Study On The Surface Functionalization Of Pvdf Hollow-Fiber Membrane Modified With Polydopamine And Poly(Ethylene Glycol)(PEG)

Membrane fouling is the main impediment for membrane technology in wastewater treatment applications.Thus, surface hydrophilic modification is one of the main ways to improve the antifouling performance of the porous membranes. Inspired by the biological adhesion phenomenon in nature, dopamine was chosen as the research object in this study. Polydopamine (PDOPA) deposition was used to improve the surface hydrophilicity of the PVDF hollow-fiber membrane. In further, PEG-NH2was grafted on the PDOPA layer through the adhesion of PDOPA to functionalize the membrane surface. The effect of modification was investigated under various reaction conditions, and was compared the different membranes fouling resistance through the filtration tests with bovine serum albumin solution, emulsified oil solution and aerobic activated sludge. The optimal cleaning methos was selected within the physical, chemical, and ultrasonic cleaning methods.The membrane surface covering a layer of brown PDOPApolymer after the dopamine coated on the PVDF hollow fiber membrane. By scanning electron microscopy (SEM), the results showed that the membrane pore reduced and membrane surface smoother after the modification. By the SEM analysis, the the changes on membranes mainly occured on the skin layer, and less changes were found in the bulk of membrane. The FTIR analysis showed that the dopamine polymerization reaction was on the surface of PVDF hollow fiber membranes, forming the polymer PDOPA layer which contained the groups, such as NH2, OH and benzene ring. In the further process of the mPEG-NH2grafting on the PDOPA layer, the contact angle (CA) of the modified membranes significantly decreased. In the mofication process,, the contact angle of membranes on the surface decreases, and the modified membrane permeability increased with the increase of the dopamine solution concentration and the coating time. The contact angle further decreased after grafting the mPEG-NH2on the PDOPA modified membranes. And with the increase of concentration of mPEG-NH2, relative molecular weight and the grafting time, the contact angle decreased, and the hydrophilic increased obviously. The lowest contact angle was detected at the grafting temperature between50～60℃. The pure water flux of PDOPA modified membranes increased firstly and then decreased with the increase of dopamine solution concentration and the coating time. The pure water flux of the modified membrane with0.5mg/mL,60min reaction time had the maximum value of219.4L/(m2h),which was27.84%higher than the unmodified membranes. The pure water flux decreased with the increase of the mPEG-NH2relative molecular weight, grafting time and the solution concentration. The pure water flux decreased at first and then increased with the increasing grafting temperature, and it has the minimal flux between50-60℃.In order to investigate the resistance of the hydrophobic material furtherly, the modified membranes filtered the emulsified oil solution. Deposition time and the concentration of dopamine solution affected the decreasing rate of flux, and all the modified membranes got a lower decreasing rate of flux. Compared to the unmodified membranes, the steady flux of modified membranes (8mg/mL dopamine,90min) increased by55.27%. The rejection rate of modified membranes (8mg/mL dopamine,120min) for the emulsified oil increased to95.1%. And all the rejection rates of modified membranes (8mg/mL dopamine,90min) grafting mPEG-NH2were above90%. Dopamine coating can slow down the flux decline rateduring the filtration of the bovine serum albumin solution. The steady-state flux of the30min PDOPA modified membrane was8.72%higher than the unmodified ones for the filtration of BSA, and the rejection rate of the60min PDOPA modified one was66.96%,35.33%higher than the unmodified ones.Finally, the membranes were cleaned by various methods to compare the flux recovery efficiencies. For filtering bovine serum solution, the chemical backwash had an ideal performance.After the backwash cleaning of1%oxalic acid+1%NaClO solution, the flux recovery of unmodified PVDF membrane was53.68%, that of the PDOPA modified membrane was69.56%, and the PDOPA-g-PEG modified membrane was nearly80%. The effect of oxalic acid cleaning for emulsified oil fouled membrane was insignificant.Rising the temperature of the cleaning solution improved the flux recovery of the membrane fouled by the emulsified oil silution. Under the condition of ultrasonic, the flux recovery rate of modified membrane was more than95%by the cleaning with surfactant.