Connecting the molecular scale to the continuum scale for diffusion processes in smectite-rich porous media.
In this paper, we address the manner in which the continuum-scale diffusive properties of smectite-rich porous media arise from their molecular- and pore-scale features. Our starting point is a successful model of the continuum-scale apparent diffusion coefficient for water tracers and cations, which decomposes it as a sum of pore-scale terms describing diffusion in macropore and interlayer "compartments." We then apply molecular dynamics (MD) simulations to determine molecular-scale diffusion coefficients D(interlayer) of water tracers and representative cations (Na(+), Cs(+), Sr(2+)) in Na-smectite interlayers. We find that a remarkably simple expression relates D(interlayer) to the pore-scale parameter δ(nanopore) ≤ 1, a constrictivity factor that accounts for the lower mobility in interlayers as compared to macropores: δ(nanopore) = D(interlayer)/D(0), where D(0) is the diffusion coefficient in bulk liquid water. Using this scaling expression, we can accurately predict the apparent diffusion coefficients of tracers H(2)0, Na(+), Sr(2+), and Cs(+) in compacted Na-smectite-rich materials.
Geochemistry Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. icbourg@lbl.gov
Ian C
IC
Bourg
Garrison
G
Sposito
D002586:Cesium / Q000737:chemistry; D004058:Diffusion; D004784:Environmental Monitoring; D007700:Kinetics; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D058608:Nanopores; D010316:Particle Size; D016062:Porosity; D017640:Silicates / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry; D013324:Strontium / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000737:chemistry*
D017640:Silicates; D014874:Water Pollutants, Chemical; D002586:Cesium; D012964:Sodium; C033214:Smectite; D013324:Strontium