Mobility of Dissolved Gases in Smectites under Saturated Conditions: Effects of Pore Size, Gas Types, Temperature, and Surface Interaction

Abstract

In a nuclear waste repository, the corrosion of metals and the degradation of the organic material in the waste matrix can generate significant amounts of gases. These gases should be able to migrate through the multibarrier system to prevent a potential pressure build-up that could lead to a loss of barrier integrity. Smectite mineral particles form a tortuous pore network consisting of larger interparticle pores and narrow interlayer pores between the platelets of the smectite minerals. These pores are normally saturated with water, so one of the most important mechanisms for the transport of gases is diffusion. The diffusion of gases through the interparticle porosity depends on the distribution of gas molecules in the water-rich phase, their self-diffusion coefficients, and the tortuosity of the pore space. Classical molecular dynamics simulations were applied to study the mobility of gases (CO2, H2, CH4, He, and Ar) in Na-montmorillonite (Na-MMT) under saturated conditions. The simulations were used to estimate the gas diffusion coefficient (D) in saturated Na-MMT as a function of nanopore size and temperature. The temperature dependence of the diffusion coefficient was expressed by the Arrhenius equation for the activation energy (E a). The predicted D values of gases were found to be sensitive to the pore size as the D values gradually increase with increasing pore size and asymptotically converge to the gas diffusion coefficient in bulk water. This behavior is also observed in the self-diffusion coefficients of water in Na-MMT. In general, H2 and He exhibit higher D values than Ar, CO2, and CH4. The predicted E a values indicate that the confinement affects the activation energy. This effect is due to the structuring of the water molecules near the clay surface, which is more pronounced in the first two layers of water near the surface and decreases thereafter. Atomic density profiles and radial distribution functions obtained from the simulations show that the interaction of the gas with the liquid and the clay surface influences mobility. The obtained diffusion coefficient for different gases and slit pore size were parameterized with a single empirical relationship, which can be applied to macroscopic simulations of gas transport.

Diffusion and Gas Flow Dynamics in Partially Saturated Smectites

“…Experimental data regarding gas diffusion coefficients in partially saturated smectite micropores as a function of gas-filled pore width are currently unavailable. However, a previous investigation of gas diffusion in saturated smectite incorporated a geometric factor and utilized an empirically derived function from a molecular dynamics study . This approach produced predictions that closely matched experimental gas diffusion data in Boom clay.…”

“…This equation allows to predict the gas-phase diffusivity for general conditions based on the geometric properties of porous media, mean pore size, and the partial water pressure via the corresponding parameters, G , h av , and t w . A further effect of temperature can be considered following the approach described in our previous study …”

“…In this study, we focused only on diffusion in the gas phase and the water–gas interface. Diffusion in the liquid phase was investigated in our previous study on the mobility of gas molecules in saturated smectites . To obtain the diffusion coefficient in different regions, we adopted the MD-calculated survival probability and the mean square displacement method of Liu et al In this study, only the x and y components of the diffusion tensor, describing the two-dimensional diffusion parallel to the gas–water interface were analyzed.…”

“…70 The interatomic interaction potentials for clay atoms, water, and gas molecules are the same as those used in our previous work, which also explains the model uncertainties. 42 The setup and equilibration of the system take full account of the gas molecules partitioning between gas-rich and waterrich phases (solubility) by implementing an iterative equilibration process under imposed PVT constraints. For the sake of data comparison, all of the simulations are conducted in the same d-spacing of 6 nm distance between the TOT layers.…”

“…Diffusion in the liquid phase was investigated in our previous study on the mobility of gas molecules in saturated smectites. 42 To obtain the diffusion coefficient in different regions, we adopted the MD-calculated survival probability and the mean square displacement method of Liu et al 74 In this study, only the x and y components of the diffusion tensor, describing the two-dimensional diffusion parallel to the gas−water interface were analyzed. Assuming a layer parallel to the basal surface of the clay particle with intervals in the z-dimension defined as {a,b}, the diffusion coefficient of particles located in the region of the interval {a,b} in one dimension is given as follows…”