| Aromatic polyamide (APA) thin-film composite (TFC) membranes are now the most dominating reverse osmosis (RO) membranes and widely used in the fields of seawater desalination and waste water treatment. The performances of the aromatic polyamide thin-film composite reverse osmosis membrane, such as separation efficiency and anti-fouling property, were mainly determined by the surface properties including surface hydrophilicity and surface charge. One of the key techniques used to modify and/or modulate the separation performance of the aromatic polyamide thin-film composite reverse osmosis membrane is surface modification, through which the membrane surface properties can be modulated for improved membrane properties. Therefore, significant interest remains in preparing aromatic polyamide reverse osmosis membranes with improved membrane properties through surface modification using different functional polymers.This study focuses on the surface modification of the commercial aromatic polyamide TFC RO membranes for improved membrane properties by coating N-isopropylacrylamide-co-acrylic acid copolymers (P (NIPAm-co-AAc)) having the property of thermo/pH-response. Firstly, the thermo/pH-responsive copolymers P (NIPAm-co- AAc) were synthesized by free radical copolymerization of monomers N-isopropylacrylamide and acrylic acid, and the compositions and thermo/pH responsive properties of the obtained copolymers were studied. The aqueous solutions of the resultant copolymers were then used to modify the commercial aromatic polyamide thin-film composite membranes by dip-coating method, the influences of the surface modification on the surface properties of the modified TFC membrane were studied in detail. The reverse osmosis performances of the modified membranes were studied through cross-flow permeation test employing a laboratory-scale test unit. Finally, the anti-fouling and cleaning properties of the modified TFC membranes were also evaluated through fouling experiments with different foulants and cleaning experiments with de-ionized water of different temperatures. The conclusions obtained from the experimental were as follows: (1) The thermo/pH-responsive copolymers P (NIPAm-co-AAc) could be successfully prepared through the free radical copolymerization of monomers N-isopropylacrylamide (NIPAm) and acrylic acid (AAc). The compositions and the lower critical solution temperatures (LCST) of the resultant copolymers could be modulated by varying the molar ratio of monomer AAc to NIPAm in the reaction system. The LCST of the P (NIPAm-co-AAc) in aqueous solution increased with the increasing of the molar ratio of hydrophilic monomer acrylic acid (AAc) in the copolymer and the increasing of solution pH. The LCST of P (NIPAm-co-AAc) in salt aqueous solution was lower than that in aqueous solution.(2) The thermo/pH-responsive copolymers P (NIPAm-co-AAc) could be successfully coated on the surface of the commercial aromatic polyamide TFC RO membranes through dip-coating method. 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 roughness of modified membranes would be lower than that of the virgin membrane when the coating solution concentration was higher.The surface hydrophilicity of the resultant modified TFC membrane was affected by the composition and amount of the deposited copolymer on the membrane surface. The surface hydrophilicity of the modified TFC membrane increased firstly with the increasing of coating solution concentration, and then kept constant for further increase in coating solution concentration. For the same coating solution concentration, the surface hydrophilicity of the resultant membrane ascended as the hydrophilic content AAc of the coated copolymers increased. Furthermore, the modified TFC membranes deposited with the surface layer of the thermo-responsive functional copolymer had the properties of hydrophilic/hydrophobic transition with the environmental temperature. The surface hydrophilicity of the modified TFC membrane decreased as the environmental temperature increased, and the hydrophilic /hydrophobic transition temperature ascended as the hydrophilic content AAc of the coated copolymers increased. The modified TFC membrane also 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.It was found from the Zeta potential measurements that at pHs lower than 3.0, the surface of modified membranes contained more positive charge than the virgin membrane, while at pHs higher than 4.0, the surface of modified membranes were more negatively charged than the virgin membrane.(3) The deposition of P (NIPAm-co-AAc) surface layer would affect the reverse osmosis performance of the polyamide thin-film composite membrane. The pure water flux of the resultant membrane was determined by the increased hydrophilicity and additional permeation resistance due to the surface coating layer. The deposition of P (NIPAm-co-AAc) surface layer would result in increased surface hydrophilicity and an additional permeation resistance for water. The pure water flux of the resultant membrane would be higher than that of the virgin membrane when the additional permeation resistance of the membrane to water could be accommodated by the increase of the surface hydrophilicity. At neutral and alkaline conditions, the membrane surface charge would be enhanced by the deposition of P (NIPAm-co-AAc) surface layer, and as a result, the NaCl permeability coefficient of the modified membrane would be lower than that of the virgin membrane.The reverse osmosis performance of the modified membrane was also affected by the pH of the coating solution. Under the same coating solution concentration, a large amount of copolymers would be deposited on the surface of polyamide thin-film composite membrane at the pH around 4.2. The modified membranes thus prepared showed improved water fluxes and salt rejections.At pH= 2.0, the MgCl2 permeation coefficient of the modified membrane was lower than that of the virgin membrane as the result of the increased surface positive charge on the surface of the modified membrane. While at pH=9.0, the modified membrane showed improved NaCl and Na2SO4 rejection property due to the enhanced surface negative charge on the modified membrane.At temperatures lower than the LCST of the deposited copolymer, the pure water flux of the modified membrane was higher than that of the virgin membrane, and the difference of the water flux between the modified and virgin membrane decreased as the temperature ascended. While at temperatures higher than that of the LCST of the deposited copolymer, the pure water flux of the modified membrane would be lower than that of the virgin membrane as the result of the hydrophilic/hydrophobic transition of the deposited copolymer with the temperature.(4) The results of the fouling experiments revealed that the deposition of P (NIPAm-co-AAc) surface layer improved the fouling resistance of the membrane to BSA as the result of the increase surface hydrophilicity and negative charge. The modified membranes also exhibited improved fouling resistance to the sodium dodecyl sulfate (SDS) and humic acid due to the increment of the surface negative charge.The P (NIPAm-co-AAc) surface coating layer would improve the cleaning efficiency of the membrane. The phase transition of the coating layer at the temperatures above its LCST would loose the layer of foulants on the membrane surface and thus facilitate the removal of foulants located on the membrane surface. |
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