Study On Preparation And Separation Performance Of Quaternary Ammonium Chitosan Modified Polyamide Composite Nanofiltration Membrane

Nanofiltration membrane separation technology have been widely used in water softening,drinking water purification,seawater desalination and other processes.However,a large number of commercial membranes monopolized by Dow and Toray are negatively charged on the surface,and the removal effect of negatively charged pollutants are better than positively charged pollutants.Furthermore,the internal pollution of nanofiltration membrane and the trade-off between flux and rejection rate have become the most prominent problem in the process of nanofiltration separation.The way to solve the problems above has become a foothold for the development of high performance nanofiltration membranes.In this paper,polyethersulfone ultrafiltration membranes?PES?were used as basement membranes.Polyethyleneimine?PEI?and trimethylbenzene chloride?TMC?were interfacially polymerized to prepare a loose polyamide composite membrane,and the interfacial polymerization products modified by quaternary ammonium chitosan?HTCC?on the surface of PES membranes were used to prepare a positive charged composite nanofiltration membrane with small molecular weight rejection.By adjusting the concentration of casting solution,reaction time,drying temperature and other factors,the composite nanofiltration membrane under the optimal conditions was obtained.The separation performance of PEI/TMC and PEI/TMC/HTCC composite nanofiltration membranes for different kinds of salts and PPCPs is studied.The effects of pressure,flow rate,salt type and concentration on membrane flux and rejection rate were discussed.The two kinds of composite nanofiltration membranes were characterized by scanning electron microscopy?SEM?,water contact angle?WCA?,atomic force microscopy?AFM?and Fourier transform infrared spectroscopy?FTIR?.The permeability coefficient and molecular weight were measured.The stability and pollution resistance of PEI/TMC and PEI/TMC/HTCC polymeric nanofiltration membranes were also tested.The experimental results indicated that the optimum conditions of PEI/TMC nanofiltration membrane were as follows:PEI concentration was 1.5%?m/v?,soaking time was 20 minutes,TMC concentration was 0.2%?m/v?,soaking time was 5 minutes.PEI/TMC/HTCC composite nanofiltration membrane was prepared by modifying the surface of the PEI/TMC composite membrane with HTCC.The optimum modification conditions were as follows:the concentration of HTCC was 0.5%?m/v?,the reaction time was 15minutes,and the heat treatment temperature was 50?.The performance evaluation results of PEI/TMC/HTCC composite membranes showed that the Zeta potential of the membrane was3.20 mV which showed that the membrane surface was positively charged.Pure water permeability coefficient was 25.37 L.m^-2.h^-1,and molecular weight was 481 Da,which belonged to the category of nanofiltration membranes.The order?sequence?of rejection to inorganic salts was MgCl₂>CaCl₂>MgSO₄>Na₂SO₄>NaCl>KCl,which conformed to the desalination rule of positively charged nanofiltration membrane in Donnon effect.The rejection rate of MgCl₂ could be as high as 90.48%.The sequence of rejection rate of 4 kinds of drugs was as follows:AML>ATE>CBZ>IBU.It was clear that AML with positive charge had the highest rejection rate,up to 90.12%.The anti-fouling performance of the nanofiltration membrane was also tested.The results showed that the recovery rate of water flux could reach 85.7%after the membrane was polluted by organic pollutants.Compared to PEI/TMC composite nanofiltration membrane,the anti-fouling performance of the nanofiltration membrane was improved.In addition,24-hour interception stability test showed that the flux and rejection rate of PEI/TMC/HTCC composite nanofiltration membranes were relatively stable.