Preparation Of Mesoporous Silica Composites And Study On Chromium Removal Properties

Chromium is an important strategic metal element,and exists mainly in the form of trivalent chromium（Cr（Ⅲ））and hexavalent chromium（Cr（Ⅵ））.Cr（Ⅲ）is toxic in a low and minor Cr（Ⅲ）is beneficial to human health.However,Cr（Ⅵ）is a strong carcinogenic,mutagenic and teratogenic substance.A large amount of chromium-containing wastewater is discharged in the industrial production process such as chromium salt chemical industry,metallurgy,electroplating,leather and so on,which is seriously harmful to human health and the environment.According to the wastewater discharge standard,the total chromium content in the water environment should be controlled within 1.5 mg/L.Therefore,it is urgent to treat Cr（Ⅵ）and Cr（Ⅲ）in the environment.In this study,mesoporous silica composites with pH response and light response were prepared by designing and regulating the pore structure and surface properties,and then they were applied to remove Cr（Ⅵ）and Cr（Ⅲ）in water environment.The adsorption and photocatalytic capacities of the composites were systematically studied by adjusting the pH,time and initial concentration of solution.Finally,the cycle performance and removal mechanism of composites were also investigated.The specific research work is as follows:（1）The double bond modified MCM-41（AM41）was prepared by in-situ method,and then the pore of AM41 was modified by 2-vinylpyridine（2-VP）to prepare poly（2-vinylpyridine）/MCM-41（PM41）composites with pH response characteristics.The PM41 nanoparticles presented spherical with the diameter of 200～300 nm.Compared with AM41,the introduction of poly（2-vinylpyridine）（P2VP）made the pore size of AM41 reduce from 2.7 nm to 1.3 nm,closer to the diameter of Cr（Ⅵ）（0.86 nm）.The effects of different conditions on the adsorption performances were investigated for the removal of Cr（Ⅵ）by PM41.PM41 completely removed Cr（Ⅵ）from the solution at the pH of 3.And during Cr（Ⅵ）adsorption,PM41 exhibited the maximum adsorption quantity of 24.5 mg/g.After 3 cycles,PM41 showed good adsorption and recycling performance.Electrostatic interaction and confinement effect were the main interaction for the adsorption Cr（Ⅵ）by PM41.In addition,pyridine N on P2VP could reduce some Cr（Ⅵ）adsorbed on PM41.（2）ZnO modified SBA-15 nanorods（ZnO/SBA）were prepared by soft template method and immersion method.Then ZnIn₂S₄/SBA-15（ZIS-SBA）was prepared by in-situ method with ZnO as Zn source,InCl₃ as In source and thioacetamide as S source.The ZnO was located on the outside surface of SBA-15 and the specific surface area of 60%ZnO/SBA was 108.1 m²/g,smaller than that of SBA-15（859.12 m²/g）.The specific surface area of 60%ZIS-SBA obtained by in-situ growth of ZnIn₂S₄ was 134.76 m²/g,close to that of 60%ZnO/SBA and about 4 times that of ZnIn₂S₄（34.34 m²/g）.The Cr（Ⅵ）solution with a concentration of 50 mg/L were completely removed by 60%ZIS-SBA,which was twice that of ZnIn₂S₄.More importantly,92.8%of Cr（Ⅲ）in the solution was trapped by 60%ZIS-SBA,reducing the total chromium content in the solution in one step.In addition,60%ZIS-SBA showed good reusability during the photocatalytic reduction process.（3）Amine-modified SBA-15（NS-15）was prepared by post-modification method,and then carboxyl spiropyran（SP-COOH）was grafted onto NS-15 by amidation reaction to prepare SPNS-15.Compared with SBA-15,the introduction of amine groups and the grafting of SP-COOH had no obvious effect on the morphology of SBA-15,but had a great influence on its pore structure.The introduction of both made the specific surface area of SBA-15 reduce by 62.8%and 66.8%,respectively,and the pore size decreased from 6.94 nm to 5.69 nm and 5.10 nm,indicating that amine groups and SP-COOH located in the inner the pores and outer surfaces of SBA-15.Under ultraviolet light irradiation,the removal rate of Cr（Ⅲ）by SPNS-15 was 100%within 10 min.Under visible light irradiation,Cr（Ⅲ）desorbed from the adsorbent,and the removal rate of Cr（Ⅲ）by SPNS-15 was 87%after repeated UV-visible light irradiation for 3 times.Under the irradiation of ultraviolet light,the SP-COOH in SPNS-15 presented ionic state,which had electrostatic interaction with positively charged Cr（Ⅲ）so it showed a good capture to Cr（Ⅲ）.On the contrast,SP-COOH appeared molecular state under visible light,meaning that it was electrically neutral,and SP-COOH had no interaction with Cr（Ⅲ）so that Cr（Ⅲ）desorbed from SPNS-15.