Study On The Modulation Of Microstructure And Separation Performance Of The Interfacially Synthesized Thin-film Composite Membrane Via Secondary Surface Cross-linking

Nowadays,environmental protection has become more and more important,and the nanofiltration（NF）membrane separation technology has become a promise technology for wastewater treatment and reclamation due to its high separation efficiency and low energy consumption.The most popular nanofiltration membrane used is the type of thin-film composite（TFC）membrane manufactured through interfacial polymerization（IP）technique.However,the separation layer obtained via IP process usually exhibits a symmetric structure and possesses the trade-off between its selectivity and permeability.Therefore,it is still of great significance to improve the separation performance of the TFC membrane through constructing a separation layer with an asymmetric structure which will greatly reduce the permeation resistance to water while maintaining high rejection ability to solute.In this study,secondary surface cross-linking was adopted to modulate the surface property and separation performance of the interfacially synthesized thin-film composite membrane.A loose separation layer was firstly laminated on the surface of porous polysulfone support through the IP reaction of triethanolamine（TEOA）and trimesyl chloride,then secondary surface cross-linking was performed with the loose separation layer of TEOA/TMC via using poly（acrylic acid）（PAA）to form a relatively dense skin and thus generate a asymmetric separation layer of TEOA/TMC-PAA with dense surface and loose bulk.Membrane properties including surface chemical structure,morphology,hydrophilicity and charge were characterized by ATR-FTIR,XPS,SEM,surface contact angle measurement and Zeta potential measurement.The molecular weight cut-off（MWCO）,permeation flux and separation characteristics to different inorganic salts and dyes aqueous solutions of the obtained membranes were systematically investigated through cross-flow permeation tests.The conclusions obtained from the experimental results are as follows:（1）The structure of TEOA/TMC separation layer could be modulated through the secondary surface cross-linking using PAA.FTIR and XPS analyses confirmed that surface cross-linking was implemented through the esterification reaction between the carboxyl group of PAA and the residual hydroxyl group of TEOA/TMC layer.After secondary cross-linking,the surface of membrane became denser,the thickness of active layer changed slightly,the molecular weight cut-off（MWCO）decreased significantly from 3380 Da of TEOA/TMC to 1640 Da of TEOA/TMC-PAA（corresponding to a decrease of pore radius from 1.31 nm to 0.96 nm）,the surface of membrane became more hydrophilic and negatively charged.（2）Parametric study showed that the separation performance of the obtained TEOA/TMC-PAA membrane was largely affected by the cross-linking conditions such as the PAA concentration and the curing temperature and time for cross-linking.With the increasing PAA concentration,the salt rejection of the membrane increased,the membrane pore size decreased and the permeate flux declined sharply.Within a certain range,the rejection performance of the membrane could be effectively improved through enhancing the curing temperature and prolonging the curing time at a sacrifice of membrane flux.The optimal conditions for secondary surface cross-linking were as follows:PAA=0.025 wt%,pH=2.00,curing temperature=125℃,curing time=7 min.（3）The TEOA/TMC-PAA membrane possesses obvious performance advantages.Compared with the TEOA/TMC membrane with the same pore size（mean pore radius=0.98 nm）,the TEOA/TMC-PAA membrane exhibited a better salt rejection ability and a water permeability higher by 1.86 times.The rejections of membrane TEOA/TMC-PAA to different inorganic salts followed the order of R（Na₂SO₄）>R（MgSO₄）>R（NaCl）>R（MgCl₂）,which is a typical separation characterics of negatively charge nanofiltration membrane.The TEOA/TMC-PAA membrane could effectively remove dyes from aqueous solution,showing removal order of Reactive red>Methyl blue>Congo red>Sunset yellow>Neutral red.It was also found that the TEOA/TMC-PAA membrane had good performance stability and industrial application potential.