Macroscopic and spectroscopic investigations on Eu(III) and Cm(III) sorption onto bayerite (ß-Al(OH)3) and corundum (α-Al2O3).

The interaction of trivalent Cm and Eu with the aluminum hydroxide bayerite (ß-Al(OH)3) and the aluminum oxide corundum (α-Al2O3) was investigated by batch sorption experiments and time resolved laser fluorescence spectroscopy (TRLFS). The experimental methods for both polymorphs show similar pH dependent sorption behavior at trace metal ion concentrations (∼10(-7) M), i.e. similar Eu sorption edges and nearly identical Cm speciation between pH=3 and 13. In this pH range the Cm aquo ion as well as the Cm(III) surface species surface⋯Cm(OH)x(H2O)(5-x) (x=0, 1, 2) can be distinguished by TRLFS. The similar sorption data point to a (surface) transformation of the thermodynamically unstable Al2O3 surface into bayerite, in agreement with the similar isoelectric points obtained for both minerals (pH(IEP)=8.6-8.8). The pH dependent surface charge is most likely due to the protonation/deprotonation of singly coordinated Al-OH surface groups, prevailing on the edge planes of the rod-like bayerite crystals and the surface of the colloidal Al2O3 particles. These surface groups are also believed to act as ligands for lanthanide/actinide(III) surface complexation. In contrast to the similar sorption behavior at trace metal ion concentrations, discrepancies are observed at higher Eu levels. While similar sorption edges occur up to 7×10(-7) M Eu for corundum, the pH edge on bayerite is gradually shifted to higher pH values in this Eu concentration range. The latter behavior may be related either to the existence of multiple sorption sites with different sorption affinities, or to the influence of an additional amorphous Al-phase, forming in the course of the bayerite synthesis.

Raman spectroscopy of uranium compounds and the use of multivariate analysis for visualization and classification.

Raman spectroscopy was used on 95 samples comprising mainly of uranium ore concentrates as well as some UF4 and UO2 samples, in order to classify uranium compounds for nuclear forensic purposes, for the first time. This technique was selected as it is non-destructive and rapid. The spectra obtained from 9 different classes of chemical compounds were subjected to multivariate data analysis such as principal component analysis (PCA), partial least square-discriminant analysis (PLS-DA) and Fisher Discriminant Analysis (FDA). These classes were ammonium diuranate (ADU), sodium diuranate (SDU), ammonium uranyl carbonate (AUC), uranyl hydroxide (UH), UO2, UO3, UO4, U3O8 and UF4. Unsupervised PCA of full spectra shows fairly good distinction among the classes with some overlaps observed with ADU and UH. These overlaps are also reflected in the poorer specificities determined by PLS-DA. Higher values of sensitivities and specificities of remaining compounds were obtained. Supervised FDA based on reduced dataset of only 40 variables shows similar results to that of PCA but with closer clustering of ADU, UH, SDU, AUC. As a rapid and non-destructive technique, Raman spectroscopy is useful and complements existing techniques in multi-faceted nuclear forensics.

High-resolution solid-state oxygen-17 NMR of actinide-bearing compounds: an insight into the 5f chemistry.

A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO2, UO2, NpO2, PuO2, and AmO2, using solid-state (17)O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the z component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study.

Reducing uncertainties affecting the assessment of the long-term corrosion behavior of spent nuclear fuel.

