Thorsten Stumpf

Sorption of Am(III) and Eu(III) onto γ-alumina: experiment and modelling

The present paper describes the surface complexation behaviour of trivalent metal ions, Am(III) and Eu(III), on well characterised γ-alumina. Experiments are conducted at different pH (4-8) and ionic strength (0.001 - 0.1 M NaClO4) either in the presence or absence of CO2. By varying the metal ion concentration from 10-9 to 10-4 mol/L, the sorption isotherm is established under each given experimental condition. Different surface complexation models are applied to the experimental results to interpret and appraise the surface sorption processes under different experimental conditions. A comparison of the present results is made with the Eu(III) sorption onto the hematite mineral surface investigated previously. It has been shown that sorption properties of hematite and γ-alumina seem to be quite similar. For both systems, a good agreement is found between experimental data and modelling, once using the two site surface complexation model and the same complexation constants for the respective monodentate surface complex. The Eu(III) sorption reaction is additionally studied in-situ by time resolved laser fluorescence spectroscopy (TRLFS). Formation of inner-sphere complexes can be deduced from the emission spectra. The continuous increase of the fluorescence life time with increasing pH starting from pH = 5.0 indicates that surface complexation is accompanied by a decreasing number of hydration water in the first coordination sphere.

Inner-sphere, outer-sphere and ternary surface complexes: a TRLFS study of the sorption process of Eu(III) onto smectite and kaolinite

Summary The surface sorption process of Eu(III) onto smectite and kaolinite was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) in the trace concentration range. The experiments were performed in 0.025 M and 0.45 M NaClO4. The sorption process of Eu(III) onto smectite was obtained by TRLFS under atmospheric conditions and in absence of CO2. The pH was varied between 3.5 and 9 at a fixed metal ion concentration of 3.3 × 10−6 mol/L Eu(III). At low pH (< 4) the metal ion keeps its complete hydration sphere indicating outer-sphere complexation. With increasing pH the formation of an inner-sphere Eu(III) surface complex was observed. The differences in the spectra and the fluorescence emission lifetimes of the surface sorbed Eu(III) in presence and absence of carbonate indicate the formation of ternary clay/Eu(III)/carbonate complexes. The different europium/clay surface complexes were characterized by their fluorescence emission spectra (5D0 → 7F1/5D0 → 7F2 intensity ratio) and their fluorescence emission lifetime.

Complexation of trivalent actinide and lanthanide ions by glycolic acid: a TRLFS study

Complexation in the Cm(III) and Eu(III) glycolate systems has been studied by time-resolved laser fluorescence spectroscopy (TRLFS). Measurements have been performed at trace Cm(III) and Eu(III) concentrations (about 10−7 and 10−6 mol L−1, respectively), at different concentrations of glycolic acid and at different pH using NaClO4 as background electrolyte. Measurements at higher Eu(III) concentrations (10−3 mol L−1) have also been performed in order to study the influence of metal ion concentration on the complexation reaction. By varying the glycolic acid concentration from 0.1 to 0.5 mol L−1 at low pH ([H+] = 10−3 mol L−1) the stepwise formation of glycolate complexes [Cm(HOCH2CO2−)n(H2O)9 − 2n]3 − n with n = 1–4 were confirmed spectroscopically. By varying the pH between 4.5 and 12.0 in 1 M glycolate three Cm(III) species were identified from the luminescence emission spectra (i) a hydrated tetraglycolate complex [Cm(HOCH2CO2−)4(H2O)]− (Cm/complex 1) with a peak maximum at 602.3 nm and a luminescence emission lifetime of 206 ± 3 µs, (ii) a mixed hydroxide–glycolate complex [Cm(HOCH2CO2−)4(OH)]2− (Cm/complex 2) with a peak maximum at 605.6 nm and the same lifetime as Cm/complex 1 and (iii) a chelate complex [Cm(HOCH2CO2−)3(−OCH2CO2−)(OH)]3− or [Cm(HOCH2CO2−)2(−OCH2CO2−)2(H2O)]3− (Cm/complex 3) (peak maximum 611.3 nm) which is generated after deprotonation of one or two of the coordinated α-OH groups of the glycolate with a luminescence emission lifetime of 295 ± 15 µs. In the europium system there is evidence only for the corresponding Eu/complex 1 and 3. The corresponding europium mixed hydroxide–glycolate complex is not detectable spectroscopically. The luminescence decay is different in the Cm and Eu systems in the pH range from 7.8 up to 10.5; a bi-exponential decay behaviour was observed for the Cm system, while the Eu system shows mono-exponential decay. This indicates that the kinetics of the chelating process is much slower for Cm(III) than for Eu(III). The rate of protonation of the coordinated α-O− group in the Eu/complex 3 is much faster than in the case of Cm/complex 3. Different spectra were observed for Eu(III)/glycolate complexes at europium concentrations of 3 × 10−6 and 1 × 10−3 mol L−1 indicating the formation of poly-nuclear Eu(III)/glycolate complexes at high metal ion concentration.

