Molecular Dynamics Simulation Study On The Application Of TpPa-1 Membranes In Dye Wasterwater Separation

With the development of technology and industrial upgrading,textile printing and dyeing wastewater has become to be the main sources of water pollution.Membrane technology is considered as the most promising method for treating dyeing wastewater owing to its environmental protection and high efficiency.Covalent organic frameworks（COFs）are crystalline porous materials connected by light elements through covalent bonds,which are considered as the most promising membrane materials with high porosity,designable structure and well chemical stability,and they have been attracted extensive attention in the field of dye separation.However,many problems still remain unclear in the experiments,such as the rejection mechanism of dye molecules by COF membranes,the transfer and separation mechanism of water molecules and dye molecules in the pores of COF membranes,and the effect of surface charges for COF membrane on the rejection rate of dye molecules.In recent years,with the vigorous development of computer science,more and more researchers have empolyed molecular dynamics simulation to investigate the transport behavior of different molecules through membranes as well as explore the transport and separation mechanism of confined fluids,it will provide theoretical guidance for the preparation of high-performance separation membranes in experiments.Therefore,nonequilibrium molecular dynamics simulation was proposed to investigate the separation performance and rejection mechanism of COF-based membranes（TpPa-1 membranes and COF/MXene composite membranes）for different dye molecules in this paper.The transfer and separation mechanisms of dye molecules and water molecules in COF membranes were revealed from the aspects of molecular configuration,transport behavior and intermolecular interaction.The main conclusions of this paper are as follows:（1）The rejection rate of dyes is mainly determined by the adsorption on the surface of TpPa-1 membranes.The thicker the TpPa-1 membrane is,the stronger theπ-πinteractions between the dye molecules and the surface of membrane become,which leads to better dye rejection.Dye molecules with good conjugated planar structures are more prone to formπ-πinteractions with the surface of the TpPa-1 membrane,and these dyes are easily adsorbed on the membrane surface,which results in the excellent rejection rate of dyes.（2）TpPa-1 membrane shows excellent recovery rate of inorganic salts.The addition of inorganic salts increases the rejection rate of dyes.The main reason is that the physical barrier of inorganic salts would affect the orientation of dye molecules when the dye molecules try to enter into the channel of membranes,which makes dye molecules more likely to formπ-πinteractions with the surface of TpPa-1 membrane,and these dyes are adsorbed on the membrane surface.（3）Ti₃C₂（OH）₂/TpPa-1 composite membrane has excellent separation performance for methyl orange（～100%）.Water flux is affected by slit width（d G）and offset（O）.The rejection rate of methyl orange decreases as the slit width increases,regardless of the offset.（4）When d G=1.6 nm,O=0 nm,H2=1 nm,the TpPa-1/Ti₃C₂（OH）₂ composite membrane shows good separation performance for potassium permanganate（PP）.Due to the electrostatic repulsion between PP and terminal groups of MXene,TpPa-1/Ti₃C₂F₂and TpPa-1/Ti₃C₂O₂ composite membranes show better rejection properties for PP than TpPa-1/Ti₃C₂（OH）₂ membrane.The surface charge on the COF layer plays an important role in the rejection performance of PP,and the TpPa-SO₃^-/MXene composite membranes with negative surface charge have excellent rejection performance for PP.