Solid phase treatment of an aged soil contaminated by polycyclic aromatic hydrocarbons

Laboratory scale tests were carried out in order to evaluate the removal efficiency of polyaromatic hydrocarbons (PAHs) during the different biological treatments of a Manufacturing Gas Plant site aged soil, heavily contaminated by high molecular weight compounds. Biodegradation studies were carried out at nearly 25 degrees C in solid phase reactors. Three tests were performed, over a period of 100 days for each test. In the first test (P1-bioaugmentated), soil was mixed with wood chips and urea at the start of the treatment and after six weeks from the beginning of the test was also periodically inoculated (at 42, 54, 69, 82, and 96 days) with selected consortia of autochthonous PAH-degrading bacteria. The second test (P2-biostimulated) was performed similarly to the previous one, but without any inoculations. In the third test (P3-control) only soil was introduced. All systems were aerated daily and humidified at the occurrence. PAH concentration, total cultivable heterotrophs, PAH-degrading bacteria, mycetes, pH, ATP concentration, and enzymatic activities were monitored every two weeks during the treatments. Tests showed that nearly 50% of light (three rings) PAHs, 35% of benzo-PAHs and 40% of the total PAHs could be removed in the reactor P2 following 100 days of treatment. Lower removal efficiency could be observed for light PAHs (28%) in the inoculated reactor (P1) at the end of the treatment: comparable abatements were obtained for benzo- and total PAHs. In the reactor P3 (control), the concentration of all polyaromatic hydrocarbons was nearly always constant, suggesting that the physical losses were negligible during the solid phase treatments. Therefore the C to N ratio balance resulted to be the key factor in promoting the biodegradation process of all PAHs.