Characterization and quantification of Sm(III)/ and Cm(III)/clay mineral outer-sphere species by TRLFS in D2O and EXAFS studies.

In order to assess the long-term safety of deep radioactive waste repositories, a precise characterization of the different sorption processes on a molecular basis and the exact definition of geochemical boundary conditions for their relevance are of immense importance. Through sorption on various minerals the migration of radionuclides will be hindered and their retention will be ensured. Using time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy, it was possible to identify outer-sphere sorbed trivalent lanthanides and actinides onto different montmorillonites and illite. Furthermore, the quantification of Cm(III)/clay outer-sphere sorption in D(2)O at different ionic strengths was shown. The results were confirmed by ion exchange model calculations. Finally, the structural parameters of a Sm(III)/clay outer-sphere complex were obtained by EXAFS measurements.

Using Eu3+ as an atomic probe to investigate the local environment in LaPO4–GdPO4 monazite end-members

In the present study, we have investigated the luminescent properties of Eu(3+) as a dopant in a series of synthetic lanthanide phosphates from the monazite group. Systematic trends in the spectroscopic properties of Eu(3+) depending on the size of the host cation and the dopant to ligand distance have been observed. Our results show that the increasing match between host and dopant radii when going from Eu(3+)-doped LaPO4 toward the smaller GdPO4 monazite decreases both the full width at half maximum of the Eu(3+) excitation peak, as well as the (7)F2/(7)F1 emission band intensity ratio. The decreasing Ln⋯O bond distance within the LnPO4 series causes a systematic bathochromic shift of the Eu(3+) excitation peak, showing a linear dependence of both the host cation size and the Ln⋯O distance. The linear relationship can be used to predict the energy band gap for Eu(3+)-doped monazites for which no Eu(3+) luminescent data is available. Finally, mechanisms for metal-metal energy transfer between host and dopant lanthanides have been explored based on recorded luminescence lifetime data. Luminescence lifetime data for Eu(3+) incorporated in the various monazite hosts clearly indicated that the energy band gap between the guest ion emission transition and the host ion absorption transition can be correlated to the degree of quenching observed in these materials with otherwise identical geometries and chemistries.

TRLFS characterization of Eu(III)-doped synthetic organo-hectorite

Europium(III) was coprecipitated with the clay mineral hectorite, a magnesian smectite, following a multi-step synthesis procedure. Different Eu(III) species associated with the proceeding synthetic hectorite were characterized by selectively exciting the 5D0-->7F0 transition at low temperature (T < 20 K). Fluorescence decay times indicated that Eu(III) ions may be incorporated in the octahedral layer of the brucite precursor as well as in the octahedral sheet of the clay mineral. The excitation spectra indicated that the substitution of the divalent Mg by the trivalent Eu induced local structural deformation. This investigation implements the molecular-level understanding of the f element structural incorporation into the octahedral layer of sheet silicates by coprecipitation with clay minerals from salt solutions at 100 degrees C.

