Immobilization Of Enzymes On ZIF-8 And Polymeric Membranes Toward Application Of Water Decontamination

Enzyme immobilization expands the practical application of the enzymes since it overcomes the drawbacks of free enzymes,such as easy deactivation,poor stability and difficulty in recovery.In recent years,as an emerging type of carriers for enzyme immobilization,metal-organic frameworks（MOFs）materials have been gaining considerable attentions in academic fields due to their large specific surface area,high porosity,adjustable structures and open metal sites.In this paper,the immobilization of two kinds of enzymes（cytochrome c（Cytc）and horseradish peroxidase（HPR））belonging to heme proteins on ZIF-8 was studied,and the morphology,structure and enzymatic characteristics of the immobilized enzymes were explored.HPR@ZIF-8 was grown in situ on the pre-grafted PAA/PTFE membranes by the co-precipitation method.The as-prepared HPR@ZIF-8/PAA/PTFE novel enzyme membranes were well characterized by using scanning electron microscopy（SEM）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,and water contact angle measurements.And the multi-function in water treatment（including filtration performance,anti-fouling and photocatalytic and enzymatic degradation activities on organic pollutants）of the enzyme membrane were studied.First,the co-precipitation method was used to immobilize Cytc on ZIF-8,and the effects of different molar ratios of Zn²⁺and dimethylimidazole（2-Ml）on Cytc@ZIF-8 were studied.SEM,FTIR,TGA and other characterizations proved the successful loading of Cytc on ZIF-8.The molar ratios of Zn²⁺and 2-Ml had obvious effects on the morphology and activitives of Cytc@ZIF-8:The crystal form of Cytc@ZIF-8（S2）was irregular;Cytc@ZIF-8（S1）and Cytc@ZIF-8（S3）presented both rhombohedral,but the former was about 10 times the size of the latter;Cytc@ZIF-8（S1）has the highest relative activity,which was 1.76 times of the free enzyme.The p H stability and storage stability of Cytc@ZIF-8 were greatly improved compared with the free enzyme.The molar ratios of Zn²⁺and 2-Ml had little effect on the stability of immobilized Cytc.At the same time,HPR was immobilized on ZIF-8 by the co-precipitation method,and the effect of the surfactant CTAB on immobilized HPR was studied.SEM,FTIR,TGA and other characterizations proved the successful loading of HPR on ZIF-8.The addition of CTAB had an obvious effect on the morphology of the HPR@ZIF-8:HPR@ZIF-8（S0）presented a rhombohedral dodecahedron,and HPR@ZIF-8（S1）obtained by adding CTAB presented a cube.The ralative activitives of HPR@ZIF-8（S0）and HPR@ZIF-8（S1）were 48%and52%of the free enzyme respectively,and the p H stability and storage stability were greatly improved compared with the free HPR.In addition,HPR@ZIF-8 exhibited excellent photocatalysis and enzymatic catalysis:the photocatalytic degradation rate for MB was about 90%within 60 min;phenol was almost completely removed by enzymatic catalysis within 5min.Finally,based on the above studies,this study pioneered the in-situ growth of HPR@ZIF-8 on a pre-grafted PAA/PTFE membrane by coprecipitation method.In other words,ZIF-8 was grown in situ on the surface of pre-grafted PAA/PTFE membranes by coordination of Zn²⁺with the carboxyl group in PAA,meanwhile,HPR was immobilized in ZIF-8framework.SEM,CLSM,FTIR and XPS demonstrated the successful loading of the nano-enzyme HPR@ZIF-8 on the membrane.The results of the multi-functional of the enzyme membrane showed that comparing with bare PTFE,the water contact angle of the enzyme membrane obviously decreased from 129°to 54°,while the water flux increased from 0 to 149L/（m2·h）.The enzyme membrane showed good anti-fouling performance in continuesou ultrafitration of the 1000 mg/L BSA feed solution.The enzyme membrane exhibited excellent photocatalytic and enzymatic catalysis for organic pollutants as well:The enzymatic removal rate of phenol,bisphenol A and estriol reached more than 80%within 60 min;the photocatalytic degradation rate of methylene blue,phenol and estriol reached 60%,90%,90%within 60 min respectively.In conclusion,the excellent properties of the as-prepared enzyme membranes plus with its easy-recovery and reusability,make the practical application in water treatment promising.