| Natural water body is a complex system consisting of water,sediments,suspended matter,organism and other solid substances.Tubificid as a typical benthic organism,the metabolism and disturbance of them in the surface of sediments can arouse the exchange and migration of substances,which increases the complexity and dynamics of water environment.In order to explore the long-term process of migration and immobilization of heavy metals effected by tubificid bioturbation in aquatic environment,with multiple phases coexist.In this study,the migration of endogenous released(exogenous added)Cu and Cd in aquatic system,including sediments,natural biofilms,and suspended particulates,under the influence of bioturbation with different tubificid biomass,were investigated,using microcosmic simulating systems.The dynamic processes of metals reaching equilibrium among different media after the change of key water conditions,were also investigated.For the migration of Cu and Cd released by the sediments,the results showed that: the content of metals in aqueous phase was extremely low due to the rapidly accumulation of various solid substances;Cu and Cd in solutions were accumulated rapidly by natural biofilms at first,then released slowly,the maximum concentrations of Cu and Cd in the biofilm of the group with tubificid increased by 15.3% and 5.4% respectively compared with the balnk group;the sediments released Cu and Cd into aqueous phase at first,and the release content increased with the increase of tubificid biomass;the total amount of particulates in aqueous phase increased significantly with the increase of tubificid biomass.Compared with Cu,the migration of Cd in the system is more affected by tubificid bioturbation.To sum up,compared to the blank group,tubificid bioturbation could promote sediments to release more Cu and Cd into water,and the more suspended particulates produced.The Cu and Cd released from sediments were mainly accumulated by biofilms and suspended particulates.Meanwhile,tubificid bioturbation could transport Cu and Cd from bottom to top in sediments,which could promote the metals in deep layer migrate to shallow lyar along with the particulates matter caused by metabolism,and join the material exchange between sediments and other media.For the migration of Cu and Cd added to water phase,the results showed that: in the experimental group with the same initial metal concentration polluted in aqueous phase,the more tubificid biomass set,the lower the equilibrium concentration of Cu and Cd in water,respectively,the lowest concentrations of Cu and Cd in the water of group with tubificid were 39.3% and 52.1% in the blank group;and the smaller the maximum concentration of metals in biofilms,the maximum concentration of Cu and Cd in biofilms decreased by 19.2% and 13.6% respectively compared with the blank group;while the accumulation rate and amount of Cu and Cd in sediments increased,the maximum concentrations of Cu and Cd in the 0-1 cm depth of sediments increased by 38.9% and 24.8% respestively compared with the blank group;the Cu and Cd concentration in suspended particulates did not change significantly,but the total amount of suspended particulates increased.In the experimental group with the same tubificid biomass set,the accumulation of Cu and Cd in solid substances increased with the increase of initial concentration of Cu and Cd in water.The migration efficiency of Cu between different media was higher than that of Cd on the whole.But the migration of Cd is more affected by tubificid bioturbation.After the migration of Cu and Cd in the system approached the first equilibrium,the new biofilms added to the system,then the equilibrium concentration of Cu and Cd in water decreased again and stabilized at a lower concentration.The Cu and Cd released from solid phase to water were mainly accumulated by new biofilms and suspended particulates.After the migration of Cu and Cd approach the second equilibrium,the pH of the water controlled to decreased,then the concentration of Cu and Cd in water increased significantly,mainly from the release of solid substances,and the release of Cu and Cd in sediments increased with the increased of tubificid biomass.To sum up,the tubificid bioturbation could significantly increase the accumulation ability of surface sediments for Cu and Cd in water,and the thickness of metals storage in sediments could also be increased,it also produced a competitive effect on coexisting biofilms.Meanwhile,the increase of total amount of suspended particulates in water phase caused by tubificid bioturbation could also increase the retention of Cu and Cd in solid phases.In conclusion,the release of endogenous heavy metals and the accumulation of exogenous heavy metals in sediments of systems with tubificid bioturbation significantly increased compared with the blank group.Meanwhile,more suspended particulates could be produced in water phase,and the suspended particulates were not only the accumulation medium of heavy metals,but also the carrier of matter exchange among the solid media in the water environment,which has great siginificance for the dynamic migration of heavy metals in coexisting systems.Furthermore,when the migration of heavy metals in water environment closing to equilibrium,more heavy metals were accumulated and stored in the solid phase of systems with tubificid bioturbation than that in the blank group.Tubificid bioturbation could directly increase the material exchange between sedimnets and water,and indirectly affect the coexisting biofilms and suspended particulates in same water phase,through the effects of water solubility and particulates transport,which made the material exchange cycle in the whole water environment more dynamic,and the heavy metal migration and fixation accompanied by these cycles were more complex and changeable. |
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