Preparation Of Superhydrophobic Polyimide Fiber Membrane And Its Application In Membrane Distillation

Membrane distillation(MD)is a new type of thermal-driven membrane separation technology,has a wide range of applications in desalination,wastewater treatment,enrichment and emission reduction.However,traditional membrane distillation films such as polyvinylidene fluoride(PVDF),polytetrafluoroethylene(PTFE)and polypropylene(PP),there are some shortcomings such as low membrane flux,easy membrane blockage and easy film wetting,etc.Therefore,the development of a hydrophobic performance,large membrane flux,and high salt rejection rate membrane plays an extremely important role in promoting membrane distillation industrialization.In this paper,ultra-hydropbic polyimide nanofiber membrane with the method of double bimodal hydrophobization modification and core elect rospinning was prepared for membrane distillation.The membrane was applied to direct contact membrane distillation(DCMD)experiment to study the MD efficiency under different experimental conditions,and explore the application then.Membrane pore structure and hydrophobic properties determine the membrane flux and salt retention rate.At present,the conventional film has the advantages of low porosity and small twist of the channel.Therefore,it is considered to use the electrospinning technique to prepare the base film with small fiber diameter,high porosity,large specific surface area and cross-linking fiber.In addition,hydrophobic modification of the base film is one of the effective measures to delay the film wetting process.At present,the methods of obtaining the hydrophobic membrane structure are in situ and post-treatment.This article uses a post-treatment on a base film.First,dopamine(DA)and polyethylene imine(PEI)in the water can be cross-linking reaction to form a layer of biomimetic plastic charged coating(C-PDA/PEI).The positive charge viscous layer was constructed without destroying the surface of the PI fiber membrane.Then,the negatively charged silica nanoparticles(Si O2 NPs)were assembled on the surface of the nanofibers by electrostatic adsorption,and the surface structure of the films was reconstructed to increase the roughness of the fiber surface.Finally,the surface energy of the fiber membrane is reduced by fluorination to form a superhydrophobic polyimide porous fiber membrane(152 °)on the imitative lotus leaf surface.The modified PI film was tested by membrane distillation,and compared with the PI film of different stages and the PTFE membrane with different pore size.It can be seen that the modified PI film not only dominates the stability,but also membrane flux and rejection rate is higher than the same pore size PTFE film.In order to test the anti-fouling properties of the membrane,the modified PI film was applied to the treatment of Na Cl saline containing surfactant(SDBS),humic acid(HA)and methyl blue dye as organic pollutants,respectively.The high membrane flux and rejection rate confirms that the modified PI film has have good resistance to pollution.Then,the modified PI film was subjected to the concentration reduction of reverse osmosis(RO)concentrated water.It was found that the volume of the feed liquid was concentrated to the original 1/4,and a higher rejection rate of membrane flux was maintained.The results showed that the modified PI film could ensure the anti-pollution and self-cleaning performance even though the concentration of the feed liquid was high.Finally,the modified PI film was subjected to a membrane distillation experiment to simulate seawater desalination.The technic al evaluation and economic analysis of the membrane distillation desalination process provided another possibility for membrane distillation in seawater desalination.