Jerome J. Bucher

EXAFS and principal component analysis: a new shell game

The use of principal component (factor) analysis for the interpretation of EXAFS spectra is described. The components derived from EXAFS spectra share mathematical properties with the original spectra. As a result, the abstract components can be analyzed using standard EXAFS methodology to yield bond distances and other coordination parameters. The number of components that must be analyzed is usually less than the number of original spectra. The method is demonstrated using a series of spectra from aqueous solutions of uranyl ions.

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.

Determinations of Uranium Structures by EXAFS: Schoepite and Other U(VI) Oxide Precipitates

We have investigated the structures of U(VI) oxides precipitated from room temperature aqueous solutions at low ionic strength as a function of pH. Using the uranium Lmedge extended Xray absorption fine structure (EXAFS) and infrared (IR) spectroscopies as probes of the local structure around the uranium, a trend is observed whereby the axial oxygen bond lengths from the uranyl groups increase from 1.80 A at pH = 7 to 1.86 A at pH = 11. Shifts in the IR spectral frequencies support this assignment. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear 0 = U = 0 group is seen directly at the Lm absorption edge where multiple scattering resonances systematically shift in energy. EXAFS curve-fitting analysis on these precipitates and a sample of synthetic schoepite indicate that the structure of the species formed at pH = 7 is similar to the structure of schoepite. At pH = 11, the precipitate structure is similar to that of a uranate.

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.

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 .

X-ray absorption fine structure spectroscopy of plutonium complexes with bacillus sphaericus

Summary Knowledge of the plutonium complexes formed with bacterial cells is critical for predicting the influence of microbial interactions on the migration behavior of actinides in the environment. This investigation describes the interaction of plutonium(VI) with cells of the aerobic soil bacteria, Bacillus sphaericus. The studies include the quantification of carboxylate and phosphate functional groups on the cell walls by potentiometric titration and the determination of the plutonium speciation by X-ray absorption fine structure (XAFS). Extended-XAFS (EXAFS) was used to determine the identity of the Pu(VI) interfacial complex with the bacteria, and the Pu(VI) was found primarily bound to phosphate groups on the cell surface. No carboxylate complexation was detected.

First results from the SpectroMicroscopy Beamline at the Advanced Light Source

The SpectroMicroscopy Facility at the Advanced Light Source is based on a high brightness, high‐resolution beamline, and includes a collection of projects designed to exploit the unique characteristics of the soft x‐ray beam. The beamline itself is comprised of a 5‐m long, 5‐cm‐period undulator, a spherical‐grating monochromator with water‐cooled gratings. Adaptive optics refocus the monochromatic beam to two ‘‘microfocus’’ experimental stations with spot sizes less than 50 μm diameter and a third ‘‘nanofocus’’ station uses a zone‐plate lens to further demagnify the microfocus spot. Experimental stations include an ‘‘ultraESCA’’ spectrometer for small‐area spectroscopy and photoelectron diffraction, a scanning transmission x‐ray microscope, and photoelectron microscopes. Commissioning experiments of microscopic actinide photoemission, surface‐core‐level photoelectron diffraction, and high‐resolution soft x‐ray fluorescence demonstrate dramatic improvements in sensitivity due to the high brightness and sma...

Holographic analysis of diffraction structure factors

We combine the theory of inside-source/inside-detector x-ray fluorescence holography and Kossel lines/ x ray standing waves in kinematic approximation to directly obtain the phases of the diffraction structure factors. The influence of Kossel lines and standing waves on holography is also discussed. We obtain partial phase determination from experimental data obtaining the sign of the real part of the structure factor for several reciprocal lattice vectors of a vanadium crystal.