Molecular Cross-linking And Separation Performance Of Highly Stable Ceramic-based Graphene Oxide Membranes For Emerging Micro-pollutants And Salt Removal

With the rapid development of modern society and industrialization,the pollution problem of micro-pollutants in the water environment has become more and more serious.As an emerging separation membrane,graphene oxide?GO?membranes have demonstrated excellent application potential in water treatment applications in recent years due to their good thermal,mechanical and chemical stability,as well as tunable two-dimensional interlayer nanochannel structure.It is still a key challenge to achieve highly efficient separation performance and excellent stability for precise separation of molecules/ions in water.Herein,a molecular-level cross-linking?thiourea,TU and m-phenylenediamine,MPD?rational structure design strategy was proposed to prepare ceramic-based graphene oxide framework?GOF?membranes with significantly enhanced sieving performance and stability for highly efficient removal of emerging micro-pollutants and salts,focusing on the structure and surface chemistry of nanochannels in GOF membranes,revealing the molecular cross-linking mechanism of GO nanosheets and the sieving mechanism of organic micro-pollutant molecules and salt ions,and clarifying relevant engineering scientific issues.The main research contents and conclusions are as follows:?1?Hollow fiber?-Al₂O₃ ceramic substrate was prepared by wet spinning-phase inversion-high temperature sintering method.The influence of spinning nitrogen pressure and ethanol content in external coagulant bath on substrate structure was investigated.The results showed that with the increase of nitrogen pressure and the decrease of ethanol content,the thickness of the sponge-like layer of the substrate gradually increased,and the proportion of the inner finger-like pores gradually decreased.As the sintering temperature increases,the average pore size of the substrate gradually decreased,and the mechanical strength gradually increased.The ceramic membrane substrate with low resistance and good permeability was obtained through systematic optimization;?2?Continuous and defect-free TU-GOF membrane and MPD-GOF membrane were prepared via vacuum filtration technology with optimized ceramic membrane as the substrate.The results showed that the interlayer channel size of TU-GOF membrane gradually decreased with the increase of TU.On the contrary,as MPD increases,the interlayer channel size of MPD-GOF membrane gradually increased.The mechanism research showed that the covalent cross-linking between TU and MPD and GO nanosheets was mainly through the dehydration condensation and nucleophilic addition reaction between amine groups and carboxyl groups and epoxy groups on the surface of GO nanosheets.Compared with GO membrane,TU-GOF and MPD-GOF membranes exhibited extraordinary stability;?3?TU-GOF membrane and MPD-GOF membrane were used for the removal of tetracycline?TC?,oxytetracycline?OTC?,chloramphenicol?CAP?and diclofenac sodium?DS?.The results showed that with the increase of TU,the CAP rejection of TU-GOF membrane gradually increased.When GO/TU is 1:3,CAP can be almost completely removed?99.9±0.1%?.When GO/TU is 1:1,the TU-GOF membrane has a high rejection of four micro-pollutants,which is significantly higher than that of pristine GO membrane.After cross-linking with MPD,the permeability of MPD-GOF membrane gradually increased.Furthermore,the MPD-GOF membranes?GO:MPD=1:6?show simultaneous increases in rejection and water permeability for CAP,DS,OTC and TC solutions,breaking the conventional trade-off issue between permeability and selectivity;?4?Na₂SO₄,MgSO₄,NaCl and MgCl₂ were selected to study the salt ion sieving performance of the TU-GOF membrane and MPD-GOF membrane.The results showed that the functionalized GOF membrane showed significantly improved separation performance.After TU cross-linking,a significant enhancement in salt rejection was achieved for TU-GOF membrane?GO:TU=1:2?,showing an almost complete rejection for Na₂SO₄ and MgSO₄ while very high rejection for NaCl?95.6±1.14%?and MgCl₂?90.2±0.95%?.After cross-linking with MPD molecules,MPD-GOF membranes?GO:MPD=1:4?exhibited a simultaneous enhancement in rejection and permeability for four salts,breaking the conventional trade-off issue between permeability and selectivity;This work provides a novel protocol for rationally designing size and surface chemistry of GO nano-channels at a molecular level to construct stable functional GOF membranes with enhanced performance for water treatment applications.