IDENTIFICATION OF REDUCED-U NANOPRECIPITATES ON THE MAGNETITE (111) SURFACE

Uranyl uptake by magnetite is likely dominated by three interactions: (1) formation of surface complexes at [Fe(O,OH)₆] sites; (2) formation of surface complexes and/or coprecipitation with ferrihydrite formed by the release of Fe(II) and subsequent oxidation; and (3) heterogeneous reduction of U(VI) to U(IV) by Fe(II). These processes have been investigated by grazing incidence U L_III-edge X-ray absorption spectroscopy (GIXAS) and atomic force microscopy (AFM) to determine the composition, structure, and size of surface precipitates from the initial exposure of a surface to aqueous U solution species, and the transformations that occur during nucleation and particle growth of U(IV)- and/or U(VI)-bearing phases on the magnetite surface. In-situ AFM images collected under batch-flow conditions (pH 5 and 10; 100 mM uranyl) indicates that particle formation occurs within an hour of exposure when CO₃ was both present and absent. However, when both CO₃ and Ca were present, no particles were observed. The particles were approximately 20 nm in diameter and 5 nm in height (at 1 hr), and particle size was independent of the initial solution composition. Within a day, particle aggregates on the order of 400 diameter nm formed, however the aggregate height remained at 5 nm. Maximum surface coverage (~15 %) was reached within 2 days. The RMS surface roughness increased from approximately 0.5 nm to 5 nm over the course of the experiment. Energy dispersive X-ray spectra measured during SEM imaging indicated that these particles were U-bearing precipitates on the magnetite surface. GI-XAS spectra indicated that approximately half of the uranium present on the surface had been reduced. Analyses of the GI-XAS spectra by linear-combination fitting indicated that U speciation consisted of ~ 60% sorbed U(VI) and ~ 40% nanoparticulate UO₂ (in the absence of CO₃ and Ca), and 100% sorbed U(VI) (in the presence of CO₃ and Ca).