Study On The Influence Of Water/organic Phase Additives On The Morphology And Performance Of Polyamide Composite Reverse Osmosis Membranes

Reverse osmosis(RO)is the key technology to solve the problem of water shortage in the world.Today's RO market is dominated by the aromatic polyamide(PA)thin film composite(TFC)membrane due to its excellent desalination performance.But the relationship between the PA structure and its separation performance is not fully understood.A variety of approaches have been explored to control the thin film forming dynamics to optimize the membrane performance.Additive approach provides a simple,yet very effective way of improving the performance of PA TFC RO membrane.However,there is a lack of correlation between physical and chemical properties of additives and the membrane separation performance.Therefore,studying how the aqueous and organic phase additives affect the morphology and properties of PA TFC RO membranes is of great significance for designing RO membranes with better performance in the future.In this paper,PA TFC RO membranes is prepared by interfacial polymerization(IP)of m-phenylene diamine(MPD)in the aqueous phase and trimesoyl chloride(TMC)in the organic phase on top of a polysulfone substrate.The RO performance of the TFC membrane was measured at 1.5 MPa and 25? with a 2000 ppm Na Cl solution.The optimized membrane forming conditions were obainted by varying the interfacial polymerization condition.When no additives were added,the water flux and salt rejection of the prepared membrane were 43.3 L/(m~2h) and 99.0%,respectively.This membrane was used as the control sample to investigatethe effects of additives on the membrane morphology and separation performance of PA TFC RO membranes by adding various additives into the aqueous or organic phases.The main research conclusions are as follows:(1)Dimethyl sulfoxide(DMSO),formamide,and acetamide were selected as aqueous phase additives.The results show that the surface roughness,water contact angle and surface charge of the TFC membrane decrease,the thickness and mass of the PA layer layer decrease,and the crosslinking degree of PA layer increase.At the optimized aquous additives content,TFC-aqu membranes show higher water flux(up to ~130%increase)with no considerable salt rejection decrease compared to the TFC-sta membrane.The mechanism of the aqueous phase additives enhancing membrane flux is mainly due to the reduction of interfacial incompatibility,which promotes the diffusion of amine-functional monomer MPD into the organic phase,thereby accelerating the rate of IP and forming thin and dense PA separation layer.(2)Cyclohexanone,anisole and benzonitrile were selected as the organic phase additives.The results show that when adding organic phase additives,the surface roughness of the TFC membrane increased,the water contact angle and the Zeta potential do not change significantly,and the PA layer has more and larger cavities,and the mass and the crosslinking degree of PA layer increase.At the optimal organic phase additives contents cyclohexanone is the most effective addtives for improving water flux of the TFC PA RO membrane.Compared to the TFC-sta membrane,the TFC-org-cyclohexanone membrane shows approximately 2 times higher water flux with no considerable decreasing of salt rejection.The effect of organic phase additives enhancing membrane flux is that the organic additive changed the composition of the solvent in the reaction zone,thereby changing the surface morphology of the separation layer and the aggregation state of the polymer chain.