U and Fe Biogeochemistry in Wetland Sediments at the Savannah River Site

Uranium production for fuel and military purposes has resulted in significant accumulation of depleted uranium (238U) at contaminated sites around the world. Understanding the factors that control the dispersal of these stocks is of paramount importance to public safety. Reactive transport models are used to predict U movement in natural systems, and they must adequately account for various interfacial and redox U reactions in the combined presence of minerals, bacteria, and dissolved ligands. As a testbed for the processes in such complex systems, we are studying the speciation of U released during past decades at a DOE field site, the Tims Branch wetland at the Savannah River Site. The organic-rich sediments in this riparian ecosystem induce changing redox conditions that cause transformations of the major elements (C, Fe, P, S), as well as of the contaminants discharged at the site (Ni, Cr, Zn, Pb, U). We characterized a number of intact sediment cores by synchrotron x-ray spectroscopy to determine the elemental distribution and the speciation of Fe and U with depth. The valence of U was dependent on local saturation state. We found U(VI) in the drier top layers of the sediment, whereas U(IV) was predominant in saturated portions of the sediment. U(IV) was present in a form different from nanoparticulate uraninite (UO2), indicating a significant influence of the minerals and organic ligands on U speciation. To better understand the effects of each, we carried out U(VI) bioreduction experiments in the presence of clay minerals and/or organic ligands (citrate, EDTA, DFOB). Addition of clays alone did not affect U(IV) speciation (i.e., uraninite formed). The presence of citrate resulted in soluble U(IV)-citrate complexes in all systems. EDTA and DFOB also solubilized U(IV), but in systems with Fe-containing clays the U(IV) complex remained in the solids. The EXAFS analysis of the samples is consistent with an interlayer uptake or a ternary bridging complex uptake mechanism. These results highlight the intricate interdependencies between the constituents of contaminated sediments, as well as the need for improved mechanistic models.