| With the municipal solid waste（MSW）incineration technology widely application,the amount of fly ash generated from MSW incineration has increased year by year.Due to the high leaching concentration of heavy metals,and the high amount of toxic and refractory polychlorinated biphenyls（PCBs）,polycyclic aromatic hydrocarbons（PAHs）and dioxin（PCDD/Fs）,fly ash was identified as hazardous waste and marked as HW18in China’s National Hazardous Waste List.The safe and effective management of fly ash has become an environmental and social problem that needs to be solved urgently.In this study,serials comprehensive experiments focus on the harmlessness of heavy metals and persistent organic compounds（PAHs,PCBs）in MSW incineration fly ash under subcritical water conditions was carried out,funded by the Chongqing Basic Science and Frontier Technology Research Project（CSTC2017jcyjAX0035）.The main contents and conclusions of this study are as follows:（1）The relative content and the activity of glass phase in fly ash is always high.On the other hand,in fly ash obtained from the grate furnace+semi-dry flue gas treatment system,the content of silicon and aluminum is low,but the content of calcium is high.In this paper,the stability of heavy metals and the degradation of PCBs in fly ash under alkaline subcritical water conditions were studied,based on adjusting the ratio of calcium,silicon and aluminum by the addition of crystal seed and compound additive.The addition amounts of compound additive were 10%and 30%,respectively,in which the mass ratio of fly ash and diatomite was 1:1.The results showed that the aluminosilicate zeolite crystals such as tobermorite,katoite and so on,which might effect on heavy metals and organics with adsorption,were synthesized with the mixed addition of silica-alumina additives after hydrothemal reaction.The leaching toxicities of Pb,Zn,Cu,Cr and Cd of the hydrothermal products（solid phase）were all significantly reduced under the standard limit value of hazardous waste identification.The decrease of heavy metals in the liquid phase after reaction showed that the mixed addition of silicon-aluminum additives and crystal could effectively inhibit the migration of heavy metals from fly ash to the hydrothermal solution during hydrothermal process.The total content and the toxicity equivalent of PCBs in hydrothermal products showed a significant decrease after the reaction.For the experimental parameters of hydrothermal process,the lowest concentration of heavy metals and the lowest toxicity of PCBs were all observed,when the addition amount of compound silicon-alumina additives was 30%and seeds of 3%under the reaction temperature of 200℃.Compared with the original fly ash,the leaching concentrations of five kinds of heavy metals（Cu,Pb,Zn,Cr and Cd）in hydrothermal products reduced to 0.08mg/L,0.27 mg/L,0.25 mg/L,0.35 mg/L and 0.03 mg/L,respectively,which were decreased by 76.2%to 98%.Meanwhile,the toxicity of PCBs decreased to4.33×10-6μg-TEQ/Kg,which was 99.9%lower than that of the raw fly ash.The results from the experiments that the ratio of the elements in the system adjusted by mixing pure silicon source and aluminum source under alkaline subcritical hydrothermal conditions,showed that the ratio of Ca/（Si+Al）and Al/（Si+Al）in the system affects the species of zeolites synthesized in the system,and also indirectly affects the stability of heavy metals and the adsorption and degradation of PAHs during hydrothermal process.With the high ratio of Ca/（Si+Al）,the crystal phase in the hydrothermal products is mainly katolite.With the high ratio of Al/（Si+Al）,the crystal phase is dominated by analcite and katoite.After the hydrothermal reaction,the leaching concentration of heavy metals in hydrothermal products decreased to the standard limit value of the identification of hazardous waste,and could effectively inhibit the migration of heavy metals to the hydrothermal solution.The contents of PAHs in hydrothermal products decreased in varying degrees.Based on the results of the leaching concentration of heavy metals in the solid phase and the concentration of heavy metals transferred to the liquid phase,it was known that the best condition for stabilization of heavy metals existed when the ratio of Ca/（Si+Al）and Al/（Si+Al）were1.10 and 0.17,respectively,which may be related to the stronger peak intensity of tobermorite and higher purity of it Meanwhile,the results that the content and the toxic equivalent of PAHs decreased by 77%and 67%respectively,indicated that the better synthesis conditions of the tobermorite could improve the degradation of PAHs.Part of the PAHs might be absorbed to the lattice of tobermorite,meanwhile,the amount of heavy metal-supported zeolite catalyst increased and thus the catalytic degradation of PAHs in the system increased correspondingly.（3）The results of the oxidation degradation of PAHs and the stability and migration of heavy metals in fly ash under the subcritical water thermal oxidation technology showed that the content and the toxic equivalent of PAHs in the hydrothermal products were effectively reduced with the oxidation reaction of 30%H2O2 used as oxidant.With the progress of the oxidation reaction,the remaining PAHs which combined with fly ash tightly,were much more difficult to degrade.Therefore,with the oxidant concentration increases,degradation rates of the total content and the toxicity equivalent of PAHs decrease.The increase of hydrothermal temperature promoted the degradation of PAHs.At 200°C and the oxidant concentration of 1 mol/L,the total amount and toxicity equivalent of PAHs decrease to the lowest level,which decrease by 86.5%and 93%respectively.In addition,the leaching concentrations of heavy metals in the hydrothermal products is reduced,and it was lower than the standard limit value of hazardous waste.At the same time,it could inhibit the migration of heavy metals into the liquid phase to some extent.After the oxidation reaction,crystalline phase of aluminosilicate was not synthesized obviously in hydrothermal products,so the stability of heavy metals was relatively lower,attributing to the imbalance of the proportion of Ca,Si and Al in the system.In this study,the stability of heavy metals and the degradation of persistent organic pollutants were investigated under a variety of subcritical water processes.The hydrothermal reaction could reduce the leaching concentration of heavy metals in the hydrothermal products and inhibit the migration of heavy metals into the liquid phase.At the same time,the content and the toxicity of PCBs or PAHs in fly ash could be degraded effectively.This study can provide technical reference and theoretical support for the subcritical hydrothermal technology to stable of heavy metals and degrade of persistent organic compounds in fly ash from MSW incineration. |
PDF Full Text Request