Fabrication Of Thin Film Composite Forward Osmosis Membranes Via Layer-by-layer Assembly Interfacial Polymerization

As a new type of membrane separation technology,forward osmosis(FO)technology has been widely applicated in the fields of desalination and deep sewage treatment.Nowadays,the preparation of thin flim composite(TFC)membranes with high water flux and excellent separation performance has became a hot spot.In this study,the support layers of FO membranes were fabricated by liquid-solid(L-S)phase inversion,then they were immersed in sodium hydroxide(Na OH)solution for surface modification.The active layers were prepared through the methods of interficial polymerization(IP).The effect of IP numbers and the concentration ofcarbonnanotubes(CNTs)on the properties of TFC membranes were studied.Finally,the optimum performance TFC membranes were utilized to treat secondary effluent from wastewater treatment plant.The results can provide technical guidance for the further urban sewage treatment.Firstly,the support layers of the FO membranes were prepared by L-S inversion method using polyacrylonitrile(PAN)as polymer solution,and then they were deposited in polyethyleneimine(PEI)solution and polyacrylic acid(PAA)solution respectively after immersing in Na OH solution,the PAN-PEI/PAA support layers were finally fabricated.Scanning electron microscopy(SEM)and atomic force microscope(AFM)were carried out to analyse the surface morphology of the support layer;infrared spectro scopy(AITR),Zeta potential,contact Angle and tensile strength wereoperated to measure materials’ properties.Secondly,layer-by-layer assembly IP method was used to fabricate active layers of the TFC membranes,the results showed that active layer density,the roughness and mechanical properties increased with IP numbers increasing.However,contact angle and Zeta potential decreased steadily while IP numbers increased.Through the test of lab-scale forward osmosis(FO)and reverse osmosis(RO)devices,the RO water flux,FO water flux(Jw)and reverse salt flux(Js)declined,while the salt rejection increased,specific salt flux(Js /Jw)firstly increased to the maximum and then decreased with the increase of IP times.The result showed that the best FO performance with water flux of 16.97 L/m2·h and reverse salt flux of 6.02 g/m2·h after twice IP.Thirdly,CNTs were added to water soluble monomer in the process of IP in order to enhance the FO membrane performance.It indicated that with the increase of CNTs concentration,flocculent substances appeared on the active layers by SEM.CNTs were successfully embedded in the surface of TFC membranes by AITR and XPS analysis,roughness and hydrophilic raised consistantly with CNTs concentration increasing.The FO system showed that water flux improved effectively,reverse salt flux showed a slightly increase.Therefore,CNTs can effectively improve membrane performance.When CNTs concentration was 0.2 wt%,FO performance was relatively superior with water flux of 25.14 L/m2·h,reverse salt flux of 8.64 g/m2·h and Js/Jw value of 0.37 g/L.The results showed that the water flux were much larger than that of the three kinds of commercial membranes from HTI and the value of Js/Jw was lower than that of the commercial membrane.Lastly,two kinds of FO membranes(TFC,TFN)were used to treat secondary effluent from wastewater treatment plant.The results showed that water flux were TFN>TFC,TFC membranes had a better rejection than TFN membranes.The rejection of two kinds of membranes for TOC,TP,TN were upon 60%,while the rejection of ammonia nitrogen rate were about 50%.