Analysis of vapor phase adsorption equilibrium of 1,1,1-trichloroethane on dry soils

Abstract

The migration and fate of volatile organic pollutants in soils are highly dependent on their vapor-phase sorptive behavior. The objective of the research presented in this paper was to investigate the vapor-phase adsorption/desorption equilibrium of 1,1,1-trichloroethane (TCA) on dry soils. A gravimetric adsorption apparatus was developed and used to generate adsorption/desorption isotherms of TCA at 288, 293 and 298 K on two soil samples, obtained from Visalia-California and Times Beach-Missouri, with different physical/chemical characteristics. The influences of temperature and soil properties were investigated. Isosteric heats of adsorption were calculated and heat curves were established. The experimental data were correlated by well-known vapor phase isotherm models including the Polanyi Potential, the BET, and the GAB models. Equilibrium isotherms of TCA on both soils were Type II, characterizing vapor condensation to form multilayers, and they exhibited hysteresis upon desorption. A positive correlation between the soil's specific surface area and its sorption capacity was observed. Clay content and pore size were also dominating factors. Thermal data showed that the adsorption of TCA vapor on soil was primarily due to physical forces and both samples exhibited energetically heterogeneous surfaces. Results followed the Potential Theory satisfactorily and led to a single temperature-independent characteristic curve for each soil-TCA pair. The BET model gave an accurate data fit for up to 40% of the saturation pressure, while the GAB model provided a superior fit of the data for the entire relative pressure range.