科研之友
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摘要
Materials with low permeability such as clays are major components in engineered or natural barriers in waste disposal sites. Diffusion of dissolved chemicals through such materials is a relatively slow process that can strongly limit the spreading of contaminants. The macroscopic diffusive transport of chemicals through barrier materials originates from Brownian motion of molecules and ions in the solution and their interaction with the surface of the solid medium. Up-scaling these molecular phenomena to the continuum scale, which is required for large-scale and long-time predictions, is particularly challenging because the considered scales differ by orders of magnitude. To address the up-scaling problem we developed a two-step simulation approach which enables us to derive macroscopic diffusion coefficients of water and ions for continuum equations from pore scale molecular diffusion coefficients. Our starting points for the up-scaling procedure were local pore diffusion coefficients derived in molecular dynamics simulations for specific local environments, such as the interlayer or edge regions of clay particles. We then assigned these local diffusion coefficients to different types of porosity of a model clay structure and obtained the structure-averaged diffusion coefficient of the sample by random walk simulations. We demonstrated the validity of the suggested approach by comparing the results of direct molecular simulations for a stack of pyrophyllite nanoparticles with the two-step up-scaling approach. We then investigated the effects of mineralogical heterogeneities and of anion exclusion on the larger-scale diffusion coefficients for a model clay structure which resembles a typical clay material. Our up-scaling concept is general and can be used for up-scaling molecular diffusion coefficients for porous materials with almost arbitrarily complex structures.
- 出版日期2011-4-14
