Antifouling Modifications And Mechanisms Of Surface Functional Layer Of Polyamide Thin-film Composite Membranes

Water crisis,consisting of water scarcity and pollution,are widely co nsidered as one of the most crucial global challenges nowadays.Membrane-based process,which exhibits many advantages such as low energy consumption,high separation efficiency,convenience in operation and wide application,plays a significant role in water purification and desalination,to address the problem of water crisis.Thin-film composite(TFC)membranes,the current state-of-the-art membrane technology for desalination due to superior water permeability and selectivity,have become the gold standard for reverse osmosis,nanofiltration,and forward osmosis.However,TFC membranes suffer from the problem of fouling,which leads to reduced water flux,increased energy consumption,shorter membrane lifetime and higher operation costs.Therefore,developing efficient antifouling membrane surface is the key to combat TFC membrane fouling and ensure long term application.This work aims to tailoring membrane surface characteristics by antifouling materials to mitigate fouling of TFC membranes,and to systematically investigate relevant antifouling mechanisms.The first section of this work investigated the antifouling efficacy of hydrophilic silica nanoparticles(Si NPs)after coated onto the surface of TFC membranes.Si NPs,functionalized with either quaternary ammonium N-Trimethoxysilylpropyl-N,N,N-Trimethylammonium chloride(TMAC)or amine moieties 3-Aminopropyl trimethoxysilane(APTMS),were grafted on the negatively charged polyamide TFC membrane via electrostatic interaction during a simple dip-coating process.In order to determine the optimal modification conditions,we conducted the dip-coating process in various concentrations(0.3 wt%,0.03 wt%,0.003 wt%,and 0.0003 wt%)and p Hs(3,5,7,9)of the functionalized Si NP suspension.As a result,0.003 wt% at p H 7 and 0.03 wt% at p H 7 were identified to be the optimal conditions for TMAC-Si NP-TFC and APTMS-Si NP-TFC modifications,respectively,resulting in the significantly reduced contact angles and dense grafting of Si NPs.Furthermore,characterizations of membrane surface properties and transport parameters were operated.Finally,dynamic fouling tests using sodium alginate as a model foulant implies that two modified membranes showed significantly reduced water flux decline,by ~29% compared to the control TFC membrane,which is mainly attributed to the enhanced surface hydrophilicity.Zwitterionic polymers have recently received considerable attention as promising antifouling materials.In the second section,we investigated the antifouling properties of polyamide TFC membranes after grafted with zwitterionic polymers.Firstly,zwitterionic monomer sulfobetaine methacrylate(SBMA)was utilized to grow a dense zwitterionic polymer brush layer(PSBMA)on TFC membrane via atom transfer radical polymerization(ATRP).Fouling resistance of the modified membrane was systematically investigated by dynamic fouling experiments mimicking real fouling conditions.Various techniques including SEM,FTIR,contact angle,and zeta potential were conducted to charact erize the membrane surface properties as well as confirm the successful grafting of PSBMA.Static adsorption tests of proteins indicate less fouling adsorption of modified membrane.In addition,lower membrane-foulant interaction forces quantified by atom force microscopy(AFM)also reveals the significant reduced fouling potential of PSBMA-TFC membrane.Fouling experiments with feed solutions containing complex organic foulants(mixture of sodium alginate,BSA,and NOM)further demonstrated the fouling resistance of the modified membrane.Tailoring surface characteristics with increased hydrophilicity is widely accepted as an efficient strategy to improve membrane fouling resistance.Typical hydrophilic materials can be categorized as hydrophilic nanomater ials and polymers.The third section of the current work focused on the previously discussed APTMS-Si NPs and PSBMA,to systematically compared the antifouling efficacies of these two hydrophilic materials after grafted on TFC membranes via dip-coating and ATRP,respectively.By controlling modification conditions,we prepared Si NP-TFC and PSBMA-TFC membranes possessing identical surface hydrophilicity and surface roughness to exclusively examine the effects of surface charge and surface chemistry on organic fouling resistance.Optimal antifouling material were selected,furthermore,the role of various membrane surface characteristics(i.e.,roughness,hydrophilicity,charge,and functional groups)on fouling propensity and relevant antifouling mechanisms are discussed in detail.PSBMA-TFC membrane exhibited significantly higher fouling resistance than Si NP-TFC membrane in adsorption tests of proteins and bacteria as well as in dynamic fouling experiments.While high surface hydrophilicity is necessary for improving antifouling property of TFC membranes,absence of membrane foulant electrostatic attraction and shielding of carboxylic groups thus reduced calcium-ion induced complexation are critical to achieve high fouling resistance.In the fourth section of this research,we investigated the antifouling architecture for combating biofouling of TFC membranes.Specifically,we demonstrated a new pathway for the fabrication of dual functionality with combination of “offensive” and “defensive”strategies,by grafting both zwitterionic polymer brushes and Ag NPs on the surface of TFC membrane,to impart both antiadhesive and antimicrobial properties.Two different membrane architectures(Ag-PSBMA and PSBMA-Ag TFC)were developed by changing the sequences of PSBMA,Ag NPs grafted on the membrane surface.Static adsorption tests of proteins and bacteria indicate that the latter architecture,PSBMA-Ag TFC membrane,shows the minimized adsorption of organic foulants,as well as maximized bacterial inactivation.Dynamic biofouling experiments and confocal laser scanning microscopy(CLSM)imaging indicated that PSBMA-Ag TFC membranes can effectively inhibit biofilm formation thereby reduce biofouling.Finally,we demonstrate the regeneration of Ag NPs on the membrane after depl etion of silver from the surface of the PSBMA-Ag TFC membrane.