Reducing the uncertainties associated with extrapolation to very long term of corrosion data obtainable from laboratory tests on a relatively young spent nuclear fuel is a formidable challenge. In a geologic repository, spent nuclear fuel may come in contact with water tens or hundreds of thousands of years after repository closure. The corrosion behavior will depend on the fuel properties and on the conditions characterizing the near field surrounding the spent fuel at the time of water contact. This paper summarizes the main conclusions drawn from multiyear experimental campaigns performed at JRC-ITU to study corrosion behavior and radionuclide release from spent light water reactor fuel. The radionuclide release from the central region of a fuel pellet is higher than that from the radial periphery, in spite of the higher burnup and the corresponding structural modifications occurring at the pellet rim during irradiation. Studies on the extent and time boundaries of the radiolytic enhancement of the spent fuel corrosion rate indicate that after tens or hundreds of thousands of years have elapsed, very small or no contribution to the enhanced corrosion rate has to be expected from α radiolysis. A beneficial effect inhibiting spent fuel corrosion due to the hydrogen overpressure generated in the near field by iron corrosion is confirmed. The results obtained so far point toward a benign picture describing spent fuel corrosion in a deep geologic repository. More work is ongoing to further reduce uncertainties and to obtain a full description of the expected corrosion behavior of spent fuel.

Complexation and thermodynamics of Cm(III) at high temperatures: the formation of [Cm(SO4)(n)](3-2n) (n = 1, 2, 3) complexes at T = 25 to 200 °C.

The formation of [Cm(SO(4))(n)](3-2n) complexes (n = 1, 2, 3) in an aquatic solution is studied by time resolved laser fluorescence spectroscopy as a function of the ligand concentration, the ionic strength (NaClO(4)) and the temperature (25 to 200 °C). The experiments are performed in a custom-built high temperature cell for spectroscopic measurements at high pressures and temperatures. The single component spectra of the individual species are identified by slope analysis at every studied temperature and their molar fractions are determined by peak deconvolution of the emission spectra. The results show a strong shift of the chemical equilibrium towards the complexed species at increased temperatures. With the determined speciation, the conditional stepwise stability constants are calculated and extrapolated to zero ionic strength, using the specific ion interaction theory (SIT). The log K(0)(n)(T) values increase by several orders of magnitude in the studied temperature range. The fitting of the temperature dependency of the first and second stability constant (log K(0)(1) and log K(0)(2)) requires an extended van't Hoff equation, taking into account a constant heat capacity of the reaction (Δ(r)C(0)(p,m) = const.). Contrarily, the temperature dependency of the log K(0)(3) is very well described by the linear van't Hoff equation, assuming Δ(r)C(0)(p,m) = 0. Thus, the thermodynamic standard state data (Δ(r)H(0)(m), Δ(r)S(0)(m), Δ(r)C(0)(p,m)) of the stepwise complexation of Cm(III) with SO(4)(2-) are determined. Additionally, the ion interaction coefficients of the stepwise complexation reactions (Δε(n)) are determined as a function of the temperature. The fluorescence lifetimes of Cm(III) are recorded at different sulphate concentrations as a function of the temperature. The results give a strong indication that at T > 100 °C the first excited state of Cm(III) ((6)D'(7/2)) is effectively quenched by a temperature dependent enhancement of the energy transfer from the metal ion to OH vibrations of first shell water molecules.

Complexation of europium(III) by bis(dialkyltriazinyl)bipyridines in 1-octanol.

The present work focuses on highly selective ligands for An(III)/Ln(III) separation: bis(triazinyl)bipyridines (BTBPs). By combining time-resolved laser-induced fluorescence spectroscopy, nanoelectrospray ionization mass spectrometry, vibronic sideband spectroscopy, and X-ray diffraction, we obtain a detailed picture of the structure and stoichiometry of the first coordination sphere of Eu(III)-BTBP complexes in an octanolic solution. The main focus is on the 1:2 complexes because extraction studies revealed that those are the species extracted into the organic phase. The investigations on europium(III) complexes of BTBP with different triazin alkylation revealed differences in the formed complexes due to the bulkiness of the ligands. Because of the vibronic sidebands in the fluorescence spectra, we were able to detect whether or not nitrate ligands are coordinated in the first coordination sphere of the Eu-BTBP complexes. In solution, less sterically demanding BTBP offers enough space for additional coordination of anions and/or solvent molecules to form 9-coordinated Eu-BTBP 1:2 complexes, while bulkier ligands tend to form 8-fold-coordinated structures. We also report the first crystal structure of a Ln-BTBP 1:2 complex and that of its 1:1 complex, both of which are 10-coordinated.

