Structure and composition of U(VI) sorption complexes at the kaolinite-water interface

Conference · Tue Oct 01 04:00:00 EDT 1996

OSTI ID:370493

Thompson, H A; Parks, G A; Brown, Jr, G E ^[1]

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1. Stanford Univ., CA (United States)

We have investigated the nature of U(VI) sorption by kaolinite by applying x-ray absorption spectroscopy (XAS) to batch sorption samples (2-10{sup -5} M total U; 0.5 g kaolinite-L{sup -1}; 1.5 mM NaNO{sub 3}; pH 6.0-8.0; presence and absence of ambient CO{sub 2}). Under these conditions, XAS shows that the U sorption complex always contains a uranyl moiety (UO{sub 2}{sup 2+}) surrounded by 5 equatorial O atoms. Si (and/or Al) is present as a second neighbor under most of the conditions studied, indicating that U is bonded in an inner-sphere fashion to kaolinite. Our and others uptake measurements demonstrate that carbonate reduces U uptake by kaolinite at high pH, suggesting that carbonate competes with the surface for U coordination. Our XAS spectra show that the presence of carbonate does not appear to alter the mode of bonding to the surface, but at pH 8, the predominant sorption complex is multinuclear when carbonate is absent and mononuclear when carbonate is present. These observations suggest that carbonate also competes with other uranyl ions for coordination of the sorbed uranyl ion at pH 8. At lower pH (8.0 > pH {>=} 7.0), carbonate competes less effectively, as demonstrated by the predominance of multinuclear sorption complexes in the presence of carbonate. At still lower pH (7.0 > pH {>=} 6.0), mononuclear sorption complexes dominate, suggesting reduced competition by carbonate and other uranyl ions for U coordination.

• OSTI ID: 370493
• Report Number(s): CONF-960376--
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ABSORPTION SPECTROSCOPY
INTERFACES
KAOLINITE
PH VALUE
SOILS
SORPTION
SORPTIVE PROPERTIES
SPECTRA
URANIUM
URANYL COMPOUNDS