Polyethersulfone/quaternized Polysulfone Positively Charged Ultrafiltration Membrane Preparation And Dye/salt Separation Selectivity

The selective separation of dye and salt is a critical issue to be solved in salt dye production and dye wastewater treatment.The development of positively charged tight ultrafiltration?UF?membranes or loose nanofiltration?NF?membranes would provide an effective way to achieve highly selective separation between dye and salt.Herein,the aim of this study is to tailor polyethersulfone?PES?/quaternized polysulfone?QAPSf?positively charged ultrafiltration membrane for selective dye and salt separation by regulating the mass ratio of PES/QAPSf using non-solvent induced phase separation?NIPS?.Polysulfone?PSf?as the raw material and trimethylchlorosilane?TMCS?and trimethylamine?TMA?aqueous solutions as chloromethylation reagents and quaternization reagents were used to synthesize.QAPSf by a two-step nucleophilic substitution reaction with chloromethylation and quaternization.The structure and properties of QAPSf were characterized by means of modern analysis methods such as H-NMR,FT-IR,TG,and Raman spectroscopy.The results show that QAPSf with a degree of substitution?DS?of 20%-96%can be successfully synthesized by controlling the reaction time of polysulfone during chloromethylation.QAPSf and PES as raw materials,N,N-dimethylacetamide?DMAc?as a solvent,lithium chloride?Li Cl?as an additive to prepare a casting soltution,and H₂O as a coagulation bath,were employed to prepare a PES/QAPSf blend UF membrane by NIPS method.The morphology and performance of the blend membrane for dye and salt selective separation were characterized.The results show that QAPSf?DS=46%?and PES casting system is completely compatible.The cross section of the resultant blend membrane gradually changed from honeycomb pores to finger pores with the increase of QAPSf content in the casting solution,and the zeta potential of the membranes surface changed from-42.2 m V?PES:QAPSf=100:0?to+1.83 m V?PES:QAPSf=30:70?.When the mass ratio of PES and QAPSf is 30:70,the pure water permeance of the blend membrane was up to 173.6 LMH bar^-1,and the rejection for the positively charged dye Alixin Blue?AB 8GX?and Basic Blue 24?BB24?was 99.6%and 91.6%,respectively.The rejection for negatively charged dyes Congo red?CR?and Methyl Blue?MB?were 99.1%and 87.8%,respectively.It indicated that the membrane exhibited a higher rejection rate for the dyes with positive charge.Simultaneously,all the rejections for the inorganic salts including Na₂SO₄,Mg SO₄,Mg Cl₂ and Na Cl were lower than 10%,displaying a high performance for the dye and salt selective separation.In particular,the blend membrane also exhibited a good separation performance and operating stability for dye and salt mixed systems under the conditions of p H 2-12 and salt concentration 2-10 g L^-1 and 30h continuous operation.The chargeability and morphology of PES/QAPSf blend ultrafiltration membranes were further tailored by using QAPSf with DS=20-96%and PVP K30 as additive in the casting solution system.It was found that the surface Zeta potential of PES/QAPSf blend membrane with PES:QAPSf?DS70%?=30:70 increased to+5.86 m V.Its pure water permeance was 154.28 LMH bar^-1,and the rejection for dye AB 8GX,BB24,CR and MB were 99.9%,88.4%,99.2%,and 85.2%,respectively.Equally,all the rejections for inorganic salts?Na₂SO₄,Mg SO₄,Mg Cl₂ and Na Cl?were lower than 7%.When the content of PVP K30 in the casting solution was 21%,the cross section of PES/QAPSf?DS=46%?/PVP blend UF membrane?polymer concentration=30 wt%?changed into a sponge-like structure from finger-like structure.The breaking strength was up to 6.5MPa.The pure water permeance was 29.2 LMH bar^-1,and the rejection for dyes AB8GX,BB24,CR and MB were 99.5%,91.6%,99.9%,and 99.8%,respectively.Similarly,the rejection for inorganic salts was lower than 9%,indicating a good performance for the dye and salt selective separation.