Ion exchange selectivity (Mg²⁺, Ca²⁺ and K⁺) in hydrated Na-montmorillonite: insights from molecular dynamic simulations

ABSTRACT

The cation exchange selectivity in the Na-montmorillonite interlayer has been extensively studied as a part of the mechanism of the cation exchange reaction, which also provides an important insight into the inhibition of montmorillonite expansion by inorganic salts. While the variation of interlayer structure associated with interlayer Na⁺ and water molecules during the ion exchange process has less been explored, to determine the possibility of replacing interlayer Na⁺ ions by the ion exchange reaction, the present study focuses on the effects of the added electrolyte cation (Mg²⁺, Ca²⁺ and K⁺) on the structure properties, diffusion features and the number of surface hydrogen bonds in hydrated Na-montmorillonite at different hydration stages by using molecular dynamic (MD) simulations. The simulation results showed that Ca²⁺ has a better tendency to exchange Na⁺ ions at a high hydration degree, and K⁺ is easier to exchange Na⁺ at a low hydration degree. The lower mobility of interlayer species and the relativity large change in the number of hydrogen bonds with the addition of Mg²⁺ reflected its weak possibility of exchanging Na⁺.

KEYWORDS:

• Na-montmorillonite
• molecular simulation
• electrolyte cation
• ion exchange

Acknowledgments

The authors would like to thank the financial support from the National Natural Science Foundation of China (grant number 52274281) and Beijing Municipal Natural Science Foundation (grant number 2192046).

Disclosure statement

No potential conflict of interest was reported by the author(s).

CRediT authorship contribution statement

Yufeng Huang: Data curation, Formal analysis, Software, Writing – Original Draft. Zhijun Zhang: Conceptualisation, Methodology, Funding acquisition, Writing: Review & Editing.

Additional information

Funding

This work was supported by Beijing Natural Science Foundation: [Grant Number 2192046]; the National Natural Science Foundation of China: [Grant Number 52274281].