T. Reich

Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)3(aq.) species

We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca2UO2(CO3)3(aq.), in uranium mining related waters [1]. We are now reporting a further validation of the stoichiometry and the formation constant of this complex using two analytical approaches with time-resolved laser-induced fluorescence spectroscopy (TRLFS) species detection: i) titration of a non-fluorescent uranyl tricarbonate complex solution with calcium ions, and quantitative determination of the produced fluorescent calcium complex via TRLFS; and ii) variation of the calcium concentration in the complex by competitive calcium complexation with EDTA4-. Slope analysis of the log (fluorescence intensity) versus log[Ca2+] with both methods have shown that two calcium ions are bound to form the complex Ca2UO2(CO3)3(aq.). The formation constants determined from the two independent methods are: i) logβ°213=30.45±0.35 and ii) logβ°213=30.77±0.25. A bathochrome shift of 0.35 nm between the UO2(CO3)34- complex and the Ca2UO2(CO3)3(aq.) complex is observed in the laser-induced photoacoustic spectrum (LIPAS), giving additional evidence for the formation of the calcium uranyl carbonate complex. EXAFS spectra at the LII and LIII-edges of uranium in uranyl carbonate solutions with and without calcium do not differ significantly. A somewhat better fit to the EXAFS of the Ca2UO2(CO3)3(aq.) complex is obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca2UO2(CO3)3(aq.) species in solution and the natural mineral liebigite, we conclude that the calcium atoms are likely to be in the same positions both in the solution complex and in the solid. This complex influences considerably the speciation of uranium in the pH region from 6 to 10 in calcium-rich uranium-mining-related waters.

ROBL – a CRG beamline for radiochemistry and materials research at the ESRF

The paper describes the Rossendorf beamline (ROBL) built by the Forschungszentrum Rossendorf at the ESRF. ROBL comprises two different and independently operating experimental stations: a radiochemistry laboratory for X-ray absorption spectroscopy of non-sealed radioactive samples and a general purpose materials research station for X-ray diffraction and reflectometry mainly of thin films and interfaces modified by ion beam techniques.

EXAFS investigation of uranium(VI) complexes formed at Bacillus cereus and Bacillus sphaericus surfaces

Uranium(VI) complex formation at vegetative cells and spores of Bacillus cereus and Bacillus sphaericus was studied using uranium LII-edge and LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. A comparison of the measured equatorial U-O distances and other EXAFS structural parameters of uranyl species formed at the Bacillus strains with those of the uranyl structure family indicates that the uranium is predominantly bound as uranyl complexes with phosphoryl residues.

The structure of U (super 6+) sorption complexes on vermiculite and hydrobiotite

The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Technetium Speciation in Cement Waste Forms Determined by X-ray Absorption Fine Structure Spectroscopy

The chemistry of technetium in cement waste forms has been studied with X-ray absorption fine structure (XAFS) spectroscopy. Using the Tc /f-edge X-ray absorption near-edge structure (XANES) as a probe of the technetium speciation, our results show that partial reduction of the pertechnetate ion, TcOj, takes place in the presence of the cement additive, blast fumace slag (BFS). The addition of the reducing agents FeS, NazS, and NaHjPOz produces more extensive reduction of TcOl, while the Compounds FeO, Fe304, and MnjOi are observed to be unreactive. The extended X-ray absorption fine structure (EXAFS) data for the BFS, Na^S, and FeS treated cements indicate the presence of Tc Clusters possessing first shell S coordination. For the Na2S and FeS additives, Tc-Tc interactions are detected in the EXAFS demonstrating an extended structure similar to that of TcSa. The EXAFS spectrum of the NaHzPOj treated cement reveals T c O and Tc-Tc interactions that resemble those found in the structure of TcO,.

