An Ex-Situ Immobilization Experiment With Heavy Metals In Dredged Marine Sediments From Bohai Bay,China

A large number of dredged marine sediments are generated worldwide due to the dredging activities in marine construction.One of the main concerns raised in relation to dredged marine sediments is their potential risk to marine environments.The marine sediment plays a role as the largest sink of different kinds of pollutants.Metals represent a crucial category of these pollutants,owing to their potential toxicity,durability,and bioaccumulation.Once the sedimentary environment changes,metals in sediments can be released into the overlying water again,causing secondary pollution to surrounding areas Therefore,it is imperative to remediate dredged marine sediments contaminated by metals prior to final disposal.Immobilization was regarded as a promising method for reducing adverse impacts on marine ecosystems.In this study,the marine surface sediment samples collected from Dashengtang Village in Tianjin,Bohai bay were used to study immobilization effect of two amendments.The physical-chemical properties of the sediments,such as the pH,electrical conductivity,salinity,and total organic carbon,were measured to better understand their influence on the three metals' mobility.In addition,the compositions of clay minerals were also analyzed to identify the transformation process of minerals in the sediments.The amendments kaolinite and limestone were used to immobilize Zn,Pb,and Cu in dredged marine sediments.The sequential extraction procedure was applied to identify the mobility of metals and,further,to evaluate the immobilization effect of the amendments.The conclusions are as follows:(1)The pH value of the sediments had no obvious variations with the addition of kaolinite and limestone,which may be related to the buffering capacity of the sediment.However,the EC,salinity,and TOC decreased moderately with time.The percentage of montmorillonite decreased to minimum value while that of chlorite increased gradually during the experimental periods for 40 days probably due to complexation reaction.(2)The results of the sequential extraction procedure indicated that the mobile fractions of three metals were converted into relatively stable fractions because of the addition of kaolinite and limestone,which meant the mobility and bioavailability of metals decreased,as well as the adverse influence to marine environments and human bodies,indicating that both amendments worked in immobilization of metals.(3)The calculation of MF%(the percentage of metals in the acid exchangeable fraction in the cumulative total extracted amount of the metal)suggested that both kaolinite and limestone effectively reduced the mobility of three metals,especially for Zn.The MF% values of Zn,Pb and Cu in kaolinite group decreased by 11.42%,2.93% and 3.55% respectively,while those in limestone group decreased by 11.61%,2.73% and 3.85% respectively at the end of the incubation period.The decrease in MF% values of kaolinite and limestone compared to control group indicated that the two amendments could be applied in the immobilization of heavy metal pollution.The MF% of limestone group is smaller than that of kaolinite group,which showed that limestone generally has a better immobilization effect,compared with kaolinite.