Role of Cations in CO₂ Adsorption, Dynamics, and Hydration in Smectite Clays under in Situ Supercritical CO₂ Conditions

Abstract

This paper explores the molecular-scale interactions between CO2 and the representative smectite mineral hectorite under supercritical conditions (90 bar, 50 °C) using novel in situ X-ray diffraction (XRD), infrared (IR) spectroscopy, and magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on understanding the roles of the smectite charge balancing cation (CBC) and H2O in these interactions. The data show that supercritical CO2 (scCO2) can be adsorbed on external surfaces and in the confined interlayer spaces of hectorite at 50 °C and 90 bar, with the uptake of CO2 into the interlayer favored at low H2O content and when the basal spacing is similar to a monolayer hydrate of hectorite (1WL, ∼12.5 Å). These results are in agreement with published spectroscopic and molecular modeling data for the related smectite Na-montmorillonite. Charge balancing cations with small radii, large hydration energies, and low polarizabilities tend to scavenge H2O from humid scCO2 or retain the H2O they held before scCO2 exposure, swelling spontaneously to a bilayer hydrate (2WL) dominated state that largely prevents CO2-ion interactions and influences the extent of CO2 intercalation into the interlayer. In contrast, ions with large radii, low hydration energies, and large polarizabilities more readily form close associations with CO2 with the energetics enabling coexistence of CO2 and H2O in the interlayer over a wide range of scCO2 humidities. Integrating our results with those from molecular dynamics simulations of wet CO2-bearing montmorillonites suggest that adsorbed CO2 in 1WL-type interlayers is oriented with its long axis parallel to the clay sheets and experiences dynamics dominated by anisotropic rotation about the axis perpendicular to the CO2 long axis at rates of at least ∼105 Hz. If appreciable CO2 is adsorbed in 2WL-type interlayers, it must experience a mean orientation and dynamic averaging affects that mimic the 1WL-type adsorption environment. External surface adsorbed CO2 is dynamically similar to the 1WL case, but the CO2 long axis samples a larger range of orientations with respect to the smectite surface and adopts a different mean angle between the long axis and the smectite surface. Our data also suggest that equilibrating hectorite with a large volume of scCO2 at 50 °C and 90 bar leads to interlayer dehydration, with the extent of dehydration correlating with the hydrophilicity of the CBC.

Competitive Adsorption of H2O and CO2 in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

“…[40][41][42] Cs + and Ca 2+ are representative of cations with greatly different hydration energies and charge/radius ratios, 48,53,58 and they have greatly different CO2/H2O intercalation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 behavior. 17 The results here are in excellent agreement with experimental data of Bowers et al 17 and provide a basis for their detailed molecular scale structural, dynamic, and energetic interpretation and understanding.…”

“…59 However, there are two important differences between the natural sample used in our experiments and the simulated model: (i) the model has 30% higher layer charge, and (ii) the simulated octahedral layer has only OHgroups, whereas the natural sample has ~ 55% Ffor OHsubstitution. 17,46,47 The hydration behavior of synthetic fluoro-hectorite has been discussed previously. 60 The simulation supercells for the Cs-and Ca-hectorite models consist of 16 crystallographic unit cells of hectorite (4 × 2 × 2) and include two interlayer galleries with surface areas of ~20.9 Å × 18.2 Å (a=b=c: 90º) that are large enough to overcome any finite size effects.…”

A review of geochemical–mechanical impacts in geological carbon storage reservoirs

“…CO 2 adsorption is thought to have little effect on mineral structure, with the exception of swelling clays (e.g., montmorillonite [MMT]). On these materials, it adsorbs within the interlayer spaces and can displace water . This process can result in shrinkage, altering the intergranular pore space and possibly the transport flow paths and mechanical properties .…”

“…On these materials, it adsorbs within the interlayer spaces and can displace water. 22,23 This process can result in shrinkage, altering the intergranular pore space and possibly the transport flow paths 24,25 and mechanical properties. 26,27 CO 2 as carbonic acid in water can also promote the dissolution of minerals 28 and, in some cases, secondary mineral precipitation; this results in changes to the pore space, which affects transport flow paths 29 and, in some cases, mechanical properties.…”

“…Cations with large radii, low hydration energies, and large polarizabilities form closer associations with CO 2 , which in turn enables coexistence of CO 2 and H 2 O in the interlayer over a wide range of RH. Bowers et al 22 reported that equilibrating hectorite with a large volume of scCO 2 may lead to interlayer dehydration (and clay shrinkage), with the extent of dehydration correlating with the hydrophilicity of the CBC. Michels et al 40 used high-pressure XRD to monitor the swelling of the structure due to CO 2 sorption into Li-, Na-or Ni-smectite clays (synthetic fluorohectorites).…”