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

Selective supercritical fluid extraction (SFE) behavior of PAHs from manufa ctured gas plant (MGP) site soils was determined on untreated soil and on s oils collected after 1/2 year and 1 year of bioremediation in a field land treatment plot. Sequentially stronger SFE conditions gave selective extract ion of PAHs associated with "fast" (or "rapidly desorbing"), "moderate," "s low," and "very slow" sites on the soil collected before and during bioreme diation. While all PAHs from the untreated soil showed "stair-step" extract ion curves (with molecules in each of the four "fast" to "very slow" SFE fr actions), 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 "sl ower" fractions. SFE comparisons of the untreated and bioremediated soils s howed that bioremediation only removed PAH molecules which were found in th e "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 goo d quantitative agreement with removals achieved after 1 year of bioremediat ion, and SFE correctly predicted that two- and three-ring PAHs would show s imilar to 90% removals, four-ring PAHs similar to 50% removals, and five- a nd six-ring PAHs <10% removals. Mild SFE reduced the total PAHs on the untr eated 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 bi oremediation. The results show that mild SFE may be a rapid and useful test to predict the bioavailability of PAHs on contaminated soil.