Modification Of Aromatic Polyamide Thin-film Composite Reverse Osmosis Membrane Through Surface Grafting

Aromatic polyamide thin-film composite membranes (APA-RO-TFC) have become the dominant reverse osmosis membranes for their excellent performance and property. However, the main drawbacks, which limit their extensive application, are the membrane fouling and sensitivity to free chlorine. Hence, the study and development of reverse osmosis membranes with improved anti-fouling property and chlorine resistance is of great interests and significance. The separation performance, perm-selectivity, anti-fouling property and the chlorine resistance of the thin-film composite reveres osmosis membrane are mainly determined by the chemical and morphological structures of the active thin-layer. Therefore, membrane surface modification has become an effective method to modulate the chemical and morphological structures of the polyamide thin-film composite membrane, and thereby improving their separation performance, antifouling property and chlorine resistance.In this work, monomers such as N-isopropyl acrylamide (NIPAm) and acrylic acid (AAc) were employed to modify the polyamide thin-film composite membrane through graft polymerization with Na2S2O5-K2S2O8system as initiator. Firstly, the PA-RO-TFC membrane was modified with PNIPAm and PAAc separately, and the obtained PA-g-PNIPAm and PA-g-PAAc membranes were studied in terms of separation performance, anti-fouling property as well as chlorine resistance. Then, both PNIPAm and PAAc were grafted onto the surface of the PA-RO-TFC membrane consecutively to prepare the modified (PA-g-PNIPAm)-g-PAAc and (PA-g-PAAc)-g-PNIPAm membranes for improved membrane properties including separation performance, anti-fouling property as well as chlorine resistance. The membrane surface was characterized by ATR-FTIR, AFM, streaming potential measurement and contact angle measurement. The separation performance of the composite membrane was evaluated through cross-flow permeation tested with electrolyte solution. The anti-fouling property of the obtained membranes was studied through permeation tested with BSA and SDS as model foulant, respectively, while the chlorine resistances was investigated through evolution the separation performance of the membrane before and after chlorination through immersing tested with chlorinate aqueous solution of different concentration. The experimental results showed that:1. PNIPAm could be grafted onto the surface of the APA-RO-TFC membrane through redox initiated polymerization with Na2S2Os-K2S2O8system as initiator. The grafting of PNIPAm was confirmed by the improved surface hydrophilicity, the intensified strengthen of the characteristic absorption peaks of amide I band and II band at1542cm-1and1662cm-1, respectively, as well as the change of surface morphology characterized by FE-SEM. The permeate flux of the membrane was improved after modification at the expensive of salt rejection. The modified membrane exhibited enhanced chlorine resistance and anti-fouling property due to its improved surface hydrophilicity and smother surface roughness. The phase transition of the grafted PNIPAm promoted desorption of the foulants on the membrane surface during cleaning using warm water, and thereby improving cleaning efficiency.2. PAAc could also be grafted onto the surface of the surface of PA-RO-TFC membrane through redox initiated polymerization with Na2S2O5-K2S2O8system as initiator. The grafting of PAAc was confirmed by the improved surface hydrophilicity and the intensified surface charged after modification. The surface contact angle was reduced from58.8°to42.0°, while the value of surface charge at neutral pH was increased from about42to52mV. The modified PA-g-PAAc membrane showed improved salt rejection for its enhanced surface charge and decreased permeate flux for its increased permeation resistance resulting form the deposition of PAAc layer. In addition, the modified PA-g-PAAc membrane also exhibited improved anti-flouing property to BSA and SDS, and chlorine resistance.3. The permeability and salt rejection of the polyamide thin-film composite reveres osmosis membrane could be improved simultaneous through grafting PNIPAm followed by PAAc ((PA-g-PNIPAm)-g-PAAc) or grafting PAAc followed by PNIPAm ((PA-g-PAAc)-g-PNIPAm), respectively. The surface contact angle of the membrane grafted with two polymers lied in between the values of PA-g-PNIPAm membrane and PA-g-PAAc membrane, and was lower than that of the unmodified membrane. The surface zeta potentials of the (PA-g-PNIPAm)-g-PAAc and (PA-g-PAAc)-g-PNIPAm membranes were much closer to that of the modified PA-g-PAAc membrane. Both the two modified membranes (PA-g-PNIPAm)-g-PAAc and (PA-g-PAAc)-g-PNIPAm showed improved anti-fouling property to BSA and SDS and enhanced cleaning efficiency compared with the unmodified membrane. Additionally, the chlorine resistance of the (PA-g-PNIPAm)-g-PAAc and (PA-g-PAAc)-g-PNIPAm membranes was also superior to the unmodified and modified PA-g-PAAc and PA-g-PNIPAm membranes.