Study On Leaching Characteristics And Numerical Simulation Of Heavy Metals In Incineration Bottom Ash As Subgrade

With the continuous increase in the production of municipal solid waste?MSW?,incineration technology with significant features such as capacity reduction and volume reduction is becoming the mainstream waste treatment technology.However,the weight of incineration ash about 20%-30%will be generated after municipal solid waste incineration?MSWI?.Bottom ash is the main component of incineration ash,and it accounts for about 80%of the volume of incineration ash.How to properly treat and dispose of incineration bottom ash has become a new problem facing China's incineration industry.Takeing the incineration bottom ash and aggregates based on wet treatment of a MSWI power plant in Shanghai as the research object.First,using heavy compaction test,improved BCR method and other test methods,the X-ray fluorescence spectrometer?XRF?,X-ray diffraction analyzer?XRD?,and inductively coupled plasma mass spectrometer?ICP?and other testing technologies to characterize and analyze the basic characteristics of incineration bottom ash,including particle size distribution,element distribution,heavy metal content and morphological distribution,etc.,and provides a rational basis for choosing a reasonable resource utilization mode.Studies have shown that the bottom ash meets the requirements of the Code for Design of Highway Subgrades and is a well-graded gravel soil that can be used as a subgrade filler;The main mineral composition of the bottom ash is Si O₂,Ca CO₃ and Ca Al Si₂O₈?4H₂O?.The wet treatment reduces the intensity of the diffraction peaks of Si O₂ and Ca CO₃ in the bottom ash;The three heavy metals with higher content in the bottom ash are Zn>Cu>Ni,and the content of heavy metals in bottom ash with a diameter of 0-5mm is the highest;Most of the heavy metals in the incineration bottm ash exist in the form of residues?about 50%or more?.This form of heavy metals is generally fixed in the silicate structure,which indicates that the characteristics of heavy metals in the bottom ash are relatively stable under natural conditions.Through batch toxicity leaching test and column leaching to simulate the natural leaching of rainwater,the leaching law and dynamic characteristics of typical heavy metals such as Cu and Zn under short-term and long-term leaching conditions under single variable were investigated.The analysis shows that the bottom ash can be disposed of as general solid waste,but its heavy metal leaching concentration has exceeded the Class V surface water standard limit to varying degrees;In the analysis of the effect of liquid-solid ratio?L/S<15:1?and temperature?20?,30?,40??on the leaching level of heavy metals,the amount of heavy metal leaching is proportional to it;Under acid leaching conditions,the decrease of p H value is conducive to the leaching of more heavy metals;The leaching rate equation of Zn and Cu in bottom ash in pure water can be described by Elovich equation and power function equation.The main speed control step in the leaching process is internal diffusion control.According to the calculation of Arrhenius diagram,the apparent activation energy of Zn leaching is 38.4 k J/mol and Cu is 19.6 k J/mol,which indicates that Cu is more easily leached than Zn in bottom ash;The level leaching heavy metals from 0-5 mm bottom ash is higher than that from 5-10 mm.It is recommended to use 5-10 mm bottom ash for backfilling while meeting the roadbed strength requirements.At the same time,anti-seepage measures should be taken on the embankment slope to reduce the environmental pollution hazards of bottom ash during long-term use;The column leaching test results show that the concentration of Cu and Zn in the leaching solution of the aggregate after wet processing is much lower than that of the original bottom ash,and the highest leaching concentration of Cu is only 1/10 of the original bottom ash.It is shown that the wet process can reduce the leaching level of heavy metals in bottom ash;The impact analysis for different p H values?p H=1 or 3 or 5?shows that when 5-10 mm of primary bottom ash is leached by rainwater,Cu has the highest cumulative leaching rate at p H=5.It can be seen that the leaching pollution of heavy metals in bottom ash is the most serious under natural acid rain.Finally,the SEEP/W module and the CTRAN/W module in the Geostudio software were used to simulate the bottom ash column leaching test under natural rainfall conditions,and it was found that the model can better reflect the leaching of heavy metals in the soil column.On this basis,a physical model of the actual subgrade of the bottom ash filler was established.Simulation results show that the peak concentration of Cu dissolution of heavy metal after 11 years of use is approximately 1 m below the foundation,which is 138 g/m³,and the lateral diffusion distance of pollutants is 0.75 m,and the concentration of Cu is less than the secondary standard limit of the Soil Environmental Quality Standard.