Microwave-Assisted Synthesis Of Functionalized 3D Silica Based Composites And The Adsorption Of Rhenium And Gallium

Mesoporous silica KIT-6(mSiO2)has high specific surface area,ordered and adjustable pore size.It is easy to modify and decorate due to the abundant silicon hydroxyl on the surface.Therefore,it is widely used in the fields of adsorption,catalysis drug transport and so on.At present,tetraethyl orthosilicate(TEOS),which hydrolysis rate slower,was generally used as the silicon source to prepare mesoporous silica,it will takes a long time on the large-scale preparation.And the silane coupling agents and organic reagents as functionalized reagent were commonly used in the modification of mesoporous silica,which was high cost and environmental pollution.Therefore,the application of cheap green biomass and mesoporous silica composites in adsorption fields has become a research hotspot.The preparation of mesoporous silica and its composites was carried out by the traditional hydrothermal synthesis method,with slow heating rate and long heating time.While the microwave-assisted synthesis method can make the materials heated quickly and uniformly in the synthesis,it also can improve the reaction rate and greatly shorten the reaction time.Therefore,in this paper,two kinds of functionalized 3D mesoporous silica composites were prepared by one-step method and post-grafting method,respectively.The structural properties of the materials by a series of characterization.The adsorption performance,adsorption selectivity and mechanism of the adsorbents for Re(?)and Ga(?)in single solution and mixed solution were analyzed.The specific research were as follows:The biomass chitosan modified mesoporous silica SS-KIT-6-M4-15-CS3 were formed by microwave-assisted synthesis.Using P123 as the template,cheap and fast hydrolysis of inorganic sodium silicate as the silicon source,non-toxic,low cost and active site more chitosan was added in the hydrolysis process of Na2SiO3,so that Si-OH could be combined with-OH on chitosan in the form of covalent bond and hydrogen bond.Then glutaraldehyde was added as the cross-linking agent to immobilize chitosan.The preparation process of the adsorbents was simple,environment friendly,and the synthesis speed was fast.The overall fabrication time was greatly reduced about 36 h than that of the common method.The results of adsorption experiments showed that the maximum adsorption capacity of Re(VII)by SS-KIT-6-M4-15-CS3 at pH 4 was 261.81 mg·g-1,which conformed to the Langmuir adsorption isothermal model and pseudo-second-order kinetic model.It was superior to most reported adsorption materials.FT-IR spectroscopy and XPS analysis suggested that the possible mechanism of this adsorption process was that the amino group on chitosan and the aldehyde group on glutaraldehyde reacted to form-C=N-,it was chelated with ReO4-to form a stable chelate.Through the experiments of selectivity and simulated material liquid,the adsorbent had a good adsorption selectivity to Re(?)in the mixed solution of Cu(?),Zn(?)and Mn(?).After eight cycles of adsorption-desorption experiments,the adsorbent still had good adsorption performance for Re(?),indicating that the adsorbent had good stability and cycling performance.The method provided a theoretical basis for the feasibility of adsorbent in practical application.According to the cationic existence and adsorption characteristics of gallium,salicylaldoxime/polydopamine/3D mesoporous silica composites were prepared by post grafting.Firstly,KIT-6 was prepared by microwave-assisted synthesis,and salicylaldoxime was loaded on the surface of KIT-6 by dopamine with the ability of fast polymerization and adhesion to to obtain the mesoporous silica material SO200/PDA50-KIT-6 modified by salicylaldoxime.The results of small-angle XRD and N2 adsorption analysis showed that the ordered mesoporous structure of KIT-6 was well preserved by the post-grafting method.With the amount of dopamine and salicylaldoxime increased,the order of materials reduced gradually,specific surface area,total pore volume and pore diameter were also reduced to 94 m2·g-1?0.14 cm3·g-1?5.9 nm after adding to 50 mg DA and 300 mg SO.It showed that the DA and SO successfully entered the mesoporous silica channel and made its pore size smaller.When 200 mg salicylaldoxime was added,the specific surface area,total pore volume and pore size of sample SO200/PDA50-KIT-6 were 171 m2·g-1,0.15 cm3·g-1,6.7nm,respectively.By titration of surface functional groups,the adsorbent SO200/PDA50-KIT-6 had the highest hydroxyl content on the surface(3.30 mmol·g-1).Therefore,when the mass ratio of KIT-6/dopamine/salicylaldoxime was 1:1:4,the pore size of SO200/PDA50-KIT-6 was 6.7 nm,and the active sites exposed on the surface were the most.According to the adsorption experiment,at pH 10,the maximum adsorption capacity of SO200/PDA50-KIT-6 for Ga(?)at pH 10 was 136.40 mg·g-1,which conformed to the Langmuir adsorption isothermal model and pseudo-second-order kinetic model.SO200/PDA50-KIT-6 had good selectivity for Ga(?).In the presence of high concentration of germanium and arsenic,SO200/PDA50-KIT-6 still can achieve an effective separation for Ga(?).FT-IR analysis suggested that the adsorption mechanism of adsorbent for Ga(?)was the ion exchange and chelation between the phenol hydroxyl and oxime functional groups in salicylaldoxime and Ga(OH)4-.The adsorbent SO200/PDA50-KIT-6 had good stability and cycling ability after six cycles of adsorption-desorption.In summary,the method discussed in this paper can be used to prepare adsorbents with high efficiency,fast,simple and low cost,it has a good prospect for practical application.