PAH release during water desorption, supercritical carbon dioxide extraction, and field bioremediation

Removal rates of polycyclic aromatic hydrocarbons (PAHs) from manufactured gas plant (MGP) soils were determined using water desorption for 120 days and mild supercritical carbon dioxide extraction (SFE) for 200 min. Both techniques were used to compare the changes in desorption rates for individual PAHs from untreated and treated soils that were obtained from a field biotreatment unit after 58, 147, and 343 days. Water desorption profiles (plotted in days) and SFE profiles (plotted in minutes) were very similar regardless of whether a PAH was rapidly or slowly removed. Water and SFE profiles were fit with a simple two-site (fast and slow) model to obtain the fraction of each PAH that was rapidly released (F). There was agreement between the F values obtained from water desorption and SFE for PAHs ranging from naphthalene to benzo[a]pyrene from all soils, with an overall correlation coefficient (r2) of 0.81. F values from water desorption and SFE also agreed with the actual removal of PAHs obtained after 147 and 343 days of field remediation (r2 ca. 0.80). The use of shorter desorption times (2-4 days for water and 20-40 min for SFE) allowed F values to be estimated for all PAHs and showed excellent agreement with the removal of individual PAHs obtained with 147-343 days of field remediation (r2 > 0.9). The comparisons indicate that short-term SFE can provide a reasonable estimate of the fraction of a PAH that is readily released and available for microbial treatment.