Study On The Preparation And Properties Of Tight Composite Nanofiltration Membrane Based On Secondary Interfacial Polymerization

Nanofiltration,as a high efficient separation technology,has been widely used in the fields of softening of drinking water,purification of micro-polluted water,separation of industrial fluids.However,nanofiltration membrane also faces the problems of low flux and poor selectivity.Therefore,it is of great significance to develop novel nanofiltration membrane with high flux and selectivity via novel material and method.In this study,a novel tight PA/TA bilayer compsite nanofiltration membrane was prepared by means of secondary interfacial polymerization of tannic acid?TA?on the surface of the loose nascent polyamide composite membrane prepared through the interfacial polymerization of piperazine?PIP?and trimesoyl chloride?TMC?on porous polysulfone?PSF?substrate.Membrane sarface properties including surface chemical composition,morphology,charge,hydrophilicity were characterized by ATR-FTIR,SEM,AFM,Zeta potential measurement and surface contact angle measurement.The molecular weight cut-off,permeation flux and separation characteristics to different inorganic salts and dyes aqueous solutions of the PA/TA bilayer composite membrane were systematically investigated through cross-flow permeation tests and compared with PA singer-layer composite membrane of the nearly same pore size.The conclusions obtained from the experimental results are as follows:?1?Tight PA/TA bilayer composite nanofiltration membrane could be prepared through the secondary surface polymerization of TA and the residual acyl chloride group on the surface of the nascent PA membrane.FTIR and XPS analyses confirmed that the TA selective layer was formed by the reaction between the hydroxyl group of TA and the residual acyl chloride group on the surface of the nascent PA membrane.The formation of TA separation layer led to a reduce of membrane molecular weight cut-off from 450 Da of the loose nascent composite membrane to210 Da of the tight PA/TA bilayer membrane,corresponding to a decrease of pore size from 1.084 to 0.778 nm.Zeta potential and water contact angle tests showed that the formation of TA layer made the membrane surface more negatively charged and less hydrophilic.AFM and SEM characterizations demonstrated that the membrane became denser after deposition of TA layer.?2?Compared with the loose PA composite membrane,the PA/TA bilayer composite membrane possessed a smaller pore size and more enhanced surface negative charge and thus higher rejections to salts.The rejections of the obtained PA/TA membrane to different salts followed the order of R_Na2SO4?98.7%?>R_MgSO4?95.6%?>R_MgCl2?78.8%?>R_NaCl?61.6%?,which is a typical separation properties of tight negatively charged nanofiltration membrane.Compared with the PA single-layer composite membrane with the same NaCl rejection,the PA/TA bilayer composite membrane had a higher permeability.In case of membranes with the rejection of 80%to 500 ppm NaCl aqueous solution,the flux of PA/TA bilayer composite membrane was 2.3 times that of PA singer-layer composite membrane.It was also found that PA/TA bilayer composite membrane had good performance stability.?3?Both the PA/TA bilayer composite membrane and PA single-layer composite membrane with nearly the same NaCl rejection exhibited similar removals to different dyes based on the effects of size exclusion and Donnan electrostatic repulsion.Under the same pressure,the PA/TA bilayer composite membrane enhibited higher initial fluxes and lower flux decline ratios of fluxes,showing better performance of dye separation.