Scale-Dependent Rates of Uranyl Surface Complexation Reaction in Sediments

Journal Article · Fri Mar 15 04:00:00 EDT 2013 · Geochimica et Cosmochimica Acta, 105:326-341

DOI:https://doi.org/10.1016/j.gca.2012.12.003· OSTI ID:1073565

Liu, Chongxuan; Shang, Jianying; Kerisit, Sebastien N; Zachara, John M; Zhu, Weihuang

Scale-dependency of uranyl[U(VI)] surface complexation rates was investigated in stirred flow-cell and column systems using a U(VI)-contaminated sediment from the US Department of Energy, Hanford site, WA. The experimental results were used to estimate the apparent rate of U(VI) surface complexation at the grain-scale and in porous media. Numerical simulations using molecular, pore-scale, and continuum models were performed to provide insights into and to estimate the rate constants of U(VI) surface complexation at the different scales. The results showed that the grain-scale rate constant of U(VI) surface complexation was over 3 to 10 orders of magnitude smaller, dependent on the temporal scale, than the rate constant calculated using the molecular simulations. The grain-scale rate was faster initially and slower with time, showing the temporal scale-dependency. The largest rate constant at the grain-scale decreased additional 2 orders of magnitude when the rate was scaled to the porous media in the column. The scaling effect from the grain-scale to the porous media became less important for the slower sorption sites. Pore-scale simulations revealed the importance of coupled mass transport and reactions in both intragranular and inter-granular domains, which caused both spatial and temporal dependence of U(VI) surface complexation rates in the sediment. Pore-scale simulations also revealed a new rate-limiting mechanism in the intragranular porous domains that the rate of coupled diffusion and surface complexation reaction was slower than either process alone. The results provided important implications for developing models to scale geochemical/biogeochemical reactions.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1073565
• Report Number(s): PNNL-SA-75094; KP1702030
• Journal Information: Geochimica et Cosmochimica Acta, 105:326-341, Journal Name: Geochimica et Cosmochimica Acta, 105:326-341
• Country of Publication: United States
• Language: English

Related Subjects

uranium
desorption
kinetics
scaling
pore-scale
simulation