| Aromatic polyamide(PA) thin-film composite(TFC) reverse osmosis membranes, as the dominat type of reverse osmosis membrane, have been widely used in desalination of brackish and sea water, wastewater filtration, and separation and purification of chemical and biological products commercial for their characterics of high selectivity and permeability. However,PA-TFC membranes still need to be improved for enhanced resistance to fouling and chlorine and removal to boron. Surface modification has been proven to be an effective approach to improve membrane antifouling property and chlorine stability through modulating membrane surface property.This study focused on the surface modification of aromatic polyamide thin-film composite reverse osmosis membrane for improve membrane properties by grafting chitosan. Chitosan(CS)was grafted on the surface of polyamide RO membrane with potassium persulfate as initiator and glutaraldehyde as cross-linking agent, respectively. ATR-FTIR was used to study the chemical composition of membrane surface. Membrane surface morphology was observed by AFM and SEM. Membrane surface zeta potential was characterized by streaming potential measurement and the surface hydrophilicity was characterized by contact angle measurement. Reverse osmosis performance and anti-fouling property of the raw and modified membranes were investigated through cross-flow permeation tests. The chlorine resistance of membranes was studied through soaking tests with the sodium hypochlorite as chlorinating agent. The azomethine-H acid spectrophotometric method was used to test the removal of boric acid of the PA-TFC membranes.The results are concluded as follows:(1) Chitosan macromolecules were grafted onto the RO membrane surface through two approaches. FTIR analysis confirmed that chitosan could be grafted onto the surface of the polyamide thin-film composite reverse osmosis membrane with potassium persulfate as initiator and glutaraldehyde as cross-linking agent, respectivley. The grafting of CS resulted in improved surface hydrophilicity, reduced surface negative charge at neutral pH and increased surfaceroughness. The chitosan-grafted membranes exhibited an improved salt rejection and a lowered permeation flux.(2) Cross-flow permeation tests with different kinds of foulants of bovine serum albumin(BSA) and sodium dodecyl sulfate(SDS) and dodecyl trimethyl ammonium bromide(DTAB)showed that the chitosan-grafted membrane possessed improved antifoulong property compared with the unmodified membranes. The strengthened anti-adsorption capacity of the modified membrane to bovine serum albumin and sodium dodecyl sulfate is maily due to its enhanced surface hydrophilicity, while the improved fouling rsistance to dodecyl trimethyl ammonium bromide is mainly attributed to its declined surface negative charge.(3) The results of chlorination experiments also illustrated that the modification performaed in this work could effectively improve the chlorine stability of the aromatic polyamide membrane. After chlorination of 36000 ppm·h sodium hypochlorite, the salt rejections of the raw membrane, potassium persulfate-initiated chitosan-grafted membrane and glutaraldehyde-linked chitosan-grafted membrane decreased by 16.5 %, 2.4% and 1.9%, respectively. The chlorine resistance of the RO membrane was improved significantly after grafting chitosa.(4) The Azomethine-H spectrophotometric method was adopted to test the removal ability to the boron acid. The boron rejection of the chitosan-modified membranes obtained through different ways was higher than that of the unmodified membranes. Compared with the the raw membrane, the boron rejections of the potassium persulfate-initiated chitosan-grafted membrane and the glutaraldehyde-linked chitosan-grafted membrane increased by 19.6% and 40.5%,respectively. |
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