| The rapid development of industrialization and urbanization has brought convenience to people's production and life,but at the same time it has also produced a lot of environmental problems.For example,a large amount of oily wastewater and water-soluble dyes and other pollutants are discharged into the environment,seriously damaging the ecological environment and endangering human health.Therefore,how to achieve efficient treatment of oily sewage has become an urgent problem to be solved.Membrane separation technology is considered to be one of the effective means of treating oily wastewater.Among them,polymer microfiltration membranes such as polyvinylidene fluoride?PVDF?membrane,polytetrafluoroethylene?PTFE?membrane,and polypropylene?PP?membrane are widely used due to their excellent chemical stability,mechanical properties,and acid and alkali resistance.In the field of water treatment.However,PVDF membrane,PTFE membrane and PP membrane are highly hydrophobic.In the process of treating oily sewage,the membrane surface is easily polluted by oil droplets,which blocks the pores of the membrane.Thereby,it leads to a significant decrease in water flux,which severely limits their application.Nevertheless,giving the super-hydrophilic property and the underwater super-hydrophobic property in the polyvinylidene fluoride?PVDF?membrane,polytetrafluoroethylene?PTFE?membrane,and polypropylene?PP?membrane has proved to be an effective way to solve the above problem.In the thesis,we used three modified strategies based on tannic acid?TA?to systematically investigate the structural composition,formation mechanism,wettability and stability of the coatings formed by different modification strategies.On this basis,the super-hydrophilic modification of various hydrophobic polymer film surfaces was realized,and the oil-water separation performance and anti-pollution performance of the obtained super-hydrophilic separation films were studied in detail.Based on tannic acid?TA?with unique adhesion properties and hydrolyzable 3-aminopropyltriethoxysilane?APTES?,we have developed a one-step modification strategy that can be applied to a variety of hydrophobic membrane surfaces.A rough coating?TA-APTES coating?consisting of a large number of microspheres can be constructed in the various hydrophobic film surface.Thus,the modified membrane possess the properities of super-hydrophilicity and underwater super-oleophobicity.Taking the PVDF film as an example,the original film and the modified film materials are respectively subjected to scanning electron microscopy?SEM?,total reflection Fourier infrared spectroscopy?ATR-FTIR?,and X-ray photoelectron spectroscopy?XPS?.These characterizations demonstrate the morphology structure and chemical composition.Studies have shown that tannic acid?TA?and hydrolyzable 3-aminopropyltriethoxysilane?APTES?form a nanosphere coating by the Michael addition reaction or the Schiff base reaction,and through adjusting the hydrolysis time of the 3-aminopropyltriethoxysilane?APTES?can control the roughness of the coating.Under the optimal conditions,the surface water contact angle of the modified membrane material is 0°,the underwater oil contact angle is greater than 150°and the rolling angle is less than 5°,which can achieve high-efficiency oil-in-water emulsion separation.The pure water flux of the modified membrane and the oil interception rate of the oil-in-water emulsion reached 6762 L m-2 h-1 bar-1 and 99.6%,respectively.The dynamic underwater oil adhesion test and the underwater oil column impact test results show that the modified polymer film has excellent anti-pollution performance in the oil-water separation.Cyclic filtration experiments show that the water flux recovery rate and oil intercept rate of the modified polymer membrane can reach more than 99%.In addition,the TA-APTES coatings on the surface of the PVDF film,PTFE film,and PP film have good stability and can withstand severe rinsing.A strategy for constructing multi-level nanoscale structures?MHS?based on TA-APTES coating as the substrate surface is proposed.The PP film modified by TA-APTES coating is used as the substrate,which is treated with tannic acid and polyethyleneimine?PEI?in sequence.The nano-nodes are builded on the TA-APTES coated nanospheres to form a multi-layer structure coating Layer?MHS?.The strategy can form a multi-stage structure on the surface of the PP film,which greatly increase the surface roughness of the coating and the surface area of the coating,and achieve the regulation of the membrane pores.The obtained functional separation membrane not only has the super hydrophilicity but also achieve efficient separation of nano-sized emulsified oil.Furthermore,the adsorption capacity of the separation membrane to dyes was also greatly promoted.Scanning electron microscopy shows that the TA-APTES coating is treated with tannic acid and polyethyleneimine,and the nanospheres on the surface of the PP film are evenly distributed and contained a large number of nano-nodes;The total reflection infrared spectrum and X-ray photoelectron spectroscopy of the coating surface indicate that the coating contains a large amount of amino groups;Element surface scanning results show that PEI is uniformly introduced into the film surface;The Zeta potential results show that under neutral or acidic conditions,the surface of the multi-layered coating is positively charged,which provides conditions for the adsorption of anionic dyes such as methyl blue under these conditions.The modified functional film not only has super-hydrophilic/underwater super-oleophobic properties,but also has smaller pores and a larger surface area.The smaller pore size allows for the separation of nanoscale emulsified oils;the higher surface area allows for a significant increase in the ability to adsorb dyes.The modified functional membrane water flux reaches 4062L m-22 h-11 bar-1,which is 46 times that of the unmodified PP microfiltration membrane;The amount of dye adsorption also reached 480 mg g-1.The modified membrane can simultaneously remove nano oil droplets and dyes in the sewage,exhibiting versatility.Moreover,the super-hydrophilic coating with multi-layer nano-scale structure also endowed excellent anti-pollution properties to the modified membrane,and has excellent stability,can tolerate severe washing and poor acid-base salt environment and maintain super-hydrophilicity/underwater super oleophobic properties.Inspired by leather tanning and hydrophobic membranes,we have further developed a super-hydrophilic modification method for completely green hydrophobic membranes.We chose safe,green,and edible ovalbumin as a co-coating modifier with tannic acid.First,the PVDF microfiltration membrane to be modified is placed in a certain concentration of neutral ovalbumin solution and allow to stand for a period of time to fully adsorb the ovalbumin on the surface of the membrane.The cured hybrid coating?OVA-TA coating?is then formed on the surface of the film by the principle that tannic acid facilitates denaturation of the protein.The above method utilizes the principle that the hydrophobic membrane easily adsorbs the protein and the tannic acid easily cures the protein to skillfully solve the problem that the tannic acid alone is difficult to realize the super-hydrophilic modification of the hydrophobic membrane surface.The modification method is simple,the reaction conditions are mild,green and non-polluting.The PVDF microfiltration membrane modified by the method has super hydrophilicity and underwater superoleophobicity,the water flux thereof reached 6231L m-2 h-1 bar-1 as well as the removal rate of the drops is over 99.5%.In the meantime,it also has excellent anti-pollution properties.Besides,the hybrid coating formed by this strategy has good stability and can withstand severe rinsing,high concentrations of saturated brine and sonication treatment while maintaining super-hydrophilicity.It is especially important that the modification strategy is applicable to a variety of hydrophobic membrane materials,including PVDF microfiltration membranes,PTFE microfiltration membranes,copper meshes,and high hydrophobic membrane materials such as woven fabrics.The modification strategy provides a new idea for the super-hydrophilic modification of hydrophobic materials. |
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