| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 87 === Recent reports have pointed out that the interaction between VOCs and clay minerals in the soils which contain low organic matter is important. Sorption/desorption mechanisms of toluene vapor on four clay minerals (montmorillonite, illite, vermiculite and kaolinite ) were investigated in situ with a Fourier transform infrared spectrometer (FTIR). The sorption/desorption kinetics were continuously monitored for more than 10 days and the levels of the relative humidity used were below 3% or above 90%.
Comparing with the spectra of toluene liquid and toluene vapor, the IR spectra for toluene on clay minerals reveal that the sorbed toluene were present in multiple states. It was comprised of sorbed labile vapor, sorbed liquid and persistent sorbed phase. The interaction between toluene and clay minerals obvious could be recognized from the changes of the wavelength and intensity of sorbed spectra, especially for montmorillonite.
Several new bands, discriminated from that of the toluene vapor or liquid, appeared on the spectra of toluene sorbed on montmorillonite and illite during long-term contamination. New peaks of 1630 cm-1, 1690 cm-1, 2881 cm-1 and 2939 cm-1 for montmorillonite were persistent to desorption. It is speculated that toluene molecules gradually move into the interlayer of montmorillonite or the chemical interaction between both has taken place, and the confined toluene molecules are responsible for the persistent phase.
Three types of toluene sorbed phases on four clays minerals show different sorption/desorption rates, and sorbed toluene phase on montmorillonite has the slowest sorption/desorption rate. The results are interpreted in the context of the sorbent porosity and the complex network of the clay minerals that limit the rates of desorption for the persistent species. Persistent sorbed phase on montmorillonite were resistant to be desorbed even the desorption course had last for 30 days. However, 24 hours was sufficient for the other investigated clay minerals. It is suggested that the existence of montmorillonite is one of the rate-controlling factors for the transport of volatile organic vapor in soil.
As the relative humidity of the system was increased to 90%, the interaction between toluene and clay minerals became weaker and the amount of sorbed toluene decreased significantly due to the interference of water. During long-term contamination, the IR spectra of sorbed toluene on montmorillonite appeared in new absorption bands and resisted to desorption . The chemical interaction of toluene with montmorillonite still occurred under high relative humidity .
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