Suchergebnisse

Soil pollution by potentially toxic metal(loid)s (PTMs) is a globally widespread problem because of most metals don't undergo microbial or chemical degradation, and their total concentration in soil persists for a long time after their introduction. For these reasons is necessary to find soil remediation technologies, also to minimize the potentially accumulation of metals in the food chain. During the last decades, the application of the "green technologies" found the interest of the scientific community because more cost-effective and easily applicable. In this thesis, the importance of an appropriate sampling scheme to assess the PTMs pollution in agricultural soils subjected to waste disposal and dumping, the relevance of bioavailability assessment for phytoremediation and the potential PTMs (im)mobilization by amendments application were studied. The choice of a most appropriate sampling scheme is a crucial step to establish soil pollution and, consequently, risk management. The sampling scheme should be able to detect the pollution level, determine its geographical extension and, possibly, the background level of pollution in the area. Usually, the high cost of the laboratories analysis and the difficulty of managing extensive soil sampling, lead to choose wide grids of sampling (from 25x25 m to 5x5 km) and to bulk discrete soil samples to obtain composite samples. However, when strong concentration gradients exist in the field compositing the discrete soil samples may significantly underestimate the risk posed by the pollutants. For this reason, the degree and spatial variability of soil pollution by PTMs, in three agricultural sites, subjected to waste disposal, were assessed applying a soil sampling scheme based on a two-level grid resolution. On the first level, a regular low-resolution 10x10 m grid was defined. On the second level, each grid was subdivided into nine high-resolution 3x3 m subplots. Discrete soil samples were taken from each 3x3 m plot. Composite soil samples were made bulking aliquots from the discrete soil samples. Soil samples were collected from both 'topsoil' (0-30cm) and 'subsoil' (30-60 cm) layers to evaluate vertical gradients. Comparing the statistical analysis and various pollution indices data from composite and discrete soil samples, only Giugliano (GI) site resulted to be polluted . Indeed, from the analysis of composite data, GI site appeared to be slightly polluted by Cu and Zn (mean content of 131 and 95 mg kg-1, respectively). When the same analysis and indices were applied to discrete soil data, the slightly polluted site became highly polluted by Cu (mean and max of 276 and 1707 mg kg-1) and Zn (174 and 972 mg kg-1), and slightly polluted by Sb and As (max of 15 and 30 mg kg-1). Furthermore, a large inhomogeneity in soil pollutant spatial distribution emerged. Most part of existing legislations on soil remediation, define an area as potentially polluted on the basis of the total or pseudototal content of PTMs. However, to establish the risk associated to soil PTMs pollution, it is crucial to determine the mobility and bioavailability of pollutants, the latter defined as the fraction of the total metal(loid)s available to the receptor organism. Based on the previous work results, the mobility and bioavailabilityof Cu and Zn in polluted discrete soil samples was investigated applying single and sequential extraction procedures. Main aim of this work was, to assess the feasibility and the effectiveness of a phytoremediation action of the soil based on Eucalyptus planting assisted by compost, Trichoderma sp. and microorganisms. 1M NH4NO3 and 0.05M EDTA extractions were applied, before and during the phytoremediation treatment, to assess the readily and potentially bioavailable amounts of Cu and Zn. The amount of both metals extracted by 1M NH4NO3 was from 0.1 to 7.9 % of respective total contents while the amount extracted by 0.05 M EDTA ranged from 13 to 64 % of total content. After one year from planting, a significant reduction of Cu and Zn bioavailable amounts was observed, without any difference between the several applied treatments. Plants uptake was not significantly correlated with both chemical extractions and a general underestimation of 1M NH4NO3 bioavailable Zn in comparison with plant uptake was observed. Both single and sequential extractions suggested an higher mobility of Zn respect to Cu in soil. A reduction of the plant uptake efficiency, as assessed by the accumulation factor in correspondence of highly polluted plots was observed. Overall, in the study sites currently under assisted phytoremediation, the chosen treatments appear to be effective in reducing at least the mobile and plant available PTMs content of the soil. When it is not possible to remove the metal(loid)s from polluted soils, other options, such as immobilization treatments should be considered as an integral part of risk management. Immobilization technique consists in an increasing of PTMs retention on soil. In agriculture, several amendments are frequently used to improve the soil properties. Many of them may modify significantly the mobility of PTMs in the soil environment. Part of the thesis work was, therefore, focused on the influence of organic and inorganic amendments on the bioavailability of As and Cd in two Australian soils with different physico-chemical properties. Soils were spiked with As and Cd and, after four weeks, incubated with poultry manure (PM), poultry manure biochar (PMBC) and coal fly-ash (CFA). After other four weeks, the incubated samples were analysed for pH and bioavailability of As and Cd, assessed by1M NH4NO3 extraction and pore-water analysis. These data were compared with plant uptake, using maize (Zea mays L.) as test crop. Cadmium was almost completely immobilized in all treated soils, mostly due to the influence that the amendments had on soil pH. Indeed, the higher pH of soil A (8.5±0.03), automatically reduced the bioavailability of Cd, independently of the applied treatment. Opposite behaviour was observed for As, whose bioavailability generally increased after amendments addition. Indeed, in soil B with pH 5.1±0.03, the As bioavailability was lower than in soil A and the application of amendments always increased the bioavailable fraction. These trends were confirmed by maize uptake. In conclusion, the application of amendments to reduce the mobility and bioavailability of metal(loid)s in polluted soil resulted efficient for Cd and inefficient for As. This different behaviour was mainly related to their different nature (cation and anion, respectively) and intrinsic properties. Hence, the amendments, modifying soil characteristics, can influence the metal(loid)s mobility and bioavailability but their effect is related to the metal properties. A similar experiment was carried out with soil and sludge from the ex-ILVA brownfield area located north of Naples city (south Italy). A phytoremediation study was carried out for two consecutive years, involving the use of humic acids to enhance the adaptability and grown of A. Donax on polluted soil and post-washing sludge with attention to the mobility and bioavailability of PTMs. The humic acid application significantly increased the biomass production, in both soil and sludge. In particular, although A. donax grown on the most polluted substrate, i.e. sludge, it was interesting to observe an increasing of plant biomass as a consequence of humic acid addition, maybe due to an improvement of rhizosphere N cycling activity consequent to more intense root activity and exudation. The addition of humic acid, although useful for the plant growth, had no influence on Pb and Zn mobility and bioavailability, in both soil and sludge, probably as a consequence of the high pH value of both substrates. Despite the low bioavailability of the metals in soil and sludge, A. donax uptaken more Pb and Zn than the amounts "predicted" by 1M ammonium nitrate extraction, confirming the high ability of A. donax to phytoextract metals in all conditions and, hence, encouraging its use for phytoremediation purposes.