Effects Of Tubificid Bioturbation On The Release And Speciation Of Heavy Metals In Sediment

Since people aware of environmental issues, the heavy metal pollution has drawn great interest. In aquatic environment, heavy metals can be accumulated in solid phases, including suspended particles and sediments. Thus, sediment is one of the most important repository of heavy metals. They play very important role in the transformation and the fate of heavy metals. However, when the conditions of water-sediment interface change the heavy metals accumulated in sediments can be re-released. Therefore sediments may also act as endogeneous contamination sources for aquatic systems. Tubificid worms are widespread ecologically and are easy to culture and grow in freshwater sediment. Many studies showed that bioturbation is one of the processes that can give rise to direct or indirect metals transfer from the sediments. The benthic animals living at the surface and/or in the sediment superficial layers are strongly influenced the storage and release capacities of pollutants. On the one hand, their burrowing, irrigating and other activities have promoted the vertical and horizontal exchange of sediment particles and the pollutants exchange between the pore water and the overlying water. On the other hand, their metabolic activities, including defecating, respiring, and feeding, absorbing oxygen and nutrient, while producing carbon dioxide and excretion. It will consequentially change the physical properties, structure, chemical composition, pH, redox conditions and pollutants distribution of sediment. All these factors will play their role in the fate, speciation and bioavailability of pollutants.In this study, tubificid was employed as the representative of bioturbators. Cd, Zn, Pb and Cu were selected as the representative of heavy metals pollution. In order to quantify the direct and indirect effects of bioturbation on Cd, Zn, Pb and Cu transfer from contaminated sediments which sampled from Nanhu Lake, Xinkai River and Yitong River in Changchun China to the water column, we set up an experimental approach based on indoor microcosms. At the same time, the main factors pH and disturbation, which were influenced observably by bioturbation, were controlled artificially to investigate the effect of pH and disturbation on the heavy metals transfer from contaminated Nanhu sediment to overlying water column. Further more, the effect of bioturbation on the speciation of heavy metals in Nanhu sediment was studied. As the results shown, the concentrations of Cd, Zn, Pb and Cu in overlying water of different sediments-water system are effected by tubificid bioturbation to some extent. In sediment Cd and Zn are mainly absorbed on carbonates and Fe-Mn oxides and in overlying water they are mainly dissolved in solution. The soluble role of Cd and Zn release from sediment to water is weaken by bioturbation. In sediment Pb and Cu are mainly absorbed on organic/sulfide matters and residuals, in overlying water they are almost insoluble and mainly combined with particles. The main contribution of bioturbation on the concentration of Pb and Cu in overlying water is the resuspension of sediment particles driven by tubificid.The effect of pH and distuebation on the concentration of heavy metals in overlying water is significant. In the studied 4.5-8.5 range of pH values, concentration of Cd in overlying water is mainly influenced by pH. Concentration of Zn in overlying water is influenced by both pH and disturbation. Concentration of Pb and Cu in overlying water are mainly influenced by disturbation, there is little effect of pH on Pb and Cu concentration without disturbation, but when disturbation at presence there is a negative correction with concentration of Pb and Cu in overlying water and pH.The speciation of heavy metals in sediment can be influenced by tubificid bioturbation, and then the concentration and speciation of heavy metals in overlying are influenced. The effect of bioturbation on Cd and Zn are more obvious than Pb and Cu, as their more active physical and chemical properties. And the effect of bioturbation on the unstable speciations including exchangeable, boned to carbonates, boned to Fe-Mn oxides are more obvious then other stable speciations. The effect of bioturbation on pH and Eh is considered to be the main factors of the speciation change in sediment.The correlation analysis to the concentration of heavy metals in overlying water and their speciation in surface sediment showed that the speciations including exchangeable, boned to carbonates, boned to Fe-Mn oxides in sediment are the main endogeneous contamination sources for aquatic system. The concentrations of Cd and Zn in overlying water are positively correlation with their exchangeable content in sediment. On the contrary, the concentration of Cd in overlying water is negative correlation with its boned to carbonate content, and there is the same relation between the concentration of Zn in overlying water and its speciation of boned to carbonate in sediment.