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Title: X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations.

Abstract

Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed uranium(VI) reduction and the formation of a Ca-UO{sub 2}-CO{sub 3} complex. However, the stoichiometry of the complex is poorly defined and may be complicated by the presence of a Na-UO{sub 2}-CO{sub 3} complex. Such a complex might exist even at high calcium concentrations, as some UO{sub 2}-CO{sub 3} complexes will still be present. The number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS) measurements of a series of solutions containing 50 {micro}M uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO{sub 2}-CO{sub 3} complex to approximately 0.6more » and enables spectroscopic identification of the Na-UO{sub 2}-CO{sub 3} complex. At nearly neutral pH values, the numbers of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted by speciation calculations.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
940696
Report Number(s):
ANL/BIO/JA-57749
Journal ID: ISSN 0016-7037; GCACAK; TRN: US200824%%216
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochim. Cosmochim. Acta; Journal Volume: 71; Journal Issue: 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
54 ENVIRONMENTAL SCIENCES; ABSORPTION; ABSORPTION SPECTROSCOPY; ATOMS; BACTERIA; BIOREMEDIATION; CALCIUM; ECOLOGICAL CONCENTRATION; FINE STRUCTURE; PH VALUE; SODIUM; STOICHIOMETRY; TRANSPORT; URANIUM; URANYL CARBONATES

