Study On CO₂ Mineralization And Sequestration Of Modified Fly Ash And Stabilization Of Heavy Metals

Municipal solid waste incineration fly ash(hereinafter referred to as fly ash)is a secondary pollutant produced by municipal solid waste incineration.The amount of fly ash produced is increasing year by year.At present,the treatment and disposal methods of fly ash are mostly solidification/stabilization treatment+landfill disposal.However,considering that most sanitary landfill sites in China are close to saturation,it is an urgent environmental pollution problem to find other disposal ways for fly ash.With the intensification of the global greenhouse effect and China’s 2030’ carbon peak’ and 2060 ’ carbon neutrality’ targets,the use of fly ash for mineralization and sequestration of CO2 can not only reduce the concentration of CO2 in the atmosphere,achieve the purpose of carbon emission reduction,but also provide a new solution for the high value utilization of fly ash.However,due to the high salt content of fly ash itself and the lack of active components such as Al and Si,the CO2 capture efficiency and the degree of mineralization and sequestration are low.Therefore,in order to improve the ability of CO2 mineralization and sequestration of fly ash,this paper discusses the modification of fly ash by water washing pretreatment,alkali heat-hydrothermal and alkali excitation-foaming methods,and evaluates the ability of CO2 mineralization and sequestration of modified fly ash and the potential mineralization mechanism.It was found that washing pretreatment greatly enhanced the mineralization and sequestration performance of fly ash.After washing the fly ash at a liquid-solid ratio of 10:1 and 600 rpm for 30 min,the amount of mineralized CO2 increased from 90 g/kg to 160 g/kg CO2.Through XRD characterization,it was found that water washing treatment can effectively remove soluble chloride salts in fly ash and increase the stability of subsequent heavy metals in fly ash.Under the action of water washing,the salt content decreases and the relative Ca content increases.In addition,CaClOH in fly ash is reduced and converted into Ca(OH)2 after water washing,which improves the alkalinity and increases the ability of CO2 mineralization and sequestration.On the other hand,BET analysis results showed that the pore volume of fly ash increased from 0.0358 cm3/g to 0.0535 cm3/g after washing treatment,and the specific surface area also increased from 6.87 m2/g to 13.25 m2/g,the specific surface area increased by about 93%.The increase of pore volume and specific surface area can provide additional CO2 sequestration sites and internal channels for fly ash,which improves the CO2 mineralization and sequestration performance.Studies have shown that the alkali treatment-hydrothermal modification method can effectively increase the CO2 sequestration capacity of fly ash.When 8 g NaOH was added for alkali heat treatment,the product was aged at room temperature for 10 h,and hydrothermal reaction at 100℃ for 10 h was the best modified fly ash.The amount of CO2 mineralization and sequestration per kilogram of the best modified fly ash reached 500.1 g,which was about 4.6 times higher than that of the original fly ash.The results of SEM and BET showed that the specific surface area of the best modified fly ash was 26 m2/g,which was about 2.8 times higher than that of the original fly ash,and the dispersion was better.In addition,compared with the original fly ash,the leaching concentration of Pb in the modified fly ash decreased from 17 mg/L to below the landfill limit standard,which was based on the fact that Pb2+ in the fly ash replaced Ca2+ in the hydrated calcium silicate due to ion exchange and inhibited the dissolution of Pb.The results showed that the performance of CO2 mineralization and sequestration was successfully improved by modifying fly ash by alkali excitation-foaming combined treatment.When the ratio of fly ash:sodium silicate:sodium aluminate was 1:0.6:0.15,the alkali-activated alkali was activated,and 13 mL hydrogen peroxide and 2 mL oleic acid were slowly added for foaming treatment.Finally,the modified fly ash had the best ability to mineralize and sequestrate CO2.The amount of CO2 mineralized and sequestrated per kilogram of modified fly ash reached 419 g,and the leaching concentrations of heavy metals Pb and Cd were successfully reduced to below the landfill standard limit.