Trace metal behaviour in riverine sediments: role of organic matter and sulphides Authors

Three sediment cores were collected in the Scheldt, Lys and Spiere canals, which drain a highly populated and industrialized area in Western Europe. The speciation and the distribution of trace metals in pore waters and sediment particles were assessed through a combination of computational and experimental techniques. The concentrations of dissolved major and trace elements and Zn) were used to calculate the thermodynamic equilibrium speciation in pore waters and to evaluate the saturation of minerals (Visual Minteq software). A sequential extraction procedure was applied on anoxic sediment particles in order to assess the main hosting phases of trace elements. Manganese was the most labile metal in pore waters and was mainly associated to carbonates in particles. On the contrary, a weak affinity of Cd, Co, Ni, Pb and Zn with carbonates was established because (1) a systematic under-saturation was noticed in pore waters and (2) less than 10% of these elements were extracted in the exchangeable and carbonate sedimentary fraction. In the studied anoxic sediments, the mobility and the lability of trace metals, apart from manganese, seemed to be rather controlled through the competition between sulphidic and organic ligands. In particular, the necessity of taking into account organic matter in the modelling of thermodynamic equilibrium was demonstrated for Cd, Ni, Zn and Pb, the latter element exhibiting the stronger affinity with humic substances. Consequently, dissolved organic matter could favour the stabilization of trace metals in the liquid phase. Conversely, sulphide minerals played a key role in the scavenging of trace metals in sediment particles. Finally, similar trace metal lability rankings were obtained for the liquid and solid phases.