| Aromatic polyamide (APA) thin-film composite (TFC) membranes are now the most dominating reverse osmosis (RO) membranes used in many desalination and water purification applications due to their excellent performance. Membrane fouling is currently one of the major problems for the aromatic polyamide thin-film composite reverse osmosis membranes, and significant research interest is attracted in discovering more fouling-resistant and easy-cleaning versions of these membranes through surface modification. This study focuses on the surface modification of the PA TFC RO membranes with thermo-responsive copolymers based on N-isopropyl acrylamide (NIPAm) for improved membrane properties, as well as the anti-fouling and easy-cleaning properties. Firstly, thermo-responsive copolymers P(NIPAm-co- Am) with certain lower critical solution temperature (LCST) were synthesized by free radical copolymerization in aqueous solution, and their thermo-responsive properties were studied. The resultant copolymers were then used to modify the commercial PA-RO TFC membranes by dip-coating in aqueous solutions, the influences of the modification conditions and the acid or base treatment on the surface properties of the modified TFC membrane were studied in detail. Finally, the effects of the modification conditions, feed temperature and acid or base treatment on the reverse osmosis performance of the modified TFC membranes were investigated through cross-flow permeation test employing a laboratory-scale test unit; additionally, the anti-fouling and cleaning properties of the modified TFC membranes were also evaluated through fouling experiments with the pollutant of BSA. The conclusions from the experimental results were as follows:(1) The LCST of P(NIPAm-co-Am) could be modulated by varying the composition of the copolymers. The LCST of the P(NIPAm-co-Am) in aqueous solution, or salt aqueous solution, or aqueous solution of different pH increased linearly with the weight ratio of hydrophilic monomer acrylamide (Am) to monomer NIPAm. The LCST of P(NIPAm-co-Am) in salt aqueous solution was lower than that in aqueous solution. The LCST in strong acid or base solution was relatively higher, while it was relatively lower in weak acid solution.(2) The thermo-responsive copolymer P(PNIAm-co-Am) could be successfully coated on the surface of the commercial TFC PA RO membranes through dip-coating method. As the coating solution concentration increased, the amide content in the skin layer of the surface of the modified TFC membrane increased, while the atomic ratio of O/N decreased. The surface morphology of resultant modified TFC membrane was somewhat compacter than the unmodified TFC membrane, and appeared to comprise a more nodular structure due to the partial assembly of the deposited copolymer. The coating solution concentration and the composition of the deposited copolymer affected the hydrophilicity of the resultant modified TFC membrane; For coating solution concentrations between 20 to 200 ppm, the hydrophilicity of the modified TFC membrane increased with the increasing of coating solution concentration; For the same coating solution concentration, the hydrophilicity of the resultant membrane ascended as the hydrophilic content Am of the coated copolymers increased. Furthermore, the modified TFC membranes deposited thermo-responsive functional copolymer had the properties of hydrophilic / hydrophobic transition. The hydrophilicity of the modified TFC membrane surface decreased as the environmental temperature increased, and the hydrophilic / hydrophobic transition temperature ascended as the hydrophilic content Am of the coated copolymers increased. The modified TFC membrane possessed the reversible thermo-responsive property. The hydrophilicity of the modified TFC membrane treated under temperatures above the LCST of the coated copolymer could be resumed after being cooled down under temperatures lower than the LCST.(3) The coating layer of thermo-responsive copolymer P(NIPAm-co-Am) had little influence on the salt rejection of the TFC membrane, while the water flux of the modified TFC membrane depended on the increased hydrophilicity and additional permeation resistance due to the coating layer. The water flux of the modified TFC membrane would be lower than that of the unmodified TFC membranes when the increased surface hydrophilicity of the coating copolymer could not compensate for reduction in membrane permeability, while the water flux of the modified TFC membrane would be higher than that of the unmodified TFC membrane when the improved hydrophilicity arising from the coating layer could compensate for the increased mass transfer resistance. For membranes coated under the same coating solution concentration, a relatively higher water flux would be expected for the modified TFC membrane deposited with more hydrophilic P(NIPAm-co-Am) of higher content of Am as the result of increased membrane surface hydrophilicity. For the same copolymer, the water flux of the modified TFC membrane would decrease as the coating solution concentration increased.(4) The water flux of the modified TFC membrane had thermo-responsive properties. At lower feed temperature, the water flux of the modified TFC membrane was higher than that of the unmodified TFC membrane due to the improved surface hydrophilicity. While, as the feed temperature increasing, the hydrophilicity of the membrane surface decreased and the hydrophobicity increased continuously, as a result, the increasing rate of the water flux with feed temperature of the modified TFC membranes was slower than that of the unmodified TFC membrane.(5) The coating layer enhanced the stability of the TFC membrane in strong acid solution. When treated by soaking in strong acid solution, the surface chemical structure of the unmodified TFC membrane changed obviously, while the chemical structure of the modified TFC membrane remained constant. The surface morphology of the modified TFC membrane treated by soaking in strong acid solution appeared more compacter and exhibited unevenly distributed features. The surface hydrophilicity of the membrane treated by soaking in acid or base solution increased as the result of the ionization of the functional groups on the surface. Additionally, the coating layer on the membrane surface would reduce the flux decline in processing acid feed aqueous solutions.(6) Surface modification with P(NIPAm-co-Am) would improve the anti-fouling property and cleaning efficiency of the TFC membrane for the pollutant of Bovine Serum Albumin (BSA). For the feed solution containing 500 ppm NaCl and 100 ppm BSA, the flux decline rate of the modified TFC membrane during the operation time of 35h was 20% lower than that of the unmodified TFC membrane, and the restoring flux ratio of the modified TFC membrane after being cleaned was 12% higher than that of the unmodified TFC membrane. Furthermore, the anti-fouling property and cleaning efficiency of the TFC membrane were affected by the feed pH and ionic strength. At pH near to the iso-electric point (IEP) of BSA, the fouling of the membrane was the most severe, the anti-fouling improvement caused by the hydrophilic coating layer was the slightest, and the flux restoring ratio was the lowest. For feed solution of low ionic strength and pH far from the IEP of BSA, the improvement of anti-fouling property and cleaning efficiency were notable, the membrane fouling was slight and the flux restoring rate was also relatively high. |
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