Application Of Tannic Acid Functionalized PAN Electrospinning Fiber Membrane In The Treatment Of Drinking Water

Research background and objective:Water is an important substance in human life activities,and the safety of drinking water is directly related to human health.The current"Sanitary Standards for Drinking Water"（GB5749-2006）stipulates that the quality of drinking water should meet the requirements including good sensory properties,epidemiological safety which means no pathogenic microorganisms and parasitic eggs,and no acute or chronic toxcicity to human body caused by chemical substances in the water.However,with the rapid development of industry and agriculture,the drinking water quality in most parts of China is difficult to meet the national standards,mainly in microorganisms,heavy metals and organic dyes and other substances seriously exceed the standard.Therefore,effective removal of harmful substances in water has very important public health significance for ensuring water safety.Electrospun fiber membrane is widely used in water treatment because of its large surface area,high porosity,various types and easy surface modification.It has been shown that polyacrylonitrile（PAN）,a polymer compound,has been widely used to fabricate fiber films because of its high viscosity solution,high surface tension,etc.Besides,the surface of PAN fiber films is easy to be modified,carbonized,activated,etc.Polyacrylonitrile also has ultrafiltration,reverse osmosis and microfiltration functions and is used in ultrapure water manufacturing,sewage treatment and recycling.However,the unmodified polyacrylonitrile fiber is poor in alkaline resistance and water absorption,which can not meet the needs of the treatment of complex pollutants in drinking water.Therefore,it is necessary to modify its surface functionalized.Tanic acid（Tanic acid,TA）as a plant extract,its molecular structure contains a large number of phenolic hydroxyl functional groups,not only has high antibacterial,antiviral and other biological activities,but also can with metal ions and organic dyes complexation,electrostatic action.MethodsFirstly,by studying the influence of different concentration of polyacrylonitrile solution on the morphology and diameter of PAN electrospinning fiber film,the suitable spinning conditions were determined to prepare PAN fiber film.Then,the PAN fiber membrane prepared under these conditions was added to the diethylene-triamine solution for 2 h to obtain amino modified fiber membrane（FPAN-NH₂）.Finally,the amino fiber membrane was placed in tannic acid solution for surface modification to obtain tannic acid modified nanofiber membrane（FPAN@TA）.Scanning electron microscopy（SEM）was used to characterize the morphology of the fiber film.Fourier transform infrared spectrometer（FITR）was used to characterize the changes of functional groups on the surface of the modified fiber film,and X-ray photoelectron spectroscopy（XPS）was used to analyze the surface elements.Then,the effects of FPAN@TA on the biological activities of Escherichia coli and Staphylococcus aureus were studied by changing tannic acid content to optimize the modified product and using bacteriostatic zone method,test tube method and plate counting method.Finally,the adsorption effect of FPAN@TA on heavy metal（Cr（VI）and methylene blue（MB）in water is studied by using the complexation of phenolic hydroxyl of tannin with metal ions and their chargability.The Box-Behnken Design（BBD）method in the reaction surface（RSM）was used to study the effects of p H of solution,initial concentration of target solution,the amount of adsorbent,adsorption time and interaction of various factors on the adsorption of Cr（VI））and MB,so as to obtain the optimal adsorption process.The adsorption mechanism of FPAN@TA was evaluated by adsorption kinetics,absorption isotherm and model.Results（1）By optimizing the experimental parameters,the optimal experimental condition for PAN preparation was 10%,the surface of PAN nanofibers was smooth,and the average diameter of the fibers was 500±0.17 nm.The results of FTIR showed that the secondary vibration of N-H at 3300 cm^-1was a weak single-peak intensity,indicating that the amino modification was on the PAN membrane.The surface functionalization modification of FPAN-NH₂fiber membrane was carried out with tannic acid.The SEM results showed that the tannic acid modified nanofiber membrane（FPAN@TA）had rough surface.The results of XPS and FTIR characterization showed that,compared with PAN and FPAN-NH₂,the benzene ring bending vibration peak appeared at 910-670 cm^-1FPAN@TA,and the tannyl contraction vibration peak appeared at 1711 cm-1（C=O stretching vibration）.XPS characterization showed that FPAN@TA had O element in tannic acid structure on the original C and N elements of PAN film.