Investigation On Nanofiltration Performance Of Tunable Structure Stability Of Graphene Oxide Composite Membrane

Membrane separation technology is widely used in the field of water treatment due to its advantages of low energy consumption,no phase change,and convenience for continuous treatment.Developing new membrane with excellent separation performance is the key to membrane separation technology.organic/inorganic composite membrane was proposed to combine the virtues of both organic and inorganic membrane materials to obtain better separation performance.Graphene oxide?GO?is a kind of 2D inorganic material with a single atomic layer thickness and it is easy to form a membrane.The laminated 2D material can form a separation film of several-atom thickness,and the stack can form micro/nano-scale channel for water permeation.Furthermore,these channels provide a critical energy barrier for repelling the solute.Therefore,GO composite membrane is expected to achieve both high water flux and high rejection.However,the hydrophilic GO laminates tend to crinkle or even detach from the supporting substrate due to the lack of a strong interaction between the GO separation layer and the organic support layer.Therefore,there is a need for a simple and convenient method without sacrificing the performance of the base film to improve the structural stability of the graphene oxide composite film.In this study,we introduced crosslinking sites on the surface of organic support layer through physical?doping GO?and chemical?alkaline hydrolysis?modification,and employed diamine molecules as crosslinking agents to establish a covalent bond between the GO separation layer and the organic support layer.In addition,the sieving ability of the GO separation layer is related to the interlayer spacing,and the interlayer spacing is affected by the degree of GO oxidation.Therefore,in this study,we also adjusted the interlayer spacing by controlling the degree of GO oxidation to further improve the sieving performance.We have prepared graphene oxide nanosheets with a high oxidation degree up to 46%through the modified Hummers method.Crosslinking sites were introduced into the polysulfone?PSf?substrate by physical methods?doping GO?.M-xylylenediamine?MXDA?was employed as the cross-linking agent to connect GO separation layer and PSf/GO substrate and construct the GO separation layer/PSf substrate interpenetrating network structure.The GO composite membrane exhibits excellent structure stability in water,acid,and alkaline solutions for several days or even longer.Such membranes show comparable water flux with the pristine GO composite membranes of4.4 L·m^-2·h^-1·bar^-1,and nearly complete rejection to dyes?i.e.methyl orange,methylene blue,congo red and reactive green?of more than 99%.We have reduced c-GO@PSf/GO to c-rGO@PSf/GO by L-ascorbic acid.The layer spacing reduced from 10.2?to 9.2?because the interlayer space occupied by hydroxyl groups is released.Such membranes show water flux of 2.88 L·m^-2·h^-1·bar^-1.The rejection of NaCl increased from 13.94%to 25.31%,and the rejection of Na₂SO₄ increased from 39.52%to71.84%.The surface of PAN was be modified by alkaline hydrolysis,and carboxyl groups appeared.M-xylylenediamine and 1,4-butanediamine was employed as crosslinking agent to connect GO separation layer and PAN substrate.The GO composite membrane exhibits excellent structure stability in water,acid,and alkaline solutions for several days or even longer.Such membranes show water flux with 7.86 L·m^-2·h^-1·bar^-1 and the rejection of Congo Red with more than97.59%.