Study On The Leaching Behavior And Environmental Risk Assessment Of Heavy Metals In Solidified/Stabilized Municipal Solid Waste Incineration Fly Ash

The rapid development of municipal solid waste incineration?MSWI?has contributed to the sharp increase of fly ash amount in China.MSWI fly ash is rich in various heavy metals,which is one of the reasons it's considered as hazardous waste.“Solidification or stabilization pretreatment + landfill disposal” is a commonly adopted fly ash management strategy in the world.GB 16889-2008?Standard for Pollution Control on the Landfill Site of Municipal Solid Waste?stipulated that the leaching concentration of heavy metals in fly ash based on HJ/T 300-2007?Solid Waste – for Leaching Toxicity – Acetic Acid Buffer Solution Method?should meet the standard limits and be landfilled in different zones with MSW.However,due to the rapid increase of fly ash production,the difficulties in site selection and construction schedule of landfills or some other reasons in management,there were some occurrences of codisposal between solidified/stabilized fly ash and raw MSW.In a realistic co-disposal environment,the stability of heavy metals in fly ash may be affected by such disposal scenarios as landfill leachate leaching,carbonation,and acid rain corrosion.However,there is a lack of simulation experiments and methodology researches on the leaching behavior,dissolution mechanism,and environmental risk assessment of heavy metals in solidified/stabilized fly ash under different disposal scenarios.Based on the investigation on fly ash treatment and disposal in China,raw fly ash?Y2?and its corresponding stabilized or solidified fly ash with chelating agent?A2?,phosphoric acid?P2?,and Portland cement?S2?from a typical MSWI plant were selected as the main experimental samples.The leaching behavior of heavy metals?Pb/Zn/Cu/Cd/Cr/Ni?,under the disposal scenarios of landfill leachate leaching,carbonation,and acid rain corrosion,was studied by column leaching tests.The dissolution mechanism of heavy metals was discussed based on the batch extraction tests of key influencing factors?pH/CO₂/DOM?.The evolution rules of?independent or comprehensive?environmental risk of heavy metals were evaluated by establishing environmental risk assessment models based on liquid-phase?OPTI model?and solidphase?RAC and STI model?.The main research contents and results are as follows:?1?The basic properties of typical solidified/stabilized fly ash were analyzed.The content of heavy metals in fly ash was found in the following trend of Zn > Pb > Cu > Cr > Cd > Ni,and Zn/Pb content was nearly 1-2 orders of magnitude higher than that of Cu/Cr/Cd/Ni.The leaching toxicity of heavy metals was not consistent with their contents,which was generally higher for Pb/Cd/Ni.Solidification or stabilization increased the proportion of stable heavy metals,but a certain proportion of leachable Pb/Cd/Cr/Ni?exchangeable or carbonate-bound?still existed.The main mineral phases in fly ash were not affected by solidification or stabilization,and no heavy metal bearing mineral phases were detected.?2?The leaching behavior of heavy metals in solidified/stabilized fly ash under different disposal scenarios was investigated by constructing three simulated realistic disposal scenarios and based on column leaching tests.Solidification or stabilization reduced the leaching of heavy metals in fly ash in different disposal scenarios,and “early leaching” was the key period to increase the leaching of heavy metals.Both landfill leachate leaching and carbonation increased the proportion of leachable heavy metals in fly ash,while the replacement of acid rain corrosion reduced the previous increase,especially the change of carbonate-bound heavy metals varied with the types of fly ash.The main mineral phases in fly ash evolved into Ca CO3 after different disposal scenarios,and the peak strength was increased.The newly detected Ca-Al-Si-O in A2/S2 after CO₂-saturated water leaching may affect the chemical speciation and leaching behavior of heavy metals.?3?The effects of different factors on the leaching behavior of heavy metals in solidified/stabilized fly ash were investigated by identifying the key influencing factors in different disposal scenarios and based on batch extraction tests.pH is the most important factor affecting the leaching of heavy metals in fly ash.The lower the end-point pH value,the higher the leaching concentration.Solidification or stabilization reduced the leaching of heavy metals,but did not change their leaching rules in the whole pH range?1-14?.The final concentration distribution of heavy metals in “fly ash-leachate” system was affected by the addition of DOM,but the influence rule varied with the types of fly ash,heavy metal,and pH.HA promoted and reduced the leaching of Pb and Zn/Cu/Cd in A2,respectively,while FA promoted the leaching of Cu in P2/S2,and the leaching behavior was less affected by pH.The leaching behavior of heavy metals in P2/S2 in the extraction tests with HA was greatly affected by pH,but the leaching of Pb/Zn/Cd/Cr/Ni at low end-point pH was reduced.The exchangeable heavy