Reaction of Pertechnetate in Highly Alkaline Solution: Synthesis and Characterization of the Nitridotrioxotechnetate Ba[TcO3 N].

The preparation of novel technetium oxides, their characterization and the general investigation of technetium chemistry are of significant importance, since fundamental research has so far mainly focused on the group homologues. Whereas the structure chemistry of technetium in strongly oxidizing media is dominated by the Tc O 4 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 4}}}\right]}^{-}}$ anion, our recent investigation yielded the new Tc O 3 N 2 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N}}\right]}^{{\rm { 2}}-}}$ anion. Brown single crystals of Ba[TcO3 N] were obtained under hydrothermal conditions starting from Ba(OH)2 ⋅ 8H2 O and NH4 [TcO4 ] at 200 °C. Ba [ Tc O 3 N ] ${{\rm { Ba[Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N]}}}$ crystallizes in the monoclinic crystal system with the space group P21 /n (a=7.2159(4) Å, b=7.8536(5) Å, c=7.4931(4) Šand ß=104.279(2)°). The crystal structure of Ba [ Tc O 3 N ] ${{\rm { Ba[Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N]}}}$ consists of isolated Tc O 3 N 2 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N}}\right]}^{{\rm { 2}}-}}$ tetrahedra, which are surrounded by Ba2+ cations. XANES measurements complement the oxidation state +VII for technetium and Raman spectroscopic experiments on Ba[TcO3 N] single crystals exhibit characteristic Tc-O and Tc-N vibrational modes.

Examination of the short-range structure of molten salts: ThF4, UF4, and related alkali actinide fluoride systems.

The short-range structures of LiF-ThF4, NaF-AnF4, KF-AnF4, and Cs-AnF4 (An = Th, U), were probed using in situ high temperature Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Signally, the EXAFS spectra of pure molten ThF4 and UF4 were measured for the first time. The data were interpreted with the aid of Molecular Dynamics (MD) and standard fitting of the EXAFS equation. As in related studies, a speciation distribution dominated by [AnFx]4-x (x = 7, 8, 9) coordination complexes was observed. The average coordination number was found to decrease with the increasing size of the alkali cation, and increase with AnF4 content. An average coordination number close to 6, which had not been detected before in melts of alkali actinide fluorides, was seen when CsF was used as solvent.

Speciation, thermodynamics and structure of Np(V) oxalate complexes in aqueous solution.

The speciation, thermodynamics and structure of the Np(v) (as the NpO2+ cation) complexes with oxalate (Ox2-) are studied by different spectroscopic techniques. Near infrared absorption spectroscopy (Vis/NIR) is used to investigate complexation reactions as a function of the total ligand concentration ([Ox2-]total), ionic strength (Im = 0.5-4.0 mol kg-1 Na+(Cl-/ClO4-)) and temperature (T = 20-85 °C) for determination of the complex stoichiometry and thermodynamic functions (log ß0n(T), ΔrH0n, ΔrS0n). Besides the solvated NpO2+ ion, two NpO2+ oxalate species (NpO2(Ox)n1-2n; n = 1, 2) are identified. With increasing temperature a decrease of the molar fractions of the 1 : 1 - and 1 : 2 - complexes is observed. Application of the law of mass action yields the temperature dependent conditional stability constants log ß'n(T) at a given ionic strength which are extrapolated to IUPAC reference state conditions (Im = 0) according to the specific ion interaction theory (SIT). The log ß0n(T) values of both complex species (log ß01(25 °C) = 4.53 ± 0.12; log ß02(25 °C) = 6.22 ± 0.24) decrease with increasing temperature confirming an exothermic complexation reaction. The temperature dependence of the thermodynamic stability constants is described by the integrated van't Hoff equation yielding the standard reaction enthalpies (ΔrH01 = -1.3 ± 0.7 kJ mol-1; ΔrH02 = -8.7 ± 1.4 kJ mol-1) and entropies (ΔrS01 = 82 ± 2 J mol-1 K-1; ΔrS02 = 90 ± 5 J mol-1 K-1) for the complexation reactions. In addition, the sum of the specific binary ion-ion interaction coefficients Δε0n(T) for the complexation reactions are obtained from SIT modelling as a function of the temperature. The structure of the complexes and the coordination mode of oxalate are investigated using EXAFS spectroscopy and quantum chemical calculations. The results show, that in case of both species NpO2(Ox)- and NpO2(Ox)23-, chelate complexes with 5-membered rings are formed.

