Experimental design approach for the extraction of polycyclic aromatic hydrocarbons from soil using supercritical carbon dioxide

Polycyclic aromatic hydrocarbons (PAHs) have been extracted from contaminated land samples by supercritical fluid extraction (SFE) with both pure and modified carbon dioxide. An experimental design approach, based on a central composite design, was used to determine which SFE operating variables affect the total recovery of 16 PAHS. Four parametes were chosen for evaluation: pressure, temperature, extraction time, and percent methanol modifier addition. Accessible levels of each parameter were dependent on instrumental constraints. A statistical treatment of the results indicated that extraction time and percent modifier addition were the only variables to significantly affect PAH recovery. The levels of these variables were then set at their maximum values, while the pressure and temperature were maintained at their midpoint values in the design. These conditions were used in a repeatability study (n = 7), which extracted an average of 458.0 mg kg 1 total PAHs from the contaminated land sample with an RSD of 3.1%. Sequential extractions on three of these samples, using identical operating conditions, did not show the presence of any PAHs. The result was compared with Soxhlet extraction and microwave-assisted extraction of the sample, which recovered an average of 297.4 (RSD 10.0%) and 422.9 mg kg-1 (RSD 2.4%), respectively. Extraction of an interlaboratory test soil yielded high RSD values because of the presence of elemental sulfur.