metals in fly ash were preferentially leached in natural pH,but the proportion of some leachable heavy metals will be re-increased due to increased acid corrosion.During carbonation,solidification or stabilization did not significantly affect the carbonation reaction activity of fly ash,but reduced the leaching of most heavy metals,and varied with the leaching method.In HJ 557-2010?distilled water extraction?,the leaching of Pb and Cd/Ni decreased and increased respectively,while the trend of Zn/Cu/Cr varied.In HJ/T 300-2007?acetic acid buffer solution extraction?,the leaching of Pb/Zn/Cu/Cd and Cr/Ni increased and decreased,respectively.After carbonation,the proportion of exchangeable Zn/Cd and Pb/Cu/Cr/Ni increased and decreased respectively,while the proportion of carbonate-bound Pb/Zn/Cu/Cd/Ni increased.Solidification or stabilization reduced the leaching of most heavy metals in MEP?Multiple Extraction Procedure?tests in the same period.The early extraction of landfill leachate or CO₂-saturated water and the replacement of simulated acid rain were all unfavorable factors for increasing the leaching of heavy metals.The prolongation of cumulative extraction time re-increased the leaching of most heavy metals?especially Pb/Cd?,but the trend varied with different fly ash samples.In addition,the evolution rules of the ratio of leachable heavy metals in fly ash after MEP extraction tests were similar to that of column leaching tests.?4?The main dissolution mechanisms of heavy metals in solidified/stabilized fly ash under different disposal scenarios were discussed based on the experimental results between disposal scenarios and key influencing factors.Complexation and adsorption or desorption of heavy metals coexisted in the landfill leachate leaching system.The destructive effect of DOM on the local “precipitation-dissolution” equilibrium at the solid-liquid interface of “fly ash-leachate”,the formation of non-adsorbable “DOM-Mn+?heavy metal ions?” complex,and the competitive effect of DOM on the adsorption binding sites?S?of Mn+ on the surface of fly ash matrix were the main influencing mechanisms,which contributing to the increased heavy metal leaching in early leaching stage.Adsorbable “S-DOM-Mn+” was the main adsorption mechanism in later leaching stage,which was affected by pH.CO₂-saturated water leaching system was an indirect carbonation process based on “CO₂-water” and “fly ash”.The reactions of “nucleation and dissolution” of carbonate minerals and “fixation and dissolution” of heavy metals coexisted in the system.The dissolution mechanism of heavy metals was mainly affected by the reaction equilibrium of “nucleation and dissolution” of carbonate minerals containing heavy metals.“Dissolution” and “nucleation” were dominant in the early and later stages of leaching,respectively.The main mechanisms affecting heavy metal leaching in simulated acid rain leaching system included dissolution of carbonate layer,corrosion/replacement of H+,and counter-ion effect of SO42-/NO3-/Cl-?SO42-> NO3-> Cl-?.?5?The evolution rules of potential environmental risk of heavy metals in liquidphase and solid-phase of solidified/stabilized fly ash in different disposal scenarios were evaluated by constructing the environmental risk assessment models.Solidification or stabilization can effectively reduce the environmental risk of leached heavy metals,but the early leaching of landfill leachate or CO₂-saturated water,acid rain corrosion,and excessive carbonation were all contributing factors to increase OPTI of leached heavy metals.Continuous landfill leachate leaching or carbonation will led to the increase of RAC of heavy metals in residual fly ash?especially Pb/Zn/Cd?.Compared with independent disposal scenario,alternative disposal scenario increased the STI of heavy metals in residual fly ash.Compared with column leaching tests,HJ/T 300-2007,as an enhanced pollutant leaching method,can effectively assess the potential environmental risk level of leached heavy metals in solidified/stabilized fly ash before being landfilled.Compared with MEP tests,HJ/T 300-2007 can effectively strengthen the simulated leaching environment for raw fly ash,and provide reliable OPTIsta under the disposal scenarios of landfill leachate leaching,carbonation,or acid rain corrosion.However,for solidified/stabilized fly ash,under the same disposal scenarios,there were some occurrences that the OPTI of leached heavy metals exceeded their corresponding OPTIsta.Therefore,in the co-landfill disposal scenarios,HJ/T 557-2007 leaching environment may underestimate the potential environmental risk level of leached heavy metals for solidified/stabilized fly ash in continuous or alternative disposal scenarios with landfill leachate leaching,carbonation,or acid rain corrosion.In a word,the research results of this study have certain theoretical and practical significance for the development of solidification or stabilization technology of MSWI fly ash,the long-term stability evaluation of solidified/stabilized fly ash in the actual disposal scenarios,and the construction of environmental risk assessment system of heavy metals in China.