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Study On The Surface Grafting Of The Aromatic Polvamide Thin-film Composite Erverse Osmosis Membrane With Hydrophilic Polymer And Properties Of The Modified Membrane

Posted on:2015-01-14Degree:MasterType:Thesis
Country:ChinaCandidate:G H YaoFull Text:PDF
GTID:2251330428964215Subject:Polymer Chemistry and Physics
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Aromatic polyamide thin-film composite reverse osmosis (RO) membrane has been thedominant reverse osmosis membrane for its high separation performance. However, themembrane fouling and chlorine susceptibility are the two drawbacks that limit its wideapplications. Surface modification, which can be used to modulate the structure of the thin-filmcomposite membrane, has become the important and effective approach to modify the surfaceproperty of the membrane and thereby improving the antifouling and chlorine resistance of thepolyamide reverse osmosis membrane.This study focused on the surface modification of commercial aromatic polyamide thin-filmcomposite reverse osmosis membrane for improving membrane properties by graftinghydrophilic polymer polyvinyl alcohol (PVA). The PVA molecules were grafted onto themembrane surface with potassium persulfate as initiator and glutaraldehyde as cross-linkingagent, respectively. Membrane surface properties including chemical composition, surfacemorphology, surface hydrophilicity and surface charge were characterized by using ATR-FTIR,SEM, AFM, Zeta potential analyzer and contact angle measurement instrument. The reveresosmosis performance and fouling behavior of the raw and modified membranes wereinvestigated through cross-flow permeation tests. The chlorine resistance was studied throughsoaking tests with the sodium hyprochlorite as chlorinating agent and the reverse osmosisperformances of the membrane before and after chlorination were used to evaluate the chlorinestability. Modification parameters were systematically studied for optimal modifying conditions.The experimental results are concluded as follows:(1) FTIR analysis confirmed that PVA molecules could be grafted onto the surface of thepolyamide thin-film composite reverse osmosis membrane with potassium persulfate as initiator.The graftation of PVA enhanced the membrane surface hydrophilicity, smoothed the surfaceroughness and decreased the surface negative charge. The graftation of PVA also lead to a muchmore compact membrane with increased permeation resistance to water, and thereby reducing the permeation flux and improving the salt rejection. Cross-flow permeation tests with differentmodel foulants showed that compared with the unmodified membrane, the PVA-graftedmembrane possessed strengthened anti-adsorption capacity to bovine serum albumin and sodiumlauryl sulfate for its enhanced surface hydrophilicity, and had weakened electrostatic attraction tocetyl trimethyl ammonium bromide due to its declined surface negative charge. The PVA-graftedmembrane showed improved anti-fouling property. Chlorination experiments also showed thatthe PVA-grafted membrane had better resistance to chlorine attack compared with the rawmembrane.(2) PVA molecules could also be grafted onto the surface of the polyamide thin-filmcomposite reverse osmosis membrane with glutaraldehyde as crosslingking agent, which chouldbe confirmed by the fact that the new peaks of1657and1026cm-1were observed in theATR-FTIR spectra after modification. The reduction of water contact angle from57.8°of theraw membrane to42°of the the modified membrane indicated the enhanced surfacehydrophilicity of the PVA-grafted membrane. The graftation of PVA with the GA ascrosslinking agent resulted in a decreased surface negative charge and an increased surfaceroughness. The membrane reverse osmosis performance in terms of water flux and salt rejectioncould be modulated by adjusting the modifying parameters. The PVA-grafted membraneexhibited good antifouling properties to the foulants of BSA, SDS and CTAB compared with theunmodified membrane. Furthermore, under the same chlorination strength, the salt rejection ofthe unmodified membrane decreased dramatically from98.2%to81.4%, while the salt rejectionof the PVA-grafted membrane declines slightly and still remained a value higher than96.5%.The chlorine resistance of the polyamide thin-film composite reverse osmosis membrane wasimproved significantly after grafting with PVA.(3) Compared with the modifying process using potassium persulfate as initiator, themodification process using GA as cross-lingking agent was much easier to be controlled and hadless impact on the perofmance of the virgin membrane, and thereby PVA-grafted membrane with better chlorine-resistance and reverse osmosis performance could be obtained. The graftation ofPVA using potassium persulfate initiator decreased the surface roughness and the modifiedmembrane eahibited better anti-fouling property to BSA. Additionally, the fouling of the smallmolecule material on the two modified polyamide membranes was more severe than that of themacromolecular substance.
Keywords/Search Tags:Aromatic polyamide, Thin-film composite reverse osmosis membrane, Polyvinylalcohol, Surface grafting, Anti-fouling, Chlorine resistance
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