Effect of Ion Concentration on Diffusion Through Clay

The applicability of an equation that suggests the effective diffusion coefficient of ions through a charged porous medium is linearly related to the square root of the concentration of ions is evaluated. Data from experiments on diffusion of NaCl and LiCl solutions through Na and Li bentonites were analyzed using the proposed equation. Plots of effective diffusion coefficient versus square root of concentration are consistent with the formulation that the effective diffusion coefficient declines linearly with the square root of concentration. The analysis also indicates that effective diffusion coefficient of an ion is dependent on the counter ion in solution as well as that at the clay exchangeable site. For solutions up to 0.07 meq/cc, the results further indicate that it is feasible to combine all the porous medium characteristics into a single factor referred to as the apparent tortuosity factor.