（2）The antibacterial properties of FPAN@TA were found to be enhanced with the increase of tannic acid content on membrane surface.FPAN@TA8 had the best inhibitory effect on E.coli,with a inhibitory zone of 10.02±0.1 mm.Similarly,FPAN@TA8 had the best inhibitory effect on Staphylococcus aureus,with a inhibitory zone of 11.24±0.5 mm.SEM characterized the antibacterial fiber membrane and found that the number of bacteria on the antibacterial fiber membrane changed according to the tannic acid concentration(with the increase of tannic acid content,the bacteria on the fiber membrane decreased.（3）The adsorption properties of dye MB and heavy metal Cr（VI）were studied.The adsorption efficiencies of FPAN@TA₁,FPAN@TA₂,FPAN@TA₄and FPAN@TA₈heavy metal Cr（VI）were 31%,39%,72%and 90%respectively.The adsorption rates of methylene blue were 16%,22%,68%and 85%,respectively.After FPAN@TA adsorption of heavy metal Cr（VI）,the fiber membrane becomes rougher and Cr（VI）element appears.FPAN@TA After adsorption of MB,the fiber membrane becomes less smooth,and the membrane becomes blue from the previous light yellow;The elements Cl and S,which are specific to MB,appear on the membrane.The adsorption rate of Cr（VI）by FPAN@TA and MB by FPAN@TA is 90%after 6 times of adsorption and desorption,and 81%after 6 times of adsorption and desorption.The adsorption rate of Cr（VI）by FPAN@TA is 90%after 6 times of adsorption and desorption,FPAN@TA the adsorption rate of MB is 81%after 6 times of adsorption and desorption.（4）Based on the established nonlinear regression model and response surface curve,it is found that the p H of solution（A）,the initial concentration of target dye（B）,the dosage of adsorbent（C）,and the adsorption time（D）have different degrees of influence on the removal rates of heavy metals Cr（VI）and MB.The optimal adsorption conditions for Cr（VI）were p H3.164,initial concentration 91.416 mg/L,dosage 90.215mg and time 8.267 h,The influence degree of the four factors is ranked from strong to weak as C>A>B>D.The optimal adsorption conditions for MB were p H 12.552,initial concentration 89.517 mg/L,and input amount 82.244 mg;The influence degree of the four factors is ranked from strong to weak as A>D>B>C.（5）The adsorption kinetics show that the adsorption of Cr（VI）on the membrane is more in line with the quasi second order kinetics.The adsorption process of Cr（VI）is the complexation of Cr（VI）on the membrane,and the adsorption process is controlled by chemical adsorption.The adsorption of MB is more in line with the quasi first order kinetic model,indicating that the adsorption process of MB on the membrane is electrostatic adsorption and the adsorption process is physical adsorption.Both isothermal models show that they conform to the Langmuir model,and the adsorption process is single layered adsorption.ConclusionsIn this study,PAN nanofiber membrane modified with tannic acid was successfully prepared,which has antibacterial effect on both Escherichia coli and Staphylococcus aureus.And FPAN@TA₈to Escherichia coli and staphylococcus aureus have strong antibacterial.From all aspects of the data,FPAN@TA is more effective against Gram-positive Staphylococcus aureus than Gram-negative E.coli.The successfully prepared FPAN@TA has the removal effect of heavy metal Cr（VI）and dye MB in sewage.Through the analysis of the nonlinear regression model and the response surface curve,the adsorption rates under the optimal adsorption conditions are96.42%±0.43 and 93.33%±0.45.Then,the adsorption mechanism of dye MB and ion Cr（VI）was determined by adsorption isotherm and kinetic model.