Sorption and Desorption Kinetics of Chlorinated Volatile Organic Vapors on Clay Minerals

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.