The Study On The Preparation Of Graphene Oxide Based Membranes And Its Separation Performance

Since the 21st century,the shortage of fresh water and the pollution of water are increasingly aggravated globally,which endanger the ecological environment of the earth and security of human beings.This propels people to be devoted to researching on seawater desalination and water purification.Membrane separation technique is a physical water treatment method with environmental friendliness,low energy consumption,high efficiency and selectivity.Graphene oxide has a unique two-dimentional structure with one-atom thickness,and is chemically stable,membranes fabricated from which are shining brightly in the domains of seawater desalination and water purification.In this paper,graphene oxide?GO?was synthesized according the modified Hummers method and GO based membranes were fabricated by the filtration-assisted assembly method.The water desalination and purification performances of GO based membranes were also investigated.The main research contents are as followings:?1?The porous g-C₃N₄?PGCN?was synthesized by the pyrolysis of the mixture of thiourea and ammonium chloride,and PGCN nanosheets were obtained by ultrasonic exfoliation.All carbon-based GO/PGCN hybrid membrane was fabricated by filtrating the dispersion of GO and PGCN under vacuum,and rGO/PGCN hybrid membrane was acquired in virtue of in situ photocatalytic reduction.The effects of UV irradiation time,the mass ratio of GO to PGCN and deposition density of GO on the NaCl desalination performance of rGO/PGCN hybrid membranes were studied,and the optimized conditions of preparing membranes were confirmed.The effect of NaCl concentration on the desalination performance was investigated.The results indicated that,without any chemicals,GO was successfully reduced to rGO in virtue of PGCN under UV irradiation.The desalination performance of rGO/PGCN hybrid membrane was the best when UV irradiation time,the mass ratio of GO to g-C₃N₄ and deposition density of GO were 30 h,6:1 and 477.7 mg/m²,respectively.With NaCl solution concentration decreasing,both the rejection towards NaCl and flux of the as-prepared hybrid membrane increased.?2?g-C₃N₄ was synthesized by the pyrolysis of melamine.The positively charged and ultrathin g-C₃N₄ nanosheets?HGCN?were obtained after ball milling,protonation and ultrasonic exfoliation.Because of the electrostatic attraction,HGCN can be intimately assembled with GO.All carbon-based GO/HGCN hybrid membrane was fabricated by filtrating the dispersion of GO and HGCN under vacuum,and rGO/HGCN hybrid membrane was acquired in virtue of in situ photocatalytic reduction.The desalination performance of rGO/HGCN hybrid membranes with different degrees of reduction was investigated utilizing NaCl,Na₂SO₄,MgCl₂ and MgSO₄.Combining the characterization results of XRD,XPS,water static contact angles and so on,the desalination mechanism was systematically analyzed.The results indicated that,with the gradual reduction of GO,the hydrophobicity of rGO/HGCN hybrid membranes was increased,which resulted in the restrained swelling effect of rGO/HGCN hybrid membranes and weakened electrostatic screening of the hydration shells.And then,the rejections towards four salts ascended gradually.For the hybrid membranes with the same reduction extent,the rejection rates towards four salts were different.In the case of the four salts,it displayed a sequence of R?Na₂SO₄?>R?NaCl?>R?MgSO₄?>R?MgCl₂?.The rGO/HGCN hybrid membranes exhibited prominent desalination performance,especially for sodium salts?67.5%for NaCl,89.2%for Na₂SO₄ at a pressure of 0.1 MPa?.The results showed the desalination performance of the hybrid membranes was dominated by physical size sieving and Donnan exclusion.?3?GO was modified using 4-nitrobenzene diazonium tetrafluoroborate and modified GO was obtained.Membranes were fabricated by filtrating modified GO dispersion under vacuum.The separation performances of GO membranes modified by different amounts of 4-nitrobenzene diazonium tetrafluoroborate were investigated utilizing EB,DR81,MB and RhB.The results indicated that the flux was greatly enhanced while preserving the high rejection of dyes when GO membranes were modified by suitable amounts of 4-nitrobenzene diazonium tetrafluoroborate.GO membranes had the better separation performance for the negatively charged dyes with large size.