Nucleation and Precipitation Processes that Affect U(VI) Sequestration following Phosphate Addition to Contaminated Sediments
EMSL Project ID
48500
Abstract
Our research investigates the potential applicability of phosphate injection as an in-situ remediation strategy to decrease aqueous U(VI) concentrations at contaminated DOE sites. Our goal is to characterize solid-associated U(VI) that forms in subsurface sediments under field-relevant conditions in response to phosphate addition. Preliminary data show that U(VI) phosphates precipitate only after a critical supersaturation is crossed, suggesting that a nucleation barrier exists. Heterogeneous nucleation may thus strongly affect U(VI)-phosphate precipitation, but it is unclear which sediment mineral(s) may serve to promote nucleation. In addition, enhanced U(VI) uptake occurs at high phosphate conditions below the onset of precipitation, and this may be associated with calcium phosphate precipitation. Such phases may adsorb U(VI) or entrap it via coprecipitation. However, the Ca-phosphate phase(s) forming in the current system have not been identified, nor have their association with sediment grains been determined. If such phases coat grains then they will alter sediment reactivity and nucleation processes differently than if they form discrete phases. The specific objectives of this project are to determine the role of sediment minerals in nucleating U(VI) phosphates and to identify the Ca phosphate phase(s) that form during phosphate addition and their effect on U(VI) fate. The proposed studies involve reacting sediments originating from uranium-contaminated DOE sites at Rifle, CO and Hanford, WA, with synthetic groundwater solutions that contain U(VI) and phosphate in batch and column systems. Single mineral phases representative of the sediment fine fractions will also be studied. These investigations require instrumentation in the EMSL Radiochemistry Annex. Thin sections of reacted sediments will be studied using electron microprobe to identify areas of U(VI)-phosphate precipitation and the nature of Ca phosphate precipitation. Cross sections of U(VI) and Ca phosphate precipitates on sediment grain surfaces will be extracted using the dual FIB/SEM and then characterized by TEM. Corresponding bulk samples will be analyzed by laser-induced fluorescence spectroscopy to determine the U(VI) species present in each sample. This research supports the DOE-BER mission by advancing the scientific basis for phosphate-based remediation at U-contaminated DOE sites. It is funded by the DOE/BER Subsurface Biogeochemical Research Program. The present study is part of the research plan described in this DOE-funded project. The samples that we propose to investigate using EMSL's capabilities originate from contaminated DOE/BER-supported field research sites at Rifle, CO and Hanford, WA.
Project Details
Project type
Special Science
Start Date
2014-06-23
End Date
2015-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
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