with indigenous microbial populations. It was also shown that the bioremediation efficiency was in proportion to the biomass of the consortium added to the soil. There was 7.1% of total PAH loss observed in the control soil, which may have been because the soil was non-sterile, and the addition of mineral salts medium stimu- lated the biodegradation capability of indigenous microorganisms in the soil (Li et al., 2008).
It is generally accepted that the biodegradation of low- molecular-weight PAHs occurs much more rapidly and exten- sively than that of HMW-PAHs (Nam et al., 2001). However, high degradation of the latter would have been obtained in some
conditions. In the bioremediation of an aged PAH-contaminated soil by a microbial consortium, Li et al. (2009) observed that the biodegradation of 5e6 rings PAHs was significantly higher than that of 2e4 rings PAHs. In our study the degradation of 3-ring PAHs was less than that of 4- or 5-ring PAHs, perhaps due to the bacterial consortium being enriched from a soil containing more than 90% of HMW-PAHs, and it had a greater capability to degrade HMW-PAHs.
Bioremediation of PAH-contaminated soil with specific isolates of bacteria or fungi has been monitored in other studies showing opposite results. Some successful examples have shown signifi- cantly enhanced degradation of HMW-PAHs (Kotterman et al.,