Synthesis Of Cationic Organic Covalent Framework Membrane And Its Application For Acid Recovery Via Diffusion Dialysis

Acid wastewater is a typical industrial wastewater,which is produced from the industrial processes（e.g.smelting,electroplating,mining）.Acid wastewater,which contains mainly the toxic metal ions(such as Fe²⁺,Zn²⁺and Cu²⁺,etc.)and the inorganic acids,discharges directly will not only pollute the environment,but also causes the inefficient resource utilisation.Membrane separation technology based on anion exchange membrane（AEMs）via diffusion dialysis is a promising technology for acid wastewater treatment due to its environmental friendliness and low energy consumption.However,the polymers are used as membrane materials in producing the traditional AEMs.Thus,the traditional modification methods such as narrowing the transmission channel or increasing the electrostatic repulsion of the channel are difficult to achieve a substantial increase in the selectivity of the membrane.Covalent organic frameworks（COFs）are considered as ideal materials for designing high-performance AEMs due to their high porosity,vertically aligned channels,tunable pore size,and modularity.In this paper,in order to overcome the challenges of traditional polymer AEMs,it is proposed to design a COFs membrane with ion channels based on the inherent properties of COFs materials.In this dissertation,a positively charged nano-ion channel for H⁺transport is designed based on cationic monomers(TG_Cland EB)by utilizing the effect of charge sieving to realize the specific sieving of H⁺.Based on the modularity of COFs,theβ-ketoenamine bonds and the functional groups（hydroxyl and methoxyl groups）is directionally designed for COFs membreane,which can improve significantly its stability in the acidic environment.First,Tp and TG_Cl are used as the building monomers,the interfacial polymerization method is used to build the continuous,uniform and defect-free COFs separation layer(Tp TG_Cl)with a thickness of 100 nm on the surface of a commercial PAN ultrafiltration membrane.The prepared COFs membrane not only has good mechanical properties,but also exhibits excellent H⁺/Fe²⁺sieving performance due to the formation of long-range ordered positively charged ion transport channels.Second,the COFs membrane(Dha TG_Cl/HPAN)with high pore size is prepared by changing the COFs monomer.The positively charged characteristics and hydroxyl-rich pore structure of the prepared COFs membrane endow it with excellent H⁺/Fe²⁺sieving performance.Thirdly,in order to promote the H⁺transfer,the strong electron withdrawing group（-OCH₃）is used in the COFs membrane(DMTATG_Cl/HPAN).It not only maintains excellent H⁺/Fe²⁺screening performance,but also exhibits excellent H⁺conductivity.Finally,a multi-component regulation strategy is used to construct the COFs membrane（COF-EBx BDy/PAN）,which exhibits the controllable charge density（0.39-0.78 mmol/g）by adjusting the ratio of charged ligands to uncharged ligands.The prepared COFs membrane not only has a stable rigid skeleton,but also exhibits extremely high selective separation performance under high acidity,high salt concentration,and high temperature（higher than4000）.The results demonstrate the great application potential of cationic COFs materials for developing a new-generation of high-performance AEMs.