%0 Journal Article
%T Modeling the Ionic Strength Effect on Diffusion in Clay. The DR-A Experiment at Mont Terri
%J -
%D 2019
%R https://doi.org/10.1021/acsearthspacechem.8b00192
%X Solute diffusion in compacted clays depends on ionic strength through its control on the thickness of the electrical double layer (EDL) on the charged clay surfaces. In the DR-A field experiment (Mont Terri, Switzerland), synthetic porewater was circulated through a borehole for 189 days, leading to the out-diffusion of a variety of tracers into the Opalinus Clay. The borehole solution was then replaced with a higher-salinity solution for an additional 540 days, leading to the diffusion of Cs+, Ca2+, Mg2+, and Sr2+ back into the borehole and to an increase in the out-diffusion of anions (I每, Br每) and 3H. The experimental results were interpreted using the CrunchClay code, which includes a mean electrostatic potential model for the EDL. The EDL corresponds to a second continuum in addition to bulk electrically neutral porewater. Species-specific diffusion (Nernst每Planck equation) occurs through both domains. A 1D radial model considered a single pore diffusion coefficient (Dp = 10每9 m2/s) for cations and 3H in the bulk porosity, and a smaller Dp (3 ℅ 10每10 m2/s) for anions. Dp values in the EDL were smaller (10每11 m2/s), except for Cs+ and K+ (5 ℅ 10每10 and 2 ℅ 10每10 m2/s, respectively). The model reproduced well the experimental results and showed the capability to consider temporal changes in geochemical conditions affecting the transport and retention of potentially important radionuclide contaminants (e.g., 137Cs+, 90Sr2+, 129I每) in underground geological nuclear waste repositories. Coupled multicomponent diffusion together with the electrostatic properties of the charged surfaces are essential in the development of predictive models for ion transport in clays
%U https://pubs.acs.org/doi/10.1021/acsearthspacechem.8b00192