title: Influence of calcite and dissolved calcium on uranium(VI) sorption to a hanford subsurface sediment.
abstract: The influence of calcite and dissolved calcium on U(VI) adsorption was investigated using a calcite-containing sandy silt/clay sediment from the U. S. Department of Energy Hanford site. U(VI) adsorption to sediment, treated sediment, and sediment size fractions was studied in solutions that both had and had not been preequilibrated with calcite, at initial [U(VI)] = 10(-7)-10(-5) mol/L and final pH = 6.0-10.0. Kinetic and reversibility studies (pH 8.4) showed rapid sorption (30 min), with reasonable reversibility in the 3-day reaction time. Sorption from solutions equilibrated with calcite showed maximum U(VI) adsorption at pH 8.4 +/- 0.1. In contrast, calcium-free systems showed the greatest adsorption at pH 6.0-7.2. At pH > 8.4, U(VI) adsorption was identical from calcium-free and calcium-containing solutions. For calcite-presaturated systems, both speciation calculations and laser-induced fluorescence spectroscopic analyses indicated that aqueous U(VI) was increasingly dominated by Ca2UO2(CO3)3(0)(aq) at pH < 8.4 and thatformation of Ca2UO2(CO3)3(0)(aq) is what suppresses U(VI) adsorption. Above pH 8.4, aqueous U(VI) speciation was dominated by UO2(CO3)3(4-) in all solutions. Finally, results also showed that U(VI) adsorption was additive in regard to size fraction but not in regard to mineral mass: Carbonate minerals may have blocked U(VI) access to surfaces of higher sorption affinity.
authors.affiliation: Department of Geography and Environmental Engineering, Johns Hopkins University, 313 Ames Hall, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Wenming
authors.initials: W
authors.lastname: Dong
authors.forename: William P
authors.initials: WP
authors.lastname: Ball
authors.forename: Chongxuan
authors.initials: C
authors.lastname: Liu
authors.forename: Zheming
authors.initials: Z
authors.lastname: Wang
authors.forename: Alan T
authors.initials: AT
authors.lastname: Stone
authors.forename: Jing
authors.initials: J
authors.lastname: Bai
authors.forename: John M
authors.initials: JM
authors.lastname: Zachara
mesh_terms: D000327:Adsorption; D002118:Calcium / Q000737:chemistry*; D002119:Calcium Carbonate / Q000737:chemistry*; D019015:Geologic Sediments / Q000032:analysis*; D007700:Kinetics; D012822:Silicon Dioxide; D013050:Spectrometry, Fluorescence; D014501:Uranium / Q000737:chemistry*; D014861:Washington
chemical_list: D014501:Uranium; D012822:Silicon Dioxide; D002119:Calcium Carbonate; D002118:Calcium