Oxidation state and local structure of plutonium reacted with magnetite, mackinawite, and chukanovite.

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

Complexation of Cm(III) with fluoride in aqueous solution in the temperature range from 20 to 90 °C. A joint TRLFS and quantum chemical study.

The formation of hydrated CmF2+ and CmF2+ species in aqueous solutions are studied in the temperature range of 20−90 °C at different fluoride concentrations and at constant ionic strength as well as at constant fluoride concentration and different ionic strengths by means of time-resolved laser fluorescence spectroscopy (TRLFS). The molar fractions of the Cm3+ aqua ion, CmF2+, and CmF2+ species are determined by peak deconvolution of the emission spectra. An increase of the mono- and difluoro complexes is observed with increasing fluoride concentration and/or increasing temperature. Using the specific ion interaction theory (SIT), the thermodynamic stability constants log K10 (CmF2+) and log K20 (CmF2+) as well as the values of Δε1 and Δε2 are determined as a function of temperature. The log K10 values increase from 3.56 ± 0.07 to 3.98 ± 0.06 and the log K20 values increase from 2.20 ± 0.84 to 3.34 ± 0.21 with increasing temperature from 20 to 90 °C. The value of Δε1 determined at 25 °C is in good agreement with literature data and shows a negligible temperature dependency in the studied temperature range. The value of Δε2 also shows only a moderate variation in the studied temperature range. The thermodynamic standard state data (ΔrHm0, ΔrSm0, ΔrGm0) are determined from the temperature dependence of the equilibrium constants at Im = 0 using the integrated Van't Hoff equation. The fluorescence lifetime of the 6D'7/2(Cm3+) state is found to be constant at 63 ± 5 µs with increasing fluoride concentration. A model based on density functional theory (DFT) calculations is introduced to account for the additional quenching occurring through the near second sphere waters in the [Cm(H2O)8F]2+(H2O)18 complex.

Phase transformation in CaCO3 polymorphs: a spectroscopic, microscopic and diffraction study.

This study presents results of the phase transformation from Cm(III) and Eu(III) doped vaterite to calcite. This transformation of one solid solution (An/Ln:vaterite) to another (An/Ln:calcite) was observed by powder X-ray diffraction and scanning electron microscopy. These observations were combined with site-selective time-resolved laser fluorescence spectroscopy (TRLFS), using Eu(3+) and Cm(3+) as atomic probes, which give an internal view of the structure. The transition from vaterite to the thermodynamically stable CaCO(3) polymorph calcite lasts several days. It could be shown that the transformation is taking place in four steps: initial precipitation of low crystalline vaterite, followed by transformation into the crystalline phase, upon suspending the vaterite in CaCO(3) solution the phase transformation to calcite starts. As third step a transition state with again partly hydrated Eu(3+) can be observed before the transformation is completed after 72h. No transition is observed in vaterite kept in vacuum, demonstrating that the transition follows a dissolution/precipitation mechanism. Comparison with Eu(3+)-doped calcite directly synthesized under near-equilibrium conditions shows that identical solid solutions are formed, independent of the reaction path. Moreover the trivalent guest cations are fully transferred to the newly formed phase. This is strong evidence for a thermodynamic driving force for the solid solution formation in these systems.

A TRLFS study on the complexation of Cm(III) and Eu(III) with 4-t-butyl-6,6'-bis-(5,6-diethyl-1,2,4-triazin-3-yl)-2,2'-bipyridine in a water/2-propanol mixture.

