Abstract:
Evaluation of the long-term health risks of uranium contamination in soils, sediments, and groundwater requires a fundamental understanding of the various processes affecting subsurface transport of uranium, including adsorption processes at mineral/water interfaces. In this study, the sites of binding and surface complexation of U(VI) adsorbed on the (11̄02) surfaces of α-Al2O3 and α-Fe2O3 have been determined using crystal truncation rod (CTR) diffraction and grazing incidence extended X-ray absorption fine structure (GI-EXAFS) spectroscopy. The available binding sites on the (11̄02) surfaces were constrained through bond valence and steric analyses. On both surfaces, U(VI) forms uranyl-carbonato ternary complexes to surface oxygens that are singly coordinated to aluminum or iron. On the α-Al2O3 (11̄02) surface, a monodentate complex results, whereas on the α-Fe2O3 (11̄02) surface, the binding is bidentate to adjacent singly coordinated oxygen sites (i.e., binuclear). Differences in protonation of the singly coordinated oxygen atoms, surface charging, U(VI) aqueous speciation, substrate structure, or the electronic structure of surface functional groups may be the cause of these differences in adsorption geometry. Both XPS and CTR diffraction reveal higher U(VI) surface coverages on the α-Fe2O3 (11̄02) surface than on the α-Al2O3 (11̄02) surface. This difference cannot be the result of differences in defect concentration alone as CTR diffraction is not sensitive to U(VI) sorbed to defect sites, implying that the α-Fe2O3 (11̄02) surface has an intrinsically higher affinity for U(VI). The surface complexes observed in this study are different from the bidentate, mononuclear complexes typically derived for U(VI) on powdered aluminum- and iron-(oxyhydr)oxides and clay minerals using U LIII-edge EXAFS spectroscopy. However, the presence of monodentate, mononuclear and bidentate, binuclear complexes may have been overlooked in past EXAFS studies on such substrates, as these complexes have U-Al or U-Fe interatomic distances that are too large to be easily detected by EXAFS spectroscopy.
