A structural coarse-grained model for clays using simple iterative Boltzmann inversion

Area of Science:

• Environmental Science
• Materials Science
• Geochemistry

Background:

• Cesium-137 (Cs+) is a hazardous byproduct of nuclear energy with long half-life.
• Cs+ readily exchanges with K+ in micaceous clays, forming interstratified Cs- and K-illite.
• Understanding this ion exchange is crucial for managing radioactive waste.

Purpose of the Study:

• To develop and validate a coarse-grained (CG) model of Cs+ and K+ ion exchange in anhydrous illite interlayers.
• To investigate the structural and energetic consequences of ion exchange at clay interlayers.
• To compare the favorability of ordered versus adjacent interstratification.

Main Methods:

• Development of a CG model using iterative Boltzmann inversion.
• Simulation of ion exchange processes in illite interlayers.
• Analysis of interlayer expansion, structural relaxations, and interstratification thermodynamics.

Main Results:

• The CG model accurately reproduces experimental observations of ion exchange and interstratification.
• A 70-fold speedup in simulation time compared to all-atom models was achieved.
• Neighboring Cs-exchanged layers induce interlayer expansion and reduce cohesion in adjacent K-illite layers.
• Ordered interstratification of Cs- and K-illite is found to be thermodynamically and mechanically favorable.

Conclusions:

• The CG model provides a computationally efficient tool for studying Cs+ remediation in clays.
• Ion exchange at clay interlayers leads to structural relaxations that differ from previous theories.
• Ordered interstratification is a preferred mechanism for Cs+ and K+ exchange in illite.