| Municipal solid waste incineration fly ash（MSWI fly ash）is classified as a hazardous solid waste since it contain toxic substances such as heavy metals and have been added into the list of national hazardous wastecategory（NO.772-002-18）.The standard for pollution control on the landfill site of municipal solid waste（GB116889-2008）stipulates that the MSWI fly ash must be treated harmlessly.Based on the National Natural Science Foundation of China（No.5157081460）supported,this paper presents an experimental study on the feasibility of using the principle of MICP to treat two kinds of municipal solid waste incineration fly ash（A and B）,which were produced from the incineration process of mechanical grate and circulating fluidized bed,respectively.The microbial bacteria used for the MICP treatment was Sporosarcina pasteurii.Firstly,the particle gradation,physical and chemical properties of the two kinds of fly ashes were tested and analyzed.Then,the leaching quantity of Ca and heavy metals from the fly ashes were measured,and the effects of leaching alkalinity and toxicity on the growth and reproduction of S.pasteurii bacteria were investigated.Finally,the fly ashes were treated by mixing with the cultured bacteria solution at the ash-liquid ratio of 1 kg : 0.3 L,and cured in a cylindrical mold for 7 days.The effectiveness of the treatment was evaluated by measuring the unconfined compressive strength,the changes in particle gradation and leaching toxicity of heavy metals on the treated ash specimens.The main findings obtained from present work are shown as follows:（1）Physicochemical properties,geotechnical properties and heavy metals leaching toxicity of MSWI fly ash.According to the analysis of SEM,the fly ash A and B were both porous,the particles of fly ash A were aggregated,and the sample B was dispersed.The results of XRD analysis showed that they had a certain cementitious properties and pozzolanic activity because of they contained more CaO and a certain amount of Al2O3,SiO2,Fe2O3.By using the horizontal vibration method and the sulphuric acid & nitric acid method to analyzing the two samples,found a variety of heavy metal leaching toxicity were more than pollution control on the landfill site of municipal solid waste（GB 16889-2008）in the required limits.The dissolution rate of Ca was analyzed by ICP-MS.The dissolution rate of Ca from sample A was 10.5%,and the sample B was 30.6%.According to the analysis of particle size distribution test and liquid plastic test,the particle size distribution of sample A is close to that of silty soil,and sample B is close to that of clay.From the permeability test,the permeability coefficient of sample A was 5.211×10-7cm/s,which is close to silty clay(5×10-710-6cm/s),and the permeability coefficient of sample B is 2.069×10-8cm/s,which was within the range of clay(<5×10-7cm/s).（2）The effects of leaching alkalinity and toxicity on the growth and reproduction of S.pasteurii bacteria.In fly ashes A and B with the leachate biochemical environment（ash-water ratio 100 g:1 L）,the reproduction rate of S.pasteurii was greater than that of the environment without fly ash.After 48 h of culture,the microbial activity indicator OD600 had exceeded 0.3.This means that strongly alkaline and heavy metal toxicity leachate environments of fly ashes A and B had no suppressive effect on S.pasteurii growth and reproduction;in contrast.（3）Experimental Study on Aggregation of MSWI fly ash based on MICP.After MSWI fly ashes A and B received the S.pasteurii solution mixing treatment,the unconfined compressive strengths of treated waste at 7 d were recorded as 0.368 MPa and 0.617 MPa,representing improvements of 31% and 118%,respectively,over the case for treated waste obtained from the deionized water mixture.This means that fly ash treatment results in S.pasteurii-induced calcium carbonate precipitation and a cementing effect.Accordingly,the resulting solidification of fly ash B with its finer particle sizes and higher level of soluble Ca was better than that of fly ash A.After fly ashes A and B received the S.pasteurii solution mixing treatment,the particle sizes of treated wastes increased significantly.After treatment,the d50 of fly ash A increased by 3 times in comparison with the deionized water mixing treatment.After fly ash B received the urea S.pasteurii solution treatment,d80,d50,and d30 increased 22 times,6 times,and 5 times in comparison with the deionized water mixing treatment.After fly ashes A and B received the S.pasteurii solution mixing treatment,the leaching toxicity values of various heavy metals were reduced.In comparison with their original leaching concentrations,the leaching reduction rates of Cu,Pb,and Hg for both types of fly ash were greater than 60%.After fly ashes A and B were subjected to the urea solution treatment,the Cu,Pb,and Hg leaching reduction rates were in excess of 45%.（4）This paper proposed a new mothed on the solidification/stabilization of MSWI fly ash by using the principle of MICP.The fly ashes were treated by mixing with the S.pasteurii solution at the ash-liquid ratio of 1 kg : 0.3 L,and cured in a cylindrical mold for 7 days under the temperature of 20±2℃ and the humidity of 95%.The size of particles were enlarged and the heavy metals leaching toxicity was reduced.It could satisfy the requirement of The standard for pollution control on the landfill site of municipal solid waste（GB116889-2008）,after the solidification treatment by adding a small amout of cement. |
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