Visible Light Catalytic Removal Of Hexavalent Chromium And Methylene Blue By Silver-loaded Titanium Dioxide Nanotubes

With the accelerated development of global industrialization,chemical products have become diversified and complex in composition,which not only brings great convenience and economic benefits to people,but also brings environmental pollution problems.A series of chemical wastewater with complex composition,high concentration of pollutants,toxic and harmful and difficult to degrade is produced in the production process of chemical products,especially in tanning and printing and dyeing industries.The wastewater discharged contains high organic dyes and heavy metal ions,which is difficult to be biodegradable by traditional water treatment methods.As a kind of green chemistry technology,photocatalysis has become a rapidly developing new research field in recent years.In the face of wastewater that is difficult to be treated by biochemistry,photocatalysis technology has obvious advantages.Organic pollutants and heavy metals discharged from the chemical industry can be removed by photocatalytic reaction.Therefore,the study on the degradation of complex mixed pollutants by light energy has received extensive attention in recent years.TiO₂ is favored by photocatalytic researchers due to its non-toxic,stable and other advantages.However,in the practical application process,there are still many problems,such as low sunlight utilization efficiency,catalyst easy deactivation and so on.In many modification methods of titanium dioxide,deposit is a kind of precious metals nano surface to improve the performance of titanium dioxide photocatalysis effective modification methods,such as noble metal nanostructures used from excimer effect,and of the titanium dioxide from excimer nano light catalyst not only has higher photocatalytic activity,and can also drive in visible light catalytic reaction.In view of the shortcomings and problems existing in the current single TiO₂photocatalytic technology,this paper explores the synergistic removal of organic compounds and heavy metals from water by the synthesis of silver-laden TiO₂nanotubes and photocatalysis.The main work is as follows:（1）Synthesis and characterization of silver-loaded titanium dioxide nanotubes（Ag/TNTs）.Firstly,TiO₂ nanotubes（TNTs）were prepared by hydrothermal synthesis using rutile TiO₂ nanoparticles as raw materials under alkaline conditions of high temperature and high pressure.Secondly,the isoplason composite photocatalytic materials（Ag/TNTs）were synthesized by loading the precious metal Ag on TNTs by the low temperature multi-alcohol oscillation method.Finally,the morphology,crystal structure,composition and light response of TNTs and Ag/TNTs were characterized by SEM,TEM,XRD,EDS and UV-Vis.The characterization results showed that both TNTs and Ag/TNTs were successfully synthesized.In Ag/TNTs,Ag was uniformly distributed on the surface of the TNTs.After loading with Ag,the diameter and size of the TNTs did not change.（2）Experimental study on the separate removal of methylene blue and hexavalent chromium with different Ag carrying ratio catalysts.According to the molar ratio of Ag/Ti（0,0.25%,0.5%,1%,2%）,five kinds of composite photocatalysts containing different Ag ratios were synthesized respectively.Using methylene blue dye and hexavalent chromium ions as simulated pollutants,the removal of methylene blue and hexavalent chromium by five catalysts was studied under UV and visible light respectively.The results show that the visible light catalytic effect of the non-Ag/TNTs is poor,and the removal effect of the synthesized Ag/TNTs under visible light is more than 20%higher than that of the non-Ag nanotubes.The results showed that 0.5%Ag/TNTs could remove methylene blue and hexavalent chromium separately under UV and visible light conditions.Therefore,0.5%was determined to be the most suitable Ag carrying ratio for the synthesis of composite photocatalytic materials.（3）Experimental study on the synergistic removal of methylene blue and hexavalent chromium by Ag/TNTs catalyzed by visible light.0.5%Ag/TNTs were selected as the catalyst,and the photocatalytic reaction conditions（dosage of catalyst,p H value of solution,light intensity）were controlled to investigate the composite pollution removal and stability of methylene blue and hexavalent chromium under visible light.The results show that the combined treatment efficiency of methylene blue and hexavalent chromium under visible light is better than that of methylene blue and hexavalent chromium treated by the catalyst alone.This is mainly due to the synergistic effect of organic matter and heavy metal photocatalytic reaction.In the photocatalytic reaction,organic matter occupies electron holes while heavy metal occupies electrons.Simultaneous catalytic degradation of organic matter and heavy metal reduces the recombination of electron hole pairs and improves the catalytic efficiency of photocatalytic reaction.In addition,after five times of reuse,the catalytic efficiency of Ag/TNTs decreased from 80.10%to 72.42%（overall decreased by 7.68%）,which indicated that the catalyst had strong stability.（4）Orthogonal test analysis and process parameter optimization of Ag/TNTs in photocatalysis.On the basis of single factor experiment,L₂₅（5³）orthogonal experimental design was adopted to explore the significance analysis of the influence of three factors,including the dosage of catalyst,p H value of solution and illumination intensity,on the effect of pollutant removal rate under the condition of constant pollutant concentration.The results showed that the order of influence of the three factors on the removal rate was the dosage of catalyst>p H value>illumination intensity;among them,the dosage of catalyst and p H value had significant effects on the removal rate,while the light intensity had no significant effect on the removal rate.Taking into account the actual catalyst cost and p H value of the solution,four groups of relatively low cost process combinations were selected.Through experimental verification,the most appropriate process combinations were obtained:catalyst amount was 0.8g/L,light intensity was 6300Lux,p H=3 and the comprehensive photocatalytic removal rate was 91.08%under these conditions.（5）Feasibility analysis and cost accounting of silver-loaded titanium dioxide nanotubes for wastewater degradation.The operating cost and operating cost of0.5%Ag/TNTs isoplason photocatalyst were calculated.It was found that the cost of the synthesized catalyst to treat one ton of wastewater was about 2.8 yuan on average.The feasibility of Ag/TNTs in the treatment of chemical wastewater was discussed and analyzed.