Investigating Mechanisms And Factors Affecting The Interaction Between Clay Minerals And Organic Compounds

Clay minerals are one of the most important constituents of soil. Interactions between organic contaminants and clay minerals are key factors affecting their fate, transport, and transformation in the environment. Therefore, it is of great importance to investigate the underlying mechanisms and controlling factors of clay-organic interactions for better understanding the environmental behavior of organic contaminants. Furthermore, it is also essential for developing novel pollution control technologies utilizing clay materials.In this dissertation, we mainly aimed to answer two questions:(1) how clay particle size and exchangeable cations affect the interactions between montmorillonites and aromatic ionizable organic compounds; (2) how clay surface properties relate to the high sorption of hexafluorobenzene on smectites. The clay-organic interactions were studied by both the macroscopic sorption experiments and the spectroscopic techniques to provide a mechanistic understanding.We examined the size effect on the sorption of two model aromatic ionizable organic compounds, tetracycline and tannic acid, on montmorillonites. The montmorillonite size fractions were prepared using a successive centrifugation method. Our results showed that the 11000 rpm fraction with sizes in the typical colloidal range had the lowest sorption of tetracycline and tannic acid. Based on the sorption data and the detailed characterization of clay size fractions, we propose that the impeded sorption on 11000 rpm size fraction was caused by its small particle size and high edge/surface ratio, which led to strong edge-to-face interactions and the formation of "house-of-cards" microstructure. The microstructure significantly lowered the availability of edge sites and part of the cation exchange sites on the clay interlayers which were active sites for tannic acid and tetracycline sorption.Two organoclays were prepared by exchanging with mono-(HDTMA) and poly-(PDADMA) organic cations. The sorption behavior and major controlling factors of six aromatic ionizable organic compounds on these two organoclays were investigated. Our results show that the electrostatic interaction was the main mechanism for the sorption of ionizable solutes on polycation-organoclay, whereas the electrostatic interaction and hydrophobic partition both contributed to the monocation-organoclay sorption. As compared to commercial activated carbon, organoclays are more advantageous in sorption kinetics and its preferential sorption of aromatic ionizable organic compounds. Moreover, the polycation-organoclays can be recycled by adjusting the solution pH.We also tested the sorption of hexafluorobenzene onto three clay minerals by batch sorption experiments and probed the mechanisms of the observed high sorption using spectroscopic techniques and molecular simulation. We propose that the strong interactions between hexafluorobenzene and clay minerals are mainly caused by the n-n electron donor acceptor (EDA) interaction between F atoms in hexafluorobenzene and Si atoms on clay surfaces. The n-n EDA interaction between O atoms on clay surfaces and C atoms in the aromatic ring of hexafluorobenzene is another possible driving force.