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«Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” Im Promotionsfach Geowissenschaften Am Fachbereich Chemie, Pharmazie und ...»

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(1.2.6.2) Importance of copper speciation in soils Speciation in soils refers to both the process and the quantification of the different defined species, forms and phases of a trace element. Copper in soil presents in different chemical forms and demonstrate various physical and chemical behaviors. The speciation of Cu in soil strongly affects its mobility, biological availability and potential toxicity. Copper can be associated with various soil components that differ in their ability to retain or release copper. The distribution of copper among different soil components can influence the mobility and as a result the bioavailability of copper. Factors also affect the forms of bioavailable metal species in soil include pH of soil, redox potential and soil organic matter content. Copper sorbed onto soil minerals and organic matter is the main species in soils. It is important to understand copper sorption mechanisms in soils and how the associated interfacial reactions contribute their roles in the fate of the metals in the environment. All the above factors are inter-correlated and depending on each other.

Copper speciation in soil is not straightforward because soils are heterogeneous with mineralogy and composition. Aging processes determine the extractability, bioavailability and toxicity of copper in soils and it is related to the contact time between the soil and the metallic cation. Copper ions can be retained in soil phases by ion exchange, outer and inner-sphere complexation processes (e.g. adsorption). The characteristics of the particle surface, bond strength and the properties of the solution in contact with the solid samples can significantly influences bioavailability of heavy metals. Concentration level of the potential pollutants alone is not enough to predict the impact of a contamination event in soils. It is also required to estimate the remobilizable fraction from the pollutant sorption-desorption pattern. The fraction demonstrates the amount of pollutant available for environmental processes and hence the potential risk for human populations can be determined. It is therefore vital to identify the exact copper species, how the species associated with mineral surfaces, and the nature of their potential precipitated phases. The assessment of soil metal bioavailability using chemical extractions is a conventional approach used in soil testing (Tessier et al., 1979).

Introduction

A number of different selective extraction procedures show a great diversity among reagents used for the determination of commonly distinguished metal species which are, in general: (a) easily exchangeable or water soluble, this fraction corresponds to the form of metals that is mostly available for plant uptake. It contains the fractions which are bound by electrostatically at charged surfaces and easily soluble salts of the metal and can be released by extraction with competing with cations in solution. (b) specifically sorbed, this fraction is bound to surfaces in soil by covalent bands as innersphere complex. (c) organically bound, (d) occluded in Fe/Mn oxides and hydroxides, this fraction can be mobilized with increasing reducing or oxidizing conditions of the environment and (e) structurally bound in minerals (residual), this fraction can only be mobilized by weathering.

However, the speciation of metals in soils is not stable, and relatively easy transformation of their forms in soils is observed. Depending upon the variability in physio-chemical characteristics of metals, their affinity to soil components governs their speciation.

Application of sequential extractions can provide detailed information about the origin, mode of occurrence, biological and physicochemical availability, mobilization and transport of trace metals. It can provide a convenient means to determine the metals associated with the principal accumulative phases in soil deposits. The following table (8) provides general descriptions of different sequential extraction fractions.

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Table (8) Table shows general characteristics of sequential extraction fractions Fraction Summary Exchangeable The adsorbed metals on the solid surface are under weak electrostatic interaction. Metals can be released by ion-exchange processes and coprecipitated with carbonates. This fraction can be mobilized by adjusting ionic composition, adsorption-desorption reactions and lowering pH.

Metals belong to this fraction is most readily available for the environment.

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Reducible Metals are scavenging by Fe and Mn hydrous oxides as coatings on mineral surfaces by co-precipitation, adsorption, surface complex formation, ion exchange and lattice penetration. Metals on these oxides are unstable under anoxic conditions and can be released by dissolution. In principle, the reducible fraction could be split into three fractions: easily reducible fraction (Mn oxides); moderately reducible fraction (amorphous Fe oxides); and poorly-reducible fraction (crystalline Fe oxides).

