Mobility and bioavailability of soil heavy metals strongly depend on their fractions. Secondary-phase fraction (SPF) of heavy metal, including acid-soluble, reducible and oxidizable fractions, is considered as direct and potential hazardous fraction to organisms. The ratio of SPF to the total concentration of heavy metal represents its bioavailability. In this study, 126 topsoil samples were collected in Kunshan, Jiangsu, China. Fraction concentrations of heavy metals, and their bioavailability and spatial distributions were determined, and relationships between their fractions and types of industry zones were analyzed. Results showed that Cd and Pb had the greatest SPFs among all metals (78.61% and 62.60%, respectively). Great SPFs of Cd and Pb were observed in the dyeing and paper-making industry zone, while great SPFs of Cr, Cu, Zn, Ni were in the smelting and plating industry zone. For most metals, fraction distributions were controlled by soil organic matter and clay contents. Spatial principal component analysis showed SPFs of heavy metals can be explained by two principle components (PCs). PC1 represented SPFs of Cd, Cr, Cu, Pb and Zn, while PC2 represented SPFs of Ni and Co. The spatial distributions of SPFs were influenced by geochemical character, industrial sewage irrigation and soil physico-chemical properties.
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