Metale ciężkie w środowisku glebowym olkuskiego rejonu eksploatacji rud Zn-Pb

Abstract

The Upper Silesian-Cracovian ore deposits are MVT (Mississippi Valley type) deposits and the ores are characterized by simple mineral compositions. In the Zn-Pb-Fe sulphidic ores, the following heavy metals are present: Zn, Pb, Fe, Mn, Cd, Tl, Ag associated with metalloid elements such as As and Sb. In the Olkusz area, though near-surface exploitation of the Pb-Ag ores was conducted ever since the 12th century, the exploitation of Zn-Pb ores began only in the 19th century. The ores were, at that time, enriched by the washing of oxidized Zn-Pb bearing deposits. In the second half of the 20th century, rapid development of Zn-Pb mining occurred with yearly outputs of ore exceeding two million tons of Zn-Pb sulphide. In parallel, oxidized ores were processed by roasting in rolldown furnances that were used until the 1980s. Since the 1970s, new techniques of hydrometallurgical ore enrichment have been developed which have enabled the recovery of high purity, electrolytic zinc. In areas of Zn-Pb ore exploitation, enrichment and metallurgical processing, heavy metals are concentrated in the superficial zone of the ground soil; these contaminate ground- and surface waters and penetrate into trophic chains. Investigations on minerals in the soil environment are of great importance in revealing the chemical transformation processes involved in the bio-concentration of particular metals in plants as well as in micro- and mezzo-fauna organisms. The source of heavy metal ions such as Zn2+, Pb2+, Fe3+, Mn2+, Cd2+ and Tl+ are metal-bearing mineral grains deriving from secondary, post-flotation waste deposits and from post-mining and post-metallurgical wastes. Further sources of these metals are high- and low-level emissions of air-borne dust from metallurgical plants and other industrial activities in the area. A significant influence on the increased concentrations of metals in the near-surface rock layer and in the in-situ created soil is the geological setting of the area and of the shallow occurrence of ore-bearing dolomites in particular (Figs 1, 2, 3). The subject of this research was the near-surface layer of the soil (depth: 0—0.05 m) in the area. This was industrially transformed by historical mining activity as well as by ore enrichment and metallurgical processing of Zn-Pb ores. The research area extends for a distance of 7 km to the north and north-east from post-flotation waste dumps (Fig. 10), which were sited several kilometers to the west of the town of Olkusz. In this research, the author of the thesis used a range of analytical techniques, including: X-ray diffraction (XRD), ASA — atomic absorption spectroscopy (for chemical composition detection) and scanning microscopy, which enabled the investigation of submicroscopic features of the mineral grains (BSE images) and their elemental composition (EDS analysis). Representative submicroscopic forms of minerals containing Zn, Pb, Fe, Mn, Cd, AS, Sb, Cu, Cu, Sn and other elements are presented on 11 illustrations comprising 101 BSE images (Photos 2—13). Elemental compositions of the mineral grains are presented on several tens of EDS spectra. Of particular interest in the research were the roots of plants which had settled on highly polluted ground. The investigation focused on the forms, elemental compositions and the manner of occurrence of both primary- and secondary minerals present in the epidermal layer of the roots of such plants as Dianthus carthusianorum, Festuca ovina, Viola tricolour, Cardaminopsis arenosa, Armeria maritime and others. Submicroscopic analysis of plant-root epiderm and of fine-grained soil indicates that metal-bearing minerals are abundant in the compositions of both. In the investigated rhizospheres, the research identified mineral grains with forms and chemical compositions different from those of the minerals occurring in both primary- and oxidized Zn-Pb ores. Roots of plants and shreds of fungi are covered by a secondary glaze, and coatings and superficial concentrations of metal-bearing minerals, all formed as a result of the biochemical activity of biotic soil components which were rich in solutions and metals. The formation of such surface forms on the roots may reflect the defensive reaction of the plants to high concentrations of heavy metals. The research focused on the origin and on the forms of Ca sulphate found on the plant roots and on other organic constituents in the soil, e.g., on needles of Pinus silvestris. The results of the research point to the major importance of submicroscopic analysis in the recognition of acid waste drainage (AWD) processes occurring in soils rich in Zn-Pb and Fe sulphides. Metal-bearing minerals are represented in the area by Zn- and Pb carbonates, Fe-Zn-Pb sulphides, Fe oxides and by unstable sulphates of Fe, Zn and Pb. Commonly, the minerals are of submicroscopic size < 20 μm) and in the form of irregular grains and aggregates with morphologies that indicate that the minerals had been subject to weathering processes during the stage of their hypergenesis. The parageneses of ore minerals recognized in the soils of the Olkusz region are identical to those found in the primary Zn-Pb ores. The simple, metal bearing sulphides present, such as sphalerite, galena, marcasite and pyrite, are typical of Zn-Pb ores of MVT type. In the near-surface layer of the soil, the chemical transformations of the sulphides are similar to those seen in the primary Zn-Pb ores. Among Zn-Pb-Fe minerals recognised in the soils, smithsonite, cerussite, and Fe oxides and hydroxides, are the most common. In the areas of both historicand contemporary post-industrial waste heaps, the processes of sulphide oxidation are very distinctly marked by the secondary crystallization of Ca and Fe sulphates on the roots of plants. A significant volume of metals occurs in polymineral aggregates characterized by complex structures and chemical compositions. In those areas close to the metallurgical plant, characteristic spherical aggregates were recognized (Photos 9—10) which, apart from Si and Al, also contain Fe, Zn, Pb, Mn, Cd and As. The presence of these elements is evidence for the major involvement of metallurgical-process emissions in the pollution of the soil. The superficial layer of soil which covers land reclaimed during the last several years does not contain spherical mineral phases. During his research, the author established that since the end of the last century, metal- bearing post-metallurgical aggregates had been no longer emitted into the atmosphere. The near-surface layer of the soil (depth; 0—0.05 m) formed on the post-mining waste deposits, in the vicinity of post-flotation tailing dumps and in places where the processing of Zn-Pb ores had taken place in the past, is very commonly characterized by exceptionally high metal contents. In areas neighboring post-flotation tailings, i.e., at distances of 0—1.2 km, analysis of numerous soil samples revealed very high concentrations of Zn (5 000—10 000 mg kg–1), Pb (1 000—5 000 mg kg–1), Fe (10 000—40 000 mg kg–1), Cd (10—100 mg kg–1) and Tl (5—50 mg kg–1). Total contents of metals in the superficial soil layer as well as the size of mineral grains containing Zn, Pb, Cd, Fe and Mn, show significant correlations with distance from the sources of the polluting emissions (Fig. 21). The distribution of metals, i.e., Zn, Pb, Fe, Mn, Cd, and Tl, in soil samples divided into specific grain size groups is also presented. The techniques of scanning electron microscopy (SEM) enable quick and easy recognition of particular groups of metal-bearing minerals occurring in the soil. Particularly good results were obtained for the mineral grains present on the epiderm of plant roots as well as on other organic material such as shreds of fungi or algae. SEM analysis, conducted in the low-vacuum working mode (0.3 Torr), makes it possible to distinguish components which are rich in Zn, Pb, Cd, Fe and Mn. The identification of Tl-bearing material is not possible with the use of EDS spectral analysis. The soils containing the significant concentrations of the metal-bearing minerals constitute a sort of reservoir for heavy metals. These metals are a potential threat to the natural environment. Commonly, however, the heavy metals are bound in stable- and difficult-to-dissolve mineral phases. Thus, the possibility for migration of the metal ions into underground waters, soil-originated solutions and trophic chains of organisms is limited and continues very slowly.