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(1.2.7) Introduction of thallium Thallium is heavy metal in Group 13 of the Periodic Table of Elements with oxidation state +1 and +3. Pure thallium is a soft, malleable, bluish-white metal and has long been discovered since 1861 however there is still relatively few literatures concerning about the chemistry, environmental fate and chronic exposure to low levels of this element in soils.





Thallium is a widely distributed and naturally occurring element with average concentrations of

0.49 ppm in the continental crust and of 0.013 ppm in the oceanic crust (Delvalls et al., 1999).

The common occurrences are ranging from lead, sulfide and zinc ores to association with potassium minerals in clays, soils and granites with mean concentrations of thallium in the earth crust are in a range of 0.1-1.7 mg/kg (John Peter, 2004). The evaluated Tl content of soils largely depend on the geological origin of the parent material across the globe with an average soils concentrations commonly found between 0.1 to 1.0 mg/kg (Kazantzis, 2000). The range of thallium concentration in non-polluted soil in general between 0.08 to 1.5 mg/kg (Wenqi et al.

1992, Tremel et al. 1997, Von Laar et al. 1994, Lukaszewski et al. 1992).Thallium exhibits both heavy metal and alkali metals chemical behavior with the degree of toxicity along with mercury, lead and cadmium. It exists primarily in a +1 oxidation state (Tl+ → Tl3+, +1.25V oxidation potential), form insoluble halide, sulphate and sulphides compound as well as soluble multihalide complexes. Tl as monovalent thallous cation is highly soluble in natural water and can be readily transported in aqueous environment. The mobility of thallium in soil samples is a crucial factor for the toxic effect of the element. Free metal ion, metal ions complexed, sorbed and exchangeable can be considered as a mobile metal. Exchangeable and sorbed metal is usually extracted with ammonium nitrate or acetate (Rule, 1998).

Introduction

Thallium is non-essential and extremely toxic to biota at trace levels in environment.

Thallium is a noticed mutagen and carcinogen. Thallium exposure can be absorbed through skin.

Tl exists at monovalent cation form in mammal’s cellular fluids. The Tl toxicity is due to similar ionic radius between Tl+ (1.49Å) and K+ (1.33Å) in which leads to nondiscriminatory uptake in metabolic processes. Symptoms of acute Tl poisoning of human include paralysis, coma and death. Thallium has been used in numerous industrial applications over the past century including human poisonous rodenticide (Tl(I) sulphate), medicine, low temperature thermometers (Tl and Hg), electronic devices (Tl(I) sulphide for semi-conductors), special glasses (Tl and Se), mercury lamps (Tl(I) halogenids), production of low-melting glasses, high refractory lenses, photoelectric cells, pigments and dyes.

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(1.2.7.1) Occurrence and properties of thallium Most of the heavy industries worldwide only consume small amounts of thallium with 8 tonnes were produced at Germany in 1975 (Zitko, 1975) and an estimated 10-15 tonnes per year in the world (Kemper and Bertram 1991). Davids et al. in 1980 showed estimated thallium emission to air from cement factories at Germany was 25 tonnes per year. Total Tl emissions from brickworks in Germany have been figured out in around 28 tonnes/year and 7.5 tonnes/year emissions from the burning of coal (Brumsack, 1977). Combustion of fossil fuels and flying-dust in Zinc, lead smelters and sulfuric acid plants are the major emitting sources of thallium.

Thallium entering along with lead, copper and zinc smelting processes is released into the atmosphere, discharged as solid waste and waste water. The Environmental Agency of WestGermany estimated thallium emissions from ferrous and non-ferrous smelters in the FRG to be ~ 45 tons/years (Davids et al., 1980). Volatile Tl compounds as trace element in large quantities of raw material in fossil fuels, oils and cement during combustion can escape through chimneys without proper filter installations and subsequently fall out on rural and urban area. An estimated of 2000-5000 tonnes Tl can be mobilized per year worldwide through the above processes (Kazantzis, 2000).

