Structural Design And Performance Study Of Multifunctional PVDF-Based Water Treatment Separation Membranes

In the application field of membrane separation technology,the traditional and monofunctional PVDF membranes are no longer able to meet the stringent requirements for water quality conditions under today’s environmental pollution control strategies.Therefore,the research,exploitation,application and promotion of new PVDF membranes with high separation efficiency and multifunctionality is the future development of the membrane industry in the forefront direction.Therefore,this thesis proposes a new strategy to endow PVDF membranes with high flux,multi-pollutant separation and self-cleaning capabilities based on the modification of the membrane’s own matrix structure modulation,involving the formulation of highly hydrophilic,tunable or catalytic membrane structures by means of grafting of PVDF chain segments,loading of functionalized nanoparticles,etc.,the aim is to make the PVDF membrane separation technology suitable for a wider and smarter range of water treatment applications.Based on the above,the research content and research methodology of this topic includes the following parts:Part I:Brominated 1-butyl-3-vinylimidazolium ionic liquid grafted poly（vinylidene fluoride）（PVDF-g-IL（Br））blends were obtained by a simple,environmentally friendly and easily industrialised electron beam irradiation grafting method;then,highly hydrophilic and high flux PVDF-g-IL（Br）functional water treatment membranes were prepared by non-solvent-induced phase separation and applied for the first time to separate multi-pollutant wastewater from cationic dyes and oil-water emulsions.The IL grafted on the PVDF-g-IL（Br）membrane improves the hydrophilicity of PVDF,enlarges the membrane pore structure and reduces the water passage resistance,which increases the membrane flux to nearly 10 times that of the original PVDF membrane.At the same time,the membranes have excellent fouling resistance and long service life,with a retention rate of over 98%and a flux recovery rate of nearly 100%for cationic dye solutions,oil-water emulsions and their mixtures.Part II:Based on the research work in the previous part,in this part,1-allyl-3-methylimidazolium acetate ionic liquid with p H-responsiveness was selected to graft poly（vinylidene fluoride）for the preparation of PVDF-g-IL（HCOO）-responsive smart membranes with the interconversion ability of hydrophilic-hydrophobicity and positive-negative electrical properties.The membrane can be used for the separation and purification of fully charged dye wastewater,and both the retention and adsorption of dyes are close to 100%.The grafted IL significantly improves the hydrophilicity of the PVDF-g-IL（HCOO）membranes,increases the porosity and average pore size,and enhances the flux,while at the same time allows the membrane flux to be adjusted to the acid-base environment to suit different separation needs.In addition,the PVDF-g-IL（HCOO）smart membrane has dye adsorption and desorption properties,which makes an outstanding contribution to solving the membrane contamination problem caused by the dye separation process.Part III:This part proposes a new strategy for preparing photo-responsive multifunctional PVDF membranes to achieve efficient and controllable multi-pollutant wastewater separation and photo self-cleaning effect.Firstly,"strawberry-like"core-shell structure boron nitride@dopamine-silver（BN@PDA-Ag）nanosheets with integrated photoconversion and thermal conductivity were synthesised,and then they were combined with temperature-sensitive PVDF-g-isopropylacrylamide（PNIPAAm）to construct a light-responsive multifunctional water treatment membrane.The prepared PVDF-g-PNIPAAm/BN@PDA-Ag membrane takes advantage of the photothermal conversion ability of Ag nanoparticles and the high thermal conductivity of BN to rapidly warm up under a short period of light conditions,and induces the temperature-sensitive effect of PNIPAAm to shrink its chain and enlarge the aperture,thus obtaining the ability of light to control the membrane flux.On the basis of this performance,the PVDF-g-PNIPAAm/BN@PDA-Ag membrane demonstrates the multifunctional effect for small pore retention and large pore cleaning,with retention rates of～82%,99%and 95%for BSA,oil-water emulsions,and various dyes,respectively,and flux recovery of close to100%.Part IV:In this part,a series of Ti O₂@PDA core-shell particles with different shell thicknesses were first prepared by using the adhesive property of dopamine to make it tightly coated on the surface of titanium dioxide,then,such core-shell particles were mixed into PVDF-g-IL matrix to obtain functionalized PVDF-g-IL/Ti O₂@PDA composite membranes with excellent dye removal and photodegradation self-cleaning ability.The combined effect of PDA encapsulated in Ti O₂@PDA core-shell particles and IL grafted on the matrix PVDF chain segments promotes the homogeneous dispersion of inorganic Ti O₂nanoparticles in PVDF membranes,as well as significantly improves the hydrophilicity and water flux of the membranes.In addition,the strong hydrophilic PDA induces more Ti O₂particles to migrate to the membrane surface during the phase transition,which promotes both dye retention and adsorption of the composite membrane,and the membrane retention and adsorption of anionic/cationic dyes are close to 100%.Finally,PDA capped on the Ti O₂surface can effectively facilitate the transfer of photogenerated electricity and holes,as well as broaden the bandgap of Ti O₂,which significantly improves the photocatalytic activity,resulting in the membrane’s degradation rate of all kinds of dyes greater than 80%.The photocatalytic degradation of PVDF-g-IL/Ti O₂@PDA composite membranes endows the membranes with excellent self-cleaning properties,and the membrane flux after photodegradation self-cleaning can be basically equal to the original flux.Part V:In this part,a composite photocatalyst with an inside-out Ti O₂-g-CS/CNTs three-layer structure was successfully prepared by utilising the electrostatic self-assembly interaction between positively charged chitosan（CS）and negatively charged carboxylated carbon nanotubes（CNTs-COOH）,and it was added into PVDF matrix to further optimise the dye degradation efficiency and self-cleaning function of PVDF photocatalytic membrane.The intermediate layer of CS adjusts the interfacial effect between the inner Ti O₂and the outer CNTs and induces the transfer of photogenerated carriers in the catalytic reaction,obtaining a wider light absorption band,which significantly improves the photocatalytic activity of Ti O₂.Ti O₂-g-CS/CNTs-PVDF composite membranes degrade more than 97%of highly concentrated dyes in 2h.In addition,a large number of hydrophilic groups in CS and CNTs-COOH greatly improve the hydrophilicity of Ti O₂-g-CS/CNTs-PVDF composite membranes and contribute to the homogeneous dispersion of Ti O₂on the membrane surface.Meanwhile,the introduction of Ti O₂-g-CS/CNTs particles also promotes the change of membrane pore structure,which enlarges the membrane flux to three times that of pure PVDF,and the composite membrane also retains high dye retention and adsorption capacity,with the dye removal rate higher than～98.5%.Finally,even after a number of light cleaning and re-filtration cycle experiments,the morphology and performance of the membrane do not change significantly,and it can always maintain a good stability.