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Title: EXAFS studies of uranium sorption on layer-silicate minerals

Abstract

The local structure of uranium sorbed on mineral surfaces was investigated by uranium L{sub 3}-edge EXAFS. Solutions of uranyl chloride, UO{sub 2}Cl{sub 2}, were exposed to vermiculite, an expansible layer silicate mineral, under conditions which favor sorption by either cation exchange or surface complexation. EXAFS of the resulting mineral samples indicates a larger distortion of the uranyl equatorial shell for cation exchange, possibly due to steric effects of interlayer sorption. The uranyl U-O axial bond distance is greater for surface complexation than for ion exchange. Uranyl sorption on talc and pyrophyllite, layer silicate minerals with essentially no cation-exchange capacity, gives results which generally support the trends for surface complexation on vermiculite.

Authors:
; ;  [1]
  1. Lawrence Livermore National Lab., CA (United States) [and others
Publication Date:
OSTI Identifier:
214985
Report Number(s):
CONF-950801-
TRN: 96:000922-0480
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: 210. national meeting of the American Chemical Society (ACS), Chicago, IL (United States), 20-25 Aug 1995; Other Information: PBD: 1995; Related Information: Is Part Of 210th ACS national meeting. Part 1 and 2; PB: 1866 p.
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; 66 PHYSICS; URANIUM; SORPTION; ION EXCHANGE; PYROPHYLLITE; SILICATE MINERALS; VERMICULITE; URANYL CHLORIDES; BOND LENGTHS; MOLECULAR STRUCTURE; ENVIRONMENT

Citation Formats

Hudson, E.A., Terminello, L.J., and Viani, B.E.. EXAFS studies of uranium sorption on layer-silicate minerals. United States: N. p., 1995. Web.
Hudson, E.A., Terminello, L.J., & Viani, B.E.. EXAFS studies of uranium sorption on layer-silicate minerals. United States.
Hudson, E.A., Terminello, L.J., and Viani, B.E.. 1995. "EXAFS studies of uranium sorption on layer-silicate minerals". United States. doi:.
@article{osti_214985,
title = {EXAFS studies of uranium sorption on layer-silicate minerals},
author = {Hudson, E.A. and Terminello, L.J. and Viani, B.E.},
abstractNote = {The local structure of uranium sorbed on mineral surfaces was investigated by uranium L{sub 3}-edge EXAFS. Solutions of uranyl chloride, UO{sub 2}Cl{sub 2}, were exposed to vermiculite, an expansible layer silicate mineral, under conditions which favor sorption by either cation exchange or surface complexation. EXAFS of the resulting mineral samples indicates a larger distortion of the uranyl equatorial shell for cation exchange, possibly due to steric effects of interlayer sorption. The uranyl U-O axial bond distance is greater for surface complexation than for ion exchange. Uranyl sorption on talc and pyrophyllite, layer silicate minerals with essentially no cation-exchange capacity, gives results which generally support the trends for surface complexation on vermiculite.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1995,
month =
}

Conference:
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  • The local geometry of uranium bound to mineral surfaces was investigated by uranium L{sub 3}-edge EXAFS. Uranium(6+) in solution was exposed to vermiculite and clinoptilolite, under conditions which favor sorption by either cation exchange or surface complexation. The aqueous speciation of uranium in each solution was predicted using the thermodynamic modeling code EQ3/6, and is dominated by complexes of the uranyl ion (UO{sub 2}{sup 2+}). EXAFS of the resulting mineral samples indicates that in all cases the sorbed species is a monomeric uranyl complex. A larger distortion of the uranyl equatorial shell is observed for cation exchange, possibly due tomore » steric effects at internal cation exchange sites. The uranyl U-O axial bond distance is greater for surface complexation than for ion exchange.« less
  • Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to investigate the reduction of aqueous Cr(VI) to Cr(III) in magnetite-bearing soils from Cr-contaminated sites. Soils from two field sites were examined, showing that mixed-valence Cr(III/VI) effluent is reduced to Cr(III) when associated with the magnetite fraction of the soil, whereas the Cr effluent associated with non-Fe(II)-bearing minerals results in mixed Cr(III/VI) adsorbates or precipitated phases. The Fe{sup 2+} in magnetite, Fe{sup 2+}Fe{sub 2}{sup 3+}O{sub 4}, may act as an electron source for heterogeneous Cr(VI)-to-Cr(III) reduction, converting magnetite topotactically to maghemite, {gamma}-Fe{sub 2}{sup 3+}. The ratio of Cr(VI)/total Cr was determinedmore » by the height of the Cr(VI) XAFS pre-edge feature, which is due to a Is to 3d electronic transition. This pre-edge feature was calibrated as a function of Cr(VI)/Cr(III) using mixtures of Cr(III) and Cr(VI) model compounds. Environmental remediation of Cr-contaminated sites requires knowledge of chromium oxidation and speciation, and XAFS spectroscopy may be used to supply both types of information with minimal sample processing or data analysis. 36 refs., 9 figs., 2 tabs.« less
  • Sodium silicate glasses containing dissolved Th, U, Np, and Pu have been studied using the EXAFS technique. Th/sup 4 +/, U/sup 4 +/, Np/sup 4 +/, and Pu/sup 4 +/ ions in the silicate glasses are 8-fold coordinated to oxygen neighbors. The higher valent U/sup 6 +/ and Np/sup 5 +/ ions have complex local symmetries. The U/sup 6 +/ ions appear in a uranyl configuration with 2 oxygen atoms at 1.85A and 4 at 2.25A from the U ion. The Np/sup 5 +/ local symmetry is more complex and difficult to determine uniquely. The U/sup 6 +/ glasses showmore » substantial clustering of the uranium atoms. A structural model, with nearly planar uranyl sheets sandwiched between alkali and silica layers, is used to explain the U/sup 6 +/ EXAFS data. This model allows us to understand why U/sup 6 +/ ions are much more soluble in the glasses than the actinide 4/sup +/ ions. 4 references, 2 figures.« less
  • Several actinide-mineral sorption systems were studied by uranium and thorium L{sub 3}-edge x-ray absorption spectroscopy. A series of layer silicate minerals, including micas, were selected for their systematic variations in surface structure, e.g. degree of permanent negative charge on the basal planes. An expansible layer silicate, vermiculite, was treated to provide several different interlayer spacings, allowing variations in the accessibility of interior cation exchange sites. The finely powdered minerals were exposed to aqueous solutions of uranyl chloride or thorium chloride. Analysis of the EXAFS and XANES spectra indicates the influence of the mineral substrate upon the local structure of themore » bound actinide species. Trends in the data are interpreted based upon the known variations in mineral structure.« less
  • Microautoradiography was used to evaluate the mineralogic basis of Pu(V) retention by tuffs from Yucca Mountain, Nevada. Altered orthopyroxenes and oxide minerals are associated with high Pu retention but are limited to specific stratigraphic horizons. A weaker but more general association of Pu with smectite occurs in most samples. Thin-sections that cross fractures allow comparative studies of Pu retention by fracture-lining versus matrix minerals. Using Ag metal in emulsions as a measure of underlying Pu concentration, electron-microprobe analysis can quantify Pu retention along fracture walls and provide mineral/mineral Pu retention factors. For smectite-lined microfractures in zeolitized tuff, the smectite/clinoptilolite Pumore » retention factor is >80.« less