Study On Dynamic Leaching Of Heavy Metals IN Cement Solidified Body Of Municipal Solid Waste Incineration Fly Ash

With the rapid development of the social economy,people’s living standards have been continuously improved and the production of household waste has also increased dramatically.Because the incineration method has the advantages of small footprint,large volume reduction,high degree of harmlessness,and the ability to recover heat energy,it has been rapidly popularized and applied in China in recent years.However,as one of the waste incineration products,fly ash is a hazardous waste,containing a large amount of toxic and harmful heavy metals such as Zn,Pb,Cr,and Cd,and highly toxic organics such as dioxins.The final disposal of the fly ash becomes a new problem.Currently,incinerated fly ash is mostly treated with cement curing or chemical stabilization to meet the requirements of the "Control Standard for Domestic Waste Landfill Contaminants" and sent to landfill disposal of domestic waste landfill,its leaching behavior and long-term potential pollution has not received enough attention.A 100mm×100mm×40mm solidified body was obtained from the incineration fly ash of Jiujiang MSW incineration power plant and the ordinary silicate 32.5R cement.The initial pH of 5.5 was used and neutral deionization(pH=7.0)was used as the extraction liquid water.The longterm leaching and dynamic leaching behavior of heavy metals in solidified blocks were simulated,and the leaching mechanism was analyzed with SEM and XRD.This study can provide an important basis for the theoretical basis for the long-term and stable disposal of waste incineration fly ash.Long-term leaching test results show: at pH=5.5,the larger the liquid table ratio,the closer the final pH of the leachate is to the initial pH.The leaching concentration of Pb,Zn,and Cd rapidly increased to the maximum value with time,then decreased and remained stable,the Cr first increased and then smaller,and finally remained stable.The leaching concentrations of Pb,Zn,Cr,and Cd were 0.0165 mg/L to 0.2219 mg/L,0.0045 mg/L to 1.7483 mg/L,0.0016 mg/L to 0.0528 mg/L,and 0.0109 mg/L to 0.1236 mg/L respectively.At pH=7.0,the final pH of the leachate is 12.73~13.06.The larger the liquid ratio is,the lower the pH is.The leaching concentrations of Pb,Zn,and Cd rapidly increased to the maximum value with time,then decreased and remained stable,the Cr leaching concentration increased first and then decreased,then increased and then decreased until it remained stable.The leaching concentrations of Pb,Zn,and Cr were 0.0012 mg/L to 0.0891 mg/L,0.0016 mg/L to 0.0170 mg/L,and 0.0012 mg/L to 0.0068 mg/L,respectively.Comparing Class III standards in the Groundwater Quality Standard(GB/T 14848-2017),if infiltrated,it may cause damage to groundwater.Dynamic leaching test results show: At pH 5.5,the pH of the leachate first increases then decreases and then remains stable.The larger the replenishment amount,the closer the pH after stabilization to the initial value,but the final stable pH is alkaline when the replenishment amount is 700 ml/d.The leaching concentration of Pb first increases with time,then decreases and then increases until it is stable.The leaching concentrations of Zn,Cd,and Cr first decrease,then increase,and then decrease until stable.The leaching concentrations of Pb,Zn,Cr,and Cd ranged from 0.0300 mg/L to 0.1628 mg/L,0.0022 mg/L to 0.6599 mg/L,0.0048 mg/L to 0.0256 mg/L,and 0.0013 mg/L to 0.0479 mg/L,respectively.The leaching concentrations of Pb,Zn,and Cr first increased rapidly to their maximum values,and then decreased to a stable level,the Cd was not detected in the extract.The leaching concentrations of Pb,Zn,and Cr ranged from 0.0233 mg/L to 0.2930 mg/L,from 0.0048 mg/L to 0.0241 mg/L,and from 0.0018 mg/L to 0.0347 mg/L,respectively.If infiltrated,pH,Pb,and Cd may cause damage to groundwater.SEM and XRD analysis of the solidified body,when the initial pH of the extract is 5.5,the dissolution phenomenon of the solidified body is serious,and the soluble crystal gradually disappears as the leaching disappears.Finally,the crystal structure of the silicate crystal is not allowed to remain.When the initial pH of the extract is 7.0,CaCO3 crystals will gradually form during the leaching process,which will hinder the leaching of heavy metals in the solidified body.