The Structure of U⁶⁺ Sorption Complexes on Vermiculite and Hydrobiotite

Abstract

Abstract--The sorption of the uranyl oxo-cation (UOz2+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized Lt-and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Synthesis of amidoximated polyacrylonitrile fibers and its application for sorption of aqueous uranyl ions under continuous flow

“…ions; however, competitive sorption with protons occurs and ion-exchange interactions are favored [6,7]. Protonation can occur on the imino group of amidoxime (pKa 5.95) [32], leading to a decrease in the chelating ability.…”

“…Several works related to uranium sorption onto naturally occurring clay minerals and soil fractions have been performed. Hudson et al [6] examined vermiculate and hydrobiotite, and Sylwester et al [7] examined silica, alumina, and montmorillonite as sorbents. Both studies investigated the pH dependence of uranyl oxo-cation (UO 2þ 2 ) sorption and determined the local structure around the sorbed uranium atom.…”

Thermal properties of sodium-exchanged palabora vermiculite

“…(Brindley et al, 1983). However, the name "hydrobiotite" is not internationally recognized as a valid mineral name, although it has long been used in literature and associated with interstratified biotite/vermiculite minerals found in commercial vermiculite deposits (Boss, 1967;van der Marel and Beutelspacher, 1976;Brindley et al, 1983;Newman, 1987, p. 106;Hudson, 1999;Zhu et al, 2008). Mixed-structure clay minerals with any appreciable degree of randomness show a seemingly irrational series of peaks that makes their identification and interpretation quite a complex exercise.…”

Determination of Uranium and Mercury Speciation in High Level Waste Tank 8F and 11H Sludge

“…The XAFS techniques have been applied to the study of U(VI) on a variety of sorbents, such as Fe oxides, silicates and sulfides 14,15,16,17,18,19,20,21,22 in HLW simulant solutions and within mineral oxides, calcite, oxyhydroxides and gels. 23,24,25,26,27 Most of these studies have an environmental focus because they were performed with naturallyoccurring minerals and U-containing minerals under conditions relevant to the geologic surface and subsurface environments.…”