Role of Cations in Adsorption of Supercritical Carbon Dioxide at Smectite Mineral–Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Simulation Studies

Abstract

The microscopic understanding of uptake and retention of supercritical carbon dioxide by expandable layered aluminosilicate minerals is of central importance for large-scale geological sequestration of CO2. At the molecular scale, the interlayer charge-balancing cations (CBCs) play a crucial role in CO2 adsorption by these cost-effective and environmentally friendly materials. This article investigates the influence of CBCs on the structure, dynamics, and energetics of CO2–H2O mixtures nanoconfined in four different montmorillonites with different CBCs (Cs+, K+, Na+, and Ca2+) using molecular dynamics and enhanced sampling simulations. The results reveal that both CO2 and H2O coexist in a single layer at the center of the interlayer, that CO2 molecules orient parallel to the basal surface and form two-dimensional percolation paths that facilitate their lateral diffusion, and that the near-neighbor CO2 molecules preferentially adopt a distorted slipped-parallel geometry in all of these systems. The calculated activation barriers for CO2 and H2O to diffuse away from the first coordination shells of cations show that Cs+–CO2 and Ca2+–H2O affinities are relatively higher than those of the other cations. The residence times in the cation coordination shells, relative orientations, and diffusion constants of intercalates are also quantified. The presented results can be useful for rational design of engineered layered minerals with improved CO2 retention capacity.

Overview of the Adsorption and Transport Properties of Water, Ions, Carbon Dioxide, and Methane in Swelling Clays

“…For example, the presence of CO 2 and CH 4 hardly affects the distribution of the interlayer water and ions in these systems. ,,,,,,,,,,,,,, The distribution of CO 2 and CH 4 normal to the clay surface shows layered structures similar to those observed, for example, for water. In general, the in-plane distributions of the interlayer water and ions almost coincide and they were positioned in mutually exclusive regions with CO 2 and CH 4 . ,,,,, The isosteric heat of adsorption of CO 2 was higher than that of CH 4 , which agrees with the higher uptake of CO 2 . , The RDFs provide insights into the interactions of the ion–water–clay system with CO 2 and CH 4 . ,,,,,− ,,, For example, as hydration/RH increases, CO 2 ,,,, and CH 4 , lose coordination to surface oxygens and become more fully solvated to water molecules. It was shown that CO 2 hardly migrates into the first hydration shell of the strongly hydrated interlayer ions (Figure ).…”

“…The molecular dynamics (MD) simulations allow one to assess the dynamic properties of the system. For the simulations of the clay systems, − the structure of the unit cell of the clay (Si 8 Al 4 O 20 (OH) 4 ) is obtained from, for example, Skipper et al The simulations mainly focus on the low-charge (Wyoming-type) montmorillonite M 0.75/ n (Si 8 )­(Al 3.25 Mg 0.75 )­O 20 (OH) 4 , where M represents a counterion (e.g., Li + , Na + , K + , Mg 2+ , Ca 2+ , or Sr 2+ ) and n is the charge on the ion. This clay contains coordinated tetrahedral silica sheet and octahedral alumina sheet forming 2:1 tetrahedral-octahedral-tetrahedral (TOT) layers.…”

“…Molecular simulations have provided important insights into the adsorption behavior of CO 2 and CH 4 in dry and wet clays. ,− It was shown that the uptake of CO 2 (Figure ) and CH 4 generally increases with increasing basal spacing. ,,,,, The total uptake is obtained in the simulations, whereas experiments usually provide the excess uptake. To convert the total uptake to the excess quantity, simulations should provide the estimates of the accessible pore volumes, for example, obtained from the GCMC simulation of adsorption of helium in clays. , However, due to the fluid–solid interactions, an enhanced adsorption of CO 2 and CH 4 is obtained at low pressures for relatively small basal d -spacings.…”

“…Molecular simulations have also provided important insights into the transport behavior of CO 2 and CH 4 in dry and wet clays. ,,,,,,− ,− ,,,,, The self-diffusion coefficients of CO 2 and CH 4 generally increased with increasing hydration/RH (Figure ) because of the associated swelling of the interlayer region. The mobility of CO 2 in the 1W state is about an order of magnitude lower than that of its bulk value, and a similar trend was observed for CH 4 .…”

“…The mobility of the interlayer water and ions generally increased with increasing RH. The adsorption and transport processes of CO 2 and CH 4 in swelling clays have also been extensively studied using molecular simulations. ,− These studies have shown that, for example, the presence of CO 2 and CH 4 hardly affects the distribution and mobility of the interlayer water and ions in these systems. The preferential adsorption of CO 2 over CH 4 plays important roles in diffusion processes.…”