Unraveling pH-dependent changes in adsorption structure of uranyl on alumina (012)

Mitigating the transport of uranium in groundwater is imperative for ensuring access to clean water across the globe. Here, in-situ resonant anomalous X-ray reflectivity is used to investigate adsorption of uranyl on alumina (012) in acidic aqueous solutions ([UVI] = 0.01–1 mM, pH 2–5), representing typical compositions of highly-contaminated water near U mining sites. Significant UVI uptake (0.18–0.27 μmol/m2) is observed even though the surface has a net positive charge at these conditions. The analyses reveal that adsorbed U is distributed at two distinct heights of 2.4–3.2 Å and 5–5.3 Å, respectively, from the terminal surface oxygens. The former species is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter species is interpreted as an outer-sphere complex that shares one equatorial H2O molecule with the terminal surface oxygen. With increasing pH, we observe increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species at the interface. The presented work provides an improved molecular-scale understanding of the sorption of uranyl on alumina that has implications for geochemistry and materials science.