X-ray diffraction study of k- and ca-exchanged montmorillonites in CO₂ atmospheres

Powder X-ray diffraction shows that K- and Ca-exchanged montmorillonites swell upon interacting with CO₂ at ambient temperatures, depending on their initial hydration state. K-exchanged montmorillonite swells rapidly to a maximum d(001) of ∼12.2 Å. In contrast, Ca-exchanged montmorillonite swells more slowly, but reaches a maximum d(001) of ∼15.1 Å. Reaction kinetics differ significantly between the K- and Ca-exchanged montmorillonite complexes. Expansion of K-exchanged montmorillonite samples was rapid, occurring on time scales of tens of minutes or less. The Ca-exchanged montmorillonite samples continued to expand over periods up to 42 h. Aging of both K- and Ca-exchanged montmorillonite complexes at elevated CO₂ pressure for 1-2 days resulted in greater stability when CO₂ pressure was released. The observed intercalation reactions have important consequences for carbon sequestration: (1) CO₂ absorption by swelling clays may represent a significant pathway for storage of CO₂. (2) The swelling of smectites under CO₂ pressure may have a significant impact on the permeability of caprock formations.