Interactions of Uranium and Neptunium with Cementitious Materials Studied by XAFS

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We have investigated the interaction of U(VI) and Np(V) actinide ions with cementitious materials that are relevant to nuclear waste repositories using X-Ray Absorption Fine Structure (XAFS) Spectroscopy. The actinide ions were individually loaded onto untreated as well as hydrothermally treated cements. The mixtures were then equilibrated at varying pH's for a period of approximately 6 months. In all cases uranium was introduced in the form of aqueous uranyl ion, UO{sub 2}{sup 2+}, and was observed to remain in this form based on the Near Edge (XANES) spectra. The uranium samples show evidence of interactions with both treated and untreated ... continued below

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410 Kilobytes pages

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Sylwester, E.R.; Allen, P.G.; Zhao, P. & Viani, B.E. November 3, 1999.

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We have investigated the interaction of U(VI) and Np(V) actinide ions with cementitious materials that are relevant to nuclear waste repositories using X-Ray Absorption Fine Structure (XAFS) Spectroscopy. The actinide ions were individually loaded onto untreated as well as hydrothermally treated cements. The mixtures were then equilibrated at varying pH's for a period of approximately 6 months. In all cases uranium was introduced in the form of aqueous uranyl ion, UO{sub 2}{sup 2+}, and was observed to remain in this form based on the Near Edge (XANES) spectra. The uranium samples show evidence of interactions with both treated and untreated cements at all pH's, with uranyl interacting with the cement mineral phases (i.e., SiO{sub 2}) through an inner-sphere mechanism where oxygen atoms in the equatorial plane of the uranyl ion are shared with the mineral surface. In contact with the hydrothermally treated cement, the uranyl ions are also observed to form oligomeric species, proving that hydrothermal treatment of the concrete has a significant effect on the structural bonding characteristics of uranyl on the concrete. Neptunium was introduced as the neptunyl ion, NpO{sub 2}{sup +}, and was observed to undergo a reduction from Np(V) to Np(IV). Percent reduction was calculated from both component analysis of the XANES region and by curve fitting to the EXAFS region. Results from both methods were in good agreement and showed ca. 15% of Np(V) is reduced to Np(IV) in the fresh sample. In comparison, the other samples showed higher reduction rates of between 40% and 65%. Reduction was thus observed to occur over a relatively slow time scale based on XAFS data collected from a ''fresh'' sample (aged for 1 month). No Np-Np interactions were observed in the EXAFS spectra which makes surface precipitation of Np{sup 4+} phases an unlikely mechanism for sorption.

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410 Kilobytes pages

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  • 1999 Materials Research Society Fall Meeting, Boston, MA (US), 11/29/1999--12/03/1999

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  • Report No.: UCRL-JC-135988
  • Grant Number: W-7405-Eng-48
  • Office of Scientific & Technical Information Report Number: 791410
  • Archival Resource Key: ark:/67531/metadc739525

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  • November 3, 1999

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  • Oct. 19, 2015, 7:39 p.m.

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  • May 6, 2016, 3:59 p.m.

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Sylwester, E.R.; Allen, P.G.; Zhao, P. & Viani, B.E. Interactions of Uranium and Neptunium with Cementitious Materials Studied by XAFS, article, November 3, 1999; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc739525/: accessed January 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.