Distribution of Cd, Pb, Cu and Zn in carbonate sediments from the Krka river estuary obtained by sequential extraction

Distribution of Cd, Pb, Cu and Zn in oxic carbonate sediments from the Krka estuary was studied by a sequential extraction technique, designed to distinguish: exchangeable cations (F1), carbonates (F2), hydrous oxides of Fe and Mn (F3), organic matter (F4) and residual fractions (F5). The procedure was performed on the sediments of selected grain size fractions: 150–300, 61–150 and 5–61 μm. Trace metal concentrations were generally low (Cd 0.156–0.399, Pb 17.3–118.6, Cu 19.1–52.1 and Zn 66.2–168.1 μg/g dry wt, even in the smallest size fraction). Distribution of trace metals differed for varying size fractions. Generally, there was no relation between metal concentrations in fractions F2, F3 and F4, and the concentrations of sediment components (CaCO3, organic matter, Fe and Mn). The highest metal concentrations were obtained in F2 (Pb in 61–5 μm and Cd and Zn in 300-150 μm size fractions) and in F3 (Cu, Zn and Cd in 61-5 μm). The concentrations in other fractions were lower. Despite the fact that carbonates are not known as trace metal collectors, such a distribution did not seem to be merely the result of the metal desorption from other sediment components.