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Μobility of geocenic chromium in Asopos river basin

Lilli Maria

Πλήρης Εγγραφή

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Περίληψη

The present thesis investigates the mobility and controlling mechanisms of chromium (Cr) release from soils obtained from an area of wide spread geogenic contamination. The objectives of this study were (1) to assess the geochemical characterization and classification of soils and river sediments with respect to the origin of Cr along Asopos river, (2) to assess the processes affecting the mobility of Cr(VI) in soils and sediments along Asopos river and to elucidate the mechanisms of Cr release from soils and (3) to develop a methodology to assess the impact of geogenic origin Cr(VI) uptake by agricultural products (specifically carrots), and the risk of human consumption of carrots grown in Asopos River basin in Greece.In Chapter 1 a comprehensive review on Cr and its presence in the environment, and specifically in soil and plant system is given. Moreover, the study area and the framework of analysis of this thesis are drawn.Chapter 2 presents the published article in the Journal of Hazardous Materials entitled: Characterization and mobility of geogenic chromium in soils and river bed sediments of Asopos basin. A field and laboratory study was conducted to assess the origin and mobility of Cr(VI) in Asopos basin. Sampling was designed in such way as to capture the spatial variability of Cr occurring in sediments and soils in different lithological units in the area. Physicochemical and geochemical characterization of surface agricultural soils obtained from river terraces and river bed sediments was conducted in order to determine the natural background of chromium. Lithologies with strong calcareous, siliceous and ultramafic components were identified using principal component analysis. Laboratory mobility studies quantified the rates of Cr sorption and release from soils and their capacity to adsorb Cr. Heavy metal analysis and local geology study support the hypothesis that the main source of Cr is of geogenic origin. Cr distribution in Asopos river bed was influenced from the eroded products derived from extensive areas with ultramafic rocks the last 5 Ma. The mobility studies showed that leaching process was very fast and sorption capacity was significant and capable to retain chromium in case of waste release in the river. Finally the mobility of Cr release is limited due to existing attenuation capacity controlled by ferric oxides coatings on the soil and sediments. Chapter 3 presents the submitted article entitled: Identifying the controlling mechanism of geogenic origin chromium release in soils. A laboratory study was conducted to assess the mobility and mechanisms of Cr release from soils obtained from Asopos river basin. The agricultural soil sample used in this study was taken from the Schimatari area in Asopos river basin. In order to refine the isolation of minerals contained in the soil, two types of separation analysis were conducted. First, a size fractionation with hydrocyclone and second, a weight fractionation with heavy liquids. The separated fractions were characterized using chemical, mineralogical and surface analysis. The results provided consistent evidence that the heavy fraction of the soil is related directly to the mobile fraction of Cr. At acidic pHs, the clay-sized fraction also plays an additional important role in the mobility of Cr, due to the fact that this fraction has high surface area and Cr reactivity index. In addition, pH-edge leaching studies showed a high correlation between Cr–Ni, Cr-Mn and Cr–Y released from the soil which also suggests that the mobility of Cr is controlled by chromite weathering which is the case observed in Asopos river basin.Chapter 4 presents the published article in the Environmental Research entitled: Assessing the impact of geogenic chromium uptake by carrots (Daucus carota) grown in Asopos river basin. A methodology was developed to assess the impact of geogenic origin Cr(VI) uptake by carrots, and the risk of human consumption of carrots grown in Asopos river basin. A field scale experiment was conducted with carrots cultivated in treatment plots, with and without compost amendment, in order to assess the impact of carbon in the mobility and uptake of Cr by plants. The results suggested that there is a trend for Cr mobilization and uptake in the surface and the leaves of the carrots cultivated in the treatment plot with the higher carbon addition, but not in the core of the carrots. Limited mobility of Cr(VI) in the soil-plant-water system is presented due to the affinity of Cr to be retained in the solid phase and be uptaken by plants. Cr(VI) tolerant bacterial strains were isolated from the carrots. These endophytic bacteria, present in all parts of the plant, were able to reduce Cr(VI) to Cr(III) form to levels below the detection limit. Finally, a site-specific risk assessment analysis suggested no adverse effects to human health due to the consumption of carrots. These findings are of particular importance since they confirm that carrots grown in soils with geogenic origin Cr does not pose any adverse risk for human consumption, but could also have the beneficial effect of the micronutrient Cr(III).In Chapter 5 the results of the present study are summarized and conclusions are drawn.The overall impact of this thesis was aimed to develop an integrated approach to outline an area with wide spread geogenic origin Cr contamination, and to give answers to questions concerning Cr existence in groundwater, soils and agricultural products. The general findings showed Cr concentrations in groundwater of Asopos basin below the drinking water standards. Increased geogenic Cr(VI) concentrations are only observed in areas directly affected by ophiolite weathering. Chromite weathering is the controlling mechanism of Cr release in soils of Asopos basin. The high adsorption and immobilization capacity of soils and sediments are important suggesting significant Cr retardation in case of wastes release with high Cr concentration and specifically for wastes with high acidity. Finally, there is a case of Cr uptake by agricultural products, however, it is not important due to the direct reduction of Cr(VI) by the endophytic bacteria. The human health risk assessment showed that the population is unlikely to experience adverse effects due to the consumption of these products.

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