Study On Construction And Properties Of Iron Metal Organic Framework-based Composites For Removal Of Contaminants In Water

The rapid population growth and development of technology in the 21st century have brought great challenges to the environmental security.Techniques which making full use of solar power to solve environmental problems especially for the removal of pollutants in water has becoming popular.Among them,semiconductor-based photocatalysis technology,as an advanced oxidation technology that effectively utilizes solar energy to achieve efficient removal of pollutants that are difficult to degrade naturally in water bodies has become one of the most promising environmentally friendly treatment technology.In recent years,with the update of synthetic methods,great breakthroughs have been made in the synthesis and application of new semiconductor photocatalysts.Among which,MIL-53（Fe）has the characteristics of flexible structure,controllable pore size and internal pore environment,good chemi cal stability,and suitable band gap position,which makes it exbibits better and wider applications.Thereby,MIL-53（Fe）has become one of the new photocatalysts that have received the most attention in recent years.However,due to the limitations of surface reaction sites and the rapid recombination of photogenerated carriers in MIL-53（Fe）during photoexcitation,pure MIL-53（Fe）cannot meet the requirements for efficient photocatalytic degradation of pollutants in water.In this paper,MIL-53（Fe）is modified through heterojunctions constructing,intramolecular structure modulation,quantum dot intercalation,and combining the advantages of both methods（elemental doping and heterostructure construction）.At the same time,the properties and mechanism of the modified photocatalyst for the adsorption/photocatalytic removal of pollutants in water were studied.In this way,theoretical basis and practical guidance has been provided to design and synthesize new composite photocatalysts based on MIL-53（Fe）,which can be applied to photocatalytic remove the pollutants in water.The main contents and the main conclusions of this paper drawn are as follows:（1）To reduce the rapid annihilation of photogenerated carriers in MIL-53（Fe）during photoexcitation,the composite of Zn In₂S₄/MIL-53（Fe）was constructed which has been taking full advantage of the good visible light absorption capacity,high specific surface area and suitable conduction band position of Zn In₂S₄,as well as the structural advantages of MIL-53（Fe）.The synthesized photocatalysts of monomers and heterojunction structures were used for efficient adsorption-photocatalytic removal of ciprofloxacin（CIP）in water.The adsorption results proved that MIL-53（Fe）/ZIS-2 had the best adsorption removal efficiency,which could reach 44.7%.At the same time,the pseudo-second-order kinetic fitting of the adsorption data indicated that the adsorption process of CIP on the surfaces of monomers and composites involve d chemical adsorption.After illumination by visible light,the removal efficiency of CIP on the MIL-53（Fe）/ZIS-2 composite structure system reached more than99%within 45 min,which was significantly higher than that of pure MIL-53（Fe）（75.7%）and pure Zn In₂S₄（51.2%）degradation system.The results of calculating the reaction constant value（k）indicated that the k value of MIL-53（Fe）/ZIS-2 photocatalytic systems was about 0.0544 min^-1,which was 5.8 and3.8 times that of the pure MIL-53（Fe）and pure Zn In₂S₄ photocatalytic systems.The trapping test and ESR test results showed that more·O₂^-and·OH were generated in the MIL-53（Fe）/ZIS-2 photocatalytic systems,which resulted in a significant improvement in the photocatalytic activity.At the same time,the reduction of electron-hole recombination and the increased lifetime of photogenerated charge carriers indicated by the transient fluorescence test results were also important reasons for the increase of photocatalytic activity.In addition,the cycling experiments and TOC test results showed that MIL-53（Fe）/ZIS-2 still exhibited good removal efficiency of CIP during the first four cycles,and the CIP adsorbed on the catalyst surface finally could be degraded through the photocatalytic process.