Exchange of trace elements at the sediment-water interface during early diagenesis processes

Abstract

The exchange processes at the sediment-water interface can significantly influence the transport of trace contaminants in a river. In order to investigate the importance of these processes in the tidal part of the River Elbe, a laboratory 'simulation' apparatus (LABOSI) was used that allows the incubation of undisturbed sediment cores with a relatively large area (1700 cm²) in a closed system under defined conditions (temperature, pH, pE, etc.). Together with measured depth profiles of the pore-water and the solids of the incubated cores before and after the experiment, this method allows all relevant processes to be investigated in all three phases (sediment, overlying water, gas) simultaneously. Under oxic conditions, a flux of heavy metals (Cu, Cd , Zn) into the water phase was observed as a result of the mineralization of organic matter in the thin oxic layer of the sediment (thickness about 3 mm) as well as a flux of manganese caused by diffusion from deeper anoxic layers. Up to 40% of Cd, Zn and Ni and 15% of Zn deposited in the sediment by settling of contaminated suspended particulate matter can be remobilized from the sediment surface by the processes of early diagenesis. Within 14 days, an enrichment of arsenic in the sediment surface caused by diffusion from deeper anoxic layers could be detected. Under anoxic conditions and when the other electron acceptors were consumed, Fe(II) was released and accompanied by a strong remobilization of As, Co and Cr and an accelerated release of Mn(II). This release was soon stopped when the water phase became oxic again, and the trace elements were removed from the water phase by co-precipitation andlor adsorption on the freshly formed hydrous ferric oxides.