| The problem of leachate leakage is widely existed in present in-service sanitary landfills of our country.Once the leachate enters into surrounding soils,it will probably result in soil and groundwater pollution after the processes of filtration,adsorption,precipitation,etc.,which may seriously threaten the eco-environment security.Due to its high efficiency,economy,and environment-friendship,in-situ soil flushing technology has been widely used in field remediation of contaminated sites.The thesis is financially supported by the National Key Research and Development Program of China(No.2018YFC1802300)and the National Natural Science Foundations of China(Nos.41877240,41672280).First,the effects of different leaching reagents in treatment of mixed-contaminated soil by landfill leachate were studied through batch tests and one-dimensional column tests.Then,the candidate leaching reagents were screened based on the analytic hierarchy process(AHP)model.Finally,the effect of the leaching reagents to repair the soil column contaminated by landfill leachate was studied through numerical simulation method.Main conclusions of the research work are summarized as follows:The adsorption properties of those pollution factors including COD,ammonia nitrogen,total nitrogen,total chromium,and hexavalent chromium from landfill leachate onto the typical silty clayey soil sampled from a landfill site were studied through batch test.Then Henry model,Langmuir model and Freundlich model were adopted to fit the results of adsorption test so as to get the characteristic parameters of different adsorption model for each pollution factor.The research work would provide experimental basis for selection of adsorption model and determination of each characteristic parameter in the subsequent numerical simulation.Batch test method was used to study the effect of different leaching reagents including SDS,SDBS,Tween 80,rhamnolipids,saponin,EDTA,EDDS,and citric acid on the remediation of soil contaminated by landfill leachate.It was found that saponin had relative good removal effect on almost all the contaminants,and the removal ratios of hexavalent chromium,total chromium,and ammonia nitrogen were all above 90%.In addition,the removal ratio of total nitrogen was the highest among all the studied reagents.Considering the nature of leaching reagent,rhamnolipid,saponin,and Tween 80 were initially selected as the applicable efficient leaching reagent for remediation of the mixed-contaminated soil in the landfill site.After that,complex reagents are manually prepared by the initially selected reagents with mixture ratios of 1 CMC: 3 CMC,1 CMC:1 CMC,3 CMC:1 CMC.Then,the contaminated soil was washed with the complex reagents through batch tests.It was found that all the complex reagents had little improvement for leaching effect on the studied soil.Therefore,the complex reagents were not further studied in the subsequent research work.The influence of p H,temperature,solid-liquid ratio on the leaching effect of rhamnolipid,Tween 80 and saponin was studied through batch test.The p H had a complicated influence on the leaching effect of the three leaching reagents,and its influence on different leaching reagents and different pollution factors were different.While the temperature had similar effect on the removal ratio of the three kinds of leaching reagents for each pollution factor,and it increased with the increase of temperature.When the temperature was 30℃,the removal ratio of total chromium,hexavalent chromium,total nitrogen,and ammonia nitrogen could be increased by53%,46%,24%,42% at most compared with the temperature of 10℃.In addition,the solidliquid ratio had similar effect on the remediation effect of the three leaching reagents,and the removal ratio increased with the decrease of the solid-liquid ratio.Compared with the solidliquid ratio of 1:5,when the solid-liquid ratio was 1:20,the removal ratio of total chromium,hexavalent chromium,total nitrogen,and ammonia nitrogen by the eluent could be increased by 44%,58%,17%,and 48% respectively.The ability of Tween 80,rhamnolipid and saponin selected in the batch test to remove the pollution factors of landfill leachate-contaminated soil was studied through one-dimensional soil column test.The removal effect could be ranked as saponin,Tween 80 and rhamnolipid,and the removal ratio in column test was quite close to that of batch test.At the same time,the effect of same dose of saponin but under different injection modes on the removal of various pollution factors was also studied.Test results indicated that the leaching effect under various injection modes are quite different.The treatment effect of different injection modes was ranked as constant rate continuous injection,stepwise injection,cyclic pulse injection,and single pulse injection.According to the analytic hierarchy process,the screening of leaching reagents is hierarchized,and the organic compound removal effect,inorganic salt removal effect,heavy metal removal effect,secondary pollution,operation cost and other criteria are comprehensively considered.Then an AHP model is established to quantitatively screen suitable leaching reagents.Through mathematical calculation,the priority of candidate reagents is ranked as saponin,rhamnolipid,Tween 80,EDDS,EDTA,and citric acid.Combining the results of batch tests,a one-dimensional column leaching model of saturated contaminated soil was established through the Darcy’s Law module,the porous media transfer module and the chemical reaction module in COMSOL Multiphysics program to simulate the effect of saponin on total chromium,hexavalent chromium,transformation and removal of total nitrogen and ammonia nitrogen.It is found that the concentration of contaminant in the upper part of the soil column decreases first in the beginning,while the it basically remains stable in the lower part.As the leaching reagent continues to pass through the soil column,the concentration of contaminant in the lower part of starts to decrease,and the contaminant removal rate gradually gets lower and finally becomes stabilized.The numerical simulation about the leaching process of the contaminated soil column by saponin is consistent with the former laboratory column tests,which verified the reasonability of the calculation model.Therefore,it provides the possibility of application into engineering practice. |
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