Abstract:

Bioturbation is one of the most important factors which influence the fate of heavy metals at the natural water/sediment interface. Laboratorial microcosm experiment was carried out to simulate the bioturbation induced by tubificids in Cd contaminated lacustrine sediment. Tessier sequential extraction was used to analyze the speciation of Cd in different sediment layers. Bioturbation effects on the vertical profiles of oxidationreduction potential (ORP) in sediment and the pH of overlying water were studied in order to analyzethe speciation of cadmium in contaminated sediment disturbed by tubificids. The results show that the mircoenvironment of sediment (including pH, redox conditions etc.) was changed by tubificid bioturbation, as a result, the main speciation of Cd was transformed. Bioturbation increased the internal pore diffusion rate from sediment to solution and the vertical diffusion of Cd in sediment and caused Cd to transform from the fraction of Cd bound to Fe and Mn oxides to
 exchangeable species. Meanwhile, the fractions of Cd bound to carbonate and exchangeable species were increased by bioturbation, which influenced the flux of Cd exchanging at the water/sediment interface. There were low fractions of Cd bound to organic/sulfide and residual in sediment, and the effect of tubificid bioturbation on these two fractions was slight.

Key words: sediment, tubificid, bioturbation, cadmium, speciation