Uranium(VI) diffusion in low-permeability subsurface materials

Liu, Chongxuan; Zhong, Lirong; Zachara, John M.

doi:10.1524/ract.2010.1773

Cited by 8 publications

(10 citation statements)

References 20 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The second scenario is hypothetical by considering intergranular multispecies U(VI) diffusion in a diffusion column. A similar scenario was experimentally explored previously and the data were reported elsewhere [ Bai et al , 2009; Liu et al , 2010]. In this scenario, the sediment was first loaded with U(VI) in equilibrium with the SGW1.…”

Section: Resultscontrasting

confidence: 77%

“…Diffusion has been identified as a major mechanism controlling uranyl adsorption/desorption and precipitation/dissolution kinetics that was rate‐limited by mass exchange to and from intragranular uranyl adsorption and precipitation locations [ McKinley et al , 2006; Stubbs et al , 2009]. Diffusion also retarded U(VI) reactive transport in low permeable materials [ Bai et al , 2009; Liu et al , 2010; Yamaguchi et al , 1997; Yamaguchi and Nakayama , 1998] and in porous media containing mass transfer‐limited domains [ Liu et al , 2008]. A fully species‐ and charge‐coupled model that integrates molecular speciation, charge, and geochemical reactions has been developed [e.g., Appelo and Wersin , 2007; Giambalvo et al , 2002; Lichtner , 1996] that can be used to describe multispecies U(VI) diffusion in porous media [ Liu et al , 2006].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Multispecies diffusion models: A study of uranyl species diffusion

Liu¹,

Shang²,

Zachara³

2011

Water Resources Research

Self Cite

View full text Add to dashboard Cite

[1] Rigorous numerical description of multispecies diffusion requires coupling of species, charge, and aqueous and surface complexation reactions that collectively affect diffusive fluxes. The applicability of a fully coupled diffusion model is, however, often constrained by the availability of species self-diffusion coefficients, as well as by computational complication in imposing charge conservation. In this study, several diffusion models with variable complexity in charge and species coupling were formulated and compared to describe reactive multispecies diffusion in groundwater. Diffusion of uranyl [U(VI)] species was used as an example in demonstrating the effectiveness of the models in describing multispecies diffusion. Numerical simulations found that a diffusion model with a single, common diffusion coefficient for all species was sufficient to describe multispecies U(VI) diffusion under a steady state condition of major chemical composition, but not under transient chemical conditions. Simulations revealed that for multispecies U(VI) diffusion under transient chemical conditions, a fully coupled diffusion model could be well approximated by a component-based diffusion model when the diffusion coefficient for each chemical component was properly selected. The component-based diffusion model considers the difference in diffusion coefficients between chemical components, but not between the species within each chemical component. This treatment significantly enhanced computational efficiency at the expense of minor charge conservation. The charge balance in the component-based diffusion model can be enforced, if necessary, by adding a secondary migration term resulting from model simplification. The effect of ion activity coefficient gradients on multispecies diffusion is also discussed. The diffusion models were applied to describe U(VI) diffusive mass transfer in intragranular domains in two sediments collected from U.S. Department of Energy's Hanford 300A, where intragranular diffusion is a rate-limiting process controlling U(VI) adsorption and desorption. The grain-scale reactive diffusion model was able to describe U(VI) adsorption/desorption kinetics that had been previously described using a semiempirical, multirate model. Compared with the multirate model, the diffusion models have the advantage to provide spatiotemporal speciation evolution within the diffusion domains.Citation: Liu, C., J. Shang, and J. M. Zachara (2011), Multispecies diffusion models: A study of uranyl species diffusion, Water Resour.

show abstract

Section: Resultscontrasting

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Multispecies diffusion models: A study of uranyl species diffusion

Liu¹,

Shang²,

Zachara³

2011

Water Resources Research

Self Cite

View full text Add to dashboard Cite

show abstract

“…2,3 In addition, ion diffusion has been shown to be a major process contributing to the preferential uranium concentration in these sediments and is predicted to control the future release of uranium from the sediments. 2,4,5 One important limitation in studies of uranium diffusion is the lack of self-diffusion coefficients for uranyl species in groundwater, 6 which has led to the development of diffusion models, referred to here as apparent diffusion models, whereby the total dissolved U(VI) concentration is considered as a single chemical species. Recently, the self-diffusion coefficients of alkaline-earth uranyl carbonate species were determined using molecular dynamics (MD) simulations.…”

