Preparation Of ZnO@PVDF Nanofiber Composite Membrane And Its Mechanism Of Action In Water Treatment

The shortage of fresh water resources is one of the important problems of human survival and development.Recently,more and more wastewater has been produced due to crude oil leakage,population growth and industrial and agricultural development,which aggravates environmental pollution and water quality deterioration.Therefore,wastewater treatment and purification of water resources have become a hot topic in international research in recent years.ZnO nanomaterials have broad application prospects in the field of wastewater treatment due to its high specific surface area,excellent photocatalytic properties,good antibacterial properties and thermal stability.At the same time,electrospun nanofiber materials with high specific surface area,unique pore structure and easy functionalization have also been paid attention to by wastewater treatment,which are considered to be one of the most promising materials to solve related problems.Based on the analysis of the application status and existing problems of ZnO nanomaterials and nanofiber materials in the field of water purification,electrospun polyvinylidene fluoride(PVDF)nanofiber membrane with high specific surface area,high flexibility,high porosity and excellent chemical stability was used as the substrate of ZnO nanomaterials to prepare ZnO@PVDF nanofiber composite membrane.The composite membrane not only solves the problem of low recycling rate of ZnO nanomaterials,but also makes up for the defects of easy pollution and easy scaling of PVDF nanofiber membrane in sewage treatment.Therefore,the purpose of this thesis is to explore the preparation method of ZnO@PVDF nanofiber composite membrane and its mechanism in wastewater treatment.Firstly,the feasibility of preparing ZnO@PVDF nanofiber composite film was explored through the combination of electrospinning technology and hydrothermal synthesis technology.The relationships between the crystalline morphology,morphology,growth location and distribution characteristics of ZnO nanowires and precursor concentration,heat treatment temperature,growth liquid substance molar ratio and ammonia water volume ratio were studied.The results showed that when the mass fraction of Zn(Ac)₂ was 1.5%,the heat treatment temperature was 130?,the mole ratio of Zn(NO₃)₂/HMTA was 3:1 and the volume ratio of ammonia was 5%,PVDF nanofiber membranes with uniform diameter and uniform distribution of ZnO nanowires was formed.ZnO@PVDF nanofiber composite membrane with Janus structure was constructed by growing ZnO nanowires on the side of PVDF nanofiber membrane by buoyancy.The oil/water separation performance and self-cleaning properties of ZnO@PVDF nanofiber composite membrane was studied.The results showed that the introduction of ZnO nanowires increased the specific surface area and surface roughness of the PVDF nanofiber membrane and changed the wetting state(Wenzel state)of water droplets on the surface of the nanofiber composite membrane.ZnO@PVDF nanofiber composite membrane showed hydrophilic/underwater oleophobicity on ZnO nanowires modified side,which could achieve"water-removing",by pre-wetting water.The other side exhibited hydrophobic,which was used to achieve"oil-removing".The separation efficiency of light oil/water mixture and heavy oil/water mixture maintained at 98.33%-99.84%under gravity driving,and the water flux and oil flux were 1210 L m^-2 h^-1 and 7563 L m^-2 h^-1.After ten cycles,the separation efficiency of heavy oil or water was higher than 97%.It showed excellent separation performance and durability.In addition,utilizing the photocatalytic property of ZnO nanowires,ZnO@PVDF nanofiber composite membrane had good self-cleaning performance under ultraviolet irradiation.Under 2h irradiation,the degradation rate of 10 mg L^-1Rh B solution could reach 98.93%.The degradation rate maintained about 95%after five cycles.Then,in order to expand the visible light absorption of the ZnO@PVDF nanofiber composite membrane,Sm-doped ZnO@PVDF nanofibers membrane was fabricated by electrospinning of PVDF/Zn(Ac)₂/Sm(Ac)₃,followed with a low-temperature heat treatment to obtain Sm-doped ZnO seeds and then a hydrothermal growth.The effect of adding Sm(Ac)₃ on the morphology,crystal structure,specific surface area,pore size distribution and photocatalytic performance of nanofiber composite membranes was studied.The results showed that when 0.12at%Sm(Ac)₃ was added,the average diameter of ZnO nanowires decreased to 45nm,the band gap decreased to 3.09 e V,the oxygen vacancy content increased,and the specific surface area of the nanofiber composite membrane increased to 17.5 m² g^-1,the average pore size decreased to0.1063?m.PS microspheres of different particle sizes were used for liquid filtration tests.The results showed that the Sm-doped ZnO@PVDF nanofiber composite membrane exhibited good filtration efficiency(98%)and water flux(1847 L m^-2 h^-1)for 0.08?m PS microspheres under permeation pressure of 60 k Pa.At the same time,the photocatalytic performance of Sm-doped ZnO@PVDF nanofibers membrane was investigated.The results showed that the degradation rate of Sm-doped ZnO@PVDF nanofiber composite membrane for anionic or cationic dyes could reach 96%under Xenon lamp irradiation.After five cycles,the degradation efficiency did not change significantly.Finally,based on the premise of growing ZnO nanowires on one side of PVDF nanofiber membrane,ZnO nanowires grown on the entire PVDF nanofiber membrane was fabricated by electrospinning technology,ethanol soaking process and hydrothermal growth.The modified P-ZnO@PVDF nanofiber composite membrane was prepared by chemical vapor deposition method and applied to the field of membrane distillation by utilizing the characteristic that the hydroxyl group of the ZnO nanowire was easy to be fluorinated.The influence of hydrothermal time on the morphology,porosity,pore size distribution,wettability and distillation performance of the nanofiber composite membrane was studied.The results showed that when the hydrothermal time was 2h,the porosity of the P-ZnO@PVDF nanofiber composite membrane increased to 91%,the average pore size was 0.31?m,and the surface was superhydrophobic.Compared with commercial PVDF membranes,P-ZnO@PVDF nanofiber composite membranes had good wetting resistance and fouling resistance.This is because modified ZnO nanowires increase the barrier of heterogeneous nucleation on the membrane surface and reduce the contact area and contact time between fluid and membrane.P-ZnO@PVDF nanofiber composite membranes with durability presented the salt rejection of 99.9%and a stable permeation flux of 15.7 L m^-2 h^-1 for simulated seawater after 60h operation.This paper opened a new way for the the preparation of organic-inorganic nanofiber composite membrane and provided a new idea for its application in high-efficiency oil-water separation,photocatalytic degradation of organic wastewater such as printing and dyeing wastewater,and membrane distillation.