Preparation And Separation Research Of Organic/inorganic Hybrid Thin-film Composite Membrane

Due to its excellent permselectivity,thin film composite（TFC）membrane has attracted much attention in research field and been commercialized and applied in various separation process,including dyeing and printing wastewater treatment,water desalination,municipal and industrial wastewater reuse,etc.TFC membrane is generally composed of three layers,the dense separation layer,the polymer substrate layer and non-woven support layer.Both the dense separation layer and the polymer substrate layer can be regulated separately to cater specific needs.However,the development of polymer membrane has apparently reached the“trade-off”limit between permeance and selectivity,a highly permeable membrane lacks of selectivity and vice versa.Therefore,searching for new material and optimization of membrane preparation method have proven to be feasible methods to address these problems.MOF materials have been recognized as a superior material in separation field due to advantages of ultra-high porosity,uniform pore size and tunable crystal structures.MOFs have been used to prepare polymer supported MOF hybrid membranes,and the performance of MOF/polymer hybrid membrane is expected to go beyond the“trade-off”limit.Utilizing the“self-sealing”property of interfacial synthesis,the“combination of self-assembly and interfacial synthesis”and“chelation assisted interfacial reaction”was used to prepare the ZIF-8/PEI-HPAN hybrid membrane.The morphology,physio-chemical property and separation performance of the prepared membrane was studied in detail.Moreover,the thermally induced phase separation combined with non-solvent induced phase separation method（TIPS-co-NIPS）was utilized to regulate the structure and property of polyether sulfone substrate membrane,aiming at reducing internal concentration polarization and enhancing the performance of thin film composite forward osmosis（TFC-FO）membrane.Layer-by-layer assembly method was used to regulate the thickness and morphology polyamide selective layer,thereby optimizing the separation performance of the TFC-FO membrane.The combination of self-assembly and interfacial synthesis method was used to prepare a thin and defect-free ZIF-8/PEI selective layer on HPAN substrate.The morphology and separation performance of the ZIF-8/PEI hybrid membrane can be easily regulated by controlling preparation conditions.The optimal preparation conditions were PEI concentration of 0.8 wt%,PEI deposition time of 30 min,interfacial reaction time of 60 min,0.1 mol/L Zn（NO3）2 in DI-water and 0.2 mol/L Hmim in hexane solution.Under this condition,the prepared ZIF-8/PEI hybrid membrane achieved a permeate flux up to 330L?m^-2?h^-1?MPa^-1 with a rejection of 99.6%for methyl blue aqueous solution,as well as excellent long-term durability,anti-fouling performance and good durability at different operation pressures.A chelation-assisted interfacial polymerization（CAIR）method was used to fabricate a MOF/polymer hybrid layer on the hydrolyzed polyacrylonitrile（HPAN）substrate to form the ZIF-8/PEI-HPAN composite membrane.Utilizing the strong chelating ability of polyethyleneimine（PEI）to metal ions,the formed ZIF-8/PEI selective layer showed good hydrophilicity and performance stability.Meanwhile,the separation performance of the membrane can be easily regulated by varying preparation conditions.The optimal preparation conditions were Zn（II）and PEI aqueous solution at the chelating temperature of 30°C,pH 6,Zn（NO₃）₂ concentration of 0.4 mol/L and synthesis time of 20 min.The prepared ZIF-8/PEI-HPAN composite membrane exhibited a rejection of 99.2%for Congo red aqueous solutions with a permeance up to 78 L?m^-2?h^-1?bar^-1,which was 5-8 times higher than literature reported polymeric membranes under the same dye rejection,as well as good long-term stability.Then,polyethersulfone（PES）substrate was prepared by a thermally induced phase separation combined with non-solvent induced phase separation method（TIPS-co-NIPS）.Polyethylene glycol（PEG）acted as a phase change material in TIPS process and then functioned as a pore-forming agent in NIPS process,and the morphology and property of the membrane can be easily regulated by controlling cooling time in TIPS process.Then,a thin polyamide layer was deposited on the aforementioned substrate surface via interfacial polymerization to form the corresponding TFC-FO membrane.The effects of different substrate on the morphology and separation performances of TFC-FO membranes were investigated.The results revealed that the substrate prepared by TIPS-co-NIPS method with10 s cooling time exhibited a fully finger-like structure,enhanced permeability and better surface hydrophilicity.As a result,the corresponding TFC-FO membrane exhibited 44%lower structural parameter（S）than the TFC-FO membrane based on NIPS substrate（684μm v.s.380μm）,high water fluxes up to 32.9/20.4 LMH and reverse salt fluxes of 14.1/8.2 gMH in the PRO/FO mode using 1M NaCl as the draw against DI-water feed,90.0%salt rejection,the lowest S value of 380μm as well as good performance stability.These potential advantages indicated TIPS-co-NIPS method can be used as a potential strategy to regulate substrate structure and property for TFC-FO membrane.Finally,layer-by-layer assembly method was used to synthesize a thin and highly selective polyamide layer on a commercial PAN substrate.The effects of different monomer concentrations and assembled cycles on the performance of TFC-FO were investigated.Compared to the control TFC-FO prepared by interfacial polymerization,the TFC-FO membrane prepared by layer-by-layer assembly method at 8 assembly cycles with MPD concentration of 1.5 wt.%and TMC concentration of 1.0 wt.%exhibited enhanced FO performance in terms of about 80%higher water flux,8.6%higher salt rejection,51%lower structural parameter.This membrane also achieved 96.1%rejection for NaCl aqueous solution and a low B/A value of 0.28.Therefore,layer-by-layer assembly method endowed the TFC-FO membrane with enhanced perm-selectivity and mitigated ICP effect,which can be used a facile and promising method to regulate the structure and performance of the TFC-FO membrane.