Interactions of Uranium and Neptunium with Cementitious Materials Studied by XAFS

Conference · Wed Nov 03 04:00:00 EST 1999

DOI:https://doi.org/10.1557/PROC-608-307· OSTI ID:791410

Sylwester, E R ^[1]; Allen, P G; Zhao, P; Viani, B E

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1. LLNL

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.

• Research Organization: Lawrence Livermore National Lab., Livermore, CA (US)
• Sponsoring Organization: USDOE Office of Defense Programs (DP) (US)
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 791410
• Report Number(s): UCRL-JC-135988
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ABSORPTION
ACTINIDES
ATOMS
BONDING
CEMENTS
CONCRETES
FINE STRUCTURE
MIXTURES
NEPTUNIUM
OXYGEN
PRECIPITATION
RADIOACTIVE WASTES
SORPTION
SPECTRA
SPECTROSCOPY
URANIUM