Abstract
The adsorption of organic micropollutants (OMPs) on high-silica zeolites is characterized by adsorption isotherms with various shapes. The occurrence of an S-shaped adsorption isotherm indicates the lack of adsorption affinity for OMPs at low, environmentally relevant equilibrium concentrations. In this study, S-shaped isotherms were observed during batch experiments with 2,4,6-trichlorophenol (TCP) and FAU zeolites. This is the first time that an S-shaped isotherm is reported for the adsorption of OMPs on high-silica zeolites. Monte Carlo (MC) simulations in the grand-canonical ensemble were used to obtain a better understanding of the mechanism of the S-shaped adsorption isotherms. From the MC simulation results, it was observed that multiple TCP molecules were adsorbed in the supercages of the FAU zeolites. It was found that the π-π interactions between TCP molecules give rise to the adsorption of multiple TCP molecules per supercage, and thus causing an S-shaped adsorption isotherm. Simulations also revealed that water molecules were preferentially adsorbed in the supercages and sodalite cages of the FAU zeolites. FAU zeolites with a higher Al content adsorbed a higher amount of water molecules and a lower amount of TCP, and showed less pronounced S-shaped isotherms.
Highlights
Zeolites are crystalline, microporous aluminosilicates with a welldefined 3-dimensional structure, composed of tetrahedral SiO4 and AlO4 clusters connected to each other by shared oxygen atoms
Monte Carlo (MC) simulations were used to explain the mechanism of S-shaped adsorption isotherms of TCP adsorption on high-silica FAU zeolites
This study shows that the occurrence of S-shaped adsorption isotherms was attributed to the lateral interaction of TCP, e.g., the π- π interaction
Summary
Microporous aluminosilicates with a welldefined 3-dimensional structure, composed of tetrahedral SiO4 and AlO4 clusters connected to each other by shared oxygen atoms. To compensate the charge imbalance caused by the Al content of the framework, exchangeable cations (usually alkali and alkaline earth cations) are located in the cavities of the structure. Due to these intrinsic characteristics, e.g. exchangeable cations and a well-defined pore structure, zeolites are widely used as catalysts, molecular sieves, and adsorbents for air-pollution remediation, gas separation and removal of volatile organic compounds [1,2,3,4,5,6]. Various families of commercially available high-silica zeolites, including the Faujasite (FAU), Mordenite (MOR), Beta (BEA) and ZSM-5 (MFI) types, have been shown to be effective adsorbents for OMP removal from water [15,16,17,18]
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