Chemical Speciation of Metals in the Top-soil and Subsoil Layers

6 Chemical Speciation of Metals in the Top-soil and Subsoil Layers 

Numerous speciation procedures have been developed in order to describe heavy metal forms in soils (Himer 1992 Rauret 1993 Zeien and Bruemmer 1989). In this study, the sequential extraction method according to Zeien and Bruemmer (1989,1991) was applied to soil samples collected from sanitary protection zones. Both surface (0-20 cm) and subsurface (30-40 cm) soil samples were examined. The method involved the sequential extraction of 7 operationally defined fractions  

Occluded in crystalline FeOx - 0.2 M NH4-oxalate + M ascorbic acid, pH 3.25 

Based on a sequence of extractants of increasing acidity, combined with reducing and complexing agents, this method was supposed to reflect quite well the chemical form of binding between metals and the soil substrate, and would also guarantee a minimum of redistribution of elements during extraction (Zeien and Bruemmer 1989, 1991 Karczewska et al. 1995). 

Fraction 1 is likely to define the actual concentration of metals in soil solution. Fractions 2-4 might be mobilised in the short and medium term by changes in soil chemistry. Fraction 2 is supposed to be easily mobilised from the surface of soil constituents (soil sorptive complex) by changes in pH. Fractions 3 and 5, which represent metals bound to Mn oxides and amorphous Fe oxides, should be sensitive to changes in redox potential. Fraction 4, reflecting the organically bound metals, will probably be mobilised by the decomposition of soil organic matter. Fractions 6 and 7 are expected to be relatively stable, particularly in well-aerated soils, as reaction kinetics of iron oxides and silicates are slow. 

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