Kathy Dardenne

Geomicrobiological Redox Cycling of the Transuranic Element Neptunium

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O(2)(+) under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

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 INEs 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.

A spectroscopic study of the hydrolysis, colloid formation and solubility of Np(IV)

The hydrolysis, colloid formation and solubility of Np(IV) are investigated in aqueous HClO4-NaClO4 solutions (log [H+] = 0 to -2.5) by absorption spectroscopy in the wavelength range of 680-1000 nm. Applying Laser induced photoacoustic spectroscopy (LPAS) in the range of 680-760 nm, the study is extended to low Np(IV) concentrations of 10-6 mol/l in DClO4-NaClO4-D2O solutions up to log [D+]=-3.3. Laser induced breakdown detection (LIBD) demonstrates the formation of Np(IV) colloids when the Np(IV) concentration exceeds the solubility of Np(OH)4(am) at given pH. The simultaneous decrease of the Np(IV) absorption bands at 723 and 960 nm cannot be ascribed to the formation of the mononuclear complex Np(OH)3+ as assumed in the literature. It is found to be caused by polynucleation. In undersaturated Np(IV) solutions below 10-4 mol/l, the position and intensity of the absorption maximum at 723 nm are practically insensitive to the pH change. In oversaturated solutions the absorption band decreases significantly. The spectroscopically determined pH-dependent equilibrium concentration of mononuclear Np(IV) species above freshly formed solid or colloidal Np(IV) particles indicates that Np(OH)22+ is the predominant species in the pH range of 1.5-3. This finding is in agreement with the Np(IV) hydrolysis constants reported in the literature from a solvent extraction study with 239Np(IV) trace concentrations. The solubility product of freshly formed Np(OH)4(am) particles is determined to be log Ksp = -54.4±0.4 in 0.1 M HClO4-NaClO4 and log K°sp=-56.5±0.4 (converted to I=0 by applying the SIT).

Combined AFM and STXM in situ study of the influence of Eu(III) on the agglomeration of humic acid

Humic acid (HA) agglomerates formed in aqueous solutions in the presence of trivalent Eu cations were investigated in situ with a combination of atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM). The micromorphologies of both natural HA and a melanoidine based synthetic HA observed by AFM in electrolyte solution are in fair agreement with previous AFM studies on humic substances. STXM micrographs of Eu(III) induced HA agglomeration reveal zones of high and low optical density with markedly distinct C K-NEXAFS, indicative of different humic functionalities. Particulate agglomerates observed by AFM can be correlated to the dense zones, whereas fibrous structures in AFM images can be associated with the low density areas. The Eu cation distribution within the agglomerates cannot be unambiguously deduced from their C K-NEXAFS spectra. The near edge X-ray absorption fine structure spectra can be correlated to a segregation of different HA fractions, possibly due to the presence of humic species with different affinities for metal cation complexation. STXM micrographs of purified Aldrich HA exhibit the presence of other yet unidentified, carbon-rich particles, independent of the addition of Eu(III). Both AFM and STXM results for the synthetic melanoidine based HA demonstrate a homogeneous morphology and chemical structure.

Identification and characterization of sorbed lutetium species on 2-line ferrihydrite by sorption data modeling, TRLFS and EXAFS

The Lu(III) sorbed species onto synthetic hydrous ferric oxide (HFO), commonly called ferrihydrite, has been identified. Characterization of the synthetic 2-line HFO shows that its synthesis is reproducible. Potentiometric titration of freshly synthesized HFO, modeled using the constant capacity model (κ1=0.5 F/m2) in the FITEQL code, yields a specific surface area Sa of 360±35 m2/g (N2-BET), a site density Nd of 2.86 sites/nm2 (concentration of hydroxyl groups, Ns=1.71×10-3 mol sites/g HFO), and acidity constants pKa1}int=6.37 and pKa2}int=9.25. Evaluation of chemical sorption data reveals the presence of two different Lu surface sorbed species, dependent on pH; a monodendate species forms at low pH and a polydentate species at pH>5. Satisfactory fits to the sorption data are obtained using a combination of monodentate and bidentate surface species. The combination of species is chosen, based on extended X-ray absorption fine structure (EXAFS) results. The sorption constants obtained from these fits are pKs=-1.89(±0.1) and pKs=-1.69(±0.1) for the monodentate species ≡FeOLu(H2O)52+ for fits to the pH edge and to the isotherm at pH°5.9, respectively. A value of pKs=3.69(±0.01) is found for the bidentate species ≡ Fe(O)2Lu(H2O)5+ for both fits. EXAFS analysis of sorption samples prepared at 4.5<pH<8 shows that Lu is surrounded by a single first shell of 7±1 oxygen atoms, at a distance of (2.30±0.01) Â in all samples. A second coordination shell of Fe neighboring atoms at a distance of (3.38±0.01) Â is observed for sorption samples pH≥5.5. This distance is associated with the formation of a bidendate complex with bonding via edge sharing to iron octahedra. The samples prepared at pH<5.1 show no Fe shell, as expected for monodentate coordination. No evidence for surface precipitation and no noticeable difference between wet paste and dried powder samples is found.

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS.

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.

Microanalysis (micro-XRF, micro-XANES, and micro-XRD) of a tertiary sediment using microfocused synchrotron radiation.

Micro-focused synchrotron radiation techniques to investigate actinide elements in geological samples are becoming an increasingly used tool in nuclear waste disposal research. In this article, results using mu-focus techniques are presented from a bore core section of a U-rich tertiary sediment collected from Ruprechtov, Czech Republic, a natural analog to nuclear waste repository scenarios in deep geological formations. Different methods are applied to obtain various, complementary information. Elemental and element chemical state distributions are obtained from micro-XRF measurements, oxidation states of As determined from micro-XANES, and the crystalline structure of selected regions are studied by means of micro-XRD. We find that preparation of the thin section created an As oxidation state artifact; it apparently changed the As valence in some regions of the sample. Results support our previously proposed hypothesis of the mechanism for U-enrichment in the sediment. AsFeS coating on framboid Fe nodules in the sediment reduced mobile groundwater-dissolved U(VI) to less-soluble U(IV), thereby immobilizing the uranium in the sediment.

TRLFS and EXAFS investigations of lanthanide and actinide complexation by triflate and perchlorate in an ionic liquid

The solvation of the Eu-perchlorate (ClO4–) and triflate (CF3SO3–, OTf–) salts as well as of Cm(ClO4)3 and Am(ClO4)3 in the ionic liquid C4mimTf2N (1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide) has been comparatively investigated by application of laser fluorescence spectroscopy and X-ray absorption spectroscopy. Moreover, the ClO4–/OTf– ligand exchange reaction for the two actinide cations has been analyzed by the same spectroscopic techniques. A structural model for the different complexes was determined by the interpretation of the spectroscopic data. The lanthanide and the two actinide cations show the same coordination in C4mimTf2N. Moreover, a sequence for the strength of complexing ligands could be deduced from the spectroscopic data for the lanthanide and the two actinides: ClO4–>OTf–≥Tf2N–>H2O.

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M4 high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V), and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L3 XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to nonstoichiometric UO2+x nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M4 HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L3 XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).