title: Fate and transport of uranium (VI) in weathered saprolite.
abstract: Batch and column experiments were conducted to investigate sorption and transport of uranium (U) in the presence of saprolite derived from interbedded shale, limestone, and sandstone sequences. Sorption kinetics were measured at two initial concentrations (C0; 1, 10 μM) and three soil:solution ratios (Rs/w; 0.005, 0.25, 2 kg/L) at pH 4.5 (pH of the saprolite). The rate of U loss from solution (μmole/L/h) increased with increasing Rs/w. Uranium sorption exhibited a fast phase with 80% sorption in the first eight hours for all C0 and Rs/w values and a slow phase during which the reaction slowly approached (pseudo)equilibrium over the next seven days. The pH-dependency of U sorption was apparent in pH sorption edges. U(VI) sorption increased over the pH range 4-6, then decreased sharply at pH > 7.5. U(VI) sorption edges were well described by a surface complexation model using calibrated parameters and the reaction network proposed by Waite et al. (1994). Sorption isotherms measured using the same Rs/w and pH values showed a solids concentration effect where U(VI) sorption capacity and affinity decreased with increasing solids concentration. This effect may have been due to either particle aggregation or competition between U(VI) and exchangeable cations for sorption sites. The surface complexation model with calibrated parameters was able to predict the general sorption behavior relatively well, but failed to reproduce solid concentration effects, implying the importance of appropriate design if batch experiments are to be utilized for dynamic systems. Transport of U(VI) through the packed column was significantly retarded. Transport simulations were conducted using the reactive transport model HydroGeoChem (HGC) v5.0 that incorporated the surface complexation reaction network used to model the batch data. Model parameters reported by Waite et al. (1994) provided a better prediction of U transport than optimized parameters derived from our sorption edges. The results presented in this study highlight the challenges in defining appropriate conditions for batch-type experiments used to extrapolate parameters for transport models, and also underline a gap in our ability to transfer batch results to transport simulations.
authors.affiliation: Civil Engineering Division, Samsung C&T Corp., Seocho-Dong, Seocho-Gu, Seoul 137-956, Republic of Korea.
authors.forename: Young-Jin
authors.initials: YJ
authors.lastname: Kim
authors.affiliation: Oak Ridge National Laboratory, Environmental Sciences Division, P.O. Box 2008, MS 6038, Oak Ridge, TN 37831-6038, USA. Electronic address: brookssc@ornl.gov.
authors.forename: Scott C
authors.initials: SC
authors.lastname: Brooks
authors.affiliation: Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China.
authors.forename: Fan
authors.initials: F
authors.lastname: Zhang
authors.affiliation: Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Jack C
authors.initials: JC
authors.lastname: Parker
authors.affiliation: Oak Ridge National Laboratory, Biosciences Division, P.O. Box 2008, MS 6036, Oak Ridge, TN 37831-6036, USA.
authors.forename: Ji-Won
authors.initials: JW
authors.lastname: Moon
authors.affiliation: Faculty of Earth System and Environmental Sciences, Chonnam National University, 300 Yongbong-Dong, Buk-Gu, Gwangju 500-757, Republic of Korea.
authors.forename: Yul
authors.initials: Y
authors.lastname: Roh
mesh_terms: D012990:Soil Pollutants, Radioactive / Q000032:analysis; D014501:Uranium / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D012990:Soil Pollutants, Radioactive; D014874:Water Pollutants, Chemical; D014501:Uranium
keywords: Modeling; Saprolite; Transport; Uranium(VI)