FIB/TEM Study of Uranium-Phosphate Mineralization in Subsurface Sediments
EMSL Project ID
47879
Abstract
Uranium contamination of the subsurface at DOE sites has resulted from past mining, milling, and waste disposal processes. Phosphate addition has been proposed as a potential remediation strategy for such sites because the precipitation of low solubility U(VI)-phosphate could significantly lower the aqueous uranium concentration without having to maintain sustained reducing conditions in naturally oxic groundwater systems. However, the geochemical factors that determine the dominant immobilization mechanisms are insufficiently understood to design efficient remediation strategies or accurately predict uranium transport in treated systems. We are currently investigating the primary mechanisms of uranium-phosphate reactions in subsurface sediments. These studies involve reacting sediments from DOE/BER-supported field research sites at Rifle, CO and Hanford, WA with groundwater solutions that contain uranyl and phosphate in batch and column systems. The current proposal aims to isolate and characterize uranium-bearing solids that may form in these reacted sediments. Specific points to be investigated are the mineralogical nature of the uranyl-phosphate solids and their local-scale associations with sediment minerals. Both pieces of information are critical to assessing the stability of such solids and determining how the site-specific mineralogy affects the efficiency of phosphate-based remediation methods. Uranyl phosphates of a range of compositions and structures can form nanosized, disordered solids. These may differ in their formation conditions and dissolution rates. Meanwhile, interactions with mineral surfaces could promote the formation of uranyl-phosphate solids through heterogeneous nucleation, which can affect the saturation state at which precipitation occurs. We propose to use the SEM/FIB for Radiological Samples at EMSL to extract cross sections of areas containing uranium precipitates from our reacted sediments. The use of FIB will allow us to preserve the fine-scale sediment geometry and physical associations of uranium and sediment minerals. These cross sections will be further studied at EMSL using the TEM for Radiological Samples. The use of transmission electron microscopy, through its high spatial resolution, will provide insight into specific associations of sediment minerals and uranium-bearing precipitates, while at the same time allowing a more detailed characterization of the uranyl-phosphate solids that forms via electron diffraction. This work will advance our understanding of the variables controlling the formation and mineralogy of U(VI)-phosphate solids under field-relevant conditions.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2014-05-01
End Date
2014-08-21
Status
Closed
Released Data Link
Team
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