EXTRACTABILITY AND BIOAVAILABILITY OF HEAVY-METALS IN SURFACE SOILS DERIVED FROM DREDGED SEDIMENTS

Accumulation of heavy metals by pla nts causes concern because it is a major pathway by which metals can enter into the food chain. Extracta bility and bioaccumulation in rye grass of Cd, Cu, Mn, Pb and Zn in dr edged sediment derived surface soils were studied in a greenhouse expe riment. The soils were extracted with 0.01 mot L(-1) CaCl2, 1 mol L(-1 ) NH4OAc, 0.02 mol L(-1) NH(4)OAC-EDTA, 0.005 mol L(-1) DTPA and 0.1 m ol L(-1) HCl. Extracted metal contents were correlated with concentrat ions in rye grass, harvested after 1 and after 2 months. The extractab ility of Cd, Mn and Zn decreased in the order NH(4)OAC-EDTA > HCl > DT PA > NH(4)OAC > CaCl2. For Cu and Pb, the order was similar except tha t DTPA extracted higher amounts than HCl. No significant differences i n dry matter yield were observed between the soils upon the first harv est. In the second harvest, the dry matter yield was significantly low er for the soils highest in clay content. Elevated concentrations of C d and Zn were observed in the plant material and were even more pronou nced in the second harvest compared to the first harvest. Despite high metal concentrations, results suggested that poor soil physical growi ng 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 extracte d metals and plant metal concentrations were significant for Cd, Mn an d Zn and were higher for NH4OAc-EDTA, DTPA, NH4OAc and HCl than for Ca Cl2. Correlations with Cu and Pb were weak for all extractants.