Structural And Interfacial Modulation Of Metal-organic Framework MIL-100(Fe) For Efficient Photo-Fenton Degradation

Tetracycline hydrochloride（TC-HCl）,as a typical representative of broad-spectrum antibiotics,has been widely used in animal husbandry and farming owing to its low price and high antibacterial activity,but the drug residue rate of up to 60%causes serious pollution to the water environment.Thus,the efficient removal of TC-HCl pollutants from aqueous systems has become a hot research topic for environmental management.The heterogeneous photo-Fenton process with Fe-based organic frameworks（Fe-MOFs）as the core materials has gained much attention in the field of environmental remediation owing to its green,simple operation,and strong oxidizing properties.However,most iron-based MOFs lack exposed coordinatively unsaturated metal sites（CUSs）to adsorb and activate H₂O₂;meanwhile,poor electron transport properties limit their Fe³⁺/Fe²⁺cycling efficiency,ultimately leading to unsatisfactory photo-Fenton degradation performance.Therefore,it is of great scientific significance and application value to design and prepare photo-Fenton materials with efficient active sites and fast Fe³⁺/Fe²⁺cycling efficiency.In this dissertation,MIL-100（Fe）with high stability was selected as the substrate,and MIL-100（Fe）-based series of photo-Fenton catalysts with efficient active sites and fast Fe³⁺/Fe²⁺recycling efficiency were designed by using the strategy of structure tailoring and material compounding.The relationship between the structure of Fe-MOFs and the interfacial structure of composites with the performance of photo-Fenton was established by studying the morphological structure,photoelectric properties,and photo-Fenton degradation performance of TC-HCl,which provide theoretical basis and experimental guidance for the development of photo-Fenton materials.The specific research contents are as follows:（1）To address the problem of insufficient exposure of coordinatively unsaturated metal sites,MIL-100（Fe）was chosen as the research object to create mixed-valence coordinatively unsaturated metal site by vacuum thermal activation.The effect of thermal activation temperature on the mixed-valence ratios and the coordinatively unsaturated metal sites was investigated in order to achieve a controlled construction of mixed-valence highly active sites.The relationship between the own structure and optical properties of MIL-100（Fe）containing different amounts of Fe²⁺coordinatively unsaturated metal sites with the photo-Fenton performance was investigated,and the synergistic effects between the Fenton-like and photocatalytic processes were investigated.Ultimately,its photo-Fenton degradation mechanism was elucidated.The results show that the vacuum-activated CUS-MIL-250 at 250℃ exhibits significant photo-Fenton activity in term of degradation of TC-HCl owing to the increased specific surface area and pore volume,improved visible light absorption,and the introduction of more Fe2+ coordinatively unsaturated metal sites,the removal rate of TC-HCl is 95.6% at p H=4,200 m M H2O2,and 0.2 g/L catalyst,and the effective separation of photogenerated electrons and holes in the photo-Fenton system results in a significant synergistic effect between the Fenton-like and photocatalytic processes.Meanwhile,·OH and ·O2-are proved to be the main active species in the degradation of TC-HCl,with ·OH playing a dominant role in the degradation mechanism.（2）In order to form more coordinatively unsaturated metal sites and optimize carrier migration behavior,defective MIL-100（Fe）（CUS-Pac-MIL-100（Fe））with mixed valence coordinatively unsaturated metal sites was synthesized by introducing missing linkers to construct ligand vacancy defects on the basis of vacuum thermal activation.In-situ diffuse reflectance infrared Fourier transform spectroscopy and electron paramagnetic resonance spectroscopy confirm the increase of coordinatively unsaturated metal sites and the formation of ligand vacancies,where the increase in the number of coordinatively unsaturated metal sites enhances the Lewis acidity on the