Response Behavior Of Heavy Metals And Dioxins In Stabilized Fly Ash By Leaching With Landfill Leachate And Alkali Washing With NaOH

Currently,municipal solid waste incineration（MSWI）fly ash is mainly treated and disposed in the form of"chelating agent stabilization+landfill"in China.There is a phenomenon of landfill leachate intruding into the stabilized fly ash landfill due to the fact that the raw MSW and stabilized fly ash are not strictly zoned for landfilling during the actual landfill disposal process.The response behavior of heavy metal（Pb,Zn,Cu,Cd,Cr,Ni）and dioxin（PCDD/Fs）pollutants in two typical DTC chelating agents（low molecular and high molecular）stabilized fly ash samples was investigated by designing landfill leachate（early and late）leaching and alkali washing（Na OH solution）dynamic leaching experiments.The main research contents and results are as follows:（1）Response behaviors of heavy metals and dioxins in stabilized fly ash leaching by early landfill leachate.During initial leaching,heavy metals in the early leachate are enriched/adsorbed to varying degrees by the stabilized fly ash matrix.However,with the increase of the cumulative liquid solid ratio,the leaching potential of heavy metals will increase.This indicates that the actual leaching behavior of heavy metals is a dynamic process of their migration in the"liquid-solid"phase.The stabilizing effects of the two chelating agents on different heavy metals are selective,and the stabilizing effects of the polymer chelating agent stabilized fly ash and the low molecular chelating agent stabilized fly ash on Cu/Cd/Pb/Zn and Cr/Ni are better when resisting early leachate leaching,respectively.The homologues of 1,2,3,4,6,7,8-H₇CDD,O₈CDD,2,3,7,8-T₄CDF,1,2,3,6,7,8-H₆CDF,2,3,4,6,7,8-H₆CDF,1,2,3,7,8,9-H₆CDF,1,2,3,4,6,7,8-H₇CDF,1,2,3,4,7,8,9-H₇CDF and O₈CDF in the early leachate showed significant enrichment/adsorption during the leaching process.There is no significant difference in the stabilization effect of macromolecular and low molecular chelating agents on dioxins in fly ash.The total leaching toxicity equivalent is 0.738 and 0.731pg I-TEQ/L,respectively.Especially,1,2,3,7,8-P₅CDD is the homologue with the largest contribution rate to the total toxicity equivalent.（2）Response behaviors of heavy metals and dioxins in stabilized fly ash leaching by late landfill leachate.During the leaching process of late leachate,the enrichment/adsorption behavior of heavy metals in the stabilized fly ash matrix is significantly stronger than that of early leachate,which is mainly affected by the content and component distribution of dissolved organic matter（DOM）in the leachate.The higher the concentration of fulvic acid（FA）,the stronger the ability of heavy metals to be enriched/adsorbed,while the concentration of FA in late leachate is significantly higher than that in early leachate.The phenomenon that Cr/Ni/Zn in the late leachate is continuously adsorbed by the stabilized fly ash matrix during the leaching process indicates that the response（adsorption or leaching）behavior mechanism of heavy metals is controlled by the dynamic interaction between the three-phases of"fly ash-DOM-heavy metals ions".The stabilization effects of macromolecular chelating agents and low molecular chelating agents on dioxins in fly ash have significant differences,with the corresponding toxic equivalents of dioxins in the stabilized fly ash leaching solution being 3.102 and 0.785 pg I-TEQ/L,respectively.This indicates that the environmental risk of dioxin leaching from polymer chelating agent stabilized fly ash in the late leachate leaching environment cannot be ignored.（3）Response behaviors of heavy metals and dioxins in stabilized fly ash after alkaline leaching.During the Na OH alkali leaching process,the initial alkali washing is a key period for DOM elution,accompanied by the leaching of most heavy metals（except for amphoteric metals Zn and Pb,Cu,Cd,Cr,and Ni are all leached）.This indicates that the stabilized fly ash matrix adsorption intercepts a large number of DOM components during the leachate leaching process,and inhibits the leaching of most heavy metal ions in the form of"fly ash matrix-DOM-heavy metal ions".However,due to its strong elution effect on DOM components,the alkali leaching process can release heavy metals enriched/adsorbed by the stabilized fly ash matrix during leachate leaching in the form of"DOM-heavy metal ions".The higher the humification degree of leachate in the landfill leachate leaching experiment,the greater the leaching toxicity equivalent of dioxins in the alkali washing experiment.The research results of this thesis can provide scientific basis and theoretical reference for understanding/controlling the environmental risks of leaching heavy metal/dioxin pollutants in landfills with a historical phenomenon of co-landfilling or mixed landfilling with"MSW and stabilized fly ash"in China,as well as for the optimization and development of fly ash chelating agent stabilization technology and stabilized fly ash landfill technology.