Speciation distribution of Cd, Pb, Cu, and Zn in contaminated Phaeozem in north-east China using single and sequential extraction procedures

Abstract

Knowledge of the total content of trace metals is not enough to fully assess the environmental impact of polluted soils. For this reason, the determination of metal speciation in soil is important to evaluate its mobilisation capacity and behaviour in the environment. The sequential extraction procedure was used to separate 4 heavy metals (Cd, Pb, Cu, Zn) from contaminated Phaeozem in north-east China into 5 operationally defined geochemical species: exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter, and residues. Neutral salts, dilute acids, and chelating reagents were used as single extractants for the functionally defined speciation. In the sequential extraction, the residual fraction was the most abundant pool for Pb, Cu, and Zn. A major portion (40–84%) of Pb, Cu, and Zn was associated with the residue. The speciation distribution of Pb, Cu, and Zn in the surface samples was similar to that in the subsurface, with residues > bound to organic matter > bound to Fe–Mn oxides > bound to carbonates > exchangeable. A significant amount (32–47%) of Cd persisted in the potential availability of the exchangeable fraction. The main part of Cd fractionation in the surface soil samples comprised exchangeable, carbonate, and Fe–Mn oxide fractions, whereas in the subsurface it comprised the residual and exchangeable phases. EDTA can be regarded as an extractant for assessing the plant-available species of Cd, Cu, and Zn; NH4Cl released electrostatically weakly bound metals and was used to estimate the mobile species. Assuming that metal mobility and bioavailability are related to their solubility and the contents in typical plants in the contaminated Phaeozem, Cd would be the main potential risk to animal health and groundwater safety in the area.