Dynamics of bacterial biofilms formation on granular activated carbon in aquatic sediments polluted with PCBs
Résumé
The intensive use of PCBs in the industry induced the contamination of many aquatic environments, leading to their bioaccumulation in the food chain. PCB are hardly biodegradable. The development of treatment processes for PCB-polluted aquatic sediments is a major issue related to the health and economic consequences of this contamination. Activated carbons (AC), widely used to remove organic pollutants (pesticides, solvents...) in the field of water treatment, proved to be efficient to decrease the ecotoxicity of PCB-polluted sediments. ACs are also good supports for bacterial attachment- and biofilm formation. The development of a biological process using AC to both adsorption and concentration of the chlorinated molecules and for in-situ bio-transformation of PCB by microbial populations might allow avoiding sediment dredging and post-treat.ment of AC. In this context, the objectives of the present work were to i) quantify and characterize the microbial communities composing biofilms on AC and ii) to evaluate the influence of the biofilm on adsorption of PCBs by AC. In the present study, a granular activated carbon (GAC), microporous, produced from coconut precursor, was used. GAC was incubated for 1 month with a sediment sample from the lake of "Le Bourget" (France). Colonization of the GAC by the indigenous bacterial community of the sediment was observed by SEM. Presence of bacteria onto the GAC was observed as soon as 2 days after the start of experiment. After 30 days, the biofilm was composed of a fine layer of bacteria embedded in extracellular matrix and strongly attached to the GAC. GAC grains previously loaded with a PCB mixture (Aroclor 1260) before the contact with sediment gave the same results, suggesting that adsorbed PCBs didn't inhibit GAC colonization by the bacterial community. In order to follow more specifically the biofilm formation at the GAC surface, a molecular approach was applied. A procedure for direct extraction of DNA from GAC was developed, that allowed the quantification of 16S DNA by qPCR and the genetic characterization of bacterial communities by CE-t-RFLP. The amount of bacteria attached to GAC and the genetic diversity of this multi-species biofilm changed with time and biofilm age. The addition of Aroclor 1260 slowed down the biofilm formation kinetics and induced a decrease of the amount of attached bacteria. The presence of adsorbed PCB molecules did not affect the biofilm formation, and the biofilm did not significantly modify the PCB adsorption capacity of the GAC.