The supercritical fluid extraction of anthracene and pyrene from a model soil : an equilibrium and mass transfer study

Supercritical carbon dioxide is used for the extraction of organic contaminants from a porous soil system. The systems studied are supercritical CO₂, white quartz sand, a model soil, and two selected polycyclic aromatic hydrocarbons, anthracene and pyrene. The adsorption and desorption studies are carried out at temperatures and pressures ranging from 308 to 328 K (35° to 55° 0), and 7.58 to 24.13 MPa (1,100 to 3,500 psia) respectively. Equilibrium phase partition coefficients and adsorption isotherms are determined for the aforementioned systems. The phase partition coefficients show temperature and supercritical fluid density dependencies. The adsorption isotherms of these polycyclic aromatic hydrocarbons on the sand and soil are modeled with an experimental linear isotherm equation. The effects of soil moisture on phase partitioning is also examined. A intraparticle mass transport model is developed to describe the complex desorption process that occurs within this supercritical extraction system. The model incorporates the mathematical representation of diffusion through the hydrodynamic boundary layer, local adsorption equilibrium, and an effective intraparticle mass transfer resistance. The intraparticle mass transfer resistance is represented by the assumed mechanism of surface diffusion. From the experimental desorption breakthrough curves, the film mass transfer and effective intraparticle diffusion coefficients are regressed. The model describes the desorption profile of anthracene with good accuracy: however, it does not for pyrene. The extraction of pyrene from soil is governed by intraparticle mass transfer resistance, a very broad mass transfer zone, while for anthracene interparticle resistance, a narrow mass transfer zone, dominates.