Structure Control Of PVDF Membrane And Separation Performance Of Composite Forward Osmosis Membrane

Forward osmosis(FO),as an effective technology not requiring high hydraulic pressure,has lower energy consumption and lower fouling tendency.Compared with other membrane separation technologies,it has important research value and broad application prospects.Forward osmosis membrane can be used as a functional packaging material for military water bags,and have special significance in food and beverage processing.However,the lack of high-performance forward osmosis membranes limits its application and development.Therefore,the high-performance thin film composite forward osmosis membrane(TFC-FO)with both high flux and high selectivity is a research hotspot and a technical difficulty in this field.In this paper,the hydrophilic modified polyvinylidene fluoride(PVDF)microporous membrane is used as the base membrane,and the polyamide(PA)active layer is prepared by the interfacial polymerization method,and three thin film composite forward osmosis membranes are obtained.The focus is on the physical and chemical properties of the base membrane surface/interface and the influence of the hydrophilicity of the active layer PA on the permeation and separation performance of the TFC-FO membrane.The main work of this paper consists of three parts:1.The PVDF/PDA-PEI blend membrane was prepared by blending dopamine(DA)and polyimide(PEI)with PVDF.The PVDF/PDA-PEI blend membrane was prepared by the non solvent phase separation method(NIPS).Then,the PA active layer was prepared on the surface of the blend membrane by the interfacial polymerization method to obtain the TFC-FO membrane.It is found that the addition of PDA-PEI can improve the pore size,hydrophilicity and roughness of the base membrane,thus significantly reducing the thickness of PA active layer and improving the uniformity of PA layer.When the concentration of PVDF is 8 wt%,DA and PEI are 2 wt%,the water flux(Jw)of the prepared composite membrane M8/PA reaches 29.98 L·m-2·h-1,and the ratio salt reverse mixing flux(Js/Jw)is only 0.018 g/L,which is obviously better than that of the Control/PA,Jw=7.23 L·m-2·h-1,Js/Jw=0.50 The separation coefficients of MG and NaCl(aMG/Nacl),J of M8/PA and Control/PA are 2.69 and 1.13 respectively.The dye is retained and NaCl is passed by the M8/PA,which has good separation ability of dye/salt.2.The PVDF/PDA-PEI solution was evenly scraped onto the non-woven fabric treated with polymethyl two siloxane(PDMS).After the NIPS process,the membrane was cured and the PA active layer was prepared by interfacial polymerization.Finally,the self supporting TFC-FO membrane was obtained by stripping the non-woven fabric.In the process of mechanical stripping,the coarse texture of the non-woven fabric and the "transfer" of PDMS to the lower surface of PVDF/PDA-PEI membrane can significantly improve its hydrophobicity,In the AL-FS mode,the reverse diffusion of solute on the absorbing liquid side is inhibited,the effective osmotic pressure is increased,and the internal concentration polarization of the composite membrane is reduced,thereby increasing the pure water flux.The water flux of the self-supporting composite membrane P8-T*treated by PDMS was 30.53 L·m-2·h-1,and the ratio of salt back mixing was 0.69 g/L.compared with the self-supporting composite membrane P8-T(Jw=24.41 L·m-2·h-1,Js/Jw=0.99 g/L)without PDMS treatment,the forward osmosis performance was significantly improved.3.PVDF/PHEMA membranes with smooth and rough surface structures were prepared by vapor combined with liquid phase induced phase separation(V-LIPS),and the TFC-FO membrane was prepared by interfacial polymerization.It is found that the smooth basement membrane is easy to form a thick PA layer,and the water flux is very low.The "willow leaf"rough basement membrane formed a thin PA layer,and the rough surface structure increased the contact area with water,so the water flux increased.After modified by AEPPS/TA,the-NH2 reacts with the PA layer further,which increases the thickness of the PA layer and makes the surface rougher.Moreover,the modification of AEPPS/TA in the channel improves the permeability of TFC-FO membrane.When 0.6 M NaCl was used as the extraction solution,the water flux was 16.22 L·m-2·h-1,and the zwitterions were both positively charged and negatively charged.The electrostatic interaction made the membrane have good selectivity for inorganic salts,the rejection rate was more than 99%,and it had good forward osmosis desalination performance.