Photocatalytic Reduction Of Hexavalent Chromium And Degradation Of Organic Pollutants By MOFs-Ag-based Composites

At present,the deterioration of water quality aggravates the situation of water resources shortage,and seriously threatens the safety of drinking water and the health of ecological environment.Among them,all kinds of organic dyes and heavy metals contained in the wastewater pose a serious threat to human health and the environment.The common methods used for wastewater treatme nt include coagulation flocculation,biodegradation,chlorination,advanced oxidation process and ozone oxidation.While they play a positive role in environmental treatment,they also have some disadvantages,such as high energy consumption,new toxic by-products and regeneration difficulties.Photocatalytic technology has been widely recognized as an environment-friendly purification technology.Photocatalyst can directly oxidize or reduce pollutants under visible light or ultraviolet light,and even degrade organic pollutants into carbon dioxide and water completely.And the photocatalyst itself has no loss in this process.The photocatalytic technology has simple operation and high reaction efficiency.It can directly use the light source to save energy consumption,no secondary pollution problems,and fast response,and can complete the reaction in a few hours or even seconds.At the same time,photocatalysis technology can also reduce the toxic pollutant Cr???with carcinogenic and mutagenic effects to non-toxic Cr?III?.At present,the common photocatalysts such as tungsten trioxide,zinc oxide and titanium dioxide have the problems of low utilization of visible light,easy recombination of photogenerated electrons and holes.It is very important to find a photocatalyst material excited by visible light and able to promote the separation of photogenerated electrons and holes for degradation of pollutants in the environment.In this paper,based on the reduction of heavy metal Cr???and the degradation of organic pollutants,a series of MOFs/Ag composite were prepared to investigate the performance of the photocatalytic reduction Cr???and oxidative degradation organic pollutants under visible light irradiation.And the influencing factors of photocatalytic reaction,the stability of reusability and the mechanism of photocatalytic reaction were explored.The main research work of this paper were as follows:1.A series of UiO-66-NH₂/Ag₂CO₃ composite were prepared by ion-exchange solution method with the classical metal organic framework material UiO-66-NH₂ and inorganic semiconductor material silver carbonate?Ag₂CO₃?as raw materials.Its structure and morphology were characterized by a series of characterization methods.The photocatalytic activity of UAC-100?mass ratio of UiO-66-NH₂:Ag₂CO₃ is 1:1?for Cr???reduction was studied under visible light.The effects of various organic compounds?tartaric acid,citric acid and oxalic acid?as hole trapper,pH value?2,3,4,6 and 8?and foreign ions?ions from tap water and lake water?on Cr???reduction were investigated.The results showed that the photocatalytic reduction of Cr???by UAC-100 was better than that by pure UiO-66-NH₂ and Ag₂CO₃,which was confirmed by photoluminescence and electrochemical impedance spectroscopy.UAC-100 also was used to degrade organic pollutants?Rhodamine B,methylene blue,methyl orange and Congo red?.In addition,the photocatalytic activity of UAC-100 on the mixture of Cr???and methylene blue under visible light was studied.Compared with the photocatalytic performance of the single component system,the degradation efficiency of Cr??? and organic dyes in the mixed system was inhibited due to the competition between them.The experiment of photocatalytic reduction Cr???cycle showd that UAC-100 as photocatalyst had good reusability and stability.The mechanism of photocatalytic reduction of Cr???and degradation of organic dyes was proposed.The mechanism was verified by electrochemical characterization,photoluminescence technology,active material capture experiment and ESR spectrum.2.Ag₃PO₄/MIL-125-NH₂?AMN-X?was prepared by ion-exchange method from the classical metal organic framework material MIL-125-NH₂ and inorganic semiconductor material silver phosphate?Ag₃PO₄?.Ag/Ag₃PO₄/MIL-125-NH₂?AAMN-X?composite was prepared by photochemical reduction deposition method.Its morphology and structure were characterized by a series of characterization methods.The photocatalytic activity of AAMN-120 and AMN-120?the mass ratio of MIL-125-NH₂:AgNO₃ is 1.2:1?for Cr???reduction under visible light were studied.It was found that the Ag nanoparticles in AAMN-120,as the electron transfer medium,promoted the effective separation of photogenerated electrons and holes,thus promoting the photocatalytic efficiency of AAMN-120 better than that of AMN-120,MIL-125-NH₂ and Ag₃PO₄.AAMN-120 was selected to investigate the effects of various small molecular organic compounds?tartaric acid,citric acid and oxalic acid?,pH?2,3,4,6 and 8?and foreign ions?ions from tap water and lake water?on Cr??? reduction.The results showd that the improvement of photocatalytic activity was attributed to the enhancement of the separation of photogenerated electrons and holes.AAMN-120 had good reusability and stability in photocatalytic reduction Cr???.The mechanism of photocatalytic reduction Cr???was verified by ESR,active species capture and Pt deposition experiments.3.Ag-UiO-66-NH₂ composites were prepared by photochemical reduction deposition.Its structure and morphology were characterized by a series of characterization methods.The photocatalytic activity of Ag-UiO-66-NH₂ for Cr???reduction under visible light was studied.The effects of various small organic compounds?tartaric acid,citric acid and oxalic acid?,pH?2,3,4,6 and 8? and foreign ions?ions from tap water,lake water and simulated seawater?on Cr???reduction were investigated.The photocatalytic reduction of Cr???by 30 min-Ag-UiO-66-NH₂ was 41.35 times higher than that by pristine UiO-66-NH₂,which indicated that the introduction of appropriate Ag nanoparticles could improve the charge transfer at the interface of the composite under visible light.In addition,30min-Ag-UiO-66-NH₂ was used to degrade organic pollutants?Rhodamine B and X-3B?.It was proved that holes and superoxide radicals were involved in the degradation of organic dyes by active substance capture experiments.The experiment of photocatalytic reduction Cr???showd that 30 min-Ag-UiO-66-NH₂ has good reusability and stability.Finally,the mechanism of photocatalytic reduction Cr???and degradation of organic dyes were proposed.The mechanism was verified by electrochemical characterization,photoluminescence technology,active material capture experiment and ESR spectrum.