Preparation And Performance Study Of Graphene Oxide-Based Hybrid Nanofiltration Membrane Based On Interlayer Spacing Regulation

Membrane technology has demonstrated significant potential in numerous fields,including wastewater treatment and seawater desalination,due to its advantages of energy efficiency,high effectiveness,and environmental sustainability.Efficient membrane separation materials are crucial in the field of membrane separation technology.Therefore,it is important to focus on the design and preparation of these materials.However,traditional membrane materials have the problem of trade-off between high flux and high retention,and the research on the mass transfer mechanism of membrane separation is not deep enough.Graphene oxide（Graphene Oxide,GO）has become one of the research hotspots of membrane materials in recent years because of its unique two-dimensional shape,honeycomb structure,and a large number of oxygen-containing functional groups distributed on the surface and edge of the sheet.In this paper,chemical and physical methods were used to modify GO membranes.First,the composite material GO-CS was prepared by cross-linking chitosan（Chitosan,CS）with graphene oxide by chemical cross-linking method,prepare a series of GO/CS hybrid nanofiltration membranes with different GO-CS ratios by vacuum filtration method,to study the membrane structure and nanofiltration performance at different p H,and explore its separation mechanism.Subsequently,GO was compounded with carbon nitride（g-C₃N₄）（GOx/g-C₃N₄）using thermal condensation method by physical modification,hybrid nanofiltration membranes with different ratios of GO and GOx/g-C₃N₄ were prepared by vacuum filtration,innovatively separated mixed dye systems with different properties,and explored their separation mechanism.The main research contents and conclusions of this paper are as following:（1）GO nanosheets with uniform size distribution were prepared by the modified Hummers method,CS molecular chains were grafted onto GO nanosheets by a one-step hydrothermal method,and different proportions of GO and GO-CS were mixed by vacuum filtration.The solution was suction-filtered to form a membrane,and GO-based hybrid nanofiltration membranes（GOCSMs）with different layer spacings were prepared.The experimental results show that the prepared GO nanosheets are uniform in size,the GO and chitosan are connected in the form of covalent bonds,and all the GO-based composite membranes have different degrees of wrinkles and layered surfaces.cross-sectional structure.（2）By testing the water permeability and retention of dye molecules of GO-based nanofiltration membranes,it was found that GO-based nanofiltration membranes with different GO-CS contents had different interlayer distances,and all GO-based composite membranes It has good hydrophilicity and molecular separation.The water permeability of the nanofiltration membrane increased from 26.5 L·m^-2·h^-1·bar^-1 to84.0 L·m^-2·h^-1·bar^-1 with the increase of the ratio of GO-CS in the membrane,the retention rate of rhodamine B decreased from 99.9%to 92.4%.The experimental results show that 4-GOCSM has the best water permeability and retention of dye molecules,with a water permeability of 75.2 L·m^-2·h^-1·bar^-1 and a water contact angle of 40.3°.The interlayer spacing of membrane is in the range of 7.5-8.6（?）.In addition,4-GOCSM also has excellent chemical stability in strong acid and strong alkali environments.（3）By studying the water permeability and dye retention performance of GO-based nanofiltration membranes at different p H,the nanofiltration performance of the membranes under different p H environments was studied.The research results on p H showed that at p H=3,4-GOCSM had the best water permeability of 102 L·m^-2·h^-1·bar^-1.In addition,4-GOCSM exhibited water permeability more than three times that of GOM at different p H environments.The retention rate of methyl orange increased from 14%to 73%,and the retention rate of methylene blue and RB exceeded 99%.The enhancement of nanofiltration performance in acidic state is attributed to the joint effect of size exclusion and electrostatic repulsion.The separation mechanism of the membrane is affected by many factors,including the size of interlayer nanochannels,the surface charge of the membrane,and the defects inside the membrane.（4）Three kinds of GOx/g-C₃N₄ composites doped with different amounts of GO were synthesized by the thermal condensation method,and they were characterized by scanning electron microscope（SEM）,fourier transform infrared（FTIR）,X-ray diffraction（XRD）,Raman,X-ray photoelectron spectrometer（XPS）tests,which proved that there wereπ-πinteractions in the composites,and the material There is no significant change in internal structural defects.A series of hybrid nanofiltration membranes with different ratios of GO and GOx/g-C₃N₄ were prepared by vacuum filtration.The surface and cross-sectional morphology of the membrane shows that the surface of the membrane is wrinkled and undulating,the degree of which is different,and the cross-section has a layer-by-layer structure.There is no obvious rule between the water contact angle of the membrane and its average surface roughness.（5）The water permeability of GOx/g-C₃N₄ hybrid nanofiltration membrane is further improved based on GOCSMs,with a maximum of 274.7 L·m^-2·h^-1·bar^-1,which is much higher than the water permeability of GOM and more than twice that of GOCSMs(102.0 L·m^-2·h^-1·bar^-1).An innovative separation of mixed dye systems,nanofiltration experiments have proved that the composite membrane has a retention rate close to 100%for the positively charged cationic dye Mn B,and a retention rate of more than 80%for EB,while for the negatively charged anionic dye MO has almost no retention performance（4%）and has a selective separation effect on cationic/anionic dyes.（6）Research and analyze the structure and physical and chemical properties of the membrane,and explore its separation mechanism.Measure the water contact angle,layer spacing,structural defects,specific surface area and pore distribution of the membrane.The results showed that the interlayer spacing of the three membranes was regulated by GOx/g-C₃N₄.Compared with GOM,the water contact angle of the composite membrane was less than 20°,and the hydr0ophilicity was greatly improved.The interlayer spacing of membrane is in the range of 6.9-8.0（?）.The Raman test proves that the structural defects in the film are 2-GO₂₅/g-C₃N₄M,2-GO₅₀/g-C₃N₄M,and 2-GO₇₅/g-C₃N₄M,which increase sequentially.The BET test showed that the four membranes had a specific surface area less than 20 m²/g,the average pore diameter of GOM was the largest（7.37 nm）,and the average pore diameter of the composite membrane was similar（1.60-2.10 nm）.Zeta potential proves that all membranes are strongly negatively charged.For dye molecules with large hydration radii,size exclusion plays a leading role in the dye retention process,and for small anionic dyes,the negative charge on the membrane surface plays an electrostatic repulsion role.