Study On The Influence Of Acid And Alkali Environment And Landfill Method About Leaching Behavior Of Heavy Metal Of Fly Ash Pretreatment Products

With the improvement of China's economic and social level,the amount of domestic waste production increases year by year.Incineration has become the main treatment method of domestic waste in China because of its advantages such as harmless thorough,reduced quantity and good reduction effect.Domestic waste incineration will produce hazardous waste of fly ash which contained a large number of dioxin-rich and heavy metal.It's disposal mode was sent into the domestic waste sanitary landfill site separate zoning landfill when meeting the"domestic waste landfill pollutant control Standards"?GB16889-2008?requirements after cured or stable pretreatment.However,fly ash pretreatment products contacted with rainwater or leachate due to the actual operation process is not strictly partitioned landfill or landfill during the operation is not standardized,accelerating the leaching of pollutants.In this paper,the effects of leaching mode,acid-base environment,pretreatment method and landfill operation mode on the leaching of Pb,Zn,Cr and Cd in fly ash pretreatment products are studied by establishing a dynamic leaching device of fly ash pretreatment products to simulate the long-term leaching behavior of fly ash pretreatment in domestic landfill.The original grey toxicity leaching test showed that the concentration of Pb exceeded the admission limit value of GB16889-2008?0.25mg/L?,so it was necessary to go through the pretreatment before entering the domestic landfill.The pretreatment methods used in this study were:25%cement curing,2%Na₂S stabilized and 15%cement curing.The dynamic and static leaching test of fly ash with stabilization+curing treatment was established respectively.The results show that the pH value rises rapidly to about 12.0 and then stabilizes during the test period in both of the dynamic and static leaching test due to the existence of a large number of Ca?OH?₂ and other alkaline substances in fly ash.In comparison,static leaching was more favorable to the leaching of heavy metals,and it's leaching concentration increases rapidly with time and then rises slowly until it stabilizes,while dynamic leaching generally shows a downward trend,although there may be an upward process in the early stage.So we should avoid the blockage of leachate diversion system in landfill and the high water level of leachate,and reduce the leaching of a large amount of heavy metals caused by the static leaching environment of fly ash pretreatment products in the actual landfill operation.The leaching solution with pH value of 4.0,8.5 and 5.5-8.5 was used to simulate acid rain,aging and the changing acid and alkali environment in the landfill period leachate leached fly ash with curing treatment.The results show that the weak acidic environment with pH value of 4.0 was more favorable to the leaching of heavy metals in fly ash,the diffusion and reaction rate of leaching process were faster,and the leaching concentration of heavy metals were larger compared with pH=8.5.Adding leaching solution with pH value changes continuously to simulated the acid-base environment of leachate during landfill.,the result showed the concentration of heavy metals in the leaching liquid showed an upward trend in the early stage,and basically reached a dynamic equilibrium when it reached about 25d,and then tended to be stable due to the large amount of extractable substances in the early-stage solidified body and the weak acidity of the leaching liquid.In actual landfill operation,the contact between weak acid leachate and fly ash should be controlled as far as possible to avoid heavy metal leaching.Dynamic leaching test were established in which the pH value of leaching solution was 5.5,the pre-processing of fly ash were 25%cement curing and 2%Na₂S stabilized+15%cement curing respectively.The results showed that the leaching rate of heavy metals was slower and the leaching concentration were smaller about fly ash with stabilized+curing treatment due to the combination of Na₂S with heavy metals in the fly ash in the formor stage.However,in the later stage of the test,the concentration of heavy metal leaching were smaller due to the formation of C-S-H hydration products in cement and high content cement curing may hinder solute diffusion.Because the leaching of heavy metals was mainly embodied in the early stage of the experiment,the stabilization+curing process was more favorable to reduce the leaching of heavy metals than the simple curing.The dynamic leaching test was established in which the fly ash after stabilization+curing treatment placed in a neat yard and scattered stacking into test device with leaching solution pH value was 5.5.The results show that the neat placement method was not conducive to solute dissolution because it reduces the contact between the curing block and the leaching solution.Therefore,we should try to use neatly placed in the form of landfill fly ash curing body in the actual landfill process.