Concentration dependence of diffusion coefficients of 22Na+ and 134Cs+ in opalinus clay rocks

We developed an inverse technique based on the Boltzmann-Matano method from which apparent coefficients of diffusion of decaying radionuclides in saturated porous media can be calculated. The technique uses the assumption that the concentration profiles (or breakthrough curves) have S shapes and can be well approximated by the error function with three adjustable parameters (the position, height, and slope). The resulting dependence of the apparent diffusion coefficient on concentration was given as a rather simple formula involving inverse error function. Here we wish to apply this straightforward technique to obtain concentration dependences of the apparent diffusion coefficients of 22Na+ and 134Cs+ for which the corresponding breakthrough curves were measured for two types of dense opalinus clay rocks. We compare our more general results with those from the original study in which the apparent diffusion coefficient was assumed to be constant and indicate that the latter approach need not be appropriate.