Study On Preparation And Properties Of Polyamide Composite Nanofiltration Membrane

The positively charged PEI-TMC and PEI/PIP-TMC composite nanofiltration（NF）membrane were formed by interfacial polymerization from polyethylenimine（PEI）、the mixture of the polyethylenimine（PEI）and piperazine（PIP）、 trimesoyl chloride（TMC）on a hydrophilic polytetrafluoroethylene（PTFE）substrate.Preparation parameters of the PEI-TMC membrane including monomer concentration,reaction time of interfacial polymerization and curing conditions were systematically discussed to get the optimum membrane preparation conditions.We study the effect of mixed mass concentration ratio on rejection and permeate flux in the process of PEI/PIP-TMC composite nanofiltration membrane preparation to get the optimum membrane preparation conditions.Surface chemical structure,surface morphology,hydrophilicity and charge of the prepared composite membrane were characterized by attenuated total reflectance-Fourier transform infrared（ATR-FTIR）,scanning electron microscopy（SEM）,atomic force microscopy（AFM）,contact angle measurement and zeta potential analyzer.Studied the rejection and permeate flux of the PEI-TMC and PEI/PIP-TMC composite nanofiltration membrane to the four kinds of ion of Mg Cl2、MgSO₄、Na₂SO₄、NaCl and the molecular weight cut-off（MWCO）by the cross-flow filtration tests.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 PEI（or PIP）and TMC on the surface of the microporous PTFE support.The SEM results showed that the PEI-TMC and PEI/PIP-TMC composite NF membranes had a smooth and level morphology.The results of zeta potential measurement indicated that both the PEI-TMC and PEI/PIP-TMC NF membranes were positively charged.The results of contact angle measurement revealed that both the PEI-TMC and PEI/PIP-TMC NF membranes possessed good hydrophilicity with the contact angle of 64.16 o and 87.27 o,respectively.（2）With the increase of TMC concentration,the MgCl₂ rejection of the PEI-TMC composite NF membrane increased firstly and then mainly remained the same,the permeate flux decreased firstly and then stay the same.When the TMC concentration is 0.6wt.%,the monomer reaches a certain degree of crosslinking form dense functional layers and the rejection and permeate flux stay the same;With the increase of PEI concentration,the MgCl₂ rejection of the PEI-TMC composite NF membrane increased firstly and then decreased,the permeate flux decreased firstly and then increased.With the increase of reaction time,the MgCl₂ rejection of the PEI-TMC composite NF membrane increased firstly and then mainly remained the same,the permeate flux decreased firstly and then stay the same.With the increasing curing temperature or curing time,the MgCl₂ rejection of the PEI-TMC composite NF membrane increased and then decreased.The optimal preparation parameters for the PEI-TMC composite NF membrane were as follows: TMC=0.6 wt%,PEI=1 wt%,polymerization time=90 s,curing temperature=70 oC,and curing time=15 min.The desired membrane possessed rejection of 95.7% to Mg Cl2 and the permeate flux of 13.5 l·m-2·h-1 and at the test condition of 0.4 MPa and pH=7.0.The rejection of the four kinds of ion is ordered by MgCl₂﹥MgSO₄﹥Na₂SO₄﹥NaCl,The obtained PEI-TMC composite NF membrane had a MWCO of around 402 Da.（3）The total mass concentration of PEI and PIP is 1.6% in the preparation of PEI/PIP-TMC composite NF membrane.With the decrease of PEI and PIP mass concentration ratio,the rejection to MgCl₂ stay the same and the rejection of MgCl₂ more than 90%.When the PEI and PIP mass concentration ratio is less than 1:3,the rejection of MgCl 2 is sharp drop in the trend and the permeate flux is increased firstly and then decreased.When the PEI and PIP mass concentration ratio is 3:1,the permeate flux achieve the maximum value to 20.3 L/m2 h.The optimal preparation parameters for the PEI/PIP-TMC composite NF membrane were as follows: PEI=1.2 wt%,PIP=0.4 wt%.The desired membrane possessed rejection of 94.8% to MgCl₂ and the permeate flux of 20.3 l·m-2·h-1 and at the test condition of 0.4 MPa and p H=7.0.The rejection of the four kinds of ion is ordered by MgCl₂（95.3%）﹥MgSO₄（86.2%）﹥Na₂SO₄（68.8%）﹥Na Cl（52.3%）.The obtained PEI/PIP-TMC composite NF membrane had a MWCO of around 402 Da and the pure water flux is 20.5 L/m2 h at 0.4 MPa.