| Fly ash is a kind of solid waste from coal-fired power plants with huge emissions,which will lead to the pollution of air,soil and groundwater.Currently,it is one of the most important solid wastes to be treated in China.In 2018,the utilization rate of fly ash in China was 71%.However,the utilization approaches are mainly concentrated in primary and low value fields such as construction and production backfill.A kind of high-alumina coal resource exists in Inner Mongolia,Shanxi and other places in China,from which the alumina content of the fly ash is more than 40%.Thus,this high-alumina fly ash is a potential aluminum-silicon resource.On the other hand,cadmium in lead and zinc smelting wastewater is a typical toxic heavy metal pollutant for environment,which can be effectively removed from water by adsorption method.The adsorbent is crucial,and the mesoporous materials are the good choices.But the high price of raw materials to prepare mesoporous materials limits their wide application in the field of adsorption,thus the cheap raw materials are highly pursuited.Based on the above situations,the high-alumina fly ash has been used as the cheap silicon/aluminum resources to prepare mesoporous alumina/silica materials to remove the Cd(?)from water in this work.Aluminum and silicon elements in fly ash mainly exist in mullite phase(Al6Si2O13)with very stable chemical properties.Alkali fusion method can effectively activate fly ash and convert the mullite phase into sodium silicate and sodium feldspar,which are easy to react with acid.Thus,sodium carbonate was used as an activator to activated the coal ash from Inner Mongolia high alumina.Then,the activated fly ash was treated by acid leaching to extract the silicon and aluminum resources to prepare the mesoporous alumina,silica and silica-aluminum adsorbents.Finally,the above three kinds of mesoporous materials were used as adsorbents to remove Cd(?)from water.Combining with the characterization of the surface properties,such as the micromorphology,phase structure,pore size distribution and so on,some conclusions have been obtained:(1)Using HNO3/HCl solution to extract aluminum from activated fly ash,and then the mesoporous alumina materials were prepared by coprecipitation method with P123 as template agent.As the conditions were the same,the mesoporous alumina from HNO3 solution leaching owned the better adsorption efficiency of Cd(?)than that from HCl solution leaching.The effects of calcinations temperature had been investigated,and it was found that 600?of calcination temperature was the best.The mesoporous alumina adsorbent calcinated at 600?from HNO3solution leaching(N/Al2O3-600)achieved 99.88%of the adsorption rate within 15 min(initial Cd(?)concentration of 20 mg/L),25 mg/g of adsorption capacity,and can be used in wider range of p H(2?7).The adsorption kinetics data were better fitted by a pseudo-second-order model,which indicated the chemical adsorption process.Characterization results showed that the specific surface area and average pore of N/Al2O3-600 were 201 m2/g and 6.5 nm,respectively with the order mesoporous channels.It was found that many elements such as Fe,Ca and Ti were also distributed on the materials,due to the retaining effect from fly ash.After the adsorption of Cd(?),the changes in the chemical shift of Al,Fe,Ti had been observed,suggesting so that Al2O3,Fe2O3and Ti2O3 were the active species for Cd(?)adsorption.(2)Mesoporous silica materials with large specific surface area were synthesized by hydrothermal method using the silica filter residue obtained after aluminum extraction by HNO3as the silicon source and CTAB as template agent.The effect of crystallization temperature was investigated and it was found that 90?of crystallization temperature was the optimal.Under this temperature,the prepared mesoporous silica(Si O2-90)achieved 99.48%of the adsorption rate within 30 minutes,and 42 mg/g of the adsorption capacity.Si O2-90 got the excellent removal efficiency in the wider range of p H(3?7).Also,the adsorption kinetics data were better fitted by a pseudo-second-order model.From the characterization results,Si O2-90 is spongy,with the specific surface area of 535 m2/g and 1.0 cm3/g pore volume.As well,the Al,Ti and other species contribute to the adsorption of Cd(?).(3)The activated fly ash was firstly treated with hydrochloric acid,then P123 was added to the solution to interact with Al,and mesoporous silica-aluminum composite adsorbent was prepared by precipitation of alumina on silica.The effect of the amount of HCl added was investigated,and it was found that the silica-alumina composite adsorbent(Si-Al-10),when the amount of HCl was 10 ml,exhibited the best removal efficiency,with 99.63%of the adsorption rate in 30 minutes and 23.6 mg/g of the adsorption capacity.The best operating range of p H was3?7.And the adsorption kinetics data were better fitted by a pseudo-second-order model.Si-Al-10 had a specific surface area of 192 m2/g,and the obvious pore structure could be observed on the pitted surface of Si-Al-10.The main components of Si-Al-10 were alumina and silica,and there was also albite phase,which indicated the strong interaction between alumina and silica.In conclusion,the three kinds of mesoporous materials,prepared from high-alumina coal ash,can remove effectively Cd(?)from the water,with the fast adsorption rate,high adsorption efficiency and wide application range of p H.Especially,the adsorption effect of Cd(?)is very good for the low Cd(?)wastewater.Thus,these three kinds of mesoporous materials will have good prospects in application. |
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