High Efficiency Photocatalytic Reduction Of Cr(Ⅵ) In Water With UiO-66 Series Metal Organic Frameworks

The widespread use of hexavalent chromium（Cr（Ⅵ））in industries such as leather tanning,paint manufacturing,and steel manufacturing has caused environmental problems that can not be ignored.Hexavalent chromium is mutagenic,carcinogenic and teratogenic to humans,and is classified as an"A"human carcinogen by the US Environmental Protection Agency（EPA）.Effective removal of hexavalent chromium in water has always been an important issue in the field of wastewater treatment research.Traditional water treatment methods for removing hexavalent chromium have many disadvantages,such as large amount of acid-base agent,high energy consumption,long treatment time,etc.In response to this defect,this paper explores and develops new materials based on metal organic framework to remove hexavalent chromium in water.Through material design and synthesis,performance characterization and optimization parameters,this paper has achieved the following three main results:1.UiO-66-NH₂ material has been developed to photocatalyze the reduction of hexavalent chromium in water under ultraviolet light.The material was prepared by a simple solvothermal method.Material characterization revealed that it has a large surface area,high chemical and thermal stability,and good adsorption performance for Cr（Ⅵ）.Compared with UiO-66,the introduction of amino group can make absorption edge red-shift to visible light region,narrow the energy gap,and lower the photogenerated electron-hole pairs recombination,which facilitates usage of the solar spectrum and promotes the photocatalytic reduction of Cr（Ⅵ）.The photocatalytic performance shows that under ultraviolet light,the initial concentration of Cr（Ⅵ）is 8mg/L,and the removal rate can reach 100%in only 25 minutes.The initial concentration of Cr（Ⅵ）and the effect of pH on photocatalytic performance were studied.The results show that the initial concentration is lower,and the smaller the pH value,the more favorable the photocatalytic reaction.The adsorption performance shows that when the initial concentration was 10 mg/L,the theoretical maximum adsorption capacity was 45.1 mg/g.In this chapter,the effects of initial concentration,adsorption time,pH and temperature on the removal efficiency of Cr（Ⅵ）by UiO-66-NH₂ is also studied.The results showed that the lower the initial concentration,the lower the pH value and the higher the temperature,the better the adsorption effect.2.Based on the above-mentioned UiO-66-NH₂ material’s narrow absorption range for visible light,wide band gap,and high recombination efficiency of electrons and holes,etc.,this chapter developed UiO-66-（OH）₂ materials for photocatalytic reduction Cr（Ⅵ）.Basic characterization studies have found that it has a large specific surface area,high chemical and thermal stability,and many active sites,which is beneficial to the removal of Cr（Ⅵ）.The light absorption performance shows that the light absorption edge of UiO-66-（OH）₂ material shifts red to the visible light region,the band gap becomes narrow,the electron and hole recombination efficiency is low,and the photocatalytic reaction can be carried out under visible light.The photocatalytic reduction performance expeiments shows that when the initial concentration of Cr（Ⅵ）is 8 ppm,the removal rate of Cr（Ⅵ）can reach 100%after10 minutes of direct exposure to visible light without going through the adsorption-desorption equilibrium.The mechanism of UiO-66-（OH）₂ photocatalytic reduction of Cr（Ⅵ）is that hydroxyl group are excited under visible light conditions,and then the excited photoelectrons are transferred to Zr-O clusters to form Zr³⁺.The formed Zr³⁺can react with Cr₂O₇^2-,in which Cr（Ⅵ）is reduced to Cr（III）.Therefore,the number of OH groups in organic molecules plays a key role in the photocatalytic reduction process.Compared with UiO-66-（OH）,the photocatalytic ability of UiO-66-（OH）₂ material is significantly enhanced.The lower the pH value,the stronger the photocatalytic reduction of Cr（Ⅵ）of UiO-66-（OH）₂.The adsorption performance experiments show that when the pH value of the aqueous solution is 3and the adsorption temperature is 35°C,the initial concentration of Cr（Ⅵ）is 10 mg/L,UiO-66-（OH）₂ has the maximum adsorption capacity of 236.41 mg/g,indicating that UiO-66-（OH）₂ has great application potential of removal Cr（Ⅵ）.3.The above two experiments confirmed that the metal organic framework material is a good template for unifying the light excitation center and the catalytic center.UiO-66-NH₂ can be excited under ultraviolet light to achieve photocatalytic reduction,UiO-66-（OH）₂ Photocatalytic reduction can be achieved under visible light,and the function of combining the high-efficiency reduction ability of nano Zr-O clusters with the visible light excitation ability of organic molecules is realized by group-modified organic connectors.It not only improves the efficiency of using visible light,but also explores a new way to avoid organic molecules agglomeration and photo-excited high catalytic capacity at the nanometer scale,which will help the research of photocatalytic materials.