Thermodynamic analysis of supercritical CO₂ as a fluid phase for naphthalene adsorption onto soil in a packed column

A comparison of supercritical CO₂ and dichloromethane as fluid phases for the adsorption of naphthalene onto a sandy loam soil is presented. In order to compare the solvents a thermodynamic study on the naphthalene-soil-supercritical CO₂ system is necessary. The isotherms for naphthalene adsorption onto soil from the supercritical CO₂ phase are modeled using the classical Freundlich isotherm model. The equilibrium partition coefficients are also determined from experimental data and they are found to increase with temperature. The heat of adsorption of naphthalene at 308K is determined by a thermodynamic analysis in terms of standard chromatographic terminology and is in the range of weak physical adsorption for the soil used in this study.

Dichloromethane is a popular solvent for producing control contaminated soils. It readily dissolves PAHs and has a low viscosity and surface tension. However, the extent of interaction with the soil matrix is unknown. Supercritical CO₂ may be a superior solvent to create a simulated in situ contaminated soil based on its enhanced solvent and mass transfer properties relative to liquid solvents. An exploration of the differences in qualitative and quantitative adsorption between these two solvents is conducted. Experimental batch desorption curves are found to be similar for both control soils when dichloromethane is used as the extraction solvent. Neither supercritical CO₂ nor dichloromethane has any apparent deleterious effect on the indigenous microbial population.