## 4 Discussion  
The impact of environmental pollution due to metals and metalloids in soil depends not only on their total concentration but also on their mobility and availability, which affects their release and interaction with other environmental components, such as water, air, and vegetables. In order to evaluate metal mobility in soil, sequential extraction procedures can be performed, in which a soil sample is treated with solutions with different extractant capacity that provoke the release of metal rates characterized by different solubility and, consequently, mobility. The fractions extracted by weak reagents are more mobile and dangerous from an environmental point of view, because they represent the metal portion which is released more easily from soil to other environmental compartments, such as superficial and ground water, or adsorbed by vegetables which grow in the contaminated site. In soil As is adsorbed mainly by minerals characterized by variable charge: in fact, it was observed by EXAFS methodologies, that AsO\({}_{4}\)\({}^{3-}\) ions were adsorbed on the surface of minerals with variable charge forming \"inner-sphere complexes\", characterized by very strong covalent bonds [29]. These bonds can be mono- or bidentate mononuclear and/or bidentate binuclear in different proportions according to the characteristics of the absorbent surfaces. Recent studies showed that AsO\({}_{4}\)\({}^{3-}\) ions are adsorbed more efficiently by Mn and Fe oxides than Al oxides, whereas AsO\({}_{3}\)\({}^{3-}\) ions are strongly adsorbed by Fe oxides and weakly adsorbed by Al oxides [30; 31; 32; 33; 34]. Sixteen of the 30 soil samples taken at the studied area were characterized by an As concentration higher than 20 ppm, the limit set by the 471/99 D.M. in the areas for public parks and private and residential use, and the highest value was 44.51 ppm (S53/1 core, depth 2.0-3.0 m). No soil sample showed an As amount higher than 50 ppm, the limit set by the 471/99 D.M. in the commercial and industrial areas. As extracted in the first step of the sequential extraction procedures was the less abundant in all the analyzed soil sample. Even if sequential extraction methods can change significantly, the first step is always performed in less drastic conditions and allows to solubilize the most available fraction of analyte and, therefore, the rate of metal or metalloid with the highest potential toxicity. Also As extracted in the second step is significantly bio-available, even if in these specific extraction conditions As species are not so water-soluble like As forms extracted in the first step. In general, As extracted at the fourth step represented an abundant fraction of total As in all analyzed soil samples. The organic matter has an important role in determining As mobility: the adsorption of As(III) and As(V) on humic acids is influenced by pH and the concentration of the element. Humic acids could be important for As adsorption in relatively acid environments, whereas in alkaline environments As release and solubilization are favored [35]. Sodium hydroxide, used at the fourth step of the sequential extractions, is able to solubilize the organic matter, as pointed out by the darkening of the extracts from the superficial soil samples, where we expect a higher concentration of humic substances. But the As fraction extracted at the fourth step also includes arsenates bound to Fe-rich minerals and, in general, to clay matrices [27]. As species extracted at the fifth step and in the final residue (dissolved in aqua regia) form the so called As occluded [26], As included in minerals such as sulphides. However, we also have to consider that in the superficial levels sulphide oxidation is possible due to the metabolism of the bacterial species present in the soil, and consequently the transformation of As in more available forms [36]. This hypothesis is not applicable to the deepest levels due to the anaerobic conditions which characterize these depths.