Study On Preparation And Properties Of PA/PTFE Composite Nanofiltration Membrane

With the application expension of the nanofiltration(NF) membrane, demands of properties of the NF membrane become much higher. Therefore, it is critical to develop better membrane materials for improving the combination property of the NF membrane which is affected by both the active layer and the substrate. In view of the facts that polytetrafluoroethylene(PTFE) has the characterics of good resistance to heat, cold, acid, alkali and solvent, this work focused on the study in term of preparation of polyamide(PA)/PTFE composite NF membrane and its application in desalination and dye separation.In the experiments, the PIP-TMC and MPD-TMC composite NF membranes were developed by the method of interfacial polymerization using the hydrophilic PTFE microfiltration membrane(obtained from hydrophilic modification of the hydrophobic PTFE flat-sheet membrane) as support, the piperazine(PIP), the m-phenylenediamine(MPD) and the trimesoyl chloride(TMC) as reactive monomers, the triethylamine(TEA) as additive. Preparation parameters including monomer concentration, additive concentration, reaction time of interfacial polymerization and curing conditions were systematically discussed to get the optimum membrane preparation conditions. Surface properties of the obtained PIP-TMC and MPD-TMC composite NF membranes such as surface chemical structure, morphology, surface charge and hydrophilicity were characterized by ATR-FTIR, FE-SEM, Zeta potential analyzer and contact angle measurement, respectively. The permeation properties including molecular weight cut-off(MWCO) and pure water permeability(PWP) were estimated by cross-flow filtration tests. Finally, removals to different dyes of the obtained PIP-TMC composite NF membrane were also discussed. The conclusions were obtained as follows:(1) The results of ATR-FTIR measurement demonstrated that an active PA skin layer had been successfully formed through the interfacial polymerization of PIP(or MPD) and TMC on the surface of the microporous PTFE support. The SEM results showed that both the PIP-TMC and MPD-TMC composite NF membranes had a dense and uniform morphology. The results of zeta potential measurement indicated that both the PIP-TMC and MPD-TMC NF membranes were negatively charged. The results of contact angle measurement revealed that both the PIP-TMC and MPD-TMC NF membranes possessed good hydrophilicity with the contact angle of 47.8o and 54.3o, respectively.(2) The separation performance of the obtained composite NF membrane was greatly affected by the monomer concentration, additive concentration, polymerization time, curing time and temperature. With the increase of PIP concentration or polymerization time, the Na2SO4 rejection of the PIP-TMC composite NF membrane increased firstly and then mainly remained the same, the permeate flux decreased firstly and then leveled off. With the increase of TMC concentration or TEA concentration, the Na2SO4 rejection of the PIP-TMC composite NF membrane decreased after an initial increase and the water flux increased after an initial decline. With the increasing curing temperature or curing time, the Na2SO4 rejection of the PIP-TMC composite NF membrane increased and then decreased. The optimal preparation parameters for the PIP-TMC composite NF membrane were as follows: PIP=1.5 wt%, TMC=0.4 wt%, TEA=1.5 wt%, polymerization time=60 s, curing temperature=70 oC, and curing time=10 min. The desired membrane possessed a pure water flux of 11.2 l·m-2·h-1 and rejection of 94.6% to Na2SO4 at the test condition of 0.2 MPa and 25 oC. The obtained PIP-TMC composite NF membrane had a MWCO of around 765 Da and good removal effect to dyes. The removals of dyes followed the order of Methyl blue(98.2%) > Congo red(96.7%) > Crystal violet(90.1%).(3)With the increase of MPD concentration or polymerization time, the Na2SO4 rejection of the MPD-TMC composite NF membrane increased firstly and then mainly remained the same, the permeate flux decreased firstly and then leveled off. With the increase of TMC concentration, the Na2SO4 rejection of the PIP-TMC composite NF membrane decreased after an initial increase and the water flux increased after an initial decline. With the increasing curing temperature or curing time, the Na2SO4 rejection of the MPD-TMC composite NF membrane increased and then decreased. The optimal preparation parameters for the PIP-TMC composite NF membrane were as follows: MPD=1.5 wt%, TMC=0.3 wt%, TEA=1.5 wt%, polymerization time=60 s, curing temperature=70 oC, and curing time=10 min. The desired membrane possessed a pure water flux of 6.8 l·m-2·h-1 and rejection of 95.6% to Na2SO4 at the test condition of 0.5 MPa and 25 oC. The obtained MPD-TMC composite NF membrane had a MWCO of around 714 Da.