Uranyl sorption species at low coverage on Al-hydroxide: TRLFS and XAFS studies |
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| Authors: | A Froideval C Gaillard I Rossini |
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| Institution: | a Institut Pluridisciplinaire Hubert Curien, Département de Recherches Subatomiques, ULP / CNRS / IN2P3, UMR 7500, 67037 Strasbourg Cedex 2, France
b Laboratoire de Cristallographie, CNRS, UPR 5031, 38042 Grenoble Cedex 9, France |
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| Abstract: | Detailed understanding of the respective roles of solution and surface parameters on the reactions at uranyl solution/Al-(hydr)oxide interfaces is crucial to model accurately the behaviour of U in nature. We report studies on the effects of the initial aqueous uranyl speciation in moderately acidic solutions, e.g. of mononuclear, polynuclear uranyl species and/or (potential) U(VI) colloids, on the sorption of U by large surface areas of amorphous Al-hydroxide. Investigations by Extended X-ray Absorption Fine Structure (EXAFS) and Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) reveal similar U coordination environments on Al-hydroxide for low to moderate U loadings of sorption samples obtained at pH 4-5, independently of the presence of mononuclear or polynuclear aqueous species, or of the potential instability of initial solutions favoring true U-colloids formation. EXAFS data can be interpreted in terms of a dimeric, bidentate, inner-sphere uranyl surface complex as an average of the U surface structures. TRLFS data, however, provide valuable insights into the complex U surface speciation. They indicate multiple uranyl surface species under moderately acidic conditions, as predominant mononuclear and/or dinuclear, inner-sphere surface complexes and as additional minor species having U atoms in a uranyl (hydr)oxide-like coordination environment. The additional species probably occur as surface polymers and/or as adsorbed true U colloids, depending on the aqueous U concentration level (1-100 μM). These results are of importance because they suggest that Al-hydroxide surface characteristics strongly control uranyl surface species in moderately acidic systems. |
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