Controllable Fabrication Of Continuous Monolayer Nanonet Membranes And Their Application In Water Filtration

The increasing worldwide contamination of fresh water by pathogenic microbes is one of the key global challenges facing public health with the acceleration of the process of urbanization and the global population explosion.According to the date from the World Health Organization that 1.2billion people lack access to safe drinking water,2.6 billion have no sanitation,and 2 million people die from diseases transmitted through pathogen contaminated water.The commonly used water purification methods involving direct disinfection and heat treatment are generally subject to critical limitations of easy secondary contamination and ineffective for spore-forming bacteria.Alternatively,membrane filtration is a safe and effective method in terms of no secondary pollution and absolute physical sieving.The existing microfiltration membranes,however,are usually limited by low fluxes(<1000 L m^-2 h^-1)even under high driving pressures?>100 k Pa?due to their inferior structures involving low porosity?<80%?,partly closed pore channels and large thickness?>100?m?.In view of their advantages of thinner fiber diameters,high porosity,favorable interconnectivity,and adjustable thickness,electrospun nanofibrous membranes?NFMs?hold great promise as exceptional materials for constructing microfiltration membranes with high permeation fluxes under low driving pressures.Despite their outstanding potential,most of these membranes were only used to size-sieve large-sized contaminants?>1?m?as a result of their relatively large pore size?>1?m?caused by the pseudo-nanoscale diameters?>100 nm?.To decrease the pore size of NFMs for the further improvement of their filtration capacity for sub-micron sized contaminants,several efforts were devoted to fabricating dual-network structured nanofiber/net membranes?average fiber diameter in the nanonets of 20?30 nm?by electro-spinning/netting technology.However,severely restricted by the principle of this technique?the multi-layer deposition of networks?,the resultant membranes are subjected to dense packing structures and low network coverage,leading to a bottleneck in increasing the flux.Therefore,it is still a great challenge to develop new approaches for constructing novel nanonet membranes with continuous monolayer nanonets on the surface of NFMs.In this thesis,we pointed out that the critical factor to solve challenges currently is to develop new methods to construct novel nanonet membranes on the basis of summarizing the electrospun and electrospun/net water filtration materials.Herein,we develop innovative nanonet membranes with NFMs as substrates by evaporation-induced selfassembly or phase separation.Researching the structure-function relationship between the nanonet membranes and water filtration to fabracate membranes with high-flux and energy-saving properties for the purification of water containing submicron-sized contaminants.The detailed contents are summarized below:?1?Bacterial cellulose?BC?nanonet membranes with stable nanonet structures were fabricated via evaporation-induced self-assembly of micro-length BC nanofibers on electrospun polyacrylonitrile nanofibrous membranes?PAN NMs?and subsequent simple cross-linking by dimethyloldihydroxyethyleneurea.The premise of our design is that the morphologies of the nanonets could be effectively controlled by controlling the size of BC nanofiber and regulating the diameter of PAN nanofibers substrate and the loading content of BC nanofibers.As a result,BC nanonet membranes with small pore size?0.22?m?and high porosity?89%?were fabricated by loading 90 mg m^-2 BC nanofibers on PAN NMs with fiber diameter of 305 nm.Consequently,by virtue of the superior structures,the resulted BC membrane exhibited high filtration performance(rejection efficiency of 99.63%,permeate flux of up to 3503 L m^-2 h^-1)for 0.3?m Ti O₂microparticles and excellent separation(log reduction value?LRV?of 8.2,permeate flux of 1557 L m^-2 h^-1)performance for E.coli.?2?PAN nanonet membranes with continuous nanonet structures were one-step constructed by vapor-induced phase separation using electrospun nanofibrous membranes as the substrates.The effect of solvents involving dimethyl sulfoxide,N,N-dimethylformamide,nitromethylpyrrolidone?NMP?,and N,N-dimethylacetamide?DMAc?on the phase inversion process of PAN solution and the morphologies of the finally membrane was systematacially investigated,and it was found that membrane with porous film was obtained by phase inversion of PAN solution with NMP as solvent.The relative humidity?20,50,90%?was further regulated to control the phase separation of PAN solution and finally PAN nanonet membranes with nanonet fiber diameter of?45 nm,pore size of0.19?m,and porosity of 93.2%were fabricated at relative humidity of 90%.Thanks to the optimized structures of small pore size and high porosity,the prepared PAN nanonet membranes showed excellent removal efficiency of 99.75%and permeation flux of 3907 L m^-2 h^-1 for 0.3?m Ti O₂ particles at an ultralow driving pressure of 5 k Pa,intriguing reusability?10 cycles?,high bacterial rejection efficiency?LRV of 8.2?and robust permeation flux(1206 L m^-2 h^-1).?3?Nonsolvent induced phase separation technique was applied to construct continuous cellulose nanonet membranes with continuous nanonet structures.The influence of nonsolvent bath?water,methanol,ethanol,isopropanol?was first studied,proving that the higher polarity of water resulted in a better intersolubility with DMAc,which caused a fast phase separation of the cellulose solution to form nanonet structures.Then structures of the nanonets were further effectually designed and optimized by means of regulating the coagulation bath temperature?10,25,40,55 ^oC?and cellulose concentration?0.002,0.004,0.008,0.016 wt%?.Consequently,cellulose nanonet membranes with ultrafine fiber diameter??42 nm?,high nanonet coverage??100%?,small pore size?0.23?m?,and high porosity?90.7%?were constructed by phase inversion of cellulose solution?concentration of 0.008 wt%?in water bath with temperature of 25 ^oC.The results of water filtration tests demonstrated that the cellulose nanonet membranes colud filtrate 0.3?m Ti O₂ particles with a removal efficiency of 99.80%and permeation flux of 4306 L m^-2 h^-1 under an ultralow driving pressure of 5 k Pa,even achieving high removal efficiency of LRV=8.0 and excellent permeation flux of 1444 L m^-2 h^-1 for bacteria.?4?To prepare nanonet membrane which can be applied in filtration of corrosive liquid,continuous polyvinylidene fluoride?PVDF?nanonet membranes with Si O₂ NMs as the substrates was first constructed by nonsolvent induced phase separation.The influence of solvent on the phase separation of PVDF and the structures of the final membrane was systematacially investigated,confirming that nanonet structures along with porous films could be obtained with DMAc as the solvent.Then Tween was further added to the PVDF solution to regulate the phase separation process to construct membranes with uniform nanonet structures.As a result,PVDF nanonet membranes with nanonet fiber diameter of 38 nm,crystallinity of 53.3%,pore size of 0.18?m,porosity of 93.7%,and water contact angle of 26^o were successfully fabricated.Benefitting from the structural advantages,PVDF nanonet membranes could remove E.coli with high efficiency of LRV=8.5,which is similar to commercial HPVDF microfiltration membranes,whereas the permeation flux(2803 L m^-2 h^-1)of PVDF nanonet membranes is 5 times higher than that of HPVDF microfiltration membranes.Additionally,PVDF nanonet membranes could endure corrosion?p H value of 2 and 12?with removal efficiency of 99.80%and permeation flux of?4.3×10³ L m^-2 h^-1.