Environ. Sci. Technol. 2009, 43, 1400–1406
Identification of Uranyl Surface Complexes on Ferrihydrite: Advanced EXAFS Data Analysis and CD-MUSIC Modeling ´ R O S S B E R G , * ,† K A I - U W E U L R I C H , ‡ ANDRE STEPHAN WEISS,† SATORU TSUSHIMA,† TJISSE HIEMSTRA,§ AND ANDREAS C. SCHEINOST† Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany, Department of Energy, Environmental & Chemical Engineering, Washington University, St. Louis, Missouri 63130-4899, and Department of Soil Quality, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands
Received June 23, 2008. Revised manuscript received November 25, 2008. Accepted December 5, 2008.
Previous spectroscopic research suggested that uranium(VI) adsorption to iron oxides is dominated by ternary uranyl-carbonato surface complexes across an unexpectedly wide pH range. Formation of such complexes would have a significant impact on the sorption behavior and mobility of uranium in aqueous environments.Wethereforereinvestigatedtheidentityandstructural coordination of uranyl sorption complexes using a combination of U LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and iterative transformation factor analysis, which enhances the resolution in comparison to conventional EXAFS analysis. A range of conditions (pH, CO2 partial pressure, ionic strength) made it possible to quantify the variations in surface speciation. In the resulting set of spectral data (N ) 11) the variance is explained by only two components, which represent two structurally different types of surface complexes: (1) a binary uranyl surface complex with a bidentate coordination to edges of Fe(O,OH)6 octahedra and (2) a uranyl triscarbonato surface complex where one carbonate ion bridges uranyl to the surface. This ternary type B complex differs from a type A complex where uranyl is directly attached to surface atoms and carbonate is bridged by uranyl to the surface. Both surface complexes agree qualitatively and quantitatively with predictions by a charge distribution (CD) model. According to this model the edge-sharing uranyl complex has equatorial ligands (-OH2, -OH, or one -CO3 group) that point away from the surface. The monodentate uranyl triscarbonato surface complex (type B) is relevant only at high pH and elevated pCO2. At these conditions, however, it is responsible for significant uranyl sorption, whereas standard models would predict only weak sorption. This paper presents the first spectroscopic evidence of this ternary surface complex, which has significant implications for immobilization of uranyl in carbonate-rich aqueous environments. * Corresponding author phone: +33 476 88 2847; fax: +33 476 88 2525, e-mail:
[email protected]. † Forschungszentrum Dresden-Rossendorf. ‡ Washington University. § Wageningen University. 1400
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 5, 2009
Introduction Due to their ubiquity in soils and sediments and high specific surface area, ferric hydroxides (Fe oxides) may limit more than any other mineral group the migration behavior of uranium (1). A thorough understanding of the sorption processes to Fe oxides and their proper implementation in surface complexation models is hence mandatory for the risk assessment of, e.g., uranium mining sites and nuclear waste repositories (2). The selection of surface complexation reactions and their corresponding thermodynamic constants should preferably be based on structural data gained from spectroscopic methods. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been proven as one of the most powerful techniques for structural investigation of aqueous and surface complexes. However, if a mixture of compounds is given where the X-ray absorbing atom occurs in different chemical environments, the shell-fitting approach is often unable to distinguish between the different species due to the inherently poor spatial and elemental resolution of backscattering neighbor atoms and the almost complete lack of symmetry information in bulk EXAFS. For the adsorption of uranium(VI) to Fe oxides, different surface complexes have been proposed to occur depending on pH, partial pressure of CO2(g) (pCO2), and Fe oxide mineral: a binary edge-sharing complex for ferrihydrite (nominal stoichiometry Fe5HO8 · 4H2O) (3, 4) and goethite (R-FeOOH) (5, 6), a double-corner sharing complex for goethite (7) and hematite (R-Fe2O3) (8), and finally ternary edge-sharing, mono-, bis-, and (dimeric) tris-carbonato complexes for hematite (8, 9). In air (pCO2 ≈ 37.5 Pa) the adsorption maximum of U(VI) is in the pH range 5-9 (3, 10). Beyond that range, U(VI) adsorption is limited by electrostatic repulsion: at pH < 5 by repulsion between positively charged uranyl and the protonated Fe oxide surface and at pH > 9 between negatively charged U(VI) carbonato complexes and the hydroxylated surface, which forms above the zero point of net charge (pHpzc = 8.5 for ferrihydrite) (11). When pCO2 increases to 1013 Pa (1% vol), a value not uncommon in subsurface environments, U(VI) adsorption declines already at pH < 8 due to the increasing concentration of aqueous uranyl carbonato complexes while the adsorption edge between pH 4 and 5 remains unaffected (12). Consistent with aqueous speciation, where carbonato complexes are absent below pH 5 even at ambient pCO2 (13), only the binary edge-sharing uranyl complex was observed by several authors in the pH range 4-5 irrespective of whether the experiment was performed in equilibrium with air or in a CO2(g)-free glovebox (3-6). In contrast to these findings, formation of ternary, mono- and biscarbonato, edge-sharing surface complexes has been proposed at pH values as low as 4.6 based on EXAFS, electrophoretic mobility, and IR spectroscopy (8, 14). EXAFS evidence for the occurrence of uranyl carbonato complexes is based on a peak in the Fourier transform at 2.3-2.4 Å (uncorrected for phase shift; in the following referred to as the 2.4 Å peak). Since this peak was also observed in experiments where carbonate was excluded, its interpretation as uranyl carbonato group is highly controversial. Walter et al. suspected that CO2(g) impurities were still present and fit C atoms with a U-C distance of ∼2.9 Å to this peak consistent with a bidentate coordination of carbonate groups to the uranyl equatorial plane (5). Redden et al. fitted this peak with Cl atoms at 2.85 Å, although Cl coordination to uranyl should be negligible at the given [Cl-] of 0.1 mol/L (6, 13, 15). Finally, in our recent study on uranyl sorption to ferrihydrite we observed 10.1021/es801727w CCC: $40.75
2009 American Chemical Society
Published on Web 01/23/2009
TABLE 1. Experimental Conditions of the U(VI) Sorption Experiments on Ferrihydriteb sample 1 2 3 4 5 6 7 8b 9 10 11
pCO2 (Pa)
