Preparation Of Polyamide-based Loose Nanofiltration Membrane And Its Application In Separation Of Dye/sodium Sulfate Mixture

With the development of China’s printing and dyeing industry,the problem of printing and dyeing wastewater has become increasingly prominent.Nanofiltration membrane process is suitable for the separation of dye/salt mixtures for its advantages of low energy consumption,simplicity,no need to add reagents,and easy to scale applications.However,most of the commercially available nanofiltration membranes have a relatively dense separation layer and negatively charged surface,which are not conducive to the permeation of inorganic salts across membrane.Therefore,it is of great significance to fabricate loose and weakly charged nanofiltration membrane for improved separation efficiency of dye/salt mixtures.In this thesis,on the base of the fabrication of traditional polyamide（PA）composite nanofiltration via interfacial polymerization technique employing monomer piperazine（PIP）,the water phase additive diethanolamine（DEA）was introduced to suppress the cross-linking reaction and construct linear structure in the selective layer,and to fabricate Loose PA/DEA nanofiltration membrane through generating more free volume and making the separation layer looser.Then,a positively charged functional polymer polyquaternary ammonium salt（PQ-10）,was coated onto the surface of the PA/DEA loose nanofiltration membrane to prepare weakly charged loose nanofiltration membrane of PA/DEA/PQ.The physical and chemical properties of the composite membrane were characterized by ATR-FTIR,XPS,FE-SEM,surface Zeta potential tester,and water contact angle tester.The cross-flow filtration experiments were conducted to evaluate membrane molecular weight cut-off,pore size distribution,as well as the separation ability of dye/sodium sulfate.The effect of salt concentration,dye concentration and operating pressure on separation performance,the separation ability of the polyamide-based loose nanofiltration membrane for different dyes,and long-term operating stability were investigated with simulated dye/salt solutions.The conclusions obtained from experiments are as follows:The study on the preparation process of composite membranes showed that the separation performance of polyamide-based weakly charged loose nanofiltration membranes was mainly affected by the concentrations of the water-phase additive DEA and positively charged polymer PQ-10.The increase of DEA content could slow down the diffusion of PIP via the hydrogen bonding effect between DEA and PIP molecules and thus affect the cross-linking reaction.Additionally,the single functional group structure of DEA endowed the membrane with more linear structure and thus pores with relatively larger sizes.The surface of the PA/DEA membrane was negatively charged,and the deposition of PQ-10 through electrostatic interaction could effectively adjust the membrane surface charge.The optimal parameters for preparing polyamide-based weakly charged loose nanofiltration composite membranes are as follows:PIP content=0.10 wt%,TMC content=0.08 wt%,SDS content=0.05 wt%,Na₃PO₄·12H₂O content=0.5 wt%,reaction time=30 s,interface polymerization temperature=room temperature（25.0°C）,heat treatment temperature=80°C,heat treatment time=4 min,PQ-10 content=0.2 wt%and PQ-10 coating time=5 min.The characterization of the microstructure of the composite membrane indicated that the introduction of DEA in aqueous phase and the surface coating of PQ-10 could regulate the microstructure of the composite membrane.ATR-FTIR and XPS analysis confirmed the absorption peaks of C-O-C at 1050 cm^-1 and N⁺（CH₃）₃ at 980 cm^-1in FTIR spectrum,and N⁺（CH₃）₃ peak at 402.1 e V in the N1s high-resolution XPS spectrum of PA/DEA-2.0/PQ-0.2 membrane,indicating the successful participation of DEA in interfacial polymerization and surface coating of PQ-10 on membrane.SEM characterization showed that the surface of membrane PA/DEA-2.0 exhibited a nodular structure and a decreased selective layer thickness compared with membrane PA.The surface nodular structure of membrane PA/DEA-2.0/PQ-0.2 became more pronounced and the separation layer became thicker.After the introduction of DEA in the aqueous phase,the surface zeta potential and contact angle decreased,the surface potential of PA/DEA-2.0/PQ-0.2 membrane coated with 0.2 wt%PQ-10 approached electrical neutrality under neutral conditions.The molecular weight cut-off and pore size evaluation showed that the molecular weight cut-off increased from 363 Da of PA membrane to 1683 Da of PA/DEA-2.0 membrane,and further to 2342 Da of PA/DEA-2.0/PQ-0.2 membrane,and the average pore radius increased from 0.28 nm of PA membrane to 0.47 nm of PA/DEA-2.0 membrane,and to 0.64 nm of PA/DEA-2.0/PQ-0.2 membrane.Dye separation experiments shown that the weakly charged loose nanofiltration membrane PA/DEA-2.0/PQ-0.2 prepared in this work possessed good separation performance to dye/salt mixed aqueous solution.To Congo red（CR）/Na₂SO₄ mixed aqueous solution,Na₂SO₄rejection of PA/DEA-2.0/PQ-0.2 decreased with increasing salt concentration and lowed to 18.6%when the salt content increased to a value closing to real wastewater,meanwhile CR rejection was up to 98.6%.The rejection of CR dye increased slightly with increasing dye concentration at a fixed salt concentration.50 h long-term stability test demonstrated that the CR dye retention of PA/DEA-2.0/PQ-0.2 kept nearly constant around 90%and the Na₂SO₄ rejection fluctuated around 40%,indicating good stability of separation performance to dye/salt mixture.