Distribution, movement and availability of Cd and Zn in a dredged sediment cultivated with Salix alba

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Willows occur as volunteer vegetation on sediment-derived soils, such as dredged sediments, landfill cover or stockpile deposits, and are used as phytoremediators on such soils. The present study evaluates growth and metal uptake by Salix alba grown on a contaminated dredge sediment for 209 days under greenhouse conditions. At the end of the study, the aerial parts of the S. alba plants had grown to heights of between 80 and 117 cm. Biomass and Cd and Zn concentration in the roots, stems and leaves, at 70, 112 and 209 days, showed that Cd and Zn had been bioaccumulated, especially in the leaves. At the three sampling dates, Cd and Zn extractability and pH measurements were also carried out on samples of two soil layers (0–15 and 15–30 cm) from both the planted and the control pots. Cd and Zn extractability were assessed using single extraction procedures (0.01 M CaCl 2; 1 M HNO 3; CaCl 2–TEA–DTPA). The two metals showed similar variations in CaCl 2 and HNO 3 extractabilities, but this was not the case for DTPA extractability. The greatest variations were observed in the upper soil layers of the control pots. In the planted pots, the CaCl 2 extractability of Zn decreased in the upper layer, and the HNO 3 extractability of Zn increased in the lower layer. The pH of the upper soil layer was always higher than the pH of the bottom layer. In addition, we monitored several parameters of the percolates from both the planted and the control sediments, including pH, Eh, conductivity, dissolved organic carbon, Zn and Cd concentrations, and presence of certain cations/anions. Dissolved organic carbon, and Cd and Zn concentrations increased steadily over time. There were no significant differences between the planted and the control pots. After 209 days, the characteristics of the control sediment reflected the effects of ageing in that the CaCl 2-extractable Cd and Zn concentrations had decreased compared with the initial concentrations. Conversely, the concentrations of HNO 3-extractable Cd and Zn had increased. A fraction of the metal initially extracted by CaCl 2 (considered as exchangeable) became less available with time. After 112 days, the plants had extracted approximately 2.8 mg of Zn. At the same time, the CaCl 2 extractability of Zn in the upper, rooted layer decreased by 2.6 mg. We can assume that S. alba extracted Zn from the pool of CaCl 2-extractable Zn.