Fate of phenanthrene, pyrene and benzo[a]pyrene during biodegradation of crude oil added to two soils

The release of (CO2)-C-14 from 9-[C-14]phenanthrene, 4,5,9,10-[C-14]pyrene and 7-[C-14]benzo[a]pyrene, added to Brent/Fortes crude oil and mixed into a pristine sand soil (0.40% organic C) and a pristine organic soil (22.9% organic C), was determined. After 244 days at 25 degrees C, 11.1 +/- 3.5% (sand) and 17.1 +/- 0.30% (organic) phenanthrene-C-14 and 9.77 +/- 2.8% (sand) and 5.56 +/- 1.4% (organic) benzo[a]pyrene-C-14 was released. After 210 days, 3.65 +/- 0.5% (sand) and 4.43 +/- 0.33% (organic) pyrene-C-14 was released. Inoculation of these two soils with DC1 and PD2 (bacteria capable of accelerating the phenanthrene and pyrene mineralisation in soil in the absence of crude oil) either at day 0 or after release as (CO2)-C-14 by indigenous degraders had ceased, failed to increase or initiate further mineralisation. Thus, aged PAH residues were non-bioavailable to these metabolically competent degrading microorganisms. At the end of the first period of incubation (210 days or 244 days), the total aromatic hydrocarbons recovered using Soxhlet extraction was 0.18% (sand) and 42.8% (organic) compared with approximately 100% from bio-inhibited soils. This confirmed that the indigenous microbiological activity not only caused a limited amount of PAH mineralisation but also reduced the extractability of residues, possibly due to the generation of metabolites which were chemisorbed and bound (and non extractable) in 'aged' soils.