Preparation And Performance Study Of PH Stimuli-responsive Nanofiltration Membranes

Environmental stimulus responsive membrane combines the advantages of traditional membrane materials and intelligent materials,and has broad application prospects.As the most common environmental factor in membrane separation process,pH is easy to control,so pH stimulation responsive membrane has become one of the hot spots in recent years.However,most pH responsive membranes are microfiltration membranes or ultrafiltration membranes,which are difficult to be used for the separation of small molecules.Therefore,it is of great significance to prepare a pH responsive nanofiltration membrane for the separation of small molecules.In this work,natural macromolecular chitosan（CS）was selected as intelligent material,and pH stimulation responsive nanofiltration membrane was prepared by CS assisted interfacial polymerization,and was applied to dye separation.The main research contents and results are as follows:（1）Preparation of pH responsive nanofiltration membranes by CS assisted interfacial polymerization and their properties.The pH responsive PIP-CS/TMC nanofiltration membranes were prepared by interfacial polymerization on polyethersulfone（PES）/sulfonated polysulfone（SPSf）/carbon nanotubes（CNTs）supporting membrane.The solution containing chitosan（CS）with ionizable amino groups and piperazine（PIP,2.0 wt%）was used as the aqueous solution,and the n-hexane solution containing trimethyl chloride（TMC,0.5 wt%）was used as the organic phase,respectively.The effect of pH of feed solution on the physico-chemical structure and performance of the nanofiltration membrane was studied in detail.The results showed that the pH response and reversibility of PIP-CS/TMC membrane were the best when CS concentration was 0.3%;the surface roughness,hydrophilicity and pore size of the composite membrane increased at pH 12.When the pH of feed solution increased from 6 to 12,the water flux increased gradually,and the water flux increased from 238.24 L·m^-2·h^-1 to 643.79 L·m^-2·h^-1at 0.5 MPa.The rejection of Na₂SO₄（pH=12）and Mg Cl₂（pH=10）decreased to less than 20%,but the rejection of Na₂SO₄ decreased to 36.80%and the rejection of Mg Cl₂ increased to 99.14%respectively at pH 2.The results show that PIP-CS/TMC membrane is pH responsive and reversible.The adjustable permeability of PIP-CS/TMC membrane is due to the shrinkage and expansion of chitosan aggregates at different pH values.（2）The dye separation performance of PIP-CS/TMC membrane under different pH values.The separation performance of the nanofiltration membrane for dye,dye/salt and mixed dye systems at different pH values was investigated.The results showed that the retention of congo red（CR）and methyl blue（MB）was higher than98%in the range of pH 2-12.When the pH of feed solution is 12,PIP-CS/TMC membrane had good separation performance for CR/Na₂SO₄ system when the concentration of Na₂SO₄ was more than 10 g/L.The rejection of Na₂SO₄ was less than 20%,while the rejection of CR was above 98%.It was found that when the feed solution pH was 12,the rejection rates of coomassie brilliant blue G（CBBG）and methyl orange（MO）were 100%and 19.65%,respectively.The separation of CBBG/MO mixture could be realized by the nanofiltration membrane,and the flux was increased three times and the separation efficiency was improved compared with the membrane at pH 6.