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/po = 1.0) ranged from 50 to 30 mgCO2/gclay and from 1.25 to 0.8 mgCO2/gclay for 1% CO2 (p/po = 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.