Speciation of Colloid-borne Uranium by EXAFS and ATR-IR spectroscopy

Molecular speciation is a prerequisite for reliable assessment of contaminant migration in the environment. We use sophisticated techniques for concentrating colloids (e.g. ultracentrifugation, ultrafiltration) in combination with Extended X-ray Absorption Fine Structure (EXAFS) and Attenuated Total Reflectance Infrared (ATR IR) spectroscopy to investigate the speciation of colloid-borne uranium in waters which occur in abandoned ore mines. Mine flooding was simulated in a 100 L scale by mixing acid mine water of elevated U concentration and near-neutral groundwater from an aquifer above the mine until pH~5.5 was reached. The generated colloids adsorbed 95% of the total uranium and consisted mainly of 2-line ferrihydrite (Fh) besides traces of aluminum, sulfur, sil-ica, and carbon compounds. EXAFS analysis at the U-LIII absorption edge suggested a bidentate surface complex of UO22+ on FeO6 octahedra, but two minor backscattering contributions in the vicinity of the absorber remained unexplained. Since only Al could be excluded as backscattering atom, we studied U sorption on Fh at pH 5.5 in presence and in absence of sulfate, silicate, and atmospheric CO2 to clarify the bond structure.

EXAFS showed the unknown backscattering contributions in all the sorption samples regardless of the presence or absence of the tested components. Contrary to structural models in the literature, bidentately complexed carbonate ligands cannot explain the results when using U concentrations around 0.1 mM. But ATR-IR spectra showed that U(VI) carbonato complexes must be involved in the sorption of uranyl on Fh. These results are not contradictory if the carbonate ligands were bound monodentately, which is currently being studied. Nevertheless, carbon cannot act as backscattering atom in carbonate-free samples prepared in N2 atmosphere. Monte-Carlo Target Transform Factor Analysis was employed to test if the EXAFS spectra could be fitted by a struc-ture including exclusively Fe, U, and O atoms. We propose a new model in which the bidentately bridged UO22+ is oriented in a way that yields a distance of ~2.9 Å to the O atom of an adjacent, edge-shared FeO6 octahedron. This model predicts a second Fe shell at ~4.35 Å which tightly fits the experimental data.

Summarizing, uranium may form different sorption complexes with colloidal Fh: a binary bidentate uranyl complex with modified orientation, and ternary U carbonato complexes with monodentate linkage of the carbonate ligands, depending on specific conditions.