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

Abstract

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 + , Ca 2+ , Mg 2+ , and Sr 2+ back into the borehole and to an increase in the out-diffusion of anions (I-, Brand nd 3 H. 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 3 H in the bulk porosity, and a smaller D p (3 × 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 × 10-10 and 2 × 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., 137 Cs + , 90 Sr 2+ , 129 I-) 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.

Multicomponent Ionic Transport Modeling in Physically and Electrostatically Heterogeneous Porous Media With PhreeqcRM Coupling for Geochemical Reactions

“…In PHREEQC, the Donnan layer thickness is calculated by assuming a cylindrical shape of the total pore space, where the fractions of the free and Donnan porosity are dynamically calculated as (Appelo, ; Appelo & Wersin, ): where n D (−) is the number of Debye lengths and r (m) is the radius of the cylindrical pore space (typically calculated as r = 2 V Tot / A s with V Tot (m 3 ) and A s (m 2 ) being the total pore volume and charged surface area) and f FW + f DL + f IL = 1. In MMIT‐Clay we also adopt this approach in order to allow a direct comparison with the PHREEQC calculations, whereas a linear relation between the Donnan porosity and the Debye length ( f DL = A s n D κ −1 / θ ), as used in other studies (e.g., Soler et al, ) can also be easily implemented in the model.…”

Reactive Transport Modeling of Coupled Processes in Nanoporous Media

“…The contribution of the Stern layer to the total charge compensation can be estimated with molecular dynamic simulations, which provides a growing set of independent information that can be used to calibrate models of ion distribution and mobility in nanopores (Rotenberg et al 2007(Rotenberg et al , 2014Jardat et al 2009;Tournassat et al 2009b;Bourg and Sposito 2011;Obliger et al 2014;Tinnacher et al 2016;Bourg et al 2017). The dual continuum model was first developed in PHREEQC, but is also now available in CrunchClay Steefel 2015, 2019;Soler et al 2019).…”

“…In practice, the geometrical factor values of diffuse layer and bulk water, as well as the concentration distribution between the bulk porosity and the diffuse layer in the dual continuum model, are fitted on experimental data. Reactive transport calculations have been essential to decipher and quantify the mechanisms explaining the contrasting diffusional behavior of cations, anions and neutral species in clay materials (Appelo and Wersin 2007;Appelo et al 2010;Glaus et al 2013Glaus et al , 2015Tournassat and Steefel 2015;Tinnacher et al 2016;Soler et al 2019). The fact that the accumulation of cations in the diffuse layer enhances their effective diffusion coefficient…”

Reactive Transport at the Crossroads

“…Another recent demonstration of the MEP model is provided by a modeling study of the DR-A borehole diffusion experiment carried out in the Opalinus Clay in Switzerland (Soler et al 2019). In this experiment, a cocktail of anions, cations, and non-reactive tracers were added to the borehole and their concentrations monitored over time as the cocktail solutes diffused out into the Opalinus Clay.…”

“…The discipline has reached a level of maturity well beyond what could be demonstrated even 15 years ago. This is shown now by the successes with which complex and in many cases coupled behavior have been described in a number of natural Earth environments, ranging from corroding storage tanks leaking radioactive Cs into the vadose zone (Zachara et al 2002;Steefel et al 2003;Lichtner et al 2004), to field scale sorption behavior of uranium (Davis et al 2004;Li et al 2011;Yabusaki et al 2017) to the successful prediction of mineral and pore solution profiles in a 226 ka chemical weathering profile (Maher et al 2009), to the prediction of ion transport in compacted bentonite and clay rocks (Tournassat and Steefel 2015;Soler et al 2019;Tournassat and Steefel 2019, this volume). Yet for those thinking deeply about Earth and Environmental Science problems impacted by reactive transport processes, it is clear that many challenges remain.…”