Study On The Influence Factors On The Transportation Of Cu And Cd From Overlying Water To Sediment With Tubificid Bioturbation

With the development of human society and the progress of technology, the emissions flux of heavy metals into the environment are also gradually increase, causing widespread water heavy metal pollution. Thus the distribution and migration regularity of heavy metals in water environment become the research hotspot. Oligochaeta benthic tubifex is dominant population in freshwater sediment, and widely distributed in the global freshwater sediments. Tubificid worms have a strong ability to adapt to the environment, so they can exist in anthropogenic pollution waters. tubificid bioturbation will consequentially change the physical properties of sediment and the pollutants’ behavior in aquatic environment.In this study, the indoor microcosms were developed and the freshwater tubificid worms were employed as bioturbating organisms. In order to investigate the effects of key variables i.e., worm density, dissolved oxygen(DO), overlying water p H and temperature, on the behavior of tubificid bioturbation and on transportation of heavy metals from overlying water to the sediment during bioturbation by tubifex.The results showed that bioturbation promoted the migration rate of both Cu and Cd to the sediment, and the effect increased with increased density of tubificid worms. The influence of bioturbation on Cu migration was more obvious than Cd migration. Bioturbation can be influenced by DO, overlying water p H and temperature in the water/sediment systems. With the increase of DO, the migration rate of Cu and Cd transport to the sediment were increased. Since tubificid bioturbation increased most obviously under the middle DO conditions, the effect of bioturbation on migration rate of heavy metals were most significant. Addition of both acid and alkali induced stress on the worms, and such stress enhanced the bioturbation. This enhancement of bioturbation was more significant in alkaline conditions. The presence of tubificid bioturbation enhanced the acid-base buffering capacity of the water/sediment systems, and p H values of systems with tubificids tended to be more uniform and neutral. Both the changes of p H and the bioturbation in the systems affected the transport of Cu and Cd. Generally, the migration rate of the metals increased with the increase of overlying water p H. With the presence of bioturbation, heavy metal migration rate were increase first and then decreases as temperature gone up. Typically, tubificid bioturbation were strongest and the migration rate of Cu and Cd were fastest, When the environment temperature is 25 ℃.The factorial experiment results indicated that tubificid bioturbation is most dominant factor affect the migration rate of Cu and Cd. As to Cu the influences follow the order of tubifex density > p H > p H×tubifex density > DO > temperature > DO×tubifex density > temperature×tubifex density > temperature×p H > p H×DO; the factors that have a significant impact on the migration rate of Cd follow order of tubifex density > p H > p H×tubifex density > temperature > DO> temperature×tubifex density > p H×DO = temperature×p H > temperature×p H×tubifex density. It was found that the tubifex density, temperature, p H and DO have significant synergic effects on the transportation of heavy mentals; tubifex bioturbation strengthen the effect of temperature, p H, DO and their interactions on the migration of Cu and Cd. Tubifex density(within a certain range) have less effect on the factors. Since the Cu2+ adsorbabiity to sediments is stronger than Cd2+, the effect value of factors increased in the order of Cu > Cd.