Effect of environmental conditions on heavy metal, binding forms

Heavy metals in surface water systems can be from natural or human sources. Sediments play auseful role in assessment of heavy metal contamination because suspended matter in thesediment preferentially adsorbs them. Their speciation in polluted river sediment particles canbe found in the following phases as cation exchangeable forms; bound to carbonates; bound toreducible hydrous Fe/ Mn oxides; bound to oxidizable sulphides and organic phases; and boundto residual phases. The migration extent of these metals from sediments to the water columnvaries with different environmental conditions. With the change of environmental conditions,the heavy metals bound to sediments may remobilize and release back to the water column, andin turn impose a negative effect on the food chain. Therefore, the bioavailability of these metalsto aquatic organism is dependent on the chemical forms of the metals.The main objectives of this study are to determine the metal binding forms to sediment fractions,and to investigate the effects of change in environmental conditions on the Cu, Zn and Pbbinding forms in the sediment.The speciation of these heavy metals in contaminated Oude Delft canal sediment wasdetermined experimentally, using a sequential extraction procedure. The effect of environmentalconditions such as pH, redox potential and additions of heavy metals was tested on bindingforms of the metals.The results show that the heavy metals have a high affinity for organic matter and Fe-Mnoxides, which are associated with the high surface areas of clay and silt particles. Acidificationand oxidation of the sediments causes the mobilization of heavy metals from the sediment to theoverlying water. The release of ions caused by low pH and oxidation may have caused theobserved increase in binding to exchangeable due to re- adsorption. The reduced binding to theorganic phase may be due to decomposition caused by both aerobic and anaerobic organisms.Fe-Mn oxides are more stable in the reduced form, which can bind more metal. The consistentincrease of metal binding to the carbonate phase under environmental changes may be a form ofbuffering in the sediment