The averaged(Y-O) distance agrees with eight-fold coordination of Y. Neighboring cationic shells of Mg atoms at 3.16−3.18 Å oriented at the magic angle and out-of-plane Si atoms at 3.48−3.51 Å were also detected. These distances and orientations are consistent with Y forming inner-sphere surface complexes at layer edges of hectorite by sharing edges with Mg octahedra and corner with Si tetrahedra. The observed angular dependence further suggests that Y is tilted away from the Mg-containing median planes of hectorite platelets.
Hydration and hydrolysys of Sm3+ and Eu3+ in a clay interlayer: a neutron diffraction study with isotopic substitution
Authors: Sobolev Oleg , Charlet L. , Cuello G. , Gehin A. , Brendle J. 
Published: November 2008

Abstract
The coordination of Sm and Eu in the montmorillonite interlayer was studied by neutron diffraction with isotopic substitution. It was found that the number of hydrogen atoms nearest to Sm and Eu were less (N
H=5.5±2.0) than oxygen nearest atoms (N
O=7.5±1.0). This means that the Sm and Eu are not surrounded only by hydration water molecules, but they are in contact with the oxygen siloxane atoms of the clay basal plane surface. It was found that the number of oxygen atoms of the clay surface giving a contribution to the total coordination number N
O should be N
O
clay≤3. Therefore, the deficit of hydrogen atoms in the first coordination shell of Sm and Eu should be explained supposing that the lanthanide cations are partially hydrolyzed.
Sorption of neptunium onto goethite in the presence of humic acids with different hydroquinone group content
Authors: Kalmykov Stepan N. , Schäfer Thorsten , Claret F. , Perminova I. V. , Petrova (Khasanova) A. B. , Shcherbina N. S. , Teterin Y. Al 
Published: November 2008

AbstractNeptunyl sorption in the ternary system of goethite–humic acid (HA)–Np was studied in a series of batch experiments with natural leonardite HA and a hydroquinone enriched derivative. The reduction of Np(V) at low pH has been observed by XPS upon interaction with hydroquinone-enriched leonardite HA–goethite suspension. Under these low pH conditions the formation of humate coatings at the goethite mineral surface has been identified using scanning transmission X-ray microscopy (STXM) at the carbonK-edge.
The influence of carbonate complexation on lanthanide/actinide sorption on montmorillonite
Authors: Marques Fernandes Maria , Baeyens B. , Bradbury Michael H. 
Published: November 2008

AbstractThe safety case for radioactive waste repositories is, to a large extend based on the physical and chemical retention of radionuclides on clay minerals which are important constituents in both the man-made engineered barriers and in argillaceous host rock formations. The presence of carbonate, one of the most important inorganic ligands for lanthanide and actinide ions in groundwaters and clay porewaters, can have a significant influence on their sorption behaviour on clay minerals. The sorption of Eu(III) on Na-montmorillonite in the absence and the presence of carbonate was investigated in batch sorption experiments. In the presence of carbonate two sets of experimental conditions were investigated. Eu(III) sorption measurements were performed as a function of pH in equilibrium with atmosphericpCO2(=10−3.5 bar) and at a variablepCO2(=10−1.4to 10−3.4 bar). The sorption data were modelled using the 2 site protolysis non-electrostatic surface complexation and cation exchange model. Both experimental data sets in the presence of carbon could be quantitatively modelled by including in the sorption model two additional surface complexation reactions on the strong sites forming ≡SSOEuCO30and ≡SSOEuOHCO3−surface complexes.
Sorption of radionuclides onto natural clay rocks
Authors: Hartmann E. , Geckeis H. , Rabung Thomas , Lützenkirchen Johannes , Fanghänel Thomas 
Published: November 2008

AbstractReactions of U(VI) and Eu/Cm(III) with natural clay rock (Opalinus Clay, Switzerland, and Callovo-Oxfordian Clay, France) are investigated by batch and spectroscopic experiments as well as geochemical calculations. The aim of the studies is to identify those minerals in the heterogeneous multiphase systems with sometimes high calcite content which control lanthanide and actinide sorption. The outcome of batch sorption studies with natural clay rocks (2 g/L natural clay rock, 0.1 mol/L NaClO4, pH 3−11) and thermodynamic calculations is compared with experimental results using a synthetic mixture consisting of purified Na-montmorillonite and calcite (ratio 80:20[wt.]%). Our studies show that U(VI) sorption decreases at intermediate pH (6.5−9) where dissolved U(VI) carbonate species predominate. In the high pH region (>9) U(VI) sorption again increases by formation of ternary hydroxo surface complexes on the clay mineral fraction, whereby calculated U(VI) sorption overestimates experimental data for the natural rock. Complete sorption to natural clay rock and the synthetic clay/calcite mixture is observed for Eu(III) at pH>7 for the conditions studied. Model calculations again point to the predominance of clay-sorbed species even though calculated speciation underestimates the experimentally observed sorption at pH 6−9 slightly. Preliminary time-resolved laser-fluorescence spectroscopy (TRLFS) studies using Cm(III) as fluorescent probe reveal the appearance of several Cm(III)-clay surface species in the pH range 5−11, but give no indications for the presence of calcite-bound Cm(III). We conclude that the clay minerals in the multiphasic clay rock act as main sorbents for tri- and hexavalent actinides.