The Speciation of Uranium in a Smectite Clay: Evidence for Catalysed Uranyl Reduction

Abstract

X-ray absorption spectroscopy (XAS) is used to determine the uranium speciation in exchanged and surface-modified clays. XAS data from uranyl-loaded bentonite clays are compared with those obtained after the particle surfaces have been coated with alkylsilanes. These silane films, which render the surface of the clay hydrophobic, are added in order to minimize the ability of extemal water to exchange with the water in the clay interlayer, thereby decreasing the release rate of the exchanged-uranium species. Mild hydrothermal conditions are used in an effort to mimic potential geologic conditions. The uranyl monomer species remain unchanged in most samples, except for those samples that were both coated with an alkylsilane and hydrothermally treated. When the clay is coated with an organic film, hydrothermal treatment results in the aggregation of a U(IV) species. This uranyl reduction, which occurs only in the presence of both the clay and the organic coating, is compared to similar Fe and Cu reductions that occur when exchanged clays are used as catalysts for organic reactions. The implications of the uranyl reduction to environmental remediation are discussed.

The Structure of U6+ Sorption Complexes on Vermiculite and Hydrobiotite

“…The absence of a substrate dependence for the local structure of uranyl at ion-exchange sites indicates that the interstratification and higher localcharge density of hydrobiotite have little influence upon the exchange mechanism. Insensitivity to charge density is also supported by a comparison to U EXAFS studies of uranyl-exchanged smectite clays (Giaquinta et al, 1997). With the low surface-charge smectite substrate, the structural parameters measured for the first two coordination shells are very similar to those obtained here for the -1El and -IE2 samples, which used higher surface-charge minerals.…”

Interactions of aqueous U(VI) with soil minerals in slightly alkaline natural systems

“…However, with Fe(II) sorbed to a surface Liger et al 1999;Boyanov et al 2007) or as a constituent in a smectite clay (Giaquinta et al 1997) and biotite (Ilton et al 2004), U(VI) reduction proceeds faster. Sorbed or structural Fe(II) may be also present or formed when strong reductants (such as dithionite, H 2 S gas and Ca-polysulfide liquid) react with Fe(III) oxides or other soil minerals, such as phyllosilicates.…”

Characterization of Montmorillonite Surfaces After Modification by Organosilane

“…We are testing these samples for leaching performance and will report the results elsewhere. In fact, Giaquinta et al (1997) have tested this method to encapsulate uranium in different oxidation states for environmental remediation purposes. This encapsulating methodology is similar to the self-assembled monolayer technique that has been developed for formation of well-ordered monolayers on a variety of functional surfaces, such as silicon dioxide (Wasserman et al, 1989;Maoz and Sagiv, 1987), gold (Nuzzo et al, 1987;Porter et al, 1987), alumina (A1-lara and Nuzzo, 1985;Ogawa et al, 1985), and mica (Kessel and Granick, 1991).…”