CONVENTIONAL BIOLOGICAL AND BIOELECTROCHEMICAL METHODS FOR THE TREATMENT OF MARINE SEDIMENTS POLLUTED BY ORGANIC COMPOUNDS

Polycyclic aromatic hydrocarbons (PAHs), mainly derived from human activities and mostly found in soils and sediments, pose a serious worldwide concern, since they are hazardous contaminants. Aerobic bioremediation approaches are usually applied, due to their fast kinetics: in this framework, two sediment slurry sequencing batch reactors, namely SBR-C and SBR-EK, treated, in 5-days cycles, marine sediments spiked with 4-mixed PAHs (fluorene, phenanthrene, fluoranthene and pyrene) and coming, respectively, from Cagliari (Italy) and El Kantaoui (Tunisia) ports. Both systems achieved the best performances (and in parallel met threshold levels of Italian regulations) when the solid-to-liquid ratio (S/L) was set at 0.1, PAHs concentration at 200 mgPAHtot/kgdw (dw = dry weight) and the volumetric organic loading rate at 0.4 mgPAHtot/(L*d). Increasing these parameters resulted in worsening the process performances; bioaugmentation was not useful to recover good process performances in SBR-C. An alternative approach could be the use of bioelectrochemical systems (BES). Sediment microbial fuel cells (SMFCs) were used for preliminarily assess phenanthrene (Phe) removal from marine sediments: glass bottles (anodic compartments, anaerobic) were batch-fed with a slurry (S/L=0.05) contaminated up to 150 mg/kgdw; cathodes were placed in a PVC tube inserted in the bottle cap and the compartments were separated by a cation-permeable membrane; both anode and cathode consisted of a piece of conductive graphite felt. BESs were operated under mechanical stirring and in static conditions, though mixing accelerated Phe removal (which was almost completed in 20 days). Since high Phe removals were achieved also in the open circuit controls, the role of sulphate (which was part of the sediment and of the culturing medium adopted) was investigated: although sulphate had a putative role in Phe degradation, the presence of the anode fastened the degradation, hence indicating the successful development and maintenance of a Phe degrading biomass on the anodic surface. However, SMFCs reached low current outputs, due to the low biodegradability of Phe and to the limited radius of influence of the bioelectrochemical treatment in the specific configuration adopted. To solve these problems, a model soil (quartz sand, density ca. 1.5 kg/dm3) was amended with 2,000 mg/kgdw of graphene oxide (GO) that, once electrochemically reduced to rGO (for 60 hours), increased the electric conductivity of the soil not amended with GO by four orders of magnitude. These percolating anodes, supplemented with an inoculum of electroactive microorganisms and acetate, outperformed the controls by delivering >30 times higher current, indicating that probably a much larger volume of soil contributed to the catalytic current production. Promising results were achieved also in preliminary tests of toluene and Phe degradation and encourage further investigations for future applications to real contaminated soils and sediments.