Water vapor diffusive transport in a smectite clay: Cationic control of normal versus anomalous diffusion
Abstract
The transport of chemical species in porous media is ubiquitous in subsurface processes, including contaminanttransport, soil drying and soil remediation. We study vapor transport in a multiscale porosity material, a smectiteclay, in which water molecules travel in mesopores and macropores between the clay grains but can also intercalateinside the nanoporous grains, making them swell. The intercalation dynamics is known to be controlled by thetype of cation that is present in the nanopores; in this case exchanging the cations from Na+ to Li+ acceleratesthe dynamics. By inferring spatial profiles of mesoporous humidity from a space-resolved measurement of grainswelling, and analyzing them with a fractional diffusion equation, we show that exchanging the cations changesmesoporous transport from Fickian to markedly subdiffusive. This results both from modifying the exchange dynamicsbetween the mesoporous and nanoporous phases, and from the feedback of transport on the medium’spermeability due to grain swelling. An important practical implication is a large difference in the time needed forvapor to permeate a given length of the clay depending on the type of intercalated cation.