Selective Layer Modification And Membrane Performance Of Thin Film Composite Forward Osmosis Membrane

Forward osmosis technology has attracted considerable attention in various fields,such as desalination,power generation,sewage treatment,food and pharmaceutical industries due to its low energy consumption,low membrane fouling tendency and high water recovery.Forward osmosis membrane is the core of the forward osmosis technology.The state-of-the-art of forward osmosis membrane is the thin film composite(TFC)membrane,which consists of a dense selective layer and a porous support layer.As an important part of the TFC membrane,the selective layer determines the membrane performance.However,the selective layer suffers from the permeability-selectivity trade-off and fouling problem.Here,the selective layer of TFC membrane were prepared by the interfacial polymerization,coating and layer by layer self-assembly method,and modified by the blending,grafting,etching and chemical crosslinking method to improve the membrane performance.The effect of selective layer property(thickness,hydrophilicity,porosity and type)on the forward osmosis membrane performance(permeability,selectivity and antifouling property)was investigated in detail.The main contents are listed as follows:(1)A hydrophilic modified polyethersulfone support layer was prepared by"in-situ crosslinking-phase conversion"method.It was found that the hydrophilic additive improves the permeability,hydrophilicity,porosity of support layer and reduces the mechanical properties of support layer.The hydrophilic modified support layer affects the interfacial polymerization process,resulting in a thinner and smoother polyamide selective layer.The resulting TFC membrane shows reduced structural parameters and improved water flux,which is mainly attributed to the optimized support layer;the good bonding force between the support layer and the selective layer endows the TFC membrane with excellent salt rejection.Moreover,the prepared forward osmosis membrane has higher flux and lower salt reverse osmosis flux than commercial CTA and TFC membranes.(2)A dual-functionalized TFC forward osmosis membrane with zwitterionic and silver nanoparticles(Ag NPs)was prepared by second interfacial polymerization and in situ synthesis method.The effects of modification on the structure and properties of the selective layer were systematically studied.The optimized TFC membrane shows an improved water flux while retaining high salt rejection.The dual-functionalized membrane surface integration of both passive and active antifouling modes,improves the antifouling efficiency of the membrane against organic pollutants and microorganisms.Moreover,the Ag NPs can be regenerated after depletion,which increase membrane life.(3)A novel forward osmosis membrane with high porosity selective layer was prepared by template method.ZIF-8-NH₂ was used as the templated nanofillers.The solvent volatilization method is used to control the amount of the nanofiller,thus reduces the preparation cost.In addition,the controlled deposition of nanofiller can be covered or wrapped by the polyamide selective layer,which reduces the risk of defects formation of the selective layer after etching the nanofiller.After removal of the nanofillers by acid etching,the water flux of the membrane in the FO and PRO modes was increased by 115%and 130%,respectively,while maintaining the salt rejection.Moreover,the membranes of this work have higher water flux and lower salt reverse osmosis flux than commercial HTI TFC and HTI CTA membranes.(4)Three different nanofiltration-like selective layers with different charge were prepared by interfacial polymerization,coating and layer by layer self-assembly methods,respectively.The resulted membranes were used to treat the wastewater containing specific contaminants.For the firstly membrane,polyethyleneimine was used to modified the polyamide layer.The positively charged surface endows the membranes with high heavy metal ions(Cu²⁺,Ni²⁺,Pb²⁺)rejection(>96%).The incorporated dopamine-modified ZIF-8 nanoparticles in the selective layer improve the permeability and selectivity of membrane.For the secondly membrane,PVA gel was used as selective layer by coating method.The resulted membrane shows excellent organic pollutants resistance due to its hydrophilic,smooth and non-charged surface.The incorporated modified ZIF-8 nanoparticles in the selective layer improve the permeability and selectivity of membrane.For the thirdly membrane,zwitterionic polymer was introduced on the surface of selective layer by LBL method.The modified membrane has good anti-fouling performance for organic pollutants with different charges.Moreover,the selectivity of membrane is also improved.