Leaching Characteristics Of Heavy Metals By Landfill Leachte From Stabilized MSWI Fly Ash

Fly ash from municipal solid waste incinerators has been known as hazardous waste,which is mostly because of the high content of heavy metal and other pollutants.Fly ash must be stabilized before landfill disposal.Co-landfilled stabilized fly ash and municipal solid waste（MSW）may increase the leaching risk of heavy metals.In this study,the leaching behaviors of heavy metals（Cd,Cu,Zn,Pb,Cr,Ni）from fly ash and stabilized fly ash with chelating agent,phosphoric acid（H₃PO₄）or cement were investigated under various liquid to solid（L/S）ratios（10:1,20:1,30:1,50:1,100:1,150:1,200:1）based on batch leaching experiments using young and old landfill leachate,respectively.（1）The change of concentration of heavy metals before and after leaching showed that,with the increase of liquid to solid ratio,the concentrations of Cd,Cu,Zn decreased while Pb increased after leaching,the leaching concentration of Cd and Cu reached maximum at the liquid-solid ratio of 20:1,leaching concentration of Pb,Zn reached maximum at the liquid-solid ratio of 150:1,and the leaching concentration Cr,Ni is smaller,but their leaching amount increased with the liquid-solid ratio increasing.（2）The speciation analysis of heavy metals showed that with the increase of liquid solid ratio,the residual form of heavy metals in leachate increased,and the Fe-Mn oxides form of Cd,Pb,Zn,Cr and Ni decreased,and the organic matters form of Cu decreased.Cd,Pb,Zn,Cr and Ni which in leachate mainly comes from Fe-Mn oxides form,the leaching of Cu mainly comes from the organic matter form;the concentration of heavy metals from all samples which leaching in young age leachate was higher than that in old leachate,and the concentration of heavy metals from fly ash which stabilized by chelating agent and phosphoric acid were both lower than that by cement.（3）The analysis of dissolved organic matter（DOM）in leachate showed that with the increase of L/S,the concentration of fulvic acid（FA）increased at first and then decreased,hydrophilic organic concentration（HyI）decreased at first and then increased,concentration of humic acid（HA）increased;the leaching concentration of Cd and Cu was closely related to FA,the concentration of FA increased and the degree of UV spectrum redshift was the highest at the liquid-solid ratio of 20:1,the combination of FA and Cd,Cu complexes lead to the concentration of Cd and Cu increased;though the concentration of HA was low,HA plays an important role on the leaching of Pb,Zn,the concentration of HA increased with the increasing of L/S,and the redshift of UV spectrum occured,the combination of HA and Pb,Zn complexes lead to the increase of Pb and Zn.Obviously,a large number of heavy metals may leach out from the stabilized fly ash.The dissolution of heavy metals is mainly affected by DOM in leachate,in which FA may promote the dissolution of heavy metals from fly ash.At the same time,although the concentration of HA is low,it plays an important role in the leaching of Pb and Zn.