MOISTURE EFFECTS ON THE CLEANUP OF PAH-CONTAMINATED SOIL WITH DENSE CARBON-DIOXIDE

The cleanup of soils contaminated with organics by extraction with sup ercritical carbon dioxide can be influenced decisively by additional s ubstances or entrainers. In most cases, the contaminated soil already contains water as a substance that can alter the extractibility of the se contaminants. In particular, the effects of soil moisture, as a kin d of discontinuous addition of water, on the extraction of polycyclic aromatic hydrocarbons (PAHs) from soil with supercritical carbon dioxi de were examined. On the other hand, a continuous addition of water by humidifying the supercritical carbon dioxide was used. Both continuou s and discontinuous addition of water elevates the extraction yield re markably by altering the adsorption phenomena for spiked silt as well as for soil samples from a former gas plant site. Furthermore, two dif ferent purity grades of carbon dioxide were used to work out the influ ence of impurities in the technical carbon dioxide serving as entraine rs. The improvement of the extraction yield by moisture indicates addi tionally that the extraction is limited by adsorption and not by diffu sion effects. Otherwise, the water must decrease the extraction rate b ecause of the formation of an additional phase. However, the contamina nt is better accessible and transported faster out of the soil with wa ter.