Optimization Of MICP Microorganisms And Their Effects On Stabilization

MSWI treatment has been paid attention to due to significant reduction,resulting in a large amount of MSWI fly ash.China's current annual output of fly ash is about 220-3.7 million tons/year,and fly ash contains various heavy metals and dioxins.It is classified as hazardous waste by toxic substances and must be solidified or stabilized before it can be safely disposed or utilized.Currently,common solidification/stabilization methods include cement curing method,high-temperature melting stabilization method,chemical agent stabilization method,etc.,but there are problems such as large capacity increase,high cost,or secondary pollution.MICP is widely found in nature.Because of its simple mechanism,low cost,high speed,high efficiency and environmental friendliness,MICP has attracted the attention of civil engineers and has been applied in the fields of sand solidification,ancient building restoration and soil pollution control.Under the auspices of the National Natural Science Foundation of China(5157081460),this paper uses MICP for high-contamination MSWI fly ash solidification/stabilization treatment.Firstly,the urease microbial separation and identification were carried out from the flowerbed soil.Based on the biochemical characteristics test of each microorganism and the environmental geotechnical characteristics test of the solidified/stabilized fly ash,the microbial performance evaluation model by entropy method was preferred to cure/stabilize the fly.Then the experiment of MICP solidifying/stabilizing MSWI fly ash under multi-factor conditions was carried out.The effects of four factors including liquid concentration,curing condition,void ratio and liquid-solid ratio on the solidification/stabilization of fly ash were studied.The optimal conditions for solidifying and stabilizing fly ash using MICP technology were determined by entropy method and Nemero index method.Finally,microscopic testing of solidified/stabilized fly ash samples was carried out to discuss the solidification and stabilization of MICP technology.This paper mainly obtained the following conclusions:(1)A variety of urease bacteria were isolated from soil and 5 dominant strains were identified.They are Sphingobacterium multivorum(UR-G53)?Raoultella ornithinolytica(UR-A51)?Bacillus aryabhattai(UR-429)?Ensifer adhaerens(UR-UA)and Pseudomonas putida(UR-422).The biochemical characteristics(ureaase activity,bacterial pH,extracellular polysaccharide)of five urease bacteria were tested.And the geotechnical properties(unconfined compressive strength,particle curve)and environmental characteristics(metal leaching concentration)of the solidified/stabilized fly ash were also tested.In view of the fact that Bacillus bacillus(UR-BA)is widely used in MICP research,the above test was carried out on Bacillus bacillus.According to the above biochemical,geotechnical and environmental characteristics test results,an evaluation model of microbial performance was established based on entropy method.Entropy analysis showed that the optimal microbial species of solidified/stabilized waste incineration fly ash was UR-UA.(2)The optimal condition that solidification/stabilization of MSWI fly ash based on MICP.Based on the above test results,UR-UA was used to test the effect of factors such as bacterial solution concentration,curing conditions,void ratio,liquid-solid ratio on the solidification/stabilization of fly ash.As the concentration of the bacterial liquid increases,the solidification effect of the waste incineration fly ash first increases and then decreases(the average particle size,the permeability coefficient,and the unconfined compressive strength first increase and then decrease),the stabilizing effect(heavy metal leaching concentration)does not change significantly.Under the conditions of constant temperature and humidity(temperature 20±0.5°C,humidity>95%),the solidification and stabilization effects of waste incineration fly ash are better than natural curing conditions.With the increase of void ratio,the solidification effect of waste incineration fly ash is gradually weakened,and the stabilizing effect is gradually enhanced.With the increase of liquid-solid ratio,the solidification effect of waste incineration fly ash is gradually weakened,and the stabilization effect has no obvious change law.Based on the above test results and entropy method,the evaluation model of fly ash solidification effect based on particle gradation,specific gravity,permeability and compressive strength of solidified fly ash was established,and the solidification treatment of fly ash by MICP was determined.The optimal conditions are bacterial liquid concentration 14.92×10~7 cells/mL,void ratio 0.552,liquid-solid ratio 0.15,constant temperature and humidity conservation.Based on the above test results and the Nemelo index method,an evaluation model for the stabilization of fly ash with stable leaching concentration of stabilized fly ash was established.The optimal conditions for stabilization of fly ash by MICP were determined.The concentration of the bacterial liquid was 22.09×10~7 cells/mL,the void ratio was 0.939,the liquid-solid ratio was 0.1,and the constant temperature and humidity were maintained.(3)Mechanism analysis of MICP solidification/stabilization of MSWI fly ash.Through the SEM-EDS test of solidified/stabilized fly ash samples,the mechanism of MICP solidification and stabilized MSWI fly ash was analyzed by combining the solidification/stabilization test results of fly ash.solidifying mechanism mainly includes:a)The pozzolanic reaction products of fly ash(calcium silicate hydrate)and calcium carbonate generated by MICP and other products are deposited on the surface of fly ash particles,or multiple fly ash particles are consolidated into clusters,or the pores of fly ash particles are filled,thus resulting in the thickening of solidified fly ash particles and the increasing of compressive strength.The stabilization mechanism mainly includes:a)In carbonation,heavy metal ions combine with carbonate ions to form carbonate precipitation.b)Physical sequestration,the generated cementitious substances wrap the fly ash particles and seal the heavy metals adsorbed on the fly ash surface together,so that the heavy metals are not easy to leach out.