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.
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IMMOBILIZATION OF HEAVY METALS IN SEDIMENT DREDGED FROM A SEAPORT BY IRON BEARING MATERIALS Pergamon War. Sci. Tech. Vol. 37. No. 4-7, pp. 379-384. 1998. 0 1998 IAWQ. Published by Ekvier Science Ltd P&ted in Great Britain. 0273-1223/98 $1900 + O+lO PII: SO273-1223(98)00221-2 IMMOBILIZATION OF HEAVY METALS IN SEDIMENT DREDGED FROM A SEAPORT BY IRON BEARING MATERIALS I. Miiller and E. Pluquet Geological Saxony, Institute of Soil Technology, Friedr. Missler Str. Survey of Lower 46/50,28211 Bremen. Germany ABSTRACT 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 NHdN03 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 -ZO%), while the uptake by plants was reduced by 20-501. 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 A13+ 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. 0 1998 IAWQ. Published by Elsevier Science Ltd KEYWORDSDredged sediments; iron oxides; heavy metals; immobilization; iron-bearing residues; pot trials; field trial. INTRODUCTIONAltogether, 700 000 m3/a of fresh sediment has to be dredged from the port of Bremen. Some of thesediment in the River Weser is strongly enriched with heavy metals and is disposed of in settling basins. Notonly the total content of heavy metals, but more important the mobile fraction should be known for riskassessment, if the sediment in the settling basin is to be reused or recultivated. One possible remedialmeasure is to immobilize the heavy metals, thus reducing their mobility and uptake by plants. A commonmethod is to apply dolomitic lime to increase the pH to neutral. Depending on the extent of pollution and thechemical properties of the soil, optimization of the pH alone may not be sufficiently effective in reducing theenvironmental impact of heavy metals (Ku&e et al., 1984). 319380 I. MiiLLER and E. PLUQUETFurthermore, the mobile portion of the total heavy-metal content can be reduced by strongly adsorptivematerials like clay, organic matter and iron oxides in soils (Brtimmer et al., 1986). The immobilizing effectsof an application of clay minerals and organic matter to contaminated soils have been described byVangronsveld er al. (1995) and Stolzer er al. (1994), respectively. Laboratory experiments have shown thatiron oxides adsorb heavy metals from solution (Grimme, 1968) and occlude them (Gerth & Brtimmer, 1983).It has also been verified by electron-microprobe studies that heavy metals in contaminated soils accumulatein iron oxides (Hiller & Brtimmer, 1995). Initial tests treating contaminated soils with iron oxides (Forster etal., 1983; Mench et al. 1994, 1995) have shown that this is a promising way to immobilize heavy metals insoils.Investigations on immobilization of heavy metals have been carried out since 1994 on dredged sediment in asettling basin and two other contaminated sites. Laboratory and pot trials were carried out using five iron-bearing materials to study the changes in mobility and particularly the uptake of heavy metals by plants; theresults were then tested in field trials. Several iron-bearing residues were also investigated in the tests. Theemphasis of these investigations is on the effectiveness of this method for long term immobilization ofheavy metals and its practical application. METHODSTest sites and soilsThe settling basin investigated is located near Bremen (northern Germany) west of the River Weser, wherethe latter is joined by the River Hunte. Up to 1992, sediment from the port of Bremen was pumped into thesettling basin, forming a layer of sediment 1.80 m thick composed of loamy sand alternating with clayey siltand ribbons of locally pure sand. Having been subsequently drained, the sediment, which is c ...