Improvement Of Performance And Self-cleaning Mechanism Of Two-dimensional G-C₃N₄-based Separation Membrane Modified By Sodium Alginate

It has been found that graphitic carbon nitride（g-C₃N₄）nanosheets can be stacked and assembled into two-dimensional nanosheet separation membranes with great separation performance,and as a polymeric semiconductor material with visible light response,it can be used to construct new photocatalytic self-cleaning membranes.However,the stacking of nanosheets in g-C₃N₄ membranes was too tight,causing the interlayer gap to be too narrow（0.324 nm）,which seriously affected the transmission performance of the membranes.Therefore,this study attempted to expand the layer spacing of the g-C₃N₄ membrane by means of organic molecular intercalation,aiming to improve its permeation performance while maintaining its high retention rate of dyes,and to investigate the photocatalytic in situ self-cleaning mechanism of the composite membrane.The main conclusions were obtained as follows.（1）The high-quality g-C₃N₄ nanosheets were prepared by concentrated hydrochloric acid etching combined with ultrasound-assisted peeling,with a thickness of approximately 1.5 nm and lateral dimensions of 2-3μm,and were successfully compounded with sodium alginate（SA）molecules by ultrasonication under electrostatic force and hydrogen bonding forces.g-C₃N₄-SA_X/PES was assembled on a porous polyethersulfone（PES）substrate membrane by vacuum filtration composite membrane.The surface and cross-sectional structures of the membranes were characterized by FE-SEM and AFM.After the introduction of SA,the thickness of the g-C₃N₄-SA_X/PES composite membrane increased while the surface roughness decreased significantly.The increase in O and Na content in the composite membrane is also evident from the EDX and XPS analyses,indicating the successful introduction of SA.The FTIR characterisation showed that the-OH on SA forms a hydrogen bonding interaction with the terminal-NH/NH₂ on the g-C₃N₄ nanosheets,anchoring SA stably between the g-C₃N₄ nanosheet layers,which can form a more homogeneous and stable membrane structure.the XRD characterisation shows that the SA molecules embedded between the nanosheet layers expand the partial layer spacing from 0.324 nm to 0.826 nm,providing a fast transport of water molecules.The synergistic effect of the two was beneficial to the enhancement of the membrane permeate flux.（2）The separation performance and transport mechanism of the g-C₃N₄-SA_X/PES composite membrane for dye molecules was investigated using Evans Blue（EB）dye solution（10 mg/L）of size 3 nm as a simulated wastewater.It was found that when the SA content was 15 wt%and the membrane thickness was 100 nm,the permeate flux could reach 165.24 L·m^-2·h^-1·bar^-1,which was 12.2 times of the water flux of the original g-C₃N₄ membrane,and the retention rate of EB dye molecules could reach 94.87%.The permeation experiments of the composite membranes on dyes of different sizes and charge properties（AB,EB,CR,MB and AF）showed that the separation of the target contaminants by the composite membranes relied mainly on the size sieving effect rather than the electrostatic adsorption effect.In addition,the composite membranes were chemically stable and structurally stable in acidic to weakly alkaline environments and organic solvent immersion.（3）The in situ photocatalytic self-cleaning performance of the composite membranes under visible light irradiation was systematically investigated and their mechanism of degrading organic pollutants was analysed.It was found that the membrane flux of the g-C₃N₄-SA₁₅/PES composite membrane with a thickness of 100 nm decreased to 50%of the initial flux and stabilised within 3 h during a 5 h dye（EB and CR）separation permeation experiment.Hydraulic cleaning experiments and photocatalytic self-cleaning experiments showed that the membrane flux could not be fully recovered by hydraulic cleaning alone.After visible light irradiation,the membrane flux was almost completely restored,and the efficient retention of dye molecules was maintained throughout the 5cycles,indicating that the g-C₃N₄-SA₁₅/PES composite membrane has great self-cleaning stability.In addition,the static photocatalytic degradation experiments showed that the introduction of SA can promote the adsorption of organic pollutants and the separation and transfer of photocatalytic carriers,resulting in the enhanced degradation efficiency of EB and TC by the composite membrane.Free radical capture experiments showed that h⁺and·O^2-were the main active components in the self-cleaning process of composite membranes,with·OH as a secondary component.