A Site Wide Perspective on Uranium Geochemistry at the Hanford Site

Abstract

Executive SummaryUranium (U) is an important risk-driving contaminant at the Hanford Site. Over 200,000 kg have been released to the vadose zone over the course of site operations, and a number of vadose zone and groundwater plumes containing the uranyl cation [UO 2 2+ , U(VI)] have been identified. U is recognized to be of moderate-to-high mobility, conditions dependent. The site is currently making decisions on several of these plumes with long-lasting implications, and others are soon to come.Uranium is one of nature's most intriguing and chemically complex elements. The fate and transport of U(VI) has been studied over the long lifetime of the Hanford Site by various contractors, along with the Pacific Northwest National Laboratory (PNNL) and its collaborators. Significant research has more recently been contributed by the national scientific community with support from the U.S. Department of Energy's (DOE) Office of Science through its Environmental Remediation Sciences Division (ERSD). This report represents a first attempt to integrate these findings into a cohesive view of the subsurface geochemistry of U at the Hanford Site. The objective is to inform all interested Hanford parties about the in-ground inventory of U and its geochemical behavior. This report also comments on the prospects for the development of a robust generic model to more accurately forecast future U(VI) migration at different Hanford waste sites, along with further research necessary to reach this goal.To accomplish the report objectives, the environmental geochemistry of U at the Hanford Site is discussed in terms of both the vadose and saturated zone, to the extent that it is known. Hexavalent uranium [U(VI)] is the dominant valence form of U under the predominantly oxidizing subsurface conditions at the Hanford Site, and the researchers' analyses consequently emphasize this species. The nature and concentration of background U in Hanford subsurface sediments is identified to place contaminant U(VI) concentrations and behavior in perspective to the natural system. In-ground U-waste inventories are quantified and characterized with regard to source term, to the extent possible, and the most important sites from an inventory perspective are identified. The U-isotopic content of various waste streams are discussed from the perspective of waste-source tracking. The geochemical attenuation processes responsible for slowing the rate of subsurface U migration, relative to the transporting water front, are illustrated through careful consideration of both field characterization studies of existing U vadose-zone and groundwater plumes, and laboratory studies of derived contaminated and uncontaminated sediments. Both empirical and more mechanistic models of these attenuation processes are considered as well as the parameters that define attenuation magnitude. Attention is given to the behavior of contaminant U(VI) that has been in contact with Hanford sediments for extended periods (circa 10-50 years), as long contact imparts unique characte...

Uranium Extraction From Laboratory-Synthesized, Uranium-Doped Hydrous Ferric Oxides

“…In all cases the model predicted the formation of hematite (Fe 2 O 3 ). In the U-HFO and Si-U-HFO systems, the model predicted that 100% of the U remains dissolved and the predominant U species are (UO 2 ) 2 CO 3 (OH) 3 -(80%) and UO 2 CO 3 0 (9%), with minor amounts of UO 2 OH + (3%) and UO 2 OH 2 0 (3%). However, in the system with P, the model predicted that sodium autinite [Na 2 (UO 2 ) 2 (PO 4 ) 2 · 10-12 H 2 O] precipitates and controls the aqueous concentration of U, and the balance of P is dissolved HPO 4 2-(47%), H 2 PO 4…”

“…Consequently, the reversibility of U sorption may change as a function of the time that it has been associated with sediment. This phenomenon is frequently observed and has been reported as an increase of the dissociation constant, or Kd, and has been attributed to diffusion into cracks or pores on the particle surface or interaggregate sequestration (2,3).…”

“…The mineral and physicochemical relationship of U with soil varies from aqueous species (2-4, 5, 6, 7) to sorbed, readily exchangeable, oxidized species, such as uranyl [U(VI)O 2 2+ ], to highly insoluble U(VI)-bearing minerals and phases containing reduced [U(IV)] (8,9). U species will participate in a range of geochemical reactions that may alter the original chemical form, resulting in more thermodynamically stable aqueous and solid phases under ambient environmental conditions (2,3,10,11). As a first step toward subsequent physicochemical transformation, sorption is a rapid mechanism that retards the translocation of uranyl within the soil profile.…”

Evaluation of Soil Flushing for Application to the Deep Vadose Zone in the Hanford Central Plateau

“…The composition from the largest tank leaks (SX-108, BX-102, and T-106) varied from alkaline to caustic with high ionic strengths and temperatures, and they contained a variety of metals and radionuclides. The discharged waste was highly reactive with the sediment as infiltration occurred, which led to a geochemically heterogeneous uranium retention and phase distribution (Zachara et al 2007a(Zachara et al , 2007bMcKinley et al 2007;Um et al 2009). Sediment samples collected near BX-102 tanks have uranium-silicate precipitates including boltwoodite and uranophane (Um et al 2009).…”

“…Ammonium carbonate was used at some ISR sites in the past because of concerns that sodium could decrease permeability due to swelling clays. However, ammonium carbonate is no longer in use at ISR sites in the United States because ammoniumcarbonate lixiviants caused problems by desorbing slowly from clays during post-ISR aquifer restoration, increasing the quantities of groundwater that must be processed (NRC 2009 Zachara et al 2007b). These stable complexes cause high uranium solubility, and a stable uranium solution that may be transported for significant distances before recovery.…”