Diffusivity and Porosity in Rock Matrix Related to the Ionic Strength in the Solution

Abstract

The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, and tracer techniques with 36Cl, 22Na+ and 3H (HTO) were used as a research method. A mathematical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Fick's second law for a case where part of the pores are so-called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. It was found that the rock-capacity factor is an increasing function of the ionic concentration of the solution in the case of Cl indicating ion-exclusion, while the opposite is true in the case of Na+ indicating ion-exchange type sorption. The effective diffusion coefficient was also found to vary as a function of the salinity in the case of 36Cl. In the case of 22Na, the effect was opposite and weaker. The diffusion of tritium through the rock samples was clearly higher than the diffusion of 36Cl. Part of the difference is explained by the smaller effective porosity for 36Cl. The rest can probably be explained by the steric effects on the chloride ion caused by the negatively charged pore surfaces in the narrow pores.