Interaction of Np(v) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl2 solutions.

The interaction of Np(v) with borate was investigated in 0.1-5.0 M NaCl and 0.25-4.5 M MgCl2 solutions with 7.2 ≤ pHm ≤ 10.0 (pHm = -log[H+]) and 0.004 M ≤ [B]tot ≤ 0.16 M. Experiments were performed under an Ar-atmosphere at T = (22 ± 2) °C using a combination of under- and oversaturation solubility experiments, NIR spectroscopy, and extensive solid phase characterization. A bathochromic shift (≈5 nm) in the Np(v) band at λ = 980 nm indicates the formation of weak Np(v)-borate complexes under mildly alkaline pHm-conditions. The identification of an isosbestic point supports the formation of a single Np(v)-borate species in dilute MgCl2 systems, whereas a more complex aqueous speciation (eventually involving the formation of several Np(v)-borate species) is observed in concentrated MgCl2 solutions. The solubility of freshly prepared NpO2OH(am) remained largely unaltered in NaCl and MgCl2 solutions with [B]tot = 0.04 M within the timeframe of this study (t ≤ 300 days). At [B]tot = 0.16 M, a kinetically hindered but very significant drop in the solubility of Np(v) (3-4 log10-units, compared to borate-free systems) was observed in NaCl and dilute MgCl2 solutions with pHm ≤ 9. The drop in the solubility was accompanied by a clear change in the colour of the solid phase (from green to white-greyish). XRD and TEM analyses showed that the amorphous NpO2OH(am) "starting material" transformed into crystalline solid phases with similar XRD patterns in NaCl and MgCl2 systems. XPS, SEM-EDS and EXAFS further indicated that borate and Na/Mg participate stoichiometrically in the formation of such solid phases. Additional undersaturation solubility experiments using the newly formed Na-Np(v)-borate(cr) and Mg-Np(v)-borate(cr) compounds further confirmed the low solubility ([Np(v)]aq ≈ 10-6-10-7 M) of such solid phases in mildly alkaline pHm-conditions. The formation of these solid phases represents a previously unreported retention mechanism for the highly mobile Np(v) under boundary conditions (pHm, [B]tot, ionic strength) of relevance to certain repository concepts for nuclear waste disposal.

Sorption of Eu(III) on Eibenstock granite studied by µTRLFS: A novel spatially-resolved luminescence-spectroscopic technique.

In this study a novel technique, micro-focus time-resolved laser-induced luminescence spectroscopy (µTRLFS) is presented to investigate heterogeneous systems like granite (mainly consisting of quartz, feldspar, and mica), regarding their sorption behavior. µTRLFS is a spatially-resolved upgrade of conventional TRLFS, which allows point-by-point analysis of single minerals by reducing the beam size of the analytic laser beam to below the size of mineral grains. This provides visualization of sorption capacity as well as speciation of a luminescent probe, here Eu3+. A thin-section of granitic rock from Eibenstock, Saxony, Germany was analyzed regarding its mineralogy with microprobe X-ray fluorescence (µXRF) and electron probe microanalysis (EPMA). Afterwards, it was reacted with 5.0 × 10-5 mol/L Eu3+ at pH 8.0 and uptake was quantified by autoradiography. Finally, the µTRLFS studies were conducted. The results clearly show that the materials interact differently with Eu3+, and often even on one mineral grain different speciations can be found. Alkali-feldspar shows very high uptake, with an inhomogeneous distribution, and intermediate sorption strength. On quartz uptake is almost 10-fold lower, while the complexation strength is higher than on feldspar. This may be indicative of adsorption only at surface defect sites, in accordance with low hydration of the observed species.

Thermodynamic description of Tc(iv) solubility and carbonate complexation in alkaline NaHCO3-Na2CO3-NaCl systems.

