Study On Modification Of Positively Charged Polyamide/polysulfone(PA/PSF) Composite Nanofiltration Membrane Based On Polyethyleneimine

To develop highly efficient separating membranes with improved rejection performances,study on modification of positively charge composite nanofiltration?NF?membranes is important for desalination of seawater from theory to application,and meaningful to solve the problems of water shoratage and energy crisis.In this thesis,aiming at the low permeate flux,poor mechanical strength and stabilities in acidic and alkaline solutions,polyethyleneimine?PEI?based polyamide/polysulfone?PA/PSF?positively charged composite NF memebranes with excellent separation performance were successfully prepared via interfacial polymerization?IP?.The surface morphology,chemical composition,roughness,wettability and surface charge of the resultant composite NF membranes were characterized by using scanning electronic microscopy?SEM?,attenuated total reflectance-Fourier transform infrared?ATR-FTIR?,atomic force microscopy?AFM?,contact angle meter?CA?,and Zeta potentiometer.The permeation flux and rejection rate of the composite NF membranes before and after modification have been to study the relationship between memebrane structure and the performances.The detail work has been done as follows:1.The positively charged PA/PSF composite nanofiltration membranes were fabricated by interfacial polymerization?IP?on the surface of polysulfone?PSF?ultrafiltration?UF?membranes,using PEI,the mixture of isophthaloyl dichloride and trimesoyl chloride?IPC/TMC?as the solutes of the aqueous phase and the organic phase,respectively.The resultant PA of the PA/PSF composite NF membranes could be adjusted via changing the mass ratio of IPC and TMC.When the mass ratio of IPC and TMC was 1:1,the permeation flux?F?of the membrane to 1 g·L^-1 MgCl₂ aqueous solution was 27.6 L·m^-2·h^-1 at ambient temperature and 0.4 MPa.The rejection rates of the membrane to the inorganic electrolyte solutions decreased in the order of ZnCl₂?97.7%?,MgCl₂?97.0%?,CaCl₂?96.9%?,CuCl₂?95.3%?,MgSO₄?88.3%?,NaCl?74.2%?,and Na₂SO₄?47.8%?.The rejection rates to the organic dyes decreased in the order of the molecular weights?Mw?of the dyes,for example,cationic red X-GTL?100.0%?,rhodamine B?94.2%?,cationic gold yellow X-GL?93.5%?,and brilliant blue KN-R?43.9%?.2.Positively charged organic/inorganic hybrid composite membranes with high pressure resistance were fabricated by IP method,in whichthe mixture of aminated multi-walled carbon nanotubes?NH₂-MWCNTs?and PEI,IPC/TMC were set as the solutes of aqueous phase and organic phase,respectively.The one-dimensional NH₂-MWCNTs shows high mechanic strength,elasticity,fatigue resistance and isotropy.The surface-NH₂ groups could react with IPC/TMC well by interfacial polymerization to achieve a stable hybrid composite membrane.The changes of permeation flux and rejection rate of the pristine PA/PSF membrance and NH₂-MWCNTs doping membrance were compared under different operation pressure.The results showed that the permeation flux and rejection rate to 1 g·L^-1 MgCl₂ aqueous solution were 75 L·m^-2·h^-1 and 94%,respectively at the ambient temperature and 2.0 MPa,which was 7.6%higher than that of pristine PA/PSF membrane.The rejection rates to different inorganic electrolytes decreased in the order of ZnCl₂?97.3%?,MgCl₂?94.1%?,CuCl₂?91.2%?,CaCl₂?89.5%?,MgSO₄?86.4%?,NaCl?78.1%?,Na₂SO₄?68.4%?.3.Using PEI aqueous solution as the aqueous phase,cyanuric chloride?CC?n-hexane solution as the organic phase,the positively charged PEI-CC/PSF composite NF membranes with anti-acid and anti-alkali characteristics were fabricated through IP on the surface of PSF UF membranes.The strong electron withdrawing and steric hindrance effects of the chloride group in the molecules of CC could protect amido bond from the attacks of H⁺or OH^-under acid or alkaline condition,thus the resultant PA composite membranes could be stable in acidic or alkali aqueous solution.A more compact PA active layer could be prepared by mixing PIP into the PEI aqueous solution,which could fill in the pores of polymer networks.There was no obvious changes of the surface morphology,chemical structure and rejection rates after immersing the resultant PEI-PIP composite NF membranes in the strong acidic solution?pH=1?and alkaline solution?pH=13?,respectively for one month.The rejection rate to Na₂SO₄ aqueous solution increased by approximately 14.8%after the addition of PIP into the active layer of the PEI composite NF membranes.