Introduction

Thallium presents in different rocks and minerals. In igneous rocks Tl is enriched in pegmatites by the magmatic differentiation process due to the large size of Tl ion (1.47 Å). Acid and intermediate magmatic rocks consisting of K-feldspar, plagioclase, biotite or muscovite are also characterized with high Tl content. In minerals Tl is commonly associated with K-minerals (e.g. micas and feldspars) and sulphides minerals with thallium as trace element associated mainly with galena, sphalerite and pyrite. During the weathering processes most of Tl compounds can be easily leached in water and transported along with alkaline metals however with great ionic radius the thallium can be immobilized by Mn, Fe oxides and clay minerals in soils especially under reducing conditions. Thallium can also be leach out and enriched in soils from the original bedrock. Thallium can also form stable humic complexes in which the thallium can be maintained in soils. Thallium associated with fly-ash in air usually deposit on the surface layer of soils and less well retained in acid soils. Thallous sulphate was found to be strongly bound in the upper 10 cm of various soil type and strongly resist leaching to lower horizons (Alloway 1990). Yang et al. (2005) showed the order for preferential immobilization of anthropogenic Tl among major soil components could be roughly summarized as: Tl(III) carbonates and hydroxidesMn oxide–hydroxidesFe oxide–hydroxidesadsorption sites on the surface of soil with the order can be significantly mediated by the pH conditions in the soils.

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(1.2.7.2) Thallium exposure in vicinity of cement factories and safety regulations worldwide Thallium poses great threat to the populations living in the vicinity of coal-burning power stations and cement works. Extensive sulfide ore mining, treatment and smelting is a major source of anthropogenic dispersion of Tl. Non-volatile Tl can exist in different compound states such as oxide (Tl2O), hydroxide (TlOH), sulphate (Tl2SO4) and sulfide (Tl2S) from anthropogenic emission. Water soluble thallium compounds (e.g. sulfate and hydroxide) can be dissolved in rain drops and precipitated out from atmosphere. Insoluble water thallium (e.g.

oxides) compounds can be transported by atmospheric dispersion and gravitational settling (ATSDR 1992).

The observed harmful potential Tl emission from cement (mixture of limestone and shale or clay) plants was not recognized until 1977 in Lengerich, Germany. The source of Tl was found to be residues of pyrite roasting added as a ferric oxide additive to powdered limestone in order to produce special qualities of cement. The emitted fly-ash thallium is bounded with particle size 0.2-0.8 µm with concentration 2.5 mg/m3 and by changing the production process can reduce the Tl level to less than 25 µg/m3 (Pielow 1979, Prinz et al. 1979 and LIS 1980).

Similar investigation of polluted soils around a cement plant at Leimen (SW Germany) had been undertaken and found out to be 3.6 mg/kg of Tl in surface soils 0-10 cm, 0.7 mg/kg at 40-50 cm and 0.1 mg/kg at depth 60-70 cm (Schoer, 1984).

Plants take up thallium and the amount of Tl being taken depends on pH and soil types.

Green rape, bush beans and rye grass were found to take up less Tl from weakly acidic soil (pH 6.2) than from more acidic soil (pH 5.6). Studies had also found out that rape plants could take up around 20% of soil Tl from a cement plant soil sample in comparison with only a range between 1.4 to 5.1% Tl in stream sediments in a mining district at Wiesloch in Germany (Scholl et al., 1982). The Tl transfer from soil to plant also depends on root system, kinetics of membrane transport and are species-specific. Uptake of Tl(I) ions can be through all parts of plant by K uptake mechanisms. The distribution of Tl varies in different vegetables, in gardens around Lengerich, leaves of kohlrabi could contain a 350-fold higher concentration than tubes, while in other vegetables the differences in concentrations between leaves and other parts could

Introduction

vary in a range between 3 to 45 times (Hoffmann et al., 1982). Plant species from Cruciferae family can accumulate Tl up to 450 mg/kg near a cement production site in Germany (McGrath, 1998). Tl in urine and hairs of residents near Lengerich area showed up to ~ 80 µg/l and 600 ppb which was significantly higher than upper normal limit of 1 µg/l (Ewers, 1988). Fruits and vegetables grow nearby the cement plant could lead to high thallium urine level for local inhabitants.



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