Supercritical CO2-cosolvent extraction of contaminated soils and sediments

DDT‐ or PCB‐contaminated topsoils of high organic content were extracted using supercritical CO2 or CO2‐5 wt% cosolvent (toluene, acetone, methanol, acetic acid, diethylamine) mixtures at 313 K and 101 bar. In separate experiments, the pure contaminants were dissolved in supercritical CO2 at the same conditions and the equilibrium solubilities determined. Most of the cosolvents only marginally improve extraction rates over the case of pure CO2. Methanol, however, increases total amounts of DDT removal from 50–80% to > 95%, and increases DDT and PCB extraction rates by as much as an order of magnitude. Methanol is a superior cosolvent probably because its hydrogen‐bonding ability is better suited to interaction with the organic matter in the soil (humic acids, fulvic acids, polysaccharides); the organic matter essentially “dissolves” the contaminants in the soil matrix. Comparing a simple fixed–bed, local equilibrium (based on pure contaminant equilibria) desorption model to the actual desorption data indicates that contaminant solubilities in supercritical fluids are enhanced (over the pure contaminant case) when desorbing from soils. The enhancements may be related to the simultaneous desorption of other low molecular‐weight organics from the soils.