PNNL

Abstract

Sorption of contaminants onto mineral surfaces is an important process that can restrict their transport in the environment. In the current study, uranium (U) uptake on magnetite (111) was measured as a function of time and solution composition (pH, [CO3]T, [Ca]) under continuous batch-flow conditions. We observed, in real-time and in situ, adsorption and reduction of U(VI) and subsequent growth of UO2 nanoprecipitates using atomic force microscopy (AFM) and newly developed batch-flow U LIII-edge grazing-incidence x-ray absorption spectroscopy near-edge structure (GI-XANES) spectroscopy. U(VI) reduction occurred with and without CO3 present, and coincided with nucleation and growth of particles; maximum sorption loadings were 23 ?mol m-2 (pH 5) and 27 ?mol m-2 (pH 10). The U sorption loading was lower when Ca and CO3 were both present and during experiments in which no U(VI) reduction occurred; the maximum U sorption loading was 17 ?mol m-2 (pH 5 and 10). In situ batch-flow AFM data indicated that UO2 particles achieved a maximum height of 4-5 nm after about 8 hours of exposure, yet lateral growth as aggregates continued up to 300 nm. U uptake is therefore divided into three-stages; (1) initial adsorption of U(VI), (2) reduction of U(VI) to UO2 nanoprecipitates at surface-specific sites after 2-3 hours of exposure, and (3) completion of U(VI) reduction after 6-8 hours, with continuing slow adsorption of U(VI). U(VI) reduction also corresponded to detectable increases in Fe released to solution and surface topography changes, indicating that reduction is coupled to Fe(II) availability at or from the magnetite (111) surface. In addition to providing molecular-scale details about U sorption on magnetite, this work also presents novel advances for collecting surface sensitive molecular-scale information in real-time under batch-flow conditions.