Discrete modelling of contaminant diffusion in porous media with sorption

A meso-scale model for diffusion of foreign species through porous media is proposed. The model considers diffusion as a continuous process operating on a discrete geometrical structure dictated by the pore size distribution. Local diffusivities and hence pore space connectivity are dependent on the size of the diffusing species and the sorption of those species onto the pore walls. The bulk diffusivity of the medium has been analysed to consider the effects of pore structure alone and in combination with sorption. The chosen medium is bentonite, which is being considered for use as a barrier to radionuclide transport in future deep geological repositories for nuclear waste. Results for transient diffusion of U(VI)-complex through bentonite are presented and very good agreement with experiments is demonstrated. Results for diffusion of larger chemical complexes are also presented to illustrate the effect of reduced pore space connectivity on steady-state and transient transport parameters. Diffusion of larger complexes can be used for experimental validation of the model. The proposed methodology can be used for any micro- and meso-porous material with known distribution of pore sizes. It can be extended to other pore space changing mechanisms, in addition to sorption, to derive mechanism-based evolution laws for the transport parameters of porous media. © 2013 The Authors. Published by Elsevier Inc. All rights reserved.