Immobilization effect and environmental impact assessment of heavy metals in pretreated fly ash
In view of the widespread problem of excessive heavy metals in waste incineration fly ash,the fly ash was pretreated with piperazine chelating agent,dimethyl dithiocarbamate and cement to reduce the landfill risk.The fixation effect of pretreatment on heavy metals in fly ash was investigated by means of acetic acid buffer Solution Method for solid waste leaching Toxicity(HJ/T300-2007),and the phase composition,microscopic morphology and heavy metal morphological distribution of fly ash were analyzed by X-ray diffraction,scanning electron microscopy and BCR morphological extraction.The RAC and STI models were used to assess the environmental risk of each fly ash.The results showed that the leaching toxicity of heavy metals Ni,Pb and Cd in raw ash exceeded the standard,and the heavy metal contents were distributed in the order of Zn＞Pb＞Cu＞Cd＞Cr＞Ni.After pretreatment,the leaching toxicity of heavy metals Ni,Cd and Pb in fly ash met the standard limit,and the fixation rate was more than 99％,and the fixation rate of heavy metals Cr,Zn and Cu was higher than 96％.The stabilization effect of SDD was better than that of TS300,but the effect of TS300 on cementing heavy metals was better than that of SDD.The effect of stabilization on Cr was better than that of solidification/stabilization.Single cement curing had the best fixing effect on Zn,Pb and Ni,but the application of cement curing was limited by 40％cement dosage and high-capacity ratio.ANC tests showed that cement solidified ANC was strongest.XRD and SEM results showed that the new minerals formed by stabilization played an important role in the fixation of heavy metals.Ca(OH)2 and C-S-H formed by cement hydration can bind heavy metals through physical adsorption.The results of BCR morphology extraction showed that pretreatment reduced the proportion of unstable heavy metals.The evaluation results of RAC and STI model showed that heavy metals Zn,Pb and Cd were the main contributors to environmental risks.Compared with the RAC index method,the evaluation results of STI model were more in line with expectations.
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