Experimental Study On Solidification And Stabilization Of Heavy Metal Dredged Sludge With Inclusion Of Fiber Reinforcement

In recent years,the international trade and the economic level of coastal areas in china have been continuously improved.In addition,a large number of new ports and coastal projects have been built,and the sediments in the navigation channels have been regularly removed to produce a large amount of dredged sludge.Because of the dense river network and the wide lake water system in China,regular dredging projects of lakes and rivers are needed to produce a large amount of dredged sludge.At present,traditional disposal methods for dredged sludge are landfill and throwing sea,but traditional disposal methods may cause lead to environmental geotechnical engineering problems such as landslide,secondary pollution and damage of ecological system.According to the current situation of dredged sludge disposal in China,from the perspective of economic feasibility,using appropriate technology to transform the sludge into a recyclable geotechnical engineering material is one of the ways to deal with dredged sludge and resource resources in line with China’s national conditions.Solidification/stabilization technology is a promising treatment method for dredged sludge,which canot only achieve rapid dewatering,but also significantly improves the mechanical properties of the sludge and fixes heavy metals in the sludge.Based on heavy metal dredged sludge as the research object,this paper adopts the fiber reinforcement and solidification/stabilization of optimization disposal technology.It was used to carry out a large number of laboratory tests to study the solidification/stabilization effect of heavy metal dredged sludge,and analyzed the fiber,solidifying agent(cement,fly ash,biochar)and heavy metal on the strength characteristics,leaching characteristics,water retention characteristics and microscopic characteristics of heavy metal dredged sludge,and the mechanism of fiber reinforcement and solidification/stabilization were discussed.The results are as follows:(1)In terms of strength characteristics,the results showed that for the strength characteristics of cement and fly ash solidified sludge,cement and fly ash significantly increased the strength of the solidified sludge.Heavy metal inhibited the solidification of cement and fly ash to reduce the strength of solidified sludge.The initial stiffness of the solidified sludge stress-strain curve decreased with the increase of heavy metal content,and its failure mode transitioned from brittle failure to plastic failure.The initial stiffness of the stress-strain curve of the solidified 2%heavy metal sludge increased with the increase of cement and fly ash content,showing plastic failure characteristics.For the strength characteristics of fiber reinforced biochar solidified sludge,the 2%heavy metal content caused the stress-strain curve to exhibit plastic failure characteristics with large deformation and low strength.The higher the biochar and fiber content leaded to the greater the initial stiffness of stress-strain curve.The strength of the reinforced solidified sludge increased with the increase of the content of biochar and fiber.The biochar promoted the solidification of cement and the reinforcement of fiber.The addition of fiber effectively beared part of the load and hindered the development of large cracks,which was an important factor to improve the toughness of soil.(2)In terms of heavy metal leaching characteristics,the results showed that for the leaching characteristics of cement and fly ash solidified sludge,the concentration of Zn2+and Pb2+ leached by solidified sludge decreased with the increase of cement and fly ash content,and the effect of cement content was more significant.In addition,the stabilization of cement and fly ash on Zn2+ was stronger than that of Pb2+.Heavy metals inhibited the stabilization of cement and fly ash,and significantly increased the Zn2+and Pb2+ concentration of the solidified sludge leachate.The pH of the solidified sludge leachate increased with the increase of cement and fly ash content,and decreased with the increase of heavy metal content.Therefore,the higher the pH of the solidified sludge leachate resulted in the lower the concentration of Zn2+ and Pb2+corresponding to the leachate.For the leaching characteristics of fiber reinforced biochar solidified sludge,biochar promoted the conversion of reinforced solidified sludge leachate from neutral to alkaline environment,and decreased the concentration of Zn2+and Pb2+ in the reinforced solidified sludge leachate.In addition,the low biochar content had a good stabilizing effect on Zn2+,while the high biochar content had a good stabilizing effect on Pb2+.(3)In terms of water retention characteristics,the results showed that for the water retention characteristics of cement and fly ash solidified sludge,the cement improved the water retention capacity of the solidified sludge and made the soil water characteristic curve slower.Heavy metal reduced the water retention capacity of the solidified sludge,making the soil water characteristic curve steeper.In the low suction(less than 10MPa)stage,the effect of cement on the soil water characteristic curve was more obvious,and the effect of heavy metal on the soil water characteristic curve had nothing to do with the suction range.For the water retention characteristics of the fiber reinforced biochar solidified sludge,the water retention capacity of the reinforced solidified heavy metal sludge increased with the increase of the biochar content.In the low suction(less than 10MPa)stage,the effect of biochar on the soil water characteristic curve was more significant,which made the soil water characteristic curve slower.(4)In terms of microstructure characteristics and modification mechanism,the fiber reinforcement mechanism was derived from the interfacial mechanical interaction of the fiber-solidified sludge,including interfacial adhesion and interfacial friction,and produced a "bridge effect" when the sample was cracked.The solidification mechanism of cement was a series of hydration reactions,carbonation reaction,pozzolanic reaction and ion exchange between cement and sludge.The stabilizing mechanism of cement was the chemical combination,adsorption,precipitation and encapsulation between cement and heavy metal.The solidification/stabilization mechanism of fly ash was the water reduction and filling effect of fly ash in the early curing age,the ion exchange and the pozzolanic reaction between the fly ash and the sludge/hydration products in the late curing age.As the activator of alkaline environment,fly ash promoted the hydration and the pozzolanic reaction of cement.The solidification mechanism of biochar is interfacial cementation.surface deposition and pore filling between biochar and sludge.The stabilization mechanism of biochar is physical adsorption,ion exchange,electrostatic interaction,complexation and precipitation between biochar and heavy metal.