Construction Of Hollow Fiber Membrane Loose Nanofiltration Separation Layer And Research On Selective Separation Performance

Loose nanofiltration membranes（LNFs）can fill in the blanks between nanofiltration membranes and ultrafiltration membranes,and can be used to efficiently treat organics with a molecular weight of several hundred and realize the separation between colored organics and salts.This paper focuses on the preparation of hollow fiber LNFs.PVDF/SMA hollow fiber membrane was used as the base,using different methods to construct LNFs with compactness-tailored separation layers on the outer surface to realize dye decolorization.The main research contents are as follows:（1）PVDF/SMA hollow fiber nanofiltration substrate was prepared by thermally induced phase separation technology.The effects of the type and content of non-diluent,SMA model and PVDF/SMA proportion on the membrane structure and permeation and separation performance were investigated to determine the process parameters of the PVDF/SMA hollow fiber membrane.It is finally determined that the non-diluent is P-OH and the content is 18%,the SMA model is 725,and the PVDF/SMA mass ratio is 20/4.（2）The loose nanofiltration separation layers with adjustable microstructure can be prepared via“chemical crosslinking and metal ion coordination”.Three kinds of LNF separation layers with different compactness were fabricated and tailored via chemical cross-linking between MA and PEI,further amidation between amine and carboxyl groups（formed by the ring opening of MA）,or the coordination of Cu²⁺with primary amine and carboxyl groups.The formation process and evolutionary regularity of the microstructures of the three separation layers were expounded.The results showed that the compactness of a separation layer can be effectively tailored by combining chemical cross-linking with metal ion coordination,the mean pore sizes is as small as 1.2 nm.The three as-prepared membranes can effectively reject dyes with different molecular weights,and the rejection for acid red 27,sunset yellow and crystal violet are 99.5%,99.3%and 96.1%,respectively,and maintain a high flux.The size sieving decolorization mechanism of the membranes was demonstrated from the relationship between the pore sizes of the LNFs and the“hydration,aggregation and absorption”process of dye molecules.In addition,the as-prepared membrane displayed excellent anti-bacterial performance and good stability in the long-term separation of dye/salt system.（3）The compactness of LNFs can be further improved by the method of anionic crosslinking and in-situ mineralization.The anionic substances with different molecular weights,functional groups and functionalities had different effect on filling pores of the PEI crosslinking layer,and two kinds of anion cross-linked membranes were chosen for Ca²⁺mineralization.The results showed that sodium tripolyphosphate with appropriate molecular weight and functionality can better fill the pores of the PEI cross-linked layer.In addition,the compactness of the two mineralized membranes is further improved,and the rejection of methylene blue which has smaller molecular weight reached up to 99.1%,and it maintained a higher permeation flux.The outer surface of the mineralized membrane has stable chemical properties and excellent hydrophilicity,and it show a good anti-pollution properties.The FRR can be as high as 97.5%under the condition of pickling.