Long-term Stability Of Cd And Pb In Solidified/stabilized Municipal Solid Waste Incineration Fly Ash

Waste incineration has become a prevalent way to treat municipal solid waste in China.However,the municipal solid waste incineration(MSWI)fly ash(FA)generated during dust reduction process is rich in heavy metals.Among these heavy metals,lead(Pb)and cadmium(Cd)have more tendency to exceed the landfill standard than other elements,and the stabilization of these two heavy metals is worth studying.After solidified/stabilized(S/S)FA was landfilled,many environmental factors will damage the stability of heavy metals in FA.In recent years,the long-term effectiveness of heavy metal stabilization technology has attracted the great attention.There are two main research methods:one is sampling and monitoring FA from landfill site;the other is laboratory simulation.But the first way takes a long time,and the induced environmental pollution is irrevocable.Therefore,in this study,ordinary Portland cement(OPC)and dithiocarbamate(DTCR)are used to solidify/stabilize the heavy metals in FA in this project.The environmental risk of heavy metals Cd and Pb in the S/S FA is investigated by simulating natural/acid rain,freeze-thaw(F-T)cycle and wetting-drying(W-D)intermittent in laboratory scale,which provides risk assessment methods and theoretical basis for the treatment and disposal of MSWI FA.The main results of this study are as follows:(1)The leaching characteristics of Cd and Pb in the cement-solidified FA under natural and acid rainfall wash were studied.In this study,the leaching concentration was determined under the two standards:USEPA Method 1311 and HJ/T300-2007.According to the leaching results of USEPA Method 1311,after aged by natural rain at first cycle,the leaching concentration of Cd and Pb in the FA were both reduced by 50%,and a large amount of soluble chlorides were lost under rainfall wash.Thus,reduction of chlorides in the FA helped enhanced the stabilization of Cd and Pb.The leaching results of HJ/T 300-2007 show that the leaching concentration of Cd in the FA aged by natural rain is 26.3%-32.5%higher than that of the FA aged by acid rain.Therefore,carbonation caused more damage to the immobilization of Cd.(2)The stability of Cd and Pb in the FA under acid rainfall wash was studied.After simulated 200 years,the leaching concentration of Cd and Pb in cement-solidified FA decreased slightly,indicating that OPC-solidification has the high durability against the rainfall wash;while the leaching concentration of Cd and Pb in DTCR-chelated FA increased significantly,indicating that DTCR-stabilization has poor durability.In the combined treatment by both OPC and DTCR,the leaching concentration of Cd and Pb after treatment increased and the durability against rainfall wash increased as OPC dosage increased.Hence,the dosage of OPC and DTCR should be adjusted properly in order to balance initial immobilization efficiency and durability against rainfall wash.In this study,a long-term ecological risk index is defined to quantitatively evaluate the long-term stability of heavy metals.200-years' long-term potential ecological risks reduced by 10.29%as the OPC dosage increased from 5%to 10%.Thus,combined treatment with 10%OPC and 1.5%DTCR dosage was recommended for the treatment of the FA.(3)The leaching characteristics of Cd and Pb in the FA stabilized by cement and chelator were studied.In combined treatment,DTCR plays the main role in stabilizing Cd and OPC plays the main role in stabilizing Pb.Under F-T cycles,the expansion of water damaged the structure of C-S-H and weakened its wrapping effect.The stabilization effect of DTCR on Cd and Pb was also damaged under F-T cycles.Therefore,the leaching toxicity of Cd and Pb increased under F-T cycles.Under W-D intermittent,the leaching toxicity of Cd increased significantly due to the damage of Cd-DTCR.However,the hydration degree of OPC is increased and fly ash particles are protected by more hydration products under W-D intermittent,which results in the decrease of Pb leaching toxicity.In addition,NaH2PO4 can effectively improve the stability of Cd and Pb under F-T cycles and W-D intermittent.