| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 90 === Abstract
The adsorption mechanisms between VOCs and nature materials have been broadly studied. However, the long- time adsorption mechanisms have not been investigated and understood well due to the restriction of the experimental period and techniques.
The long- time adsorption mechanisms of toluene on three different clay minerals (STx-1b, IMt-2, KGa-2) were investigated by using batch experimental systems under different temperature. The kinetics of adsorption were observed at different time period. We also discussed the changes of free energy and enthalpy when toluene was adsorbed by STx-1b for a longer time. The XRD spectroscopy were performed to identify whether the toluene molecules entered the interlayer of clay minerals, so as we could reveal the actual mechanisms of the slow sorption.
The amount of toluene adsorbed has a good linear relationship with the specific surface area (the micropore specific area) of clay minerals. STx-1b adsorbed the highest amount of toluene, IMt-2 did the second, and KGa-2 adsorbed the less. The BET equation could describe the adsorption of IMt-2 and KGa-2 well. However, the adsorption isotherm of STx-1b was deviated from the model at higher relative pressure, resulted from the micropore adsorption. The adsorption isotherm looks like a Type I adsorption isotherm.
Under both dry and wet conditions, the adsorption rate of toluene on three clay minerals had two stages. At dry conditions, the adsorption rate of the first stage (0~1 week) was 100 times higher than the adsorption rate of the second stage (1~16 weeks) of STx-1b and IMt-2, and was about 10 times higher for KGa-2. There was also about 40 times higher for STx-1b when the system was wet. Besides, it could reach about 80% of the ultimate adsorbed amount at the first stage by STx-1b. This may be due to the high specific surface area of STx-1b. Therefore, STx-1b can adsorb almost 80% of toluene at a short time. The second adsorption rates of IMt-2 and KGa-2 increased at 40 oC. The different adsorption rates between dry and wet conditions could be the result of competition between water and toluene. At dry condition, IMt-2 and KGa-2 were easily to become aggregates which required higher temperature and longer time to let toluene to penetrate.
It is suggested that the effect of " active entry" and adsorption heat make the adsorption isotherm jump at 30oC. From the adsorption free energy and the adsorption enthalpy, we also found that no matter at what temperature the pores which adsorb toluene the most are of 20 Å. The adsorption mechanism at 40oC is suggested to be condensation. The adsorption mechanism at 30oC is the surface adsorption of micropores (10 Å) or the adsorption of higher energy sites. The adsorption mechanism at 25oC would be the adsorption of mesopores (20Å∼30Å) or condensation of toluene at pores with pore radius larger then 30Å.
According to the result of XRD spectroscopy, we were not able to directly figure out whether toluene molecules had entered into interlayer of STx-1b or not. However, it is still possible that toluene molecules can enter the interlayer of STx-1b.
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