Molecular Simulations Of The Montmorillonite Interlayer Microstructure And The Sorption Towards Organics

Clay minerals are a class of hydrated phyllosilicates which are abundant in earth surface system, geographically ubiquitous and low-cost. They contain special nanoscale two-dimension layer structure and high surface reaction activity. Therefore clay minerals not only play a key role in reserving and migrating various natural elements and molecules, but are also widely used in pollution control areas such as sewage treatment and soil remediation. The interface reaction activity between clay minerals and various molecules/ions has attracted great attention and the related interface reaction mechanism is research hotspot in multidisciplinary research of mineralogy, geochemistry and environmental science. However, due to the complicated structure of clay minerals and limit of experimental methods, the interaction mechanism of molecules/ions on the surface of clay minerals still remain obscured. In this thesis, montmorillonite was chosen to represent the clay minerals and molecular dynamic simulations have been performed to study the microstructure of montmorillonite interlayer space and the adsorption characteritics of organic molecules. The hydrated methane structure in the interlayer space of montmorillonite and the main factors influencing the stability of this structure has been elucidated. Then the effect of surfactant loading level to the microstructure and sorption characteristics of typical long chain organo-montmorillonite (CTMA-Mt) and shot chain organo-montmorillonite (TMA-Mt) in water saturated condition has been illustrated. On the basis of present study, the following conclusions are made: (1) Methane molecules are fixed in the center of siloxane six-member ring on montmorillonite surface and solvated by nearly water molecules, which present a hydrate-like structure. It was found that sufficient interlayer water representing three water layer in the interlayer space is necessary for coordinating with methane and forming stable hydrate-like structures. The amount and distribution of layer charge can remarkably affect the behavior of methane hydration by confining interlayer water mobility. Montmorillonite with larger layer charge and tetrahedral negative charge site is more effective in confining water and lead to more stable hydrate structure. (2) The interlayer microstructure of CTMA-montmorillonite in water saturated condition was elucidated. The sorption mechanism of hydrophobic organic molecules is partition process coexisting with various special interaction such as H-bonding. The arrangement of interlayer CTMA transformed from bilayer to inclined paraffin type, forming organic aggregates in water saturated condition. The size and pack density of aggregate increased with increasing CTMA loading level and phenol was adsorbed into the aggregates from montmorillonite siloxane surface, indicating a partition process. In addition, high CTMA loading level decreased the sorption affinity of CTMA-Mt toward phenol by increasing the packing density and cohesive characteristic of the aggregates. The oxygen atoms on siloxane surface and water molecules around Br- serve as H-bond acceptor while water molecules around Ca2+ serve as H-bond donor, affecting the interaction between phenol and CTMA-Mt. (3) The influence of organic ions loading level on the interlayer structure, hydration and adsorptive characteristic of TMA-Mt with Ca2+ or K+ in the interlayer has been clarified. When Ca2+ was in the interlayer of TMA-Mt, the conformation and hydration characteristic changed with increasing loading level. The arrangement of TMA transformed from bilayer to single layer and went into siloxane six-member ring gradually. The hydration energy of TMA-Ca-Mt decreased with increasing loading level. Though the hydrophobicity increased, the effective adsorption space decreased with increasing loading level. The adsorption site of phenol is the top of siloxane surface oxygen. When K+ was in the interlayer of TMA-Mt, the conformation and hydration characteristic did not change much with increasing loading level. The arrangement of TMA was single layer and trapped into the siloxane six-member ring and hydration energy of TMA-K-Mt only had little change. The effective adsorption space decreased with increasing loading level, which obstructed the uptake of phenol. Phenol located into the center of siloxane six-member ring. The hydration energy of TMA-K-Mt is smaller than TMA-Ca-Mt with the same TMA loading amount which lead to a more hydrophobic interlayer environment, indicating that the adsorption capacity of TMA-K-Mt is better than TMA-Ca-Mt.