| With the development of the textile,printing and dyeing industries,the emission of organic dyes has attracted widespread attention.Organic dyes are not only harmful to the environment but also hazardous to aquatic life as well as humans,because of their potential toxicity,bioaccumulative,deep colors,complex components,hard-to-degrade,large emissions and wide distribution.Therefore,it is very meaningful to remove organic dyes from wastewater.In this study,the SiO2-ZrO2 composite aerogel with high specific surface area and high pore volume was selected as an adsorbent,and the organic dyes were removed from the aqueo us solution through adsorption for the purpose of purifying the wastewater.SiO2-ZrO2 aerogel was prepared via a sol-gel method followed by ambient pressure drying.In order to analyze the effect of Si/Zr molar ratio,PEG-400 addition amount,heat treatment and other conditions on the aerogel preparation and structure,the aerogel was characterized by X-ray diffraction,thermal analysis,fourier transform infrared spectroscopy,scanning electron microscopy,and nitrogen adsorption/desorption analysis.The effects of adsorption conditions,such as pH,aerogel dose,adsorption time and initial dye concentration,on the adsorption of rhodamine B and methylene blue by aerogel were studied.Pseudo-first-order kinetic equations,pseudo-second-order kinetic equations,Elovich equations and Webber-Morris kinetics models were used to analyze the adsorption kinetics.The adsorption isotherms were fitted using Langmuir and Freundlich isotherm models.And the adsorption thermodynamics were studied.The optimized preparation process of SiO2-ZrO2 composite aerogel is as follows:the volume ratio of alcohol to water is 3:1,the concentration of zirconyl nitrate in the aqueous alcohol solution is 0.1 mol/L,the concentration of ethyl orthosilicate in the aqueous alcohol solution is 0.3 mol/L,the molar ratio of precursor?zirconyl nitrate and tetraethylorthosilicate are used as precursors together?:formamide:propylene oxide is 1:1:6,the concentration of PEG-400 in the aqueous alcohol solution is 15 ml/L and the drying temperature is 60?.The prepared SiO2-ZrO2 composite aerogel is amorphous with a BET specific surface area of 336.8 m2/g and a pore volume of 0.74 cm3/g,which is a typical porous material.After heat treatment at 600?,the aerogel retains the amorphous s tructure.At the same time,its pore volume increases to 1.16 cm3/g and its BET specific surface area increases to 525.6 m2/g due to the removal of organics remaining in the aerogel.Adsorption thermodynamic studies indicate that the adsorption processes of Rhodamine B and methylene blue onto SiO2-ZrO2 composite aerogels are feasible and spontaneous in nature.Both of the adsorption processes are entropy reduction and exothermic processes,and increasing the temperature is not conducive to adsorption.Adsorption kinetics studies show that the adsorption of Rhodamine B and methylene blue onto SiO2-ZrO2 composite aerogel mainly depends on the physical adsorption.The adsorption rate at room temperature is mainly controlled by two processes of surface diffusion and intraparticle diffusion,which are determined by the square of the number of unoccupied adsorption sites.The adsorption rate of Rhodamine B and methylene blue onto SiO2-ZrO2 composite aerogel is fast,and adsorption equilibrium can be achieved in 30-50 min.When pH?4,the adsorption of Rhodamine B onto aerogel is facilitated.And the adsorption of methylene blue onto aerogel is facilitated when pH>5.The adsorption isotherms of aerogels for the two dyes are in line with the Langmuir adsorption isotherm model,which reveals that the aerogel surface is uniform and the adsorption of both dyes onto SiO2-ZrO2composite aerogel is monolayer adsorption.The adsorption capacity of rhodamine B and methylene blue onto SiO2-ZrO2 composite aerogel are 177.7 and 63.13 mg/g,respectively.SiO2-ZrO2 composite aerogel is expected to be used as an adsorbent for the treatment of printing and dyeing wastewater. |
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