Correlating selective supercritical fluid extraction with bioremediation behavior of PAHs in a field treatment plot

Selective supercritical fluid extraction (SFE) behavior of PAHs from manufactured gas plant (MGP) site soils was determined on untreated soil and on soils collected after 1/2 year and 1 year of bioremediation in a field land treatment plot. Sequentially stronger SFE conditions gave selective extraction of PAHs associated with "fast" (or "rapidly desorbing"), "moderate," "slow," and "very slow" sites on the soil collected before and during bioremediation. While all PAHs from the untreated soil showed "stair-step" extraction curves (with molecules in each of the four "fast" to "very slow" SFE fractions), two- and three-ring PAHs were found mostly in the "fast" fraction, while the five- and six-ring PAHs were found almost completely in the "slower" fractions. SFE comparisons of the untreated and bioremediated soils showed that bioremediation only removed PAH molecules which were found in the "fast" fractions by SFE and that remediation for 1 year did not result in the migration of PAHs from "slower" to "faster" sites. One hour SFE of the untreated sample at the mildest condition (120 bar, 50 degrees C) gave good quantitative agreement with removals achieved after 1 year of bioremediation, and SFE correctly predicted that two- and three-ring PAHs would show similar to 90% removals, four-ring PAHs similar to 50% removals, and five- and six-ring PAHs <10% removals. Mild SFE reduced the total PAHs on the untreated soil from 6860 mg/kg to 2360 mg/kg (after SFE), which is in excellent agreement with the reduction to 2420 mg/kg achieved following 1 year of bioremediation. The results show that mild SFE may be a rapid and useful test to predict the bioavailability of PAHs on contaminated soil.