PNNL

Abstract

Diffusive exchange of solutes between intragranular pores and flowing water is a recognized but poorly understood contributor to dispersion. Intragranular porosity may also contribute to the slow sorption phenomenon. Intragranular pores may be sparsely interconnected, raising the possibility that accessible porosity and diffusive exchange are limited by pore connectivity. We used a pore-scale network model to examine pore connectivity effects on accessible porosity, diffusivity, and tortuosity in spherical particles. The diffusive process simulated was release of a non-sorbing solute initially at equilibrium with the surrounding solution. High-connectivity results were essentially identical to Crank’s analytical solution. Low-connectivity results were consistent with observations reported in the literature, with solute released more quickly at early times than indicated by the analytical solution, and more slowly at late times. Values of accessible porosity, diffusivity, and tortuosity scaled with connection probability, distance to the sphere’s exterior, and/or the sphere’s radius, as predicted by percolation theory. The scaling relationships should be useful in conventional modeling of diffusive processes in porous spherical particles.