Riuso dei sedimenti dragati nelle aree portuali: tecniche di trattamento, valutazione delle variazioni tessiturali, morfoscopiche e geochimiche ed ipotesi di intervento

SUMMARY Dredged sediments management requires a good knowledge of their textural, chemical and ecotoxicological features in order to select the best decontamination techniques allowing their reuse for environmental and civil interventions (Gardner et al., 2004; Michael et al., 2002; USACE / POAK, 1994; Burt and Murray, 2004) according to the national legislation (for instance in Italy the Legislative Decree no. 152/2006 and 1/2012). The study aims to analyze and discuss the modifications of the chemical and physical characteristics of the sediments dredged in the harbors of Ravenna, Livorno, Porto Garibaldi, Cervia, Cesenatico, Bellaria (Italy) and Midia (Romania) after decontaminations treatements. The results of the analysis have been then used to discuss the sediment reuse according to the national regulations. The sediments are generally constituted by fine or very fine sand and are characterized by a percentage of the fine fraction reaching 40 – 50% (silt and clay). Almost all the samples are contaminated by organic and inorganic pollutants with Lead, Vanadium, Mercury, Arsenic, IPA, C < = 12 and total hydrocarbons C> 12 that exceed the limits imposed by the Legislative Decree n. 152/2006. Only the sediments sampled in the harbor of Cervia and Bellaria did not present contamination. To obtain adequate quality standards (as defined by ICRAM - APAT, 2007), the contaminated sediments were treated with different experimental techniques (soil washing, sorting, landfarming, attrition and fusion with plasma torch). The results were used to quantify the performance of the decontamination’s treatment by measuring the changes and modifications in the sediments’ compositions. From the comparisons of the sediments’ characteristics before and after treatments, it appears that: -- Sediments presenting the minor textural variations were only treated with sorting that allows the separation of the sand fraction from the clay. -- The abrasion process, performed with attrition cell, has only induced a slight increase of silt and clay clearly due to the abrasion of particles. -- Sediments treated with plasma torch present a mean grain size greater than the native sediments. -- The landfarmig treatment, only used for the fine fraction of the sediment (< 63 μm) has not induced significant textural variations. The results of the geochemical analysis show that the sediments treated with soil washing, sorting and landfarming were totally or partially decontaminated, presenting a reduction of organic contaminants (for instance, the landfarming induce reductions of organic contaminants from 44% to 70%) and heavy metals. A change in the content of heavy metals has been observed in the sandy sediment during the test of the abrasion treatment and therefore the treatment does not allow establishing a unique trend of decontamination. Furthermore, the experiment realized with the plasma torch has demonstrated the possibility to extract silicon from the sediments as Iron-Silicon components (the Iron derives from the ferrous minerals of the sand) .The analysis carried out by SEM/EDS also allowed to characterize the molten material and minerals crystallized. The morphological changes of the particles due to the decontamination treatments have been observed using the Microscope Morphologi G3 Malvern and the optical microscope. The results highlight that only the plasma torch causes a radical change of the particles morphology, while the other treatments alter the aspect ratio, circularity and intensity mean parameters but less the convexity parameter. The characterizations allowed to establish, the reuse of decontaminated sediments. To assess the possible reuses in coastal areas and hinterland, several compatibility analyses (Krumbein and James, 1965) and morphological particles comparisons were performed. The results obtained indicate that the treated sediments can be used for submerged nourishment and filling interventions of quarries in the Ravenna province. Considering the proposed interventions, the results show that the total amount of sediment reusable of Ravenna port is about 3,500,000 m3 (sand, silt/clay), from a total of about 10 million m3. Taking into account the sediments treated by fusion (classified as inert) used in the construction industry, it is possible to hypothesize a total reuse of sediment. The study also showed that the use and combination of different decontamination treatments and the development of accurate management plans allow the reuse of all fractions of the dredged sediments, even the highly contaminated fine fraction. Finally, it has been demonstrated that decontaminated sediments, morphologically suitable for environmental reuse, were obtained from the treatments of soil washing, sorting and attrition. The fusion with the plasma torch allow the extraction of silicon, which has economical returns and a reuse of the sediments in the civil-infrastructure as well.