  A non-equilibrium thermodynamics model able to analyze the combined effect of
diffusion and adsorption in porous materials is proposed. The model considers
the coupled dynamics of the diffusive phase, described by a diffusion type
equation, and the adsorbed phase governed by a generalized Langmuir equation.
It is shown that the combination of diffusion and adsorption can be treated as
diffusion in an effective medium, with an effective diffusion coefficient
depending on the characteristics of the adsorption kinetics. The interplay
between these effects leads to the appearance of different regimes in which the
effective diffusion coefficient exhibits peculiar behaviours such as a decrease
due to the presence of the adsorption process and a non-monotonous behavior
resulting from particle interactions. We outline applications of the model
developed to diffusion in porous materials such as zeolites and bentonites, and
to engineered highly porous materials.
