Supercritical extraction of polychlorinated biphenyls from a solid matrix

Abstract

The contaminated soil around the uncontrolled waste sites in the United States is considered to be a potentially hazardous environmental pollution. At present, over 1,000 waste sites are on the National Priorities List (NPL) which are eligible for research funds from the Superfund. The major environmental problem of the NPL sites is that the site ground water is contaminated by the leaching of hazardous pollutants from contaminated soil. Currently, burial and incineration are used to decontaminate soil both of which will not be a viable solution in the future. Burial is banned in 1990 and incineration, on the other hand, is expensive and in many cases it poses an immediate environmental risk because of the highly toxic gaseous products of incomplete combustion of toxic contaminants. The objective of this study was to investigate the feasibility of supercritical fluid extraction as an alternative technology for soil decontamination and to determine the relevant extraction parameters. This project used supercritical carbon dioxide to extract an organic pollutant (hexachlorobenzene) from soil using a dynamic extractor equipment. Three parameters (partition coefficient, mass transfer coefficient, axial dispersion coefficient) were investigated as a function of the temperature, pressure, and flow rate of carbon dioxide. The partition coefficient behavior depended on the temperature and pressure, while it was independent of the flow rate. This result indicated that the carbon dioxide solvent power is directly related to its density. The axial dispersion coefficient, on the other hand, was not only dependent on pressure and temperature, but also on the flow rate. These results suggest that the contribution by convection is more important than molecular diffusion under supercritical conditions. The overall mass transfer coefficient was independent of the pressure and temperature, but it changed with flow rate. Methanol when used as an entrainer, did not offer any significant advantages over that of pure carbon dioxide. This result confirms the fact that polar entrainers do not increase the solubilities of non-polar contaminants in carbon dioxide...