Study On The Deterioration And Mechanism Of Engineering Characteristics In Solidified Composite Heavy Metal Contaminated Soil Under The Long-term Freeze-thaw Cycles

Due to the advantages of low cost,wide applicability and strong operability,solidification/stabilization technology has gradually become the preferred technology for remediation of heavy metal contaminated soil.However,due to the limitation of solidified/stabilizing technology,the harmful heavy metals in the solidified/stabilized contaminated soil have not been effectively removed,so the long-term stability of the solidified/stabilized contaminated soil has attracted the attention of scholars at home and abroad.There is a large area of seasonal permafrost in China,and the freeze-thaw cycle,as a kind of external force commonly exists in the seasonal permafrost area,will undoubtedly affect the long-term stability of the solidified/stabilized heavy metal contaminated soil.And there are few researches considering the long-term freeze-thaw cycle effect and the complexity of soil heavy metal pollution,so the further study of curing composite soil heavy metal pollution in the long run the change law of engineering characteristics under the action of freeze-thaw cycle and its mechanism,to ensure the repaired in seasonal frozen soil heavy metal pollution of solidification/stabilization long-term effectiveness has very important practical significance.The article is supported by the National Natural Science Foundation of China"Research on the Deterioration Effect Mechanism of the Solidification and Modification of High Concentration Heavy Metal Contaminated Soil under Long-term Freezing and Thawing（Grant No.41772306）",and the main research content is the effect of freeze-thaw cycles on the engineering characteristics of solidified lead-zinc-cadmium composite heavy metal contaminated soil.Through triaxial compression test,toxicity leaching test,tracer solute soil column leaching test,seven-step extraction of heavy metal occurrence analysis,X-ray diffraction（XRD）analysis technology,scanning electron microscope（SEM）analysis technology,energy dispersive X-ray spectrometer（EDS）analysis technology,and Fourier Transform Infrared Spectroscopy（FTIR）analysis technology,the engineering characteristics degradation law and microscopic mechanism of solidified lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles were explored.The main research results obtained are as follows:（1）Triaxial compression test results of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles show that the stress-strain relationship curve of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles can be divided into four stages:linear elastic stage,plastic yield stage,stress softening stage,and residual stage;long-term freeze-thaw cycle action makes the brittleness characteristics of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil more obvious when it is destroyed;under the action of long-term freeze-thaw cycles,the elastic modulus,ultimate compressive strength and shear strength of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil have a certain degree of deterioration,but the deterioration of shear strength is mainly reflected in the cohesive force.（2）Under the action of long-term freeze-thaw cycles,the toxicity characteristic leaching test of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil showed that the leaching concentration of Pb²⁺,Zn²⁺,and Cd²⁺is significantly positively correlated with the number of freeze-thaw cycles;the electrical conductivity of the leaching solution is generally positively correlated with the number of freeze-thaw cycles;long-term freeze-thaw cycles will reduce the p H of the leaching solution;the leaching concentration of Pb²⁺,Zn²⁺,and Cd²⁺in the leaching solution is roughly linearly correlated with the conductivity of the leaching solution,and has a non-linear negative correlation with the p H value of the leaching solution.（3）Under long-term freeze-thaw cycles,the tracer solute soil column leaching test of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil showed that after long-term freeze-thaw cycles,the tracer ion Br^-under stable low-velocity seepage conditions appeared obvious adsorption non-equilibrium during the migration of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil column;the longitudinal migration coefficient of the tracer ion Br^-in the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil is positively correlated with the number of freeze-thaw cycles.（4）Under the action of long-term freeze-thaw cycles,the seven-step extraction of heavy metals in solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil showed that the percentage of carbonate-bound Pb,Zn,and Cd in solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil is negatively correlated with the number of freeze-thaw cycles;the percentage of humic acid-bound Pb and Zn are positively correlated with the number of freeze-thaw cycles;the percentage of ion-exchange Zn,Cd is positively correlated with the number of freeze-thaw cycles;long-term freeze-thaw cycles make the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil tend to be active..（5）Qualitative analysis of the mineral composition of solidified/stabilized lead-zinc-cadmium composite contaminated soil under long-term freeze-thaw cycles based on X-ray spectroscopy（XRD）diagrams showed that the solidification effect of cement/lime/fly ash on lead,zinc,and cadmium is mainly realized by the precipitation of Pb（OH）₂、Zn（OH）₂、Cd（OH）₂;the effect of long-term freeze-thaw cycles on the mineral composition of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil is not significant.（6）Qualitative analysis of the microstructure of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles based on scanning electron microscope（SEM）images showed that the hydration reaction in the solidified contaminated soil is still going on in the early stage of the freeze-thaw cycle;there are a large amount of CAH,CSH,ettringite and Ca（OH）₂ in it;at the later stage of the freeze-thaw cycle,a large amount of ettringite appears in the solidified/stabilized lead-zinc-cadmium composite contaminated soil and the expansion of ettringite with water causes a loss of strength,which is manifested in the macroscopically as a decrease in ultimate compressive strength and shear strength indexes.（7）Quantitative analysis of the particle structure and pore structure of the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under the action of long-term freeze-thaw cycles based on SEM images by MATLAB and Image Pro Plus（IPP）software showed that the freeze-thaw cycle effect increases the percentage of small pores（<1um）and small particles（<1um）in the solidified contaminated soil,and the soil structure is destroyed,resulting in a decrease in the ultimate compressive strength and shear strength of the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil.（8）Combining energy dispersive X-ray spectrometer（EDS）with SEM,the EDS analysis of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles based on SEM images showed that there are a large amount of CAH,CSH,ettringite and Ca（OH）₂ in the solidified contaminated soil,which is compared with XRD and SEM analysis results;with the increase in the number of freeze-thaw cycles,the contents of CAH and CSH are significantly reduced,which in turn causes the deterioration of the mechanical properties of the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil;moreover,as the content of hydration products decreases,the heavy metals physically wrapped by the hydration products will precipitate again,which leads to an increase in the leaching concentration of heavy metals and an increase in the content of ionic heavy metals,thereby threatening the safety of the soil environment.（9）The analysis of functional groups of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles based on Fourier Transform Infrared Spectroscopy（FTIR）chart showed that although long-term freeze-thaw cycles cannot significantly change the types of main functional groups of solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil,it can significantly reduce the number of its main functional groups,which in turn leads to deterioration of the mechanical properties and environmental effects of the solidified/stabilized lead-zinc-cadmium composite heavy metal contaminated soil..