Unveiling Supercritical CO ₂ and CH ₄ Uptake Mechanisms in Natural and Acid-Treated Clays through Advanced Six-Parameter SLD Analysis

Supercritical methane and carbon dioxide adsorption mechanisms on five clay samples were studied through volumetric single-component adsorption experiments at 313 K using an indigenously designed volumetric setup, followed by the modeling of excess and absolute adsorption, pore characterization using low-pressure N 2 (at 77K) and CO 2 (at 273 K) adsorption, and morphological and elemental composition study using scanning electron microscopy coupled with energy dispersive spectroscopy. Excess adsorption data were fitted to a simplified local density model to obtain the gas density profile within the clay pores. To solve this model, a novel six-parameter optimization technique was used. The obtained density profile indicated multilayer formation near the clay surface. The structural parameters obtained from this model, including slit length and specific surface area, were in good agreement with the pore characteristics derived from low-pressure gas adsorption. Further, the absolute adsorption was fitted using Langmuir, Freundlich, Toth, and Sips models to obtain the relevant model parameters for the adsorption capacity and affinity, surface heterogeneity, and the CO 2 separation coefficient. Langmuir and Sips models described the experimental data better than Toth and Freundlich models. The modeling results showed two to three times higher adsorption capacity for CO 2 than for CH 4 . Acid-treated montmorillonite exhibited the highest adsorption capacities for both gases, reaching 64 cc/g for CO 2 and 27 cc/g for CH 4 at STP. Gas uptake increased with surface area, total pore volume, and micropore volume but decreased with mean pore diameter. All materials exhibited a higher Langmuir affinity for CO 2 compared to CH 4 . The study shows that clay mineral composition and pore architecture strongly control CO 2 and CH 4 adsorption, supporting their potential use for enhanced methane recovery, carbon dioxide sequestration, and CO 2 separation from natural gas.