Identification of Uranyl Surface Complexes on Ferrihydrite: Advanced EXAFS Data Analysis and CD-MUSIC Modeling

Abstract

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. We therefore reinvestigated the identity and structural 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 complexwith 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 pC0O. 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.

Vanadate complexation to ferrihydrite: X-ray absorption spectroscopy and CD-MUSIC modelling

“…[43] There may be a similar difference for uranium(VI), as a mixture of edge-sharing and corner-sharing complexes is observed on goethite, whereas edge-sharing complexes predominate on ferrihydrite. [44][45] Further, arsenite is bound only as a corner-sharing bidentate complex on goethite, but partly also as an edge-sharing bidentate complex on ferrihydrite. [46] Arsenate, however, is bound predominantly as corner-sharing complexes on both ferrihydrite and goethite.…”

Resonant x-ray spectroscopy of uranium intermetallics at the M4,5 edges of uranium

“…The quantitative empirical analysis for the 5f electron count (n f ) was performed using the HERFD spectrum at the U M 4 edge of URu 2 Si 2 by an iterative transformation factor analysis program 39,40 , which has been successfully applied to the studies of the actinides by the extended Xray absorption fine structure (EXAFS) technique. In the present paper, the fractions of the 5f 2 and 5f 3 configurations in the U M 4 edge spectrum of the URu 2 Si 2 sample have been derived.…”

<i>In</i>-<i>Situ</i> XAFS Characterization of PtPd Nanoparticles Synthesized by Galvanic Replacement

“…The evolution of the in-situ XAFS spectra was studied using a program developed by A. Rossberg, A. C. Scheinost et al [19] [20], permitting the analysis of the XAFS spectra via the Iterative Transformation Factor Analysis (ITFA) with a calculus of Varimax and Iterative Target Test (ITT). The first step of the ITFA is the Principal Component Analysis (PCA) [21].…”