Abstract
With the aim to determine the influence of dominant interlayer cation on the sorption and diffusion properties of bentonite, diffusion experiments with Sr on the compacted homoionous Ca- and Na-forms of Czech natural Mg/Ca bentonite using the planar source method were performed. The bentonite was compacted to 1400 kg·m−3, and diffusion experiments lasted 1, 3 or 5 days. Two methods of apparent diffusion coefficient Da determination based on the analytical solution of diffusion equation for ideal boundary conditions in a linear form were compared and applied. The determined Da value for Ca-bentonite was 1.36 times higher than that for Na-bentonite sample. Values of Kd were determined in independent batch sorption experiments and were extrapolated for the conditions of compacted bentonite. In spite of this treatment, the use of Kd values determined by batch sorption experiments on a loose material for the determination of effective diffusion coefficient De values from planar source diffusion experiments proved to be inconsistent with the standard Fickian description of diffusion taking into account only the pore diffusion in compacted bentonite. Discrepancies between Kd and De values were measured in independent experiments, and those that resulted from the evaluation of planar source diffusion experiments could be well explained by the phenomenon of surface diffusion. The obtained values of surface diffusion coefficients Ds were similar for both studied systems, and the predicted value of total effective diffusion coefficient De(tot) describing Sr transport in the Na-bentonite was four times higher than in the Ca-bentonite.
Highlights
If the time-lag method is used for the evaluation of through-diffusion experiments, the maintaining of the required ideal boundary conditions might be challenging because the flux into the compacted clay layer is large due to the sorption, and changing of solutions in reservoirs might become a source of uncertainties [4]
10−3 m3 ·kg−1, and (47.4 ± 1.8) × 10−3 m3 ·kg−1 on Na-BaM in NaCl background electrolyte, respectively. Such a difference between Sr sorption on clays with different dominating interlayer cations is in agreement with the previous findings on the natural bentonite [9]
Two methods of apparent diffusion coefficient Da determination based on the analytical solution of diffusion equation for given boundary conditions in a linear form were introduced and both were found suitable and comparable because both provided similar values of Da and their uncertainty estimates
Summary
If the time-lag method is used for the evaluation of through-diffusion experiments, the maintaining of the required ideal boundary conditions might be challenging because the flux into the compacted clay layer is large due to the sorption, and changing of solutions in reservoirs might become a source of uncertainties [4]. From this point of view, non-steady-state methods appear to be more suitable. In all methods using either an inlet and/or outlet reservoir, it is necessary to deal with separating filters, which affect the evaluation of the experiments, as was shown, e.g., by Glaus et al [4,5] or Samper et al [6]
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