CO₂ Adsorption on Variably Hydrated Cation-Exchanged Montmorillonite-Rich Clays

Layered swelling clay minerals like montmorillonite (MMT) can competitively and synergistically adsorb CO₂ and H₂O in their interlayer galleries. This work examines how different interlayer cations, relative humidity levels (and amount of cosorbed H₂O), and (de)hydration history affect CO₂ adsorption on MMT and MMT-rich bentonite at near-ambient pressure and temperature. For CO₂ to be adsorbed, the MMT requires either large (e.g., Cs⁺) or hydrated interlayer cations to provide a sufficiently wide interlayer gallery, and must not have too much H₂O adsorbed competitively with CO₂. Na-MMT and initially anhydrous Mg- and Ca-MMT studied under increasing relative humidity conditions adsorb little CO₂. However, Mg- and Ca-MMT can effectively adsorb CO₂ if first hydrated and then mildly dried such that the cations remain hydrated while the competitively adsorbed excess H₂O is removed. Because of their high stability and the favorable shape of their CO₂ adsorption isotherms, low-cost (near-)natural Mg- and Ca-bentonite can be used for (cyclic) CO₂ adsorption and separation purposes, similar to the more expensive Cs-bentonite.