Structural Designs And Performance Studies Of Loose Nanofibrous Composite Nanofiltration Membrane For “Salt-color” Separation

The issue of sustainable recovery of resources from textile wastewater urgently requires a solution.Asa membrane separation technology developed on the basis of nanofiltration（NF）membrane,the loose nanofiltration（LNF）membrane uses pressure as the driving force to realize the separation of liquids.It has the characteristics of high selective separation and low operating pressure,which has developed rapidly in the fields of sugar separation,polyphenol fractionation,product decolorization,textile wastewater treatment,etc.The ideal LNF membrane should have sufficient water permeability,selective separation and stability to ensure long-term operation of membrane separation.In addition,it need to mitigate the negative effects of membrane fouling on membrane permeability,energy consumption,separation performance,membrane life and other aspects.In this article,functional two-dimensional nanomaterials with photocatalytic activity were constructed on the surface of nanofibrous membranes by combining filtration-assisted interfacial polymerization and electrostatic spinning to prepare LNF membranes with loose but strong barrier layers for sustainable treatment of textile wastewater.In this paper,polyacrylonitrile（PAN）nanofibrous porous membrane is used as the base membrane,and two-dimensional g-C₃N₄ nanosheets（GNs）obtained by acid etching and hybrid composite materials ZIF-67@g-C₃N₄ nanosheet（ZGN）,as a functional membrane material,has designed and prepared two kinds of LNF membranes with different membrane structures and nanofiltration separation properties（GN-PA/PAN composite LNF membrane,ZGN-PA/PAN composite LNF membrane）.The details of the study are as follows:（1）In order to improve the permeability of composite membrane,hydrophilic electrospun PANnanofibrous membrane was selected as the porous base membrane.And a novel loose thin film nanofibrous composite NF membrane was fabricated by filtration-assisted interfacial polymerization for dye/salt separation,in which GNs modified with polyethylenimine（PEI）were sturdily integrated together via reaction with organic phase trimesoyl chloride（TMC）to form a polyamide（PA）soldered GNs barrier layer onto polyacrylonitrile（PAN）nanofibrous substrate.Benefiting from the inimitable photocatalytic activity of GNs,the organic dyes adhered and accumulated on the membrane surface could be easily cleared up under visible light irradiation for the great recyclability of the composite membrane.The optimized robust GNs composite membrane possessed high water permeability(40.9 L m^-2 h^-1 bar^-1),superior dyes rejections（>94%）for Congo Red,Methyl Blue and Reactive Black 5 and low salt rejections（<7%）for Na Cl and Na₂SO₄ for excellent salt/dye separation performance（the dye/Na Cl separation factor was up to 236）.The resultant GNs barrier layer endowed the composite membrane with long-term stability and sustainability,the dye permeability could still recover to 93%of the initial membrane during three cycles of visible light irradiation.（2）In order to further construct a high permeability composite nanofiltration membrane withphotocatalytic dye degradation capability,we started from the structural design of the composite membrane,and used ultrasonic assisted in-situ stripping of metal organic skeleton modified g-C₃N₄ to prepare ZIF-67@g-C₃N₄composite nanosheets（ZGN）.Compared with acid etched GNs,ZGN has significantly enhanced photocatalytic activity,while avoiding the chemical waste generated during the stripping synthesis of functional two-dimensional nanomaterials.In this paper,ZGN barrier layer was prepared on the surface of PAN nanofibrous substrate by filtration assisted interfacial polymerization,and a series of composite membranes were prepared by changing the deposition amount of ZGN.The results showed that when the deposition amount of ZGN was600 mg/m²,the pure water permeability of the ZGN-PA/PAN composite membrane was 83.5 L m^-2 h^-1 bar^-1,and it had good separation and permeability for various dye solutions and salt solutions（the dye/Na Cl separation factor was 86）.It also has a certain separation performance for the mixed salt/dye system,the ZGN-PA/PAN composite membrane prepared has good stability in the test pressure range（1-9 bar）,and can still maintain good separation performance after 24 hours of long operation.