Improving Chloride Resistance Of Polyamide Reverse Osmosis Membrane By Graphene Oxide And Oxidizied G-C₃N₄

With the development of society,industry and agriculture,the demand for fresh water resources have increased significantly and the pollution of freshwater resources has intensified.Therefore,the shortage of available fresh water resources has become a huge problem faced by all countries in the world.Several approaches have been investigated to obtain usable fresh water resources through brackish water and seawater desalination.Among them,reverse osmosis technology has become the most popular technology for water treatment processes such as seawater desalination due to its excellent desalination and low cost.Polyamide?PA?reverse osmosis membrane is one of the most widely used membranes,which has the advantages of high desalination efficiency,acid and alkali resistance.In the water treatment process,active chlorine is often present in the feed water,and the active chlorine can destroy thin separation layer of PA membrane,resulting in a decrease in the separation performance of the reverse osmosis membrane.Therefore,the chlorine resistance modification of PA membrane is a hot issue in the research of reverse osmosis membrane.In this study,graphene oxide?GO?and oxidized g-C₃N₄?CNO?were used as modified materials to improve the chlorine resistance of PA membranes.And different modified membranes were prepared by chemical grafting,layer by layer self-assembly and bulk doping.The specific work is as follows:1.Graphene oxide was synthesized by Hummers method and two-dimensional thin layer morphology of GO was observed by SEM and TEM.Infrared spectroscopy?FT-IR?shows that GO contains many functional groups,and X-ray photoelectron spectroscopy?XPS?demonstrates that the oxidation degree of GO can reach 47.1%.Subsequently,the reactions of carboxyl group and amino group were catalyzed by EDC/NHS activation,and GO was grafted on the surface of PA membrane.Chlorine resistance tests show that as the number of grafting increases,the flux of grafted membrane decreases more and more.The water flux of the modified PA membrane grafted 5 times decreased from 50.2 L/m²h to 37.2L/m²h and its rejection increases.This is because the GO layer blocks the passage of water molecules and salt ions on the surface of PA membrane.With soaking in 1000 ppm sodium hypochlorite solution,the modified membrane showed better chlorine resistance than pristine membrane.The rejection change of the modified membrane was smaller than that of pristine membrane before and after chlorine resistance tests.Moreover,the greater number of grafting,the greater the reduction in the modified membrane and the better chlorine resistance.The amino group at the end of grafting experiments can slightly improve separation performance and chlorine resistance of the modified membrane.The amino group changes the surface charge of PA membrane and provides some active sites to react with active chlorine.The chemically grafted GO can adhere tightly to the surface of PA membrane.PA membrane with different separation performances and chlorine resistance can be obtained by changing the number of grafting.2.The surface of GO and titanium dioxide?TiO₂?have different charges,and the surface of PA membrane is modified by layer self-assembly of GO and TiO₂.The presence of GO and TiO₂ on the modified membrane was observed by SEM characterization and XPS analysis,and contact angle testing demonstrated that the coating layer effectively improved the hydrophilicity of the PA membrane.The PA membrane coated with different layers has different separation performances,and the modified membrane shows an increased water flux when the number of coating layers is less than 6.Excessive number of coating layers can result in impeding the transport of water molecules,resulting in a decrease in water flux.The modified PA membrane also showed better chlorine resistance in chlorination experiments,and the chlorine resistance of the modified membrane was also analyzed by XPS.TiO₂ has the ability to sterilize.Therefore,after 3 days of bio-fouling experiments,the change of water flux of the modified PA membrane is smaller than that of pristine PA membrane,showing better bio-fouling resistance.Moreover,the modified membrane exhibits good anti-fouling resistance because the coating layer can improve the hydrophilicity and roughness of the surface of the PA membrane.3.CNO was synthesized from g-C₃N₄.Through observation and analysis of SEM,TEM and XPS,it can be confirmed that CNO is a two-dimensional sheet nanomaterial with oxygen-containing functional groups.The test of separation performances of different doping concentrations shows that the modified membrane with a doping concentration of0.01 g/L has the best separation performance.In the chlorine resistance test,the membrane modified by CNO shows the best chlorine resistance.After 40 hours of active chlorine treatment,the rejection of the CNO-doped membrane decreased by 15%,while the rejection of pristine membrane decreased by 27%.The XPS analysis demonstrated the C-N bond number of the membrane modified by CNO and pristine membrane after chlorination was reduced by 1.2%and 7.6%,respectively,indicating that CNO has the ability to react with active chlorine.