Study On The Surface Chlorination And Rejuvenation Of The Aromatic Polyamide Thin-film Composite Reverse Osmosis Membrane

Aromatic polyamide thin-film composite reverse osmosis membrane has become thedominant type of reverse osmosis membrane for its advantages of high salt rejection andpermeability. However, the major problems facing with the aromatic polyamide thin-filmcomposite membrane are membrane fouling and susceptibility to chlorine, which will lead to thedegradation to membrane performance and the shortening of membrane using life. Therefore, inorder to extend the application of the aromatic polyamide reverse osmosis membrane in the areaof wastewater treatment, it is of great importance to investigate the degradation mechanism ofthe aromatic polyamide thin-film composite membrane under different conditions and to explorethe methods for the performance rejuvenation of the degraded membrane.The work of this study includes two parts:(1) Chlorination of the fresh aromatic polyamidethin-film composite membrane by investing the degradation mechanism and the effects ofdegradation conditions on the structure and separation performance of aromatic polyamide thin-film composite reverse osmosis membrane.(2) Rejuvenation of the chlorinated membrane withsericin as the functional material and glutaraldehyde as the crosslinking agent through exploringthe effects of different rejuvenating parameters on the structure and separation performance ofthe membrane. Membrane surface properties including chemical composition, surfacemorphology, surface hydrophilicity and surface charge were characterized by using ATR-FTIR,XPS, AFM, SEM, zeta potential analyzer and contact angle measurement instrument. Thereverse osmosis performance in terms of water flux and salt rejection and fouling behavior of thevirgin, chlorinated and rejuvenated membranes were investigated through cross-flow permeationtests. The chlorine resistance of virgin and rejuvenated membrane was evaluated through soakingtests with the sodium hypochlorite as chlorinating agent. The experimental results contained areas follows:(1) Chlorination of aromatic polyamide composite reverse osmosis membrane: ATR-FTIRanalysis revealed that the degradation process of polyamide membrane with the sodium hypochlorite as chlorinating agent was as follow: N-H of amide bond presented in the polyamideskin layer was firstly chlorinated to N-Cl and then the interstrand hydrogen bond was broken.Finally, the occurrence of Orton rearrangement of N-Cl led to aromatic ring chlorination, whichresulted to a deeper polymer chlorination and deformation of the segments of polymer chains.After chlorination, no obvious difference was observed with the membrane, while thehydrophilicity of membrane surface decreased and the isoelectric point became lower comparedwith the virgin membrane. It was found that the aromatic polyamide membrane was moresusceptible to chlorine under acidic condition compared with neutral condition, while underalkaline condition nearly no degradation was detected with the membrane. Lower chlorinetreatment intensity would lead to the chlorination of the amide bond and the damage of hydrogenbond, while higher chlorine treatment intensity would lead to the Orton rearrangement of N-Cland thereby the chlorination of aromatic ring.(2) Rejuvenation of the degraded aromatic polyamide composite membrane: Sericin couldbe grafted to the surface of the chlorinated polyamide composite membrane with glutaraldehydeas cross-linking agent. ATR-FTIR analysis showed that new absorption peaks at1718cm-1and1657cm-1were appeared after rejuvenation. XPS results also demonstrated that the contents ofN, O atoms markedly increased after the rejuvenation. The results of ATR-FTIR and XPS provedthat sericin has reacted with the amino groups from the chlorinated membrane surface throughthe glutaraldehyde as linking agent and a new crosslinking structure has formed. Afterrejuvenation, the membrane surface became more hydrophilic, negatively charged and smooth.Additionally, both of the water flux and salt rejection of all the membranes rejuvenated underdifferent conditions were higher than those of the chlorinated membrane.(3) Properties of the rejuvenated aromatic polyamide composite membrane: the chlorineresistance of the rejuvenated membrane was better than that of the virgin membrane under acidic,neutral and alkaline conditions. The rejuvenated membrane exhibited better antifoulingproperties to BSA compared with the virgin membrane for its smoother membrane surface after rejuvenation. Compared with the virgin membrane, the rejuvenated membrane possessed betteranti-fouling property to the anion surfactant SDS, but poorer fouling resistance to the cationicsurfactant CTAB for its improved surface negative charger after rejuvenation.