CHARACTERIZATION OF HYDROUS URANYL SILICATE BY EXAFS

Extended X-ray absorption fine structure (EXAFS) analysis was performed on uranyl orthosilicate, (U02)2Si04 · 2H20, and uranium(VI) sorbed onto silicic acid and silica gel. Uranyl orthosilicate was investigated as a reference for EXAFS studies of similar but non-crystalline uranium, oxygen, and silicon containing samples. Fitting the EXAFS spectrum yields the following distances for the first four coordination shells of uranium: U— Oax = 1.79 Á, U-O e i l = 2.38 λ , U S i = 3.16 Â, and U U = 3.88 A. These values agree well with results from single-crystal X-ray diffraction (XRD) measurements. Structural parameters of light elements such as oxygen and silicon at distances greater than 3.5 A could not be detected without a priori knowledge of their presence. The EXAFS spectra of uranyl species sorbed at pH 4 onto silicic acid and silica gel are identical indicating similar uranyl coordination. The main characteristic of the surface species are two well-separated oxygen coordination shells in the equatorial uranyl plane at 2.27 and 2.50 A. The results of the EXAFS analysis favor the interpretation of the uranyl surface species as an inner-sphere, mononuclear, bidentate complex.

The Rossendorf beam line ROBL - A dedicated experimental station for XAFS measurements of actinides and other radionuclides

X-ray absorption fine structure (XAFS) spectroscopy is a powerful tool for obtaining basic molecular-level information, which is required for a better understanding of the mechanisms responsible for radionuclide transport in the environment. A unique experimental station dedicated to the study of actinides and other radionuclides by XAFS spectroscopy has become operational at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The main characteristics of the Rossendorf Beamline, ROBL, and its radiochemistry end station and selected results obtained on Tc and Np solutions are presented.

Determination of structural parameters of uranyl ions complexed with organic acids using EXAFS

Abstract Two compounds of known crystal structure, sodium triacetatodioxouranium(VI), Na[UO 2 (CH 3 COO) 3 ], and dibenzoatodioxouranium(VI), UO 2 [C 6 H 5 (COO)] 2 , were studied by uranium L III -edge extended X-ray absorption fine structure, EXAFS, spectroscopy to differentiate between bidentate and monodentate coordination of carboxylate ions on the basis of the uranium–equatorial oxygen, O eq , bond lengths. Bidentate coordination can be verified by detecting carboxyl carbon atoms and the neighboring distal carbon atom of the organic rest. In contrast, EXAFS spectra for monodentate carboxylate complexes show no evidence of carbon atoms beyond the O eq coordination shell. The mode of coordination was determined by EXAFS analysis for solid uranyl complexes with humic, methoxybenzoic, and salicylic acids. A correlation between the U L III -edge X-ray absorption near-edge structure, XANES, and the U–O eq bond distance according to the relationship Δ E · R (O eq ) 2 =constant was observed. For the samples studied, the constant was determined to be 197±8 eV A 2 .

EXAFS Investigations of the Interaction of Humic Acids and Model Compounds with Uranyl Cations in Solid Complexes

By M. A. Denecke*, T. Reich, S. Pompe, Μ. Bubner, Κ. Η. Heise, Η. Nitsche**, P. G. Allen, J. J. Bucher, Ν. Μ. Edelstein, D. Κ. Shuh and Κ. R. Czerwinski 1 Forschungszentrum Rossendorf e.V., Institute of Radiochemistry, Dresden, Germany 2 Lawrence Berkeley National Laboratory, Chemical Sciences Division, Berkeley, USA 3 Massachusetts Institute of Technology, Nuclear Engineering Department, Boston, USA

EXAFS structural analysis of aqueous uranium(VI) complexes with lignin degradation products

Uranium LIII-edge extended X-ray absorption fine structure (EXAFS) analysis was carried out on aqueous uranium(VI) complexes with monomeric intermediates of the natural wood-degradation process such as protocatechuic acid (3,4-dihydroxybenzoic acid), catechol (2-hydroxyphenol), pyrogallol (1,2,3-trihydroxybenzol), and vanillic acid (4-hydroxy-3-methoxybenzoic acid). The structural parameters of the 1:1 complexes with protocatechuic acid and vanillic acid in weakly acidic solution indicate that the carboxylic group coordinates the uranyl cation in a bidentate mode in the equatorial plane. Based on the identical structural parameters observed for protocatechuic acid, catechol and pyrogallol in alkaline media, we conclude that the coordination with uranium occurs via the neighboring OH groups of the phenols under formation of a five-membered ring.