Spectroscopic confirmation of uranium(VI)-carbonato adsorption complexes on hematite

Evaluating societal risks posed by uranium contamination from waste managem ent facilities, mining sites, and heavy industry requires knowledge about u ranium transport in groundwater, often the most significant pathway of expo sure to humans. It has been proposed that uranium mobility in aquifers may be controlled by adsorption of U(VI)-carbonato complexes on oxide minerals. The existence of such complexes has not been demonstrated, and little is k nown about their compositions and reaction stoichiometries. We have used at tenuated total reflectance Fourier transform infrared (ATR-FTIR) and extend ed X-ray absorption fine structure (EXAFS) spectroscopies to probe the exis tence, structures, and compositions of=FeOsurface-U(VI)-carbonato complexes on hematite throughout the pH range of uranyl uptake under conditions rele vant to aquifers. U(VI)-carbonato complexes were found to be the predominan t adsorbed U(VI) species at all pH values examined, a much wider pH range t han previously postulated based on analogy to aqueous U(VI)-carbonato compl exes;, which are trace constituents at pH < 6. This result indicates the in adequacy of the common modeling assumption that the compositions and predom inance of adsorbed species can be inferred from aqueous species. By extensi on, adsorbed carbonate complexes may be of major importance to the groundwa ter transport of similar actinide contaminants such as neptunium and pluton ium.