Section: ■ Introductionmentioning

confidence: 99%

“…7 This allowed for the development of a species-based diffusion model 6 that explicitly treats the variations in mass, charge, and diffusion coefficient of aqueous uranyl species, which proportions can be determined from speciation reactions. In test cases, the species-based diffusion model was shown to give a good description of the uranyl release from and diffusion into a saprolite sediment from the DOE Oak Ridge Site 6 and of the mass transfer limited uranyl adsorption in Hanford sediments. 8 However, several research groups 9−17 have shown, using MD simulations, that the diffusion of aqueous species can be significantly influenced by the presence of a mineral surface.…”

Section: ■ Introductionmentioning

confidence: 99%

Diffusion and Adsorption of Uranyl Carbonate Species in Nanosized Mineral Fractures

Kerisit

Liu

2012

Environ. Sci. Technol.

Self Cite

View full text Add to dashboard Cite

Atomistic simulations were performed to study the diffusion and adsorption of Ca 2 UO 2 (CO 3 ) 3 and of some of its constituent species, i.e., UO 2 2+ , CO 3 2− , and UO 2 CO 3 , in feldspar nanosized fractures. Feldspar is important to uranium remediation efforts at the U.S. Department of Energy Hanford site as it has been found in recent studies to host contaminants within its intragrain fractures. In addition, uranyl carbonate species are known to dominate U(VI) speciation in conditions relevant to the Hanford site. Molecular dynamics (MD) simulations showed that the presence of the feldspar surface diminishes the diffusion coefficients of all of the species considered in this work and that the diffusion coefficients do not reach their bulk aqueous solution values in the center of a 2.5 nm fracture. Moreover, the MD simulations showed that the rate of decrease in the diffusion coefficients with decreasing distance from the surface is greater for larger adsorbing species. Free energy profiles of the same species adsorbing on the feldspar surface revealed a large favorable free energy of adsorption for UO 2 2+ and UO 2 CO 3 , which are able to adsorb to the surface with their uranium atom directly bonded to a surface hydroxyl oxygen, whereas adsorption of CO 3 2− and Ca 2 UO 2 (CO 3 ) 3 , which attach to the surface via hydrogen bonding from a surface hydroxyl group to a carbonate oxygen, was calculated to be either only slightly favorable or unfavorable. ■ INTRODUCTIONUranium is a major radionuclide contaminant in groundwater systems at sites where nuclear materials were processed and stored. 1 Consequently, one of the main challenges for uranium remediation at nuclear facilities is to develop a comprehensive understanding of the reactivity and stability of uranium in order to predict its fate in subsurface environments. Uranium has been found predominantly in microscopic intragrain domains in contaminated sediments such as from the US Department of Energy (DOE) Hanford Site. 2,3 In addition, ion diffusion has been shown to be a major process contributing to the preferential uranium concentration in these sediments and is predicted to control the future release of uranium from the sediments. 2,4,5 One important limitation in studies of uranium diffusion is the lack of self-diffusion coefficients for uranyl species in groundwater, 6 which has led to the development of diffusion models, referred to here as apparent diffusion models, whereby the total dissolved U(VI) concentration is considered as a single chemical species. Recently, the self-diffusion coefficients of alkaline-earth uranyl carbonate species were determined using molecular dynamics (MD) simulations. 7 This allowed for the development of a species-based diffusion model 6 that explicitly treats the variations in mass, charge, and diffusion coefficient of aqueous uranyl species, which proportions can be determined from speciation reactions. In test cases, the species-based diffusion model was shown to give a good description of the uranyl release fro...

show abstract

“…However, at this point, this strategy has been applied less frequently (Bai et al, 2009;Liu et al, 2010).…”

Section: Investigation Of Reactive Transport and Coupled Thmc Processmentioning

confidence: 99%