Uranyl sorption species at low coverage on Al-hydroxide: TRLFS and XAFS studies

Abstract

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

Uranyl and arsenate cosorption on aluminum oxide surface

“…4 with their shell-by-shell fitting results in Table 2. EX-AFS fit results for sample U-2-0.1-pH4 include a split equatorial oxygen shell with $3.7 O eq at $2.34 Å and $2.4 O eq at $2.48 Å , similar to the EXAFS results from previous work on uranyl sorption on alumina (e.g., Sylwester et al, 2000;Froideval et al, 2006), and can be attributed to the formation of an inner-sphere sorption complex. In addition to a similar split O eq shell (fit as $2.3 O eq at $2.36 and $1.5 O eq at $2.51 Å ), sample U-10-1-pH9 can also be fit with $1.7 U at $3.91 Å , suggesting the formation of uranyl polymeric species or (oxy)hydroxide precipitates at this higher concentration and pH (Sylwester et al, 2000;Froideval et al, 2006).…”

“…U EXAFS of sample cosp-2-0.4-0.2-pH6 shows a uniform O eq distance as well as As backscattering at $3.7 Å , suggesting uranyl arsenate precipitates as the predominant species, consistent with corresponding As EXAFS results. Previous studies of uranyl sorption on alumina at this pH range suggest the formation of a mononuclear inner-sphere sorption complex and possibly minor amounts of surface-sorbed or precipitated polynuclear uranyl species depending on solution concentrations (Sylwester et al, 2000;Froideval et al, 2006). Although no significant sorption enhancement was observed, our results reveal possible enhanced structural stability because of the formation of uranyl arsenate precipitates at this pH range, which are more stable and less susceptible to desorption and remobilization than surface-sorbed uranyl species.…”

“…In addition to the formation of mononuclear/binuclear inner-sphere sorption complex(es) when sorbed onto mineral surfaces, uranyl is also known to form polymeric species at neutral to basic pH ranges or at high concentrations (e.g., Sylwester et al, 2000;Kowal-Fouchard et al, 2004;Baumann et al, 2005;Froideval et al, 2006). Therefore, parallel experiments were carried out with different U concentrations and corresponding alumina loading and As concentrations.…”

“…Sorption onto mineral surfaces is one of the most important means in retarding uranyl mobility (e.g., Duff et al, 2002). Numerous studies have looked at uranyl sorption onto different geological materials, such as carbonate minerals (Geipel et al, 1997;Elzinga et al, 2004), iron oxyhydroxides (Hsi and Langmuir, 1985;Ho and Miller, 1986;Duff and Amrhein, 1996;Giammar and Hering, 2001;Missana et al, 2003), manganese oxyhydroxides (Han et al, 2007) and aluminum oxyhydroxides (Prikryl et al, 1994;Sylwester et al, 2000;Froideval et al, 2006;Pandey, 2006).…”

“…Uranyl has been shown to sorb moderately on aluminum oxides (Prikryl et al, 1994;Sylwester et al, 2000;Froideval et al, 2006;Pandey, 2006), depending on solution pH and U concentration. Phosphate and arsenate adsorb strongly to alumina at acidic to neutral pH values (e.g., Arai et al, 2001;Goldberg and Johnston, 2001;Guo et al, 2005).…”