Experimental Study On Heavy Metals Adsorption/Desorption From Sediments Of Rivers

Heavy metal is one of the common pollution in river sediments.Because of its enrichment and difficult to degrade,most of the heavy metal gradually absorbed on the sediment surface with the water flow.Heavy metal pollutants on sediments will release to water under certain conditions,and will cause secondary pollution to water.The adsorption and desorption of heavy metal in sediment/water interface is an important way to affect their migration,transformation,and biological utilization in river environment.Therefore,it's very important to study the adsorption and desorption behaviour of heavy metal on city's river sediments.In this paper,the adsorption and desorption behaviors of heavy metal on Shanghai main river sediments were comprehensively studied through laboratory experiments.The sediment sampling were freeze-drying for measured the total amount of heavy metal,analyzed speciation of heavy metal,studied the sediments adsorption and desorption kinetics of heavy metals,explore the effect of temperature and pH.The main conclusions are as follows:(1)Through analyzed the physical and chemical characteristics of eight sampling sediments,learned that the pH of Shanghai river's sediments are mainly weakly acidic or neutral,organic matter content is less than 10%.The particle size distribution conformed normal distribution,the particle size range is mainly aleurite and caly.(2)The highest contents of heavy metals in river reach sediment are Zn and Cr,followed by the content of Cu,Pb,Ni.The worst of ecological harm was Cd,its content exceed soil national standard Grade ? and the pollution in Yunzao river-Hutai Road(S7)was the most serious.(3)The heavy metal of Cu in sampling sediment was priority to residual and organic,the Fe-Mn oxidesiron ratio was lager.Zn was mainly Fe-Mn oxidesiron and residual.There was more than 65%residual of Ni.Pb was mainly Fe-Mn oxidesiron,the residual ratio was lager.Cr was mainly residual,its content was more than 70%.Cd was mainly Fe-Mn oxidesiron and residual,organic ratio was lager.(4)The adsorbing capacity of heavy metal in sediments was increased with time passe,eventually reach equilibrium.The adsorption kinetics was divided into fast,medium and slow three stages,and it accords with the pseudo-second order kinetic model.Adsorption rate of heavy metals from fast to slow is Cu>Cd>Ni>Zn>Cr>Pb.Meanwhile,the adsorption rate increased with the reaction temperature rised,and the adsorption of six heavy metals in sediments were well described by both Langmuir and Freudlich equations,especially Langmuir equation.The order of partition coefficients between sediments and water for heavy metals are Cu>Pb>Cd>Zn>Ni>Cr.Therefore Cu and Pb transport into sediments more easily.The pH on adsorption quantity of changes had a major influence.With the pH increasing,adsorption of metal ions had obviously increased.The order of sensitivity to pH is Ni?Cd>Zn?Cu>Pb?Cr.With the decrease of sediment particle size,the amount of adsorption of heavy metals was increased.(5)As the time went on,the desorbing capality of heavy metals in sediments was increased,eventually reach equilibrium.The desorption process of heavy metals in sediments can be divided into fast and slow reaction stage,and it more accords with the Elvich equation,indicating that the heavy metals desorption process is heterogeneous di fusion.Desorption rate of heavy metals from fast to slow is Cr>Cu>Ni>Pb>Zn>Cd.With the reaction temperature rised,the desorbing capality was no significant change,but the desorption rate increased slightly.In addition Cr,the rest of five heavy metals desorbing capacity decreased with pH increased,change significantly.With the decrease of sediment particle size,the desorbing capality of heavy metals is also reduced.