Supercritical fluid extraction of organic pollutants from soil combined with adsorption onto activated carbon

Abstract

A novel process has been developed for the remediation of soils contaminated with organics. In this process, the organics are continuously extracted from soil and deposited on to activated carbon. The feasibility of this process is governed by the extraction efficiency of organics from soil by supercritical carbon dioxide and the affinity of activated carbon to the organics in the presence of supercritical carbon dioxide. These factors are controlled by the adsorption/desorption phenomena in supercritical fluids. Four model pollutants, naphthalene, phenanthrene, hexachlorobenzene and pentachlorophenol were investigated in this study. The adsorption isotherms of these organics on soil and carbon was measured using frontal analysis chromatography and modeled using the Freundlich isotherm. The amount of organic adsorbed on soil and carbon increased with temperature and was found to be independent of the solubility of the organic in supercritical carbon dioxide. The heats of adsorption of the organics on soil and carbon were found to be 1-5 and 4-8 kcal/mol, respectively, suggesting physical adsorption. The desorption profiles of these organics from soil and carbon were obtained by the same technique. The mass balance closures indicated that the organics could be completely extracted from soil and carbon indicating physical adsorption. The decontamination process was investigated by two models. A simple model, neglecting mass transfer resistances and axial dispersion, was able to predict the amount of carbon dioxide required to attain a certain decontamination level. However, it could not simulate the shape of the experimental desorption profiles satisfactorily. A rigorous model, which accounted for the axial dispersion, effective diffusion, and external mass transfer was highly successful in predicting the shape of the desorption profiles. A perturbation analysis of the parameters indicated that the adsorption equilibrium was the most significant parameter in all the systems investigated. An economic feasibility analysis of the proposed process indicated the process was feasible and the cost for 99% decontamination of a ton of soil contaminated with 1000 ppm of organic was found to be 60-200 dollars, which is lower than the competing technologies.