The Mechanisms Of Ultrafiltration Membrane Aging And Interfacial Behavior During Membrane Fouling Process

Membrane fouling,cleaning and consequent membrane aging have been paid more and more attention.Membrane fouling leads to the degradation of membrane filtration performance,and frequent chemical cleaning and disinfection were proved to have significant impacts on the inherent structure,properties and performance of UF membranes,these irreversible changes happened on membranes were regarded as membrane ageing.Membrane fouling and membrane aging are important obstacles to the sustainable application of membrane processes in water treatment.Firstly,in this research,PVDF ultrafiltration(UF)membranes were exposed to Na Cl O,then the surface chemical composition,physicochemical properties and structural changes of the control and exposed membranes were compared based on the material characterization method and the membrane compression performance test.In addition,the effects of membrane aging on fouling behaviors and filtration performances were investigated using simulated natural organic matter(NOM)compounds in surface water.Results showed different exposure doses lead to variations in PVDF membrane structural/surface characteristics according to different mechanisms,followed the order: the removal of the surface modification substance,polymers chain cross-linking,defluorination and oxygenation reaction.The mechanical properties of membrane were seriously decreased under a certain exposure dose,which was shown that the inherent membrane resistance increased significantly when the transmembrane pressure(TMP)exceeded 0.1 MPa.The irreversible flux decline of the membranes could not only caused by physically irremovable fouling but also irreversible compaction of the membranes due to the formation of macro-voids in the membrane skin layer.Besides,the interaction energy between foulants and membranes with different ageing degrees was assessed by the extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory and the distribution of deposited foulants was explored through confocal laser scanning microscopy(CLSM)to reveal the evolution of aged membrane fouling behavior,cake layer structure and competitive adsorption behavior of mixed foulants.Results demonstrated that the changes in energy barrier between aged membrane and foulants affects fouling behaviors,and membrane aging has the least impact on HA fouling,while BSA fouling of the short-term aged membrane was more serious.When BSA and SA exist together,competitive adsorption and synergistic fouling occur.Secondly,the effects of dopamine coatings on the structural/surface characteristics,the filtration performance and fouling behaviors of aged membranes were investigated in this study.Results indicate that the polydopamine(PDA)coating structures for PSF and PVDF membranes are different,which are regulated by the different aperture ranges of aged membranes.PDA deposited on/in aged PVDF membrane that had large aperture could achieve the synchronous repair of membrane external and pore surface,which not only enhance their surface hydrophilicity and zeta potential but also these properties of the pore channel surface and narrow the enlarged pore size simultaneously,in turn improved the anti-fouling and rejection performances.By contrast,the PDA was formed as an individual layer on the surface of aged PSF membrane with smaller pore sizes and blocked some pores,less reduction of fouling was observed during fouling experiments.Thirdly,the real-time behavior and dynamic characteristics of the interfacial region during filtration are analyzed by fiber-optic reflectance UV-vis spectrophotometry fiber-optic UV-vis spectroscopy and electrochemical impedance spectroscopy(EIS).On the one hand,using fiber-optic reflectance UV-vis spectrophotometry enable real-time and in-situ measurement of the surface foulant concentrations in ultrafiltration,which could qualitatively and quantitative quantitatively monitor and analyze the dynamic behavior of organic fouling.On the other hand,combining with EIS technology,the influence of ionic strength on BSA fouling of membrane was investigated in detail.Results indicate that the flux decline rate decreased slightly when the ionic strengths increased from 0 to 10 and 100 m M during initial filtration stage,however,membrane fouling at higher ionic strengths became more serious at longer fouling duration.The main reason was the hydration forces experience from enhancement to disappearance,accompanied by salting-out dehydration of protein on membrane surface.