The solubility of 99Tc(iv) was investigated in dilute to concentrated carbonate solutions (0.01 M ≤ Ctot≤ 1.0 M, with Ctot = [HCO3-] + [CO32-]) under systematic variation of ionic strength (I = 0.3-5.0 M NaHCO3-Na2CO3-NaCl-NaOH) and pHm (-log[H+] = 8.5-14.5). Strongly reducing conditions (pe + pHm≈ 2) were set with Sn(ii). Carbonate enhances the solubility of Tc(iv) in alkaline conditions by up to 3.5 log10-units compared to carbonate-free systems. Solvent extraction and XANES confirmed that Tc was kept as +IV during the timeframe of the experiments (≤ 650 days). Solid phase characterization performed by XAFS, XRD, SEM-EDS, chemical analysis and TG-DTA confirmed that TcO2·0.6H2O(am) controls the solubility of Tc(iv) under the conditions investigated. Slope analysis of the solubility data in combination with solid/aqueous phase characterization and DFT calculations indicate the predominance of the species Tc(OH)3CO3- at pHm≤ 11 and Ctot≥ 0.01 M, for which thermodynamic and activity models are derived. Solubility data obtained above pHm≈ 11 indicates the formation of previously unreported Tc(iv)-carbonate species, possibly Tc(OH)4CO32-, although the likely formation of additional complexes prevents deriving a thermodynamic model valid for this pHm-region. This work provides the most comprehensive thermodynamic dataset available for the system Tc4+-Na+-Cl--OH--HCO3--CO32--H2O(l) valid under a range of conditions relevant for nuclear waste disposal.

CAT-ACT-A new highly versatile x-ray spectroscopy beamline for catalysis and radionuclide science at the KIT synchrotron light facility ANKA.

CAT-ACT-the hard X-ray beamline for CATalysis and ACTinide/radionuclide research at the KIT synchrotron radiation facility ANKA-is dedicated to X-ray spectroscopy, including "flux hungry" photon-in/photon-out and correlative techniques and combines state-of-the-art optics with a unique infrastructure for radionuclide and catalysis research. Measurements can be performed at photon energies varying between 3.4 keV and 55 keV, thus encompassing the actinide M- and L-edge or potassium K-edge up to the K-edges of the lanthanide series such as cerium. Well-established X-ray absorption fine structure spectroscopy in transmission and fluorescence detection modes is available in combination with high energy-resolution X-ray emission spectroscopy or X-ray diffraction techniques. The modular beamline design with two alternately operated in-line experimental stations enables sufficient flexibility to adapt sample environments and detection systems to many scientific challenges. The ACT experimental station focuses on various aspects of nuclear waste disposal within the mission of the Helmholtz association to contribute to the solution of one of the greatest scientific and social challenges of our time-the safe disposal of heat producing, highly radioactive waste forms from nuclear energy production. It augments present capabilities at the INE-Beamline by increasing the flux and extending the energy range into the hard X-ray regime. The CAT experimental station focuses on catalytic materials, e.g., for energy-related and exhaust gas catalysis. Characterization of catalytically active materials under realistic reaction conditions and the development of in situ and operando cells for sample environments close to industrial reactors are essential aspects at CAT.

Selenide retention by mackinawite.

The isotope (79)Se may be of great concern with regard to the safe disposal of nuclear wastes in deep geological repositories due to its long half-life and potential mobility in the geosphere. The Se mobility is controlled by the oxidation state: the oxidized species (Se(IV)) and (Se(VI)) are highly mobile, whereas the reduced species (Se(0) and Se(-II)) form low soluble solids. The mobility of this trace pollutant can be greatly reduced by interacting with the various barriers of the repository. Numerous studies report on the oxidized species retention by mineral phases, but only very scarce studies report on the selenide (Se(-II)) retention. In the present study, the selenide retention by coprecipitation with and by adsorption on mackinawite (FeS) was investigated. XRD and SEM analyses of the samples reveal no significant influence of Se on the mackinawite precipitate morphology and structure. Samples from coprecipitation and from adsorption are characterized at the molecular scale by a multi-edge X-ray absorption spectroscopy (XAS) investigation. In the coprecipitation experiment, all elements (S, Fe, and Se) are in a low ionic oxidation state and the EXAFS data strongly point to selenium located in a mackinawite-like sulfide environment. By contacting selenide ions with FeS in suspension, part of Se is located in an environment similar to that found in the coprecipitation experiment. The explanation is a dynamical dissolution-recrystallization mechanism of the highly reactive mackinawite. This is the first experimental study to report on selenide incorporation in iron monosulfide by a multi-edge XAS approach.

The INE-Beamline for actinide science at ANKA.

