Bioavailability of benzo[a]pyrene during NAPL-enhanced biodegradation in soil and in liquid culture

The high molecular weight polycyclic aromatic hydrocarbon (HMW PAH) benzo[a]pyrene is generally persistent in the environment and its persistence may be due to bioavailability limitations. However, the presence of degradation-capable microorganisms and a suitable cosubstrate are also necessary. This is especially the case for benzo[a]pyrene because it may only be degraded by fortuitous metabolism. Non-aqueous phase liquid (NAPL)-enhanced benzo[a]pyrene biodegradation and indicators of bioavailability were measured in soil and liquid culture. In soil, 14CO2 from 7-[14C]benzo[a]pyrene mineralisation and overall CO2 production were monitored for 83 d after treatment with different types of NAPLs in biometer flasks. Monitoring was followed by soil extraction and measurement of 14C residues and of the remaining NAPL by gravimetry. In liquid culture, 7-[14C]benzo[a]pyrene mineralisation was monitored after treatment with different NAPLs and followed by a radiocarbon mass balance of 14C residues. Results indicated that although benzo[a]pyrene may have been bioavailable in both media types, benzo[a]pyrene mineralisation only occurred when a suitable NAPL cosubstrate was present to facilitate biodegradation. In soil, rapid increases in the rate and onset of benzo[a]pyrene mineralisation were shown to occur in benzo[a]pyrene-contaminated soils that were treated with mineral oil, which was a relatively non-biodegradable NAPL cosolvent, plus a hexane fraction-NAPL which was biodegradable and contained suitable cosubstrate(s).