Molecular-scale structures of uranyl surface complexes on hematite facets

Abstract

As the fate, transport and bioavailability of U(VI) in subsurface environments are strongly influenced by its adsorption structures on iron minerals such as hematite, we systematically studied the molecular-scale structures of U(VI) complexes formed at the interfaces of hematite and water with periodic density-functional theory (DFT) calculation, extended X-ray absorption fine structure (EXAFS) measurements, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy dispersive spectroscopy (XEDS). The combined theoretical and experimental results revealed that U(VI) was complexed on three hematite facets in an inner-sphere coordination, but edge-sharing bidentate mononuclear (²E) complexes were formed on {001} facets, and corner-sharing bidentate binuclear (²C) ones were on both {012} and {110} facets. Moreover, the U(VI) adsorption site densities on the {012} and {110} facets of hematite were both about 0.32 #U nm⁻², significantly higher than the adsorption site density (0.18 #U nm⁻²) on the {001} counterpart, which was consistent with the results of STEM-XEDS quantification. The results suggest that the U(VI) adsorption performance with hematite was strongly dependent on the coordination environment of U(VI) on the hematite facets. This study clarifies the molecular-scale structures of the U(VI) surface coordination on different hematite facets and also provides new insights into predicting the long-term fate and transport of highly radiotoxic actinyl ions in subsurface environments.