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Polycyclic aromatic hydrocarbons (PAH) are aromatic compounds persistent in the environment because of their hydrophobic nature, low water solubility and high adsorption onto soils and sediments. Bacteria can degrade phenanthrene dissolved in water but not when adsorbed onto soil particles. The bioavailability of phenanthrene for bacteria used for pollutant degradation was evaluated as a function of ageing of a soil polluted by phenanthrene. Ageing and biodegradation were studied in batches containing a sterile contaminated soil (soil H) and kaolinite coated with heavy fuel oil (KF2), both spiked with 14C-labelled phenanthrene. The solid matrix was kept in contact with the pollutant for 2, 48 h, 1 week, and 1, 2 and 4 months for soil H, and 2, 48 h and 2 months for KF2. After each contact time, a bacterial culture able to degrade phenanthrene and the nutrient medium was added. For 25 days, 14CO2 produced and 14C-residual activity in the liquid phase were monitored using liquid scintillation. The metabolites formed during biodegradation were determined by HPLC/UV. Adsorption of phenanthrene on soil H or KF2 was shown by comparing assay without ageing and those with different ageing times. For Soil H, 8%, 40% and 55% of the phenanthrene had been adsorbed after 2, 48h and 4 months of ageing, respectively. After the addition of degrading bacteria, 25% of phenanthrene had been mineralized in non-aged soil H and only 8% after a 4-month ageing time. This indicates that the amount of substrate available is a limiting factor for bacterial activity. For KF2, phenanthrene adsorption onto the solid was more rapid than that observed with the soil H since 55% of the pollutant had been adsorbed after only 2 h of ageing. This is due to the different nature of the organic matter in KF2 (only one type of organic matter and numerous available adsorption sites) and in the soil H (heterogeneous organic matter and adsorption sites). For both soil H and KF2, 60% of phenanthrene had been adsorbed after 2 months of ageing. With KF2, the percentage of phenanthrene mineralized reached 20% after 2 and 48 h of ageing and decreased to 6% after 2 months of ageing. These experiments show that the availability of the pollutant for bacteria decreased with soil ageing.

Influence of Ageing of Polluted Soils on Bioavailability of Phenanthrene