title: Cryogenic laser induced fluorescence characterization of U(VI) in Hanford Vadose Zone pore waters.
abstract: Ambient and liquid helium temperature laser-induced time-resolved uranyl fluorescence spectroscopy was applied to study the speciation of aqueous uranyl solutions containing carbonate and phosphate and two porewater samples obtained by ultracentrifugation of U(VI)-contaminated sediments. The significantly enhanced fluorescence signal intensity and spectral resolution found at liquid helium temperature allowed, for the first time, direct fluorescence spectroscopic observation of the higher aqueous uranyl complexes with carbonate: UO2(CO3)2(2-), UO2(CO3)3(4-), and (UO2)2(OH)3CO3-. The porewater samples were nonfluorescent at room temperature. However, at liquid helium temperature, both porewater samples displayed strong, well-resolved fluorescence spectra. Comparisons of the spectroscopic characteristics of the porewaters with those of the standard uranyl-carbonate complexes confirmed that U(VI) in the porewaters existed primarily as UO2(CO3)3(4-) along with a small amount of other minor components, such as dicalcium-urano-tricarbonate complex, Ca2UO2(CO3)3, consistent with thermodynamic calculation. The U(VI)-carbonate complex is apparently the mobile species responsible for the subsurface migration of U(VI), even though the majority of the in-ground U(VI) inventory at the site from which the samples were obtained exists as intragrain U(VI)-silicate precipitates.
authors.affiliation: Pacific Northwest National Laboratory, Richland, Washington 99352, USA. Zheming.Wang@pnl.gov
authors.forename: Zheming
authors.initials: Z
authors.lastname: Wang
authors.forename: John M
authors.initials: JM
authors.lastname: Zachara
authors.forename: Wassana
authors.initials: W
authors.lastname: Yantasee
authors.forename: Paul L
authors.initials: PL
authors.lastname: Gassman
authors.forename: Chongxuan
authors.initials: C
authors.lastname: Liu
authors.forename: Alan G
authors.initials: AG
authors.lastname: Joly
mesh_terms: D002254:Carbonates / Q000032:analysis / Q000737:chemistry; D004785:Environmental Pollutants / Q000032:analysis*; D005618:Fresh Water / Q000032:analysis*; D019015:Geologic Sediments / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007834:Lasers; D010710:Phosphates / Q000032:analysis / Q000737:chemistry; D017640:Silicates / Q000032:analysis / Q000737:chemistry; D013050:Spectrometry, Fluorescence / Q000379:methods*; D017974:Uranium Compounds / Q000032:analysis* / Q000737:chemistry
chemical_list: D002254:Carbonates; D004785:Environmental Pollutants; D010710:Phosphates; D017640:Silicates; D017974:Uranium Compounds