Study On The Rejuvenation Of Hypochlorite-degraded Polyamide Thin–film Composite Reverse Osmosis Membrane With Tannic Acid And The Properties Of The Restored Membrane

Polyamide reverse osmosis composite membrane has become the dominant reverse osmosis membrane for its high water flux,salt rejection,mechanical strength and chemical stability and has been widely used in the fields of seawater desalination and wastewater treatment.However,this kind of membrane still faces two major problems of fouling and chlorination,which can seriously reduce its separation performance and lifetime.Thus,it is of great practical significance to rejuvenate the degraded polyamide reverse osmosis membrane for prolonging membrane lifespan.In this study,the hypochlorite-degraded polyamide membrane was restored through surface deposition of tannic acid.Three approaches of physical coating,physical coating followed by cross-linking with ferric chloride(FeCl3),and physical coating followed by chemical crosslinking using glutaraldehyde(GA)were adopted to apply the tannic acid molecules on membrane surface.The efficiency of each restoration treatment process was compared in terms of the reverse osmosis performance of the restored membrane.The effects of tannic acid concentration,coating time,pH,Fe3+ concentration,glutaraldehyde concentration,curing time and curing temperature on membrane surface physico-chemical and permeation properties systematically studied to optimize the rejuvenating treatment processes.Physico-chemical properties of the rejuvenated membranes including surface chemical structure,morphology,hydrophilicity,charge and roughness were characterized by attenuated total reflectance Fourier transform infrared spectroscopy(ATR-FTIR),Raman spectroscopy,X ray photoelectron spectroscopy(XPS),field emission scanning electron microscopy(FE-SEM),atomic force microscopy(AFM),static contact angle measurement and zeta potential measurement.Additionally,the reverse osmosis performance in terms of water flux,salt rejection,fouling behavior and performance stability of the rejuvenated membranes were investigated through cross-flow permeation tests and compared with the virgin membrane(before chlorination).The following conclusions were obtained from the experimental results:(1)Tannic acid molecules could be deposited on the surface of the degraded membrane through the restoration approach of physical coating,physical coating followed by cross-linking with FeCl3 or physical coating followed by chemical cross-linking using GA.The peak at 1715 cm-1 of ATR-FTIR spectra indicated the presence of tannic acid molecules on membrane surface,the peaks at 562~630 cm-1 of Raman spectra revealed the coordination-chelation interaction between Fe3+ and tannic acid,while the peak at 1051 cm-1 of ATR-FTIR spectra attributed to CO-C indicated the reaction between TA and GA.XPS results demonstrated that the contents of C and O atoms increased appreciably after the rejuvenation.The peaks at 531.3 eV attributed to O 1s of Fe-O and the peak at 711.6 eV attributed to Fe 2p confirmed the presence of Fe3+ and its coordination-chelation interaction with TA,while the peak at 286.1 eV attributed to C 1s of C-OC confirmed the occurrence of the reaction between TA and GA.The results of SEM and AFM showed that the rejuvenation performed in this study had no evident influence on membrane surface morphology.The deposition of tannic acid with luxuriant phenolic hydroxyl groups also made the membrane surface more negatively charged and hydrophilic after rejuvenation.(2)The salt rejection of the degraded membrane could be resumed to the same as or a little higher than that of the virgin membrane(before chlorination)after restoration through each restoring process,while the flux of the restored membrane was always higher than that of the virgin membrane.The optimum parameters for physical coating process were tannic acid concentration=3000 ppm,coating time=30 min,pH=4.2.The optimum parameters for the process of physical coating followed by cross-linking with FeCl3 were tannic acid concentration=3000 ppm,coating time=40 min,pH=4.2,FeCl3 concentration=480 ppm.The optimum parameters for the process of physical coating followed by chemical cross-linking using GA were tannic acid concentration=1000 ppm,glutaraldehyde concentration=2.0 w/v%,curing time=90 s,curing temperature=60?.The salt rejections of the restored membranes of three restoring processes followed the order of physical coating < physical coating followed by cross-linking with FeCl3 < physical coating followed by chemical cross-linking using GA,while the fluxes of the restored membranes followed the reversed order.The rejuvenated membranes obtained through the restoring processes of physical coating followed by cross-linking with FeCl3 and physical coating followed by chemical cross-linking using GA exhibited satisfactory long-term performance stability.(3)The rejuvenated membranes exhibited better antifouling properties to BSA and SA compared with the virgin membrane for its enhanced surface hydrophilic and negatively charged membrane surface after restoration.However,the rejuvenated membranes had worse fouling resistance to the cationic surfactant DTAB compared with the virgin membrane due to its strengthened electrostatic attraction between membrane surface and DTAB molecules resulting from the improved surface negative charge.