Preliminary Study On Remediation Of Heavy Metal Pollution In Sediment By Washing Combined With Composting

With the development of industrialization and the increase of human activities,a large amount of industrial wastewater containing heavy metals is discharged into the ecological environment due to improper disposal,causing serious environmental problems.After entering the water body,heavy metals are easily stored in river sediments through complexation,adsorption,ion exchange,and coprecipitation.Heavy metals in river sediments pose risks to aquatic organisms and human health through the food chain.Therefore,the control of heavy metals in river sediments has received extensive attention.The washing technology can effectively remove heavy metals in the sediment.The composting technology has achieved excellent efficiency in passivating heavy metals.Therefore,the combination of washing and composting was used to remediate the heavy metal-contaminated river sediment（Cr,Cu,Zn,As,Cd,Pb）in the study.Here are the key findings of this study:（1）The effects of washing combined composting on the basic parameters of the pile（pH,NH₄⁺-N,NO₃^--N,total nitrogen,organic matter,carbonnitrogen ratio,and dehydrogenase activity）were studied.The washing of CA and EDTA-2Na led to a decrease in the pH,while the washing of UW increased the pH.The washing of the three washing solutions reduced the content of NO₃^--N.EDTA-2Na significantly changed the pH,total nitrogen,and dehydrogenase of the pile,thus affecting the basic parameters of the pile to a greater extent.CA significantly decreased the pH and dehydrogenase activity of the pile.The results show that the EDTA-2Na washing has a significant effect on various parameters in the composting process.In addition,changing the order of washing and composting caused different changes in some parameters in the pile.（2）The changes in humus,humic acid,and fulvic acid in the process of combined washing and composting were analyzed.UW washing reduced humus and fulvic acid.The sequence of washing and composting affected the changes of humus,humic acid,and fulvic acid.However,the effects of UW and CA washing on humus,fulvic and humic acids were not significant during the composting process.EDTA-2Na contributed to the formation of humus,humic acid,and fulvic acid in the pile during the washing process and the composting process,and made the structure of humus more stable.（3）The effects of different washing solutions on the removal efficiency and bioavailability of heavy metals in sediments,as well as the effects of washing on the total amount and form of heavy metals in the composting process were investigated.Before composting,EDTA-2Na washing had the most significant removal efficiency on heavy metals,and the removal rates of Cr,Cd,and Pb by EDTA-2Na were as high as 31.82%,61.14%,and 31.08%,respectively.After composting,the removal rate of Cr,Cd,and Pb by EDTA2Na washing was the highest,the removal rate of Cu and Zn by UW washing was the highest,and the removal rate of As was the highest by CA washing.After washing,except for Cr and As,heavy metals were activated,and the activation effect of EDTA-2Na washing was the most obvious.Subsequent composting effectively passivated heavy metals and reduced the side effects caused by washing to a certain extent.（4）The important parameters affecting the bioavailability of heavy metals in the process of combined washing and composting were discussed by using redundancy analysis and structural equation modeling.The results showed that the environmental parameters affecting the bioavailability of heavy metals were mainly humic acid,organic matter,total nitrogen,pH,and dehydrogenase activity.The activation of heavy metals caused by EDTA-2Na washing could be attenuated by increasing the dehydrogenase activity and pH.The combined washing and composting can effectively remove and passivate heavy metals in this study,which provides a theoretical reference for the remediation of heavy metal-contaminated sediment,and expands new remediation methods.