Surface Functionalization Of Polyethersulfone Membranes With Zwitterion And Quaternary Ammonium Salts For Anti-biofouling Property

Backgrounds: Membrane separation technology is a very energy-saving and environment friendly water purification treatment,which could meet the requirements of army on-site water collection and ensure the safety of water.But in the practical use it often encounters many problems,one of which is the membrane fouling.During the water treatment,the waterborn bacteria would adhere on the membrane surface and form a biofilm resulting the membrane permeability decreased,and even the membrane material degradation.Although the antibacterial and anti-adhesion materials could inhibit the growth and adhesion of bacteria on the surface by different mechanisms of action,they still had their own disadvantages.The anti-adhesion materials could prevent the bacteria adsorption on the early stage,but it would not achieve 100%.As the bacteria constantly reproducing in the environment,the biofilm would be formed and ultimately cover the entire surface of the membrane.The antibacterial materials could kill bacteria to inhibit the formation of biofilm,but the dead bacteria body and the released biological substances would still cause material contamination.Therefore,it is of great significance to find an ideal anti-fouling membrane for water purification treatment.Objective: Polyethersulfone(PES)is a common organic membrane material with characteristics of heat and corrosion resistance,good mechanical performance,high flexbility,etc.Because of the hydrophobic nature,it is easy to be adsorbed by bacteria on the surface resulting the membrane fouling.In order to solve the problem,this paper had grafted several functional monomers onto the surface of polyethersulfone membrane to improve the anti-fouling ability,and studied on the factors influencing the bacteria contamination.Method:(1)The zwitterion compound MEDSA was grafted onto the surface of commercially PES microporous membrane by plate compressing-UV photografting reaction.The optimal grafting condition was obtained by calculating the garfting rate with different reaction time and monomer concentrations.The modified PES membranes were characterized by ATR-FTIR,XPS,contact angle test and Zeta potential measurement.24 h bacterial static adsorption experiment and bacterial solution filtration test were used to evaluate the anti-adhesion ability of zwitterion.(2)Four quternary ammonium compounds(QACs)were synthesized from DMAEMA by quaternizing with butyl bromide(BB),octyl bromide(OB),dodecyl bromide(DB),or hexadecyl bromide(HB).The products were purified and confirmed by FTIR and 1H-NMR.PES membranes were modified by grafting the QACs respectively and characterized.The antibacterial activities of the modified PES membranes were evaluated by contact antibacterial test.The best antibacterial QAC was chose by analyzing the quantitative structure-activity relationship.24 h bacterial static adsorption experiment and bacterial solution filtration test were used to evaluate the anti-adhesion ability of QACs.(3)The selected QAC was grafted on the surface of PES-g-MEDSA by photografting,and the modified membrane was characterized.By comparing the antibacterial and anti-adhesion effects of three modified membranes,the effects of zwitterions and/or QACs on bacterial contamination were evaluated comprehensively.Results:(1)The plate compressing-UV photografting can achieve PES membrane surface modification.The grafting rate of MEDSA monomer on PES surface was positively correlated with the UV irradiation time and the monomer concentration.When the monomer concentration was 6% and the irradiation time was 30 min,the grafting rate changed little.The surface hydrophilicity and wettability of PES-g-MEDSA increased,and the Zeta potential was neutral.After 24 h bacteria static adsorption,bacteria aggregated on PES surface and formed larger colonies which could produce many metabolites and form biofilms.But little bacteria could be found on PES-g-MEDSA surface.In the bacterial solution filtration experiment,the water fluxes of PES-g-MEDSA decreased slower and the flux recovery rates(RFRs)were higher(90.7% and 85.4%)compared with PES.(2)After modification with four QACs,the membrane pores were narrowed and blocked.We supposed that the QACs have both an electro provider(C=O)and receiver(quaternary ammonium),so the hydrogen bonds could be created inter-or intramolecularly.That might lead the monomers or polymers assembling and forming a skin on the membrane surface.DMAEMA-BB and DMAEMA-OB had stronger the steric hindrance which weak the hydrogen bonds,and the membrane pores were retained.As QACs grafted on the surface,the modified membranes had certain antibacterial activities and the surface Zeta potential became positive.Bacteria testing found the antibacterial activities of the membranes increased with the length of the substituted alkyl chain.Correspondingly,little bacteria were observed on PES-g-DMAEMA-DB and PES-g-DMAEMA-HB by SEM.In the bacterial solution filtration experiment,the water fluxes of PES-g-DMAEMA-DB and PES-g-DMAEMA-HB decreased slower and the RFRs were 88.3% and 92.7%,respectively.(3)The hydrophilicity of the modified membrane was slightly decreased after secondary photografting of PES-g-MEDSA with DMAEMA-HB,but higher than that of PES and PES-g-DMAEMA-HB.The surface Zeta potential was positively status.Antibacterial experiments found that PES-g-MEDSA had almost no antibacterial activity although it had quaternary ammonium structure,and the antibacterial activity of PES-g-SA-HB was higher than that of PES-g-DMAEMA-HB.Bacteria static adhesion experiment showed no obvious bacterial attaching on PES-g-SA-HB.In the BSA adsorption experiment,PES-g-DMAEMA-HB had the highest adsorption capacity on per unit area,while PES-g-MEDSA and PES-g-SA-HB had lower adsorption capacity than PES.In the bacterial solution filtration experiment,the water flux of PES-g-SA-HB decreased slower,and the RFR was 86.5%.Conclusion: The plate compressing-UV photografting was an inexpensive and convenient grafting method,which worked without photoinitiator and oxygen-free operation.The modified PES membrane had some antibacterial and anti-adhesion properties.MEDSA could improve the hydrophilicity of PES,and form a water barrier layer on the surface to inhibit the bacteria and protein adsorption.The surface charge of PES-g-MEDSA tend to be neutral that weak the electrostatic interaction between membrane and bacteria.QACs synthesized from DMAEMA empowered PES with antibacterial activity,which depend on the surface positive charge density and increased with the alkyl chain length of QACs.Although the contact antibacterial effect could inhibit the bacteria adhesion,the high positive charge made the protein more easily adsorbed on the membrane surface.Zwitterions could not only compensate for the lack of hydrophilicity of QACs,which was useful for protein resistance,but also improved the antibacterial activity.PES-g-SA-HB killed the bacteria on the surface by contact-active antibacterial activity of QACs groups,and inhibitted dead bacteria and other biological macromolecules adhesion by zwitterion groups.It was a good anti-fouling membrane with antibacterial and anti-adhesion dual functions.