Abstract
In addition to the more reactive forms, metals can occur in the structure of minerals, and the sum of all these forms defines their total contents in different soil fractions. The isomorphic substitution of heavy metals for example alters the dimensions of the unit cell and mineral size. This study proposed a method of chemical fractionation of heavy metals, using more powerful extraction methods, to remove the organic and different mineral phases completely. Soil samples were taken from eight soil profiles (0-10, 10-20 and 20-40 cm) in a Pb mining and metallurgy area in Adrianópolis, Paraná, Brazil. The Pb and Zn concentrations were determined in the following fractions (complete phase removal in each sequential extraction): exchangeable; carbonates; organic matter; amorphous and crystalline Fe oxides; Al oxide, amorphous aluminosilicates and kaolinite; and residual fractions. The complete removal of organic matter and mineral phases in sequential extractions resulted in low participation of residual forms of Pb and Zn in the total concentrations of these metals in the soils: there was lower association of metals with primary and 2:1 minerals and refractory oxides. The powerful methods used here allow an identification of the complete metal-mineral associations, such as the occurrence of Pb and Zn in the structure of the minerals. The higher incidence of Zn than Pb in the structure of Fe oxides, due to isomorphic substitution, was attributed to a smaller difference between the ionic radius of Zn2+ and Fe3+.
Highlights
Metals preserved in soils in different forms can be separated by fractionation techniques that allow inferences on the element concentration in several organic and mineral fractions
There is no method of sequential extractions of heavy metals for basic studies to determine the composition of the soil constituents, by the total extraction of mineral and organic constituents indicating the actual contribution of each phase to the total heavy metal contents
The loss of sample mass during the sequential extractions by traditional methods for environmental purposes does not represent a total extraction of constituents at each step and the less reactive metal forms are computed in the residual phase
Summary
Metals preserved in soils in different forms can be separated by fractionation techniques that allow inferences on the element concentration (reactive or total forms) in several organic and mineral fractions. The extraction of mineral and organic constituents is only partially due to the nature of the treatments and the fractions normally considered are: exchangeable; carbonates; Fe, Al and Mn oxides and hydroxides; organic matter and residual fractions. These methods are applied to studies of environmental diagnostic since the extraction of heavy metals at each step is limited to more reactive forms. To determine the reactive heavy metal fraction associated with Fe oxides diluted solutions of ammonium oxalate are commonly used This extractor is only efficient in solubilizing low crystalline Fe oxides (e.g. ferrihydrite) (Norrish & Taylor, 1961). The loss of sample mass during the sequential extractions by traditional methods for environmental purposes does not represent a total extraction of constituents at each step and the less reactive metal forms are computed in the residual phase
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