Fabrication And Performance Of Nanofiltration Membranes Based On Covalent Organic Frameworks

Nanofiltration,as a new membrane separation technology,has the advantages of high efficiency and low cost compared with traditional separation technology.Thus it gets attention in purification of dyes wastewater and desalination of brackish water.However,traditional polymer nanofiltration membranes possess low permeability due to their poor porosity and compact structure.So it is urgent to find suitable membrane materials to prepare nanofiltration membranes with high performance.Covalent organic frameworks(COFs)are a kind of porous crystalline polymers which are completely built by organic building blocks via strong covalent bonds.Possessing physicochemical properties of high hydrophilicity,low density as well as abundant and ordered pores,COFs have been regarded as promising candidates for nanofiltration membranes.In this study,based on COFs,nanofiltration membranes with high performance were prepared via direct growth method and secondary interfacial polymerization.Firstly,aldehyde groups were introduced on the surface of porous support layer of polyacrylonitrile(PAN)as the growth site for COFs.Then a continuous defect-free Tp Pa-1(a kind of COFs)layer formed on the support via direct growth method to fabricate Tp Pa-1/HPAN nanofiltration membrane.Tp Pa-1 layer had high hydrophilicity and abundant pores,which gave the membrane good permeability.The pores of Tp Pa-1 could play an significant role in sieving,endowing it high selectivity.The effects of reaction time and monomer concentration on the structure and properties of the membranes were investigated.The pure water flux could reach 897.2L/(m~2 h MPa),and the rejection rates of dyes maintained over 95% with superior chemical stability.Secondly,we prepared Tp Pa-1 layer and polyamide(PA)layer on the surface of porous support layer of polyethersulfone(PES)successively via secondary interfacial polymerization,and PA/Tp Pa-1/PES nanofiltration membrane was fabricated.The abundant pores of Tp Pa-1 increased the concentration of local amine monomers by adsorbing piperazines,while hydrogen bonds between hydrophilic groups of Tp Pa-1 and piperazines slowed down their diffusion rate.Both above regulated the interfacial polymerization process.By optimizing the structure of Tp Pa-1 and the time of interfacial polymerization,the flux of the nanofiltration membranes could reach171.35 L/(m~2 h MPa),increased by 125.4% compared with PA/PES membranes.The rejection rate of sodium sulfate and dyes solution remained more than 90% and 99% separately.