Effects of tubificid bioturbation on realease of Cd and Zn from capping sediments

Laboratorial microcosm experiments were carried out to estimate the effects of the bioturbation induced by tubificid disturbance on release of Cd and Zn from capping lacustrine sediment. The farmland soil (5mm, 20mm) and uncontaminated sediment (5mm, 20mm) were collected as cap materials. Bioturbation effects on the pH of overlying water and the concentration of cadmium and zinc in the overlying water were studied in order to analyze the release flux of heavy metal in contaminated sediment disturbed by tubificid. The results showed that the capping decrease the release of heavy metals and the resuspension of sediment significantly and bioturbation could accelerate the release of Cd and Zn from capping sediment. With increasing of biomass, the release flux of Cd and Zn showed an upward-downward-upward tendency with no capping. The concentrations of Cd and Zn in overlying water declined with capping, and the release flux of Cd and Zn from sediment to overlying water decreased when the thickness of covering layer increased. The effects of capping and the transfer of heavy metal in sediment depended on the types and characteristics of cap materials. Generally, the inhibition for the release of heavy metal from polluted sediment capped by uncontaminated sediment is more significant. The release flux of Cd and Zn goes up with disturbance time increases. Meanwhile, there was adsorption of heavy metal by particles resuspending into overlying water under the tubificid reworking.