Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids

To investigate the molecular structure of U(VI) species associated with colloids forming by the gradual flooding of abandoned uranium mines, we mixed acid mine water rich in Fe(II/III) and U(VI) with near-neutral groundwater at oxic conditions. Iron K-edge and U LIII-edge Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy, and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy were applied to elucidate the structure of the surface complexes at the molecular level. Since the resulting colloidal bulk consisted mainly of 2-line ferrihydrite, we tested by simplified batch experiments the influence of other matrix ions such as sulfate, silicate, and carbonate on the U(VI) sorption structure at the ferrihydrite surface.
In all samples, U(VI) was bound to ferrihydrite by formation of a mononuclear, edge sharing inner-sphere complex. Monte Carlo simulation yielded a slightly tilted orientation of the UO22+ pentagonal bipyramid relative to the edge sharing Fe(O,OH)6 octahedron. This tilted orientation allows to attribute a previously unexplained EXAFS peak at a distance of ~2.87 Å to a third O-atom of the Fe octahedra at the ferrihydrite surface. The experimental results allow us to reject formation of ternary U(VI) sulfate, silicate, and carbonate surface complexes as source of this peak. Furthermore, both spectroscopic approaches do not give convincing evidence for the formation of ternary U(VI) carbonato surface complexes at pH 5.5 and pH 8.0 and U(VI) concentrations of ≤ 0.012 mM. At U(VI) concentrations of ≥ 0.05 mM, however, ATR-FTIR spectra show an increased splitting of the symmetric and antisymmetric carbonate stretching modes due to a frequency shift which can be assigned to a bidentate linkage of carbonate to the adsorbed UO22+, hence suggesting formation of a ternary surface complex at these experimental conditions. At the higher U(VI) concentrations, the formation of ternary complex species and changes of the coordination may cause a decrease in U(VI) binding strength with increasing carbonate concentration. These effects should be considered in hydrogeochemical scenarios of elevated pCO2 conditions prevalent in many groundwater aquifers.