P. G. Allen

An EXAFS study of uranium(VI) sorption onto silica gel and ferrihydrite

Abstract Structural parameters of the near-neighbor surrounding of uranium(VI) sorbed onto silica gel and ferrihydrite surfaces have been determined by U L III -edge extended X-ray absorption fine structure (EXAFS) analysis. The sorption on silica gel was studied at pHxa04 with uranium(VI) concentrations in the range of 5×10 −2 –2×10 −5 xa0mol/l. Sorption experiments with ferrihydrite have been performed at pHxa05.8 with uranium(VI) concentrations of 10 −4 and 10 −5 xa0mol/l and at pHxa07.8 with 10 −5 xa0mol/l uranium(VI). The structural parameters for the uranium coordination shells (U-O ax , U-O eq , and U-Si/Fe) indicate formation of inner-sphere, mononuclear uranyl complexes at the silica gel and ferrihydrite surfaces.

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 .

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

Characterization of the Interaction of Uranyl Ions with Humic Acids by X-Ray Absorption Spectroscopy

Humic substances are present throughout the environment in soil and natural water. They are organic macromolecules with a variable structural formula, molecular weight, and a wide variety of functional groups depending on their origin. In natural waters, humic substances represent the main component of the “dissolved organic carbon” (DOC). The DOC may vary considerably from 1 mg/L at sea water surfaces to 50 mg/L at the surface in dark water swamps.1 There is strong evidence that all actinides form complexes with humic substances in natural waters.2 Therefore, humic substances can play an important role in the environmental migration of radionuclides by enhancing their transport. Retardation through humic substance interaction may be also possible due to formation of precipitating agglomerates. For remediation and restoration of contaminated environmental sites and risk assessment of future nuclear waste repositories, it is important to improve the predictive capabilities for radionuclide migration through a better understanding of the interaction of radionuclides with humic substances.

The Effects of Na 2 O, Al 2 O 3 , and 3203 ON HfO 2 Solubility in Borosilicate Glass

A single borosilicate glass composition has previously been shown to dissolve 10 and 25 mass% PuO 2 under oxidizing and reducing conditions, respectively. A simplified version of this glass has been thoroughly investigated to determine the effect of increasing the alkali:aluminum ratio on the HfO 2 solubility in borosilicate glasses. We are investigating HfO 2 solubility because specific Pu wastes are being considered for disposal in glass, and Hf(IV) serves as a structural surrogate for Pu(IV) and as a neutron absorber in glass. Three series of base glasses were produced using the same initial composition, but varying the oxides B 2 O 3 , Al 2 O 3 , or Na 2 O one at a time. In a fourth series of the same initial composition, both Na 2 O and A1 2 O 3 were varied. Hafnia was added to these glasses and the mixture equilibrated for 2 hours: 1 hour at 1450°C after 1 hour at 1560°C. A wide range of HfO 2 additions were made to the base glasses, and the solubility of HfO 2 determined to within ±1 mass%. The highest solubility determined was 14 mol% (35 mass%) HfO2 in a low-Al glass. We conclude that increasing Na 2 O/Al 2 O 3 increases the HfO 2 solubility, and increasing the B 2 O 3 content apparently has little effect on HfO 2 solubility in the borosilicate glasses studied.

Application of Synchrotron Radiation Techniques to Chemical Issues in the Environmental Sciences

The problem of hazardous waste contamination in the environment has become an issue of central importance in society. As the number of remediation efforts taking place worldwide grows and the public awareness of these problems increases, it is clear that the problems faced are enormous in terms of effort and cost. With existing chemical technologies being applied to well over one thousand contaminated sites within the U.S. alone, cost estimates for the cleanup efforts range well into the tens of billions of dollars. Furthermore, because many of the problems are still being assessed and identified, there is no accurate time estimate for the completion of these projects, with currently proposed restoration programs ranging well into the mid-21st century.

Exafs spectroscopic studies of uranium (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 L{sub III} - edge extended x-ray absorption fine structure (EXAFS) as a probe 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 {Angstrom} at pH=7 to 1.86 {Angstrom} at pH=11. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear O=U=O group is seen directly at the L{sub III} 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.

EXAFS studies of lanthanide coordination in crystalline phosphates and amorphous phytates

As part of the DOE Efficient Separations and Processing Integrated Program, techniques are being developed to stabilize radioactive and hazardous contaminants to prevent their migration from buried wastes. This report is part of a study to immobilize actinide ions in the near-surface environment by reacting them with organophosphorus complexants that decompose to phosphates. Nd and Gd ions were used as models for Pu, Am, and Cm ions; phytic acid was used as the complexant. Solid Nd and Gd phytates were prepared and the coordination characterized by EXAFS.