Citation Formats

Kelly, S. D., Kemner, K. M., Brooks, S. C., Biosciences Division, and ORNL. X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations.. United States: N. p., 2007. Web. doi:10.1016/j.gca.2006.10.013.
Kelly, S. D., Kemner, K. M., Brooks, S. C., Biosciences Division, & ORNL. X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations.. United States. doi:10.1016/j.gca.2006.10.013.
Kelly, S. D., Kemner, K. M., Brooks, S. C., Biosciences Division, and ORNL. Mon . "X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations.". United States. doi:10.1016/j.gca.2006.10.013.
@article{osti_940696,
title = {X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations.},
author = {Kelly, S. D. and Kemner, K. M. and Brooks, S. C. and Biosciences Division and ORNL},
abstractNote = {Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed uranium(VI) reduction and the formation of a Ca-UO{sub 2}-CO{sub 3} complex. However, the stoichiometry of the complex is poorly defined and may be complicated by the presence of a Na-UO{sub 2}-CO{sub 3} complex. Such a complex might exist even at high calcium concentrations, as some UO{sub 2}-CO{sub 3} complexes will still be present. The number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS) measurements of a series of solutions containing 50 {micro}M uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO{sub 2}-CO{sub 3} complex to approximately 0.6 and enables spectroscopic identification of the Na-UO{sub 2}-CO{sub 3} complex. At nearly neutral pH values, the numbers of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted by speciation calculations.},
doi = {10.1016/j.gca.2006.10.013},
journal = {Geochim. Cosmochim. Acta},
number = 2007,
volume = 71,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed uranium(VI) reduction and the formation of a Ca-UO2-CO3 complex. However, the stoichiometry of the complex is poorly defined and may be complicated by the presence of a Na-UO2-CO3 complex. Such a complex might exist even at high calcium concentrations, as some UO2-CO3 complexes will still be present. The number of calcium and/or sodiummore » atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS) measurements of a series of solutions containing 50 lM uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO2-CO3 complex to approximately 0.6 and enables spectroscopic identification of the Na-UO2-CO3 complex. At nearly neutral pH values, the numbers of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted by speciation calculations.« less
  • No abstract prepared.
  • An in situ procedure for quantifying total organic and inorganic Cl concentrations in environmental samples based on X-ray absorption near-edge structure (XANES) spectroscopy has been developed. Cl 1s XANES spectra reflect contributions from all Cl species present in a sample, providing a definitive measure of total Cl concentration in chemically heterogeneous samples. Spectral features near the Cl K-absorption edge provide detailed information about the bonding state of Cl, whereas the absolute fluorescence intensity of the spectra is directly proportional to total Cl concentration, allowing for simultaneous determination of Cl speciation and concentration in plant, soil, and natural water samples. Absolutemore » Cl concentrations are obtained from Cl 1s XANES spectra using a series of Cl standards in a matrix of uniform bulk density. With the high sensitivity of synchrotron-based X-ray absorption spectroscopy, Cl concentration can be reliably measured down to the 5-10 ppm range in solid and liquid samples. Referencing the characteristic near-edge features of Cl in various model compounds, we can distinguish between inorganic chloride (Cl{sub inorg}) and organochlorine (Cl{sub org}), as well as between aliphatic Cl{sub org} and aromatic Cl{sub org}, with uncertainties in the range of {approx}6%. In addition, total organic and inorganic Br concentrations in sediment samples are quantified using a combination of Br 1s XANES and X-ray fluorescence (XRF) spectroscopy. Br concentration is detected down to {approx}1 ppm by XRF, and Br 1s XANES spectra allow quantification of the Br{sub inorg} and Br{sub org} fractions. These procedures provide nondestructive, element-specific techniques for quantification of Cl and Br concentrations that preclude extensive sample preparation.« less
  • {sup 13}C and {sup 17}O NMR and Raman spectroscopies were used to monitor the fractions of UO{sub 2}(CO{sub 3}){sub 3}{sup 4-} (1) and (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup 6-} (2) in aqueous carbonate solutions as a function of pH, ionic strength, carbonate concentration, uranium concentration, and temperature. The multinuclear NMR and Raman data are consistent with the formation of (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup 6-}. The pH dependence of the {sup 13}C NMR spectra was used to determine the equilibrium constant for the reaction 3UO{sub 2}(CO{sub 3}){sub 3}{sup 4-} + 3H{sup +}{r_equilibrium} (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup 6-} + 3HCO{submore » 3}{sup -}, log K = 18.1({plus_minus}0.5) at I{sub m} = 2.5 m and 25{degrees}C, and corresponds to log {beta}{sub 36} = 55.6({plus_minus}0.5) for the reaction 3UO{sub 2}{sup 2+} + 6CO{sub 3}{sup 2-} {r_equilibrium} (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup 6-} under the same conditions. Raman spectra showed the uranyl v{sub 1} stretching band at 831.6 cm{sup -1} for monomeric 1 and at 812.5 cm{sup -1} for trimeric (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup -6} (2). EXAFS data from solid [C(NH{sub 2}){sub 3}]{sub 6}[(UO{sub 2}){sub 3}(CO{sub 3}){sub 6}] and a solution of (UO{sub 2}){sub 3}(CO{sub 3}){sub 6}{sup 6-} suggest that the same uranium species is present in both the solid and solution states, Fourier transforms of the EXAFS spectra of both solid and solution samples revealed five well-resolved peaks corresponding to nearly identical near-neighbor distances for solid and solution samples.« less
  • The x-ray linear dichroism of the uranyl ion (UO{sub 2}{sup 2+}) in uranium {ital L}{sub 3}-edge extended x-ray-absorption fine structure (EXAFS), and {ital L}{sub 1}- and {ital L}{sub 3}-edge x-ray-absorption near-edge structure (XANES), has been investigated both by experiment and theory. A striking polarization dependence is observed in the experimental XANES and EXAFS for an oriented single crystal of uranyl acetate dihydrate [UO{sub 2}(CH{sub 3}CO{sub 2}){sub 2}{center_dot}2H{sub 2}O], with the x-ray polarization vector aligned either parallel or perpendicular to the bond axis of the linear uranyl cation (O-U-O). Single-crystal results are compared to experimental spectra for a polycrystalline uranyl acetatemore » sample and to calculations using the {ital ab} {ital initio} multiple-scattering (MS) code FEFF 6. Theoretical XANES spectra for uranyl fluoride (UO{sub 2}F{sub 2}) reproduce all the features of the measured uranyl acetate spectra. By identifying scattering paths which contribute to individual features in the calculated spectrum, a detailed understanding of the {ital L}{sub 1}-edge XANES is obtained. MS paths within the uranyl cation have a notable influence upon the XANES. The measured {ital L}{sub 3}-edge EXAFS is also influenced by MS, especially when the x-ray polarization is parallel to the uranyl species. These MS contributions are extracted from the total EXAFS and compared to calculations. The best agreement with the isolated MS signal is obtained by using nonoverlapped muffin-tin spheres in the FEFF 6 calculation. This contrasts the {ital L}{sub 1}-edge XANES calculations, in which overlapping was required for the best agreement with experiment. {copyright} {ital 1996 The American Physical Society.}« less