Seasonal changes of metals in willow (Salix sp.) stands for phytoremediation on dredged sediment

Fast growing biomass plants such as Salix species are promising for use in phytoremediation of contaminated land. This study assessed the seasonal variations and changes with stand age in metal concentrations of S. fragilis L. and S. triandra L. grown in field conditions on contaminated dredged sediment substrates with comparable properties. A lesser proportion of total soil Cd was extractable by ammonium-acetate in the 6-year-old stand (6%) compared to the 1-year-old stand (17%). This suggests that the potential to remove metals from the site declines with tree age. Metal concentrations in willow biomass compartments decreased with stand age. Concentrations of Cd and Zn in leaves, wood, and bark increased toward the end of the growing season, irrespective of the species. Only Cd behavior offered limited prospects for targeting effective removal of the metal from the sediment through repeated harvest. The most efficient removal of Cd would require the combined harvest of stems and leaves; at the same time the risk of spreading Cd and Zn to the surroundings with leaf fall would be avoided

Metal uptake by young trees from dredged brackish sediment: limitations and possibilities for phytoextraction and phytostabilisation

Five tree species (Acer pseudoplatanus L., Alnus glutinosa L. Gaertn., Fraxinus excelsior L., Populus alba L. and Robinia pseudoacacia L.) were planted on a mound constructed of dredged sediment. The sediment originated from a brackish river mouth and was slightly polluted with heavy metals. This preliminary study evaluated the use of trees for site reclamation by means of phytoextraction of metals or phytostabilisation. Although the brackish nature of the sediment caused slight salt damage, overall survival of the planted trees was satisfactory. Robinia and white poplar had the highest growth rates. Ash, maple and alder had the highest survival rates (>90%) but showed stunted growth. Ash, alder, maple and Robinia contained normal concentrations of Cd, Cu, Pb and Zn in their foliage. As a consequence these species reduce the risk of metal dispersal and are therefore suitable species for phytostabilisation under the given conditions. White poplar accumulated high concentrations of Cd (8.0 mg kg-1) and Zn (465 mg kg-1) in its leaves and might therefore cause a risk of Cd and Zn input into the ecosystem because of autumn litter fall. This species is thus unsuitable for phytostabilisation. Despite elevated metal concentrations in the leaves, phytoextraction of heavy metals from the soil by harvesting stem and/or leaf biomass of white poplar would not be a realistic option because it will require an excessive amount of time to be effective