The complexation of Cm(III) and Eu(III) with 4-t-butyl-6,6'-bis-(5,6-diethyl-1,2,4-triazin-3-yl)-2,2'-bipyridine (t-Bu-C2-BTBP) in water/2-propanol solution is studied. With increasing ligand concentration, 1 : 2 complexes [M(t-Bu-C2-BTBP)(2)(H(2)O)](3+) form from the solvated metal ions. The stability constants are log K(Cm(III)) = 11.1 and log K(Eu(III)) = 9.0. For both Cm(III) and Eu(III), the complexation reaction is both enthalpy and entropy driven. DeltaH(Cm(III)) is 11.7 kJ mol(-1) more negative than DeltaH(Eu(III)), whereas the entropy difference is negligible. This is in good agreement with t-Bu-C2-BTBP's selectivity in liquid-liquid extraction.

Incorporation versus adsorption: substitution of Ca2+ by Eu3+ and Cm3+ in aragonite and gypsum.

The aim of the present work is to understand the interaction behaviour of trivalent f-elements with Ca2+-bearing mineral phases on a molecular level. This is achieved by use of time-resolved laser fluorescence spectroscopy (TRLFS) with Eu3+ and Cm3+ as atomic sensors on a trace concentration level. These ions are of special interest as models for trivalent actinides (e.g., Cm(III), Am(III) or Pu(III)) which strongly contribute to radiotoxicity in high level nuclear waste. Results from TRLFS with these trivalent ions show a significantly different mode of interaction in the two cases: while in aragonite only structural incorporation is observed, on gypsum nothing but inner-sphere surface complexes can be found. This shows how the anions forming the coordination sphere of the foreign ion control the formation of solid solutions as opposed to adsorption complexes.

Theoretical investigation of the water/corundum (0001) interface.

For the reliable long-term modeling of the actinide migration in geological formations, the adsorption/desorption properties and the reactivity of mineral surfaces must be understood at the molecular level. The adsorption of radioisotopes at mineral surfaces of the aquifer is an important process that leads to the retention of contaminants such as radionuclides. Their transport by the ground water is either retarded or even completely inhibited by the presence of such a surface. Accordingly, this subject is of main importance for the safety assessment of nuclear waste repositories. As part of a joint theoretical/experimental effort, the interaction of water with the corundum (0001) surface is studied using several theoretical methods (Moller-Plesset perturbation theory, coupled cluster singles doubles with triplet corrections, as well as density functional theory). We focus in this study on the determination of the bond lengths and tilt angles of the surface OH species and their respective vibrational frequencies. The theoretical results are confirmed by subsequent simulation of the interface selective nonlinear sum frequency spectra. The excellent agreement of the simulated with the experimental spectra allows an assignment of the observed peaks in the sum frequency spectra of the water/corundum (0001) interface on the basis of our theoretical data. In this theoretical study we are able to give a unique interpretation of the observed sum frequency spectra of the water/corundum (0001) interface.

Hydration of mineral surfaces probed at the molecular level.

By employing the nonlinear optical, interface selective experiment of sum frequency spectroscopy together with independent ab initio and density functional theory calculations, we determine the functional species of a corundum (001) surface: doubly coordinated OH groups which differ in their bond tilt angles. The interaction of the functional species with the adjacent water molecules is also observed. In a large pH range around the point of zero charge, the interaction is not controlled electrostatically but by hydrogen bonding. The functional species' tilt angles are crucial parameters, determining whether the species act as hydrogen bond donors or acceptors.

Sorption of Am(III) onto 6-line-ferrihydrite and its alteration products: investigations by EXAFS.

For the long-term performance assessment of nuclear waste repositories, knowledge about the interactions of actinide ions with mineral surfaces such as iron oxides is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at these surfaces and on their incorporation into the mineral structure. In this study the interaction of Am(III) with 6-line-ferrihydrite (6LFh) was investigated by EXAFS spectroscopy. At low pH values (pH 5.5), as well at higher pH values (pH 8.0), Am(III) sorbs as a bidentate corner-sharing species onto the surface. Investigations of the interaction of Am(III) with Fh coated silica colloids prove the sorption onto the iron coating and not onto the silica substrate. Hence, the presence of Fh, even as sediment coating, is the dominant sorption surface. Upon heating, Fh is transformed into goethite and hematite as shown by TEM and IR measurements. The results of the fit to the EXAFS data indicate the release of sorbed Am(III) at pH 5.5 during the transformation and likely a partial incorporation of Am into the Fh transformation products at pH 8.0.

