| While municipal solid waste incineration(MSWI)technology has become increasingly popular in recent years,there is a significant amount of incineration fly ash generated in China,ranking the first in the world.Therefore,it is urgent to employ proper methods to treat this kind of hazardous waste.The characteristics of incineration fly ash,such as high salt content and high potential toxic metal leachability,affect its further treatment and disposal.Chelation stabilization method,which has the advantages of simple operation and low cost,is one of the main technologies currently adopted to treat incineration fly ash in China.However,this technique is at the early stage of development in China.Thus,it is crucial to accurately evaluate the treatment efficiency and environmental durability of this technology,as well as smartly synthesize novel chelating agents to improve the treatment performance.This dissertation firstly analyzed the long-term dynamic changes in the heavy metal contents and leaching concentrations of incineration fly ash that was collected from a MSWI plant in south China and identified the major heavy metals that should be concerned in incineration fly ash.Secondly,the applicability of traditional sequential extraction procedures for the environmental risk assessment of heavy metals in incineration fly ash was assessed,given that the sequential extraction schemes are specifically developed for soil or sediment,and the properties of incineration fly ash differ from soil or sediment.Thirdly,a quantitative extraction of metal chelates in stabilized incineration fly ash was developed,and the possible environmental factors which influence the environmental durability of commercial chelating agent stabilized fly ash were discussed.Finally,based on the results above,two new types of macromolecule chelating agents were synthesized.The stabilization effect of different chelating agents on incineration fly ash was compared,and the influence of washing pretreatment on the fly ash chelation efficiency was explored.According to the rapid quantitative analysis results of heavy metal contents obtained by the high-resolution X-ray fluorescence spectrum(HDXRF),it is found that the monthly average contents of heavy metals remained consistent with coefficient of variation(CV)less than 0.14,while their daily average contents fluctuated significantly with CV over 2.54.The heavy metal leaching concentrations of incineration fly ash are affected by the final p H of the leachate.Among the heavy metals being studied,Pb(shows the highest probability(68.9%)beyond the limit specified in Standard for Pollution Control on the Landfill Site of Municipal Solid Waste(GB 16889-2008).By comparing the metal fractionation results under different acid extraction conditions,it is found that only easily soluble salts,but not the heavy metals in the hydroxides or carbonates was extracted in the traditional sequential extraction procedure with one-time acid washing.Based on the developed chelation efficiency evaluation method that uses ethyl acetate to directly extract metal chelates in stabilized fly ash,the results demonstrate that the chelating efficiency of heavy metal increases with the increasement of SDD addition dosage.Although the heavy metal leaching concentration of stabilized fly ash meets limits stipulated in GB 16889-2008 under the condition of 3% of the commercial chelating agent,the metal chelating efficiency is not high(e.g.9.7% for Pb).The 300-day environmental durability experiment result shows that the leaching concentration of heavy metals in chelated fly ash gradually increases and exceeds the leaching concentration limit in GB 16889-2008,even under dark and sealed condition.Combined with the quantitative extraction of chelates and mineral analysis,it is found that the environmental durability of chelated fly ash is affected by the decomposition and possible redistribution of metal chelates,as well as the gradual release of non-chelated metals along with the reduction of alkaline substances.Through batch tests,acid stability experiment,scanning electron microscope,Fourier infrared spectrometer,and X-ray photoelectron spectroscopy analysis,the properties of two types of novel macromolecule chelators,including polyethyleneimine chelating agents and soluble modified lignin chelating agents were investigated.The results suggest that the molecular weight change of polyethyleneimine or the change of small-molecule amines can affect the chelating performance of macromolecule chelators.The increasement of molecular weight of raw materials may not only reduce the electronegativity of sulfur atoms in-CSSfunctional group,but also enhance the steric hindrance effect.In general,the use of polyethyleneimine with a molecular weight of 600 for polyethyleneimine chelating agent and the use of diethylenetriamine for modified lignin chelating agents can improve the ability to selectively remove Pb2+ from solution compared to the commercial chelating agent.Based on the optimization of water washing pretreatment condition,and the comparison of the chelating efficiency of raw fly ash and water-washed fly ash by different chelating agents,the results illustrate that under the condition of the liquidsolid ratio of 1:1 and continuous washing four times,most of the soluble chlorine salts can be removed and above 35% of mass reduction can be achieved.For raw fly ash,two new macromolecule chelators enable the stabilized fly ash to meet the limits stipulated in GB 16889-2008 at a lower dosage(1%).For the water-washed fly ash,at least 5% of the commercial chelating agent or new polymer chelating agent is required to meet the limits stipulated in GB 16889-2008.According to the quantitative extraction of heavy metal chelates and the analysis of mineral morphology,the decrease of the chelating efficiency of heavy metals can be attributed to the enrichment of heavy metals in the washed fly ash and the formation of calcium sulfate on the fly ash surface with low solubility. |
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