Diffusion and Gas Flow Dynamics in Partially Saturated Smectites

JP Owusu and K Karalis and NI Prasianakis and SV Churakov, JOURNAL OF PHYSICAL CHEMISTRY C, 127, 14425-14438 (2023).

DOI: 10.1021/acs.jpcc.3c02264

Clays and clay rocks are considered good natural andengineeredbarriers for deep geological disposal of nuclear waste worldwide.Metal corrosion and organic waste degradation in underground repositoriesgenerate significant amounts of gas that should be able to migratethrough the multibarrier system to avoid potential pressure buildup,which could be compromising the integrity of the barriers and hostrocks. The gas is expected to accumulate in larger pores and eventuallyform an interconnected network. Under such conditions, the migrationof gas molecules takes place both in pore water films and gas-filledmacropores. Therefore, mass fluxes depend on the distribution of gasmolecules between the water-rich and gas-rich phases and their mobilityin both compartments. Classical molecular dynamics (MD) simulationswere employed to investigate the mobilities of He, H-2,CO2, Ar, and CH4 in a Na- montmorillonite mesoporeas a function of the degree of saturation, as well as evaluate thehydrodynamic behavior of the pore fluid in partially saturated clays.The diffusivity of the gas molecules was determined by observing theasymptotic behavior of the mean square displacement in the gas-richphase and at the gas-water interface. The partition coefficientand Gibbs free energy were analyzed to investigate the transfer ofgas molecules between the gas-rich and water-rich phases by observingthe molecular trajectories as they cross the vapor-liquid interface.The results revealed that the diffusion coefficient in the gas phaseincreased with increasing gas-filled pore width and converged asymptoticallytoward the diffusion coefficient in the bulk state. It could be shownthat the diffusion coefficient of gas molecules dissolved in the waterfilms remained constant as long as the interacting water surface wasin the bulk-liquid-like phase. This behavior changes in very thinwater films. It was observed that the partitioning coefficient ofgas molecules at the solid-liquid interface is nearly the sameas that in the bulk-liquid-like phase. Partitioning is observed tobe strongly dependent on the temperature and gas molecular weights.In the second part of the study, nonequilibrium molecular dynamics(NEMD) simulations were performed to investigate the mobility of gasesin pressure-driven decoupled gas-phase dynamics (DGPD) and coupledgas and water phase dynamics (CGWPD) in a partially saturated Na-montmorilloniteslit mesopore. The dynamic viscosity of the gas phase was calculatedfrom NEMD simulations and indicated that the viscosity of the gasphase was almost the same in both methods (DGPD and CGWPD). The averageslip length for gas molecules at the gas-water interface wasalso calculated, revealing that the slip-free boundary condition assumedin continuum models is generally invalid for microfluidics and thata slip boundary condition exists at the microscale for specific surfaceinteractions. Finally, a Bosanquet-type equation was developed topredict the diffusion coefficient and dynamic viscosity of gas asa function of the average pore width, gas mean-free path, geometricfactor, and thickness of the adsorbed water film.

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