| Summary: | 碩士 === 國立中興大學 === 土壤環境科學系所 === 94 === Clay and organoclay are usually used to prevent the spread of organic contaminants in landfills and remediation sites and to clean the organic pollutants by sorption processes. Sorption of organic pollutants by organo-inorganic complexes can occur via different mechanisms. The aims of this thesis were to evaluate the effects of carbon contents of clays and relative humidity (RH) on the sorption of volatile organic compounds (VOCs) on pure clay, montmorillonite, and organoclays, montmorillonite modified with hexadecyltrimethyl ammonium (HDTMA), and to elucidate the sorption and desorption mechanisms. The sorption and desorption kinetics of toluene on clay and organoclays was investigated by a gravimetric method. The inverse gas chromatography was used to obtain the equilibrium sorption coefficients of a diverse of VOCs on clay and organoclays. The interactions between VOCs and clays were characterized via linear solvation energy relationships (LSERs).
The results of kinetics experiments showed that, at low relative humidity (~0% RH), the equilibrium sorption coefficients for toluene vapor sorbed on clay were larger than those on organoclays, and the sorption coefficients for toluene sorbed on organoclays decreased with decreasing surface area. The results indicated that the sorption capacities of clay and organoclays were determined by the specific surface areas of the sorbents under dry conditions. The apparent diffusivities for toluene sorption and desorption on clay and organoclays increased with increasing carbon contents of sorbents. The organic fraction of organoclays resulted in the expansion of clay and accelerated the diffusion process of toluene molecules into organoclay particles. The rates for toluene desorpion from clay and organoclays were smaller than the rates for toluene sorption, respectively, which were attributed to the difficulties for toluene to desorb from the interlayers of clay and organoclays. In addition, a great quantity of toluene residual, after the equilibrium for toluene desorption was reached, was observed in the pure clay. It was further attributed to the micropores the pure clay had only.
In the sorption equilibrium experiments, the results showed that the equilibrium sorption coefficients for VOCs sorption on clay and organoclays decreased with increasing relative humidity, which proved that the existence and amount of water indeed suppressed the sorption capacities for clay and organoclays to sorb VOCs. At low relative humidity (~0% RH), the capacities for pure clay were usually greater than those for organoclays, which consisted with the kinetics experimental results. Under ambient (~55% RH) and high relative humidity (~90% RH), the equilibrium sorption coefficients of VOCs on clay and organoclays generally increased with increasing carbon contents of sorbents, indicating that the organic moieties of organoclays may behave as a partition medium and play the predominant role for the sorption. Characterization of sorption mechanisms of VOCs on clay and organoclays using LSERs revealed the main interaction force for VOCs vapors sorption on clay and organoclays was London dispersion force. At the same time, VOCs interacting with clay and organoclays through π-/n-electron pairs always displayed a negative contribution to VOCs sorption.
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