Modeling the Ionic Strength Effect on Diffusion in Clay. The DR-A Experiment at Mont Terri

Soler, Josep M.; Steefel, Carl I.; Gimmi, Thomas; Leupin, Olivier X.; Cloet, Veerle (2019). Modeling the Ionic Strength Effect on Diffusion in Clay. The DR-A Experiment at Mont Terri. ACS earth and space chemistry, 3(3), pp. 442-451. ACS Publications 10.1021/acsearthspacechem.8b00192

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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-, Br-) and H-3. 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-Planck equation) occurs through both domains. A 1D radial model considered a single pore diffusion coefficient (D-p = 10(-9) m(2)/s) for cations and H-3 in the bulk porosity, and a smaller D (3 X 10(-10) m(2)/s) for anions. D-p values in the EDL were smaller (10(-11) m(2)/s), except for Cs+ and K+ (5 x 10(-10) and 2 x 10(-10) m(2)/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., Cs-137(+), Sr-90(2+), I-129(-)) 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.

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