Research Electron Microbeam Investigation of Uranium-Contaminated Soils from Oak Ridge, TN, USA J O A N N E E . S T U B B S , †,* DAVID C. ELBERT,† DAVID R. VEBLEN,† AND CHEN ZHU‡ Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405
Two samples of uranium-contaminated soil from the Department of Energy’s Oak Ridge Reservation in Oak Ridge, Tennessee were investigated using electron microprobe analysis and transmission electron microscopy. The objectives of this research were to identify and characterize soil particles and rock chips with high uranium concentrations, to investigate the extent of uranium penetration into chips of parent material, and to identify solidphase hosts for uranium in the samples. Three distinct solidphase hosts for uranium have been identified: (1) iron oxyhydroxides, including goethite and ferrihydrite; (2) mixed Mn-Fe oxides; and (3) discrete uranium phosphates. In all three, uranium is associated with phosphorus. The ubiquitous U-P association highlights the influence of phosphate on the environmental fate of uranium. Uraniumbearing phases are found well within chips of weathered shale, as far as 900 µm from fractures and chip edges, indicating that uranium has diffused into the shale matrix.
Introduction Since construction in 1943, the Y-12 plant at the Department of Energy’s (DOE) Oak Ridge Reservation has been home to numerous activities related to the production, reclamation, and storage of nuclear weapons components (1). From 1951 to 1983, wastes from these activities were discharged to four unlined disposal ponds, known as the S-3 ponds, adjacent to the plant. The wastes comprised mainly acidic uranium nitrate, but they also contained other heavy metals (e.g., B, Ba, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sr, V, and Zn), anions (Cl, F, and sulfate), radionuclides (e.g., 99Tc, 230Th), and organic compounds. In 1983, the ponds were closed. The remaining liquids were biologically denitrified and neutralized prior to filling and capping of the ponds. The former ponds are underlain by soil and saprolite developed from interbedded shale and limestone bedrock. Contaminants from the ponds have leached into the groundwater and geologic materials at the site (1-3). Because uranium poses a threat to human health and to the health of other organisms, there are strong incentives to understand, predict, and control its fate in soils and aquifers. * Corresponding author phone: 410-516-7135; fax: 410-516-7933; e-mail:
[email protected]. † Johns Hopkins University. ‡ Indiana University. 2108
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 7, 2006
FIGURE 1. Map of NABIR FRC. Samples are from Area 3, hydrologically downgradient from the former S-3 Ponds. Uranium occurs in two common oxidation states in the environment. Uranium(IV) is stable under reducing conditions and relatively insoluble. Uranium(VI), which typically occurs in the uranyl (UO22+) ion or in uranyl complexes, dominates under oxidizing conditions. It is relatively soluble and, therefore, mobile (4, 5). The DOE has established a Field Research Center (FRC) at Oak Ridge as part of its Natural and Accelerated Bioremediation Research (NABIR) Program. The FRC includes the S-3 ponds area, as well as a geologically similar, uncontaminated background area. The NABIR mission includes development of in situ bioremediation methods for contaminant metals such as uranium. Subsurface conditions at the FRC are oxidizing; thus, most of the uranium is likely in the U(VI) oxidation state. Among the goals of NABIR researchers is the stimulation of microbial reduction of U(VI) to U(IV) and consequent uranium immobilization (2, 6). Understanding mineral-fluid-microbe interactions is central to such research and necessarily includes thorough characterization of the solid phases involved. Furthermore, significant quantities of uranium have partitioned to the solids (3, 7). Identification of solid-phase hosts for uranium in FRC soils is the subject of this paper. Adsorption of U(VI) by soil minerals has been the subject of extensive research (8-10). Iron oxides and oxyhydroxides, such as hematite (Fe2O3), goethite (R-FeOOH), ferrihydrite, and amorphous ferric oxyhydroxides (hereafter referred to simply as iron oxides), are important sorbents for uranium, and their role in the uptake of U(VI) from solution has received particular attention (4, 10-12). Manganese oxides, while modally less significant than iron oxides, are common in soils and are noted for their high adsorption capacities and trace element scavenging capabilities (13). Both classes of oxides are ubiquitous in the shale saprolite at the FRC (14). The groundwater at the FRC is acidic, and at low pH the presence of phosphate results in the formation of ternary U-P surface complexes on iron oxides. Thus, uranium sorbs at lower pH values than in systems with little phosphate (15-17). While sorption processes are generally thought to govern the mobility of U(VI) (4, 8, 9), the precipitation of uranyl minerals is also possible. Uranyl minerals can precipitate as a result of local saturation, even when bulk groundwater is undersaturated with respect to these phases (18-20). 10.1021/es0518676 CCC: $33.50
2006 American Chemical Society Published on Web 02/21/2006
FIGURE 2. (a) BSE of iron oxide pseudomorphs after pyrite in shale. Fine-grained matrix consists primarily of aluminosilicate clays and micas. Bright regions in (b), (c), and (d) indicate high concentrations of Fe, U, and P, respectively. Rims around reacted particles at cluster edges contain substantially more U and P than do central, unreacted particles.
Materials and Methods Two drill core samples, numbered FWB103-00-40E and FWB103-00-42D, were provided by David Watson of the FRC.
The samples are from Area 3, a study area immediately adjacent to and hydrologically downgradient from the former S-3 ponds (Figure 1). The samples are from 12.2- and 12.8meter depths, at which the parent material is shale of the Nolichucky formation. These depths correspond to a zone of fast groundwater flow just above the saprolite/bedrock interface, approximately 9 m below the water table, where solid-phase concentrations of U, Fe, Mn, and P are the highest found in this core. Although bulk chemical analyses of these samples were not available, uranium concentrations in other samples from similar depths in this drill core are 272-388 mg/kg (3, 7). Uncontaminated C-horizon soils developed from Nolichucky shale contain 3-5 mg/kg uranium (21). Conditions are acidic, with soil and groundwater pH values of 3.60-3.80. The groundwater in this bore hole contains 22 mg/L uranium (7). Approximately 10 g of each sample were subdivided into soil-sized (
