Synthesis Of Covalent Organic Framework(COF) Separation Layers By Interfacial Polymerization Method On Polyacrylonitrile Mixed Matrix Membranes For Nanofiltration

The worldwide freshwater growing scarcity is a big concern to everyone.The growing number of people in the world also demand more supply of freshwater than the present.Freshwater is needed for many purposes from drinking to industrial applications.But the sources of fresh water are limited on the earth.97%of total water in the world is in the sea,which is in the form of saline water.The remaining 3%of water could not also use directly without any further purification,because of the impurities,and pollution.A large amount of water is used for industrial purposes which will waste along with other by-products without further purification.Besides the piles of problems,the concerns of awareness were also raised together.So,to overcome these issues,the industrial wastewater must be treated,repurposed,and reused,the saline water of the sea must be treated,and the water from natural sources also must be treated to protect from the contaminations.Among the many conventional treatment processes such as filtration,distillation,ion exchange,etc.the membrane-based separation technologies have shown to be effective in accomplishing these goals.Membrane technology refers to the scientific and engineering approach that is used to separate,transport,and reject the components,substances via the membrane.The nanoscale sizes of particles and substances are separated by using the nanofiltration membrane.The membrane materials are the key to making the membrane that works in the range of nanoscale range i.e nanofiltration.The new material,covalent organic frameworks（COFs）have emerged as interesting candidates for the next-generation membrane materials.The COFs are porous crystalline polymers with uniformly ordered covalently bound networks with customizable design properties.The high crystallinity and chemical stability are the key features of COFs which are seen in imine-based COFs than in other COFs materials.The interfacial polymerization（IP）route was adopted to synthesize the COFs layer on the support membranes.Firstly,four different kinds of support mixed matrix membrane（MMMs）namely PAN,Ti O₂/PAN,GO/PAN,and Ti O₂@GO/PAN were fabricated by the NIPS process.Then,the Tp Pa COFs layer was synthesized on support membranes.In this way,the influence of pore structure,morphology,and chemical structure of the support membrane on the structure and performances of Tp Pa COFs composite membranes were studied.The results showed that the hydrophilic and smooth top surface,homogenous pore distribution,and the large pore size of support membrane i.e.Ti O₂@GO/PAN was more suited.This was because the homogenous diffusion of aqueous solutions were promoted to infiltrate from the bottom surface to the top surface of the support membrane,where COFs layer fabrication takes place.A thin,crystalline Tp Pa layer with mean pore size of 1.78nm on the top surface of Ti O2@GO/PAN endowed the COF composite membranes with excellent filtration performance,with pure-water permeance of 89.08 L·m^-2·h^-1·bar^-1and the congo red rejection of 94.83%.Later the next work focused to minimize the thickness and make a stable layer of COFs which improved the filtration performances.The thickness of COFs could only be controlled when limited the stable reaction area for monomers on the top surface of supports.So here,we have developed a novel negative surface charge support membrane Ti O2@GO/PAN and used the cationic ethidium bromide（EB）as aqueous phase monomers.There exist the electrostatic force,which can limit the interface reaction area on the top surface of the membrane during the interfacial polymerization（IP）process.The hydrophilicity and large pore size of support membranes allowed ethidium bromide monomers（EB）from the aqueous phase to pass through and reach the surface of the negatively charged and homogeneously distributed pores.This assisted the fixation of the EB monomers at the interface with highly electrostatic interaction between membrane surface and EB monomers.The organic phase’s Tp monomer was polymerized with surface entangle EB,resulting in Tp EB COFs with very thin,hydrophilic,stable,homogeneously distributed tiny pores,and cationic surface charge.The prepared Tp EB COFs composite membrane had the water permeability of 32.34 L·m^-2.h^-1.bar^-1,highly selective sieving performances for dye molecules of different charges and sizes.This membrane could be efficiently applied to reject～98%of anionic dyes maintaining high solvent permeability for a long duration of time.The required Tp monomers and graphene oxide（GO）nanosheets were also synthesized and characterized by ourselves.The chemical and crystal structures of COFs membranes were characterized by FTIR and XRD,respectively.The morphological structures were characterized by SEM and FESEM.The surface roughness of membrane surfaces was measured by AFM.The filtration performances were conducted by using the dead-end cell and cross-flow filtration apparatus.The absorption and concentration of filtrated dye and BSA were measured by UV-vis spectrometer.