（2）To take full advantage of the structural advantages of MIL-53（Fe）and improve the separation efficiency of its photogenerated charges,the intramolecular structure modulation of MIL-53（Fe）was realized by introducing sulfur into the MIL-53（Fe）framework through the thermal reaction process under vacuum conditions,resulting replacing the oxygen atoms in the framework.The as-synthesized sulfur-modulated MIL-53（Fe）（S-MIL-53（Fe））crystals had regular spindle morphology.Significantly increased degradation efficiency for tetracycline（TC）was exhibited under visible light excitation.During the degradation process,the calculated apparent quantum efficiency（AQE）of S3-MIL-53（Fe）can reach about 27.82%,which was more than 30times that of pure MIL-53（Fe）（0.903%）.The reacting constant was 1.088mmol/g/h,which was higher than the degradation efficiency of TC for other reported MIL-53（Fe）-based photocatalysts,indicating that intramolecular structure modulation could significantly improve the photocatalytic performance of MIL-53（Fe）.The enhanced photocatalytic performance could be attributed to the structural advantages and more efficient photoexcitation.Specifically,the introduction of sulfur formed an intermediate energy level in the S3-MIL-53（Fe）structure,making the photoexcitation of electrons jumping from the valence band to the intermediate energy level much easier.At the same time,the Fe-S-O framework formed by the introduction of sulfur exhibited an enhanced charge effect,which provided a faster path for electron transport during photoexcitation and accelerates the electron transport,resulting in better charge separation.It wass obviously that ingenious cooperation the charge transfer steering and active site enrichment can effectively promote the improvement of photocatalytic activity.In addition,the photocatalytic degradation experimental results and cycle test results in different water environments indicated that S3-MIL-53（Fe）exhibited high TC removal rates and good stability in different water environments.（3）To further expand the application of MIL-53（Fe）in the removal of pollutants in water and improve its light absorption ability,nitrogen-doped quantum dots（N-CQDs）were successfully intercalated into MIL-53（Fe）structure by a one-step hydrothermal method.Trivalent arsenic（As（III））in water was oxidized to As（V）through photocatalytic process and then removed by adsorption using the synthesized N-CQDs modified MIL-53（Fe）with unique structure（oil vine-like）.The oxidation and adsorption removal of As（III）by N-CQDs3/MIL-53（Fe）was 6 times higher than that of pure MIL-53（Fe）.At the same time,the effects of different p H and common anions and cations in water on the oxidation and adsorption and removal of As（III）were studied,and the optimal removal p H value could be set to 6.9,while the common cations Na⁺,Ca⁺and Mg²⁺in water were obtained.There were no significant effects on the removal efficiency of As（III）,but the common anions CO₃^2-and SO₄^2-could significantly inhibit the removal efficiency of As（III）.Combined with theoretical calculation results and XPS characterization analysis results,it was proved that As（III）was oxidized to As（V）through photocatalytic process on the surface of the photocatalyst,and then C-O-As bonds were formed on the surface of N-CQDs3/MIL-53（Fe）,Finally,the efficient removal of As（III）was achieved.Furthermore,the N-CQDs3/MIL-53（Fe）with unique structure exhibited excellent removal efficiency and good durability in high concentration of As（III）.The in situ intercalated N-CQDs could accelerate the separation of photogenerated electrons and holes,and significantly improve the photocatalytic oxidation and adsorption capacity of N-CQDs/MIL-53（Fe）.（4）To adjust the electron transfer path of MIL-53（Fe）and accelerate the photogenerated charge separation.Doping with nickel element and coupling nitrogen-deficient carbon nitride（g-C₃N₄-N）at the same time to construct a heterojunction interface composite structure of MIL-53（Fe）/Ni/g-C₃N₄-N.The photocatalytic performance of the modified MIL-53（Fe）/Ni/g-C₃N₄-N composite structure was evaluated by photocatalytic degradation of TC,CIP and Rh B in water.The results showed that the degradation efficiencies of CIP,TC and Rh B by MIL-53（Fe）/Ni/g-C₃N₄-N-2 could reach 96.8%、90.8%and 98.7%within40 min,respectively,which were significantly higher than those of monomers.Through the capture experiment combined with the ESR test results,it c ould be concluded that oxygen molecules in water or air could be oxidized into·O₂^-and·O₂¹ during the photoexcitation process,which was the main active species of the MIL-53（Fe）/Ni/g-C₃N₄-N-2 photocatalytic degradation system.The improved photocatalytic performance test results,combined with solid ultraviolet（UV-vis）and photoluminescence（PL）analysis results indicated that by nickel element doping and simultaneously coupling g-C₃N₄-N could increase visible light absorption and improve the separation of photogenerated electrons and holes.In this way,its photocatalytic activity was significantly improved.In addition,total organic carbon and cycling experiments demonstrate that the MIL-53（Fe）/Ni/g-C₃N₄-N composite exibtied excellent mineralization ability and reusability.