Carbonate effects on surface speciation of uranyl at the ferrihydrite-water interface: New insights from advanced EXAFS data analysis

The assessment of uranium migration requires the proper use of geochemical specition and reactive transport models due to the complexity of reactions and phase interactions. In the past decades the main focus has shifted from improving the modelling algorithms to a proper formulation of reaction mechanisms and the validation of thermodynamic data [1]. Thermodynamic constants should only be used if the corresponding reaction mechanism is supported by structural data (spectroscopic data, or theoretical calculations) [1]. This recommendation assumes that the spectroscopic studies provide reliable structural models. However, a comparison of recent literature on U(VI)-iron(hydr)oxide sorption complexes showed that XAS data enable a variety of interpretations and structural assemblies. Most data analysis routines do not address the multivariate character of such systems. The EXAFS spectra of the aqueous system U(VI)/ferrihydrite(Fh)/carbonate show a strong dependence on pH and CO2 partial pressure (pCO2). We used advanced multi-factorial analysis such as ITFA (Iterative Target Transformation Factor Analysis) [2] for the extraction of end-member spectra from the EXAFS spectral mixtures; and MCTFA (Monte Carlo Target Transformation Factor Analysis) [3] for the 3-dimensional structure analysis of the sorption complexes. We found that the whole set of EXAFS spectra derived from the experimental system U(VI)/Fh/pH/pCO2 supports only two structurally different sorption complexes. The two isolated species are: (i) a binary inner-sphere U(VI)-Fh sorption complex in which U(VI) shares the edge of an FeO6 octahedron, and (ii) an outer-sphere U(VI)-carbonato complex. The structure of the outer-sphere complex matches the structure of the aqueous UO2(CO3)34- complex. For the 3-dimensional analysis of the UO2(CO3)34- complex we included multiple scattering calculations in the MCTFA algorithm. By this combination the radial pair distribution functions and the spatial disorder of the backscattering atoms were determined at the first time for an isolated aqueous metal complex. We compare the structure of the UO2(CO3)34- complex and the spatial atomic disorder with quantum chemical calculations. The EXAFS spectra of the binary inner-sphere complex are more difficult to interprete. A Fourier Transform (FT) peak at 2.4 Å seems to be present in many U(VI) systems and may disturb the analysis of the EXAFS data. Up to now, this feature was either neglected, or attributed to C-atoms at a radial distance of 2.9 Å. Based on this latter interpretation, ternary U(VI)-iron(hydr)oxide carbonato complexes have been proposed even at pH as low as 4.6, where carbonato complexes are highly unlikely according to thermodynamic considerations [4]. Implications for the modelling of sorption data are manyfold. For example, Lenhart et al. (1999) [5] modelled the sorption of U(VI) onto hematite under atmospheric conditions and found that their sorption edges can be explained without the inclusion of inner-sphere ternary carbonato complexes as proposed by Bargar et al. (2000) [4]. However, Lenhart et al. (1999) [5] followed the general recommendation that the reaction mechanism should be supported by structural data [1], hence included a ternary U(VI)-iron(hydr)oxide carbonato complex into their sorption model. We present a tentative structural model for the binary inner-sphere complex and discuss the origin of the FT peak at 2.4 Å.

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