Saturday, 15 July 2006

Heavy Metals in Contaminated Calcareus Soils: Distribution, Mobility and Bioavailability.

Ivanka Anguelova1, Gueorgui Anguelov1, and Ivan Atanassov2. (1) Florida A&M Univ, Banneker Tech Complex C-12, Tallahassee, FL 32307, (2) Institute for Sustainable Development, Yaroslav Veshin Str., Bl. 10 A, Ap.13, Sofia 1408, Bulgaria

The assessment of soil pollution by Heavy Metals (HM) has mainly been based on the total content of the elements. The total content does not give indication of elements' mobility and bioavailability. The metal input in polluted soil is in non-silicate bounded forms, i.e. potentially available. The mobile fractions extracted with different aquatic solutions have been used to estimate HM impacts on plants and microorganisms, as well as migration of metals within soil-plant-water system. However, many of the soil extractants need very specific setups and may not be equally suitable for different climatic and soil conditions. Depending on the pH of extraction used (from 7.0 to 4.6), ammonium acetate extractant can be used as an accurate indicator of HM mobility in excessively polluted soils. The purpose of this study was to evaluate the mobile as well as available forms of Pb, Zn, Cd and Cu in contaminated calcareus soils in vicinity of Pb-Zn smelter near Plovdiv, South Bulgaria. The soils were defined as Calcaric Alluvial Meadow Soils (Calcaric Fluvisols) with follow main characteristics: organic carbon 1.5 -1.9% (A horizon); pH (H2O) 7-7.5; clay 20 -24%; and CaCO3 presence in all soil horizons. Winter wheat, burley and alfalfs were used as test plants. The total soil-HM content was determined after HNO3+HCLO4+HF digestion; the mobile fraction of HM after extraction with 1M-ammonium acetate solution (soil: solution =1:10); and water-soluble fraction in soil: distil water=1:10. Wheat, barley and alfalfa plants were analyzed after washing, drying, and digesting (HNO3+HClO4+H2SO4). Heavy metals were predominantly deposited in the upper soil horizons near the smelter, and no metal migration through the soil profiles was observed. The content of the mobile forms of Pb, Zn, Cd and Cu, determined by an extract of ammonium acetate can reach up to 90% for Cd, up to 55% for Pb, and 20-25% for Zn and Cu of the total element's content. The mobile forms tend to diminish when the total content of the elements in the soil decreases and when the distance from the source of contamination increases. The water-soluble fractions were very low (between 0.1 -0.4% for Pb, Zn and Cu) in comparison to their total contents. The transfer coefficient (Tc) of Pb, Zn, Cd and Cu in the tested winter wheat, barley, and alfalfa varied between 0.1-0.8 depending on soil content of the metals, their bioavailability and plant species. According to the Tc values, the bioavailability of the metals was in the order Cd>Cu>Zn>Pb, corresponding to the degree of their solubility in 1-M ammonium acetate. The relative amount of the metal uptake by the tested crops, however, agreed with their total content in the soil (Zn>Pb>Cu>Cd). The highest bioavailability of Cd suggested that this metal is unable to form insoluble complexes in the investigated calcareus soil and it is largely taken up by the plants. A strong positive correlation between total-content and ammonium acetate extracted-fraction of Pb, Cd and Cu in soil and plant-biomass was established; no significant relationship between the concentrations in the soils and plants was found for Zn. The results suggest that the extracted with ammonium acetate solution Pb, Cd and Cu in metal-contaminated soil can be considered as indicative for assessment of their solubility and plant availability. It needs to be taken into consideration that the transfer of trace elements to plants is a very complex process and the analytical advantages and diagnostic importance of ammonium acetate should be studied further using different soils, under different ecosystems in both laboratory and field experiments. Keywords: soil pollution, heavy metals, mobile forms, water-soluble fractions

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