Preparation And Performance Of Thin Film Composite Nanofiltration Membrane Via Pre-Diffusion Interfacial Polymerization

Membrane materials are the core of the high-precision separation of nanofiltration technology.The most widely commercialized nanofiltration membranes are mainly thin film composite nanofiltration membranes,which have the advantages of high water flux,simple preparation process,and low raw material cost.The active separation layer of polyamide,which plays the main role of separation,is prepared by interfacial polymerization of polyamine monomers and polyacid chloride monomers on a porous substrate.During the interfacial polymerization process,the uncontrollable diffusion behavior of amine monomers often makes defects in the polyamide layer causing uneven pore size distribution and reducing the solute retention performance of the nanofiltration membrane.Therefore,rational design of the interfacial polymerization process and the development of new interfacial polymerization methods are expected to greatly improve the separation efficiency of nanofiltration membranes and promote the transformation of membrane separation technology.From the perspective of the process of interfacial polymerization,this paper proposes a new method of interfacial polymerization,namely,pre-diffusion interfacial polymerization to prepare thin-film composite nanofiltration membranes.The main research contents are described below:(1)Preparation of thin film composite nanofiltration membrane by pre-diffusion interfacial polymerization and research on its desalination performance.From the perspective of interfacial polymerization mechanism,it is designed to make the amine monomer diffuse to the oil-water interface before the polymerization reaction occurs,and then provide sufficient amine monomer in the initial stage of film formation,reduce the defects of the prepared polyamide layer,and improve the salt retention performance.Using SEM,AFM and other characterization methods to study the difference between the surface morphology,roughness and thickness of the nanofiltration membrane and the traditional interfacial polymerization.The interception performance of six salts such as Na2SO4,MgSO4,MgCl2,CaCl2,NaCl,etc.were tested.Among them,the interception rate of Na2SO4 is up to 99.7%,and the interception rate of MgSO4,MgCl2,CaCl2 has reached more than 95%,far exceeding the traditional interfacial polymerization.Prepared nanofiltration membrane.In addition,the monovalent/divalent ion sieving performance was studied.The optimal nanofiltration membrane has a Cl-/SO42-separation selectivity of 256,and a Na+/Mg2+ separation selectivity of 54,which exceeds that of most reported nanofiltration membranes.(2)Study on the mechanism of pre-diffusion interfacial polymerization process and monomer universality.Two hypotheses are proposed for the mechanism of the pre-diffusion process to increase the salt rejection rate:First,the piperazine monomer is pre-enriched at the oil-water interface during the pre-diffusion process,which is equivalent to increasing the concentration of piperazine molecules in the initial stage of the reaction.Therefore,the experiment was designed on the basis of traditional interfacial polymerization,thus the concentration of piperazine molecules was increased,and the retention performance of the prepared nanofiltration membrane was studied.Second,since the acid chloride monomer was added dropwise to the pure n-hexane solvent before the polymerization reaction,the concentration of the polyvalent acid chloride monomer at the interface was lower in the initial stage of the polymerization reaction,which is equivalent to reducing the concentration of the acid chloride monomer,so the low concentration was studied.The desalination performance of the nanofiltration membrane prepared by the traditional interfacial polymerization method under the condition of acid chloride monomer.The nanofiltration membranes prepared under the two conditions have obtained higher salt rejection rates,verifying the above two points based on the mechanism hypothesis of the pre-diffusion interfacial polymerization process.In addition,the pre-diffusion interfacial polymerization nanofiltration membrane was prepared with polyethyleneimine instead of piperazine as the amine monomer,which proved the universality of the monomer by pre-diffusion interfacial polymerization.