Preparation And Separation Performance Of Quaternary Ammonium Salt Modified Polyamide Nanofiltration Membrane

Nanofiltration is a membrane separation technology that features high efficiency,energy efficiency and easy operation,holding great application potentials in fields of water treatment and ion sieving.Membrane materials play important roles in determining membrane separation performance.The interfacial polymerization of acyl chloride monomers and amino monomers is the most widely used method to prepare polyamide nanofiltration membranes.However,monomers for preparing polyamide nanofiltration membranes are mostly weak electrolytes with poor charge adjustability and hydrophilicity,and there is trade-off relationship between the rejection rate and water permeability of state-of-the-art nanofiltration membranes.In this thesis work,we designed and synthesized quaternary ammonium salts for the surface modification of polyamide nanofiltration membranes.By doing so,both the hydrophilicity and charge characteristics of the pristine membranes were improved,leading to excellent nanofiltration performance in organic desalination and magnesium/lithium separation.Thin film composite nanofiltration membranes were prepared by interfacial polymerization of piperazine and 1,3,5-trimesoyl chloride（PIP-TMC）,which was surface modified by 1-aminoethyl-3-methylimidazolium bromide salt（AMIB）to prepare the PIP-TMC-AMIB membrane.Infrared spectrum and XPS characterizations indicate that the AMIB molecules were firmly grafted onto the surface of PIP-TMC membranes by the amidation reaction of amine gropus（from AMIB）and the acyl chloride group（from the PIP-TMC membrane）.The molecular weight cut-off values of the pristine and modified membranes are 280 Da and 442 Da,respectively.After modification by the AMIB molecule,the mean pore diameter of the membrane increased from 0.49 nm to 0.68 nm,while the water contact angle decreased from 58.3°to 43.0°,showing improved hydrophilicity.The salt rejection and permeability of PIP-TMC-AMIB membrane to Na₂SO₄ are 95±1%and 135±5 L m^-2 h^-1 at 0.6 MPa pressure.Thus the water flux of the modified membrane is about 4 times higher than that of the PIP-TMC membrane.The modified membrane effectively rejected 50 ppm ciprofloxacin,tetracycline and other antibiotics（>90%）.In addition,the modified membrane allowed rapid separation of antibiotic/sodium chloride mixtures to achieve efficient desalination of antibiotics.Last,the modified membrane exhibited good antimicrobial properties by killing 99%of E.coli colonies.PIP-TMC nanofiltration membrane modified by AMIB is negatively charged and shows low rejection rate to cations.To improve the positive charge of the membrane surface,we prepared nanofiltration membrane（PEI-TMC）by interfacial polymerization of polyethylenimine（PEI）and TMC.We then designed and synthesised N,N’-（2-aminoethyl）-tetramethylenediamine（BATB）bearing double amino groups and positive charges.BATB molecules were grafted onto the surface of PEI-TMC by amidation reaction between the amine and acyl chloride groups.The surface roughness of the modified membrane increases from 5.1 nm to 7.3 nm,and the surface energy increases from 30.7 m J/m² to 38.9 m J/m².The rejection rate and water permeability of the modified membrane（PEI-TMC-BATB）to Mg Cl₂ at 0.6 MPa are 93%and 115 L m^-2 h^-1,respectively.The water flux of the modified membrane is about 3.5 times as high as the unmodified PEI-TMC membrane,the modified membrane has excellent operating stability（86 h）.The modified membrane was exploited to separate the Mg²⁺/Li⁺mixture with different mass ratio.The membrane’s water flux was 108 L m^-2h^-1 with the separation factor around 9,which holds considerable potential for efficient separation of lithium and magnesium ion mixtures.