The Leaching Characteristics And Chemical Stabilization Of Heavy Metals In MSWI Fly Ash

Municipal solid waste incineration(MSWI)is considered to be the best way of waste disposal with significant reduction,harmlessness and resource utilization.However,the MSWI fly ash is rich in heavy metals and other hazardous substances.When being disposed improperly,heavy metals are easy to leach and threaten the environment.In this paper,the physicochemical properties and leaching characteristics of heavy metals in fly ash in a municipal solid waste incineration plant in Chongqing were analyzed.Thereafter,FACAR and triple superphosphate were used to stabilize heavy metals in fly ash to meet the requirements of landfill admission.The stability mechanism was also explored by the analysis of XRD,SEM/EDS and FTIR.The main conclusions are as follows:(1)XRF measurement demonstrated that due to excessive lime adding for removal of acidic matters from flue gas,the content of calcium oxide(CaO)in fly ash was high,while the content of silica was low in comparison to the fly ash in other cities of China.XRD analysis indicated that the main crystal forms of compound in original fly ash were KCl,NaCl,CaCO3,CaSO4,CaClOH and SiO2.But the main crystal forms of the fly ash changed to CaCO3 and CaSO4 after the fly ash was extracted by acetic acid buffer solution.(2)The surface morphology of the fly ash observed by SEM indicated that the particles were mostly irregular sphere or floc with the high porosity.After the leaching with acetic acid buffer solution,it changed to amorphous form,and the porosity decreased significantly.EDS measurement demonstrated that the contents of S and Si in the fly ash after leaching were significantly improved,while the contents of O,Ca and C were the highest and the content of heavy metal elements was less than 1%.The results also showed that the heavy metals in fly ash were evenly distributed in fly ash particles before and after leaching.FTIR analysis showed that the main functional groups in the original ash are O-H,-CO32-,-SO42-and Si-O-Si.(3)The leaching concentrations of Pb,Cd,Ni,Cr,Cu and Zn decreased with the increase of the leaching agent pH.However,when the pH exceeded a certain value,the concentration of leached Pb and Zn increased with the increase of pH,and Cr and Cd also increased slightly.While the leaching concentration of Ni remained unchange,and the leaching concentration of Cu decreased slowly.(4)The effects of Na H2PO4,TSP and Na2 S on Pb,Cd and Ni stabilization in fly ash was in order of TSP> Na2S> NaH2PO4.The stabilization of Pb,Cd and Ni by using FACAR as a stabilizer was significantly better than using sodium diethyldithiocarbamatre(DDTC)and potassium ethyl xanthate(EXT).At the addition of 3% FACAR,the immobilization ratios of Pb,Cd and Ni in fly ash were 91.52%,100% and 100%,respectively.For Pb stabilization,EXT was better than DDTC,while DDTC was better than EXT for Cd and Ni stabilization.(5)When 3% FACAR and 3% TSP mixture were used to immobilize the heavy metals in fly ash,the stabilization ratio of Pb was 98.06%,and Cd and Ni were completely stablilized which could meet the requirement of landfill admission.Thus it could be the optimal addition ratio for heavy metals immobilization in fly ash.Stabilized fly ash presented a large proportion of steady state of Pb,Cd and Ni.So that the environmental impacts of heavy metals in fly ash were greatly reduced.(6)The combined stabilization did not change the main crystal form of the fly ash.It was found that the porosity of the fly ash surface decreased significantly after stabilization.The content of P and S was significantly increased.FTIR analysis indicated that the stabilized fly ash contained C=S and phosphate functional groups.These results showed that the reactions of precipitation and chelation between stabilizers and heavy metal in fly ash were responsible for prevention heavy metals from leaching.