Modeling structure-composition feedbacks during ion exchange driven swelling of smectite clay minerals
Abstract
Smectite-rich geomaterials provide excellent sealing capabilities for subsurface storage of nuclear waste and CO2. The same properties that make smectites ideal seals - their nanporosity, structural charge, and propensity to swell - also make it difficult to model mass transport through clays. In particular, the relationship between counterion composition and microstructure is poorly understood, and progress in this area has been limited by our inability to study coupled ion exchange and water adsorption with sufficiently high spatial and temporal resolution. We use X-ray scattering, cryo-TEM imaging, and molecular simulations to investigate the structural evolution of montmorillonite (MMT) suspended in aqueous KCl + NaCl mixtures. The population distribution of coexisting 2- and 3- water layer hydrates of MMT is used to derive a thermodynamic model for ion exchange driven collapse of saturated MMT that treats the clay phase as a mixture of homoionic end-member layer states in equilibrium with the same aqueous fluid reservoir.
We apply our model to calculate the clay microstructural response to perturbations in aqueous electrolyte composition. The calculated variation in bulk clay ion exchange selectivity and swelling pressure is consistent with experimental data over a range of dry densities relevant to subsurface systems. Increasing aqueous K/Na decreases the proportion of more hydrated states, decreasing the swelling pressure. A simple relationship is found to link water adsorption and ion exchange thermodynamics, such that the layer state more selective for a given ion becomes more abundant with increasing proportion of that ion. This finding can explain why mechanical compaction increases clay selectivity for low hydration enthalpy ions like Cs+. Our results provide a thermodynamic basis for the widely observed relationship between clay composition and structure, providing a path forward for developing mechanically coupled models for mass transport in clay.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H44B..02S
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 1805 Computational hydrology;
- HYDROLOGY;
- 1847 Modeling;
- HYDROLOGY;
- 3947 Surfaces and interfaces;
- MINERAL PHYSICS