surface of the materials and improves its adsorption and activation ability of H2O2.The formation of the ligand vacancies promotes the separation and transport efficiency of photo-generated carriers of the system,thereby improving the Fe3+/Fe2+ cycle efficiency in photo-Fenton system.CUS-Pac-MIL-100（Fe）exhibits excellent photoFenton removal performance at p H=4,100 m M H2O2,and 0.2 g/L catalyst,with the removal rate of TC-HCl（50 mg/L）reaching 96.5% and the removal rate of TOC reaching 52.3% at 80 min.The synergistic degradation mechanism of mixed-valence coordinatively unsaturated metal sites and ligand vacancies for TC-HCl in this photo-Fenton system was analyzed and revealed.The specific degradation pathway of TC-HCl in this photo-Fenton system was investigated in detail.The final degradation products were confirmed to be small molecules,with significantly reduced toxicity as confirmed by toxicological simulations.（3）To further improve the Fe3+/Fe2+ cycling efficiency in the system,M-D-MIL（Fe）/CuC3N4 heterojunctions of Cu-doped C3N4（Cu-C3N4）compounded with mixed-valence defect MIL-100（Fe）（M-D-MIL（Fe））were designed and synthesized by adopting defect,doping,and heterojunction engineering.The results show that the ligand vacancies in M-D-MIL（Fe）and the copper ions in Cu-C3N4 form double electron capture sites,which improve the carrier migration behavior of the respective single components.The suitable energy band structure,oriented built-in electric field,and tight interfacial contacts provide pathways and dynamics for the continuous transport and transfer of photogenerated charges.Meanwhile,the built-in Cu+/Cu2+ and Fe3+/Fe2+ form a double redox center.Ultimately,the efficiency of Fe3+/Fe2+ cycling is significantly increased,thus the activation of H2O2 is promoted.The removal rate of TC-HCl（50 mg/L）in a system of 50 m M H2O2 at 80 min reaches 97.7% with a kinetic rate constant of 0.045 min-1.The removal rate of TC-HCl reaches 94.2% with low concentrations of H2O2（5 m M）,and the kinetic rate constant is 0.035 min-1,which realizes the rapid degradation under low concentration of H2O2.The photo-Fenton degradation mechanism of ligand vacancy,copper ion doping,and Cu-C3N4/M-D-MIL（Fe）heterojunction coupling was elucidated.（4）In order to maximize the use of photogenerated carriers in the system to accelerate Fe3+/Fe2+ cycling and promote H2O2 activation,reduced graphene oxide（r GO）with broadband light absorption and high electron migration rate was introduced between the M-DMIL（Fe）/Cu-C3N4 interface to construct r GO-mediated M-D-MIL（Fe）/r GO/Cu-C3N4 ternary heterojunction.The results show that the broadband absorbing property of r GO significantly redshifts the optical absorption band edge of the ternary composites,broadens the response to visible light and improves the yield of photo-generated carriers.The high electron migration rate of r GO promotes the charge transfer between the M-D-MIL（Fe）and Cu-C3N4 phases and suppresses the photogenerated charge recombination in the system.As a result,the Fe3+/Fe2+ cycling efficiency is significantly improved under the dual action of wide spectrum absorption and low photogenerated electron-hole recombination rate,which promotes the rapid activation of H2O2.The removal rate of TC-HCl（50 mg/L）by M-D-MIL（Fe）/r GO/Cu-C3N4 is 99.1% at 50 m M H2O2,with the kinetic rate constant being 0.049 min-1.The removal rate of TC-HCl reaches 97.4% with a kinetic rate constant of 0.043 min-1 at low concentrations of H2O2（5 m M）,further achieving rapid degradation at low H2O2 concentrations.The photo-Fenton degradation mechanism of M-D-MIL（Fe）/r GO/Cu-C3N4 heterojunction enhanced by r GO as an electronic bridge was revealed.The degradation pathways of TC-HCl and the toxicological changes of the intermediates were elucidated.In conclusion,MIL-100（Fe）-based series of photo-Fenton catalysts with efficient active sites and fast Fe3+/Fe2+ recycling efficiency prepared in this dissertation have potential applications in organic pollutant removal,which provides new ideas and new insights for the development of Fe-based MOFs photo-Fenton materials.