Mechanistic understanding of the dehydroxylation reaction of smectites: Insights from reactive force field (ReaxFF) molecular dynamics simulation

Abstract Smectite, a major barrier material for contaminants in Earth’s critical zone, is a layered aluminosilicate mineral with cis-vacant (cv) and trans-vacant (tv) configurations. High-temperature transformations of smectite are directly related to smectite’s thermal reactions. However, the precise thermal reaction mechanisms and thermally induced structural transitions of cv and tv smectite remain debated. In this study, we systematically investigated the mechanism of thermal reactions of cv and tv smectite models using reactive force field (ReaxFF) molecular dynamics. We explored the deprotonation and the intralayer dehydration steps of cv and tv smectites at 700 and 900 K. The results revealed that the dehydroxylation reaction of cv smectite exhibited more difficulty than tv smectite at 700 K, while demonstrating less difficulty at 900 K. Furthermore, it was found that the dehydroxylated cv and tv smectites evolved to a consistent structure spontaneously. Our findings further confirmed that cv smectite has a higher dehydroxylation temperature than tv smectite and thus provided a theoretical basis for distinguishing cv and tv smectites using thermogravimetric analysis (TGA). Moreover, we gathered TGA data of smectites from different regions, confirming that montmorillonites have a cv structure and almost all smectites from China (over 11 provinces) are cv. This study provides a molecular-level understanding of the thermal reaction mechanisms of smectites and a physical basis for further study and application of smectites.