Molecular Dynamics Study of the Interactions between Cations/Carbon Dioxide and Selected Mineral Surfaces

  • Author / Creator
    Naderi Khorshidi, Zeinab
  • Undoubtedly, we need to become increasingly aware of the changes and problems introduced to our environment. Oil sands industrial waste disposal and greenhouse gas emission are the two major by-products of the industrial processes that cause environmental concerns, such as huge tailing ponds and global warming. Reducing the impact of these harmful by-products on our environment is inevitable and based on many studies clay minerals can be useful in this regard. However, due to the complex structure of the minerals, experimental procedures cannot always provide a decent insight into the interaction and reaction of minerals with the environment. Nowadays, with advancements in computing facilities, computer molecular simulation is a powerful tool which enables us to describe the behavior of the materials in different conditions in more details in higher resolution in molecular levels. For treatment of oil sands tailings, alkali-activation of these tailings is considered as an appropriate solution which turns tailing ponds to the geopolymer through geopolymerization. The geopolymerization involves the dissolution of aluminosilicates in an alkali solution followed by the polymerization of the dissolved aluminate and silicate oligomers to form an amorphous geopolymer. It is generally accepted that the dissolution determines the properties of the resultant geopolymer. Accordingly, in the first part of the study (chapter four, five and six), a series of molecular dynamics (MD) simulations were carried out in the isothermal-isobaric (NPT) ensemble at 298 K and 1 atm to study the initial stage of dissolution process that takes place at the two basal surfaces (tetrahedral and partially deprotonated octahedral) of kaolinite in alkali media. Two different alkali media containing Na+ and K+ cations were modeled at 1 M, 3 M, and 5 M concentrations. The influence of structural vacancies on the interaction of the basal surfaces of kaolinite exposed to alkali media was studied in chapter five and the chapter six is particularly aimed at elucidating the dissolution mechanism of kaolinite in alkali media with the presence of aqueous medium contaminants. The MD results showed that cations migrated to the vicinity of the deprotonated sites, trigging the dissociation of the nearby surface hydroxyl and aluminates groups into the solution. However, Na+ and K+ exhibited different dissolution mechanisms. In particular, Na+ induced more dissociation of the surface hydroxyl groups, whereas K+ resulted in more dissociation of aluminate groups. Regarding the defect sites, Al vacancies on the octahedral surface promoted the dissolution of aluminate groups into the solution compared to the surface without Al vacancies. However, there existed a vacancy concentration (2 Al vacancies per 576 Al atoms) at which the dissolution amount was the maximum and above which the dissolution decreased with increasing Al vacancy concentration. In presence of inorganic salts contaminants (i.e., CaCl2 and MgCl2) atomic density profiles show that all cations including those from the contaminants adsorbed on the two basal surfaces intensifying the dissociation of the aluminate groups from the deprotonated octahedral surface. The number of the aluminate groups dissociated decreased with increasing contaminants concentration. Structural analyses of the deprotonated octahedral surface indicated that the crystallinity of the surface decreased with increasing simulation time and alkali solution concentration. No dissolution of the tetrahedral surface was observed for all systems studied. In chapter seven, MD simulation was used to study the role of water in the intercalation of carbon dioxide in a model Mg-Al-Cl-hydrotalcite mineral at ambient pressure and temperature. It was observed that high CO2 storage capacity could be achieved at low water concentrations or even without the presence of water. However, high water concentrations could also yield similar CO2 storage capacity but in this case, the presence of water led to a significant interlayer spacing expansion. The expansion was due to the change in the orientation distribution of the CO2 molecules. Analyzing the orientation of CO2 molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO2 molecules exhibited a wider range of orientations with a significant fraction of them oriented perpendicular to the mineral surface, especially at high CO2 concentrations. Also, it was observed that water molecules formed extensive hydrogen bonds network.

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  • Degree
    Doctor of Philosophy
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