Study On Controllable Construction Of Graphene Oxide-based Separation Membranes And The Performance

Membrane separation is recognized as a high-efficiency,green,and low-carbon separation technology,which is thus widely used in the field of wastewater treatment,drinking water cleaning,as well as gas separation and so on.High-performance membrane is the key to realize low-energy-consumption and high-efficiency application,which highly depends on the membrane material and the manufacture process.Graphene oxide（GO）is considered as the next generation high-performance membrane materials,due to its intrinsic merits,e.g.,ultrathin 2D nanostructures,superior water dispersibility,membrane-forming ability,stacked-structure controllability,and distinctive 2D nanochannels.However,there are still several issues challenging the development of GO membranes,including the inability of the ideal stacking structure model in precisely description of the mass transfer mechanism and the membrane compaction induced performance degradation during running.Therefore,establishing a model for description of GO membrane structure and controlling the GO membrane stacking structure are essential in preparation of high-performance membrane for practical applications.To solve above issues,in this thesis,we fabricated GO membranes by the bottom-up method,i.e.,self-assembling the nanosized GO nanosheets.The microstructures of GO membrane were confirmed by using low-field nuclear magnetic resonance（LF-NMR）combined X-ray diffraction（XRD）technique.Besides,we also established the critical concentration model on the basis of the spatial volume repulsion effect,which guided us to construct a series of GO membranes with different microstructures by optimizing the size,concentration,assembly method,and post-hybrid modification strategy of GO nanosheets.The fabricated GO membranes were applied for nanofiltration and gas separation according to their microstructure characters.Combining the experimental results with the theoretical simulation,we finally elaborated the structure-efficiency relationship between GO membrane structure and separation performance.（1）The GO-based loose nanofiltration（LNF）membrane was prepared by ice crystal template method.Utilizing XRD coupled LF-NMR technique,we,for the first time,verified the hierarchical pores in the GO membrane,i.e.,ordered 2D interlayer subnanometer channels and orderless stacking defective nanopores.Moreover,by tuning the water content and freeze drying treatment,the size of 2D interlayer channels of GO membranes can boost from 0.75 to 0.93 nm,and the volume of the defective pore can enhance around 10 times.The such-prepared GO membrane was used for desalination of dye/Na Cl solutions.The optimized membrane demonstrated more than98.0%rejection to negatively charged dyes（M_w>700 Da）,<20.0%rejection to Na Cl,as well as a high water permeability of 25.8 LMH/bar.（2）A two-step strategy,called‘freeze-drying and in-situ crystallization’,was adopted to construct connected nanoporous and sub-nanopororous channels in the GO membrane,which offered both superior water transportation and high-pressure resistance.In detail,the ZIF-8 nanoparticles was in situ and selectively crystallized at the defective pores of GO membrane created via the ice-crystal template method,taking advantages of the selective permeation of ZIF-8 precursor methanol solution in the GO membrane.Resultantly,the ZIF-8@f-GO hybrid membrane was obtained.The 2D nanochannels originated from the interlayer space and discontinuous ZIF-8 channels synergistically enhanced the structural stability,mass transfer,and selectivity of the GO membrane.The membrane was employed for nanofiltration application and the optimized ZIF-8@f-GO hybrid membrane offered a superior water permeability of 50LMH/bar（30-fold enhancement compared with pristine GO membrane）,a low flux attenuation rate（less than 10.0%）,and long-term running stability（180 h）.（3）A novel strategy named‘confined gelling and rigidified by crosslinking’to construct low-pressure-resistance loosen GO nanofiltration membrane（X-s GOm）by using the self-assembled supra-GO-nanosheets（s-GO）as the building unit.In detail,the GO gel membrane was firstly fabricated;subsequently,the single-layer GO nanosheets in the membrane were in situ self-assmebled to form s-GO with different chemical reagents;finally,the GO membrane can be obtained after drying.The interaction forces between s-GO were intensified with the chemical stimulus,which thus enhanced the stability of s GO membranes.Moreover,the size of s GO building unit and loosen degree of X-s GOm can be tuned with different chemical reagents.The optimized X-s GOm showed 3.5-foled increased volume in nanopores,compared with pristine GO membrane,which was subsequently empolyed for nanofiltration.The result showed that the optimized X-s GOm provided a high water permeability of 51.9LMH/bar（20-fold enhancement）,98.0%rejection to negatively charged dyes（Mw>400 Da）and<10.0%rejection to Na₂SO₄.（4）We establised the critical concentration model for GO membrane on the basis of the spatial volume repulsion effect proposed in Onsager model.According to the model,two kinds of limited stacking structures in GO membrane were evolved:C-GOm and O-GOm corresponding to nanoscale culvert structure and nanoscale overlapping slit type structure,respectively.Guided by the structure model,the controllable assembly experiment of GO membrane was explored.Then,two types of GO composite membranes with different microstructures were prepared by vacuum-filtration method and spin-coating method,respectively.The two kinds of GO composite membranes were applied for CO₂ separation.The results showed that the ideal selectivity of H₂/CO₂ and N₂/CO₂ for C-GOm were 33 and 11,respectively,with applied pressure of 0.1 bar;however,the CO₂ could not penetrate through the O-GOm when the applied pressure lower than 0.8 bar,which exhibited pressure-gate effect.Besides,the mass transfer process of CO₂ in GOm was also confirmed by MD simulation.