Developing Novel Thin-film Composite Polyamide Membranes For Forward Osmosis Applications

Forward osmosis(FO),a novel promising technology for water treatment,has not progressed significantly beyond the conceptualization.The desirable FO membrane with high separation performance,good antifouling properties and chemical stability is the key prerequisite for the development of FO technology.Among various FO membranes,the thin film composite(TFC)membrane fabricated by the interfacial polymerization on a porous substrate is the dominant membrane type,due to its easy fabrication and excellent separation performance.However,traditional TFC membrane prepared by aromatic amines and acyl chlorides may suffer the relative low water flux,a high fouling tendency and relative weak chemical stability,due to its relatively hydrophobic,rough and vulnerable nature of the polyamide(PA)selective layer.In this thesis,various surface and in-situ modifications are performed on the PA layer of TFC membrane to solve above problems.As for the surface modification,we firstly utilize hyper-branched polyethyleneimine(PEI)to conduct the surface modification of TFC membrane.In comparison with the control TFC membrane,modified membranes exhibit much higher hydrophilicity,larger free volume of PA network,improved water flux with mild sacrifice of salt rejection and enhanced fouling resistance.In view of the low reactivity of PEI with large steric hindrance,a smaller amine molecule of tris(2-aminoethyl)amine(TAEA)is also employed for the surface modification of TFC membrane,which plays a dual-role of reactive amine monomer and catalyst for the surface modification reaction,contributing to the higher utilization percentage of residual acyl chloride groups with less TAEA and thus better modification effect.Improved membrane hydrophilicity of the selective layer is therefore achieved,contributing a higher water flux and fouling resistance.Based on previous two works,the surface functionalization by organic phosphonic aicd(OPa)on aminized membranes is further conducted,followed by mineralization and metallization treatment.N-containing organic phosphonic acid,an excellent anti-scaling agent,can provide plentiful complexion sites for further mineralization and metallization to chelate various metallic ions by O and N atoms stably.In comparison with the control membrane,OPa-modified membranes show significantly improved separation performance,fouling resistance(inorganic,organic and microbial fouling,especially for gypsum scale),antibacterial properties and chemical stability simultaneously.In-situ modifications of the PA layer are also explored to tune bulk properties of resultant TFC membranes.Three different tertiary amines of tri-ethylamine(TEA),TAEA,and hexamethylenetetramine(HMTA)are firstly employed as aqueous phase additives,which can act as the catalyst to accelerate the reaction rate between the two reactive monomers by absorbing the by-product(hydrogen chloride)during interfacial polymerization process,resulting in the denser PA layer with a high crosslinking degree.All modified membranes exhibit optimized morphology and microstructure,enhanced separation performance and fouling resistance,and their performance are compared and investigated in terms of the different chemical structures of the tertiary amines.In addition,a high-performance thin-film nanocomposite(TFN)membrane is also developed by incorporating nano-material graphene oxide(GO)in MPD solution,and achieved a higher hydrophilicity and a smoother surface of the PA layer,as well as the higher water permeability,lower salt permeability and lower fouling propensity.Besides the chemical modification,a "green" ultrasound-assisted interfacial polymerization approach is put forward for the first time to fabricate TFC membranes with desirable separation performances.Ultrasound in IP process enlarges the mixing area of reactive monomers,facilitates the mass transport of the amine monomer,therefore contributing to an efficient monomer mixing and a higher IP reaction degree.Accordingly,resultant TFC membranes exhibit much higher water flux and even lower reverse salt flux as compared to the control TFC membrane.