Interaction of trace elements in acid mine drainage solution with humic acid.

The release of metal ions from a coal mining tailing area, Lamphun, Northern Thailand, is studied by leaching tests. Considerable amounts of Mn, Fe, Al, Ni and Co are dissolved in both simulated rain water (pH 4) and 10 mg L(-1) humic acid (HA) solution (Aldrich humic acid, pH 7). Due to the presence of oxidizing pyrite and sulfide minerals, the pH in both leachates decreases down to approximately 3 combined with high sulfate concentrations typical to acid mine drainage (AMD) water composition. Interaction of the acidic leachates upon mixing with ground- and surface water containing natural organic matter is simulated by subsequent dilution (1:100; 1:200; 1:300; 1:500) with a 10 mg L(-1) HA solution (ionic strength: 10(-3) mol L(-1)). Combining asymmetric flow field-flow fractionation (AsFlFFF) with UV/Vis and ICP-MS detection allows for the investigation of metal ion interaction with HA colloid and colloid size evolution. Formation of colloid aggregates is observed by filtration and AsFlFFF depending on the degree of the dilution. While the average HA size is initially found to be 2 nm, metal-HA complexes are always found to be larger. Such observation is attributed to a metal induced HA agglomeration, which is found even at low coverage of HA functional groups with metal ions. Increasing the metal ion to HA ratio, the HA bound metal ions and the HA entities are growing in size from <3 to >450 nm. At high metal ion to HA ratios, precipitation of FeOOH phases and HA agglomeration due to colloid charge neutralization by complete saturation of HA complexing sites are responsible for the fact that most of Fe and Al precipitate and are found in a size fraction >450 nm. In the more diluted solutions, HA is more relevant as a carrier for metal ion mobilization.

Large ground-state and excited-state crystal field splitting of 8-fold-coordinate Cm3+ in [Y(H2O)8]Cl3.15-crown-5.

The optical spectra of Cm(3+) incorporated into the crystalline host structure of [Y(H(2)O)(8)]Cl(3).15-crown-5 (1) is investigated by using laser spectroscopic methods at temperatures between 20 and 293 K. The coordination geometry of the [Y(H(2)O)(8)](3+) entity in 1 is a distorted bicapped trigonal prism with approximately C(2) point symmetry, as confirmed by single-crystal X-ray diffraction at 200 K. The crystal-field splitting of the (8)S'(7/2) ground state and the (6)D'(7/2) and (6)P'(5/2) excited states of the hydrated Cm(3+) ion are measured by high-resolution fluorescence emission and excitation spectroscopy at various temperatures. The transitions between the ground state and the respective lowest crystal-field levels of the excited states exhibit narrow fluorescence lines, resolving the four crystal-field levels of the ground state as sharp, well-resolved lines at about 0, 10, 19, and 35 cm(-1). The total splittings of the (6)D'(7/2) and (6)P'(5/2) states are 670 and 170 cm(-1), respectively. Thermal population of the ground-state crystal-field levels is observed and quantified in the excitation spectra in the temperature range of 20-70 K. All spectroscopic results are consistent with the presence of one unique [Cm(H(2)O)(8)](3+) site. The ground-state splitting of Cm(3+) in 1, 35 cm(-1), is comparable to that of Cm(3+) in solid ThO(2), 36 cm(-1), which shows the strongest crystal field for Cm(3+) reported so far. For this reason the present results are different than the findings for Ln(3+) aqua ions, which show rather weak crystal field strengths.