Research On The Migration And Transformation Mechanism Of Typical Heavy Metals In Sediments In The Water-level-fluctuating Zone Of The Three Gorges Reservoir

Heavy metals are the most common environmental pollutants in sediments in water level fluctuating zone of Three Gorges Reservoir area.It is the main destination of heavy metals during periodical change of drought-rewetting cycles.And the physiochemical properties of sediments could impact the migration and transformation of heavy metals.The research selected sediments in Wanzhou district water level fluctuating zone of Three Gorges Reservoir area as study area.Firstly contents of heavy metals in sediments were determined and risk assessment methods were used to evaluate the ecological risk of heavy metals and to confirm the main ecological contribution factor.Secondly,an indoor simulation experiment was built up and added heavy metal solution(Cd and Pb)to reveal the mechanisms for sediments on the migration and transformation of heavy metals during drought-rewetting cycles.Thirdly,adsorption properties and physiochemical properties(including physical properties,chemical properties and surface characteristic)of sediment particles were determined and analyzed the correlation between physiochemical properties of sediment and heavy metal fractions to reveal the mechanisms for sediments on the migration and transformation of heavy metals.The main results were as follows:(1)The risk assessment results of sediments in water level fluctuating zone of Three Gorges Reservoir area indicated that Cd showed the moderately pollution,Pb,Zn,and Cu showed the light-moderately pollution,Cr and Hg showed no risk.Wanzhou had a large proportion of rural water level fluctuating zone,the application of pesticides and fertilizers were the main source of Cd.Pb pollution may be caused by atmospheric deposition and the direct discharged of oily wastewater into the basin during the course of ship navigation.There were many mechanical manufacturing industries in the wanzhou basin,and the discharged of industrial waste water such as iron and steel smelting machinery manufacturing were the main source of Cu and Zn in sediments in water level fluctuating area.The Wanzhou water level fluctuating zone in the Three Gorges Reservoir area had a higher pollution level,and Xintian water level fluctuating zone was more serious pollution,Cd and Pb were the main ecological contribution factors.(2)The sediments water level fluctuating zone were subjected to seasonal flooding,which led to the gradual migration of exchangeable heavy metals from layer to layer.Cd was mainly in Fe-Mn oxide-bound and exchangeable fraction in different depths.Pb and Zn were mainly in Fe-Mn oxide-bound and residual fraction in different depths.Cr and Cu were mainly in residual fraction in different depths.The contents of Fe-Mn oxide-boundand exchangeable fraction in Cd were increased during flooding period.The contents of organic matter-bound fraction in Pb were increased while the contents of carbonate-bound fraction in Pb were decreased during falling dry period.Fe-Mn oxide-bound and organic matter-bound fraction were not stable under the reduction and oxidization environment,that was,they were easy to migrate during the flooding and falling dry period in water level fluctuating zone.When affected by environmental factors like pH and redox potential,Fe-Mn oxide-bound fraction were benefited to form.Fe-Mn oxide-bound fraction reflected the pollution of human activities to the environment.When exchangeable fraction in heavy metals change in water environment,they will be released into water,causing ecological effects.(3)The physiochemical properties of sediments had great influence on the heavy metals fractions.Combined with surface analysis technology,The sediments grain size were mainly in silt.The content of heavy metals in the sediments of 20-30 cm in Xintian water level fluctuating zone reached the maximum,and the content of clay and silt reached the maximum.The specific surface area of silt in sediments was large and its morphology were rough and the pores with different sizes and shapes were developed,so it was easier to absorb pollutants.At the same time,under the seasonal flooding,the sediments flowed vertically and the heavy metals migrated downward.And the migration rate became slow after reaching a certain depth.when pH was changed,heavy metals will be released,causing secondary pollution.The main minerals in the sediments mainly included quartz,feldspar,zeolite,mica,kaolinite,plagioclase.As the environment changed,The heavy metals adsorbed on the crystal surface of the original sediment will be released along with the changed of mineral phases.High concentrations of heavy metals in the environment increased environmental mobility and ecological health risks.The adsorption of heavy metals by sediments tended to be single-layer adsorption and it was easy to occur.Pb was more stable than Cd in sediments and was not easy to be released into water.