Since its inauguration in 2005, the INE-Beamline for actinide research at the synchrotron source ANKA (KIT North Campus) provides dedicated instrumentation for x-ray spectroscopic characterization of actinide samples and other radioactive materials. R&D work at the beamline focuses on various aspects of nuclear waste disposal within INE's mission to provide the scientific basis for assessing long-term safety of a final nuclear waste repository. The INE-Beamline is accessible for the actinide and radiochemistry community through the ANKA proposal system and the European Union Integrated Infrastructure Initiative ACTINET-I3. Experiments with activities up to 1 × 10(+6) times the European exemption limit are feasible within a safe but flexible containment concept. Measurements with monochromatic radiation are performed at photon energies varying between ~2.1 keV (P K-edge) and ~25 keV (Pd K-edge), including the lanthanide L-edges and the actinide M- and L3-edges up to Cf. The close proximity of the INE-Beamline to INE controlled area labs offers infrastructure unique in Europe for the spectroscopic and microscopic characterization of actinide samples. The modular beamline design enables sufficient flexibility to adapt sample environments and detection systems to many scientific questions. The well-established bulk techniques x-ray absorption fine structure (XAFS) spectroscopy in transmission and fluorescence mode have been augmented by advanced methods using a microfocused beam, including (confocal) XAFS/x-ray fluorescence detection and a combination of (micro-)XAFS and (micro-)x-ray diffraction. Additional instrumentation for high energy-resolution x-ray emission spectroscopy has been successfully developed and tested.

Sorption of Eu(III) onto titanium dioxide: measurements and modeling.

In the present study, the sorption of europium and lutetium onto titanium dioxide from aqueous solutions is presented, as a function of pH, ionic strength and concentration. An acid base model for the titanium dioxide surface was determined from potentiometric titrations and zeta-potential measurements. The common intersection point of potentiometric titrations coincided with the isoelectric point from electrokinetic experiments, resulting in a pristine point of zero charge of about 6.1. The experimental data were in agreement with previously published results and a previously published MUSIC-type model was used as the basis to model the acid-base behavior. Comparison of europium and lutetium showed no difference in the adsorption behavior. Furthermore, no difference was observed both in uptake and spectroscopic studies whether carbonate was absent or present. The absence of a noticeable effect of the ionic strength on the adsorption behavior was indicative of strong binding. EXAFS revealed rough conservation of the coordination with 9-8 water and surface hydroxyl groups upon sorption. EXAFS results suggested the existence of different metal-oxygen distances, more varied than that observed for the respective aquo complex and thus indicative for inner-sphere surface complexation. A clear differentiation of surface complexation denticity was not possible based on spectroscopic data. A multisite surface complexation model approach was applied by assuming monodentate and multidentate binding to describe the trivalent metal uptake data. It is conceivable that mono- and multidentate species contribute to lanthanide sorption to titanium dioxide. In other words a distribution of states occurs in cation surface complexation reactions.

Time-resolved laser fluorescence spectroscopy and extended X-ray absorption spectroscopy investigations of the N3- complexation of Eu(III), Cm(III), and Am(III) in an ionic liquid: differences and similarities.

The complexation of the lanthanide Eu(III) and the actinides Cm(III) and Am(III) by N3- was investigated by application of time-resolved laser fluorescence spectroscopy (TRLFS) and X-ray absorption spectroscopy (XAFS) in the ionic liquid solution of C4mimTf2N (1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide). TRLFS measurements show that the interaction of azide with Eu(CF3SO3)3 and Eu(ClO4)3 results in both dynamic luminescence quenching by collisional encounters of N3- with Eu(III) and static luminescence quenching by inner-sphere complexation of Eu(III) by N3-. Hereby, the complexation of Eu-triflate by azide starts at a lower N3- concentration as compared to the perchlorate salt. The authors ascribe this phenomenon to a stronger bonding of ClO4- toward the metal ion than triflate, as well as to a stronger electrostatic repulsion of N3- by the perchlorate ligand. In both actinide samples (Cm(ClO4)3, Am(ClO4)3), the complexation with azide exhibits a clear kinetic hindrance. Nevertheless, mixed actinide-perchlorate-azide complexes are formed after several days in C4mimTf2N. The different reaction kinetics for the Ln- and An-complexation by azide may provide the opportunity for an effective separation of lanthanides from actinides in the nuclear fuel cycle by the use of N-based extractants in ionic liquid solution.

Incorporation of Eu(III) into hydrotalcite: a TRLFS and EXAFS study.

