Molecular Dynamics Study of Clay-Polymer Interactions in the Treatment of Mature Fine Tailings

  • Author / Creator
    Sun, Wenyuan
  • In this dissertation, all-atom molecular dynamics (MD) simulations were performed to study the mechanisms of clay-polymer interactions and the effects of solution chemistry in the treatment of mature fine tailings (MFT). The fine solids in MFT were mainly clay particles such as montmorillonite and kaolinite. Polymers with different properties were widely used to flocculate the clay particles.
    Chitosan, polyacrylamide (PAM), and anionic polyacrylamide (APAM) as commonly used cationic, neutral, and anionic polymer flocculants were firstly introduced to the proximity of montmorillonite (Mt) surface. A monolayer coating of chitosan was observed to form quickly on the Mt surface, driven by coulombic attraction between the cationic polymer and anionic Mt surface. PAM and APAM did not show effective adsorption by themselves. However, when PAM or APAM was added after chitosan, their adsorption was facilitated by the pre-adsorbed chitosan. PAM could adsorb either directly on Mt or on chitosan, in the form of clusters or individual molecules. On the other hand, APAM only adsorbed on chitosan, forming a two-layer structure above the Mt surface. Adding chitosan simultaneously with PAM or APAM, instead of sequentially, did not change the characteristics of the adsorption. The synergetic adsorption of polymers was attributed to the interplay of electrostatic attraction between Mt and chitosan, hydrogen bonding between chitosan and PAM, as well as electrostatic attraction between chitosan and APAM.
    In colloidal systems such as polymer-modulated MFT, the influence of ions on the solid-polymer interactions cannot be neglected, in particular the interaction between similarly charged polymer and solid. The adsorption of anionic polyacrylamide (APAM) on anionic Mt, in an aqueous solution containing monovalent or divalent salts was simulated to address the effects of ions. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl2) was significant, which could not be explained by the Poisson−Boltzmann theory alone. APAM coordinated to the solvated Ca2+ by displacing 1-2 water oxygens in the first coordination shell of Ca2+. Ca2+ ions in the adsorbed Ca2+−APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to Mt. The number of adsorbed Ca2+−APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of Mt and APAM by Ca2+. Increasing adsorption of Ca2+−APAM complexes also promoted APAM adsorption through direct hydrogen bonding with Mt.
    Besides ions, the presence of asphaltene in MFT greatly influences the flocculation, through interacting with both the clay surfaces and the polymers. A model asphaltene, C5Pe, was simulated to study such effects. Potential of mean force (PMF) calculations demonstrated the interplay between enthalpy-driven adsorption of C5Pe on the hydrophilic alumina surface of kaolinite and entropy-driven adsorption on the hydrophobic siloxane surface of kaolinite. When added between different types of clay surfaces in water, participating in the hetero coagulation of clay particles, C5Pe tended to adsorb on the hydrophilic surface. In the adsorbed aggregate, the hydrophobic parts of C5Pe molecules stacked in a parallel manner, which aligned perpendicularly to the surface, while the hydrophilic part formed hydrogen bonds with the surface.
    When APAM was added simultaneously with C5Pe to an adjacent Mt basal surface, the clustering of APAM molecules was suppressed by C5Pe and the adsorption of APAM was enhanced by the concurrent interaction of C5Pe with APAM and the Mt basal surface. In contrast, near a kaolinite edge surface, unadsorbed C5Pe in the bulk served as growth nucleus for APAM cluster, attracting APAM to the bulk solution and reducing their adsorption.
    Together, the molecular insights derived from the simulation results in this dissertation complemented experimental investigations and can further help the polymer treatment of MFT.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.