| With the rapid development of society,heavy metal pollution in water has become a serious environmental problem worldwide.Heavy metals that enter the water can migrate to the riverbed sediments through adsorption,precipitation and other forms,causing potential harm to the sediment ecosystem.When the environment around the sediment fluctuates,the accumulated heavy metals ma y once again threaten the water ecosystem and harmed human health pass through the food chain.In the face of serious heavy metal pollution of sediment,people have adopted a variety of remediation strategies（e.g.,soil washing,plant extraction,fixation,etc.）.Among them,soil washing is an economical and efficient remediation method,which transfers heavy metals in the solid phase to the liquid phase,thereby reducing the total amount of heavy metals in the solid phase and reducing the potential exposu re risk of heavy metals in the sediment.However,a single remediation method is often not a good solution to the compound pollution of heavy metals in sediment.Therefore,in this study,the cheap and green humic substances were used to wash the sediment polluted by heavy metals,followed by zeolite fixation to further improve the sediment quality.Based on the surface sediments of East Dongting Lake,this research discussed how the different combinations of sediment washing（with humic substances derived from green waste compost）and in situ fixation（with zeolite modified with Na Cl）play a valuable role in remediation of cadmium（Cd）-contaminated sediments.In order to evaluate the effectiveness of this remediation strategy,the changes of basic physicochemical properties,Cd speciation,enzyme activity,microbial biomass and Cd bioavailability of sediments in different treatment groups were discussed.The results showed that the p H of the sediment remained between 7.62–8.03,and a slight decrease was observed only in the treatment group associated with humic substances washing.The electrical conductivity（EC）and sediment organic matter（SOM）were significantly improved with the use of humic substances washing and/or zeolite fixation.The BCR sequential extraction procedure showed that the removal efficiency of Cd by humic substances washing reached 11.4%–26.0%.At the same time,compared with the humic substances washing alone,the residual Cd（F4）was further increased by 9.0%–16.4%by adding zeolite fixation,that is,the residual Cd after leaching was further stabilized.The combined treatment of humic substances washing and zeolite fixation had statistically significant effect on enzyme activity.In particular,the change of dehydrogenase was the most sensitive(the maximum increase was 220times in the 2000 mg L-1 humic substances+100 g kg-1 zeolite treatment),which can be regarded as an indicator of the restoration of sediment biological function.In the presence of humic substances or zeolite,microbial biomass carbon（MBC）and microbial biomass carbon nitrogen（MBN）showed varying degrees of increase,suggesting that the microenvironment of the sediments was significantly optimized.An in vitro digestion assay（PBET）was designed to characterize the oral bioavailability of Cd.The results showed that the PBET extractable Cd was significantly reduced by 3.9%–67.9%.Correlation analysis and principal component analysis showed that the optimal biol ogical properties were obtained after 2000 mg L-1 humic substances washing and 100 g kg-1 zeolite fixation.In addition,we also compared the cost of the combined treatment strategy.The cost of humic substances is only 0.85 US$t-1 for 1%Cd removal,and the cost of zeolite is only 0.03–0.12US$kg-1,which were far cheaper than many other washing agents and stabilizers.Therefore,the combined treatment of humic substances washing and zeolite immobilization is a feasible and effective technology for remediation of Cd contaminated river sediments and restoration of their ecological functions. |
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