Biotic and abiotic disappearance, mainly in terms of biodegradation and volatilisation respectively, of flooded soil contaminated with acenaphthene, acenaphthylene, fluorene and anthracene, was studied in erlenmeyer flasks and in bench-scale bioreactors. The erlenmeyer experiments were conducted under four different redox conditions. Disappearance kinetics followed zero order. Under aerobic and denitrifying environments, biodegradation of all four compounds occurred; the rates observed ranged from 0.38 to 0.53 ppm/day for the aerobic environment and from 0.29 to 0.35 ppm/day for the denitrifying one. However, no significant biodegradation occurred under the sulphate-reducing nor methanogenic environments. Aerobic abiotic losses were very significant; the ratio of the volatilisation to the biodegradation rates ranged from 2.4 to 3.6. The ratio of the aerobic to the denitrifying volatilisation rates ranged from 5.0 to 10.1. In the light of these results, the denitrifying environment was chosen for a further experiment to investigate the performance of a bench-scale bioreactor. Results showed that by enlarging the scale of the bioreactor approximately 8 times and simultaneously reducing the mixing intensity of the soil/water system, the biodegradation rates remained virtually unchanged. This study suggests that the denitrifying environment could play an important role in the development of an effective, economical and environmentally safe decontamination technology for treating PAH-contaminated soils.

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