Lab-scale feasibility tests for sediment treatment using different physico-chemical techniques

The fairly high amounts of sediments dredged in coastal or internal water bodies for navigational and/or environmental purposes claims for the identification of appropriate management strategies. Dredged sediments are frequently affected by organic and inorganic contamination, so that their reuse, as an alternative to final landfill disposal, could need remediation. In this framework, a two-year joint research project was carried out to assess the feasibility of different remediation technologies for the treatment of polluted sediments. The sediment used in this study was similar in composition and contaminant loading to many sediments around the world. It was dredged from the northern canal of the Porto Marghera industrial area (Venice, Italy) and it was homogenized and characterized for the physical properties and chemical composition. The material was found to exceed the regulatory limits established for the reuse of dredged materials in the Venice lagoon for a wide range of pollutants, but this study specifically focused on heavy metals and polycyclic aromatic hydrocarbons (PAHs). Homogeneous samples were subjected to a number of physico-chemical remediation techniques including chemical oxidation, electrochemical oxidation, and electrokinetics under different operating conditions. The treated material was characterized for the residual contaminants in order to determine the remediation efficiency. The treatments investigated produced a variety of effects in terms of removal of heavy metals and PAHs. For total PAHs, the best results were obtained using H2O2 only as the oxidizing agent (45% removal), and chemically + thermally activated persulfate (up to 72% removal). The kinetics of these chemical oxidation processes was rapid and almost complete in a few hours. Electrooxidation produced up to 44% of total PAHs degradation, whereas no appreciable PAH removal was attained by the electrokinetic treatment. Metal extraction by means of electrokinetics was the highest when both the anodic and the cathodic chambers were conditioned with the complexing agent ethylenediamine tetraacetic acid (EDTA). The following removal yields were obtained: 81% for As, 69% for Cr, 40% for Cu, 33% for Pb, and 22% for Zn. The modified Fenton-like reactants were not capable of improving the PAH removal compared to the conventional H2O2-based oxidation process; as a consequence, the latter should be suggested. Comparable PAH removals were obtained using the electrooxidation treatment, whereas the activated persulfate treatment exhibited the highest removal efficiency. Electrokinetics was found to be effective towards heavy metal mobilization only when the process was enhanced by EDTA. Specific modifications should be applied to the electrokinetics treatment to enhance the PAH remediation yield, such as enhancement of the electroosmotic flow in order to achieve a higher liquid to solid ratio; for metals, performance might be optimized by using an EDTA solution as the catholyte and an inorganic salt solution as the anolyte. For all the tested technologies, it would also be mandatory to evaluate whether the presence of residual chemicals or undesired by-products in the material treated might pose any ecotoxicological risk at the site.