Research On Chemical Stabilization Of Heavy Metals From Municipal Solid Waste Incineration

In the last few years,Municipal Solid Waste Incineration was increasingly becoming a main method to treat municipal solid waste because of its advantages including reduction,harmlessness and recovery,but municipal solid waste incineration can generate secondary pollution.For example,Municipal Solid Waste Incineration Fly Ash accumulates heavy metals and dioxin so fly ash is defined as hazardous waste and need to subject harmless treatment.At present,the technologies of fly ash stabilization include solidification/stabilization,extraction and heat treatment.Cement solidification and chemical stabilization are widely used in China to treat fly ash.Fly ashes sampled from Foshan municipal solid waste inc ineration plant and Zhejiang municipal solid waste incineration plant were taken as the research object.Based on its physical and chemical characteristic,chemical stabilization of fly ash was studied.The main researches and conclusions are as follows.As for physical and chemical characteristic of fly ash,the main researches included content of heavy metals,leaching concentration of heavy metals and speciation distribution of heavy meatals in fly ash.In a conclusion,the main metals in Foshan fly ash were Ca,Al and Fe and its main heavy metals were Zn,Pb and Cu.The main metals in Zhejiang fly ash were Ca and Al,and its main heavy metals were Zn and Pb.The contents of most heavy metals were a lot higher than GB 15618-1995-Environmental quality standard for soils.The leaching concentrations of Pb and Cd were much higher than the limit of GB 16889-2008-standard for pollution control on land site of municipal solid waste.The leaching concentrations of Pb and Cd in Foshan fly ash were 1.24 and 1.93mg/L and the leaching concentrations of Pb and Cd in Zhejiang fly ash were 1.90 and 2.67mg/L.The main speciation of Cd in Foshan fly ash and Zhejiang fly ash was reducible fractions about 77% and 84%.The distribution of Zn in Foshan fly ash and Zhejiang fly ash was mainly in reducible fraction and residual fraction.The main speciation of Pb in Foshan fly ash was oxidisable fraction and residual fraction,but in Zhejiang fly ash residual fraction was the main speciation for Pb.Little acid soluble heavy metals existed in Foshan fly ash or Zhejiang fly ash.Phosphoric acid,sodium phosphate and diethyldithiocarbamate were taken to stabilize fly ash in order to research stabilizing effect.In a conclusion,the stabilizing efficiency of 5% of phosphoric acid on Pb in Foshan fly ash was 81% and the stabilizing efficiency to Pb in Zhejiang fly ash was 88%.Also,their leaching concentractions of Pb in Foshan fly ash or Zhejiang fly ash were below the limit of standard for pollution control on land site of municipal solid waste.But the concentractions of C u,Ni,Hg and As increased.Sodim phosphate could effectively stabilize Zn.After fly ash was treated,the leaching concentraction of Zn in Zhejiang fly ash decreased to 41mg/L,and its stabilizing efficienc y was 48%,but the leaching concentrations of C u,Pb,N i,Cd,Hg and As increased.The efficiency of stabilization of diethyldithiocarbamate to heavy metals was comprehensively better than phosphoric acid and sodium phosphate.After treated by 1% of diethyidithiocarbamate,leaching concentractions of heavy metals in Foshan fly ash were below the limit of standard for pollution control on land site of municipal solid waste.Also,the stabilizing efficiencies on Pb and Cd were higher than 90%.However,the effect of stabilization of diethyldithiocarbamate on heavy metals in Zhejiang fly ash was worse than the effect on heavy metals in Foshan fly ash.After treated by 3.5% of diethyldithiocarbamate,the stabilizing efficiencies on Pb and Cd was 87% and 78%,and the leaching concentraction of Cd was still higher than the limit of standard for pollution control on land site of municipal solid waste.Diethyldithiocarbamate couldn't effectively stabilize Zn in Foshan fly ash or Zhejiang fly ash.With the increase of dose of diethyldithiocarbamate,leaching concentrations of Zn in Foshan fly ash totally increased,and the highest stabilizing efficiency to Zn in Zhejiang fly ash could only reach 25%.Diethyldithiocarbamate and phosphoric acid,diethyldithiocarbamate and sodium phosphate,diethyldithiocarbamate and sodium sulfide were taken as complex agents to stabilize heavy metals in fly ash.Also,acetate acid and sodium acetate were taken as additives for effect on stabilization of diethyldithiocarbamate on heavy metals in Foshan fly ash.In a conclusion,the complex agent of 1.5% of diethyldithiocarbamate and 2.5% of phosphoric acid could stabilize Pb,Cd,Cr and other heavy metals in Foshan fly ash.Also,the leaching concentractions of heavy metals in Foshan fly ash were below the limit of standard for poluution control on land site of municipal solid waste.However,the complex agent of diethyldithiocarbamate and phosphoric acid stabilized heavy metals in Zhejiang fly ash weakly.The complex agent of 0.5% of diethyldithiocarbamate and 5% of sodium phosphate had effective stabilization on Pb and Zn in Zhejiang fly ash,whose efficiencies were 84% and 46%,but it couldn't stabilize N i,Pb,As.The complex agent of 1% of diethyldithiocarbamate and 3% of sodium sulfide had best stabilization on heavy metals in Zhejiang fly ash and its stabilizing efficiency was much better than 3.5% of diethyldithiocarbamate.Espencially it could effectively stabilize Zn and Cd on which stabilizing efficiencies reached 84% and 95%.Except for Pb,the leaching concentractions of heavy metals in Zhejiang fly ash were below the limit of standard for pollution control on land site of municipal solid waste.Acetate acid or sodium acetate couldn't improve the stabilizing effect of diethyldithiocarbamate on heavy metals in Foshan fly ash.After adding acetate acid,stabilizing efficienc ies of diethyldithiocarbamate on Cd and Pb were changed unstably,and stabilizing efficienc ies on Zn and Cr had been worse.After adding sodium acetate,stabilizing efficiencies of diethyldithiocarbamate to Zn,Cr and Cd were changed unstably,and stabilizing efficienc y of diethyldithiocarbamate to Pb had been worse.