含氯揮發性有機物氣體於黏土礦物上之吸/脫附動力

Abstract

含氯碳氫物(二氯乙烯、三氯乙烯、四氯乙烯、三氯乙烷等)是土壤及地下水污染場址中最常見的污染物，其具毒性之特性，對人體健康與環境危害甚鉅。欲正確評估整治復育方案之可行性，必須充分了解污染物在土壤的吸脫附行為。本研究採用薄膜-傅立葉轉換紅外線光譜儀法(Film-FTIR method)，主要針對土壤無機固相的主要成份-黏土礦物進行吸脫附含氯碳氫物之動力實驗，並同時監測含氯有機物吸附於黏土礦物上是否有發生化學結構轉化反應。 由吸/脫附動力之實驗結果顯示，二氯乙烯、三氯乙烯(黃，2003)、四氯乙烯以及三氯乙烷於鈣飽和蒙特石上之吸/脫附行為沒有不可逆的現象發生。在高相對濕度（RH＞95%）下，此五種含氯有機化合物於鈣飽和蒙特石上之吸/脫附速率，以1,2-順、反-二氯乙烯最快，三氯乙烷次之，三氯乙烯、四氯乙烯最慢。此外，不同相對濕度下，有機物的吸脫附行為有明顯差異，以雙吸附基模式來描述高/低相對濕度下之吸脫附行為，推估在高相對濕度下是由較快的黏土礦物表面之水膜吸/脫附作用，和較慢的有機物分子溶入水膜進而吸附於黏土礦物表面所組成；在低相對濕度下是由快的表面吸/脫附和較慢的微孔脫附所組成。由三氯乙烯在不同相對濕度之等溫吸附曲線，亦可發現在低相對溼度（RH<1%）時，吸附曲線呈現BET型式，而在高相對溼度（RH＞95%）時，其吸附量下降且吸附曲線成線性。 藉由紅外線光譜，發現三氯乙烷在高/低相對濕度下於鈣飽和蒙特石上可能發生化學結構轉化反應。利用IC分析三氯乙烯於鈣飽和蒙特石(4個月)之脫氯程度，結果顯示三氯乙烯有脫氯反應，可視為三氯乙烯於鈣飽和蒙特石已發生化學結構轉化之證據。

Chlorinated hydrocarbons, such as dichloroethylene, trichloroethylene, tetrachloroethylene, and trichloroethane, are common contaminants found in soil and groundwater. Due to the characteristics of their toxicity, they are extremely harmful to human health and the environment. In order to correctly evaluate the feasibility of remediation programs, it is necessary to fully understand the adsorption/desorption behaviors of these compounds in soil. In present study, we use Film-FTIR method to analyze the adsorption/desorption kinetics of the chlorinated hydrocarbons in clay, the major inorganic soil component. In addition, this technique also allows possible transformations (degradation) of the adsorbed chlorinated organic compounds to be detected. The results of the adsorption/desorption kinetics show that the adsorption/desorption behaviors of dichloroethylene, thrichloroethylene (Huang,2003), tetrachloroethylene ,and 1,1,1-thrichloroethane on Ca- montmorillonite are not irreversible. Under high relative humidity (RH > 95%), the adsorption/desorption rates of these five chlorinated organic compounds on Ca-montmorillonite decrease in the following order: cis or trans 1,2-dichloroethylene > 1,1,1-trichloroethane > trichloroethylene and tetrachloroethylene. Moreover, the adsorption/desorption behavior is obviously different under different relative humidity. A two-site model can be used to describe the adsorption/desorption behavior under different relative humidity. At high relative humidity, the adsorption of the organic molecules onto water film surface is faster, while the transferring of the organic molecules through the water layer onto the surface of clay minerals is slower. In contrast, the adsorption/desorption mechanism consists of fast surface adsorption/desorption and slower porous adsorption/desorption steps under low relative humidity. The adsorption isotherm is BET type at low relative humidity (RH<1%), while a lower adsorption capacity and a linear adsorption isotherm was observed at higher relative humidity (RH>95%). Furthermore, we analyzed FTIR spectra and found that trichloroethane undergoes chemical transformations on Ca-montmorillonite at either low or high relative humidity. The IC analysis of trichloroethylene on Ca- montmorillonite (4 months) further indicates that trichloroethylene was dechlorinated. Therefore, this provides another evidence of the chemical transformation of trichloroethylene on Ca-montmorillonite.