| The number of informal landfills in China is high,and the soil and groundwater around them are often compounded by leachate leakage,posing a threat to human health and the ecological environment.In-situ flushing technology has broad application prospects in the field of contaminated site remediation due to its advantages of various types of flushing agents,effective removal of adsorbed phase contaminants,controllable cost,and the possibility of combining with other technologies.Among them,the biosurfactant saponin has the ability to elute both organic matter and heavy metals,and is an environmentally friendly flushing agent with good application prospects in landfills.In this paper,the remediation effect of distilled water and different concentrations of saponin solution on landfill composite contaminated soil was investigated by soil column flushing test based on the National Key R&D Program of China(2018YFC1802300)and the National Natural Science Foundation of China(41877240;41672280).Subsequently,the biological toxicity of the landfill composite contaminated soil before and after flushing treatment was compared by different testing methods.Finally,numerical simulation was used to simulate the process of saponin flushing the landfill composite contaminated soil column.The main findings of the paper are as follows.(1)A comparative analysis of the effect of distilled water and different concentrations of saponin on landfill composite contamination of sandy fine-grained soils was performed by soil column flushing tests.The results showed that distilled water and different concentrations of saponin solutions had elution effects on Zn,ammonia nitrogen and COD in the landfill composite contaminated sandy fine-grained soil,and the elution rate of Zn was most significantly improved by saponin compared with distilled water.In the p H at 8.27-9.4conditions,1-10CMC saponin solution for Zn elution rate is generally higher than that of distilled water,the removal rate of ammonia nitrogen are lower than that of distilled water,COD removal rate and distilled water is basically the same;2.5CMC saponin solution for landfill composite contaminated sandy fine-grained soil in the best removal effect of pollutants,the removal rate of Zn,ammonia nitrogen and COD were 20%,90.1%,95.8%.90.1%and 95.8%,respectively.(2)A comparative analysis of the elution effect of distilled water and saponin on landfill composite contaminated chalky sand with high organic matter content was performed by soil column flushing tests.The results showed that both of them mainly showed the elution of heavy metals in the weak acid extracted and reducible states at p H 7.48-8.03;compared with distilled water,the elution rate of pollutants was significantly enhanced by 2.5CMC saponin solution,and the removal rates of ammonia nitrogen,COD,Cu,Zn,and Cd by saponin were 90.40%,47.00%,30.80%,23.50%,18.65%,compared with distilled water,respectively,25.90%,5.15%,11.95%,9.00%,9.75%;both for ammonia nitrogen,COD removal mainly concentrated in the first 1 PV of the leachate,the removal of copper mainly concentrated in the first 2 PV of the leachate,the removal of zinc,cadmium mainly concentrated in the first 3 PV of the leachate.(3)The variation patterns between COD,ammonia nitrogen and conductivity during the flushing process were investigated by data fitting analysis.The results showed that there was a significant positive correlation between the electrical conductivity of leachate and COD and ammonia nitrogen in the flushing process of composite contaminated soil in landfill,and the electrical conductivity could be used as a rapid and effective indicator to evaluate COD and ammonia nitrogen.In the flushing process of composite contaminated sandy fine-grained soil,the R~2 of the linear fit between the electrical conductivity of the dewatered liquid and COD was0.9837;in the flushing process of composite contaminated chalky sand,the R~2 of the linear fit between the electrical conductivity of the dewatered liquid and ammonia nitrogen and COD were 0.9741 and 0.9929,respectively.(4)Biotoxicity changes of landfill complex contaminated soil before and after flushing were evaluated by plant and animal toxicity testing tests.The results showed that the biotoxicity of the landfill complex contaminated soil was significantly reduced after flushing with distilled water and saponin solution.Saponin and distilled water flushed soil in earthworms 7d lethality,48h avoidance rate,7d weight loss rate,pea germination rate,grass seed growth are significantly better than the soil before flushing;compared with distilled water,saponin flushed soil in earthworms,peas and grass seed growth and development are better;compared with chemical analysis methods,biological toxicity test results more accurately reflect the real risk of soil contamination.(5)Finite element simulation software was used to simulate and analyze the process of saponin flushing the landfill composite contaminated soil column,and the simulation results showed that the simulation results were basically consistent with the measured results of indoor tests,which laid a theoretical foundation for future flushing optimization.The simulation results showed that the concentration of contaminants in the lower part of the liquid phase of the soil column was higher than that in the upper part during the flushing process;compared with the initial flushing head and the initial ammonia nitrogen concentration,the simulation results of the elution rate of ammonia nitrogen under 2 times the initial flushing head were reduced by18.9%;the simulation results of the elution rate of ammonia nitrogen under 2 times the initial ammonia nitrogen concentration were increased by 3%. |
