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SOIL PHYSICS

Sorption of Trichloroethylene in Humic Acid Studied by Experimental Investigations and Molecular Dynamics Simulations

Department of Soil and Environmental Sciences, Center of Nanoscience and Technology, National Chung Hsing Univ., 250, Kuo-Kuang Rd., Taichung 402, Taiwan, R.O.C

^* Corresponding author (yhs{at}nchu.edu.tw).

Gas transport of volatile organic compounds through soils is important for understanding the fate of these organic compounds. Soil organic matter plays a key role in the transport of organic compounds in soils; however, studies have not been performed at the molecular level. A gravimetric experimental method and a computer simulation method were used to study the sorption–desorption of organic contaminants in humic substances. The average apparent diffusivities of trichloroethylene (TCE) in soil humic acid are 1.1 x 10^–8 cm²/s for sorption and 3.0 x 10^–9 cm²/s for desorption. The activation energies are 29.3 and 59.5 kJ/mol for sorption and desorption, respectively. The molecular simulation results of the kinetics and the activated energy of TCE sorption in humic acid are in good agreement with the experimental data. Both results indicate that the sorption rate of TCE to humic acid increases with the environmental temperature. The sorption of TCE into humic acid is mainly diffusion controlled. Molecular dynamics of chlorinated volatile organic compounds in natural humic substances does yield meaningful results, which can help with understanding the sorption mechanism of organic chemicals in soils at the molecular level.

Abbreviations: MD, molecular dynamics • TCE, trichloroethylene • VOC, volatile organic compound