Modelling carbon dioxide adsorption behaviour on montmorillonite at supercritical temperatures

Expandable clay minerals play a pivotal role in the geological sequestration of greenhouse gases due to their contribution to storage capacity and caprock integrity. The charge-balancing cations in the interlayer space are known to influence carbon dioxide adsorption. The present study investigated the general adsorption behaviour and characteristics of montmorillonite towards CO₂ adsorption at temperatures above critical point, typical to geological sequestration. Modelling on the excess isotherm and absolute isotherm was used to compare the variation of the adsorption behaviour of sodium, potassium and calcium montmorillonite. Excess isotherm modelling using the monolayer Ono-Kondo (O-K) model successfully captured the experimental adsorption. The lateral interaction of the adsorbed molecules on the montmorillonite surface, a variable with the type of cation, remarkably affected the strength of adsorption. The derived adsorbed phase density data confirmed the onset of swelling in montmorillonite at pressures below the critical point. Additionally, the study comprehended the influence of approximations used to derive the absolute isotherm from the experimental isotherm and investigated the validity of common theoretical models to represent adsorption. The study recommended the application of Dubinin-Astakhov (D-A) isotherm with reciprocal van der Waals density approximation or liquid density approximation to model the adsorption of CO₂ on montmorillonite.