Preparation Of SiO₂ Aerogels And TiO₂/SiO₂ Photocatalytic Materials Derived From Acid-extraction Residues Of Fly Ash

The chemical composition,annual emissions and stockpile of fly ash determine its important position in the field of renewable resources.The extraction of alumina from high-alumina fly ash is a major model for its high value utilization,but this pattern will produce a lot of waste residue.If these waste residue dipose improperly,it will not only produce secondary pollution of solid waste,but also waste a lot of silicon components,which can not really use of fly ash efficiently.Based on the urgent need of environmental protection and resource development,the application of extraction of aluminium acid residue from fly ash is expanded from the two aspects of pore material and photocatalytic material in this paper,so as to provide theoretical and data support for the subsequent research and industrial production.In view of the characteristics of high content of amorphous SiO₂ in acidextraction residues,preparing sodium silicate solution using alkali dissolving process,and providing silicious material for later experiments.According to the Box-Behnken experimental design method,the process conditions are optimized with the highest SiO₂ extraction efficiency as the goal.The results show that Box-Behnken experimental design method is reliable for preparing sodium silicate solution.It can effectively predict the influence of reaction temperature,reaction time and NaOH concentration on the extraction efficiency of SiO₂.The significant relationship from high to low is the reaction temperature,reaction time,and NaOH concentration.The optimal solution is NaOH concentration of 15 wt.%,reaction time of 2 h,and reaction temperature of 90 °C,and the corresponding SiO₂ extraction efficiency is 73.95±0.65 %,the concentration of sodium silicate solution is 79.63±0.69 g/l.SiO₂ aerogels have a wide range of applications in the field of thermal,acoustic and environmental protection.In this paper,porous hydrophobic SiO₂ aerogels are prepared by solvent exchange/surface modification and ambient pressure drying using sodium silicate solution.Firstly,the effect of surface modification on the preparationof aerogel is analyzed.The results indicate that the surface modification process makes the gel reversible shrink in drying process,which is the key to prepare porous and low density aerogel.Then,the influence of the molar ratio of silica/water and pH value on the gel time of the sol and the density of the aerogel has been investigated.The results show that when the molar ratio of silica/water and pH value are 0.021 and5⁶ respectively,gel time is short,the prepared aerogel has a low density.On the basis of this,the effects of the dosage of modifier on the structure and properties of aerogel has been studied.The results reveal that when the volume ratio of the modifier and hydrogel is 0.7¹,the microstructure of the aerogels is loose and uniform,the density is lower than 0.076 g/cm3,the porosity is more than 96.53 %,the specific surface area can be up to 731⁸39 m2/g,showing strong hydrophobic.This hydrophobicity can be maintained at room temperature,if the heat treatment temperature rises up to 370 °C,silica aerogels show hydrophilic.Photocatalytic materials are widely used in the field of environmental protection because of their advantages of simple operation,no two pollution,high efficiency,reusable and low cost.In order to further explore the potential value of acid-extraction residues,the TiO₂/SiO₂ photocatalytic materials are prepared by using sodium silicate solution as the silicon source and titanium sulfate as the titanium source.Then under irradiation with and without 250 W high pressure mercury lamp,the decolorization performance of the composite catalyst is investigated using organic pollutant rhodamine B as target degradation agent.The decolorization effect of sample is the result of photocatalytic degradation and adsorption.On the one hand,TiO₂/SiO₂ composite powders are synthesized by sol-gel and calcination process using sodium silicate solution and Ti?SO4?2 as raw materials.The results of light experiment show that when the molar ratio of silica/water is 0.022,aging temperature is 80 °C,TiO₂ content is 40 % and calcination temperature is600 °C,the TiO₂/SiO₂ powder has the best effect on the decolorization of rhodamine B.On the other hand,TiO₂/Si O₂ composite aerogels are prepared by adding surface modification to the above process using the same source.Due to the pore structure and hydrophobicity of aerogel and the crystal form and degradation properties of TiO₂ changed in the calcination process,the influence of calcination temperature on the photocatalysis and adsorption of TiO₂/SiO₂ composite aerogel has been studied emphatically.The results indicate that the composite aerogel with the irradiation time of less than 4 h and the calcination temperature of 400 °C shows good decolorization performancefor rhodamine B;more than 4 h,the composite aerogel calcined at600 °C has highest decolorization rate.The structure and properties of the composite catalyst are analyzed,it is confirmed that the combination of TiO₂ and SiO₂ have formed Si-O-Ti bond,significantly improved the thermal stability of TiO₂,prevented the growth of TiO₂ crystal grains,increased the specific surface area of the product,which is helpful to enhance the photocatalytic oxidation and adsorption properties of the composite powders.Comparing the two kinds of composite catalysts,when the irradiation time is short,the photocatalytic degradation of TiO₂/SiO₂ powder is faster,and the adsorption of TiO₂/SiO₂ aerogel is stronger;the decolorization rate of two composite catalysts for rhodamine B is more than 95%,when the irradiation time was 8 h.The reproducibility of TiO₂/SiO₂ powder and aerogel prove the repeatability of catalyst.Composite photocatalysts are prepared by hydrolysis-calcination method using Ti?SO4?2 and silica wet gel as TiO₂ carrier.The corresponding test results show that the addition of silica wet gel can improve the transition temperature of anatase to rutile,slow down the growth rate of TiO₂ grains and increase the specific surface area of the product;the chemical action between TiO₂ and carrier silica wet gel occurred to form Ti-O-Si bond.These factors create a composite photocatalyst with high decoloring performance.The results of photocatalytic degradation and adsorption indicate that the TiO₂/silica wet gel catalyst has the best decolorization effect for Rhodamine B and the decolorization rate is as high as 96 %,when the hydrolysis pH is 4,hydrolysis temperature is 80 °C,TiO₂ loading is 40 % and calcination temperature is 600 °C.The repeatability of the catalyst shows that the prepared composite photocatalyst can be recycled and reused.After three recycle,the decolorization rate reach to 91%.Further inquiry shows that more than 8 hours of light,not only thetarget degradation dyes are nearly completely decolorized,but also the surface of TiO₂/SiO₂ composite powder,composite aerogel and TiO₂/silica wet gel photocatalyst are no obvious staining,which proves that three kinds of composite catalysts can achieve the goal of completely degrading the dyes,and their adsorption properties can improve the photocatalytic degradation efficiency.