Removal Of Cr(Ⅵ) And RhB By MOFs And Its Modified Materials

The coexistence of inorganic heavey mentals and organic contaminants increases the difficulty of wastewater treatment,how to efficiently remove these two pollutants simultaneously is the key to sewage treatment and resource utilization.Adsorption,as a simple,efficient,and low-cost technology,has achieved achievements in removing heavy metals and organic pollutants,but it still has the disadvantage of not being able to completely removal the enriched pollutants.As an efficient and environmentally friendly technology,photocatalysis has received increasing attention in recent years.In addition,the key to the removal of pollutants by photocatalysis is the oxidation-reduction reaction between pollutants adsorbed on photocatalysts and photogenerated electrons and holes generated on the surface of photocatalysts.Therefore,it is great important to find photocatalysts with strong adsorption performance.NH₂-MIL-125（Ti）,as a kind of Metal-Organic Frameworks（MOFs）,not only has the characteristics of large specific surface area,high porosity and adjustable structure,but also can carry out photocatalytic reaction under visible light.However,its shortcomings such as short photogenerated carrier lifetime and low charge utilization rate limit its photocatalytic efficiency.Therefore,a large number of studies are devoted to regulating its own structure and performance to improve its photocatalytic ability.The current modification strategies for MOFs are generally quite complex,which seriously restricts the application of this technology in practice.Hence,how to enhance the visible light catalytic activity of NH₂-MIL-125（Ti）in a simpler way has theoretical and practical significance.Based on the above background,this article first explored the adsorption performance and mechanism of original NH₂-MIL-125（Ti）on Hexavalent chromium（Cr（Ⅵ））and Rhodamine B（RhB）under dark conditions,as well as the photocatalytic efficiency and mechanism of reducing Cr（Ⅵ）and degrading RhB under visible light.On this basis,based on principles such as dye sensitization,the co-existing Cr（Ⅵ）and RhB were used as modifiers to explore the changes in adsorption and photocatalytic properties of NH₂-MIL-125（Ti）,in order to provide new ideas for improving the adsorption performance and visible light catalytic activity of materials.The main research results of this article are as follows:（1）The adsorption experiments show that:In the single system,the maximum adsorption capacity of NH₂-MIL-125（Ti）for Cr（Ⅵ）was 325.644 mg_·g^-1 while for RhB it was 443.648mg_·g^-1.While in a binary system,the maximum adsorption capacity of NH₂-MIL-125（Ti）for Cr（Ⅵ）and RhB changed to 264.212 mg_·g^-1 and 1828.507 mg_·g^-1,respectively.Electrostatic attraction,reduction,chelation and surface complexation were the primary mechanisms of Cr（Ⅵ）adsorption.Electrostatic attraction,hydrogen bonding andπ-πinteraction were the primary mechanisms of RhB adsorption.The adsorption capacity of Cr（Ⅵ）was decreased by the presence of RhB,however,the adsorption capacity for RhB increased in the coexistence with Cr（Ⅵ）.The formation of Cr（Ⅵ）-RhB and Cr（III）-RhB complexes was the cause that provided facilitation for adsorption of RhB in the binary system.（2）Photocatalytic experiments show that:After 120 minutes of photocatalytic reaction,in the single system,the reduction efficiency of NH₂-MIL-125（Ti）for Cr（Ⅵ）was 79.9%,and the degradation efficiency of NH₂-MIL-125（Ti）for RhB was 94.8%.In the binary system,the reduction efficiency of NH₂-MIL-125（Ti）for Cr（Ⅵ）was 91.9%,and the degradation efficiency for RhB was 99.9%.When Cr（Ⅵ）and RhB coexisted in the system,Cr（Ⅵ）adsorbated on NH₂-MIL-125（Ti）can rapidly reduction by photogenerated electrons,and RhB acted as hole scavenger to be oxidized.The main reason for the improvement of Cr（Ⅵ）and RhB removal efficiency in binary systems is that Cr（Ⅵ）and RhB can effectively prevent the recombination of photo generated electrons and holes.Moreover,in the binary system,Cr（Ⅵ）adsorbed on NH₂-MIL-125（Ti）can be used as the active sites to adsorb more RhB,thus accelerating the degradation rate of RhB,and finally realizing the synergistic and efficient removal of Cr（Ⅵ）and RhB under visible light.（3）RhB-NH₂-MIL-125（Ti）（RhB-NM）and Cr（Ⅵ）-NH₂-MIL-125（Ti）（Cr（Ⅵ）-NM）were prepared by loading RhB and Cr（Ⅵ）onto NH₂-MIL-125（Ti）,respectively,The maximum adsorption capacity of RhB-NM for Cr（Ⅵ）is 535.471 mg_·g^-1,and that of Cr（Ⅵ）-NM for RhB is 892.515 mg_·g^-1.Compared with NH₂-MIL-125（Ti）adsorption of Cr（Ⅵ）and RhB,the adsorption capacity of Cr（Ⅵ）by RhB-NM and RhB by Cr（Ⅵ）-NM was significantly increased.Under visible light irradiation,after 120 minutes of photocatalytic reaction,the reduction efficiency of RhB-NM for Cr（Ⅵ）was 99.1%,and the degradation efficiency of Cr（Ⅵ）-NM for RhB was 100%.The results of ESR showed that superoxide radical（·O₂^-）and hydroxyl radical（·OH）were the main active substances affecting the degradation of RhB.RhB-NM and Cr（Ⅵ）-NM formed the surface structure and transport channel that are conducive to absorption of visible light and electron migration.Thus,the spectral response range of NH₂-MIL-125（Ti）was expanded,and the photocatalytic performance of RhB-NM and Cr（Ⅵ）-NM was significantly improved.