| Hexavalent chromium(Cr(VI))is a highly toxic heavy metal pollutant with teratogenic,carcinogenic and mutagenic effects.It has been listed as one of the five key prevention and control objects in the 12th Five-Year Plan.Heavy metal pollution incidents occur frequently in recent years.The treatment of water body Cr(VI)is urgent.mesoporous materials have excellent properties as adsorbents,but the traditional mesoporous materials are mostly powder particles,which have high synthesis cost,complex preparation process,easy agglomeration and high flow resistance in the adsorption process,it is difficult to realize secondary recovery and reuse.However,large particle size mesoporous silica(particle size>1 mm)is non-toxic and pollution-free,with wide sources and high porosity.In the process of pseudomorphic transformation,the silicon source can be dissolved by using alkaline solution and precipitated in situ around the surfactant micelles.The ordered mesoporous template SiO2 particles with hexagonal and cubic symmetrical structure and uniform pore size are formed,which reduce the cost of silicon source,and are easy to prepare,the pore size is adjustable,and the structure is stable.Therefore,it is widely used in the treatment of heavy metals in water.m-phenylenediamine and oxidant(such as sodium persulfate)can be oxidized and polymerized to form poly(m-phenylenediamine)(PmPD),which self-polymerizes on the surface of silica spheres to increase the number of active sites;the macromolecular chain of polyethylenediamine(PEI)contains a large number of primary,secondary and tertiary amino functional groups,which can bind to heavy metals by means of electrostatic,ion exchange,chelation and coordination.The mesoporous silicon materials synthesized by pseudomorphic transformation can further increase the active sites after grafting the functional groups such as sulfhydryl and amino groups and make them react with heavy metals in water to improve the adsorption properties of Cr(VI).In this paper,two kinds of particle size(1.5-2.5mm and 2.5-4.0mm)commercial silica sphere were used as silicon source,and the series of alkyl ammonium bromide as template,different preparation methods and modification methods were adopted.It was found that the optimal condition was as follows:nsiO2:nNaOH:ncTAB:nH2O=1:0.1:0.27:140,VH2O:Vethanol=302:75.6.and there are a large number of wormlike mesoporous channels in the materials,and the specific surface area of parent silicon can be increased to 567.018 m2/g.The specific surface of the materials treated by hydrothermal treatment have improved better from 268.449 m2/g to 717.849 m2/g(1.5-2.5mm parent silica spheres)and from 344.855 m2/g to 629.465 m2/g(2.5-4.0 mm parent silica spheres),meanwhile its pore diameter adjusted from 12.045 nm to 3.027 nm and 3.390 mm respectively.mPD and PEI were used to modify 1.5-2.5mm mesoporous silica spheres.It was found that the best experimental temperature for mPD modification was in ice bath at 0℃;the highest Cr(VI)removal rate was about 93%when the concentration of PEI methanol solution is 30 g/L and the optimum molecular weight of PEI is 10000.It was found by FT-IR that the amino functional groups had been successfully grafted into the pore interior of the material.In the study of static adsorption properties,it was found that the adsorption efficiency of Cr(VI)was the best in the strong acid condition,its main form was HCrO4-in this moment,the surface of adsorbent was protonated with positive charge and Cr(VI)was removed by electrostatic action.The adsorption performance of mesoporous silica spheres of small particles(1.5-2.5mm)is much higher than that of large particles(2.5-4.0mm)in static adsorption.And for large mesoporous silica spheres,the best adsorption effect is obtained when the template agent is CTAB and hydrothermal synthesis in 110 ℃.While in dynamic adsorption,the mesoporous silica spheres of large particles are superior to those of small particles,which could maintain low concentration for a long time.Three adsorption isotherms and five adsorption kinetics models of Langmuir,Freundlich,Tempkin and pseudo-first order,pseudo-second order,Elovich,intraparticle diffusion,bangham diffusion were carried out for the materials of MSSl60℃ CTAB,MSSl110℃ CTAB and MSSs-27 respectively.It is found that the adsorption process is in accordance with the Langmuir model and R2 is all above 0.97,indicating that the adsorption process is monolayer adsorption.Comparing the R2 values derived from models,it is found that the pseudo-second-order kinetic model fits better with the kinetic data,R2>0.98,implying the complexity of the adsorption process and more inclined to chemical adsorption.The regression coefficients of the internal particle diffusion model to the adsorption process of the three materials have reached more than 0.94,showing a good correlation.The Elovich model can also fit the adsorption process of the three materials with R2>0.97.Increasing the temperature can improve the adsorption effect of Cr(VI)on these materials,but the effect is not obvious.In thermodynamic studies,the correlation coefficients of the three materials are R2=0.77,R2=0.94 and R2=0.64 respectively.The fitting effect of MSSl110℃ CTAB is the best.The values of ΔH0 of the three kinds of mesoporous silicon spheres are 21.369,27.909 and 16.911 KJ/mol,respectively,and the positive values of ΔH0 show that the adsorption process of the three kinds of materials is endothermic process.MSSl110℃ CTAB has the largest values ofΔH0,which indicates that the adsorption capacity of Cr(VI)increases with the temperature improved.ΔS0>0 show that the adsorption process is a process of entropy increase,and the degree of confusion increases.ΔG0 is a negative value,which indicates that the reaction process is a spontaneous reaction.The regeneration of adsorbent is better when NaOH is used as desorption solution.The removal of Cr(VI)was about 60%by the mesoporous silica spheres with small particles and 40%by the mesoporous silica spheres with large particles,which was basically consistent with the adsorption effect of the materials before desorption.And the adsorption effect is better after three cycles under alkaline condition.The dynamic adsorption process is affected by particle size,flow rate and pH value.The higher the flow rate,the lower the adsorption amount of Cr(VI).After 78 h of dynamic adsorption,the adsorption capacities of samples MSSl60℃ CTAB、MSSl110℃ CTAB and MSSs-27 on Cr(VI)were reduced from 14.596,17.691 and 12.572mg/g to 11.600,12.099 and 6.395mg/g,respectively,in the process of raising the flow rate from 2.2 mL/min to 10 m/min,respectively,indicating that the suitable flow rate can ensure the good adsorption performance of the adsorbent.The adsorption capacity of adsorbent in dynamic experiment is generally lower than that in static experiment,it may owing to the short contact time between adsorbent and Cr(VI)and the difficulty of reaching adsorption equilibrium,which indicates that the flow rate will affect the residence time of liquid in the adsorption process,so the adsorption performance of the adsorbent was affected. |
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