title: Surface complexation modeling of U(VI) adsorption by aquifer sediments from a former mill tailings site at Rifle, Colorado.
abstract: A study of U(VI) adsorption by aquifer sediment samples from a former uranium mill tailings site at Rifle, Colorado, was conducted under oxic conditions as a function of pH, U(VI), Ca, and dissolved carbonate concentration. Batch adsorption experiments were performed using <2 mm size sediment fractions, a sand-sized fraction, and artificial groundwater solutions prepared to simulate the field groundwater composition. To encompass the geochemical conditions of the alluvial aquifer at the site, the experimental conditions ranged from 6.8 x 10(-8) to 10(-5) M in [U(VI)](tot), 7.2 to 8.0 in pH, 3.0 x 10(-3) to 6.0 x 10(-3) M in [Ca(2+)], and 0.05 to 2.6% in partial pressure of carbon dioxide. Surface area normalized U(VI) adsorption K(d) values for the sand and <2 mm sediment fraction were similar, suggesting a similar reactive surface coating on both fractions. A two-site two-reaction, nonelectrostatic generalized composite surface complexation model was developed and successfully simulated the U(VI) adsorption data. The model successfully predicted U(VI) adsorption observed from a multilevel sampling well installed at the site. A comparison of the model with the one developed previously for a uranium mill tailings site at Naturita, Colorado, indicated that possible calcite nonequilibrium of dissolved calcium concentration should be evaluated. The modeling results also illustrate the importance of the range of data used in deriving the best fit model parameters.
authors.affiliation: University of Michigan, Ann Arbor, Michigan 48109, USA. sphyun@umich.edu
authors.forename: Sung Pil
authors.initials: SP
authors.lastname: Hyun
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Fox
authors.forename: James A
authors.initials: JA
authors.lastname: Davis
authors.forename: Kate M
authors.initials: KM
authors.lastname: Campbell
authors.forename: Kim F
authors.initials: KF
authors.lastname: Hayes
authors.forename: Philip E
authors.initials: PE
authors.lastname: Long
mesh_terms: D000327:Adsorption; D003120:Colorado; D019015:Geologic Sediments / Q000737:chemistry*; D008906:Mining; D008956:Models, Chemical; D012990:Soil Pollutants, Radioactive / Q000737:chemistry; D014501:Uranium / Q000737:chemistry*; D014875:Water Pollutants, Radioactive / Q000737:chemistry; D014881:Water Supply / Q000032:analysis
chemical_list: D012990:Soil Pollutants, Radioactive; D014875:Water Pollutants, Radioactive; D014501:Uranium