Studio delle forme chimiche e mineralogiche e della mobilità/biodisponibilità di metalli pesanti in suoli, piante (Pistacia lentiscus L.) e soluzioni del suolo del Bacino del Rio San Giorgio (Iglesias – Gonnesa, Sardegna sud - occidentale, Italia) finalizzato allo sviluppo di strategie di soil remediation

This study was addressed to the environmental consequences that the intense extractive activity of lead and zinc has produced in the district around the towns of Iglesias and Gonnesa (southwestern Sardinia, Italy), with particular reference to the soil and to possible actions of phytoremediation. The research project, performed in cooperation with various national and international institutions, including ENEA, CNR and NTU (Nottingham Trent University), focuses the investigation on different aspects related to environmental pollution by elements such as Zn, Pb and Hg associated with the mines of the district. The study is located in the area of the Rio San Giorgio basin, where the following representative sites were chosen: one of the ‘hazard centers’ of the district, consisting of mining flotation wastes at Campo Pisano, close to the town of Iglesias; three natural sites, not directly affected by morphological alterations due to mineral extraction, located along the ‘metalliferous ring’, that is the Cambrian limestones-dolomites that host the Pb-Zn mineralisation; the Sa Masa swamp, that is the final collector of drainage waters of the river basin, closed by the Iglesias valley to the north and the Sulcis coal district to the south. The aims of this work are: i) to quantify the total content of ‘heavy metals’ in the various abiotic sectors (soils and soil pore waters), and in the biotic sector, specifically Pistacia lentiscus L., a spontaneous vegetal species; ii) to identify the chemical and mineralogical forms of heavy metals, and to define their mobility and bioavailability in the interactive environmental system soil - plant - soil pore water; iii) to study the relationship between the different concentrations of heavy metals in the various environmental media considered, to understand the mobility mechanisms; iv) to establish how much Pistacia lentiscus L. takes up and translocates each metal from roots to leaves, through the relationship of the contents measured in each plant parts, hypogeans and epigeans; v) to provide a full description in situ of the spontaneous system of vegetal species selected, that characterizes these contaminated zones, as an example of phytostabilization and soil remediation techniques, aimed to reclamation and stabilization of the abandoned mining areas. In the sampling points, soil profiles were opened, making a distinction in pedological horizons according the standard procedures of soil investigations; samples were collected for each horizon for subsequent laboratory analysis. These included the determination of chemical and physical characteristics according to the official Italian soil study methods. In general, the soils studied in the mining wastes are not pedogenised; those on dolomitic limestones are sparsely and moderately deep, in lithic contact with the underlying rock; the soils in the marshy deposits are characterized by a sequence of different sedimentary cycles (marked by textural differences). The pH reaction is generally from neutral to weakly alkaline (from 7.1 to 8.1), with variations and exceptions attributable to different parent materials from which the studied soils originate. The mineralogical and chemical analyses point out a great abundance of calcite and dolomite, in agreement with the geological substrate of the area. The organic carbon contents are generally higher in the surface horizons, with minimum value of 11 g/Kg and maximum of 202 g/Kg, and they decrease at increasing depths. The study of thin sections allowed to refine the attribution of acronyms to the horizons, according to the international soil classification systems of the Soil Taxonomy and IUSS Working Group. On the same soil samples were carried out further mineralogical determinations (X-ray powder diffraction), loss on ignition (LOI), and chemical composition (major elements with X fluorescence; trace metals with acid digestion followed by analysis with ICP-OES). In agreement with thin sections, calcite and dolomite result abundant, as well as quartz; in some samples the presence of barite, pyrite and gypsum was detected, as well as trace 'heavy metals' minerals (hemimorphite, cerussite, hydrozincite, anglesite). Consistent with mineralogy, the chemical composition is dominated by CaO-MgO-SiO2. Among the analysed heavy metals, Zn, Pb, As, Cd, and Hg very often occur in higher concentrations than Italian law limits (D.Lgs 152/2006) for the metal contents in soils for industrial use; particularly high are the contents of Zn (1655-220946 mg/Kg), and Pb (1360-54914 mg/Kg). These values are found also in the soils developed on natural substrates, presumably not much interested by anthropic impacts, reflecting therefore a natural geochemical anomaly; however, in the sites impacted by the mining activity all values are higher, demonstrating the influence of anthropic action. The mobility of metals was estimated through two different single extraction chemical methods: with DTPA solution, followed by analysis of the extracted metals by ICP-OES, and with sodium citrate/hydroxylamine hydrochloride/TEA followed by colorimetric titration with Dithizone (DZ) solution. The data comparison showed a good correlation between these two methods (R2=0.93), with values obtained from the DZ method roughly double with respect to those from the DTPA method. The mobile fraction extracted with both methods reveals a good correlation also with the total metal contents in soils (R2=0.98 for DZ; R2=0.92 for DTPA). As mentioned previously, a particularly innovative aspect of this study was the sampling and analysis of soil pore waters. Their chemical composition is variable among the various sites, and one can identify at least two principal compositional types (Ca-SO4, Ca-Mg-HCO3): to this last type, a Na-Cl-SO4 component of possible seawater origin is locally associated. Regarding the trace elements, the metals more abundant are Ba in some sites, and Zn in others; this last is usually the more abundant trace metal, particularly if compared to Pb. The modelisation with the speciation programs PHREEQC and WHAM shows in general an approach to equilibrium of the solutions with the minerals present in soils; specifically, the concentrations of Ba, Zn and Pb appear substantially controlled by barite, hydrozincite and cerussite, respectively. As concerns the transfer of metals to the biosphere, it was carried out also the sampling of P.lentiscus, on which the contents of Zn, Pb and Hg were determined, for roots, stems and leaves separately. In general, the metal concentrations decrease according to the order roots>stems>leaves, with the partial exception of Hg, for which there is locally an enrichment in leaves (TF>1), perhaps influenced from foliar absorption of Hg volatilised from soils. The metal contents in plants are considerably lower than those in soils (BAC<<1), and nevertheless roughly proportional to them. The P.lentiscus seems in fact to behave as a tolerant species with strategies of exclusion, and with character of 'indicator'. It is confirmed therefore that this species is particularly apt to revegetation actions aimed at the phytostabilisation of soils.