Fundamental theoretical study of nanoscale ion transport through charged porous rocks: Part II - Diffusion-advection

Depending on the rock permeability, different mass transport mechanisms have to be distinguished. Whereas mass transport through porous rocks characterized by low permeabilities is governed by diffusion, mass transport through highly permeable rocks is governed by advection. Coupled diffusive-advective mass transport then plays an important roll in many geoengineering systems, such as mineralization, ore body genesis and pollutant migration (including the transport of bacteria, viruses and other microorganisms). Additionally a large number of porous materials are characterized by the presence of surface charge which affects the permeability of the porous medium. In this paper the fundamental equations describing micro-scale multi-ion transport (i.e., the generalized Nernst-Planck equations are coupled with the Navier-Stokes equations describing advective mass transport and the Poisson equation of electrostatics) are given. Various effects such as (i) pore size, (ii) concentration of ionic species, and (iii) the size of particle charge on solvent transport are studied at the nanoscale by means of finite element simulations. The nanoscale findings provide insight into events encountered at the macroscale