Remobilization events involving Cd and Zn from intertidal flat sediments in the elbe estuary during the tidal cycle

Abstract

In laboratory experiments, the effect of periodic alternations between aerobic and anaerobic conditions on the mobilization of Zn and Cd from the surface layers of an intertidal sand and mud flat sediment was studied. A percolation technique was used to simulate submersion and exposure to the air by controlling the concentrations of oxygen and nitrate within undisturbed 2 cm sediment layers. Zn and Cd released from the sediment were removed from the pore water under anerobic conditions and maintained in the percolating water when aerobic conditions were established. Zn was mobilized immediately after re-oxidation, whereas Cd mobilization was observed after a retention time of 53 to 180 min. and 25–72 h in sand and mud flat sediments, respectively. From 0·3 to 1·8% of the total Cd was then lost from these sediments. The rates of particulate organic matter degradation calculated from the production of ΣCO2 under aerobic and anaerobic conditions were similar. The pH values in sand flat sediments used in the experiments were significantly higher than in mud flat sediments. Neither the rates of organic matter mineralization nor the variations in pH controlled the observed trace metal mobility. The results strongly indicate that trace metals were precipitated as sulphides under anaerobic conditions. The release of trace metals under aerobic conditions was attributed to the oxidation of sulphides and the mineralization of organic matter. In the surface layer of the mud flat, the rate of Zn mobilization reached 8·79 × 10−7 μmol × g−1 × s−1 in summer but was only 1·82 × 10−7 μmol × g−1 × s−1 in winter. Cd was released from the same layer during the whole year at a constant rate of 5·89 × 10−9 μmol × g−1 × s−1. Calculations based on the percolation experiments indicate that in mud flat sediments from the Elbe Estuary, a loss of up to 50% of the total Cd occurs within one month.