Competitive Sorption of H₂O and CO₂ in Clay Mineral Interlayers

The first-reported competitive CO₂-H₂O sorption isotherms on a naturally occurring expandable clay mineral (saponite) exchanged with Na, K, and Cs at near ambient CO₂ concentrations (1% CO₂) are reported by combining ATR-FTIR spectroscopy with gravimetric sorption methods. Little CO₂ sorption was observed at high relative humidity (RH) but increased significantly at low RH. Unlike prior work where CO₂ sorption was reported on samples that were previously subjected to heating and/or evacuation treatment to remove sorbed H₂O, this study manipulated CO₂ sorption by controlling RH. CO₂ sorption increased in the order Cs- > K- > Na-exchanged saponite and was anticorrelated with the Gibbs energy of hydration of the exchangeable cation. As expected, H₂O sorption followed the opposite trend, with increased H₂O sorption occurring in the order Na > K > Cs. The amount of CO₂ sorbed using 1 atm CO₂ (p/p_o = 1.0) ranged from 50 to 30 mg_CO2/g_clay and from 1.25 to 0.8 mg_CO2/g_clay for 1% CO₂ (p/p_o = 0.01) in N₂. The sensitivity of the ATR-FTIR measurements is demonstrated by detecting the CO₂ stretch of adsorbed CO₂ from air containing 500 ppmv CO₂. The spectral features of sorbed CO₂ were minimally affected by the nature of the exchangeable cation or variations in H₂O content. However, the position of the HOH bending band shifted significantly from 1638 to 1610 cm^-1 as H₂O content decreased, reflecting changes in intermolecular hydrogen bonding between H₂O molecules in the saponite interlayer. The shift in position of the HOH bending band to lower energy was coincident with increased CO₂ sorption. CO₂ sorption in clay mineral interlayers is consistent with sorption on weakly hydrated partially hydrophobic sites found on low charge density smectites and is enhanced by lower RH and by the presence of weakly hydrated exchangeable cations like Cs⁺ or K⁺. The greatest CO₂ sorption occurs on Cs-saponite because this is the least hydrated cation of the three used in the study and indicates that CO₂ sorption occurs on the neutral portion of the siloxane surface.