Extractability and bioavailability of heavy metals in surface soils derived from dredged sediments

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Abstract
Accumulation of heavy metals by plants causes concern because it is a major pathway by which metals can enter into the food chain. Extractability and bioaccumulation in rye grass of Cd, Cu, Mn, Pb and Zn in dredged sediment derived surface soils were studied in a greenhouse experiment. The soils were extracted with 0.01 mol L−1 CaCl2, 1 mol L−1 NH4OAc, 0.02 mol L−1 NH4OAC—EDTA, 0.005 mol L−1 DTPA and 0.1 mol L−1 HCI. Extracted metal contents were correlated with concentrations in rye grass, harvested after 1 and after 2 months. The extractability of Cd, Mn and Zn decreased in the order NH4OAC-EDTA > HCI > DTPA > NH4OAC > CaCl2. For Cu and Pb, the order was similar except that DTPA extracted higher amounts than HCI. No significant differences in dry matter yield were observed between the soils upon the first harvest. In the second harvest, the dry matter yield was significantly lower for the soils highest in clay content. Elevated concentrations of Cd and Zn were observed in the plant material and were even more pronounced in the second harvest compared to the first harvest. Despite high metal concentrations, results suggested that poor soil physical growing conditions rather than metal toxicity accounted for the decrease in dry matter yield. No single extractant provided a reliable prediction of plant concentrations for all metals. Correlations between extracted metals and plant metal concentrations were significant for Cd, Mn and Zn and were higher for NH4OAc-EDTA, DTPA, NH4OAc and HCI than for CaCl2. Correlations with Cu and Pb were weak for all extractants.

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