Effect Of Seepage Path Of Invaded Aqueous Phase On Leaching Characteristics Of Heavy Metals In Landfill Stabilized Fly Ash

"Solidification/stabilization + landfill" is the mainstream technical route of MSW incineration fly ash treatment in China.In view of the possible aging and damage of the overburden,imperfect waterproof,rainproof and seepage prevention measures in the stabilized fly ash landfill area,two intrusion external environments of "simulated acid rain(using sulfuric acid and nitric acid solution)" and "simulated landfill leachate(using acetic acid solution)" were constructed.The effects of them on the leaching behavior of heavy metals(Pb,Cd,Zn,Cu,Cr,Ni)in landfill chelating agent stabilized fly ash under six typical seepage paths were investigated by means of phased influent.The potential environmental risk evolution laws of heavy metals in liquid-phase and solid-phase were evaluated.The main research contents and results are as follows:(1)Leaching behavior of heavy metals in stabilized fly ash under different seepage paths under simulated acid rain intrusionInitial acid rain leaching was the main stage of heavy metal leaching,and most of the heavy metals leached in the middle and later stages were concentrated in the early period of each stage.The over standard rate of Pb was the highest,especially in the initial leaching stage.From the seepage path,the leaching level of heavy metals generally showed a trend of "top in and bottom out > top left in and bottom right out > bottom in and top out > top left in and top right out".Dynamic leaching after initial soaking showed a certain impact on the leaching of heavy metals,and the overall trend was "dynamic scouring > dynamic soaking > static soaking".The process of acid rain corrosion will gradually destroy the surface matrix of fly ash,and then expose more strip-shaped or rodshaped crystal Ca-based mineral structure,affectting the chemical form distribution of heavy metals.This corrosion behavior affected the leaching behavior of heavy metals.(2)Leaching behavior of heavy metals in stabilized fly ash under different seepage paths under simulated landfill leachate intrusionThe leaching concentration of heavy metals showed an increasing trend in varying degrees in the middle and later stages.The environmental risk of Cd,Cr,Ni,and Zn leaching should be focused on the later stage of acetic acid leaching,while Pb should be focused on both the early and later leaching stages.From the seepage path,the overall change trend of heavy metal leaching amount was consistent with that of simulated acid rain invasion.The leaching level of heavy metals leached after the fly ash column was overflowed was significantly lower than that of heavy metals leached directly.The occurrence forms of some heavy metals in the fly ash matrix leached by continuous acetic acid solution tended to change from stable state to unstable state.Combined with the changes of surface morphology and mineral phase of fly ash matrix,the surface matrix of fly ash will be continuously eroded and peeled off in the leaching process with acetic acid solution,thus exposing the internal mineral components.It was the destruction of the stable structure formed by chelating agent and heavy metals that led to the increase of heavy metal leaching in the later stage.(3)Potential environmental risk assessment of heavy metals in stabilized fly ash under different seepage paths in two invasive environmentsUnder the intrusion environment with simulated acid rain,the environmental risk level(OPTI value)of heavy metals in each fly ash leachate generally showed the trend of "initial stage > middle stage > later stage" and "top in and bottom out > top left in and bottom right out > bottom in and top out > top left in and top right out".Compared with the initial stabilized fly ash without leaching,the potential environmental risk level(RAC value)of heavy metals contained in the solid-phase fly ash matrix after simulated acid rain leaching showed an increasing trend,and the higher risk value basically appeared in the middle and later stages.Under the intrusion environment with acetic acid solution,the OPTI value of heavy metals in each fly ash leachate also showed a trend of "initial stage < middle stage <later stage".Generally,the environmental risks(liquid-phase OPTI value and solid-phase RAC value)caused by simulated landfill leachate intrusion were much higher than those caused by simulated acid rain intrusion.This study can provide theoretical reference for the development of solidification or stabilization technology of fly ash,the construction of heavy metal stability evaluation system and the environmental risk control of landfill stabilized fly ash in China.