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International audienceDetailed 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

The complexation of uranyl ion by the bacterial siderophore pyoverdine A in aqueous solution has been monitored with the aid of UV-visible spectroscopy. The complexing molecule was employed either in intact form or in a partially modified state by the action of laser illumination prior to complexation. In both cases, models were developed which took into account hydrolysis of uranium and permitted an excellent fit with the experimental data. The complexation constants are AT, = (8.2±0.5) · 10" 4 M in the case of unmodified pyoverdine and K\ = (2.0±0.3) · ΙΟ" 3 M for partially degraded pyoverdine.