The behavior of radionuclides in the environment (geo-, hydro-, and biosphere) is determined by interface reactions like adsorption, ion exchange, and incorporation processes. Presently, operational gross parameters for the distribution between solution and minerals are available. For predictive modeling of the radionuclide mobility in such systems, however, individual reactions and processes need to be localized, characterized, and quantified. A prerequisite for localization and clarification of the concerned processes is the use of modern advanced analytical and speciation methods, especially spectroscopy. In this study, Eu(III) was chosen as an analogue for trivalent actinides to identify the different species that occur by the Ln(III)/hydrotalcite interaction. Therefore, Eu(III) doped Mg-Al-Cl-hydrotalcite was synthesized and investigated by TRLFS, EXAFS, and XRD measurements. Two different Eu/hydrotalcite species were obtained. The minor part of the lanthanide is found to be inner-sphere sorbed onto the mineral surface, while the dominating Eu/hydrotalcite species consists of Eu(III) that is incorporated into the hydrotalcite lattice. Both Eu/hydrotalcite species have been characterized by their fluorescence emission spectra and lifetimes. Structural parameters of the incorporated Eu(III) species determined by EXAFS indicate a coordination number of 6.6 +/- 1.3 and distances of 2.41 +/- 0.02 A for the first Eu-OH shell.

Structural characterization of Am incorporated into calcite: a TRLFS and EXAFS study.

Calcite homogeneously doped with Am(III) and Cm(III) was synthesized in a mixed-flow reactor. The mechanism of incorporation of these actinides (An) into calcite was investigated by time-resolved laser fluorescence spectroscopy. Two different An(III)/calcite species were found. One has been identified as ions bonded onto the calcite surface. The second An(III) species has lost its complete hydration sphere and is incorporated into the calcite bulk structure. Both Cm(III)/calcite and Am(III)/calcite complexes have been characterized by their fluorescence emission spectra and lifetimes. Structural parameters of the incorporated Am(III) species determined by EXAFS indicate a coordination number of 6.3+/-0.6 and distances of 2.40+/-0.01 A for the first AmO shell.

Low temperature XAFS investigation on the lutetium binding changes during the 2-line ferrihydrite alteration process.

The time dependent changes of Lu speciation (used as Am(III) homologue), initially sorbed onto 2-line ferrihydrite at pH 5.9, during tempering (70 degrees C) to stable crystalline transformation products, goethite and hematite, is studied. Microscopies (AFM, SEM), XRD and FTIR spectroscopy confirm transformation to both goethite and hematite, with a predominance of hematite. XRD investigation of another transformation series at pH 8.0 (75 degrees C, [Lu(III)initial] 7 times higher) shows that the cell volume of hematite increases, suggesting the incorporation of Lu in the crystal structure. Extended X-ray absorption fine structure (EXAFS) (pH 5.9 series, 70 degrees C) reveals a shortening of the Lu-O bond distance and an increase in asymmetry of the first shell with increasing tempering time in the intermediate temper time samples. The intensity of the second peak in the Fourier transform (FT) of the EXAFS increases and splits into two components. The EXAFS data of the end product can be modeled well using a hematite-like cluster, with an isotropic expansion of distances to account for incorporation of Lu into the hematite structure. These results demonstrate that the Lu is incorporated in the crystal lattice of the transformation product, as opposed to being occluded or remaining a sorbed species on the surface.

Soft X-Ray Spectromicroscopy Investigation of the Interaction of Aquatic Humic Acid and Clay Colloids.

Soft X-ray spectromicroscopy investigations of the interaction of aquatic humic acid (HA) and montmorillonite colloids have been performed in situ at the NSLS X1-A STXM endstation. Images have been recorded of montmorillonite particles, HA aggregates, and mixed suspensions of both montmorillonite + HA and montmorillonite + carboxyl polystyrene microspheres, as reference organic colloids. Special emphasis has been placed on the sample preparation technique in order to keep the colloid particles hydrated during all measurements. C 1s near edge absorption fine structure extracted from STXM image stacks reveals electronic transitions corresponding to functional -COOH and -C(6)H(5) groups present in HA and polystyrene. XANES peak intensities reflect the relative amounts of these two carbon functional groups in the organic particles. For example, the greater amount of carboxyl groups in HA compared to the latex reference particles affects a larger 1s-->pi* transition intensity. A specific form of montmorillonite-HA particle agglomeration at near-neutral pH has been observed. Under these conditions, we found no separate clustering of HA. Instead, STXM images show the HA to coat the clay mineral surface, leading to nearly a fractal manner of aggregation. Copyright 2000 Academic Press.