Changes in the leachability of metals from dredged canal sediments during drying and oxidation

The behaviour of metals in canal sediments after their disposal to land has important implications for the environmental management of canal dredgings. The leaching behaviour of trace metals was investigated in a laboratory-based experiment using sediment from a canal in the UK (139 mg Zn kg(dry sediment)(-1), 1.1 mg Cd, kg(dry sediment)(-1), 31.5 mg Cr kg(dry sediment)(-1), 20.6 mg Cu kg(dry sediment)(-1), 48.4 mg Ni kg(dry sediment)(-1), 43.4 mg Pb kg(dry sediment)(-1), and 7.6 mg As kg(dry sediment)(-1)). The sediment was allowed to dry. Cores (10 cm. long) of the drying canal sediment were taken over a period of 12 weeks. A simple water extraction procedure was used to investigate changes in metal leachability at varying depths through the cores. Metal leachability increased over the first five weeks of drying and then subsequently decreased between weeks five and twelve, (e.g. Cd increased from similar to0.006 to 0.018 mg/kg(sediment) then decreased to similar to0.006 mg/kg(sediment), Zn increased from similar to1.5 to 3 mg/kg(sediment) and then decreased to similar to1.5 mg/kg(sediment)). These results were combined with sulphide/sulphate ratios, which showed a decrease as the sediment dried (e.g. at 2-4 cm depth from similar to1 to 0.49), and BCR sequential extraction data. Most metals (except Cd and As) showed a redistribution from the residual phase into more mobile phases as the sediment dried and oxidised. Metal leachability was strongly correlated with the sulphide/sulphate ratio with leachability normally increasing with decreasing sulphide/sulphate ratio. The combined results were used to infer the likely behaviour of dredged material upon disposal to land. (C) 2001 Elsevier Science Ltd. All rights reserved.