Visualising the molecular alteration of the calcite (104) - water interface by sodium nitrate.

The reactivity of calcite, one of the most abundant minerals in the earth’s crust, is determined by the molecular details of its interface with the contacting solution. Recently, it has been found that trace concentrations of NaNO3 severely affect calcite’s (104) surface and its reactivity. Here we combine molecular dynamics (MD) simulations, X-ray reflectivity (XR) and in situ atomic force microscopy (AFM) to probe the calcite (104) – water interface in the presence of NaNO3. Simulations reveal density profiles of different ions near calcite’s surface, with NO3− able to reach closer to the surface than CO32− and in higher concentrations. Reflectivity measurements show a structural destabilisation of the (104) surfaces’ topmost atomic layers in NaNO3 bearing solution, with distorted rotation angles of the carbonate groups and substantial displacement of the lattice ions. Nanoscale AFM results confirm the alteration of crystallographic characteristics, and the ability of dissolved NaNO3 to modify the structure of interfacial water was observed by AFM force spectroscopy. Our experiments and simulations consistently evidence a dramatic deterioration of the crystals’ surface, with potentially important implications for geological and industrial processes.

Multistage bioassociation of uranium onto an extremely halophilic archaeon revealed by a unique combination of spectroscopic and microscopic techniques

The interactions of two extremely halophilic archaea with uranium were investigated at high ionic strength as a function of time, pH and uranium concentration. Halobacterium noricense DSM-15987 and Halobacterium sp. putatively noricense, isolated from the Waste Isolation Pilot Plant repository, were used for these investigations. The kinetics of U(VI) bioassociation with both strains showed an atypical multistage behavior, meaning that after an initial phase of U(VI) sorption, an unexpected interim period of U(VI) release was observed, followed by a slow reassociation of uranium with the cells. By applying in situ attenuated total reflection Fourier-transform infrared spectroscopy, the involvement of phosphoryl and carboxylate groups in U(VI) complexation during the first biosorption phase was shown. Differences in cell morphology and uranium localization become visible at different stages of the bioassociation process, as shown with scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. Our results demonstrate for the first time that association of uranium with the extremely halophilic archaeon is a multistage process, beginning with sorption and followed by another process, probably biomineralization.

Formation of Cm humate complexes in aqueous solution at pHc 3 to 5.5: The role of fast interchange

Abstract Safety assessment of nuclear waste disposal includes determination of the possible impact of natural dissolved organic matter on the transport of actinide ions via groundwater into the biosphere. Thereby, much attention is paid to americium as it dominates the radiotoxicity of the nuclear waste after about 300 y and up to about 1000 y (spent fuel) or 100000 y (vitrified reprocessing waste). A trustworthy description of the influence benefits from a sound chemical process understanding of the americium humate complexation and transport processes. A problem in this respect is that studies by TRLFS lead to inconclusive results with respect to the nature of the complexes involved. In the present study the outcome of TRLFS measurements in H2O and D2O, and at room temperature and in liquid nitrogen are compared. It is shown that the Cm3+ ion interchanges between aquo ion (Cm3aq) and humate complex (CmHA) on a time scale of milliseconds in a pH range between 3 and 5.5. Taking this interchange into account, the process can be described in the absence of ternary complexes by the 1:1 stoichiometry formation of one curium humate complex, or a sufficiently narrow distribution of complexes to be represented by one average complex.

Dihydridotetrakis(4-picoline-N)­silicon dichloride chloro­form hexasolvate

The title compound, [SiH2(C6H7N)4]Cl2·6CHCl3 or C24H30N4Si2+·2Cl−·6CHCl3, contains a hexacoordinated Si atom located on a crystallographic centre of inversion. The coordination of the Si atom can be described as a slightly distorted octahedron, with the 4-picoline ligands in the equatorial plane and the two H atoms occupying axial positions. The title compound is isomorphous with its analogue where the Cl ions are substituted by Br ions.