Modelling of Redistribution of Toxic Elements in River Sediments

Abstract

Mobility and redistribution of Fe, Mn, Cd, Cr, Cu, Ni, Pb and Zn in anoxic stream sediments of the grain size 63 mm and less were studied after air bubbling, freezing and oven-drying at 60 °C. Quantitative speciation of metals associated with the solid phase was obtained by a sequential six-step extraction technique. The combined effects of organics and the presence and formation of soluble carrier phases on the metal redistribution are indicated. Under anoxic conditions, large proportions of Cu, Cd, Pb together with minor amounts of Zn, Cr and Ni are associated with organics, which widely corresponds to their complex stability order, but also to the stability of the respective metal sulfides: Cu > Cd > Pb > Zn > Ni > Cr > Fe > Mn. Fe and Mn occur in large amounts in iron oxides and hydrated oxides (Fe) and in adsorbed carbonate fractions (Mn, Fe). Partition of the elements between the phases is summarized. Changes from reducing to oxidizing conditions, which involve transformations of sulfides and a shift to more acidic media, are greater for cadmium, followed by zinc, copper, and lead. These elements are an environmental risk due to their high amounts in the more easily remobilized fractions. There was little change in mobility of nickel and chromium. On the other hand, manganese and iron showed a characteristic lowering of mobility.