Preparation Of Composite RO Membrane With High Chlorine Resistance

Crosslinked aromatic polyamide composite reverse osmosis (RO) membranes have become the mainstream of RO membrane products. However, such membranes are susceptible to free chlorine. The free chlorine in RO feed stream can cause degradation of crosslinked aromatic polyamides and ultimately result in membrane failure, which shortens membrane life and greatly limits the further application of RO membranes. Therefore, based on the further study of degradation mechanism by chlorination for crosslinked aromatic polyamide membranes, development of composite RO membranes with high chlorine resistances is of great theoretical and practical significance.The degradation mechanism by chlorination for crosslinked aromatic polyamide membranes was investigated mainly with the self-made composite RO membranes which were prepared under optimized experimental conditions. It was found that by free chlorine treatments, the crosslinked aromatic polyamides in the surface of composite RO membranes followed multi-step degradation process, that is, the end amine groups of crosslinked aromatic polyamides were first chlorinated. Then, with increasing chlorination intensity or with decreasing pH value of free chlorine solution, the reversible chlorination of amide N-H groups to form amide N-Cl groups occurred for crosslinked aromatic polyamides, accompanied by weakening or destruction of the hydrogen bonds among polymer chains. After that, irreversible chlorine substitutions in aromatic rings were formed through the Orton rearrangement of the amide N-Cl groups, and at the same time, hydrogen bonds among polymer chains were seriously damaged.Further crosslinking treatments were performed for crosslinked aromatic polyamide composite RO membranes. In the treatment processes, three crosslinkers 1,6-hexanediol diglycidyl ether (HDGE), adipoyl choride (APC), and hexamethylene diisocyanate (HDI) which all contain flexible aliphatic chains were used. HDGE mainly reacted with the end amine groups in aromatic polyamides. APC and HDI could react with both the end amine groups and the amide N-H groups in aromatic polyamides. After further crosslinking treatments, the membrane surface contact angles increased, surface zeta potentials decreased, but there was no obvious change in membrane surface morphology. The membranes with further crosslinking treatments had improved salt rejections but decreased water fluxes. Moreover, the membranes were shown to possess high chlorine resistances with small changes in permselectivity after free chlorine treatments, which is much more obvious for the membranes crosslinked by HDI.3-monomethylol-5,5-dimethylhydantoin (MDMH) molecules were grafted onto composite RO membrane surfaces by the reactions with active groups (mainly acyl chloride groups) in the nascent crosslinked aromatic polyamides. After grafting with MDMH, the membrane surface contact angles decreased, but there was no obvious change in membrane surface morphology. After free chlorine treatments, the permselectivity of MDMH-grafted membranes changed slightly, and it was almost constant when treatments with free chlorine lower than 500 ppm·h. These results suggested that MDMH-grafted membranes had high chlorine resistances. Moreover, free chlorine treatments endowed MDMH-grafted membranes with regenerable sterilization functions and substantial preventions against biofouling.3-allyl-5,5-dimethylhydantion (ADMH) molecules were introduced into commercially available aromatic polyamide RO membrane surfaces by free-radical graft polymerization. After graft polymerization with ADMH, the membrane surface contact angles decreased, and the absolute values of zeta potentials decreased. After free chlorine treatments, the permselectivity of ADMH-grafted membranes changed slightly. After the ADMH-grafted membranes were treated with 1500 ppm·h chlorine at pH 4, the chlorination of crosslinked aromatic polyamides can be neglected as compared with the chlorination of grafted ADMH chain blocks. This confirmed that the grafted ADMH chain blocks played as sacrificial pendant groups when crosslinked aromatic polyamides suffered from chlorine attacks, which resulted in increases in the chlorine resistances for ADMH-grafted membranes. Moreover, free chlorine treatments endowed ADMH-grafted membranes with regenerable sterilization functions and substantial preventions against biofouling, which was similar to that for MDMH-grafted membranes.