Immobilization of heavy metals in sediment dredged from a seaport by iron bearing materials

Pot and field trials were carried out using sediment dredged from the port of Bremen (Germany) and deposited in a settling basin near Bremen; the sediment is polluted with Cd and Zn. Five iron-bearing materials were added to the soil (1% pure Fe in soil dry matter) to immobilize the heavy metals: ‘Red mud’ from the aluminium industry, sludge from drinking-water treatment, bog iron ore, unused steel shot and steel shot waste. The pH and CEC were little influenced by any of these treatments, but the NH4N03 and DTPA extractable amounts of Cd and Zn, and particularly the uptake of Cd and Zn by plants, were markedly reduced. It was demonstrated that red mud and precipitated Fe-bearing sludge were the most effective materials. They caused an increase in the adsorption capacity of the dredged sediment with respect to Cd of about 50%. In the pot trials, NH4N03 soluble amounts of Cd and Zn in samples of soil treated with these materials were reduced by 50% (DTPA: −20%), while the uptake by plants was reduced by 20–50%. In the field trial, Cd and Zn were immobilized in the soil to a certain extent, but less effect was observed on the concentrations in plants and soil extracts compared with the pot trials. In practice, red mud is unsuitable as it contains large amounts of Cr and Al3+ ions. Therefore, only sludge from drinking-water treatment, as long as the As concentration in it is low, remains as a useful material for immobilizing heavy metals in polluted sediment dredged from a seaport.