Preparation Of Persistent High-Performance Ultrafiltration Membrane Based On Reaction Enhanced Surface Segregation

Membrane technology has become the key technology for wastewater treatment and water resource recovery,but it is restricted by membrane fouling problem.Surface segregation is an in situ membrane modification method.Amphiphilic segregation agents are enriched at membrane surfaces to form antifouling molecular brushes during the non-solvent induced membrane formation process,which renders surface segregation with advantages of simple operation,uniform modification and no pore blockage.The antifouling property of membranes depends on the enrichment of segregation agents on membrane surfaces,while,the persistence of antifouling property depends on the stability of segregation agents on membrane surfaces.In previous studies,the enrichment and stability of segregation agents were usually controlled by tuning the hydrophilic and hydrophobic segment ratios of segregation agents.However,the lack of flexibility and accuracy usually results in the enrichment problem of“undersegregation”and“oversegregation”as well as the stability problem of“leaching out”.In this study,pre-reaction and in situ reaction were introduced to manipulate surface segregation process,in order to synergistically increase the surface enrichment and stability of segregation agents,and synchronously improve the flux and rejection of membranes.As a result,persistent high-performance ultrafiltration membranes were prepared.The pre-reaction enhanced surface segregation method:The segregation agent precursor and polyethylene glycol were added into the casting solution.Before the surface segregation process took place,the segregation agents,which had high hydrophilic and hydrophobic segment ratio and interpenetrating structures with the membrane bulk polymer,were generated by the pre-reaction.The high hydrophilic and hydrophobic segment ratio led to the enhanced enrichment of hydrophilic segments on membrane surfaces,while the interpenetrating structures led to the improved stability and pore-forming ability of segregation agents.The resultant membrane could effectively reject bovine serum albumin(rejection of 97.3%),and kept a persistent pure water flux of 230 Lm^-2h^-1bar^-1 in multiple fouling cycles.The in situ reaction enhanced surface segregation method:Polyvinylpyrrolidone(good hydrogen bond receptor)was added in the casting solution as the segregation agent,and tannin or polyacrylic acid(strong hydrogen bond donor)was added in the coagulating bath as the crosslinking agent.During the surface segregation process,hydrogen-bonding induced in situ crosslinking reaction took place and generated a hydrogel layer at the membrane surface,rendering the membrane surface with antifouling property.The thickness and density of the hydrogel layer could be manipulated by the type,molecular weight and concentration of crosslinkers.The dense hydrogel layer could reject small organic dyes like Congo red,and kept a stable pure water flux of 135 Lm^-2h^-1bar^-1 in multiple fouling cycles.The loose hydrogel layer could completely reject emulsified oil,and kept a persistent pure water flux of 580 Lm^-2h^-1bar^-1 in multiple fouling cycles.The in situ reaction enhanced surface segregation method with etching post treatment:The in situ reaction of block copolymer Pluronic F127 and tannic acid during the surface segregation process could manipulate the"nucleation and growth"process of near surface pores of PES membrane,leading to the decrease of pore sizes and increase of pore density.The generated hydrogel layer could be removed by alkaline etching,leaving the pores of PES membrane.The resultant membrane could effectively reject bovine serum albumin(rejection of 99.3%),and kept a persistent pure water flux of 390Lm^-2h^-1bar^-1 in multiple fouling cycles.The pre-reaction and in situ reaction enhanced surface segregation method:Based on the surface segregation method enhanced by the pre-reaction of fluorinated GO and Pluronic F127 and the in situ reaction of Pluronic F127 and tannin,discrete low surface energy domains and continuous large-size hydrophilic domains were constructed on membrane surfaces.By this way,the membrane flux was remarkably increased with the antifouling property of the low surface energy effect.The resulting membrane could completely reject emulsified oil,keeping a persistent pure water flux of 1500 Lm^-2h^-1bar^-1 and a persistent fouling flux of 1260 Lm^-2h^-1bar^-1 in multiple fouling cycles.