Simultaneous estimation of effective and apparent diffusion coefficients in compacted bentonite

Abstract

Effective diffusion coefficients ( D e) are usually measured by means of “through-diffusion” experiments in which steady state is reached, and the “time-lag” methods are used to estimate the apparent diffusion coefficient ( D a). For sorbing radionuclides (as caesium), the time needed to reach steady-state conditions is very large, and the precision in D a determinations is not satisfactory. In this paper, a method that allows determining simultaneously effective and apparent diffusion coefficients in compacted bentonite without reaching steady-state conditions is described. Basically, this method consists of an “in-diffusion” experiment in which the concentration profile in the bentonite sample is used to estimate D a, and the temporal evolution of the solute concentration in the reservoir is used to estimate D e. This method has several advantages over the typical “through-diffusion” experiments, in particular: (a) experiment duration is significantly shorter, (b) D a values are measured with greater precision and (c) it is not necessary to maintain a constant solute concentration in the reservoir. This new method has been used to estimate the effective and apparent diffusion coefficients for caesium in FEBEX bentonite and in order to validate it, the results have been compared with results previously obtained with standard methods. Experimental results have been satisfactorily modelled using a simple model of diffusion in porewater and the measured value of D e(Cs) is very similar to D e(HTO) in the same bentonite. There is no evidence of “surface diffusion” in FEBEX bentonite for caesium.