MIL-88B(Fe) Immobilized Horseradish Peroxidase To Remove Bisphenol A And 2,4-dichlorophenol

Phenols are one of the most common organic pollutants in the water environment,and most of them are neurotoxic,with high stability,strong enrichment effect,and difficult to be biodegradable.Therefore,it is urgent to remove phenolic substances in water.Compared with the incompleteness of physical methods,the high energy consumption of chemical methods and the complexity of microbiological methods,enzymatic methods have come into the public’s view due to its green,high-efficiency,non-toxic byproducts.However,due to the poor stability and unrecyclable disadvantages of free enzyme,its application is greatly limited,so researchers put forward the concept of immobilized enzyme.In this study,horseradish peroxidase（HRP）was immobilized on iron-based organic frameworks MIL-88B（Fe）material to prepare MIL-88B（Fe）/HRP biocatalyst.Different characterization methods were used to compare the changes of the structure and properties of the enzyme before and after immobilization,and the single factor experiments of the immobilization process were carried out.The differences of temperature,p H,and storage stability between free HRP and immobilized HRP were compared,and the enzymatic reaction kinetics was analyzed.After that,MIL-88B（Fe）/HRP was applied to the removal of bisphenol A（BPA）and 2,4-dichlorophenol（2,4-DCP）,and the single factor degradation experiments,degradation kinetics and enzymatic catalysis mechanism were mainly analyzed.Specific research results are as follows:（1）XRD results showed that the introduction of HRP did not change the crystal structure of MIL-88B（Fe）.SEM showed that the two materials were fusiform.The introduction of enzyme did not change the morphology of MIL-88B（Fe）.The presence of N element in the EDS test proved the successful synthesis of MIL-88B（Fe）/HRP.Fluorescence analysis further confirmed the successful loading of HRP and the uniform distribution of HRP on the material.FTIR results showed that the immobilized enzyme material possessed both MIL-88B（Fe）and the characteristic peaks of the enzyme.The secondary structure analysis showed that the rigidity and stability of the enzyme were enhanced after immobilization.TGA test showed that both materials had good thermal stability.According to the BET results,MIL-88B（Fe）had many microporous structures,and the pore volume decreased after HRP immobilization.（2）The results of single factor immobilization experiments showed that the best effect could be achieved under the conditions of 25℃,immobilization time of 6 h and crosslinking agent concentration of 30 mg/m L.The maximum load of MIL-88B（Fe）/HRP material was 38.9mg/g,and the maximum enzyme retention activity was 73.2%.The stability experiment showed that the immobilized enzyme had better temperature stability,p H stability and storage stability than the free enzyme.Because immobilization could protect the spatial conformation of the enzyme,so that the enzyme was not easy to be inactivated due to environmental changes.The kinetics of the enzymatic reaction showed that the affinity between the enzyme and the substrate decreased after immobilization due to limited mass transfer.（3）The degradation experiments showed that MIL-88（Fe）/HRP and polyethylene glycol system had the fastest degradation rate of BPA and 2,4-DCP,and the reaction rate constants were k=0.1651 min^-1 and k=0.3698 min^-1,respectively.The addition of polyethylene glycol（PEG）could protect the catalytic center of HRP.The optimum dosage of m（PEG）:m（BPA/2,4-DCP）was 0.4 and 0.1,respectively.From the single factor experiments of degradation of BPA and 2,4-DCP,it could be concluded that under the conditions of n（H₂O₂）:n（BPA/2,4-DCP）=1.5,temperature 25℃,initial concentration 20 mg/L,p H=7,MIL-88（Fe）/HRP could degrade98.4%of BPA in 3 h and 99.0%of 2,4-DCP in 1 h.（4）The degradation of BPA and 2,4-DCP by free and immobilized HRP showed that the synergistic effect between the iron ion in material and the iron active site of HRP further enhanced the catalytic effect.Meanwhile,the framework of MIL-88B（Fe）had a protective effect on HRP.At the same time,MIL-88（Fe）/HRP material also had good reusability,and the XRD patterns of the material did not change much after several cycles,indicating the strong structural stability of the synthesized sample.The catalysis mechanism of HRP is due to the oxidation and reduction of its ferric active center.