## 4 Results and Discussion
### Assessment of Hg Delivery to the Augusta Harbor: Direct Inputs or Resuspension?
As no Mg industry had ever been present in the Industrial Area of Porto Marghera, a new question arose: Was it possible to identify another industrial metal process that could explain the observed PCDD/F signatures in the 1 st Industrial Area? At that point, it became necessary to verify whether the sediments of the most contaminated canals of Porto Marghera presented associations between PCDD/Fs and metals that could lead to specific industrial processes actually operating in the area. Using data from the survey carried out by the Venice Water Authority and the Port Authority [67], a good correlation (R\({}^{2}\): 0.742) was found between PCDD/Fs and As (Figure 5). Since the formation of PCDD/Fs requires oxygen, a hydrocarbon compound, Cl, a catalyst (e.g., Cu or Fe), an ideal temperature (400-600 \({}^{\circ}\)C), and an adequate residence time [68], there was the necessity to identify the industrial processes operating under these conditions and simultaneously related to As pollution. One of these processes occurred regularly in the 1 st Industrial Area and involved the extraction of Cu from pyrite ashes (very rich in As [69]), through the addition of 10% NaCl (chlorination) and the subsequent roasting at temperatures of about 450-550 \({}^{\circ}\)C with the formation of CuCl\({}_{2}\) and FeCl\({}_{2}\)[70]. Both compounds (and also NaCl) are regarded as catalysts for the formation of dioxins and especially furans [68; 71], through chlorination and oxidation reactions of carbonaceous particles with degenerate graphitic structure, present in the combustion gases. Reactions take place on metal oxides in the flying ashes and are catalyzed by transition metal compounds. Chlorination takes place essentially by means of inorganic salts (KCl, CuCl, CuCl\({}_{2}\), NaCl) while the oxygen supply is given both by that present in the gaseous phase and by that existing in the aromatic nuclei present in the degenerate graphitic structures. Table 1 summarizes the main reactions that occur when roasting pyrite  
Figure 4: (**a**) PCDD/F congener composition (as chlorination classes) in the Venice Lagoon (E1 and M3) and in the industrial canals of Porto Marghera (C9 and C12). Total PCDD/F concentrations are also reported. Data are from [28; 45; 46]. (**b**) PCDD/F congener composition (as chlorination classes) retrieved in literature and relative to specific industrial processes. Total PCDD/F concentrations are also reported. Data are from [63; 65; 66]. ashes. It is evident that a high amount of metal compounds potentially active as PCDD/F catalysts is produced. The residues of the roasting are then leached and metallic Cu, small amounts of Ag, Au, and also Zn are recovered from the resulting solution. Furthermore, the residual Fe oxides (called \"purple ore\") are agglomerated into tiles through a sintering process that, due to the presence of residual chlorine from the previous treatment, can produce dioxins and furans. The industrial process able to produce PCDD/Fs as by-product was identified, but the so-called smoking gun had still to be found, i.e., a case study that linked undoubtedly the process of Cu extraction from pyrite ashes to PCDD/F production. A new literature search brought to light the \"Kieselrot\" case [59]. Kieselrot (German for red pebble) is the name given to the red silicous slag obtained from Cu extraction processes which were carried out in Marsberg (North Rhine-Westphalia, Germany), until 1945. The extraction process involved the chlorination, roasting, and leaching of a \"black silicous mineral\" containing Cu called Schwarzkiesel. During the 1950s-60s, the Kieselrot was used to build sport fields, recreational areas, roads, and paths. In the following years, this material was found to be highly contaminated by chlorinated aromatic hydrocarbons, in particular PCDD/F, HCB, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, as well as by heavy metals [65].