An integrated sorption-diffusion model for the calculation of consistent distribution and diffusion coefficients in...

Area of Science:

• Geochemistry
• Materials Science
• Chemical Engineering

Background:

• Understanding ion transport in porous media like bentonite is crucial for applications such as nuclear waste disposal.
• Existing models often treat porewater chemistry, surface reactions, and charged pore wall effects separately.
• A unified approach is needed for consistent prediction of solute behavior in compacted clays.

Purpose of the Study:

• To develop and validate an integrated surface chemical model for predicting ion diffusion and sorption in bentonite.
• To consistently account for porewater chemistry, surface reactions, and electric double layer (EDL) effects on ion transport.
• To assess the model's predictive capability for various ions (cations, anions, neutral species) and conditions.

Main Methods:

• Integration of a thermodynamic sorption model with a diffusion model based on electric double layer (EDL) theory.
• Optimization of the model's key parameter (Stern and diffuse layer contribution to surface charge compensation) using Cs diffusion data in Kunipia-F bentonite.
• Direct prediction of apparent diffusivities (Da), effective diffusivities (De), and distribution coefficients (Kd) for multiple ions and varying dry densities.

Main Results:

• The integrated model successfully predicted apparent diffusivities (Da) for Cs, Sr, Cl-, I-, and TcO4-.
• Calculated distribution coefficients (Kd) for Cs and Sr were consistent with model predictions across different bentonites and dry densities.
• Effective diffusivities (De) for Cs, HTO, and TcO4- were calculated and found to be consistent with Da and Kd values.
• Model predictions showed good agreement with published experimental data for both diffusion and Kd values.

Conclusions:

• The developed surface chemical model provides a consistent framework for understanding and predicting ion transport in compacted bentonite.
• The model effectively captures the influence of charged pore walls and surface chemistry on the diffusion of various ionic species.
• This approach offers a reliable tool for assessing the long-term performance of bentonite barriers in geological disposal systems.